Membrane contactor onsite piloting for nutrient recovery from mesophilic digester reject water: The effect of process conditions and pre-treatment options

Nutrient recovery is an important segment of the circular economy, and it significantly contributes to sustainable development goals. This work reports on the outcomes of a field testing pilot-scale membrane contactor system designed for nitrogen (N) and phosphorous (P) recovery in the form of high purity ammonium salts and high-quality P containing sludge. The pilot testing was conducted at the Viikinmaki WWTP using digester reject water under different treatment conditions. Our system showed a high tolerance for solids (concentration > 500 mg/L). Field test trials showed that the higher the feed flow, the better the ammonia transfer rate. Decreasing the retention time from 4 h to 2 h increased the ammonia mass transfer rate constant by >150 %. Among the tested feed pH levels, a pH of 10 had the highest solids removal, which in turn resulted in the highest ammonia recovery percentage. A high acid concentration lowered the ammonia transfer rate. Strong acids such as HNO3 and H2SO4 had a higher ammonia recovery than that of H3PO4. Pre-treating feedwater with starch resulted in the same ammonia accumulation rate as a poly-aluminum chloride (PAX)/polymer pre-treatment. The highest PO4-3 removal of 99 % was achieved with a PAX/polymer treatment at pH 10, whereas the highest total phosphorous removal of 77 % was achieved with a starch treatment. The produced sludge consists mainly of CaCO3 emanating from the used lime, which can be used as a soil amendment. The produced ammonium salts were of high purity and have a nutrient content comparable to that of commercial fertilizers. This study provides important insights into the selection of process parameters of membrane contactor systems based on the goal of the treatment, whether it be nutrient removal or recovery.