Investigating the relationship between nitrification and pathogen reduction in biological wastewater treatment systems
This research was conducted to assess the potential of biological wastewater treatment systems in contributing to disinfection of municipal wastewater. The research focused on the microbial mediated processes occurring at secondary treatment and their effect on faecal coliform and E. coli numbers. The activated sludge system which is the most common biological treatment system was the case study. Focus was on the concurrent occurrence of nitrification and pathogen reduction processes at secondary treatment stage during the treatment process of municipal wastewater in which nitrogen and pathogens are already important pollutants of concern of domestic source. Specifically, the research was seeking to establish any links between both contaminants or the effects they could have on each other by assessing physico-chemical and microbial parameters. As such, municipal wastewater was extensively aerated in lab-scale bioreactors for nitrification at secondary treatment and during this period enumeration of faecal coliforms and E. coli was carried out. Nitrification was observed between 4 to 11 days during which time reduction of faecal coliforms quantities was observed as well. The reduction was initially observed to be because of carbonaceous matter reduction and later by the oxidation of inorganic nitrogen which are both processes involved in wastewater nitrifying systems. Reduction of average FC numbers from 6.5 log to 4.5 log was observed during organic carbon reduction and thereafter further down to 2 log average when nitrogenous oxidation occurred. These results therefore indicated that nitrogenous oxidation was significantly negatively correlated to faecal coliform numbers (-0.907, p=0.013<0.05) in the thereby implying that the presence of the process of nitrification could lead to reduction of faecal coliform numbers in the system. Also, API 20E identification of persistent FC assessed and final stages of treatment process, revealed the presence of E. coli and FC including Salmonella, Klebsiella, Enterobacter, Pseudomonas, Citrobacter and Chromo spp. Some of which are important as pathogens as well as relevant for both biological processes. In another experiment investigating the effects of nitrogenous oxides on faecal coliform quantities in municipal wastewater extensively aerated in the lab-scale bioreactors, it was observed that faecal coliform quantities reduced. However, predation by protozoa was already known to be a cause of depletion of bacteria in aquatic system and therefore to confirm the observation above the effect of predation on the system was assessed. This was carried out by assessing the effects of the inhibition of protozoa activity in the system by the addition of cycloheximide, a protozoa inhibitor into the same system. Results revealed that the inhibition of protozoa resulted in an increase in the number of faecal coliforms enumerated thereby indicating that protozoan activity was a contributing factor in the reduction of faecal coliforms in this system. Further investigations revealed that the effect of protozoa on faecal coliform reduction at time of carbonaceous removal while that of nitrification was evident thereafter. It was therefore concluded that the occurrence of nitrification in and extended aerated biological treatment system, would result in the reduction of faecal coliforms. However, further research would be required to investigate the extend and effect of this method of disinfection on other processes in the system.
Nitrification pathogens, Protozoan predation, Carbonaceous oxidation, Nitrogenous oxidation, Wastewater microorganism, Wastewater treatment
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