Wastewater is not just sewage. All the water used in the home that goes down the drains or into the sewage collection system is wastewater. This includes water from baths, showers, sinks, dishwashers, washing machines, and toilets. Small businesses and industries often contribute large amounts of wastewater to sewage collection systems; others operate their own wastewater treatment systems. In combined municipal sewage systems, water from storm drains is also added to the municipal wastewater stream. The average American contributes 265-568 liters (66 to 192 gallons) of wastewater each day. Wastewater is about 99 percent water by weight and is generally referred to as influent as it enters the wastewater treatment facility. "Domestic wastewater" is wastewater that comes primarily from individuals and does not generally include industrial or agricultural wastewater.

At wastewater treatment plants, this flow is treated before it is allowed to be returned to the environment, lakes, or streams. There are no holidays for wastewater treatment, and most plants operate 24 hours per day every day of the week. Wastewater treatment plants operate at a critical point of the water cycle, helping nature defend water from excessive pollution. Most treatment plants have primary treatment (physical removal of floatable and settleable solids) and secondary treatment (the biological conversion of organics to carbon and carbon dioxide in the presence of dissolved oxygen).

Unit Processes In Wastewater Treatment

A. Preliminary Treatment

• Screening - Similar to water, but mechanically operated too.
• Grit removal - Removing grit and inorganic material (e.g., sand)
• Grinding/Shredding - Using mechanical cutting equipment that reduce all particles to small sizes as wastewater passes through and over cutting surfaces.
• Storm overflow - Diverting sewage in excess of capacity of treatment plant to storm water treatment storage tanks.

B. Primary Treatment

• Primary Clarification
• Separation (Settling) of suspended solids (only 40to 60% are removed) from primary influent, chemicals are not usually added.
• Primary sludge is a mixture of settled suspended solids(TSS) and wastewater( typically 3 % or more TSS dry weight).
• Primary effluent passes on to secondary treatment.

C. Secondary Treatment

• Carbon (organic matter) Oxidation - Biodegrading organic matter through the growth process of microorganisms in a biological reactor treatment unit such as aeration basins, contactor media, lagoons or ponds, etc.
• Nitrification - Oxidation of ammonia by microorganisms or the oxidation of ammonium ion to nitrate.
• Secondary Clarification - Settling out of a biological sludge containing microorganisms to produce a secondary effluent which passes on to further treatment and disinfection.

Additional Treatment

• Advanced Treatment - Polishing of secondary effluent by treatment processes such as chemically aided clarification and filtration similar to drinking water treatment.
• Disinfection - Reduction of total microorganism population in treated effluent through exposure to germicides such as, residual chlorine compounds, oxidants, or ultraviolet light irradiation.

Sludge Treatment

• Aerobic Digestion - Mixing of thickened sludges in the presence of dissolved oxygen to oxidize organics and separate out supernatent (water with lower solids content).
• Anaerobic Digestion - Separation of oxidized organic and inorganic solids from sludge mixture producing both a supernatent and thickened (higher solids content) sludge under anaerobic conditions, resulting in the formation of methane gas and carbon dioxide.

Sludge Management

• Composting - Further Biodegredation of organics in piles of a mixture of sludge (digested only) and bulking agent, followed by curing and screening to produce a safe material that can be used as a soil conditioner if exceptional quality characteristics are achieved.
• Drying Beds - Draining and drying sewage sludge in open atmosphere on porous media with return of filtrate back to treatment process.
• Gravity Thickening - Removal of supernatent (thickening) of primary and secondary sewage sludges by settling.
• Incineration - Combustion of sludge, (normally undigested but dewatered) by exposure to high heat, with burial of ash.
• Land Application - Spreading on land, properly treated sludge, that has acceptable levels of trace elements, at controlled rates, to improve the soils ability to grow plants.
• Landfilling - Placing sludge in excavated areas (burial - preferably digested and dewatered or dried) by co-disposal with municipal solid waste.
• Mechanical Dewatering - Thickening of sewage sludges by mechanical methods such as centrifuges, pressure or vacuum filters, etc.

Primary Treatment Involves:

Screening

• Removes large objects, such as stones or sticks, that could plug lines or block tank inlets.

Grit Chamber

• Enhances settling to allow grit (sand, rock, metal, plastic, etc.) to be removed from the sewage influent.

Primary Clarification (Gravity)

• Separates settleable solids from primary influent resulting in accumulation of primary sludge on basin floor (sloped) that is pumped away, while a float or scum accumulates on the wastewater surface and is skimmed off.

Secondary Treatment Includes:

• Secondary treatment typically utilizes a biological treatment process, in which microorganisms convert non-settleable solids to settleable solids in a biological reactor unit operation. Sedimentation typically follows, allowing the settleable solids to be removed from reactor effluent. Three options include:

Aeration Reactors

• The most common option establishes a population of bacteria and protozoa microorganisms in a basin that is provided with a continuous supply of air. This biomass provides oxidation of organic matter. The aeration basin is usually followed by clarification to remove the biomass by settling. Some of the bacteria containing biomass(activated sludge) is continually recirculated back to the aeration basin to increase the rate of organic decomposition. Excess biomass is wasted to sludge treatment.

Bio Contactors

• These are reactors containing media (mineral or plastic) typically placed in a vertical arrangement that provides a specific depth and surface area. Wastewater is allowed to pass around the media. The contactor media can also be a circular arrangement that is revolved through the wastewater(rotating contactor). Microorganisms attached to and growing on the media, decompose organic material in the wastewater. Vertical media contactors usually are operated as downflow basins that drain at the bottom; the wastewater is collected and then undergoes clarification.

Lagoons

• These are large impoundments containing a small concentration of biomass, including algae and bacteria.
• They rely on the interaction of sunlight, algae, microorganisms, and oxygen (sometimes mechanically aerated) to oxidize organics in the wastewater.

Additional Treatment Includes:

• After primary and secondary treatment, municipal wastewater is usually disinfected using chlorine (or other disinfection methods, i.e., ultraviolet light and occasionally ozone). An increasing number of wastewater facilities also employ tertiary treatment, often using advanced treatment methods.
• Tertiary treatment may include processes to remove nutrients such as nitrogen and phosphorus, and carbon adsorption to remove organic chemicals. These processes can be physical, biological, or chemical.
• Settled solids (sludge) from primary treatment or secondary treatment settling tanks are given further treatment and several options are used for management including uses as fertilizer, or disposal with solid waste in a landfill. Treated sewage sludge that is acceptable for land application is defined as Biosolids.