Development and application of a physico-chemical equilibrium model for systematic struvite precipitation from post-aerated municipal anaerobically digested sludge

Abstract

Phosphorus is a crucial fertilizer for modern agriculture and therefore a key component of our nutritional security, but due to its limited availability, phosphorus recycling is becoming increasingly important. Thus, given the high dissolved phosphorous concentrations (ortho–P) of treatment processes such as anaerobically digested sludge from anaerobic sludge treatment, municipal wastewater treatment with enhanced biological phosphorus removal (EBPR) creates an economically viable opportunity for phosphorus recovery. One approach involves precipitating dissolved phosphorus as struvite (NH₄MgPO₄*6H₂O) by adding magnesium such as MgCl₂*6H₂O in post-aerated, anaerobically digested sludge. The phosphorus can then be recovered from the ash after sludge incineration. However, the physico-chemical processes governing struvite precipitation in post-aerated anaerobically digested sludge have not been thoroughly investigated. To address this gap, the present study develops a physico-chemical equilibrium model for struvite precipitation in post-aerated anaerobically digested sludge using the SIMBA# 3.2 software. The model aims to predict phosphorus recovery in post-aerated anaerobically digested sludge and prevent unwanted precipitation that could clog pipes in downstream processes. The study used three batch experiments with real anaerobically digested sludge (a’ 260 L) to calibrate and validate the model. Results showed good agreement between simulated and measured values for pH, Mg²⁺, Ca²⁺, PO₄-P, and NH₄-N, with deviations generally below 5 %. These findings suggest that the model can support the efficient operation and design of struvite precipitation plants, improving phosphorus recovery and minimizing operational issues without costly trials.