Wastewater

Wastewater is not just sewage. Defined as domestic, industrial, agricultural, and storm water flows that drain into sewage collection systems, wastewater reflects the geographic character of communities and environments. Sewage, or refuse liquid and waste matter produced by residences and commerce, is often labeled “wastewater;” yet sewage is technically limited to discharge channeled by sewer pipes. Wastewater, however, pulls from a broader array of social and environmental sources: Storm drains, overflowing creeks, septic tank leaks, and runoff from parking lots and pavements, the crop field, and the industrial dump site. Wastewater quality and quantity are thus related to the patterns and politics of water availability, governance, and waste-making practices.

Wastewater composition is approximately 99 percent water by weight, but it contains numerous biological, chemical, and material compounds ranging from pathogenic bacteria to pharmaceutical compounds and trash. In large quantities, these compounds produce adverse effects on human and ecological systems. For example, in municipalities with combined storm drains and sewer infrastructure, storm water mixes with wastewater after severe rainfall events, often resulting in combined sewer overflows. These overflows, in tandem with renegade wastewater flows and increased urban runoff, frequently result in poor water quality. For this reason, many laws and regulations (such as the U.S. Clean Water Act) mandate wastewater treatment to decrease environmental contamination and improve water quality.

Wastewater treatment plants intervene at critical points in the water cycle. Although septic tanks are still common in rural areas, the majority of municipal wastewater is treated in large-scale plants. There are no holidays for wastewater treatment: Most plants operate 24 hours per day, seven days per week. Treatment plants are designed to reduce harmful substances and pollutants in wastewater before flows are returned to rivers, oceans, or the broader environment. In general, there are three stages of wastewater treatment: (1) primary treatment (physical removal of floatable and settleable solids; (2) secondary treatment (biological removal of dissolved solids); and (3) tertiary or advanced treatment (removal of nutrients and chemicals).

Primary treatment extracts solid particulates and oils from wastewater. First, influent is screened to remove large objects, such as rocks, corpses, or condoms, which could plug sewer lines or block tank inlets. Next, flows enter a grit chamber and decrease in velocity, allowing sand and grit to fall out. Macerators (revolving cylinders with rotating knife edges) are sometimes used in place of screens to cut solids into smaller, collectable particles. Finally, wastewater is slowly moved through sedimentation tanks (also called clarifiers or settling tanks). Fecal solids settle out in the tanks and are pumped away, while oils, grease, and plastics float to the surface and are skimmed off.

Secondary treatment typically utilizes aerobic biological processes to further degrade the supernatant (remaining flows after primary treatment) and convert nonsettleables to settleable solids. This level of treatment removes approximately 85 percent of the total suspended solids (TSS) in wastewater and is the minimum level of treatment required by the U.S. Clean Water Act. Secondary treatment is a balance of engineering, siting politics, budgets, and local environmental conditions. Secondary systems are classified either as suspended growth or fixed film, although systems may use elements of both. The most common suspended growth option, activated sludge, uses microorganisms to break down organic material via aeration, agitation, and settling. The sludge, which contains fungi, protozoa, and aerobic bacteria, is continually recirculated through the aeration basins to speed the process of organic decomposition. In general, suspended growth systems require less space, but may not be able to handle shocks in biological loading.

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