Chemical purification of peat harvesting runoff water

Transcription

1 Process and environmental engineering department Water resources and environmental engineering laboratory Master s thesis Chemical purification of peat harvesting runoff water Oulu Author: Supervisor: Advisor: Advisor: Elisangela Heiderscheidt Prof. Bjørn Kløve University of Oulu D.Sc. (Chem.) Jaakko Saukkoriipi Finnish Environment Institute - SYKE D.Sc. (Tech.) Anna-Kaisa Ronkanen University of Oulu

2 UNIVERSITY OF OULU Abstract of thesis Faculty of technology Department Laboratory Process and environmental engineering department Author Heiderscheidt, Elisangela Name of the thesis Water resources and environmental engineering laboratory Supervisor Kløve, B., professor Chemical purification of peat harvesting runoff water Subject Level of studies Date Number of pages Water protection engineering Master s Thesis 21/04/ Abstract Peat production activities such as drainage of the peatland area and the exposal of peat layers are known to cause an increase on the runoff water discharging from the production sites and an increase on the leaching of pollutant substances into water bodies located downstream. The leaching of pollutant substances such as suspended solids, nutrients, toxic metals and organic matter may result in the eutrophication and siltation of the receiving water bodies causing water quality deterioration and a series of negative impacts to the local aquatic ecosystem. Among the treatment methods developed over the past decades for the treatment of peat harvesting runoff water is the chemical purification via application of metal salt coagulants. The high costs connected to the implementation of chemical purification via addition of liquid pre-hydrolyzed coagulants have until now rendered this treatment method economically feasible only to large production sites. Nevertheless, recent reports have estimated that the costs for the implementation of chemical purification via addition of solid metal salts coagulants are 50 to 75% lower which would also enable the application of this treatment method to smaller peat production sites. The study carried out for this thesis work aimed on developing the chemical purification of peat harvesting runoff water via application of solid metal salts coagulants. The objective was to evaluate the treatment efficiency achieved by the application of four metal salt coagulants (aluminium sulphate, aluminium chloride, ferric sulphate and ferric aluminium sulphate) in the purification of water samples from two different locations via a series of laboratory (jar test) experiments. The goal was to identify among the tested chemicals the best coagulant, optimum dosages, and the influence of process parameters such as ph, temperature and mixing, on load removal efficiency. The obtained purification efficiencies were evaluated based on the percentage removal of total nitrogen (tot-n), total phosphorous (tot-p), phosphate phosphorous (PO 4 -P), suspended solids (SS) and organic matter (TOC) from the water samples being purified. All tested solid coagulants were found to produce high load reduction levels which are in agreement with the reported purification efficiencies achieved by the chemical purification treatment method. The purification efficiencies achieved regarding the removal of concerning substances from the tested water samples were: tot-n (15 44%), tot-p (67 90%), PO 4 -P (63 93%), SS (48 96%) and TOC (20 62%). It was also determined that the purification of peat harvesting runoff water is highly dependent on a series of process parameters such as: coagulant type and dosage, ph, temperature, mixing and the physicochemical characteristics of the water. Ferric sulphate (Ferix-3) proved to be the best performing coagulant. It required, under optimized mixing conditions, around 15% lower dosages (60 to 70 mg/l) than the other tested coagulants (70 to 80 mg/l) for the purification of both water samples. Furthermore, even at lower dosages, it achieved slightly higher load removal efficiencies. Ferric sulphate also produced the best settling characteristics among the tested coagulants. It presented the fastest sedimentation rates and the best final clarification of the supernatant water. However, solid ferric sulphate presented a very narrow optimum dosage range and appears to have suffered to a higher extend the effects of low temperature (5 C) applied to the purification process. Library location University of Oulu, Science and Technology library Tellus Additional information 2

3 Acknowledgements My deepest gratitude goes to those who made this thesis possible; the support I received from my work colleagues, friends and family members has been nothing short than unbelievable. First and foremost I would like to thank my supervisor Prof. Bjørn Kløve for the opportunity of participating in this project and the guidance he has provided. A huge thank you to my advisers Jaakko Saukkoriipi and Anna-Kaisa Ronkanen who have throughout this process offered me priceless advises and moral support. My sincere gratitude goes also to Vapo Oy the commissioner of this project. Vapo s financial support has not only brought this project to its very existence but assured that I could concentrate all my efforts to this thesis work. An especial thank you to: Laëtitia Depre, Tiina Leiviskä and Tuomo Reinikka without whom the laboratory phase of this project would not have succeeded. Thank you also to my colleagues and friends in the Water Resources and Environmental Engineering laboratory who helped me in the thesis writing process. Your feedback and tips have enriched the contents of this work. Last but not least, thanks to my Brazilian and Finnish families. To my mom, dad and three beloved sisters who are geographically so far but so close to my heart. Thank you for every word of encouragement and for your understanding of my distance. Thank you to my partner for believing in me when I mostly didn t. There are no words to describe my love for you, thank you for your support and for making me feel at home here. I cannot forget to mention my four legged boy, thank you for not eating my shoes when I didn t have time to take you for walks! Elisangela Heiderscheidt April,

4 Table of contents Acknowledgements... 3 Symbols and abbreviations Introduction Peat harvesting industry and related environmental impacts Peat harvesting industry in Finland Environmental impacts of peat harvesting and usage Peat harvesting runoff water Peat harvesting runoff water treatment methods Chemical treatment of peat harvesting runoff water Processes involved in chemical purification Coagulation process Removal of particles Removal of dissolved substances Coagulation via addition of metal salts Flocculation process Sedimentation Chemical removal of phosphorous Chemical removal of dissolved organic matter Parameters influencing chemical purification Influence of temperature and ph Influence of mixing Environmental impacts of metal residual discharge Materials and methods Coagulants

5 5.2 Sample collection Laboratory analyses Jar test experiments Results Optimum dosage range and purification efficiency Optimum dosage range of tested coagulants Purification efficiency Settling characteristics Influence of temperature Influence of temperature on purification efficiency Influence of temperature on settling characteristics Influence of mixing Discussion Conclusion and future aspects References

6 Symbols and abbreviations Al Aluminium AlCl 3. 6H 2 O Aluminium chloride Al 2 (SO 4 ) 3. 14H 2 O Aluminium sulphate BAT Best available techniques C Colour [mg l -1 Pt] ca Circa, meaning around, approximately. Ca Calcium C 0 COD D d DOC Fr Fe Fe 2 (SO 4 ) H 2 O Initial concentration of particles in suspension Chemical oxygen demand Diameter of mixer impeller [m] Effective particle diameter [cm] Dissolved organic matter Fraction of particles removes from solution Iron Ferric sulphate g Acceleration due to gravity [cm s -2 ] G Velocity gradient [s -1 ] L Length of rectangular basin [m] Me Metal element n Mixer rotational speed [revolutions per second] Nav. Navettarimpi sample NOM Natural organic matter P Power consumed by mixing device [Nm s -1 ] Piip. Piipsanneva sample PO 4 -P Phosphate phosphorous Q Flow rate [m 3 s -1 ] rpm Revolutions per minute SS Suspended solids sol Coagulant in solution form t Retention time [s] 6

7 T Turbidity [NTU] ti Elapsed time [s] TOC Total organic carbon tot-n Total nitrogen tot-p Total phosphorous T t Turbidity at time t [NTU] ν Horizontal flow velocity [m s -1 ] V Volume of tank [m 3 ] v i Particle settling velocity for elapsed time ti [cm s -1 ] ν s Settling velocity [cm s -1 ] W Dissipation function [N m -2 s -1 ] Yi Fraction of particles in suspension at time ti Y T Z α Total amount of particles removed from solution Depth [cm] Dimensionless power number (mixer and tank geometry) Dimensionless factor reflecting particle shape ρ s Density of the particle [g cm -3 ] ρ w Density of the water [g cm -3 ] µ Dynamic viscosity [g cm -1 s -1 ] or [Ns m -2 ] Formation of precipitate ph Sample with raised ph 7

8 1 Introduction The quality of the water of our rivers and lakes has a direct impact in our environment and water protection measures are an important factor on the road to sustainable development. Several industrial activities produce point and diffuse load sources which discharge into our streams resulting in water quality deterioration and the environmental impacts related to it. It is extremely important that sound research coupled with the best available technologies is applied in the development and optimization of water protection and pollution control measures, so that good water quality characteristics can be maintained or returned to our water systems. Peat production is an important industry to the Finnish economy. However, the drainage of peatlands and other peat harvesting activities are known to increase the amount of water discharging from the catchment areas as well as the amount of pollutant substances being leached from the sites into water courses. (Heikkinen & Ihme, 1995; Kløve, 2001) Over the past decades several treatment methods have been developed and are now applied in the purification of peat harvesting runoff water. Throughout this period, improvements to the different methods surfaced from the increasing awareness within the industry regarding the environmental impacts of the imposed loads and from stricter emission limits imposed by Finnish authorities. Nevertheless, due to factors such as load concentration and volumetric discharge variations, the purification levels achieved by the applied treatment methods do not reach, in all production sites, the requirements set by the Finnish legislation. (Silvan, et al. 2010) Further developments to all peat harvesting water treatment methods are required to ensure the appropriate level of load reduction and the protection of water resources surrounding peat production sites. The aim of this Master s thesis project was to develop the chemical purification of peat harvesting runoff water via application of solid metal salt coagulants. Although chemical purification is considered one of the best available technologies for the treatment of peat harvesting runoff water, it is mostly applied to large production sites due to its economic viability. (Kløve, 1997) Nevertheless, the high investment costs linked to chemical purification process are mainly due to the special treatment structures and facilities required 8

9 in conventional chemical treatment using liquid pre-hydrolyzed metal salt coagulants. According to Vapo Oy inter-company project report prepared by Alatalo and Peronius (2004) the costs of chemical purification treatment implementation are 50 to 75% lower for smaller scale treatment structures using solid coagulants. The optimization of chemical purification via addition of solid coagulants can then render this treatment method feasible also to smaller production sites. The objective was to evaluate and optimize through a series of laboratory experiments the purification efficiency achieved by the application of four different metal salt coagulants to water samples collected from two different locations. The evaluation of the purification efficiency was based on the percentage removal of concerning substances such as phosphorous, nitrogen, suspended solids and organic matter from the water samples being purified. Of interest were the identification of the best chemical among the tested coagulants, the optimum dosages to be applied and the influence of process parameters on load removal. The optimization of the purification process was based on analyzes of the influence of different process parameters on the purification efficiency. Including the influence of coagulant dosage, the mixing effect applied upon and after coagulant addition and the temperature and ph of the water. The main goal of the performed laboratory experiments was the development of guideline information that could be used for the design and implementation of new treatment structures and for the improvement of already existing treatment facilities. 9

10 2 Peat harvesting industry and related environmental impacts Peatlands are estimated to cover around 10 % of the globe surface area. The drainage of peatlands for forestry, agriculture and peat harvesting is an activity which has been performed for centuries resulting in economically important increase in land use for e.g. wood and bio-energy fuel production. However, despite the economical advantages peatland drainage produces a series of negative impacts on the environment. (Marttila, 2010, p. 15) 2.1 Peat harvesting industry in Finland Peatlands cover around 30% of Finland s surface area summing up to 9.3 million ha and although 1.2 million ha are technically suitable for the peat industry and contain 29.6 billion m 3 of peat in situ, less than 1% of the total peatland area is used for industrial purposes. Nevertheless peat is a very important fuel source in Finland, 17 to 20% of the district heat and combined heat and power energy is produced with peat. About one million Finns live in areas where district heat is generated by combined peat and wood combustion. Peat is also widely used in horticulture as a growing medium. Furthermore peat products are suitable for many other purposes such as litter or absorbent peat, composting peat, frost insulation, landfill structures and soil improvement. (Association of Finnish Peat Industries, 2010) The peat harvesting industry is worth hundreds of millions of Euros to the Finnish economy. At the moment, there are eight large and middle-sized peat producing companies, about 250 small-size family businesses and hundreds of private contractors and entrepreneurs in peat harvesting and logistics (Association of Finnish Peat Industries, 2010). Peat harvesting and usage employs, directly and indirectly, more than twelve thousand workers a year. (Paappanen & Leinonen, 2010) Vapo Oy, the commissioner of this thesis work, is the world s leading supplier of peat. The company is owned mostly by the Finnish state (50.1%) and by Suomen Energiavarat Oy a consortium of Finnish energy companies (49.9%). (Vapo, 2010) 10

11 Regarding the usage of peat as a fuel source, Finland is in the up most positions in Europe and in the world. The use of peat as a fuel source in Finland during the past decade has fluctuated but was on average around 25 TWh, value which it is expected to increase to about 29 TWh by Peat provides over 6% of Finland s primary energy requirements decreasing Finnish dependency on energy production from imported fuels such as coal and natural gas. Peat s role in the security of energy supply in Finland is of most significance. (Paappanen & Leinonen, 2010) 2.2 Environmental impacts of peat harvesting and usage According to the World Wide Fund for Nature (WWF) the burning of peat in the production of power in Finland is responsible for approximately 10 million tons of greenhouse gas emissions per year. The emissions caused by the burning of peat are almost as large as the carbon dioxide emissions of the whole traffic sector in the country. (WWF, 2008) In addition to green house gases emissions of peat usage, peat harvesting activities are also responsible for a series of negative impacts imposed to the surrounding environment of peat production sites. The immediate environmental impacts of peat production projects are local direct impacts due to preparation of the mire and the production. These may include for instance; removing of vegetation from the mire, alterations in the landscape, disappearance or changing of bird nesting environments, disturbance of hydrological balance, as well as noise and dust emissions. (Sopo, et al., 2002) As a result of these direct impacts, indirect impacts are also observed. The removal of vegetation results for example in changes to the area landscape causing loss of local biodiversity and has a direct impact on the population living in the area. The drainage of the site via ditching and the exposal of peat layers cause an increase in run-off water from the site as well as changes in ground water supply in the area. The water discharging from peat production sites, if left untreated, will carry with it organic substances, toxic metals, nutrients and particulates. These pollutants will result in the eutrophication and siltation of 11

12 the receiving water bodies causing among other negative effects water quality deterioration, loss in fishing resources and increased risk of floods. (Sopo, et al., 2002) Due to the importance and the scale of the peat harvesting industry in Finland, the environmental impacts related to peat production have not only been observed in the past decades but have also received special attention from the Finnish environmental protection authorities. Legislation has been put into place in order to minimize the impacts, encourage the use of more sustainable and environmentally friendly extraction methods as well as pollution control measures. (Silvan, et al., 2010) 2.3 Peat harvesting runoff water Drainage and other peat extraction activities are known to increase the amount of water discharging from the catchment area as both base-flow and storm-flow (Foundation for Water Research (FWR), 1993). Peat extraction activities are also responsible for the increase in the leaching of suspended solids (SS), dissolved organic carbon (DOC) and nutrients, especially phosphorous (P) and nitrogen (N) into watercourses located downstream of the extraction site. (Silvan, et al., 2010) Although the phosphorus and nitrogen load caused by peat harvesting is only about 1% of the total load to water systems in Finland, locally it can have a significant effect on water quality. (Vieltojärvi, 2005) The leaching of nutrients and suspended matter into sensitive water bodies can cause adverse impacts such as eutrophication, siltation, loss of biodiversity and other symptoms of water quality deterioration. (Heikkinen & Ihme, 1995; Kløve, 2001) Finnish national water protection authorities proposed back in 1998 that by year 2005 a 65% reduction in SS and a 30% reduction in nutrients loads should be achieved over emission levels of (Vieltojärvi, 2005) But according to Silvan, et al. (2010) overall, only a reduction of ca 30% in SS load and 20% in nutrient load has actually been achieved. This leads to the conclusion that although advances have been made improvements to all applied pollution control measures or runoff water treatment methods are still needed. 12

13 2.3.1 Peat harvesting runoff water treatment methods Water and wastewater treatment methods are purification methods designed to remove substances from the water that are harmful to the environment and to human health. Most common harmful substances or contaminants found in natural waters in general are: SS, oxygen demanding substances, nutrients and heavy metals (Lindquist, 2003, p. 36). As previously mentioned discharge waters from peat harvesting areas contain SS, nutrients and dissolved substances. In order to prevent the deterioration of the water quality in the receiving water bodies it is necessary to efficiently treat or purify the runoff waters from peat harvesting areas. The contamination load imposed by peat harvesting areas in watercourses is somewhat difficult to predict due to the large load fluctuations observed in the monitored sites. However, studies have linked the load dependence to: the moisture content of the peat in the extraction area, variation in runoff and peaks in discharge (Kløve, 1997); peat types and their degree of humification (Svahnbäck, 2007); and peat extraction methods (Silvan, et al., 2010). Most important it is to note that different sites will require different treatment methods designed to satisfy the requirements of the peat production process, site hydrology and geology, the sensitivity of the receiving water bodies and the current legislation. According to Kløve (2001), over the past decades several methods have been developed to reduce the SS and nutrient load from peat harvesting runoff waters. However, Kløve also affirms that the nutrient load being leached from peat harvesting areas, even with the developed treatment methods in place, is still high and does not always meet the requirements established by the Finnish authorities. Some of the developed and most used treatment methods for peat harvesting runoff water are: constructed sedimentation ponds; overland flow fields; peak runoff control dam and chemical purification. (Kløve, 2001; Central Finland Regional Environment Centre (CFREC), 2004) - Constructed sedimentation pond: consists of a pond dug in the proximities of the peat harvesting area into which the runoff water is discharged. The retention time is 13

14 designed to allow the removal of suspended solids (and nutrients attached to it) from the water. It has an average SS reduction of 30 to 40%. (CFREC, 2004) - Overland flow field: consists of directing the discharge water of a peat harvesting site into the surface layer of a natural bog or a peat bog. The vegetation of the surface layer works as a mechanical filter separating solids and sludge from the water. Dissolved nutrients are believed to be removed in the peat layer as a result of chemical and biological processes. The average load reductions are: 55% (ditched wetlands area) to 92% (natural wetland area) of SS, up to 49% of total nitrogen (tot- N) and up to 46% of total phosphorous (tot-p). (CFREC, 2004) - Peak runoff control dam: consists of a weir structure which controls the volume of water discharged from the production site. While controlling water discharge rates this structure also traps peat particles, erosion generated substances and nutrients as well as acting as an auxiliary in ditch erosion reduction. Marttila and Kløve (2009) reported the following load reductions for peak runoff control dams: 61-94% reduction in SS, 45-91% reduction in tot-n and 47-88% reduction in tot-p. - Chemical purification: chemical treatment method consists on the addition of chemicals used for treatment of drinking water to the runoff water of a peat harvesting area. The chemicals cause sedimentation of solids and dissolved substances which deposit in the bottom of the sedimentation pond. The average load reductions are: 30-90% of SS; 30-60% of tot-n and 75-95% of tot-p. (CFREC, 2004) Peat harvesting companies are required to attain an environmental permit from Finnish environmental authorities in order to be able to establish a new peat production site. The permit is given on the basis of the Environmental Protection Act and the Water Act. For sites over 150 ha an Environmental Impact Assessment (EIA) is carried out. The issued environmental permit stipulates according to the best available technologies (BAT) and site characteristics the water pollution control method to be applied and monitoring requirements. (Hellsten, et al., 2008) 14

15 Overland flow field is the most used treatment method since (CFREC, 2004) Chemical treatment is also considered one of the best available technologies for peat harvesting runoff water purification and is now applied to sites where overland flow fields cannot be constructed (e.g. lack of space) or do not achieve the required purification levels. The author finds it important to emphasize here that although overland flow fields are considered the most cost effective has the highest utilization rate, there is a huge potential encased within the use of chemical purification treatment. High load removal efficiencies are most certainly achievable if the process parameters are optimized. Chemical purification can easily be combined to other treatment methods serving as the main load reducer or as a final water quality polishing process Chemical treatment of peat harvesting runoff water The chemical treatment of runoff deriving from peat production areas is based on the ability of the chemicals to precipitate SS and dissolved substances, such as nutrients and organic matter, present in the water. The development in the use of chemicals for the purification of peat harvesting wastewater has been slow. Mostly to blame are the observed high costs involved in the treatment implementation and maintenance, as well as its seasonal application due to technical requirements such as the freezing of applied chemicals at temperatures below 0 C. (CFREC, 2004) Kløve (1997) affirmed that the chemical treatment of peat harvesting wastewater, as developed to date, is economically viable only for extraction areas larger than 200 ha. All over the world metal salts of aluminium and iron are widely used in the chemical purification of water and wastewater related to industrial and domestic usages. Specific research in the application of metal salts coagulants for the purification of peat harvesting wastewater is though scarce. A 2004 Vapo Oy internal project report regarding the use of chemical coagulants (Alatalo & Peronius, 2004) was made available to this project. The report contained results achieved by direct field application of two coagulants: aluminium sulphate and ferric aluminium sulphate. Field experiments were carried out at Navettarimpi 15

16 peat production site over the summer periods of 2002 and 2003 with aluminium sulphate and during summer 2003 with ferric aluminium sulphate. Reported dosages of 51 to 107 g/m 3 of aluminium sulphate produced average removal efficiencies of 40 to 45% in SS, 27 to 32% in tot-n, 72% in tot-p and 48% in COD (chemical oxygen demand) concentrations. Dosages of 66 to 165 g/m 3 of ferric aluminium sulphate were required to achieve average load reductions of around 78% in SS, 39% in tot-n, 88% in tot-p and 66% in COD. (Alatalo & Peronius, 2004) The use of iron based coagulants in field tests have also been reported by the Finnish Environmental Institute (SYKE) (2005), but unfortunately no information is presented about the field application conditions, applied dosages or achieved results. It is assumed that the purification levels reported by CFREC (2004) and previously described in this thesis work are also results obtained in field applications. As previously stated, of interest here is the development in the use of solid metal salts coagulants. According to Alatalo and Peronius (2004) the costs of implementation for the application of solid coagulants is around 50 to 75% lower when compared to the conventional purification stations using liquid coagulants. It is of the author s opinion that, via the optimization of chemical purification process parameters it is also possible to create process design guidelines which can reduce the chemical purification method maintenance costs, by reducing chemical dosages and enhancing purification efficiency. 16

17 3 Processes involved in chemical purification Coagulation, flocculation and sedimentation processes are chemically and physically induced purification methods used to remove contaminant substances from the water. Contaminant substances may occur as SS, including colloidal particles with diameter of 0.08 to 1 µm, and dissolved matter. (Lindquist, 2003, pp ) 3.1 Coagulation process Coagulation process is a chemically induced destabilization process used in water and wastewater treatment with the objective of removing from solution non settleable contaminant substances occurring as particulates and dissolved matter. The coagulation of a given solution is achieved via addition of chemical coagulants such as metal salts of iron and aluminium, activated silica, clays, lime, natural and synthetic organic polymers, etc. (Sincero & Sincero, 2003, pp ) Removal of particles There are two main classes of colloidal particles named hydrophobic (low degree of affinity with water) and hydrophilic (high degree of affinity with water) colloids. The amount of water bounded to hydrophilic particles can account to up to ten times the particles dry mass. Although the hydrophobic term establishes no affinity with the water, hydrophobic particles also possess a layer of water molecules strongly bounded to their surface. (Bratby, 2006, pp. 9-10) Colloidal and smaller size particles when in solution are capable of remaining in a disperse state due to electrical repulsive forces acting between them and to some extend to the hydration of the particles surface layer. The term stability refers to this ability of colloidal particles to remain as independent entities within a given dispersion. (Bratby, 2006, pp. 3-4) 17

18 The repulsive property of colloidal particles is due to the electrical forces that they posses. The electrical forces are produced as a result of charges, called primary charges, that the particles posses at their surfaces. The particles primary charges may originate due to two factors; the dissociation of the polar groups and the preferential adsorptions of ions from solution. The primary charges of hydrophobic colloids are mainly due to the adsorption of ions from the medium while the primary charges of hydrophilic colloids are mostly related to polar groups such as carboxylic and amine. Depending on the ph of the solution colloids may attain positive or negative surface charges (Figure 1). (Sincero & Sincero, 2003, pp ) Figure 1 Primary charges of hydrophilic colloid as a function of ph (Sincero & Sincero, 2003, p. 548) The colloidal particles, if their primary charges are strong enough, attract counter ions in the solution which form a compact layer, called stern layer, around the particle surface. Note that the stern layer also contains water molecules and adsorbed hydrated ions. The ions forming the stern layer then attract their own counter ions from the solution and form a looser layer called the diffuse layer (Figure 2). The stern and diffuse layers form the so called electrical double layer of the colloidal particle. When the colloidal particle moves not all charges move with it, only a part of the diffuse or outer layer moves with the particle shearing at a shear plane. Because the surface charges are electrical they posses electrostatic potential which is greatest at the particle surface (Nernst potential) and decreases to zero at the bulk of the solution (Gouy Chapman layer). The electrostatic 18

19 potential at a distance from the particle surface at the location of the shear plane is called zeta potential. The greater the zeta potential the greater is the force of repulsion between the colloidal particles and more stable is the solution. (Sincero & Sincero, 2003, pp ) To destabilize a colloid its zeta potential must be reduced. The reduction of the zeta potential can be achieved by the addition of chemicals. The added chemicals should contain counter ions of the colloidal particles primary charges, which upon addition will neutralize these charges and consequently reduce the zeta potential and enable the occurrence of the coagulation process. (Sincero & Sincero, 2003, pp ) Figure 2 - Conceptual representation of the electrical double layer. (Bratby, 2006, p. 18) A complete coagulation process is a combination of four destabilization mechanisms which include; double layer compression, charge neutralization, entrapment in a precipitate and intra-particle bridging. (Lin & Lee, 2007, p. 376) For the compression of the electric double 19

20 layer to occur and culminate on the coagulation of the colloidal particles, counter ions of the primary charges must be added until the Van der Waals force of attraction between the particles exceed the repulsion forces due to their primary charges. Direct charge neutralization is triggered by the addition of ions of opposite charges that have the ability to direct adsorb to the colloid surface. Entrapment in a precipitate, also known as sweep coagulation occurs when cations of a metal salt forms hydroxide precipitates using colloidal particles as nucleation sites entrapping the colloid in the precipitates. Furthermore as the precipitate sediments it carries down with it a large number of other colloidal particles. Intra-particle bridging or patch coagulation takes place when bridging molecules, mainly polymeric molecules, attach a colloidal particle to one active site and a second colloidal particle to another active site. If the active sites of the polymeric molecule are close to one another, coagulation of the colloidal particles then occurs. (Faust & Aly, 1999, pp ; Sincero & Sincero, 2003, p. 551) Chemical coagulants are substances referred to as electrolytes and polyelectrolytes. Electrolytes are materials that when placed in solution cause the solution to be conductive of electricity due to the charges they posses. Polyelectrolytes are polymers possessing more than one electrolytic site. Electrolytes and polyelectrolytes are able to coagulate and precipitate colloids due to the charges they posses. In natural waters, due to their acidic nature, most particles are negatively charged therefore they repel each other and remain disperse in the liquid if no destabilizing substance or electrolyte is applied. Metal salts of iron and aluminium are commonly used coagulants in water and wastewater treatment. (Lindquist, 2003, pp ) An important phase of the coagulation process is the addition of the chemical coagulant into the stable solution or wastewater it needs to destabilize. The mixing of the coagulant is an important operation for the coagulation process, rapid and throughout mixing provides complete and uniform dispersion of the coagulant added to the water, enabling the four destabilization processes to occur and effective coagulation to be achieved. (Sincero & Sincero, 2003, p. 553; Lin & Lee, 2007, p. 377) 20

21 3.1.2 Removal of dissolved substances Dissolved substances in water and wastewater include: orthophosphates; natural organic matter (NOM) including humic substances and other organic dissolved material such as carbohydrates and sterols, etc. (Lindquist, 2003, p.116) Although traditionally the coagulation process is described in terms of the destabilization of colloidal solutions (as it has been done in the previous section), coagulation process is also responsible for the removal of dissolved substances via direct precipitation or adsorption onto precipitates of metal hydroxide. (Lamsal, 1997) The precipitation of particulates and dissolved matter from wastewater follow different chemical rules. Consequently, different coagulants will present different relative efficiencies on the removal of particulates and dissolved substances. Metal salts of aluminium and iron have also been showed to efficiently remove dissolved substances such as NOM and phosphates from wastewater. (Lindquist, 2003, pp ; Jiang & Wang, 2009) The efficiency observed on the removals of dissolved substances will depend on the type and dosage of coagulants, coagulation ph, water temperature, concentration of NOM and other wastewater characteristics such as alkalinity. (Omoike & Vanloon, 1999; Jiang & Wang, 2009) Coagulation via addition of metal salts The most used chemical coagulants in water and wastewater treatment are salts of aluminium and iron, not only for their effectiveness but also for their ready availability and lower cost (Bratby, 2006, p. 32). Hence the effectiveness of selected aluminium and iron salts as coagulant agents for the chemical purification of runoff water from peat harvesting sites is the focus of this work; the mechanisms or reactions involved in the coagulation processes of aluminium and iron salts are further detailed in the following sections. 21

22 Coagulation mechanism of metal salts The mechanisms or reactions involved in the coagulation process with metal salts will be represented here by reactions related to aluminium sulphate (alum) and ferric sulphate. Alum is the most used salt of aluminium in water and waste water treatment and its chemical formulation is: Al 2 (SO 4 ) 3. xh 2 O, with x assuming values from 13 to 18 and referring to the hydration of the salt. Ferric sulphate chemical formulation is: Fe 2 (SO 4 ) 3. xh 2 O, with x assuming values from 7 to 9. (Sincero & Sincero, 2003, p. 568) For brevity aluminium sulphate and ferric sulphate will be referred to as Me 2 (SO 4 ) 3 without the water of hydration and with Me representing the salt s metal element. When metal salts are added to water they dissociate and react with water molecules (hydrolysis) according to equations (1) to (7). These reactions occur very quickly and all reactions are completed within few seconds forming between other complexes and polymeric species the precipitate metal hydroxides Me(OH) 3 (the downward pointing arrow represents the formation of the precipitate). (Sincero & Sincero, 2003, p ; Gregory & Duan, 2001 Me 2 (SO 4 ) 3 2[Me(H 2 O) 6 ] SO 4 (1) Note that in the subsequent reactions the H 2 O molecules of the formed complexes are omitted for simplicity Me H 2 O Me(OH) 3 + 3H + (2) Me H 2 O Me(OH) H + (3) Me 3+ + H 2 O Me(OH) 2+ + H + (4) Me(OH) H 2 O Me(OH) 3 + H + (5) - Me(OH) 3 + H 2 O Me(OH) 4 + H + (6) 22

23 2[Me(OH)] H 2 O [Me 2 (OH) 2 ] H + (7) Further polymerization of metal complexes also occurs As it can be seen, these reactions are complex and involve dissolution, hydrolysis and polymerization of the metal salt. It is important to note that the hydrolysis reactions are primarily dependent on the ph of the solution (Figures 3 and 4). According to Saukkoriipi (2010, p. 22) for example, ph affects not only to the speciation of the mononuclear aluminium species but also to the speciation and formation of polynuclear aluminium hydroxide complexes The complete but simplified equation including the reactions of the metal salt with the alkalinity present in the water is shown below in equation (8). According to this equation an alkaline substance is required when metal salts are added to the water. The bicarbonate alkaline is used since it is the alkalinity that is always found in natural waters. (Sincero & Sincero, 2003, p. 568) Me 2 (SO 4 ) 3. xh 2 O + 3Ca(HCO 3 ) 2 2Me(OH) 3 + 6CO 2 + 3Ca SO 4 + xh 2 O (8) The complex ions Me 3+, MeOH 2+, Me(OH) + - 2, Me(OH) 4 together with the formed polymeric metal species are effective charge neutralizers. Due to the fact that the hydrolysis reactions occur in fast rates and that the likelihood of the trivalent metal cations finding and reacting with water molecules is much greater than the likelihood of them first reacting with contaminants particles or molecules, the coagulation mechanism which prevail under natural water conditions is sweep coagulation via metal hydroxide (Me(OH) 3 ) precipitate formation. (Lindquist, 2003, p. 126) Sincero and Sincero (2003, p. 555) affirmed that for the effective removal of colloids as much metal sulphate as possible should be converted to the solid precipitate Me(OH) 3 and as much of the concentration of the complex and polymeric ions formed should neutralize the primary charges of the colloids to induce their destabilization. The diagrams proposed by Amirtharajah and Mills (1982, cited in Bratby, 2006, p. 85) and Johnson Amirtharajah (1983, cited in Bratby, 2006, p. 86) shown in Figures 3 and 4 also suggest that the best 23

24 coagulation condition for conventional treatment with aluminium and iron salts would be in the region of Me(OH) 3 precipitation and optimum sweep floc formation. Figure 3 - Coagulation domain diagram for aluminium sulphate (Bratby, 2006, p. 85) Lindquist (2003, p. 20) wrote that in conventional water and wastewater treatment where the objective is the removal of particulates the focus should be directed to the sweep coagulation mechanism hence it is often difficult to achieve rapid and through mixing required for the charge neutralization mechanism to occur. The aforementioned author also established that the optimum ph for the sweep coagulation mechanism to occur with aluminium and iron salts varies consecutively between 5.5 and 6.5 and 5.5 to 8. According to Sincero and Sincero (2003, p. 569) it is nevertheless impossible to attain the optimum coagulation ph and aluminium dosage for the purification of a particular water from the presented metal salts reactions due to their complexity. Consequently these coagulation parameters must be determined in laboratory via jar test experiments. 24

25 Figure 4 Coagulation domain diagram for ferric chloride (Bratby, 2006, p. 86) 3.2 Flocculation process The flocculation process follows the rapid mixing stage where the destabilization reactions occur and the primary flocs are formed. Flocculation is a physically induced process which aims to promote the growth of the primary flocs by enabling them to aggregate and form larger agglomerates. These agglomerates can easily be removed by a subsequent separation process such as sedimentation or flotation. (Vigneswaran & Visvanathan, 1995, p. 61; Bratby, 2006, p. 240) Flocculation is a very important process within chemical purification. An effective flocculation process will produce flocs with good settling characteristics enabling an effective subsequent solid liquid separation. There are two distinct processes within the flocculation process; perikinetic flocculation which arises from thermal agitation of the fluid and orthokinetic flocculation which arises from induced velocity gradient in the liquid. (Bratby, 2006, pp ) 25

26 Perikinetic flocculation Perikinetic flocculation is the aggregation of particles due to the random Brownian movement of fluid molecules. Particles under Brownian motion move and collide with other particles, forming progressively larger agglomerates until the flocs reach a size beyond which Brownian motion has little or no effect. This flocculation process starts immediately after destabilization and it is complete within seconds. (Vigneswaran & Visvanathan, 1995, p. 61; Bratby, 2006, pp ) Orthokinetic flocculation Orthokinetic flocculation is the agglomeration of particles due to induced fluid motion. By inducing gentle motion and creating velocity gradients within the water the suspended particles are encouraged to make contact and form larger agglomerates. (Vigneswaran & Visvanathan, 1995, p. 62) According to Bratby (2006, p. 243) the greater the velocity gradients the more particle contacts there will be in a given time. However, velocity gradients above a critical value will result in small flocs due to the higher rate of breakage of the larger formed flocs. Bratby also affirmed that, the lower the velocity gradient the larger will be the final floc size, although it will take longer for the larger flocs to form. Velocity gradients can be induced by setting the liquid in motion using a wide range of available mixers, among them are: mechanical mixers such as back mixers; hydraulic mixers such as baffled channel and gravel bed mixers. (Vigneswaran & Visvanathan, 1995, p. 64) Lin and Lee (2007, p. 380) reported that a mean velocity gradient ranging from 20 to 70 s -1 together with contact times from 20 to 30 minutes should be kept during the orthokinetic flocculation process. 26

27 3.3 Sedimentation Sedimentation is the removal of settleable solids by the effect of gravity. It is essentially, a solid-liquid separation process which follows the coagulation and flocculation processes. The process takes place in a sedimentation basin which design parameters are obtained from purification process characteristics and requirements such as water inflow rates, settleability of the suspended solids formed during flocculation and space availability. (American Water Works Association (AWWA), 2003, p. 83) The seatleability of the suspended solids is characterized by the settling velocity of the particles (flocs) in suspension. Sedimentation aims to remove the flocs formed and produce a clarified overflow liquid. The sedimentation process or the settling of the suspended solids contained in the water is directly influenced by the characteristics of the water, the system hydraulics and the characteristics of the particles in suspension. These characteristics include the temperature of the water (which influences its properties), the settling basin geometry and overflow rate, the specific gravity of the material in suspension, and the size and shape of the suspended particles. (AWWA & American Society of Civil Engineers (ASCE), 1990, p. 111) The various regimes observed within the settling of particles are mostly referred to as settling types 1 to 4 which are defined according to AWWA (1990, pp ) as follow: - Type 1: Settling or sedimentation of discrete particles in low concentration, with flocculation and other inter-particle effects being negligible. - Type 2: Settling or sedimentation of particles in low concentration but with flocculation. As flocculation occurs, particles masses increases resulting in faster settling rates. - Type 3: Zone settling or sedimentation under the condition where the particles concentration cause inter-particles effects to the extent that the rate of settling is a function of particle concentration. Zones of different particle concentration may develop because of the differences in the particles settling velocity. 27

28 - Type 4: Compression settling or subsidence under the layers of zone settling. The rate of settling depends on the residence time and weight of the solids in the above layers. The sedimentation of flocculent systems is a complex process where the settling velocities of the particles in suspension change with time and depth as the particles agglomerate and form larger flocs. An accurate theoretical analysis of the sedimentation process is also made complicated by the fact that the particles involved are not regular in shape, density or size. The theory related to the settling of particles in ideal systems is nevertheless applied and can serve as a useful guide in the interpretation of such complex systems. (AWWA, 1990, p. 372) For most theoretical computation of settling velocity an ideal system of discrete particle settling with flocculation and other interparticle effects being negligible is assumed. (AWWA, 1990, pp ) The settling velocity of a discrete particles for Re <1 is given by equation (9) known as Stokes law. (Chapra, 1997, p. 300) (9) Where: ν s = settling velocity (cm s -1 ) α = dimensionless factor reflecting the particle s shape (for spheres α = 1) g = acceleration due to gravity (cm s -2 ) ρ s = density of the particle (g cm -3 ) ρ w = density of the water (g cm -3 ) µ = dynamic viscosity (g cm -1 s -1 ) d = effective particle diameter (cm) In practical applications when suspensions of non-uniform particles in flocculent systems are concerned, where the particles or flocs densities are mostly unknown, settling velocities cannot be determined via Stokes law. Practical tests should then be applied in order to determine the particles settling rate. The most common test performed is the column 28

29 settling test which proceeds as follows: The water sample or suspension of interest is placed in a tall clear column and the descending level between the suspension and clear water interface is recorded at frequent intervals. The results are then plotted producing what is known as the suspension settling curve. (AWWA, 1990, p. 383) Bratby (2006, pp ) affirms that jar tests are an important tool in determining unit process design parameters. As aforementioned, jar tests may be used for the determination of coagulant dosage, coagulation process ph, etc. Bratby also affirms that due to the fragility of the flocs formed via chemical induced coagulation flocculation processes, the results obtained by column settling tests are not as reliable, for design purposes, as the results obtained from settling tests performed in jar tests reactor with samples taken from one depth point only. The procedure for evaluating particle settling velocities using a jar test reactor is as follows: The jar test reactor (beaker) is filled with the correct volume of the water sample (suspension). The selected coagulant is then added to the sample and optimized mixing parameters for coagulation and flocculation processes are adopted. After the flocculation time has elapsed and the mixing is stopped the sample is allowed to stand. Samples are taken at the same depth at suitable periods determined by previous observation of the suspension under study. The samples are then analyzed for turbidity and the mean settling velocity of the particles (flocs) in the sample is determined by standard procedure as follow (Bratby, 2006, p. 294): - Samples are taken at one depth Z at different times t - Particles with settling velocity great enough to carry then passed the sampling point within a time t i will not be present in the sample taken at time t i. Hence all particles in the sample have settling velocities ν i less than or equal to: (10) - Y i is the fraction of particles in the original suspension with settling velocity less than ν i and is defined by equation (11) below where C i is the concentration of 29

30 particles in the sample taken at time t i and C 0 is the initial concentration of particles in the suspension (11) - If Y 0 is the fraction of particles with velocity less than ν 0 then the fraction 1 Y 0 of particles will be completely removed. For slow settling particles they will be removed in the ratio ν i /ν 0 and the fractional removal Fr of these particles will be: (12) - The total removal is thus: (13) - The settling test yields a distribution curve like the one presented in Figure 5. The integral expression of equation (13) corresponds to the shaded area in the figure and may be determined graphically. (Bratby, 2006, p. 294) Figure 5 Settling test distribution curve, settling curve. (Bratby, 2006, p. 294) 30

31 3.4 Chemical removal of phosphorous Phosphorus is normally present in natural waters and it is often the limiting nutrient for plants and microorganisms. Therefore the leaching of phosphorous into fresh water system such as lakes and rivers can cause eutrophication. Phosphorous can be found in soluble or particulate form in drainage waters and the aim of phosphorous precipitation is to convert its soluble form into insoluble so that a separation process such as sedimentation can be applied removing phosphorous from solution. Soluble phosphorous forms include orthophosphates, polyphosphates, pyrophosphates and organic phosphates. Orthophosphates and other condensed forms are known to form insoluble salts with a number of metal ions including aluminium and iron. The actual form of the reactions between metal ions and phosphorous depend on a number of factors such as: the concentration of the metal and phosphate ions, ph of the solution, and presence of other reactants such as sulphates, carbonates and organic species. Chemical precipitation removes phosphorous as orthophosphates and particulates the easiest. Polyphosphates and organic phosphorous are also know to participate in precipitation and adsorption reactions although not as readily as the other phosphorous species. It is important to note that orthophosphate is the type of phosphorous that plants can readily assimilate. (Bratby, 2006, pp ) According to Sincero and Sincero (2003, pp. 631) when aluminium and iron salts are added to the water, the dissociated trivalent metal cations (Al 3+ and Fe 3+ ) will react with phosphate ions to precipitate the metal phosphates while also reacting with water molecules to precipitate the metal hydroxides and to form complexes. Significant direct precipitation of metal phosphate salts via reaction with the trivalent metal cations is nevertheless restricted under the slightly acidic ph of natural waters. As it can be seen in Figure 6 the concentration of phosphate ions in solution is only significant at ph values well above neutral. 31

32 Figure 6 Distribution of phosphorous species as a function of ph. (Bratby, 2006, p. 124) Georgantas and Grigoropoulou (2006) stated that the main mechanisms of phosphorous removal are: 1) the incorporation of phosphate to the solids in suspension their subsequent removal; 2) The direct adsorption of phosphate ions on the hydrolyzes products of the added metal salt coagulants; 3) The formation of insoluble metal (Me) phosphates salts where the basic reactions occurring are described in equation (14). Me 3+ +H n PO 4 n-3 MePO 4 + nh + (14) Lindquist (2003, p. 127) affirms that the efficiency of phosphate removal via adsorption by metal hydroxides is considerably less than that of direct precipitation with Al 3+ and Fe 3+ and their hydrolysis products. Figures 7 and 8 consecutively illustrate the influence of ph when precipitating orthophosphates with constant dosages of aluminium and ferric sulphate. Orthophosphates were precipitated by adding concentrated aqueous solution of metal salts (0.25 mmol of Al 3+ and Fe 3+ as aluminium and ferric sulphate) to solutions of orthophosphate (0.25 mmol) and by adding solution of orthophosphate to 20 minute old formed metal hydroxides. It is clear that more orthophosphates were precipitated via direct precipitation than via hydroxides. Furthermore, orthophosphates were also more effectively removed by both aluminium and iron salts under acidic conditions or at ph values lower than 6. (Lindquist, 2003, p. 127) 32

33 Figure 7 The influence of ph on the Figure 8 The influence of ph on the percentage of precipitated orthophosphate percentage of precipitated orthophosphate via aluminium sulphate addition via ferric sulphate addition (Lindquist, (Lindquist, 2003, p. 127) 2003, p. 127) 3.5 Chemical removal of dissolved organic matter The presence of NOM in natural waters is mostly associated with humic substances originating from the extraction of living wood substances, the solution of incomplete degradation products in decaying wood and the solution of soil organic matter (Bratby, 2006, p. 87). These substances assign the water a characteristic dark brownish colour referred to as organic colour. The use of chemical induced coagulation processes for the removal of NOM is widely applied especially in the purification of water for drinking purposes. Just as for the removal of particulates, the most common coagulation agents used are salts of aluminium and iron. The removal efficiency of NOM is dependent on factors such as: the nature and concentration of the NOM, the type and dosage of coagulant and the ph and temperature of the solution. (Libecki & Dziejowski, 2008) 33

34 The coagulation mechanisms which contribute to the removal of NOM include (Bratby, 2006, p 93): - Charge neutralization-precipitation, consisting of the reaction between soluble polynuclear metal coagulants species and humic substances. - Simultaneous precipitation consisting of charge neutralization-precipitation reactions, and reaction with metal hydroxides precipitates, occurring simultaneously. - Adsorption of humic substances to metal hydroxides surface by van der Waals interactions, etc. According to Jiang and Wang (2009) studies conducted on the efficiency of organic colour removal via coagulation using metal salts have concluded that the optimum ph for ferric salts are within the range of 3.7 to 4.2, the optimum ph for aluminium sulphate is within the range of 5.0 to 5.5 and the optimum dosage for achieving the lowest residual colour, in molar terms, are mostly the same for aluminium and iron. However, Bratby (2006, pp ) emphasizes that optimum conditions should be determined via comprehensive jar test experiments, so that the best combination of ph, chemicals and dosages can be found not only for the removal of the NOM but also for achieving the overall desired purification levels. 3.6 Parameters influencing chemical purification It has become clear throughout the previous sections where individual aspects of the coagulation, flocculation and sedimentation processes were evaluated that a series of factors or process parameters can exert great influence on the purification process outcome. Among these parameters are: temperature, ph of the solution, mixing effect applied upon and after chemical addition and the type and dosage of added coagulant 34

35 3.6.1 Influence of temperature and ph The ph of the water sample to be treated may be the single most influential parameter affecting the chemical purification. As previously stated in this work, the primary charges of particles and molecules present in water are directly dependent on the ph of the solution. Under slightly acidic conditions such as in the case of natural waters, particles and molecules posses mainly negative charges. When the metal salts coagulants are added to the water they cause the ph of the solution to decrease due to the consumption of alkalinity during the coagulant hydration. The ph of the solution upon metal salt coagulant addition will affect the dissolution of the coagulant, the molar fraction of the different hydroxide species formed and their charges and consequently will directly affect the occurring coagulation mechanisms. (Bratby, 2006, pp ) Low temperatures affect coagulation, flocculation and sedimentation processes by altering the coagulants solubility, increasing water viscosity, and retarding the kinetics of hydrolysis reactions and particle flocculation. (Bratby, 2006, pp ) As previously mentioned, when metal coagulants are added to the water they dissociate and the trivalent metal cations undergo hydrolysis forming not only the precipitate hydroxides Al(OH) 3 and Fe(OH) 3 but also other dissolved hydroxides. The molar fraction distribution of dissolved species in solution is not only ph but also temperature dependent (Pernitsky & Edzwald, 2006). In consequence the coagulation process which is dependent on the concentration distribution of the species formed is also temperature and ph dependent. Figure 9 illustrates general trends in the distribution of dissolved aluminium species as a function of ph and temperature while Figure 10 presents the molar fractions of iron species as a function of ph. 35

36 Figure 9 - Theoretical distribution of Al species in solution as a function of ph and temperature. (Pernitsky & Edzwald, 2006, p. 124) Figure 10 - Distribution of monomeric Fe hydrolysis products as a function of ph. (Gregory & Duan, 2001, p. 2019) 36

37 It is important to highlight that the species distributions showed in Figures 9 and 10 are not meant to be definite under all conditions. The actual distribution is affected by the degree of polymerization of the primary formed monomeric species and the presence in the solution of other aluminium and iron complexing species such NOM, phosphate and sulphate ions. (Pernitsky and Edzwald, 2006) The solubility of the formed aluminium and iron hydroxides species is as well dependent of the ph and temperature of the solution. According to Bratby (2006, p. 172) with decreasing temperature, the minimum solubility of aluminium hydroxide precipitate shifts to a higher ph, just as the optimum coagulation ph also shifts towards a higher value. The shift in ph for the minimum solubility of aluminium and iron hydroxides (Al(OH) 3 and Fe(OH) 3 ) was also reported by Pernitsky and Edzwald (2006) and Lim-Seok Kang and John (1995) where the theoretical solubility diagram for aluminium at 20 C and 5 C and for Iron (III) at 25 C and 5 C in deionised water were extracted and are presented in Figure 11. The applied dosage of coagulant also influences the ph at which the lowest residual iron and aluminium concentration is obtained. Increasing the amount of coagulant added increases the ph range which gives the lowest residual concentration of aluminium or iron and also shifts it in the basic direction. Therefore, the ph which gives the lowest residual concentration of aluminium or iron should be determined experimentally for a specific water type at the actual process temperature and applied metal salt dosage. (Lindquist, 2003, p. 145) Regarding the effects of temperature, studies have shown that different coagulants are affected differently by temperature fluctuations and, although overall low temperatures affect both aluminium and iron salts coagulation performances, it appears that iron salts are affected to a lesser extent than aluminium salts. (Bratby, 2006, p. 171) 37

38 -log concentration (a) (b) 25 C 5 C Fe(OH) 3 (s) Figure 11 (a) Theoretical solubility diagram for aluminium at 5 and 20 C (Pernitsky & Edzwald, 2006, p. 124). (b) Theoretical solubility diagram for iron (III) at 5 and 25 C (Lim-Seok Kang & John, 1995, p. 894) 38

39 Morris and Knocke (1984, cited in Bratby, 2006, p. 171) affirmed that the effect of temperature appears to be more pronounced when the coagulation mechanism relies on enmeshment or sweep coagulation by the metal hydroxides precipitate. Bratby (2006, p. 171) complemented this statement by affirming that low temperatures (1 C) do not inhibit the rate of metal-hydroxide precipitation but have a detrimental effect on floc formation characteristics. Low temperatures result in smaller flocs inhibiting the enmeshment mechanism of particle removal, which is especially important for low turbidity waters Influence of mixing The effective mixing applied to the solution throughout the coagulation and flocculation processes have direct impact on the purification levels achieved by the chemical purification process. The mixing effect applied during the coagulation process is referred to as rapid mixing while slow mixing is the term used for the mixing applied during the flocculation process. Velocity gradients within a liquid mass can be induced by setting the liquid in motion using a variety of mixers such as: mechanical mixers, hydraulic mixers, pump mixers, etc. (Vigneswaran & Visvanathan, 1995, p. 64) The root mean square velocity gradient (G) for any given type of mixer is determined using equations (15) and (16) presented below (Bratby, 2006, p. 261). (15) (16) Where: G = velocity gradient (s -1 ) V = volume of flocculation tank (m 3 ) 39

40 P = power consumed by mixing device (Nm s -1 ) W = dissipation function (N m -2 s -1 ) µ = absolute viscosity, 10-3 Ns m -2 for water at 20 C For a mechanical mixer with rotating blades the power P drawn by the device is determined by its rotational speed and the geometry of the tank in which it operates. The power consumed by such devices was defined by Leentvaar and Ywema (1980, cited in Bratby, 2006, p. 261) and is given by equation (17). Equation (18) is the resulting root mean square velocity gradient equation for mechanical mixers with rotating blades. (17) (18) Where: G, V, P, W and µ are defined as above and; = dimensionless power number related to mixing device and tank geometry ρ = liquid density (kg m -3 ) n = mixer rotational speed, revolutions per second D = diameter of mixer impeller (m) The time of contact or retention time in the mixing unit or basin is given by equations (19) and (20) described below. (Lin and Lee, 2007, p. 377) (19) For plug flow, (20) 40

41 Where: t = retention time of the basin (s) V = volume of basin (m 3 ) Q = flow rate (m 3 s -1 ) L = length of rectangular basin (m) ν = horizontal velocity of flow (m s -1 ) Rapid (or flash) mixing is one of the most important stages involved in the chemical purification process. It is the stage in which a coagulant is rapidly and uniformly dispersed through a mass of water. (Lin & Lee, 2007, p. 377) During the rapid mixing stage the destabilization reactions occur (coagulation) and the primary flocs are formed. The characteristics of the formed primary flocs strongly influence the flocculation process which follows. (Bratby, 2006, p. 219) The time and intensity of mixing required for the fast mixing stage has been intensely reported but different sources usually recommend different values. (Rossini, et al., 1999) The required fast mixing time has generally been assumed to fluctuate between 30 and 60 s. However, according to Bratby (2006, p. 220) due to the fast rate within which the hydrolysis and destabilization reactions occur, fast mixing times over 5 s may not improve the subsequent flocculation process efficiency. Furthermore Griffith and Willians (1972, cited in Bratby 2006, p. 220) stated that, beyond a certain optimum rapid mixing time, a detrimental effect on the flocculation efficiency may be observed. In water and wastewater treatment plants the rapid mixing unit is specifically equipped with the most convenient type of mixer to provide the mixing effect required. Bratby (2006, pp ) reported that depending on the desired coagulation destabilization mechanism different types of mixers may be chosen and different retention times applied. The aforementioned author also affirmed that, for an efficient rapid mixing stage to be achieved high velocity gradients must be applied together with high turbulence effect. However, there is an upper limit for the applied velocity gradient depending on the coagulation process requirements. Too high velocity gradients during rapid mixing can cause a delay on floc formation in the flocculation process which follows (Bratby, 2006, p. 225). 41

42 According to Lin and Lee (2007, p. 379) velocity gradient (G) values of 500 to 1000 s -1 are required for the rapid mixing stage in order to produced effective flocculation. Lin and Lee also affirm that the product G*t should produce values from to with time (t) generally in the range of 60 to 120 s, a much higher range than that recommended by Bratby (2006). However, Bratby (2006, p. 226) states that: the best way of determining the appropriate rapid mixing time for a particular water is to conduct laboratory scale and/or pilot scale tests. When slow mixing and the flocculation process are concerned suitable mixers are also chosen according to the process requirements. The reported velocity gradient and retention times to be applied are also conflicting. Nevertheless, Vigneswaran and Visvanathan (1995, p. 64) and Lin and Lee (2007, p. 380) reported that a mean velocity gradient ranging from 20 to 70 s -1 together with contact times from 10 to 30 minutes should be kept during the flocculation process. When optimizing the mixing parameters for the flocculation process one must bear in mind the solid liquid separation process which follows. Larger and denser flocs may be more suitable for sedimentation process while smaller and lighter flocs may provide a more efficient flotation process. According to Bratby (2006, p.243) the greater the velocity gradient the smaller will be the final floc size due to the higher rate of breakage of the larger formed flocs. And the lower the velocity gradient the larger will be the final floc size although it will take longer for the larger flocs to form. 42

43 4 Environmental impacts of metal residual discharge Aluminium and iron are abundant substances in Finnish soils and are naturally leached into our water systems. However human activities such as metal ore mining, intensified forestry, peat production and agricultural draining have increased the load of iron and aluminium in many of the Finnish rivers ecosystems. (Vuorinen, et al., 1998; Vuori, 1995) The concentration of toxic aluminium and iron compounds in the aquatic system is directly dependent on the ph as well as on the concentration and type of organic matter present in the water. Under acidic conditions the predominant species of aluminium and iron are soluble bio-available ionic metal hydroxides. Humic substances form stable complexes with metals reducing the concentration of ionic bio-available metals hydroxide in the water, therefore lowering their toxicity. (Vuorinen, et al., 1998) The biggest concern regarding aluminium residuals is related to human health and the possible link between aluminium and adverse neurological effects, specifically the adverse effects manifested in Alzheimer s disease. (Bratby, 2006, p. 173) However, aluminium has as well for a long time been recognized as toxic for aquatic systems when found in high concentrations. According to Rosseland, et al. (1990) aluminium acts as a toxic agent on gill-breathing animals such as fish and invertebrates and may also as an organically complexed form, be absorbed by mammals and birds and interfere with their metabolic processes. The above mentioned authors also affirm that some inorganic monomeric forms of aluminium have adverse effects on plants root systems and that aluminium can accumulate in invertebrates and plants making its way up to the terrestrial food chain. According to Vuori (1995) high concentration of iron in fresh water systems have long been considered a deteriorating factor. Increasing evidence is available suggesting that iron concentration has a significant impact on the structure and function of river ecosystems. Vuori also stated that the direct action of toxic iron compounds can cause impairment of aquatic life survival, reproduction and growth rates. Edén, et al. (1999) reported that acidic waters (4.5 < ph < 5.5) together with high contents of iron (> 1 mg/l) and aluminium (> 100 µg/l) may cause severe impact on the rivers biological system. Of special concern is 43

44 the impacts imposed by these conditions on several fish species which metabolism, reproduction, growth and mortality rates have been affected by these deteriorated water quality characteristics. A study carried out by Poléo, et al. (1997) evaluated the sensitivity of seven common Scandinavian fresh water fish species to aluminium rich waters (0-300 µg/l of inorganic monomeric aluminium species). The investigations concluded that aluminium is acutely toxic to freshwater fish species under neutral and acidified water conditions but more in acidified conditions. Vuorinen, et al. (1998) affirm that aluminium and iron in concentrations found in the Finnish fresh water systems (150 to 800 µg/l of total Al and 500 to 4000 µg/l of total Fe) may be toxic to fish such as trout and grayling and that the toxic effect increases with increasing acidity of the water. Chemical purification of peat harvesting runoff water has the potential to decrease but also to increase the concentration of metal residuals discharging from peat production sites. The coagulation and sedimentation of metal containing particulates and dissolved substances may reduce the metal concentration of the discharge water. However, a considerable amount of aluminium or iron is added to the water during the purification process. Most of the added aluminium and iron during the coagulation process should be removed from solution via precipitation of their respective hydroxides and other insoluble formed compounds. (Bratby, 2006, p. 173) However, for this to occur, all process parameters should be optimized. Effective coagulation and subsequent flocculation processes will result in low metal residual in the discharging water. As previously stated in this work, the coagulant dosage, the ph and the temperature of the water are directly linked to the solubility of the various iron and aluminium compounds in solution. Therefore the coagulation process should be closely monitored and controlled to assure as low metal residuals as possible in the discharge water. 44

45 5 Materials and methods The selection of the four coagulants to be tested and evaluated in this study was made based on the analyses of previous research project report prepared by Vapo Oy (Alatalo & Peronius, 2004) from which further developments were required. The water samples to be purified were collected from two different locations with the objective of also evaluating the influence of varying water quality characteristics on the purification levels achieved by chemical treatment. Jar test experiments were used to evaluate the coagulation, flocculation and sedimentation process parameters thus providing a way to access chemical treatment purification efficiency under laboratory conditions. The laboratory experiments were performed in four distinctive phases: - Phase 1 Optimum dosage range and purification efficiency - Phase 2 - Settling characteristics - Phase 3 - Evaluation of temperature influence on the purification efficiency and settling characteristics - Phase 4 - Evaluation of mixing parameters influence on the purification efficiency 5.1 Coagulants Four solid metal salt coagulants were studied in this thesis project: aluminium sulphate, aluminium chloride, ferric sulphate and a mixture of ferric (15%) and aluminium sulphate (85%). The selected coagulants chemical composition, manufacturers, and main characteristics can be found in Table 1. With the objective of evaluating the effects of coagulants solubility, aluminium sulphate and ferric sulphate were also tested in predissolved (solution) form. For that, stock solutions of 10 g/l of coagulant were prepared and used over a 3 day period. It is necessary to highlight that the aluminium chloride (AlCl 3. 6H 2 O) utilized in our experiments was of analytical quality with 99% purity while the other coagulants were of 45

46 commercial quality with around 90% purity. No corrections have been made to the presented coagulant dosages to compensate the higher purity of aluminium chloride. Table 1 Tested coagulants and their main properties. Coagulant Aluminium sulphate (ALG) Chemical composition Physical form Manufacturer Purity Al 2 (SO 4 ) 3. 14H 2 O Small to medium size granules Kemira Oyj, Kemwater > 90% Aluminium + Ferric sulphate (ALF-30) Al 2 (SO 4 ) 3. 14H 2 O + Fe 2 (SO 4 ) H 2 O Mixture of small to large size granules Kemira Oyj, Kemwater > 90% Ferric sulphate (Ferix-3) Fe 2 (SO 4 ) H 2 O Medium to large size granules Kemira Oyj, Kemwater > 90% Aluminium chloride AlCl 3. 6H 2 O Small size crystal granules Alfa Aezar Gmbh & Co 99% 5.2 Sample collection With the objective of evaluating the influence of variations in water quality characteristics on the purification efficiency to be achieved by the tested coagulants, water samples from two different locations were used to conduct the laboratory experiments. The samples were taken from two peat harvesting sites under exploration by Vapo Oy. The first sample was collected from Navettarimpi peat harvesting site located between Vaala and Kestilä (E: , N: ) in August The second was collected from Piipsanneva peat harvesting site in Haapavesi (E: , N: ) in October In both occasions around 400 litres of water was pumped from ditches prior to chemical treatment into 35 l plastic gallons which had previously been acid treated to eliminate any source of contamination. The gallons were stored in cold room (5 10 C). Prior utilization each gallon was vigorously shaken to provide thorough mixing of the sample and to eliminate changes in water quality due to sedimentation of particulate matter or adsorption of substances to the plastic surface. 46

47 5.3 Laboratory analyses The purification efficiency achieved by the tested coagulants under different process parameters was firstly monitored and evaluated in the laboratory via measurements of colour, turbidity, ph, temperature and conductivity. The standards followed and used equipment is given in Table B of appendix 1. From the results of these measurements, samples of the treated water were then selected and sent for further analyzes to the Finnish Accreditation Service (FINAS) accredited Environment Measurement and Testing Laboratory (T003 and T164). The performed outsourced analyzes (standard methods in Table A of appendix 1) were: total organic carbon (TOC), total nitrogen (tot-n), total phosphorous (tot-p), phosphate phosphorous (PO 4 -P), suspended solids (SS), aluminium (Al) and iron (Fe). To obtain the prior treatment water quality characteristics (Table 2) or initial conditions, samples of the untreated water were sent for the above mentioned set of analyzes and in addition for analyzes of conductivity (Cond.) and ph. Furthermore, to eliminate possible errors while evaluating the purification efficiency, samples of the untreated water were sent for analyzes on the day of the sample collection and at regular intervals during the testing period. Errors while evaluating the purification levels could arise from possible changes in water quality during the storage period. Table 2 Water quality characteristics of Piipsanneva and Navettarimpi samples. Water quality parameter Average value ± standard deviation (Navettarimpi) n Average value ± standard deviation (Piipsanneva) ph (range) (range) 8 Cond. [ms/m] (range) (range) 8 SS [mg/l] 17.1 ± ± tot-n [µg/l] 1720 ± ± 0 6 tot-p [µg/l] 58 ± ± PO 4 -P [µg/l] 24 ± ± TOC [mg/l] 27 ± ± Fe [µg/l] 3830 ± ± 50 4 Al [µg/l] 4230 ± ± n = number of samples analyzed. n 47

48 5.4 Jar test experiments The jar test experiments were performed using a six jars paddle stirrer equipment from Kemira Kemwater named Flocculator 2000 (Figure12). The rotational speed and mixing time for each individual stirrer (mixer) is program-controlled allowing the application of different parameters to each individual one litre cylindrical beaker. Figure 12 - Jar test equipment The dimensions of the individual beakers and stirrers are as follow: - Beakers - Stirrers Height = 18 cm Length = 15 cm Diameter = 9 cm Width of paddle = 3 cm Volume = 1 l Length of paddle = 5.6 cm Based on the dimensions of the beakers and on the type and dimension of the mixer (stirrer) an estimation of the velocity gradient (G) imposed to the liquid within the jars for the fast and slow mixing stages was calculated using equation (18) were: 48

49 - (dimensionless power number related to mixing device and tank geometry extracted from Bratby (2006, p. 264) according to jar test equipment dimensions varies from 3.5 to 4.0) - m (diameter of mixing impeller) - m 3 (volume of beaker) - µ = Ns/ m 2 (absolute viscosity of water at 20 C) - ρ = 1000 kg/m 3 (density of the water) For the coagulation or fast mixing stage: - n = 400 rpm = rps (adopted mixer rotational speed) - Calculated G = 756 s -1 - For contact time t = 60 s; G*t = For the flocculation or slow mixing stage: - n = 70 rpm = rps (adopted mixer rotational speed) - Calculated G = 55 s -1 - For contact time t = 15 min = 900 s; G*t = The adopted rotational speeds and contact times for both mixing stages were based on preliminary jar tests experiments and on literature recommended values cited in this work. Jar test procedure for laboratory experiments phase 1 - determination of optimum dosage range and purification efficiency All four selected coagulants were tested in this phase of experiments according to the jar test procedure which follows: 1- One litre of the untreated water was transferred to each of the six jars. For establishing the tests initial conditions an extra sample of the untreated water was separated and analyzed for turbidity, colour, temperature, ph and conductivity. 49

50 2- Constant and equal mixing parameters were applied to the six individual mixers. Solid coagulants: 400 rpm for 60s, 70 rpm for 15 min followed by 30 minutes of sedimentation time where no mixing was applied. Coagulants in solution form: 300 rpm for 10s, 50 rpm for 25 min followed by 30 minutes of sedimentation time where no mixing was applied 3- Increasing dosages of coagulant was added in sequence to the water samples in the jars simultaneously to the start of mixing. 4- After the mixing and sedimentation period elapsed samples of the supernatant water were extracted from each jar and analyzed for turbidity. Based on the obtained results an optimum dosage range composed of six different dosages for each tested coagulant was determined. The optimum dosage range of each coagulant was then applied on 3 consecutive experiments for each solid coagulant and 2 consecutive experiments for each of the coagulants in solution form following procedures 1 to 3 and 5 to 7: 5- The samples extracted after the sedimentation time had elapsed were analyzed for turbidity, colour, ph and conductivity. Purification efficiencies were evaluated and reported as the removal of colour and turbidity with applied dosage and were expressed as the ratios: T/Ti (turbidity of the purified water or final turbidity) / (turbidity of the raw water or initial turbidity); C/Ci (colour of the purified water or final colour) / (colour of the raw water or initial colour). 6- From the dosage range applied the dosage which presented the best removal of colour and turbidity was identified as the optimum dosage and the dosage which resulted in the removal efficiency of 40 to 60% in colour and/or turbidity was identified as the limit dosage or lowest working dosage. The procedure of the identification of a limit dosage was a tentative of obtaining the lowest dosage to be applied in order to achieve reasonable purification levels of ca 50% removal efficiency of total phosphorus, phosphates and suspended solids. 7- Further 400 ml of supernatant water were extracted from the samples treated with the optimum (coagulants in solid and solution form) and limit dosages (solid 50

51 coagulants) and sent to the laboratory for analyzes of TOC, tot-p, tot-n, PO 4 -P, Fe, and Al. Procedures 1 to 7 were carried out for both water samples at field ph conditions. Piipsanneva water sample was also tested after it s ph was increased by one unit from 5.8 to 6.85 via the addition of slake lime (Ca(OH) 2 ). After the addition of Ca(OH) 2 the sample was thoroughly mixed and left to stand for 12 hours before tests were performed. According to analyzes of the results obtained in this first phase of tests, it was decided that only three of the four coagulants would be tested in the second, third and fourth phases of experiments: ALG, ALF-30 and Ferix-3. Jar test procedure for laboratory experiments phase 2 settling characteristics For the phase 2 of laboratory experiments the procedures 1 and 2 of phase 1 (detailed above) were followed and in addition: 1- Each experiment used only one jar. The outside wall of the cylindrical beaker was marked across its circumference with a line at a point 8 cm from its bottom. 2- Optimum dosages of each coagulant were applied in individual tests. The coagulant was added simultaneously to the start of mixing. After coagulation and flocculation mixing period had elapsed eleven 30 ml samples were extracted from the jars with a volumetric pipette inserted in the water until the 8 cm mark. The samples were collected in sequence and in time intervals as follow: 13 minutes inside flocculation time (or 2 minutes before mixing ceased) and then at 1, 2, 3, 4, 6, 8, 11, 13, 17 and 25 minutes inside sedimentation time. 3- The extracted samples were subsequently analyzed for turbidity. Since turbidity is a measure which can be correlated to the concentration of particulates in the water it was used to monitor how the concentration of particulates changed over time at a constant depth point in the jars. The obtained results were expressed as the ratio 51

52 Tt/Tt 0 = (turbidity at time t) / (turbidity measured during flocculation period 2 minutes before mixing was ceased). Two replications of the settling test were performed for each of the three studied solid coagulants tested in this phase of laboratory experiments. Furthermore two replications were also performed for ALG and Ferix-3 in solution or pre-dissolved form. Jar test procedure for laboratory experiments phase 3 evaluation of temperature influence on purification efficiency and settling characteristics The tests performed in this phase of laboratory experiments were carried out in two stages. First the influence of low temperature on the overall purification efficiency was evaluated and in sequence the influence of temperature on the settling characteristics of the formed flocs was also investigated. For the evaluation of temperature influence in the purification efficiency the procedures followed mostly replicated the procedures described for laboratory phase 1 and in addition: 1- The jars containing the untreated water samples were placed inside a temperature controlled insulated water tank (Figure 13). The temperature of the pre-determined volume of water in the tank was controlled via a Lauda RK KS low temperature thermostat. The jars were left in cold water bath and slow mixing was applied to keep the samples homogenized until a constant temperature of 5 (± 1 C) was achieved in all jars. 2- Three increasing dosages of each coagulant were applied. The dosages were selected from the optimum dosage range determined in phase 1 and consisted of the lowest working dosage, the optimum dosage and a higher than optimum dosage for each coagulant. 3- Only from the sample treated with the optimum dosage of each coagulant extra 400 ml of supernatant water was extracted and sent to the laboratory for further analyzes. 52

53 Procedures 1, 2 and 3 were performed twice for all coagulants in solid and in solution form, however, only samples from one repetition for the coagulants in solution form were sent to the laboratory for analyzes. Figure 13 Temperature controlled insulated water tank. For the determination of the influence of low temperature on the coagulants settling characteristics, all procedures from laboratory phase 2 (where the settling characteristics of the flocs formed at 20 C was evaluated) were followed and in addition procedure 1 detailed previously in this phase of laboratory experiments. Jar test procedure for laboratory experiments phase 4 evaluation of mixing parameters influence on purification efficiency For the evaluation of the influence of mixing parameters a series of jar test experiments were performed which basically followed the procedures applied in phase 1 except: 1- Each experiment used three jars. The three stirrers were programmed individually where one of the four mixing parameters (fast mixing time and speed, slow mixing time and speed) was different in each jar and the other three remained constant. 53