Enhancing nutrient recovery by optimizing phosphorus stripping of bio-P sludge: Experimental analysis and modeling

Phosphorus (P) from bio-P sludge can precipitate uncontrolled in the sludge treatment line and decrease the potential for phosphorus recovery. The presence of an anaerobic unit (or equivalent) before the pre-dewatering stage could favor the early release of phosphorus and minimize uncontrolled struvite precipitation in the anaerobic digestion (AD) and post-digestion processes. In this study, a series of batch experiments in a phosphorus stripping unit (P- stripping unit) were conducted to investigate the phosphorus release rate and efficiency of phosphorus being stripped, when a bio-P sludge was exposed to different amounts of carbon and different temperatures (~20 °C and between 11 and 13 °C). The results indicated that with a dose between 10,000–17,000 mg sCOD (mg P mg TS−1)−1 at ~20 °C and between 15,000–25,000 mg sCOD (mg P mg TS−1)−1 at 11–13 °C, can be released 40–65% and 30–38% luxury phosphorus, respectively. Calculations showed that 80% percent of the soluble phosphorus present in the P-stripping unit (468 Kg d−1) was following the reject water of the pre-dewatering facility. A hybrid mathematical model was developed using the data obtained from the batch kinetic experiments to explain the phosphorus release kinetics in the stripping unit. A good fit between the experiment and model-predicted data demonstrates the ability of the model to predict the phosphorus release under varying initial conditions. Mass balance calculations on P and Nitrogen (N) for the sludge pre and post-dewatering side streams, showed that struvite precipitates 1.6 times more in the existing than in the modified configuration which includes a P-stripping unit. Implementing a P-stripping unit would minimize the precipitation in the AD, enhance P-recovery, and reduce the phosphorus content of EBPR biosolids. The information gained in this study can be used to generate and optimize phosphorus-rich streams early in the sludge line and thereby enhance phosphorus recovery.