Research On Soil Infiltration Bioreactor Incorporated With Sulfur/Sulfide-based Autotrophic Denitrification For Domestic Wastewater Treatment

Accumulation of nitrogen and phosphorus in natural water body, which was caused by uncontrolled discharging of domestic wastewater, would eventually lead to the occurrence of eutrophication, deteriorating water quality, damaging ecological balance and putting human health at risk. The majority of domestic wastewater from cities or towns could be purified in wastewater treatment plants, however, most of them were barely built in rural areas due to their cost, high energy consumption and large land occupation, especially for a developing country like China, which resulted in that a great amount of dispersive point-source of wastewater was directly poured into natural water body without any treatment. Therefore, it was essential to develop an effective and convenient on-site treatment for domestic wastewater in rural areas. By combing conventional soil treatment and bioreactor process, a novel series of soil infiltration bioreactor with sulfur/pyrite-based autotrophic denitrification(SIBSAD/SIBPAD) was newly designed and developed for for treating domestic wastewater, and the detailed research results of which were above:In terms of the adsorption batch tests for quartz sands, zeolites, activated carbon and volcanic rocks, the results indicated that activated carbon owned the best adsorption ability for COD, while zeolites were capable of adsorbing ammonium and nitrate the most, and volcanic rocks showed best adsorption performance on TP. Besides, dynamic adsorption column tests for synthetic wastewater demonstrated that the mixed-packing material had a best adsorption efficiency of ammonium, which reached 53.46% and the maximum saturation adsorption time was over 9 h.Those natural porous infiltrating materials which owns excellent adsorption ability and water permeability served as packing structure and reaction bed. The top aerobic stage(AES) was artificially aerated in order to remove organic matter by respiration and anabolism as well as converting ammonium into nitrate by nitrification, while the bottom anaerobic stage(ANS) took advantage ofsulfur/pyrite-based autotrophic denitrification reducing nitrate to nitrogen gas. Besides, SIBPAD was capable of removing phosphorus and entrapping suspended solid. Conducting by synthetic wastewater during a 120-day experimental period(including 14-day of real wastewater), SIBSAD exhibited a good removal performance showing 86.32% for COD, 92.56% for ammonium, 77.40% for TP and 89.74% of nitrate was removed in ANS. While SIBPAD demonstrated 87.14% removal efficiency for COD, 89.65% for ammonium, 82.58% for TP and 80.72% of nitrate was removed in ANS. SIBSAD had better nitrate removal than SIBPAD, however the p H value of final effluent kept decreasing to 3.47 and the highest sulfate concentration was up to 145.46 mg L-1. SIBPAD showed better TP removal than SIBSAD, moreover, the p H value of final effluent stayed over 7 and the highest sulfate concentration was only 63.22 mg L-1.In addition, according to the microbial community diversity analysis of ANSs for two bioreactors, we discovered: denitrification in SIBSAD was contributed by Thiobacillus, and there were many species of acid-producing sulfur-oxidizing bacteria in ANS of SIBSAD as well, such as Sulfobacillus, Acidiphilium, Thiomonas and Acidithiobacillus, which was the reason of poor diversity in SIBPAD, and also explained the reason why the effluent p H kept decreasing and sulfate was excessively produced. However, denitrification in SIBPAD was not only contributed by Thiobacillus, but also much more contributed by two species of heterotrophic denitrifying bacteria Thauera and Halomonas, which indicated that denitrification in SIBPAD turned out to be a mixotrophic process combing autotrophic and heterotrophic ones. This mixotrophic process provided stable and alkalescence effluent and low sulfate production, enriching the microbial diversity in ANS of SIBPAD.The series of SIBSAD/SIBPAD was proved to be an innovative wastewater treatment with practicability and feasibility, research of which give us an alternative and experimental experience in the studying area of on-site wastewater treatment.