Nitrogen recovery from digester reject water using selective gas-permeable membrane

A continuous flow pilot reactor was built and run to test nutrient recovery from mesophilic digester reject water of a large wastewater treatment plant (WWTP). Nitrogen as ammonia gas was recovered in acid using gas permeable hydrophobic membrane. Hydraulic retention time had a linear effect on the recovery efficiency up to 60 % and both acid recirculation rate and pH had a linear effect on the efficiency. The pilot reactor was used as a basis for process cost calculation and the costs were compared to those of conventional nitrogen removal using nitrification-denitrification process. For the WWTP this recovery process was economically feasible if the recovery efficiency is between 50 % and 85 %. Membrane reactors are susceptible to clogging and require efficient pretreatment. The suspended solids in the reject water were characterised with different methods to find a suitable pretreatment. Centrifugation, settling and filtering after increasing pH with two different alkali, were tested. Generally, increasing pH improved solid removal already at pH 10. The emphasis of process optimization was end-product quality and resource optimization. The use of lime products could potentially combine several aspects in the process - increase the pH, improve solid separation and add value to the end-products.