The Role of Algae in Disinfection Processes in Wastewater Stabilization Ponds

Abstract

This thesis is an investigation of pathogen removal in wastewater stabilization ponds (WSPs). WSPs are sustainable alternatives to conventional wastewater treatment, requiring minimal energy input and low costs. They have the capability to effectively attenuate nutrients as well as pathogenic microorganisms. Disinfection in WSPs is a complex process, which can be influenced by a number of mechanisms and environmental factors, such as pH, dissolved oxygen (DO) and temperature. Due to long retention time in these systems, algal activities are often expected during warm seasons. Their presence may have effects on the complex interaction between biological communities and water chemistry. This thesis aimed to assess the role of algae in disinfection in WSP systems.

Two algae (Mougeotia sp. and Hydrodicty sp.) that were collected from the Amherstview WSP were noted to increase both pH and DO when cultivated under laboratory conditions. This finding was corroborated by the multivariate statistical analysis where the correlation relationships between a wide range of water quality parameters (pH, DO, temperature, chlorophyll, phycocyanin, Escherichia coli (E. coli), total coliforms (TC), Enterococci, Clostridium perfringens (C. perfringens), ammonium, nitrate, total nitrogen, phosphate total phosphorus and COD) were investigated. As such, the presence of algae is believed to contribute to the high pH and DO levels in the WSP system.

The inactivation of E. coli and total coliforms were found to increase with increasing pH. Intermediate DO level (8.6 mg/L) appeared to support the survival of both E. coli and total coliforms, while both high and low DO levels (20 mg/L and 1 mg/L, respectively) could facilitate their removal and inactivation. This would suggest that the presence of algae leading to high pH and DO could potentially promote the removal of E. coli, total coliforms and Enterococci. Two additional indicator organisms (Enterococci and C. perfringens) showed different inactivation trends compared to traditional indicator organisms (E. coli, fecal coliforms or TC). C. perfringens were tolerant to the pH, DO and temperature ranges investigated. They exhibited weaker correlations with both E. coli and TC. This finding suggested that Enterococci and C. perfringens can be proposed as potential indicator organisms.