Canal wall brushing—a control measure for taste and odour problems in drinking water supplies in arid environments

Canal wall brushing, accomplished by a tractor-mounted custom-designed rotating metal brush, was an effective means of removing nuisance periphytic cyanobacterial growth and consequently reducing MIB and geosmin production in the Arizona Canal, a major water conveying open channel in the metropolitan Phoenix (Arizona) water supply system. On average, c. 80% of the periphyton biomass was removed from the canal walls, resulting in immediate reduction in MIB and geosmin concentrations. Recolonization of periphytic cyanobacteria and other microalgae on the canal walls occurred following brushing, and algal biomass (chlorophyll a concentration) reached pre-brushing levels within 2 weeks. However, the production of MIB and geosmin was significantly reduced in the brushed section of the canal during this period of time. The extended duration of the effectiveness of brushing therefore did not appear to be due to the reduced total periphytic biomass, but rather the influence on species composition and population density of MIB and geosmin producers. Thus, slow recovery of MIBand geosmin-producing cyanobacterial populations probably accounts for the reduced MIB and geosmin production. The brushing technique may be particularly applicable to open concrete-lined canal water supply systems and fish culture impoundments that contain point sources of periphyton-associated MIB and geosmin production.