A NOVEL METHOD FOR REMOVING PHOSPHORUS FROM AGRICULTURAL RUNOFF AND SEWAGE EFFLUENT

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1 A NOVEL METHOD FOR REMOVING PHOSPHORUS FROM AGRICULTURAL RUNOFF AND SEWAGE EFFLUENT K V Heal 1, P L Younger 2, K A Smith 1, P Quinn 2, S Glendinning 2, J Aumônier 2, K E Dobbie 1, H McHaffie 1, D Dimoliatis 1, A M Bush 1, E Bozika 1, E Tatsi 1, A E Simpson 1, R Sweetman 2 1 School of GeoSciences, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JU, UK E.mail: k.heal@ed.ac.uk 2 School of Civil Engineering and Geosciences, University of Newcastle upon Tyne, NE1 7RU, UK Summary: Treatment of polluting discharges from abandoned mines is producing large quantities of ochre (mainly iron (III) oxides) for which no major end-use has yet been identified. At the same time, phosphorus pollution is a serious threat to the water environment in industrialised countries, causing eutrophication, algal blooms, fish kills and loss of water resources. Newcastle and Edinburgh Universities are currently conducting research to develop and test novel field-scale methods to use ochre for phosphorus removal from agricultural runoff and sewage effluent. Our laboratory experiments have demonstrated that ochre is an excellent adsorbent of phosphorus from solution (up to 30 g P kg -1 ). Phosphorus removal is unaffected by phosphorus form (total, inorganic, organic) and solution ph. Potential applications of ochre for phosphorus removal from agricultural runoff and sewage effluent are discussed, including treatment of agricultural drainage with ochre-filled filter and dosing units and ochre-filled reaction vessels for sewage effluent treatment. INTRODUCTION The flooding of abandoned mines frequently results in the formation of acidic, ferruginous water due to the oxidation of pyrite in the mine workings. To prevent the pollution of surface watercourses, mine water treatment plants (MWTPs) are employed to treat the most serious discharges of this polluted water. MWTPs accumulate large quantities of Fe(OH) 3 and FeO.OH precipitate, collectively known as ochre (of the order of tens of tonnes per annum at a single site). Typically, this is stockpiled and, although a number of possible end-uses have been considered, no single end-use has yet been identified which could consume the projected future production. Phosphorus pollution from point and diffuse sources is a serious threat to the water environment in the UK and other industrialised countries. Ochre has the potential to be used in novel technologies for the removal of phosphorus from wastewater due to its high sorption capacity for phosphorus. The predominant mechanism of phosphorus removal from water by ochre is sorption onto iron oxides and the other components present. This paper presents results from laboratory experiments that show the potential of ochre for phosphorus removal from agricultural runoff and sewage effluent and also discusses current field demonstrations of the technology. A companion paper in the Proceedings (Dobbie et al.) reports on the recycling of phosphorus-saturated ochre as a slow-release fertiliser. 1

2 DEMONSTRATION OF THE POTENTIAL OF OCHRE FOR PHOSPHORUS REMOVAL: LABORATORY RESULTS Ochre extracted from MWTPs has a very high water content (80-95 %) unless it has been stored in drying beds in good weather conditions. Ochre which has not been air-dried is difficult to handle and transport and consequently most investigations of its phosphorus removal properties have used the air-dried form. Dried ochres from different MWTPs have similar chemical properties but may have different physical properties. Experiments have focused on ochres from two MWTPs in Scotland, Polkemmet and Minto, that have very different particle-size distributions (Figure 1). Polkemmet ochre dries into clods that are readily crushed to a coarse, granular texture which has a high saturated hydraulic conductivity (26-32 m day -1, equivalent to coarse sand). In contrast, Minto ochre dries to a fine powder with a considerably lower saturated hydraulic conductivity ( m day -1 ) % Polkemmet Minto Upper particle size diameter (mm) Figure 1. Particle-size distribution of air-dried ochres from two MWTPs, Scotland (after Bozika, 2001) The maximum phosphorus adsorption capacities for Polkemmet and Minto ochres, determined in laboratory saturation experiments involving the repeated addition of artificial phosphorus solutions, are orders of magnitude higher than those measured in other adsorbents (Table 1). The removal of phosphorus by both ochres is also rapid (within minutes). Solution ph was found not to affect the removal rate of phosphorus (Bozika, 2001) due to the buffering capacity of the ochre (8-11 % CaO and 8 % MgO content). Further laboratory batch experiments with sewage effluent showed that all phosphorus forms (total, inorganic and organic) are removed rapidly (< 1 to 15 minutes) by both ochre types. For a more detailed discussion of laboratory experiments and methods see Heal et al. (2003). 2

3 Table 1. Maximum phosphorus adsorption capacities of different materials (after Drizo (1998) and Mann (1997)) Material Adsorption capacity (mg P (g substrate) -1 ) Gravel Bottom ash 0.06 Steel slag 0.38 Blast furnace slag Fly ash 0.62 Shale 0.75 Laterite 0.75 Zeolite 1 Polkemmet ochre 26 Minto ochre 30.5 Different physical properties of ochres influence their suitability for different phosphorus removal applications. Coarse-grained ochres are more suitable for phosphorus removal in filter units or in the substrate of constructed wetlands. Fine-grained ochres are difficult to contain and easily clog filter units, but rapidly remove phosphorus from wastewater because of the larger surface area of the ochre particles. Such ochres are more suitable for dosing applications to waters with undesirable phosphorus concentrations, as long as adequate sedimentation is provided. Laboratory dosing experiments with 10-litre columns of sewage effluent and agricultural runoff showed that a single addition of ochre settled completely within eight hours and removed up to 80 % of the phosphorus present in the water column (Tatsi, 2003). A method for producing robust, pea-sized spherical granules of fine-grained ochre by the addition of Portland cement, a surfactant and water has been developed by Newcastle University to facilitate the handling and use of this form of ochre. The potential applications of ochre for removing phosphorus in the aquatic environment are summarised in Table 2. Table 2. Methods of using different forms of ochre to reduce phosphorus concentrations in agricultural runoff and sewage effluent Ochre type Agricultural runoff Sewage effluent Coarse-grained Fine-grained Instream barrier Filters on field drains Dosing agent in powder form, then settlement Granules in instream barrier Reaction vessel Wetland substrate Dosing agent in powder form, then settlement Granules in reaction vessel Granules in wetland substrate 3

4 FIELD DEMONSTRATIONS OF PHOSPHORUS REMOVAL BY OCHRE Treatment of agricultural runoff with ochre Approximately 40 % of agricultural land in the UK (excluding rough grazing) is underlain by subsurface field drains which have been shown to be a major conduit of soluble and particulate phosphorus export to watercourses in storm events, even when best management practices are implemented (Dils and Heathwaite 1996). Although it is not feasible to install wastewater treatment works or constructed wetlands for every field drain, treatment with ochre may form a cheap, low-maintenance means of reducing phosphorus exports from field drains. Treatment at the field scale could take the form of instream barriers or filter cartridges containing coarsegrained ochre or ochre granules. Alternatively, dosing with powdered ochre, followed by settlement, has already been demonstrated to be effective in reducing phosphorus concentrations in simulated agricultural runoff in laboratory-scale flume experiments (Sweetman, 2001). Short-term pilot studies in a Scottish river affected by agricultural runoff have demonstrated that instream barriers, containing the coarse-grained Polkemmet ochre, can reduce phosphorus concentrations. Discrete instream filter units, containing 6 kg of ochre, deployed for one month reduced phosphorus concentrations (from a mean inflow concentration of 0.18 mg P l -1 to 0.06 mg P l -1 ) and altered the algal community composition, from one dominated by algae typical of eutrophic conditions to one dominated by algae typical of mesotrophic conditions (Bush, 2001). In a separate study of an instream barrier containing five different ochre-grit mixtures, those containing 100 % and 50 % ochre reduced phosphorus concentrations to less than half the inflow value over a one-month period (Dimoliatis, 2002). Longer-term field investigations comparing the effectiveness of coarse-grained ochre and ochre granules for phosphorus removal from agricultural runoff are being conducted at farms in Northumberland and Midlothian. Treatment of sewage effluent with ochre With stricter controls on sewage discharge into receiving waters (e.g., the EU Urban Wastewater Treatment Directive (Farmer, 2001)), the development of new methods for phosphorus removal are required. Currently, phosphorus removal at sewage treatment works is often achieved by the addition of costly dosing agents such as iron (III) chloride and iron (III) sulphate. Fine-grained ochre is an alternative dosing agent as it has a high capacity for phosphorus adsorption. On the basis of laboratory-scale experiments, fine-grained ochre compares favourably with other dosing agents, apart from the generation of larger volumes of sludge for disposal (Table 3). Ongoing field trials at sewage treatment works are investigating the effectiveness of reaction vessels, containing coarse-grained ochre or ochre granules, in removing phosphorus. Preliminary results show that good reductions in phosphorus concentrations (from 3.04 to 0.33 mg P l -1 ) are achieved if the hydraulics are satisfactory. 4

5 Table 3. Comparison of fine-grained ochre with other dosing agents used for phosphorus removal in sewage treatment works (Tatsi, 2003) Reagent Dosage (mg l -1 ) P removal (%) Comments FeCl Most often employed Al 2 (SO 4 ).16H 2 O High cost Ca(OH) Precipitation occurs at high ph Fine-grained ochre Not ph dependent. Use of waste material, but larger sludge quantities. Constructed wetlands are effective in removing nitrogen from sewage effluent but are less efficient in removing phosphorus (Cooper et al., 1996). The incorporation of ochre into the substrate of constructed wetlands is therefore anticipated to improve their treatment efficiency for phosphorus. Estimates (Table 4) show that ochre-based wetlands have a long operational life time for phosphorus removal from sewage effluent compared to other substrates. Table 4. Estimated operational lifetimes for constructed wetlands using 3 or 5 m 2 wetland area per person with shale or Polkemmet ochre as a substrate with three concentrations of sewage effluent, 20, 5 or 3 mg P l 1 (McHaffie et al., 2000) Substrate Operational lifetime / years 3 m 2 per person a 5 m 2 per person a 20 mg l 1 5 mg l 1 3 mg l 1 20 mg l 1 5 mg l 1 3 mg l 1 Shale b Ochre c Constructed wetland substrate assumed to be 0.6 m deep. a Each person assumed to produce 0.2 m 3 sewage a day. b From data in Drizo (1998). c Calculated with half the measured maximum phosphorus adsorption capacity to take account of the anticipated decline in flow rates. CONCLUSIONS Laboratory experiments have demonstrated that ochre, formed as a by-product from mine water treatment, has a high capacity for phosphorus removal from agricultural runoff and sewage effluent, as a result of sorption to the high concentration of iron oxides and hydroxides 5

6 contained within the ochre. Potential applications of phosphorus removal by ochre are widespread, including sewage effluent treatment in constructed wetlands and by dosing systems, and treatment of agricultural runoff by instream filter units and/or dosing and settlement systems. The use of ochre for removal of excess phosphorus in the aquatic environment is particularly attractive not only because treatment costs are potentially lower than more traditional methods, but also due to the sustainability advantage of using a byproduct of treating one form of water pollution (i.e. mine water pollution) to address another water problem (eutrophication). ACKNOWLEDGEMENTS The authors are grateful to the Coal Authority, Northumbrian Water Limited and the Engineering and Physical Sciences Research Council (EPSRC grant nos. GR/R73522/01 and GR/R73539/01) for funding. The technical support of Andy Gray and Rab Howard is acknowledged. REFERENCES Bozika E (2001). Phosphorus removal from wastewater using sludge from mine drainage treatment settling ponds. MSc thesis, University of Edinburgh. Bush A M (2001). An investigation into the use of iron ore sludge to remove phosphorus in the River Leet, in an attempt to mitigate eutrophic conditions. MSc thesis, University of Edinburgh. Cooper P F, Job G D, Green M B and Shutes R B E (1996). Reed beds and constructed wetlands for wastewater treatment, Water Research Centre, Swindon, 184pp. Dils R M and Heathwaite A L (1996). Phosphorus fractionation in hillslope hydrological pathways contributing to agricultural runoff. In: Anderson, M.G. and Brookes, S. (eds), Advances in Hillslope Processes, John Wiley & Sons Ltd., Chichester: Dimoliatis D (2002). Potential use of ochre in-stream filters to reduce phosphorus concentrations in the Leet Water, Berwickshire. MSc thesis, University of Edinburgh. Drizo A (1998). Phosphate and ammonium removal from waste water using constructed wetland systems. PhD thesis, University of Edinburgh. Farmer A M (2001). Reducing phosphate discharges: the role of the 1991 EC urban wastewater treatment directive. Water Science and Technology 44: Heal K V, Younger P L, Smith K A, Glendinning S, Quinn P and Dobbie K E (2003). Novel use of ochre from mine water treatment plants to reduce point and diffuse phosphorus pollution. Land Contamination and Reclamation: 11, Mann R A (1997). Phosphorus adsorption and desorption characteristics of constructed wetland gravels and steelworks by-products. Australian Journal of Soil Research 35: McHaffie H, Heal K V and Smith K A (2000). Using sludge from mine drainage treatment settling ponds for phosphorus removal from wastewaters: Report to Coal Authority. Institute of Ecology and Resource Management, University of Edinburgh. Sweetman R (2001). The removal of phosphate from agricultural runoff with blast furnace slag and ochre. MSc thesis, University of Newcastle upon Tyne. Tatsi E (2003). Investigating the settling and phosphorus removal properties of ochre in water columns. MSc thesis, University of Edinburgh. 6

7 Refer to as: Heal, K.V., Younger, P.L., Smith, K.A.,Quinn, P., Glendinning, S., Aumônier, J., Dobbie, K.E., McHaffie, H., Dimoliatis, D., Bush, A.M., Bozika, E., Tatsi, E., Simpson, A.E. and Sweetman, R. (2004) A novel method for removing phosphorus from agricultural runoff and sewage effluent. In: D. Lewis and L. Gairns (eds), Agriculture and the Environment Water Framework Directive and Agriculture, Proceedings of the SAC and SEPA Biennial Conference, March 2004, Edinburgh, pp ISBN