Study of growth rates and hydrogen sulphide production rate of D.Vulgaris and E.coli in pure and mixed cultures under varying conditions to aid in the understanding of odour and corrosion issues in sewer systems

In the maintenance of sewer systems, bacteria are important. In sewer systems, bacteria that break down organic material produce toxic gases like hydrogen sulphide, which has a distinct smell of rotten eggs. For sewer workers and nearby residents, the release of these offensive gases frequently results in unpleasant and potentially dangerous conditions. Additionally, the corrosion of concrete and metal sewer infrastructure caused by sulphate reducing bacteria (SRB's) compromises sewer pipes and infrastructure which often result in expensive repairs or infrastructure failure. Municipalities can ensure effective and sustainable management of sewer systems while preserving the environment and public health by examining and understanding the metabolic processes which take place within sewers to mitigate the problems associated with the interaction of bacteria in the sewer systems.

This thesis examined the impact of varying growth parameters such as temperature, pH and carbon source on the metabolic activity of D.Vulgaris and also the influence of co-culturing D.Vulgaris with a heterotroph like E.coli, representing a metabolically diverse organism.

Results from the pure culture study of D.Vulgaris showed that the highest average sulphide production rate (0.04 h-1) and average D.Vulgaris growth rate (0.01h-1) was observed at pH 6 and temperature of 40 degrees celsius. The data suggests that pH and Temperature were observed to influence the growth rate of the strain although some of the experimental results showed no reproducibility. The carbon source used was found to influence the growth rate as sodium lactate was a preferred carbon source to sodium acetate. These findings were compared to typical sewer environments and it was suggested that high temperatures often occur due to microbial activity, exothermic reactions and heat dissipation from industrial processes. This implied that one of the way of controlling the concentration of sulphide gas emissions in sewer systems could be monitoring the microbial processes which occur in the system for example through microbial sampling, microscopic analysis and online monitoring systems which will provide valuable insight into the microbial processes occurring within the system.

The co-culture studies suggested that the inherent complexity of microbial interactions and the dynamic nature of co-cultures can lead to variations in outcomes. The experiments were not reproducible and showed no clear trend or type of relationship existing between both species. However a key trend from this study was the correlation between increase in acetate concentrations and decrease in sulphate reduction observed in some of the experiments. Although not conclusive, the results suggested a competitive relationship could potentially exist between both species given that they are capable of utilising sodium acetate as a carbon substrate under anaerobic conditions.

Understanding these relationships becomes relevant when managing complex microbial processes in sewers systems, optimising wastewater treatment, and mitigating potential problems like the production of harmful by-products.