COMMUNITY WASTEWATER TREATMENT AND DISPOSAL

Wastewater Removal

A community wastewater treatment and disposal system, a net work of pipes that transport wastewater to treatment plants where it’s treated and released to the environment, serves most buildings in the United States. At a typical wastewater treatment plant, several million gallons of wastewater are treated each day, about 50 to 100 gallons for every person using the system.

In these systems, wastewater is carried from the building to the community treatment facility through below-ground piping systems that are generally classified according to the type of wastewater flowing through them. Sanitary sewage is typically separated from storm water (from rain and snowmelt) by a separate piping arrangement. In less modern systems, known as combined systems, the system carries both sanitary water and storm water.

The building drain in a building sanitary drainage system is connected to a building sewer about 2 to 5 ft (0.6 to 1.5 m) outside of the building. The building sewer connects the building sanitary drainage system to the community sanitary sewer main located several feet below the surface of a street or alley.

Sewer lines must be sloped to permit gravity flow of waste water at a velocity of at least 1.5 ft/s (0.46 m/s), because at lower velocities the solid waste tends to settle in the pipe.

Storm water mains are similar in design to sanitary sewers except that they have a much larger diameter. Certain types of sewers, such as inverted siphons and pipes from pumping stations, flow under pressure, and are thus called force mains.

Urban sewer mains generally discharge into interceptor sewers that join to form a large trunk line, which discharges wastewater into the community sewage treatment plant where it’s treated.

After being treated at the community wastewater treatment plant, the treated wastewater is usually released into a lake or stream, where it flows toward the ocean. Reconditioned waste water will generally be used again and again along the way for irrigation, by industry, and as drinking water, or it will evaporate into the atmosphere and return again as rain as part of the hydro logical cycle. Wastewater treatment plants operate at a critical point in the water cycle to protect against excessive pollution.

Sewage Treatment and Disposal

Sewage treatment is a multistage process designed to restore the quality of wastewater before it reenters a body of water such as a stream, river, or lake. The objective is to reduce or entirely remove organic matter, solids, nutrients, disease-causing organisms, and other pollutants from wastewater.

Community wastewater treatment plants are operated by a municipality or special district and serve a community.

Processes involved in large wastewater treatment plants are usually classified as being part of preliminary, primary, secondary, or tertiary treatment.

Preliminary Wastewater Treatment

Preliminary treatment to screen out, grind up, or separate debris is the first step in wastewater treatment. Sticks, rags, large food particles, sand, gravel, toys, and so on are removed at this stage to protect the pumping and other equipment in the treatment plant. Treatment equipment such as bar screens, comminutors (a large version of a garbage disposal), and grit chambers are used as the wastewater first enters a treatment plant. The collected debris is usually disposed of in a landfill.

Primary Wastewater Treatment

Primary treatment is the second step in wastewater treatment. It separates suspended solids and greases from wastewater. Waste water is held in a quiet tank for several hours, allowing the solid particles to settle to the bottom and the greases to float to the top. The solids drawn off the bottom and skimmed off the top receive further treatment as sludge. The clarified wastewater then flows on to the next stage of wastewater treatment. Clarifiers and septic tanks are usually used to provide primary treatment.

Secondary Wastewater Treatment --- Secondary treatment is a biological treatment process to remove dissolved organic matter from wastewater. Sewage microorganisms are cultivated and added to the wastewater. The microorganisms absorb organic matter from wastewater as their food supply. Three approaches are used to accomplish secondary treatment: fixed film, suspended film, and lagoon systems.

• Fixed film systems grow microorganisms on substrates such as rocks, sand, or plastic. The wastewater is spread over the substrate, allowing the wastewater to flow past the film of microorganisms fixed to the substrate. As organic matter and nutrients are absorbed from the wastewater, the film of microorganisms grows and thickens. Trickling filters, rotating biological contactors, and sand filters are examples of fixed film systems.

• Suspended film systems stir and suspend microorganisms in wastewater. As the microorganisms absorb organic matter and nutrients from the wastewater, they grow in size and number. After the microorganisms have been suspended in the wastewater for several hours, they are settled out as sludge. Some of the sludge is pumped back into the incoming wastewater to provide "seed" microorganisms. The remainder is wasted and sent on to a sludge treatment process. Activated sludge, extended aeration, oxidation ditch, and sequential batch reactor systems are all examples of suspended film systems.

• Lagoon systems are shallow basins that hold the wastewater for several months to allow for the natural degradation of sewage. These systems take advantage of natural aeration and microorganisms in the waste water to recondition wastewater.

Final Treatment (Disinfection)

Final treatment involves disinfection; the removal of disease causing organisms from wastewater. Treated wastewater can be disinfected by adding chemicals to the water such as chlorine, bromine, iodine and ozone, or by exposing it to ultraviolet light.

Chlorination has become the standard method of disinfection because it remains in the water after the treatment. Al though chlorine introduces an unnatural taste and smell to the water, it does remove or mask tastes and odors caused by other potential ingredients. High levels of chlorine may be harmful to aquatic life in receiving streams. Treatment systems often add a chlorine-neutralizing chemical to the treated wastewater before stream discharge.

Other treatments include the following: aeration, the exposure of treated water to air, which removes odors and improves taste; corrosion removal, to balance the pH (acidity) of the treated water to prevent corrosion damage to pipes, remove odors, and improve taste; and to soften the water. The softening process removes calcium and magnesium that have dissolved in water, making the hard water soft. Hard water leaves deposits on plumbing fixtures and does not allow soap to clean as effectively.

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