Assessing the relationship between water quality parameters and toxic cyanobacterial abundance in a hospital wastewater treatment plant

Abstract

Cyanobacteria are a group of prokaryotes that thrive in various aquatic environments under favourable conditions such as high levels of nutrients, optimal water temperature, and light. The presence of cyanobacteria in wastewater treatment plants (WWTPs) is concerning, as certain species can produce toxins, posing significant health and environmental problems. Understanding the conditions that promote the growth of cyanobacteria in WWTPs is crucial for developing strategies to prevent or minimise their proliferation. Therefore, this study assesses cyanobacterial abundance with different water quality parameters at a WWTP in Matara District, Sri Lanka, which treats hospital wastewater generated from both black water and grey water. The aim of the study was to identify both toxic and non-toxic cyanobacteria, quantify their abundance, and analyse the relationship between toxic cyanobacterial abundance and various water quality parameters. Wastewater samples were collected from three sampling sites (Site 1- the sedimentation tank; Site 2 - post-chlorination tank; and Site 3 - the discharge field) in the WWTP for five months. Water temperature, pH, dissolved oxygen, total dissolved solids, salinity, electrical conductivity, total suspended solids, nitrate, and phosphate were analysed using standard protocols. Cyanobacteria cells were collected using a plankton net, identified with identification keys, and enumerated using the Sedgewick-Rafter counting method. Water quality parameters and cyanobacterial abundance were measured in triplicate at each sampling site. The relationship between the abundance of cyanobacterial species and water quality parameters was determined using Pearson’s correlation test in Minitab 19.0. Five toxin-producing cyanobacterial species (Oscillatoria sp., Phormidium sp., Limnothrix sp., Pseudanabaena sp., and Cylindrospermopsis sp.) and one non-toxic species (Spirulina sp.) were identified, with Phormidium sp. as the most abundant. The identified species showed varying correlations with water quality parameters. The abundance of Oscillatoria sp., Phormidium sp., Limnothrix sp., and Pseudanabaena sp. showed a strong positive correlation with total dissolved solids, electrical conductivity, and phosphate, indicating they thrive in nutrient-rich environments with abundant dissolved salts and phosphate. Phormidium sp. also showed a positive correlation with dissolved oxygen and nitrate, while Cylindrospermopsis sp. did not show any significant correlation with any parameter, indicating its growth was influenced by other factors. No cyanobacterial species showed a significant correlation with total suspended solids. Limnothrix sp. abundance correlated positively with water temperature, whereas Oscillatoria sp. and Phormidium sp. showed a negative correlation with water temperature. This study highlights that the WWTP is a potential source of toxic cyanobacteria, with water quality parameters such as water temperature, electrical conductivity, total dissolved solids, and phosphate significantly influencing their growth. Hence, it is crucial to monitor and manage these water quality parameters to control the harmful cyanobacterial proliferation in WWTPs. Future research should include additional water quality parameters to gain a more comprehensive understanding of the factors affecting cyanobacterial growth in WWTPs.