A full-scale evaluation of biological phosphorus removal using a fixed and suspended growth combination
Creators: name: Gibb, Allan James
Publisher: name: University of British Columbia
Year: 2010

description: A study was undertaken to assess the feasibility of using a combination trickling filter-activated sludge (fixed growth-suspended growth) treatment process for enhanced biological phosphorus removal from municipal wastewater, and to evaluate the operating conditions at a full-scale fixed growth-suspended growth (FGR-SGR) demonstration facility in Salmon Arm, British Columbia, Canada. The results of the study, based on full-scale plant data and bench-scale batch test results obtained over the first year of operation, showed that enhanced biological phosphorus removal was established in the combined FGR-SGR process. The phosphate release and uptake rates of the biomass cultured in the full-scale FGR-SGR system were comparable to the findings of others for activated sludge-type biological phosphorus removal systems. The study was designed to include an assessment of the effects of plant operating MLSS concentration on effluent quality; the average effluent total phosphorus concentration increased from 2.1 mg P/L (75% removal) to 2.6 mg P/L (79% removal) to 4.6 mg P/L, for average operating MLSS concentrations of 4090 mg/L, 3250 mg/L, and 2360 mg/L, respectively, over an 11 month operating period. However, the effects of the planned changes in MLSS may have been confounded with the effects of (unknown) seasonal variations in plant operating conditions. Seasonal changes in process organic loading appeared to have a significant effect on bacterial phosphate release and uptake rates in the full-scale process, but had no apparent effect on effluent quality. The average effluent concentrations of total suspended solids and BOD₅ were both in the range 8-14 mg/L over the entire 11 month period. Process liquid temperatures as low as 8° C had no detrimental effect on effluent quality. The average phosphorus content of the SGR total suspended solids was 4.4% by dry weight over the 11 month study period. Diurnal fluctuations in flow and load to the full-scale process were found to have a significant effect on phosphorus removal. The concentration of total phosphorus in the plant final effluent was consistently less than 1 mg P/L during the morning low flow-low organic load condition; after the onset of the afternoon high flow-high organic load condition, plant effluent orthophosphate concentrations were generally greater than 1 mg P/L. Batch test simulations indicated that lowering the secondary sludge return flow rate would increase bacterial PO₄ release in the anaerobic phase, but would have no short-term effect on aerobic bacterial PO₄ uptake rates, or on the aerated volume required for complete PO₄ removal. Batch test results also indicated that the biomass cultured in the full-scale FGR-SGR process had an average total PO₄ uptake capacity of 40-60 mg P/L (19-21 mg P/g MLSS), compared to the plant design phosphorus loading of 7-8 mg P/L (the aeration periods for the batch tests used to calculate the average total PO₄ uptake capacity of the biomass were 2-3.5 times longer than the actual aeration time available in the full-scale process, and the initial PO₄ concentration used in the batch tests was approximately 10 times the plant design loading).