Wastewater Essay

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Wastewate r 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.

In many older plants, fixed film processes are used. For example, wastewater is sprayed into the air (a process called aeration) and allowed to trickle down through coarse media, such as beds of stones or plastic. Microorganisms, attached to and growing on the media, break down organic material as wastewater seeps past. These secondary systems provide higher removal rates for BOD (biological oxygen demand: an indicator of pollutant quality) and are better able to cope with quantity variability, but require large tracts of land and are often rejected by nearby communities for aesthetic and political reasons.

Tertiary treatment is the polishing stage of wastewater treatment. In response to successful litigation by environmental groups and higher regulatory standards, many wastewater facilities increasingly employ advanced tertiary methods to improve effluent quality. Tertiary treatment includes a broad range of methods, such as physical, biological, or chemical processes to remove nitrogen and phosphorus, carbon adsorption to remove chemicals, and disinfection using chlorine, ozone, or ultraviolet light.

Tertiary treatment is needed to produce reclaimed water: Highly treated and recycled wastewater commonly used for nonpotable and nonagricultural uses (such as the irrigation of parks and public spaces). However, due to increasing population, water consumption patterns, and demand for new supplies, many areas are now considering broader uses for reclaimed water. For instance, water providers in Orange County, California, use reclaimed water for indirect potable recharge: The method of blending reclaimed water with other drinking sources through groundwater recharge or reservoir augmentation. For better or for worse, the debates over reclaimed water use in the municipal sector have focused attention on wastewater treatment plants as key links between water quality and quantity.

Despite advances in engineering and treatment, problems associated with wastewater pollution, management, and disposal continue to plague communities and environments worldwide. Approximately 2.6 billion people lack access to improved sanitation: A broad category that includes ventilated pit latrines, composting toilets, and toilets connected to septic tanks or piped sewers. Although global sanitation coverage rose from 49 percent in 1990 to 58 percent in 2002, access to potable water supply still outstrips sanitation access. The United Nations (UN) Millennium Development Goals aim to halve the proportion of people without access to basic sanitation by 2015; yet the UN estimates that if the 1990-2002 sanitation trend continues, roughly 2.4 billion people will be without improved sanitation in 2015, almost as many as are without today.

Developed nations have not escaped the problems of wastewater either. Many European and North American countries feature excellent rates of sanitation access, well-established institutions, and strong regulatory mechanisms; yet, non-pointsource pollution, high rates of water consumption, and excessive waste-generating practices have contributed to wastewater problems in many major cities. For example, on a daily basis, California sends billions of gallons of partially treated sewage into the Pacific Ocean. The sewage usually meets state and federal effluent standards, but increased nutrient loading, urban runoff, and wastewater discharges have caused massive algae blooms, turning coastal waters into toxic soup for marine mammals, fisheries, and recreational users.

Bibliography:

  1. Robert L. Droste, Theory and Practice of Water and Wastewater Treatment (J. Wiley, 1997);
  2. George T chobanoglous, Franklin L. Burton, and David Stensel, Wastewater Engineering: Treatment and Reuse (McGraw-Hill, 2003);
  3. United Nations (UN) World Wide Water Assessment Program (WWAP), Water, A Shared Responsibility: The United Nations World Water Development Report 2 (UN Educational, Scientific, and Cultural Organization and Berghahn Books, 2006);
  4. Kenneth R. Weiss, “Sentinels Under Attack,” Los Angeles Times (July 31, 2006).

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