The inhibitory effect of inorganic carbon on phosphate recovery from upflow anaerobic sludge blanket reactor (UASB) effluent as calcium phosphate.

After treatment of the wastewater from the potato processing industry in an upflow anaerobic sludge blanket reactor (UASB) the effluent is rich in phosphate and dissolved inorganic carbon (IC). Increasing the pH of the UASB effluent with NaOH to precipitate phosphate as calcium phosphate leads to contamination with magnesium phosphate. Increasing the pH with Ca(OH)₂ had a positive effect on phosphate precipitation, but after increasing the pH with Na₂CO₃ no precipitate was formed. After prior nitrification of the UASB effluent to remove IC, less NaOH was needed to increase the pH and the ions precipitated in a ratio that agreed with calcium phosphate formation. When the pH of the nitrified effluent was increased with Na₂CO₃ neither calcium nor phosphate precipitated. This inhibitory effect of IC on phosphate precipitation as calcium phosphate could not be derived from the saturation indexes calculated by the geochemical modelling program PHREEQC.