Date of Award
Master of Science (MS)
Environmental Engineering and Earth Science
Rendering plant wastewater is generally characterized by high ammonia concentration owing to the usage of Quaternary Ammonium Compounds (QACs) as disinfectants in the plant. The high ammonia concentration needs to be treated and converted into harmless nitrate before being discharged into water bodies. Nitrification is a standard process carried out to facilitate ammonia removal, but the high QAC concentrations in these wastewaters are known to inhibit the nitrification process. The two-staged nitrification process is carried out by ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB), collectively called nitrifiers. The most commonly found AOB and NOB in wastewater systems are Nitrosomonas and Nitrobacter respectively, which are pH sensitive and function effectively within the optimum range around neutral pH. To assess the inhibitory effect of QAC on nitrification, a mixed nitrifying culture was developed. The enriched microbes were used to perform two short-term batch assays for 4 to 5 days. Various concentrations of benzalkonium chloride (BAC), a type of QAC commonly used in industries as a disinfectant, were mixed with the enriched microbes, and the nitrification process was continuously monitored by measuring the ammonia, nitrite and nitrate levels. The results of both the short-term batch assays indicated that even 5 mg BAC/L had a significant inhibitory effect on nitrification, whereas the 20 mg BAC/L dosed sample exhibited almost complete inhibition. It was hypothesized that long-term exposure to QACs would cause the microbial communities to adapt to the inhibitory conditions thereby lowering the resistance to the nitrifying process. Furthermore, adjusting the dissolved oxygen concentration or the pH conditions can alter the microbial communities, which would be an added advantage to overcome the inhibition. To substantiate this, a long-term assay was performed with two semi-continuous reactors, a control and an assay reactor, at an SRT and HRT of 15 days and an optimal pH range of 5.5 + 0.2. BAC concentrations at 5, 10 and 20 mg/L were dosed in the assay reactor and each concentration was maintained in the reactor for 60 days (4 SRTs). DNA was extracted from the reactors at every stage and the microbial communities were analyzed along with pH, ammonia, nitrite, and nitrate levels. The results showed that the microbial communities had indeed adapted to the changed conditions in the reactor and the lowered pH had modified the microbial community, thereby making Nitrosospira and Nitrospira the dominant AOB and NOB, respectively. Even though the BAC laden system was not 100% effective in nitrification, it showed more than 90% effectiveness of the nitrification process, thus achieving ammonia removal. Hence it can be concluded that modifying the microbial community at a lower pH combined with long-term exposure to QACs can help overcome the inhibitory effect of QACs on nitrification systems.
Rama Thulasiram, Mrinal, "Evaluation of the Effect of Benzalkonium Chloride on Nitrification for Application in Rendering Wastewater" (2022). All Theses. 3965.