The impact of sunlight on the inactivation of bacteria

A review of Schultz-Fademrecht, C., Wichern, M., Horn, H., (2008), The impact of sunlight on inactivation of indicator microorganism both in river water and benthic biofilms, Water Research, 42(19):4771-4779.

Sewage effluents and agricultural run-off are some of the sources of high pathogen levels in rivers. Despite all sewage treatment plants discharging into the river Isar (Germany) implementing disinfection techniques to maintain water quality levels expected by the EU, faecal bacteria levels still exceed the limits on a few days each year. Establishing specific inactivation rates of indicator microorganisms in the river and thus creating a model predicting decay of indicator bacteria in the river depending on the run-off would help to understand the river’s decomposition capability concerning faecal bacteria. Recent studies have looked at light intensity, temperature and pH and their effect on bacterial inactivation rate, however there are few reliable reaction constants for the decay of faecal coliforms and intestinal Enterococci in rivers. Specific inactivation kinetics of indicator microorganisms is vital for assessing a river’s decomposition capability so that correct water management techniques can be implemented in the bathing areas of river Isar. This study aims to determine the specific inactivation rates of faecal coliforms and Enterococcci, concentrating on the effect of sunlight intensity and biofilm attachment on inactivation and accumulation.

A flume was built containing Isar water and sediment. A pulse of pre-treated sewage water was added and the changes in bacteria levels were measured by samples taken on an hourly basis. Different intensities of artificial sunlight radiation were used to replicate; sunlight intensity at midday sun in June and annual mean radiation in Germany. A further trial with no artificial light was conducted. Turbidity, dissolved oxygen, pH and chemical oxygen demand (COD) were also noted.

They found no difference in turbidity, dissolved oxygen and pH, however COD increased after adding pre-treated sewage and then decreased over time during all trials. They reported inactivation rates between 21.403.4d^-1 for coliforms and 20-1.7d^-1 for Enterococci, finding that cultivable indicator bacteria levels significantly decreased, thus inactivation levels increased, with higher intensities of sunlight radiation, with the half-life of coliforms generally being higher compared to that of Enterococci. Coliforms also increased in concentration 12hours after artificial sunlight was removed, unlike Enterococci, suggesting that coliforms were able to survive for a period of time in benthic biofilms with protection from radiation.

Both coliforms and Enterococci are also faster inactivated in the bulk phase compared to those attached to benthic biofilm and with high light intensities, there was permanent inactivation of cultivable pathogens even without UV radiation in the bulk phase due to the suboptimal conditions, with re-growth and stabilisation of concentrations detected in periods without UV radiation, only in the biofilms where they are partially protected from radiation. The results of this paper give reason to concentrate on faecal bacteria accumulated in biofilms, which are suggested to act as reservoirs, instead of just focusing UV treatment on those present in the water column when considering disinfection techniques.