Pilot scale struvite recovery trials from a full-scale anaerobic digester supernatant at the City of Penticton Advanced Wastewater Treatment Plant
Creators: name: Britton, Ahren Thomas
Publisher: name: University of British Columbia
Year: 2009

description: Pilot-testing of a fluidized bed reactor used to recover phosphate in the form of struvite from a full-scale anaerobic digester supernatant was conducted on site, at the City of Penticton Advanced Wastewater Treatment Plant (AWWTP). The main objective of this study was to demonstrate the ability of the reactor, developed by the UBC phosphate recovery team, to remove at least 70 % of the phosphate in the supernatant from a full-scale digester fed with a combination of primary and secondary sludge, from a biological nutrient removal wastewater treatment plant. Results showed that the reactor was capable of removing over 80 % of the phosphate from the digester supernatant. The operation of the reactor could easily be controlled to achieve any desired level of phosphorus removal up to 90%. Reactor operation was relatively trouble free after an initial commissioning period. By the end of the experiment, it was possible to leave the reactor unattended for periods of up to 5 days without incident. Analysis of the recovered struvite crystals showed essentially pure struvite (&gt;99 % by weight) with small amounts of calcium (&lt;0.5 % by weight) and traces of potassium and iron. The recovered crystals had mean diameters increasing from 0.5 to 1.8 mm over the course of the study. This increasing diameter is believed to be due to changes in the crystal structure that caused them to become stronger over the course of the study. The causes of this change in crystal structure remain unknown, and require further investigation. A model was developed which was able to predict the effluent quality of the reactor based on the concentrations of magnesium, ammonia and phosphate in the reactor influent and the operating pH of the reactor. The model is based on the assumptions that the reactor effluent is at equilibrium with respect to struvite, and that magnesium, ammonia and phosphate are removed in equimolar amounts. The system equilibrium was described by an equilibrium conditional solubility product curve, developed for a sample of digester supernatant taken during the study Phosphate release from the anaerobic digestion of waste activated sludge was found to be 13% of the total phosphorus load to the treatment plant, when digesting only 40% of the secondary sludge, significantly lower than predicted in a previous study (Niedbala, 1995). This is probably due to the recent practice of discharging aluminum-rich sludge from the city drinking water treatment plant to the wastewater treatment plant. Changing this practice could result in the production of significantly greater masses of product at similar costs, thus increasing the economic viability of the process. Further studies at larger scale and of longer duration would be required to determine the steady state struvite product qualities produced by this process. The market that the product will target will also be important in order to produce a desirable and profitable product.