In general, removal of phosphorus is not often required for wastewaters that receive lagoon treatment, although a number of exceptions can be found for systems in the northcentral United States and Canada. If such a requirement is imposed, the experiences described in the following text will provide some guidance.
In order to meet a phosphorus requirement of 1 mg/L for discharge to the Great Lakes, an approach using in-pond chemical treatment in controlled-discharge ponds was developed in Canada. Alum, ferric chloride, and lime were all tested by using a motorboat for distribution and mixing of the chemical. A typical alum dosage might be 150 mg/L, which should produce an effluent from the con-trolled-discharge pond that contains less than 1 mg/L of phosphorus and less than 20 mg/L BOD and SS.
The sludge buildup from the additional chemicals is insignificant and would allow years of operation before requiring cleaning. The costs for this method were very reasonable and much less than those for conventional phosphorus removal methods. This method has been applied successfully in several midwestern states.
Studies of in-pond precipitation of phosphorus, BOD, and SS were conducted over a 2-year period in Ontario, Canada. The primary objective of the chemical dosing process was to test removal of phosphorus with ferric chloride, alum, and lime. Ferric chloride doses of 20 mg/L and alum doses of 225 mg/L, when added continuously to the pond influent, effectively maintained pond effluent phosphorus levels below 1 mg/L over a 2-year period. Hydrated lime at dosages up to 400 mg/L was not effective in consistently reducing phosphorus below 1 mg/L (1 to 3 mg/L was achieved) and produced no BOD reduction while slightly increasing the SS concentration. Ferric chloride reduced effluent BOD from 17 to 11 mg/L and SS from 28 to 21 mg/L; alum produced no BOD reduction and a slight SS reduction (from 43 to 28-34 mg/L). Consequently, direct chemical addition appears to be effective only for phosphorus removal. A six-cell pond system located in Waldorf, Maryland, was modified to operate as two three-cell units in parallel. One system was used as a control, and alum was added to the other for phosphorus removal. Each system contained an aerated first cell. Alum addition to the third cell of the system proved to be more efficient in removing total phosphorus, BOD, and SS than alum addition to the first cell. Total phosphorus reduction averaged 81% when alum was added to the inlet to the third cell and 60% when alum was added to the inlet of the first cell. Total phosphorus removal in the control ponds averaged 37%. When alum was added to the third cell, the effluent total phosphorus concentration averaged 2.5 mg/L, with the control units averaging 8.3 mg/L. Improvements in BOD and SS removal by alum addition were more difficult to detect, and at times increases in effluent concentrations were observed.
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