Faecal indicator organism die-off rates in UK rivers

•Objectives: The project aim was to assess the die-off rates of faecal indicator organisms (FIO) within a range of UK rivers, enabling a more accurate assessment of the extent to which wastewater treatment works upstream of bathing areas have the potential to cause harm.
•Approach: Three study rivers were selected that had a single WwTW in their headwaters. These were the River Wye (Buxton WwTW), the Tetbury Avon (Tetbury WwTW) and the River Thame (Aylesbury WwTW.
Growth and decay rates of sewage-derived coliforms were assessed using dialysis microcosms. These clear plastic bags allowed river microbial communities to be isolated, but still exposed to natural light, UV levels, and water temperatures. Dialysis membranes allow dissolved substances, including nutrients, to diffuse into the microcosms from the surrounding river water, thereby avoiding nutrient depletion as the bacterial and algal community grow. The microcosms were used in two types of experiment. Firstly, they were deployed directly in the study rivers, either positioned just below the surface in full sunlight, or in reduced light conditions (by deploying at 1m depth or by covering with shading mesh). A second set of lab experiments were conducted in a temperature-controlled greenhouse to investigate the impact of water temperature on coliform die-off rates. For both the in-river and lab-based experiments, microcosms were sampled at 3h, 6h, 24h, 48h and 72h, and total coliforms and E. coli were quantified by culturing using Petrifilms. Longitudinal surveys were also conducted at 8 sampling sites along each river at the same time as the experiments. Samples were analysed for total coliform, E. coli and total bacterial concentrations and general water quality parameters. Bulk (1L) water samples were also taken for DNA extraction and microbial source tracking analysis, to identify the relative signals from humans, birds and ruminant animals.
•Conclusions: This study clearly showed that sewage-derived coliforms do not usually die-off on entering the river, and often have a growth phase, usually within the first 24 hours, before subsequently decaying. This sudden increase in coliform numbers could be due to the increase in nutrient concentrations associated with the WwTW effluent input, which can help microbes to withstand other environmental stressors. Some of these growth peaks reach high E. coli concentrations (with a maximum of 1900 CFU/ml in the River Thame) and this could increase human health risks downstream of the WwTW.
Coliform growth and die-off rates were also greatly affected by light intensities. Microcosms held in reduced light conditions had longer decay times (T90) than their equivalent microcosms in full light. This shows that E. coli survives longer during dull weather conditions, at lower depths of the water column, or potentially if shaded by bankside vegetation. In some experiments, coliform concentrations were still increasing after 72-h, by which time they could be between 20km to 50km downstream of the WwTW. There was a marked seasonal effect, with decay rates being much faster in the summer experiments, when visible and UV light intensities were highest. Light (and potentially UV) intensity also had an impact on the magnitude of the initial growth phases, with low light treatments having larger peaks in coliform concentration than the equivalent full-light treatment.
Increasing the ambient river temperature by 4oC often resulted in large and rapid increases in coliform concentrations over the first 24-h, followed by a rapid decay phase. Ambient temperatures also usually exhibited a coliform growth phase, but often its peak was smaller and later than the +4oC treatment. The -4oC treatments usually showed no significant growth phase.
The longitudinal river surveys always had the highest total coliform and E. coli concentrations in the upper catchment as the river passed through the towns in the headwaters. However, this wasn’t always associated with the input of the WwTW final effluent. There were clearly other coliform inputs to the river from within the towns themselves. Coliform concentrations decreased downstream of the WwTWs, implying that the coliforms emitted from the WwTWs rapidly decay. This seems to be in contradiction with the light and temperature experiments, which showed that coliforms often go through an initial growth phase overnight. Some of the decrease will be due to dilution by small tributaries and groundwater inputs, but this is not sufficient to explain the rapid reduction along the transect. The downstream decrease in coliform concentration could be due to sewage-derived coliforms quickly attaching to sediment or algae, and depositing on the riverbed, especially during the low flow conditions through spring to autumn 2025. Another important factor is that the surveys took place during daylight hours in full light conditions, and therefore the growth rates would be greatly reduced, compared to the overnight growth seen in the experiments. Molecular analysis suggests that, in comparison to the natural background microbial community, E. coli were in very low abundance. Microbial source tracking highlighted that human source indicators were relatively low, but avian and bovine tracers were at the limits of detection, and therefore unlikely to be a significant source of E. coli in these catchments at the time of sampling.
Recommendations
• Sewage-derived coliforms have the potential to survive for many days in the river environment. To reduce human health risks at river bathing sites, the impacts of more-distant WwTWs in the upstream catchment may need to be considered.
• The wider use of membrane bioreactor technology at WwTW could be a key tool to reduce FIO loads from WwTWs impacting bathing sites with high human health risk.
• In some of the experiments, coliforms were still growing after 72-h. Future experimental work should be carried out for five days, so that all decay rates could be captured. We also advocate sub-daily sampling frequencies, so that the daily dynamics in growth rates could be assessed.
• Options for future work include investigating other pathogens, such as Enterococci, and the impacts of zooplankton grazing of pathogens by rotifers and protozoa.
Benefits
• The observation that sewage-derived coliform concentrations often increase in in river environments will assist more effective planning and decision-making, leading to increased recreational opportunities for the public.
• This research has highlighted membrane bioreactor technology as an extremely effective means to reduce coliform loads entering rivers from WwTWs.
• The large quantities of monitoring and experimental data generated within this project could provide important inputs to develop and refine catchment-scale FIO models.