Technologies for Recovering Nutrients from Wastewater: A Critical Review

Phosphorus (P) is a non-renewable resource, production of nitrogen (N) fertilizer is energy intensive, and discharge of these nutrients in treated wastewater causes environmental eutrophication. Hence, recovery of nutrients from municipal wastewater has attracted attention. In this article, current technologies for such recovery are reviewed, with synthesis in terms of wastewater characteristics, recovery goals, effluent discharge limits, constraints on chemical usage, treatment plant scale, operational complexity and applicability, and analysis of energy demands. Phosphorus recovery processes applicable for centralized plants include enhanced biological phosphorus removal (EBPR) combined with chemical and electrochemical struvite precipitation and chemical precipitation alone, whereas electrochemical and chemical precipitation and ion exchange (IE) may be adapted to onsite and packaged treatment plants. Many processes can be used for N concentration; however, N recovery has been reported only by struvite precipitation and acid absorption following separation by gas stripping or gas permeable membrane. Only chemical and electrochemical precipitation can produce fertilizer requiring minimal post-processing beyond filtration. Electrochemical precipitation of struvite and calcium phosphate is further capable of such recovery with minimal chemical addition. Direct microbiological recovery as protein is an emerging technology, and algal recovery is being developed for livestock and fuel production. Although reactive filtration can achieve very low P discharge concentrations, the only processes reported to be capable individually of removing P in secondary effluent to below 10 μg/L, for example, for discharge to surficial waters, were adsorption and IE. Several authors point to EBPR as a currently preferred approach, and further development of electrochemical processes appears warranted.