Study of decay of faecal coliform in aquatic environment

Abstract

Prediction of microbial water quality in surface water bodies is essential to provide an indication of health risks posed by faecal contamination. Due to the great difficulties involved in the detection and enumeration of individualpathogens, indicator organisms are usually used to assess the microbial water quality. Faecal coliform bacteria is widely used as an indicator of microbial water quality in water supply engineering. Faecal coliform enter water sources mainly through discharge of sewage and waste water or urban and rural runoff. Once in water, various factors act to enhance or inhibit their growth. For accurate prediction of microbiological water quality a thorough understanding of the effect of various factors on the survival of faecal coliform is essential. This study was devoted to acquire a better understanding of the decay mechanism of faecal coliform in natural water environment. The decay of faecal coliform was observed under various laboratory conditions and also in actual lake and stream environment. The effect of temperature, pH and nutrient deficiency on the decay of faecal coliform was observed in the laboratory. Decay rate of faecal coliform in Dhanmondi lake water and in the Buriganga river water was determined both in the laboratory and in the actual aquatic environment. Temperature was found to have a major impact on the survival of faecal coliform in . I. aquatic environment. At very low temperatures the bacterial population remained nearly constant. But as the temperature increased above 200e the decay rate started increasing rapidly. At about 500e faecal coliform cannot survive formore than 6 hours. pH of the aquatic environment also significantly affects the decay rate of faecal coliform. However, near neutral condition, the variation of decay rate with change in pH level is very small. But as the deviation becomes larger the change in decay rate also becomes greater. Survivalis more strongly affected by acidic environment than by alkaline environment. Nutrients usually retard the decay of faecal coliform. At a very high nutrient concentration, a small after-growth was observed. However, the nutrient concentration and pH of the natural water bodies rarely reach those levels to significantly affect bacterial decay. Decay rate of faecal coliform in both river water and lake water under laboratory condition is significantly higher than that in sterilised distilled water. Presence of toxic substances and trace metals are responsible for this. Decay rate in actual river environment was 10% higher than that in the laboratory. It indicates that the combined effect of sedimentation, resuspension, biological extraction and irradiance in the ultraviolet range is not very significant on the loss rate of faecal coliform in the actual stream environment.