PHOSPHATE RECOVERY BY THE CRYSTALLISATION PROCESS: EXPERIENCE AND DEVELOPMENTS

Transcription

1 PHOSPHATE RECOVERY BY THE CRYSTALLISATION PROCESS: EXPERIENCE AND DEVELOPMENTS Peter Piekema and Andreas Giesen DHV Water BV, P.O. Box 484, 3800 AL Amersfoort, The Netherlands, Tel: , Fax: , Abstract An advanced alternative for phosphate removal by conventional precipitation is to apply crystallisation. DHV, a multi-national group of consulting engineers and general contractors with headquarters in The Netherlands, has developed and patented a fluid-bed type of crystallizer in which phosphate is removed and recovered from the wastewater while phosphate pellets with a typical diameter of 1 mm are produced. The major advantage of the crystallisation technology, the so-called Crystalactor 1, is that in the compact installation high-purity phosphate pellets are produced which have proven to be reusable. Phosphate has been successfully recovered in the form of calcium phosphate, magnesium phosphate and magnesium ammonium phosphate. Up to now Crystalactor plants were constructed for the removal/recovery of heavy metals, phosphate, fluoride and the softening of drinking and process water. 1 CONVENTIONAL PHOSPHATE REMOVAL In municipal water applications and many industrial unit operations phosphate-polluted wastewater is generated. In general, conventional phosphate removal techniques are applied for the wastewater treatment. These conventional processes are based on the phosphate precipitation as calcium or iron salt or fixation in activated sludge. These processes unfortunately generate huge amounts of a water-rich sludge which has to be disposed off at continuous increasing costs. To minimise disposal costs, the sludge is often mechanical dewatered prior to disposal. Typically, even after dewatering the water content of the sludge still amounts to 60% to 85% and a relatively large part of the disposal costs comes from the expensive disposal of water. Due to high water content and the low quality of the waste sludge, reuse of phosphate is not an economically attractive option. Furthermore the operation of mechanical dewatering equipment is often troublesome. Also the area requirement for conventional phosphate precipitation processes is relatively high because of the four process steps are performed serially. They are (also refer to Figure A): 1. coagulation 2. flocculation 3. sludge/water separation 4. sludge dewatering 1 The Crystalactor is the registered trademark of the pellet reactor systems developed by DHV Water BV, Amersfoort, The Netherlands, for water treatment

2 2 CRYSTALLISATION IN A CRYSTALACTOR An advanced alternative is to apply crystallisation instead of precipitation. The Crystalactor, a fluid-bed type of crystallizer, has been developed for this purpose. Instead of bulky sludge, this process generates high purity phosphate crystal pellets that can be re-used in many ways. Recovery of phosphate because more and more important since it is a sustainable solution to the environmental problems related to the mining and processing of natural phosphate resources. The Crystalactor enables phosphate removal and recovery by means of several process routes. The most important routes are: 1. crystallisation as calcium phosphate (CP) 2. crystallisation as magnesium phosphate (MP) 3. crystallisation as magnesium ammonium phosphate (MAP) 4. crystallisation as potassium magnesium phosphate (KMP) 2.1 Process description The chemistry of the process is comparable to the conventional precipitation. By dosing a calcium or magnesium salt to the water (e.g. lime, calcium chloride, magnesium hydroxide, magnesium chloride), the solubility of CP, MP, MAP or KMP is exceeded and subsequently phosphate is transformed from the aqueous solution into solid crystal material. The primary difference with conventional precipitation is, that in the crystallisation process the transformation is controlled accurately and that pellets with a typical size of approx. 1 mm are produced instead of fine dispersed, microscopic sludge particles. Principle effluent The Crystalactor is a cylindrical reactor, partially filled with a suitable seed material like sand or minerals. The phosphatecontaining wastewater is pumped in an upward direction, maintaining the pellet bed in a fluidised state. In order to crystallise the phosphate on the pellet bed, a driving force is created by a reagent dosage and sometimes also ph-adjustment. By selecting the appropriate process conditions, co-crystallisation of impurities is minimised and high-purity phosphate crystals are obtained. The pellets grow and move towards the reactor bottom. At regular intervals, a quantity of the largest fluidised pellets is discharged at full operation from the reactor and fresh seed material is added. After atmospheric drying, readily handled and virtually water-free pellets are obtained. pellet discharge reagent feed

3 2.2 No Residual Waste A major advantage of the process is its ability to produce highly pure, nearly dry phosphate pellets. Table A shows the typically characteristics of the pellets in comparison with precipitation sludge. No copious amounts of waste sludge, but compact reusable pellets (shown magnified) Table A: comparison of characteristics Parameter morphology water content seed material content CP, MP, MAP, KMP-content crystallisation in pellet reactor round pellets mm 1-5 % < 5 % % conventional precipitation sludge % (after dewatering) % (after dewatering) Due to their excellent composition, the pellets are easily reusable, resulting in no residual waste for disposal. Several reuse options are: raw material for the production of phosphoric acid in either the wet or thermic production processes intermediate product for fertiliser formulation raw material for kettle food direct use as (slow-release) fertiliser Phosphate processing industries in the Netherlands have been using Crystalactor calcium phosphate pellets since a few years, as it has proven to be an attractive and clean (low in heavy metals) secondary phosphate source. In the rare event that pellets have to be disposed of by other means, the advantage of low-volume secondary waste production still remains: water-free pellets, not bulky sludge. 2.3 Capacity and effluent concentrations The reactivity of phosphate is reflected in the crystallisation process and high reactor loadings can be applied. The reactivity for MP, MAP and KMP is even substantial higher (factor 3-5) than for CP, as well as the reactor loading. Depending on the ph and the calcium or magnesium dosage rate, phosphate can be removed from the wastewater down to low concentration levels. The phosphate concentration in the effluent from a pellet reactor when treating a typical wastewater stream depends on the applied process route. With the CP-route effluent concentration below 0.5 mg P/l can easily be obtained, while the other routes result in effluent containing typically 5-10 mg P/l. If the Crystalactor is applied in a sidestream, as is the case in combination with biological phosphate removal, the actual effluent quality discharged by the Crystalactor-unit is less important. In this case the crystallisation capacity determines the overall performance. As result of the high reactor capacity, the high surface loading rates ( m/h) and since the coagulation, flocculation, separation and dewatering processes are combined into one by the crystallisation process, the unit often is compact (refer to Figure A).

4 Figure A: The four steps found in conventional treatment processes are combined into one by the crystallisation process conventional Feed Coagulation Flocculation Sludge/water separation Filtration Effluent Sludge dewatering Optional Pellet Reactor Waste sludge Feed Pellet Reactor Filtration Effluent Reusable pellets Optional 2.4 Selectivity In general the crystallisation process enjoys a substantial higher selectivity than conventional precipitation. This is caused primary by the selectivity related to the crystal structure and secondly by the fact that adsorption of impurities to the phosphate sludge flocs is minimal. 2.5 Process parameters The efficiency of phosphate removal for the pellet reactor depends upon the following three process parameters 1. reagent overdose and ph 2. supersaturation 3. hydraulic reactor load Reagent overdose and ph The pellet reactor effluent contains dissolved phosphate and suspended micro-crystals from nucleation. Nucleation is effectively minimised by the particular construction of the crystallizer and the choice of the appropriate degree of supersaturation. The dissolved phosphate concentration is fixed by the solubility product, the ionic reagent concentration and the process ph. This means, that the desired phosphate effluent concentration can be obtained by selection of the ph and reagent dosage. In practice at the optimal process ph, an overdose of mol/m 3 is applied Supersaturation At a given ph and overdose, the degree of supersaturation depends only upon the phosphate concentration of the wastewater. The phosphate concentration at the bottom of the reactor has to be maintained below a critical value in order to prevent primary nucleation. Moreover, the mechanical strength of the crystals decreases with increasing supersaturation. In practice, it has been observed that negligible nucleation occurs at a phosphate concentration of mg/l P. This concentration is obtained in the pellet reactor by the correct selection of the circulation ratio, irrespective of the phosphate concentration in the wastewater.

5 Recycle solution is collected in a mixing box underneath a bottom distribution plate and reintroduced into the heart of the Crystalactor through a series of nozzles. Influent and reagents are also fed through a specially designed manifold and nozzle system. To ensure easy maintenance, reagent injection system can be inspected or serviced without interrupting pellet reactor operation. Recycle solution, influent and reagent nozzles are placed according to a pattern so as to maximize mixing efficiency and to control the crystallization process. Hydraulic reactor load The hydraulic reactor load is the supernatant liquid velocity in the pellet reactor. This hydraulic load has to be selected in such a way that the pellet bed is fluidised. An increase in hydraulic load will result in an increase in secondary nucleation. In practice, good results are obtained for phosphate crystallisation with a hydraulic load of m/h. 3 EXAMPLES OF APPLICATIONS 3.1 Phosphate recovery municipal wastewater treatment plants In 1988 the first full-scale application was realised at the municipal wastewater treatment plant of Westerbork, The Netherlands. Phosphate was recovered by crystallisation of CP in the effluent of the biological treatment, followed by filtration. The plant operated successfully and removed phosphate from 10 mg/l P (in effluent of biological treatment) to below 0.5 mg/l P. The pellets were re-used by the phosphate processing industry. Phosphate recovery plant Westerbork The plant was built in a time that the raw municipal wastewater in the Netherlands contained around 20 mg/l P. Since phosphatefree detergents were introduced in Dutch households, the phosphate concentration in raw municipal wastewater has decreased sharply to around 10 mg/l P, leaving 3-4 mg/l P in the effluent of a normal biological treatment unit (without luxury uptake or chemical dosing). Due to the low P concentration in the Crystalactor feed, the direct phosphate removal from municipal effluent was not economically attractive anymore, and the Westerbork crystallisation unit was closed. In view of the lower inlet P concentration a combination of biological phosphate removal and recovery by the Crystalactor was developed. The biological phosphate removal is used to concentrate the phosphate in a side stream, which is treated in the Crystalactor. In 1993 two fullscale demonstration plants applying this process for the treatment of municipal wastewater were built: Geestmerambacht ( p.e.) and Heemstede ( p.e.). The process set-up is as follows:

6 - A part of the return sludge is pumped to an anaerobic tank where acetic acid is dosed (other lower fatty acids are also possible). Phosphate is released by the sludge in this anaerobic tank, reaching high P- concentrations (60-80 mg/l); - The sludge is separated from the P-rich water by a gravity-thickener in Geestmerambacht and a decanter in Heemstede; - The thickened sludge is returned to the aeration tank where it takes up phosphate again; - The phosphate is recovered from the P-rich water by CP crystallisation in the Crystalactor. Since the Crystalactor is located in a side stream no filter step is required; - The effluent of the Crystalactor is returned to the aeration tank. Crystalactors at Geestmerambacht Overview of P-recovery plant at Geestmerambacht In this process set-up the Crystalactor typically removes 70-80% of a feed concentration of mg/l P, at an effluent recirculation ratio between 2 and 3. The working ph in the reactor is kept relatively low to avoid a high level of supersaturation which may promote nucleation. Suspended solids and inorganic carbon should be kept below 250 mg/l and 1 mmol/l respectively to avoid disturbance of the crystallisation. In order to achieve this, measures are included to reduce suspended solids and protect the reactor, such as scum baffles in the thickener, sludge blanket detection in the thickener and turbidity monitor in the reactor influent line.

7

8 Inorganic carbon is removed from the influent by acid dosage to form carbon dioxide, and subsequent stripping in a cascade tower. Since several years the produced pellets are being re-used by the phosphate processing industry. 3.2 Phosphate recovery in the food industry In the food industry waste waters with a high organic load are released. The waste water of a Dutch potato processing plant is treated in an anaerobic biological reactor because of the low sludge production, the low energy consumption and the biogas production. The effluent is polished in an aerobic biological treatment plant. Cost effective phosphate removal by struvite crystallisation in the Crystalactor was implemented on full-scale. MgCl 2 and NaOH solutions were dosed into a part of the effluent of the anaerobic stage and in a rapid reaction strong MAP crystals were formed. No filter is required because of the high crystallisation efficiency and the fact that rest phosphate uptake takes place in the aerobic stage. Phosphate recovery at potato processing plant. competitive investment and operational cost. A flow of maximum 150 m 3 /h with 120 mg/l PO 4 -P was successfully treated in a reactor with a diameter of 1.8 m. The effluent contained about 10 mg/l PO 4 -P at a ph of The MAP was accepted by a fertiliser producer to be used directly in the granulation process for fertilisers. The advantages of the Crystalactor for the user were:. complete elimination of the existing struvite scaling problems downstream of the anaerobic reactor;. compact plant which is easy to insert between the anaerobic and the aerobic stage;. production of compact reusable pellets; 4 RECENT DEVELOPMENTS For a chemical industry, a pilot plant was operated on a wastewater stream that contains a very high P- and K-concentration (several grams per litre). It seemed that the production of KMP was not successful for this specific wastewater. In order to reduce P to the desired target concentration MP was easily produced. Currently, the costs for chemical demands are compared with alternatives. In a pilot plant test for a food industry on wastewater from a biological anaerobic pre-treatment unit, containing around 150 mg/l P and 10 mmol/l of inorganic carbon, MP was successfully produced without disturbance by the presence of inorganic carbon. The effluent soluble P concentration was around 10 mg/l. The results of this pilot test strongly indicate the advantage of MP production with respect to the influence of inorganic carbon. In the near future more tests will be implemented, in which the production of CP and MP will be compared, in environments were inorganic carbon and/or calcium is already present.

9 A promising adaptation still to be further developed, is based on the improved use of the height of the reactor. Laboratory tests have shown a large increase in the crystallisation capacity by injection of the reagents at multiple levels. However, due to closely related mixing requirements, crystal shear, supersaturation and nucleation, up-scaling is not straight forward, and there is still extensive research to be done with respect to this improvement. If successful, this research can result in super compact and low-cost crystallizers. 5 OTHER APPLICATIONS The pellet reactor crystallisation technology is not only applied for phosphate recovery, but also for water softening, fluoride removal and heavy metal recovery. In principle all crystalline salts can potentially be removed from wastewater. As shown in table B, there is an extensive experience in removing most heavy metals and major anions, and the number of applications continues to grow. Metals are generally removed as hydroxide, carbonate or sulphide compounds. Table B: Periodical system showing pellet reactor experience 1 2 H He 3 4 successfully Li Be recovered B CO3 NH4 O F Ne Na Mg Al Si PO4 SO4 Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe / Cs Ba La-Lu Hf Ta W Re Os Ir Pt Au Hg Ti Pb Bi Po At Rn / Fr Ra Ac-Lr Rf-Ku Ha-Ns Unh Uns Uno Une Anions are usually removed as calcium salts. Occasionally it is more desirable to form complex salts (for example MAP). The following pictures show some samples of produced pellets, and installations used for recovery of other components. A recent opportunity is recovery of fluoride and phosphate in the semiconductor industry, as part of a comprehensive strategy to reach zero discharge (ref. 7).

10 Examples of produced pellets Pellet reactors for softening of drinking water, Municipal Drinking Water Company of Amsterdam, The Netherlands Capacity: 8,500 m 3 /h Pellet reactor for nickel and aluminium recovery, Shell Chimie, Berre, France

11 6 ENVIRONMENTAL SOUND OPERATION Municipal wastewater treatment plants and commercial industries need cost-effective, compact and reliable technology to reduce waste emissions. Moreover, this technology has to provide a sustainable solution to the problem by avoiding secondary emissions. Secondary emissions such as waste sludge represent a growing environmental concern. Furthermore, they will increasingly be subject to ever-rising charges levied by the authorities, and ultimately their disposal will be prohibited altogether. Consequently, industry has embarked on a new strategy to tackle environmental load problems. Alongside waste recovery - often referred to as reuse or recycling - waste prevention is now a key feature. The Crystalactor offers a sustainable solution to above mentioned problems and combines an environmental sound production or wastewater treatment with attractive economics. 7 CONCLUSIONS 1. The Crystalactor technology is for many applications, including heavy metal, fluoride and phosphate recovery, a proven sustainable and frequently cost-effective technology. 2. Various options are available with regard to the phosphate recovery product. Most full-scale experience (especially in municipal wastewater treatment) is based on the formation of calcium phosphate. In industrial wastewater treatment full-scale experience exists with the crystallisation of magnesium ammonium phosphate (struvite). 3. Recent pilot testing on the recovery of magnesium phosphate from industrial wastewater indicates that crystallisation at a high loading rate is possible without disturbance by high carbonate concentrations in the influent. REFERENCES 1. Giesen, A., Eliminate sludge, Industrial Wastewater, 6 (1998). 2. CEEP, Phosphates, a sustainable future in recycling, Centre Européen d' Polyphosphates (1999). 3. Van Dijk, J.C., Braakensiek, H., Phosphate removal by crystallization in a fluidized bed, Water Sci. Technol., 17, (1984). 4. Giesen, A., Bouwman, J.G.M.A., de la Rie, T., Cost-effective phosphate removal by MAPcrystallisation (in Dutch), VMT, 1996, No Giesen, A., Removal of amorphous components in a fluidized bed type crystallizer, Proc. 7th Gothenburg Symp., Edinburgh, Scotland, September (1996). 6. Piekema, P.G., Gaastra, S.B., Upgrading of a wastewater treatment plant in The Netherlands: combination of several nutrient removal processes, European Water Pollution Control, Vol. 3, No. 3, (1993). 7. Bouwman, J.G.M.A., Luisman, A.H.C., Aiming at zero discharge and total reuse, European Semiconductor, September, (2000).