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THE FATE AND EFFECTS OF DIQUAT DIBROMIDE IN BIOLOGICAL WASTEWATER TREATMENT PLANTS AFTER ITS UTILIZATION AS A ROOT KILLING AGENT IN SEWERS

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The fate and effects of diquat dibromide in the influent wastewater to activated sludge wastewater treatment systems was investigated and evaluated. Diquat dibromide is the active ingredient in formulations used to control the growth of roots into sewers. This practice can result in calculated diquat concentrations ranging from 0.67 mg/L for a flow of 10 mgd up to 6.7 mg/L for a flow of 1 mgd, given that the amount of formulation applied in a workday would be four gallons, which is typical. The sorption of diquat by both raw sewage particles and activated sludge suspended solids was determined. Diquat concentrations that ranged from 0.93 to 12.6 mg/L in the influent flow to two continuous flow pilot plant activated sludge systems were used for investigative purposes. One of the systems was a full biological nutrient removal (BNR), VIP process configuration system, and the other was a conventional fully aerobic system. Both systems were fed municipal sewage spiked with diquat, and operated at a mixed liquor temperature of 10 °C and an MCRT of 10 days. A control BNR system also was maintained.

The results show that only about 20% of the diquat in raw sewage flow will be removed by adsorption to the sewage solids, but 80% or more of the diquat will be removed in activated sludge systems. When the influent diquat concentration was approximately 1 mg/L, over 99.3% of the diquat was removed by the activated sludge process. Some of the removal was believed to be by biodegradation. The concentration reduction by activated sludge was 83.3 and 91.7 % for influent diquat concentrations of 10 and 12.6 mg/L, respectively.

The diquat used had no observable detrimental effects on any of the biological processes of the continuous flow fully aerobic and VIP BNR activated sludge systems. However, an influent diquat concentration of 12.6 mg/L to the fully aerobic system clearly increased the oxygen uptake rate by as much as 19%, and the total oxygen consumed over a four hour period by 9.3%. This increase was much more than could be explained by the additional COD of the added diquat, and it is clear that the presence of diquat stimulates the OUR of activated sludge. It presence also increased denitrification during the experiments, probably because the increased OUR reduced the DO concentrations in the flocs. A 10 mg/L diquat concentration slightly reduced the nitrifier growth rate in a batch test determination when compared to a control, but the rate measured, 0.764 d−1, is an excellent rate and is higher than the 0.65 d−1 value that is typically used for design.

The observed effluent diquat concentrations from the continuous flow pilot plant systems either had no or very mild toxicity for Ceriodaphnia dubia (water fleas) during acute toxicity testing, and over 80% reproduction continued to occur. This included effluent concentration samples when the influent diquat concentration was 12.6 mg/L. It was not possible to calculate an LC50 for diquat because of the low toxicity, however, it was noted that diquat toxicity increases as the dissolved solids concentration decreases, for conductivity measurements of 250 units or less. The typical effluent conductivities were 400 to 600 units, and toxicity was very low.
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Document Type: Research Article

Publication date: 2001-01-01

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