What works? The influence of changing wastewater treatment type, including tertiary granular activated charcoal, on downstream macroinvertebrate biodiversity over time

This study reviewed the impacts of wastewater on macroinvertebrates over four decades in a UK lowland river. This involved examining changes in chemicals, temperature, flow and macroinvertebrate diversity from the 1970s until 2017 for a wastewater‐dominated river downstream of Swindon in the UK (population about 220,000). When the wastewater treatment process changed from trickling filter to activated sludge in 1991, biological oxygen demand was nearly halved (90%ile 8.1 to 4.6 mg/L), ammonia peaks dropped more than 7‐fold (90%ile 3.9 to 0.53 mg/L) whilst dissolved oxygen climbed consistently above 60% saturation (10%ile went from 49% to 64%) at a sampling point 2 km downstream of the wastewater treatment plant. A sustained increase in the number of macroinvertebrate species was evident from that point. River flow did not change, temperature rose slightly, whilst the major metal concentrations declined steadily over most of the monitoring period. Neither the introduction of phosphate stripping in 1999, nor the use of tertiary granular activated charcoal from 2008 to 2014 had strong positive effects on subsequent macroinvertebrate diversity. That the diversity still had not reached the ideal status by 2016 may be related to the modest habitat quality, agricultural pesticides and the limited recolonization potential in the catchment. The results indicate that urban wastewaters, with their chemical pollutants, are today probably not the biggest threat to the macroinvertebrate diversity of multiple‐stressed lowland rivers in the UK.