The solubilisation of N and P during primary sludge acid fermentation and precipitation of the resultant P

The solubilization of phosphorus and ammonia, as it relates to the yield of volatile fatty acids (VFA), was investigated during the acid digestion of 4 primary sludges from biological nutrient removal (BNR) plants in Johannesburg. The experimental work was carried out in anaerobic batch reactors at a room temperature with a daily maximum of 18-28C, and the effect of using lime to precipitate the phosphorus in the fermented sludge liquor was evaluated, as was the influence of adding settled sewage elutriant for VFA recovery on the lime dosage requirements of fermented sludges. The amount of phosphate and ammonia release from primary sludge was dependent on the wastewater solids composition and detention period, and possibly, temperature and mixing effects. Lime addition was an effective and stable method of removing phosphate from the liquid phase of acid-rich primary sludges, and of minimizing phosphorus release from fermented sludge during storage periods of up to 2 d.