One step further to closed water loop: Reclamation of municipal wastewater to high-grade product water

Abstract

Due to the lack of natural water resources and the rapid growth of population and economy, China is currently facing the pressing challenge of water shortage. Current evidence suggests that the traditional water resources including surface water and groundwater could not be enough to sustain increasing water demand which has been projected to hit 8.80×1011 m3 by 2030. Facing to such a critical situation, it is absolutely necessary to explore non-traditional water resources in order to meet future water demand. As such, municipal wastewater should be regarded as an emerging water resource rather than a waste. Presently various biological processes with activated sludge as the core have been commonly adopted for municipal wastewater treatment in China. However, these conventional activated sludge processes have been suffering from high energy consumption, large amount of excess sludge, low resource recovery efficiency, greenhouse gas emission, etc. On the other hand, the process upgrading is becoming inevitable in order for the effluent to meet increasingly strict national discharge standards. However, it should be pointed out that the current practice of process upgrading is mainly based on a simple add-on-driven philosophy, i.e., the existing biological treatment processes are expanded by including various post-treatment units for polishing soluble COD, nitrogen and phosphorus. Obviously, such an add-on-based practice inevitably leads to extremely complex process configuration, difficulty in process operation, increasing treatment costs, energy consumption and large footprint. In fact, a fundamental question to be addressed is what the primary function of municipal wastewater treatment is in the future. This article thus argues that future wastewater treatment plant should primarily serve as a nontraditional water factory that is able to supply high-quality product water, with the ultimate goal of closing wastewater-water loop. One of such examples is the integrated anaerobic membrane bioreactor and reverse osmosis process that offers a feasible engineering solution for producing high-grade water from municipal wastewater, with substantially reduced energy consumption and excess sludge production. In this integrated process, almost all soluble COD in municipal wastewater could be directly converted to biomethane gas with minimized sludge production, while the majority of soluble nutrients (e.g., ammonium and phosphate) could be effectively rejected by reverse osmosis membrane. However, in some circumstances, the ammonium concentration in reverse osmosis permeate might not meet the standards for high-grade product water when treating municipal wastewater with high ammonium concentration. As such, adsorption could be considered as a promising technology for polishing the effluent from the proposed integrated process, and different kinds of adsorbents for ammonium removal were also reviewed, ranging from conventional zeolite, biochar, ion exchange resin to advanced nanomaterials. It appears that more efforts are needed to develop novel adsorbents with high adsorption capacity and selectivity for ammonium, which are becoming possible with advances of material science and engineering. Consequently, it is demonstrated in this review that municipal wastewater as an important fresh water resource should first be fully collected and then reclaimed to high-grade product water which is a game changer in closing water loop, while helping to prolong water life cycle and alleviating the pressure of urban water shortage.