THE REMOVAL OF PHOSPHORUS FROM MUNICIPAL WASTEWATERS WITHIN THE CITY OF CAPE TOWN

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1 THE REMOVAL OF PHOSPHORUS FROM MUNICIPAL WASTEWATERS WITHIN THE CITY OF CAPE TOWN R.W. Moollan Scientific Services Department, Water Services, City of Cape Town. ABSTRACT This paper provides information on the status of phosphorus removal at the twenty wastewater treatment works and three marine outfalls in the City of Cape Town. Average influent total phosphorus concentrations range from 3.3 to 35 mgp/l with a flow-weighted average of 12 mgp/l. The average final effluent ortho-phosphate concentration ranges from 2 mgp/l to 23 mgp/l and amounts to a combined load of 2570 kg of phosphorus discharged to the environment per day. Although not all the wastewater treatment facilities are designed for nutrient phosphorus removal, the average percentage removal ranged from 11 to 70 %. If the most recent annual performance (2003) prevails, none of the wastewater treatment facilities will be in a position to consistently comply with the Department of Water Affairs and Forestry s (DWAF s) General Standard Guideline limit of 1 mgp/l. Of the 50 plus tests done, the percentage compliance with the DWAF guidelines ranged from zero to 38 %. Current phosphorus loads to the environment were found to be approximately six times higher than would be permissible under the new guidelines. Placing a total ban on detergent phosphorus is not likely to solve the problem of compliance with legislation and the payment of fines according to the proposed Waste Discharge Charge System appears to be a cheaper option than dosing with chemicals to remove ortho-phosphate from treated effluents. Other possible remediation measures include, reducing the amount of phosphorus at source via community involvement and media attention, upgrading of wastewater treatment facilities to nutrient removal systems and the application for exemptions from the guideline where feasible. INTRODUCTION The discharge of waste effluents that contain phosphorus contributes to the eutrophication (nutrient enrichment) of water bodies. This shifts the ecological balance in the receiving environment by promoting excessive plant and algal growth, the removal of which, escalate water treatment costs, especially if the water has to be treated for human consumption or remediated to an environmentally acceptable water quality standard. Relatively high levels of phosphorus (i.e., above mgp/l) in natural water systems is therefore considered to be a form of pollution because it lessens the value of water to humans and other life forms in some way. In line with the polluter pays principle and the envisaged Waste Discharge Charge System (WDCS), Water Services Authorities (WSAs) and Water Services Providers (WSPs) will be under increasing pressure to comply with the ortho-phosphate limit of 1 mg P/l, as set by the Department of Water Affairs and Forestry (DWAF) in the 2010 General Standard guidelines for the discharge of treated wastewater effluent. This concern has prompted an investigation into status of phosphorus removal from municipal wastewater at the various wastewater treatment facilities within the City of Cape Town. Strategies for the long-term minimization of this form of water pollution are also proposed. The Phosphorus Composition in Municipal Wastewater Phosphorus plays an important role in many bio-chemical processes due to the versatility of its chemical structure and its ability to form various organic and inorganic polymers. Approximately 85% of phosphoric acid, the most basic phosphorus compound produced, is used in the fertilizer industry; 5% is used in the detergent industry; 5% in the animal feed industry and the remaining 5% in the food, beverage and other industries. Proceedings of the 2004 Water Institute of Southern Africa (WISA) Biennial Conference 2 6 May 2004 ISBN: Cape Town, South Africa Produced by: Document Transformation Technologies Organised by Event Dynamics

2 According to the USEPA, approximately 50 to 70% of the phosphorus in municipal wastewater originates from detergents that contain phosphate builders (multi-purpose cleaners, laundry detergents, etc.); and the other 30 to 50% originates from human faeces, urine and food waste disposal. These percentages cover a wide range depending on the amount of industrial effluent entering the sewage reticulation system, which may either dilute or concentrate the phosphorus content of municipal domestic sewage. In a South African study detergent phosphorus was found to contribute between 35 and 51%of the phosphorus load to the wastewater treatment works. Methodology for the Measurement of Phosphorus Removal In municipal wastewater, phosphorus is found in three forms, namely, ortho-phosphate, polyphosphate and organically bound phosphorus, all of which are measured as mg P/l. Because phosphorus is present in a variety of forms, a satisfactory measure of its removal efficiency in wastewater treatment is based on the total phosphorus entering and leaving the wastewater treatment works. However, the phosphorus limitation with regard to the DWAF guidelines for treated effluent is on the ortho-phosphate form because during biological wastewater treatment the phosphorus content is converted to the ortho-phosphate form, which is the soluble biologically active form that is readily used up by plants and algae. At the end of the treatment the total and ortho-phosphate concentrations are also similar, hence the calculation of the desired phosphorus removal percentage is based on the difference between the influent total phosphorus and the orthophosphate concentration in the final effluent. Chemical Analysis Methods (i) Ortho-phosphate: samples were filtered through 0.22 µm filters and analysed spectrophotometrically using a Lachat Quickchem Series 8000 flow injection analyser. {detection wavelength = 880 nm, reagents: ammonium molybdate, antimony potassium tartrate, sulphuric acid; at 36 o C} (ii) Total phosphorus: samples were digested in ammonium persulphate and sulphuric acid at 121 o C and analysed as described for ortho-phosphate. (Refer to Ref.7 for more detail) Flocculation Experiments Lab scale flocculation tests using ferric chloride or alum were done using 1 litre samples placed on a Phipps and Bird stirrer into which varied doses of the flocculant was added. Samples were stirred at 100 rpm for 2 minutes (during which the flocculant was added) followed by 40 rpm for 18 minutes and allowed to settle for 30 minutes before the supernatant was analysed for ortho-phosphate. Control tests (without addition of flocculant) were also carried out. THE STATUS OF PHOSPHORUS REMOVAL AT WASTEWATER TREATMENT WORKS WITHIN THE CITY OF CAPE TOWN Total phosphorus concentrations in the influent sewage to the 20 wastewater treatment works and 3 marine outfalls range from 3.3 to 35 mg P/l, with a flow-weighted average concentration of 12 mg P/l. Whilst not all the treatment facilities are designed to remove phosphorus, removal figures range from 11 to 70% with an average percentage removal being 53%. The desired percentage removal was calculated for characteristic influent quality for each WWTW as shown in Figure 1.

3 Figure 1. Actual and desired phosphorus removal. The desired percentage phosphorus removal ranges from 70 to 97 percent, which is consistently above what the individual works are achieving on average. The higher figures for the desired removal imply that these works receive relatively higher total phosphorus concentrations. (See Figure 2). For the year 2003, there were only eight of the twenty works that had on occasion complied with ortho-phosphate concentration of less than 1 mgp/l, with the Potsdam activated sludge plant having the highest percentage compliance of 38%.

4 Wastewater Treatment Works Type of plant Average influent flow Influent total phosphorus Final effluent orthophosphate Average removal Number of final effluents sampled Percentage of final effluents sampled that complied with the 1 mgp/l standard Ml/d mgp/l mgp/l % n % Athlone A/S da Bellville A/S da Borcherds Quarry A/S da Cape Flats A/S da Gordons Bay A/S da Klipheuwel RBC Kraaifontein T/F Llandudno ARBC Macassar A/S sa Melkbosstrand A/S sa Miller's Point RBC Mitchells Plain A/S da Oudekraal RBC Parow A/S sa Potsdam AS A/S sa Potsdam Biofilter T/F Scottsdene AS sa Simons Town T/F Wesfleur Domestic A/S da Wesfleur Industrial A/S da Wildevoelvlei A/S sa Zandvliet A/S sa Camps Bay M/O n/a n/a n/a n/a Green Point M/O n/a n/a n/a n/a Hout Bay M/O n/a n/a n/a n/a A/S = activated sludge T/F = trickling filter RBC = rotating biological contactor da = diffused aeration sa = surface aeration n/a = not applicable Figure 2. Statistics on the annual figures for 2003 (January to December). STRATEGIES TO REMEDIATE THE PROBLEM OF NON-COMPLIANCE THE DWAF STANDARDS There are at least four possibilities: reduce the amount of point source pollution, i.e., impose restrictions on the use of phosphate detergents modify wastewater treatment processes to chemically and/or biologically remove phosphorus evaluate the cost / need for phosphorus removal and apply for an exemption from the DWAF guidelines a mixture of the above three Reducing Phosphorus at its Source This can be in the form of restricting or prohibiting the use of phosphate-containing detergents by substituting the use of sodium tripolyphosphate (STPP) as the detergent builder. Possible substitutes include zeolite A, nitriloacetic acid, silicates, carbonates, carboxylic acids, soaps and ethylenediaminetetraacetic acid. Internationally this strategy has had some success, but there is no detail as to whether they were used in conjunction with other remediation measures. Of interest to the South African context would be: should phosphate-containing detergents be completely banned in all regions and is that an appropriate strategy given the economic disparity of the population? Will there upgrades to the existing wastewater treatment plants in conjunction with the phosphorus restrictions? Whilst in Western Europe long-term improvements to the impoundments were noted, in studies conducted in the USA, where a total ban on phosphorus detergents was imposed, it was found that there was no significant improvement to the eutrophic status of the receiving water bodies. This shifts the focus from point to non-point (or dispersed) sources of nutrient pollution, namely fertilizer and farmland run-off, storm water infiltration, etc.

5 Figure 3. Actual and desired influent total phosphorus concentrations. This intervention alone is unlikely to solve the problem of compliance with legislation. If there is no alteration to the wastewater treatment processes, the desired extent of this intervention will require that the total phosphorus entering the various works be reduced by 50 to 96 %. The difference of between the actual and desired concentration for the influent sewage to each works is shown in Figure 3. If detergents contribute approximately 50 % of the phosphorus load, then a total ban on phosphate-containing detergents may solve the problem for the Athlone wastewater treatment works only. However, the social aspect regarding the readiness of change towards higher household cost solutions (from buying phosphate-free detergents) for an environmental problem (that could possibly be treated at the wastewater treatment stage) may deter efforts. Personal communication with authorities in the Unilever group (detergent manufacturers) pointed out that there are no phosphorus free laundry detergents available on the market in South Africa. Previous substitutes for phosphorus in detergents were investigated in Europe in the 1980s and were not that successful. It was also mentioned that the substitutes were possibly more harmful to the environment and are either relatively abrasive or corrosive. At best, the move towards lowering the

6 use of phosphate detergents can be encouraged amongst environmentally conscious industry groups and individuals through catchment management forums and media education. This can come in the form of reducing the amount of detergents used where possible, e.g., washing bigger loads of clothing, reducing the frequency, using laundry water in the gardens, etc. Reducing Phosphorus at the Wastewater Treatment Stage If the phosphorus cannot be reduced effectively at the point of origin, the other possibility is to reduce the phosphorus at the wastewater treatment stage. As stated earlier, not all the wastewater treatment facilities in the city are equipped to remove phosphorus, however, their treatment processes can be modified to some extent. This can be done by converting the existing works to activated sludge nutrient removal plants or by the addition of alum, lime or iron salts to the primary, secondary or tertiary stage in the wastewater treatment process. Activated sludge plants that are capable of biologically reducing the phosphorus will require increased chemical monitoring as well as more management and operator attention. Characteristics of the influent wastewater and process sludge need to be carefully observed to ensure an optimum flow balance; influent readily biodegradable chemical oxygen demand (RBCOD) fraction; TKN:COD ratio; appropriate dissolved oxygen concentrations; wasting rates; sludge age; recycle flows; etc., all of which depend on the availability of fully trained and competent operators and the availability of funds. In many cases, the WWTW design and operation vary and the conditions for optimal phosphorus removal will be plant specific. Hence it will require detailed process knowledge into the plant capabilities, contributing industrial effluent discharges and the development and spatial planning of the area being serviced. Modified primary treatment involves chemical precipitation with lime, iron or aluminium salts by adding it before the primary settling tank. Secondary treatment requires the addition of these salts to the activated sludge process (where applicable), either the aeration basin or before the secondary settling tank. Wastewater Treatment Works Average influent flow Final effluent orthophosphate Phosphorus load onto the environment FeCl 3 required to reduce the ortho phosphate to 1 mgp/l Approximate associated cost per day (excl. V.A.T.) Ml/d mgp/l kg/d mg/l Rands Athlone Bellville Borcherds Quarry Cape Flats Gordons Bay Klipheuwel Kraaifontein Llandudno Macassar Melkbosstrand Miller's Point Mitchells Plain Oudekraal Parow Potsdam AS Potsdam Biofilter Scottsdene Simons Town Wesfleur Domestic Wesfleur Industrial Wildevoelvlei Zandvliet TOTAL per day Figure 4. Phosphorus loads onto the environment. Tertiary treatment involves the use of chemicals to precipitate the phosphorus in the final stages. This method has had reported successes, however, additional financial burdens on the operating

7 cost of wastewater treatment will be incurred. In lab scale tests using secondary settling tank effluent and ferric chloride coagulant ( % FeCl 3 ; without polyelectrolyte), it was established that the coagulant dose required to bring the concentration to 1mgP/l increased proportionately with the initial ortho-phosphate concentration (1-10 mgp/l) of the test solution. Figure 4 lists the required doses for the individual works. Precipitating the ortho-phosphate using the relatively cheapest option of adding ferric chloride without polyelectrolyte at R1150 per tonne (2003) to the secondary settling tanks at the various WWTWs is likely to cost the operations department approximately R per day (excluding V.A.T.) or R 22m per annum (incl. V.A.T.) for all the WWTWs. Besides the cost of the chemicals, additional structural and equipment costs will be required - for the installation of storage facilities and chemical dosing facilities, etc. Extra handling costs may also be incurred with these additional processes. Comparing the additional treatment cost to the envisaged tariff on waste discharges, it will be more economically feasible not to treat the effluent any further. The proposed draft charge for the discharge of effluent phosphorus is between R 2,70 to R 5,40 per kg. The 2003 annual discharge from the twenty WWTWs amounted to approximately 2721 kgp/d, whereas the maximum allowed would have been in the region of 472 kgp/d. The difference plus the load from the marine outfalls (ca. 320 kg/d) leaves the tariff load at approximately kg P/d. Therefore, should the WDCS have been in operation in 2003, the cost in tariff charges would have been between R2.5m and R5.1m per annum. In the short term it may be more feasible for authorities and service providers to pay the discharge charges but the long term environmental damage may be even costlier when considering the pollution of underground and other potential potable water sources. The cost to the environment can at best be estimated by the subsequent cost of remediation or treatment of the receiving water body. However, estimating the ecological damage is not easily determinable. The phosphorus loading from the various works are shown in Figure 5, which may possibly serve as a guide for prioritisation. Applying for Exemptions from the DWAF Guidelines Considering the location of the various wastewater treatment works and the uses of the effluent, it may not be necessary for all the wastewater treatment works to remove phosphorus. A good example would be the Parow WWTW where the final effluent is used solely for the irrigation of the adjacent golf course. The phosphate content, in this case, is considered a beneficial nutrient for plant growth and removing it would be pointless as the phosphorus is normally added back via fertilizers. The Parow plant itself only removes 50% of the influent phosphorus as opposed to the desired 92% required to comply with the guideline limit of 1 mg P/l. To modify the design of the works or to treat the wastewater with chemicals to remove the excess phosphorus is not likely to be cost-effective. In this and similar cases, it will be in the authority s best interest to apply for an exemption from the permit regulation.

8 Figure 5. Average daily phosphorus loads from the various WWTWs. CONCLUSION Research conducted internationally and by the Water Research Commission (WRC) in South Africa have established that the bulk of the phosphorus load entering municipal sewage works originates from laundry detergents. However, the cost of eliminating phosphorus in detergents by substituting it with other chemical compounds does not appear to solve the problem of compliance with the DWAF effluent guideline of 1 mgp/l. Besides there being no phosphorus-free laundry detergents available in South African stores, the WRC study also indicated that the cost of detergent reformulation was higher than the cost of treating the consequent higher phosphorus loading, which includes the removal of nuisance algae. Other cost implications include reduced lifetime of fabrics and the corrosion of washing machine equipment, as they are more abrasive or corrosive.

9 The alternative is to remove phosphorus at the wastewater treatment stage before it enters the environment. Figures for 2003 show that the daily phosphorus loads entering the environment average approximately 2721 kg, which is roughly six times higher than the envisaged permissible load of 472 kg. Recommendations for improved wastewater treatment range from increased management and operator attention to the problem, chemical precipitation using lime, alum or ferric salts and community involvement. Some of these interventions will involve additional capital and operational expenditure. Further detailed treatment costs will have to be investigated depending on the remedial action that will be adopted. For the City of Cape Town chemical dosing alone is likely to cost in the region of R22m per annum which is about 4 to 9 times higher than paying the envisaged tariff charge. For the smaller works with relatively negligible impact on the environment it may be advisable to apply for permit exemptions from the phosphorous compliance with the 2010 General Standard guideline. REFERENCES 1. Viljoen P. and Oberholzer J., Towards the formulation of waste discharge system for South Africa, Department of Water Affairs and Forestry, IWA Conference, Cape Town, September Pillay M. and Buckley C., Detergent Phosphorus in South Africa: Impact on Eutrophication with Specific Reference to Mgeni Catchment, WRC Report 465/1/ Wiechers H.N.S. and Heynicke J.C., Sources of Phosphorus which give rise to Eutrophication in South African Waters, Water SA, 12 (2). 4. Shreve's Chemical Process Industries, 5 th Edition, McGraw-Hill Book Company, Process Design Manual for Phosphorus Removal, USEPA, Theory, Design and Operation of Nutrient Removal Activated Sludge Processes, WRC Report TT16/ QuikChem Method E, Zellweger Analytics, Lachat Instruments Inc. 8. Plumbley R., Unilever, pers.comm.