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How can tertiary treatment remove more than 99 percent of impurities from sewage?
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Tertiary treatmentāalso known as advanced treatmentāis employed when environmental conditions or regulatory standards require the removal of more than 85 percent of suspended solids and biochemical oxygen demand (BOD), or the reduction of dissolved nutrients like nitrates and phosphates. By utilizing these specialized processes, tertiary treatment can remove more than 99 percent of all impurities, resulting in effluent of nearly drinking-water quality.1
Methods to achieve this high level of purification include:
- Nutrient Removal: Chemical precipitation is used to remove phosphorus, while nitrification-denitrification or ammonia stripping removes nitrogen compounds.2
- Effluent Polishing: Granular media filters or microstrainers are used to remove additional suspended solids and BOD from secondary effluent.3
- Disinfection: As a final step, disinfection using chlorine or ultraviolet radiation destroys remaining pathogens to protect public health.1
While highly effective, these processes are significantly more expensive than secondary treatment and are typically reserved for special circumstances where the receiving water is highly vulnerable to pollution.41