Insect life has rebounded in a river receiving all of Swindon's wastewater thanks to substantial investments to improve sewage treatment, according to a DEFRA-funded study.
The River Ray in Wiltshire, which lies downstream of Swindon’s major sewage plant, is largely formed of treated wastewater and had seen steep declines in wildlife populations from the 1960s onwards.
However, EU regulation, in the form of the 1991 Urban Wastewater Directive, led to water company Thames Water adopting the more efficient “activated sludge” method for processing the town’s sewage, which helped to improve the quality of processed water entering the river.
“There was a marked increase in the diversity and abundance of freshwater invertebrates on the River Ray immediately after 1991 and there has been a steady improvement since then,” observed Professor Andrew Johnson of the Centre for Ecology and Hydrology, who led the study.
Since 1991, ammonia levels in the river has dropped more than seven-fold, the study notes, while dissolved oxygen has risen to above 60% saturation. Levels of all metals except iron have fallen since that time, although that is likely to be the result of “reduced industrial and domestic consumption and discharge of metals over time”, the study suggests.
The return of animal families sensitive to water pollution, such as caddisflies, black flies, damselflies and riffle beetles, suggests that the improvement in water quality has led to a recovery in the river habitat since reaching its lowest ecological point in the 1980s.
“If macroinvertebrates are improving in an effluent-dominated river like the Ray, this may go some way to explaining the general improvement across the United Kingdom for macroinvertebrate diversity in urban areas reported from the 1990s,” reads the study.
Additional sewage treatments - phosphate stripping and the use of granular activated charcoal - had little discernible impact on the insect populations in the river. Instead, the study suggests improving habitats and reducing agricultural pollution may be more effective.
A national census carried out by Salmon & Trout Conservation has warned industrial, agricultural and domestic pollution have led to declines of up to 58% in some species of aquatic insects in English rivers.
“Our freshwater fish and invertebrates are being choked by fine sediments which should be on fields, not in rivers, and are subjected to chemical cocktails we don’t understand or try to monitor,” warned Dr Janina Gray, the group’s head of science and environmental policy.