Controlled struvite precipitation from belt press filtrate of anaerobic digester in a CSTR

Struvite (MgNH₄PO₄·6H₂O) is known to precipitate and clog pipes and pumps, causing operational difficulties and expenses downstream of the anaerobic digester in wastewater treatment plants around the world. This work focused on determining the most appropriate reactor operational conditions (focus on hydraulic retention time and aeration rate) required for attaining the most homogeneous controlled struvite precipitation in order to reduce pipes and pumps clogging downstream of the digester. To this end a continuous laboratory-scale completely stirred tank reactor aerated (for CO₂ stripping) reactor was operated at different hydraulic retention times and aeration rates and precipitate composition was determined. A minimum pH of 8.0 was calculated and experimental observed as necessary to attain struvite precipitation to its maximum potential to prevent clogging. To reach this pH value a minimum HRT of 0.5 hours with aeration flow rate of 11.2 m ³/h/m³ and/or 15-min HRT with an air flow of 46.7 m ³/h/m³ was necessary for the reactor studied. Struvite precipitation kinetics was observed to be much faster than other minerals that can precipitate and a decrease in HRT promoted a precipitate richer in struvite. However, for all HRT and air flows studied the precipitate obtained can be considered to be poor in struvite and can hardly be reused in agriculture.