Sunlight inactivation of faecal coliforms in domestic wastewater

Abstract

A B S T R A C TThe relationship between sunlight effect, algal biomass and faecal coliform inactivation in wastewater pond treatment systems is still not clearly understood. Increased pH and dissolved oxygen concentration in treatment ponds results in an increased destruction of faecal coliforms. Increased algal growth however results in a decreased destruction of faecal coliforms due to light attenuation. Algae also releases variable amounts and types of organic matter at various rates and quantities depending on environmental conditions and this can either aid or retard faecal bacteria destruction. We investigated the effect of algal density on faecal coliform destruction under field conditions in sunlight and darkness and how this can be affected by light intensity. In darkness, increased inactivation of faecal coliform occurred with increasing algal density. Rates of decay of faecal coliforms were much faster in sunlight than in darkness even in the absence of algae. In sunlight, rates of decay of faecal coliforms increased with increasing algal density up to a chlorophyll-a concentration of 1.3 ± 0.1 mg/L after which rates of decay decreased. Increased decay rates of faecal coliforms occurred with increasing light intensity or light input. With decreased light input of 20% of 213 W/m 2 , the optimum algal density for maximum faecal coliform decay decreased to a value which is 6-7 times the value of that under normal insolation of 213 W/m 2 . It is recommended that in future studies relating to the assessment of performance and estimation of rate of Escherichia coli or faecal coliform inactivation, one of the parameters that need to be reported as well is the insolation.

Submerged Pond Sand Filter—A Novel Approach to Rural Water Supply

“…The water quality was lower at the surface than at the bottom level, which could indicate that contamination enters the pond from the surface e.g., as road runoff (samples were taken during rainy season), or that mucilage of cyanobacteria in the surface water act as habitat for bacteria [27]. It could be expected that the surface water would be less contaminated due to bacterial inactivation by sun-exposure, as sunlight has been shown to increase decay rates of faecal coliforms in water treatment ponds [28], but this was not the case. It is a general perception that the raw water source used for filtration should be as uncontaminated as possible to ensure the highest water quality [26].…”

Effect of sunlight on three stratifications of waste water from a waste stabilization pond

“…A number of authors have sought to explain the processes that influence pond performance. Some researches [4] focused on the impact of light on pathogenic organism as well as the mechanism of removal, while others [2,[5][6][7] studied the influence of thermal stratification on pond performance.…”

Review on Challenges and Limitations for Algae-Based Wastewater Treatment

“…Such conditions can be formed by extended hydraulic retention time, insufficient mixing and algae sedimentation. In addition, it was observed that elevated algae cell density reduces light attenuation, thus lowering their contribution to coliform neutralization [84]. Also, it is hypothesized that algae are promoting decay of fecal coliforms and producing a neutralizing substance.…”