The impact of sunlight on inactivation of indicator microorganisms both in river water and benthic biofilms

A detailed knowledge on decay or inactivation kinetics of faecal indicator microorganism in rivers is essential for control of bathing water quality. Both reliable inactivation rate coefficients for such kinetics and the knowledge on pathogen accumulation in benthic biofilms are needed for the assessment of river self purification capability and for being able to make decisions for an optimized water management. Therefore, the inactivation kinetics for main indicator microorganisms like faecal coliforms and intestinal Enterococci in water and on stones of the river Isar (Germany) were measured at artificial sunlight radiation. A flume was built, containing Isar water and sediment. By adding a pulse of pre-treated sewage water, the subsequently change of indicator bacteria levels in the bulk and biofilm phase is measured. Bacterial inactivation was mainly dependent on sunlight intensity. Mean inactivation rate coefficients in the experimental set up with river water obtained for a radiation intensity comparable to average midday sunlight in June in 50° north latitude (I(290–390 nm) = 40.0 W/m2) were found to be 21.4 d−1 for faecal coliforms and 20.0 d−1 for intestinal Enterococci. For a radiation intensity conform with the annual mean radiation in Germany (I(290–390 nm) = 8.0 W/m2) inactivation rate coefficients were 12.7 d−1 for faecal coliforms and 9.3 d−1 for intestinal Enterococci. A measurement without direct artificial sunlight (I(290–390 nm) = 0.08 W/m2) yields inactivation rate coefficients of 3.4 d−1 for faecal coliforms and 1.7 d−1 for intestinal Enterococci, which were similar to those found in wastewater treatment ponds. Due to re-growth and better environmental conditions the concentrations of faecal coliforms and intestinal Enterococci within the biofilm were 102 to 104 fold higher compared to the bulk water depending on the exposure time.