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Why does primary treatment not remove most dissolved impurities?
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Primary treatment does not remove most dissolved impurities because it relies exclusively on physical processesāsuch as screening, grit removal, and sedimentationādesigned to target materials that float or settle out by gravity.1
These methods are effective at removing bulky debris, sand, and suspended solids, but they are not equipped to address soluble organic matter. Because dissolved impurities do not settle out of the water under the conditions provided in primary clarifiers, they remain in the wastewater stream. The removal of these soluble organic impurities requires secondary treatment, which utilizes biological processes where microbes consume the dissolved matter as food.213