This article examines the optimisation of recovery of phosphorus and nitrogen (via struvite) in small treatment units for high strength wastewaters using biologically assisted precipitation. The particular focus was the synergetic effect between removal of orthophosphate and the biological reactions occurring during nitrogen removal. The most sensitive parameter influencing the phosphate solid forms (HAP, MAP) is pH. Sequencing batch mode helps maintain high gradients in ammonia concentration, which encourages struvite precipitation. Nitrification has a key effect on the precipitation, through its influence on pH and ammonia concentration, determining the remaining soluble phosphorus concentration. Denitrification and CO2 stripping, by increasing pH also improve precipitation of phosphorus. Optimal operating conditions will therefore depend on the chosen strategy: thus nitrification may help to keep phosphorus in a soluble form (as needed in direct urine reuse), whereas if co-precipitation is desired in the biological reactor, nitrification should be controlled in relation with others processes responsible for pH increase.

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