Sewage Treatment

Pollutants

Sewage treatment is focused on reducing in-wastewater discharges, so-called conventional pollutants: oil, grease, organics like nitrogen and phosphorous, total suspended solids, and settleable matter. Most chemical removal is incidental and is dictated by volatility, solubility, and hydrophobic properties, rather than by treatment processes.

Elements commonly found in wastewater, such as nitrogen and phosphates, reduce the available oxygen in water and become organic pollutants in the receiving waters. Organic pollutants can profoundly alter aquatic ecosystems to the point of eventually making them unable to support aquatic life.

Measurement

The principal metric used to measure the quality of treated wastewater is its biological oxygen demand (BOD). BOD measures the oxygen used by microorganisms to decompose organic waste. The higher the BOD, the more polluted the water and—when measured in wastewater—the less effective the wastewater treatment.

Since BOD can be uniformly measured regardless of the heterogeneous and complex nature of wastewater, it has been the preferred benchmark for wastewater quality. Inorganic pollutants, such as synthetic organic chemicals and other chemicals of concern, are much more difficult to measure and their impact on aquatic and terrestrial life is more difficult to gauge.

Regulation

The Environmental Protection Agency (EPA) regulates toxic chemicals in sewage if three conditions are met: (1) the pollutant is present in high amounts, (2) technology for its control is available, and (3) the implementation of that technology is economically feasible.

Pollution control is implemented by end-of-pipe limitations, meaning by effluent limitations on specific constituents in the discharge. These are based on current available technologies (what the treatment plant is actually capable of doing).

To address indirect discharges from industries to WWTPs, the EPA established the National Pretreatment Program as a component of the National Pollutant Discharge Elimination System (NPDES) Permit Program. The Pretreatment Program is plagued with permit backlogs, gaps in program coverage, huge amounts of nonregulated pollutants, lack of monitoring, inadequacy of regulatory compliance, lack of WWTP incentives for implementation and enforcement, and the fundamental fact that it is cheaper to dump hazardous wastes in the publicly owned and financed sewer than to properly dispose of them.

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