A Study on the Behavior Evaluation and Control of Midge Larvae in the Filtration Process

Abstract

Recently, midge larvae, which are indicators of aquatic environmental pollution, have been detected in tap water in some areas, resulting in controversy. Therefore, the objective of this study is to analyze the behavioural features of midge larvae in sand, GAC (granular activated carbon) and GAC/sand processes, which are mainly operated in domestic water purification plants, and suggest effective operating methods for larval control. In this study, a pilot-scale square column (W300×L300×H3500~5000 mm) was used for the filtration experiment and was used with a filter height of 2.5 m as well as in the field. The water to be treated was the sand filtered from the water purification plant G, and midge larvae from the Han River were injected periodically and operated for an estimated 40 to 50 days. As a result, no larvae were detected during the first five operational days in the sand filter and GAC and 14 days in the case of GAC/sand. However, after backwashing, larvae were detected from the 9th day of operation for sand and GAC, and from the 15th day for GAC/sand. In the case of sand laying under the GAC, larvae detection was delayed by approximately 6 days compared to the operation of the GAC alone. Following examination of larval distribution in relation to the height of the activated carbon layer after approximately 50 days of operation, it was found that the larvae were distributed on 17% in the upper layer, 7% in the middle layer, 19% in the lower layer and 0.2% in the lower water collection device. Therefore, it is necessary to shorten the backwashing cycle from the initial stage of larval introduction and to increase the rate of expansion of GAC during the backwashing to quickly flush out the larvae. The results of this study also confirmed that when the backwash speed was 0.57 m/min or more, the rate of expansion of GAC was 30% or more, and larval rejection increased approximately twice when compared with a washing rate of 0.45 m/min. Consequently, as a method of effective functioning of the filtration process for larval control, in the case of sand filtration, when it is difficult to expand the filter media, it is necessary to increase the rinsing time, and in the case of GAC, it is necessary to quickly discard the larvae by increasing the backwashing rate and shortening the backwashing cycle.