AN EVALUATION OF SIX DIFFERENT METHODS FOR IMPROVING PHOSPHORUS REMOVAL AT WASTEWATER TREATMENT FACILITIES

The City of Boise's West Boise wastewater treatment facility (WWTF) is in the process of upgrading its treatment train in order to reduce its discharge of phosphorus. This work is a part of a larger effort by the City of Boise to meet upcoming total maximum daily load- (TMDL) based limitations on the discharge of phosphorus into the Boise and Snake Rivers. As part of this effort, a whole plant simulator was developed for the West Boise WWTF. The simulator was used to determine the benefits and effects of different treatment options, and combinations thereof, on the plant effluent phosphorus levels. The processes investigated are: 1) Enhanced Primary Treatment, 2) Metal salt addition to secondary clarifier influent, 3) Biological phosphorus removal, 4) Primary sludge fermentation, 5) Metal salt addition to the dewatering recycles stream, and 6) Intentional struvite formation within the dewatering recycles stream.The West Boise WWTF influent contains inadequate volatile fatty acids (VFAs) and biological oxygen demand (BOD) to allow full biological removal of the bioreactor influent ortho-phosphate. This problem is compounded since the West Boise WWTF must treat the anaerobically digested biosolids from the other major facility in the City of Boise, which almost doubles the phosphorus load to the West Boise WWTF (by way of the dewatering recycles stream) without any additional BOD load to help remove the additional phosphorus.The modeling effort indicates that there is no single technology that can cost-effectively meet the City's effluent phosphorus goals. It was found that metal salt (alum or iron) addition will probably be required in order to reduce the plant effluent to the target ortho-phosphate level of 0.5 mg P/l. The alum dosage point that resulted in the lowest overall use of alum was in the dewatering feed stream. This is a direct result of adding the alum to a high-strength phosphate stream and getting the most efficient usage out of the metal. In all cases, primary sludge fermentation reduced either the effluent ortho-phosphate level and/or the required alum dosage to meet the effluent target.The combination of alum addition in the primary clarifier (chemically enhanced primary treatment, [CEPT]) and primary sludge fermentation appeared to have a number of advantages. This process required only about 9 percent more chemical, but reduced plant air requirements by 13 percent and the mixed liquor suspended solids (MLSS) by about 25 percent. These latter two effects directly translate to more plant capacity.