Integration of chemically enhanced primary sedimentation and advanced biological nitrogen removal for treating low C/N ratio municipal wastewater

Abstract

Conventional wastewater treatment is an energy-intensive and resource-wasting process, in which the oxidation of organics and ammonium nitrogen consumes energy and external carbon sources are often needed for enhanced biological nitrogen removal. Chemically enhanced primary sedimentation (CEPS) is an effective process for removing organics and phosphorus from wastewater, significantly improving the potential of resource recovery in wastewater treatment. However, CEPS is not effective in nitrogen removal and ammonium remains in the CEPS effluent. In this study, two innovative chemical-biological processes, the CEPS-fermentation-denitrification combination and the CEPS-partial nitritation-anammox (PNA) system, were developed to enhanced nutrient removal in wastewater treatment. In the CEPS-fermentation-denitrification combination, CEPS effectively removed and concentrated over 70% of organics and over 90% of phosphorous in municipal wastewater into sludge, which significantly reduced the organic and phosphorous loads in the following biological treatment. In the side-stream with the anaerobic acidogenesis of the CEPS sludge, nearly 20% of organic carbon in municipal wastewater could be converted to volatile fatty acids (VFAs). The VFAs-rich fermentation liquor of the CEPS sludge provided a suitable organic carbon source for heterotrophic denitrification, with a high denitrification rate (410.3 mg N/g VSS/d) and a low nitrous oxide emission (1.14%). In the CEPS-fermentation-denitrification combination, the overall energy consumption of wastewater treatment can be effectively reduced and organic carbon self-sufficiency can be achieved for enhanced nutrient removals. Anaerobic ammonium oxidation (anammox) was utilized for autotrophic nitrogen removal treating the wastewater with a low C/N ratio. The packed bed biofilm reactor (PBBR), instead of the commonly used moving bed biofilm reactor (MBBR), was developed as an effective means for the formation and growth of anammox biofilms in the reactor. The low hydraulic turbulence and strict anaerobic condition in the PBBR, as well as the control of free ammonia, were conducive to the anammox biofilm formation. Mature anammox biofilm was well developed and the nitrogen removal capacity increased rapidly from 77.6 to 876.8 mg N/L/d in less than 2 months. By using the pre-acclimated anammox biofilms, a new sludge seeding and operating strategy was developed for the rapid start-up of the PNA-integrated fixed-film activated sludge (IFAS) system for autotrophic nitrogen removal. Different from the common inoculation of suspended PNA sludge with bare biocarriers, the seeding of anammox biofilms on biocarriers, together with the suspended PNA sludge, could shorten the start-up time of PNA-IFAS with well-developed biofilms from more than half a year to one month or so. Moreover, compared to the control bioreactor with the conventional seeding and operating approach, the bioreactor seeded with anammox biofilms achieved a higher nitrogen removal capacity (707.3±45.1 vs. 395.1±11.1 mg N/L/d), a lower nitrate yield (9.4±1.1% vs. 19.5±4.6%), and improved stability in long-term operation. Overall, the research findings present novel technologies to achieve energy-efficient and organic carbon self-sufficient wastewater treatment, particularly for the enhanced and more sustainable nutrient removals.