Investigation of high-rate algal ponds for the treatment of domestic sewage with low carbon-to-nitrogen ratio

Sewage with a low carbon-to-nitrogen (C/N) ratio can be effectively treated using high-rate algal ponds (HRAPs), consisting of a combined symbiotic system of algae and bacteria. However, HRAPs have been predominantly used in laboratories, but their application in wastewater treatment plants is yet to be realized. Herein, laboratory and pilot-scale experimental studies were conducted using wetland effluent from the Baitabao estuary and effluent from the ecological wastewater treatment plant in Liaozhong County. The ammonia nitrogen (NH4+–N) and chemical oxygen demand (COD) concentrations in the effluent treated using the lab-scale system decreased to <4 and <45 mg L−1, respectively while those in the effluent treated in the pilot-scale test reduced to 1.51 and 9.15 mg L−1, respectively. The primary bacteria in HRAPs were Pseudomonas sp., Massilia sp., Kocuria sp., Bacillus cereus group, and Exiguobacterium acetylicum group, and the dominant algae were Chlorella. Results confirm that HRAPs can effectively treat domestic sewage with low-C/N ratios while meeting China's Pollutant Emission from Urban Sewage Treatment Plants (GB 18918-2002) Level 1A standard. Studies on growth and degradation kinetics reported that (1) the specific proliferation rate of the algal–bacterial mixed system stemmed from the joint action of the specific proliferation rate of single bacteria and Chlorella, (2) there was no mutual inhibition between Chlorella and bacteria in the mixed system, (3) bacteria were responsible for a greater proportion of COD removal, and (4) Chlorella removed NH4+–N primarily via adsorption, absorption, and transformation. This study demonstrates the promising potential of HRAPs for practical implementation.