Chemically enhanced primary sedimentation and acidogenic sludge fermentation for improved nutrient removal and resource recovery from municipal wastewater

Abstract

In the age of environmental conservation and sustainable development, municipal wastewater has been regarded as a source of water, energy, fertilizer nutrients and other valuable materials, rather than a stream of waste flow. In this study, a new chemical-biological process has been developed to achieve effective energy-saving and resource recovery in municipal wastewater treatment. The new system utilizes iron and aluminum-based chemically enhanced primary sedimentation (CEPS) to concentrate organics and P into the sludge and hence reduce the pollutants load on the downstream treatment process. A side-steam module for acidogenic sludge fermentation was applied to convert wastes in CEPS sludge to valuable resources, including volatile fatty acids (VFAs) and nutrient fertilizers. After nutrients extraction, the VFAs-rich sludge liquor was then used for denitrification in wastewater treatment. The influencing factors, including the coagulant types, dosage and the pH condition were investigated and optimized for CEPS treatment and sludge fermentation. The experimental results show that the efficiency of polyaluminum chloride (PACl, at 16 mg-Al/L) was about 20% higher than that of ferric chloride (FeCl3, at 20 mg-Fe/L) in terms of the removal of suspended pollutants, whereas FeCl3 was more effective in removing soluble phosphate. The FeCl3 addition had little influence on the organic hydrolysis and acidogenesis of the CEPS sludge, whereas a significant inhibitory effect was observed for the PACl-based sludge. Alkaline pretreatment at pH 10 effectively relieved the inhibition effect of Al coagulant on sludge hydrolysis, attributed to the sludge flocs disintegration, enzyme activity improvement, and enrichment of protein-fermenting bacteria (Treponema and Bacteroides). The VFAs yield of Fe-sludge was improved by 20% when controlling the fermentation pH at 6.0 constantly, but VFAs decreased by 10% at pH 8 condition. An innovative technology of electro-fermentation in a two-chamber bioreactor equipped with cation exchange membrane was developed. With 0.5-1 V applied, enhanced phosphorus dissolution and VFAs accumulation together with iron and ammonium separation were all achieved during the sludge fermentation. The microbial community structures in the fermentation reactors for the Fe- and Al-based sludge were also analyzed using the high-throughput sequencing technique. Utilizing the fermented Fe-sludge liquor to provide the carbon source, the denitrification efficiency was increased to over 99%, instead of 35% for the simple primary sedimentation followed by the oxic/anoxic process without the addition of fermented sludge liquor. Through the electrodialysis treatment at 6 V for 8 hours, 50% of ammonium in the sludge liquor was separated into the catholyte, while 35-43% of phosphate, acetate and propionate were recovered into the anolyte. Mass balance and cost-benefit analysis showed that this innovative chemical-biological treatment system can achieve effective pollutant removal, energy-saving and resource recovery in municipal wastewater treatment, providing an environmentally-friendly process that will transform the current wastewater treatment practice into a resource-mining opportunity.