The origin and population dynamics of annually re-occurring Paratanytarsus grimmii (Diptera: Chironomidae) colonising granular activated carbon (GAC) adsorbers used in potable water treatment.

Various sampling techniques were employed to study the population dynamics and identify the origin of annually re-occurring infestations of Paratanytarsus grimmii in granular activated carbon (GAC) adsorbers. Larvae overwintered in all adsorbers studied and are the main source of endemic persistent infestations. Significant differences in larval densities were identified between the down-flow cell (mean of 61 larvae per 0.3 l of GAC) and the up-flow cell (mean of 14 larvae per 0.3 l of GAC) of each adsorber. Larvae were distributed uniformly with no significant difference in density at any depth through the 2-m carbon column. Application of anaerobic treatment as a control measure was ineffective at low temperatures due to a slow down in chironomid metabolism. During summer months, ovipositing females have access to all locations within the GAC adsorber building by flight, leading to immediate re-colonisation of anaerobically-treated adsorbers. Regeneration of GAC in individual cells served only to reduce larval numbers but not remove them completely, particularly when only one of the two cells is regenerated at any one time.