Chemical Flocculation And Oxidation

Since pesticides are used mainly in unsewered agricultural areas, they reach lakes and streams without passing through treatment facilities. Consequently, ease of removal in conventional water supply treatment processes (when water is withdrawn for processing to produce potable water) is important. A study used pilot water supply treatment plants to evaluate conventional and auxilliary treatment process effectiveness in removing pesticides from natural surface water (Robeck, Dostal, Cohen and Kreiss 1965). The results showed that each part of the water treatment plant had some potential for reducing certain pesticides. The effectiveness of the standard process of coagulation and filtration is shown in Table 8.2.6. Removals ranged from 98% for DDT to less than 10% for lindane. The only pesticide affected significantly by the application of chlorine or potassium permanganate (1-5 mg/l) was parathion, 75% of which was oxidized to paroxon, a more toxic material. At high dosages, ozone (10-38 mg/l) reduced chlorinated hydrocarbons; by-products of unknown toxicity were formed.

In full-scale evaluations (Nicholson, Grzenda and Teasley 1968), the standard processing steps of coagulation, settling, rapid sand filtration, and chlorination were successful in reducing DDT and DDE levels but not toxaphene and lindane levels. Side tests with a 25-p, filter removed DDT and DDE more effectively than toxaphene and lindane, indicating that the latter materials were transported in solution.

Chemical degradability of frequently used chlorinated hydrocarbon insecticides has also been investigated (Leigh 1969). Lindane and endrin were not removed by either chlorine or potassium permanganate at oxidant dosages ranging from 48 to 61 mg/l, contact times of 48 hr and a wide range of pH values. Heptachlor was removed by KMnO4 to the extent of 88% with only slight variation due to pH adjustment. Heptachlor and DDT were both partially removed by chlorine, and DDT was partially removed by KMnO4 with slightly higher removals at lower pH levels. Maximum removals by potassium persulfate, attained only for lindane and DDT, were 9.4% and 18.5%, respectively, at higher pH values.

Several physical and chemical treatments for removing the herbicide 2,4-D and its ester derivatives from natural waters have also been investigated (Aly and Faust 1965). Chemical coagulation of 1 mg/l solutions by 100 mg/l aluminum sulfate showed no promise with the herbicides and derivatives studied. Activated carbon studies indicated carbon requirements for reducing 2,4-D concentrations from 1 to 0.1 mg/l were 31 mg/l for sodium salt, 14 mg/l for isopropyl ester, 15 mg/l for butyl ester and 16 mg/l for isooctyl ester. Potassium permanganate dosed at 3 mg/l did not oxidize 1 mg/l of these same compounds. However, 0.98 mg/l of 2,4-DCP was completely oxidized by 1.25 mg/l KMnO4 in 15 min. Ion exchange studies indicated that strongly basic anion-exchange resins more effectively removed the compounds studied than cation exchange resins.

Strong oxidants to degrade chlorinated hydrocarbon pesticides (Buescher, Dougherty and Skrinde 1964) have also been studied. Preliminary studies with lindane and aldrin showed negligible removals with hydrogen peroxide and sodium peroxide at 40 mg/l dosages and four-hr contact times. Chlorination had negligible effects on lin-dane, but completely oxidized aldrin, while potassium permanganate (KMnO4) oxidized lindane to approximately 12% and aldrin, fully. Further studies of potassium permanganate added in varying doses from 6 to 40 mg/l to lindane solution indicated that the excessive time and ox-idant dosages required for removals greater than 40% made this treatment unfeasible. Complete removal for aldrin could be attained in 15 min at 1 mg/l dosage of KMnO4.

Due to the relatively small fraction of ozone in the air stream used for ozonation, pesticide removals from air stripping were measured, as well as removals from oxidation. Up to 75% of lindane was removed by ozonation, whereas aeration alone had no measurable effect. Dieldrin and aldrin were completely removed almost at once, but aeration studies also showed fairly rapid removals.

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