Wastewater Engineering - General Information

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:


  • 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.


  • 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.


activated sludge: Biomass produced by the growth of microorganisms in aerated reactors as a part of the secondary treatment process.

aeration: exposing to circulating air; adds oxygen to the wastewater and allows other gases trapped in the wastewater to escape (the first step in secondary treatment via activated sludge process.)

anaerobic: oxygen is not available in significant amounts.

biochemical oxygen demand(BOD): a laboratory measurement of oxidation in wastewater that is one of the main indicators of the quantity of pollutants present; a parameter used to measure the amount of oxygen that will be consumed by microorganisms during the biological reaction of oxygen with organic material.

biomass: mixed population of microorganisms contained in a bio-reactor as measured by the total suspended solids content.

biosolids: sludge that is suitable for beneficial use. Biosolids must meet certain government-specified criteria depending on it's use (e.g., fertilizer or soil amendment).

conventional treatment: treatment processes with established performance and capacities

decomposition: the process of breaking down (degrading) into constituent parts or elements

domestic wastewater: wastewater that comes primarily from individuals, and does not generally include industrial or agricultural wastewater

effluent: treated wastewater, flowing from a lagoon, unit operation basin, or treatment process

grit chamber: a chamber or tank used in primary treatment where wastewater slows down and heavy, large solids (grit) settle out and are removed

influent: untreated wastewater flowing into a treatment plant or partially treated wastewater flowing into a downstream unit operation

lagoons: (oxidation ponds or stabilization ponds): a wastewater treatment method that uses ponds to treat wastewater. Algae grow within the lagoons and utilize sunlight to produce oxygen, which is in turn used by microorganisms in the lagoon to break down organic material in the wastewater. Some wastewater solids settle in the lagoon, resulting in an effluent that is relatively well treated, although it does contain algae.

municipal: of, or related to a municipality (city, town, etc.). Municipal wastewater is primarily domestic wastewater.

non-conventional treatment: alternative or new treatment processes whose performance and capacity is not well established.

oxidation: biochemical decomposition of organic subtances in the presence of molecular oxygen, which produces and reduces BOD.

primary treatment: the first stage of wastewater treatment that removes settleable or floating solids only; generally removes 40% of the suspended solids and 30% of the BOD in the wastewater.

secondary treatment: a type of wastewater treatment used to convert dissolved and suspended pollutants into a form that can be removed, producing a relatively highly treated effluent. Secondary treatment normally utilizes biological treatment processes (activated sludge, trickling filters, etc.) followed by settling tanks and will remove approximately 85% of the BOD and TSS in wastewater. Secondary treatment for municipal wastewater is the minimum level of treatment required by the Federal Clean Water Act.

sedimentation: the process used in both primary and secondary wastewater treatment, that takes place when gravity pulls particles to the bottom of a tank ( also called settling).

settling basin (sedimentation tank or clarifier): a vessel in which solids settle out of water by gravity during wastewater or drinking water treatment processes.

sludge: any solid, semi-solid, or liquid waste that settles to the bottom of sedimentation tanks (in wastewater treatment plants or drinking water treatment plants) or septic tanks.

tertiary treatment: any level of treatment beyond secondary treatment, which include filtration, nutrient removal (removal of nitrogen and phosphorus) and removal of toxic chemicals or metals; also called 'advanced treatment' when nutrient removal is included.

total suspended solids (TSS): a laboratory measurement of the quantity of suspended solids that are filtered from wastewater that is one of the main indicators of the presence of organic matter and biomass present in a treatment process.

trickling filter/biofilm process: a biological treatment process that uses coarse media (usually rock or plastic elements), contained in a tank, that serves as a surface on which a biofilm(attached microbiological growth) occurs on the media. Wastewater trickles over the media and microorganisms remove the pollutants (BOD and TSS). Trickling filters are followed by settling tanks to remove microorganisms that wash off and pass through the media.

turbidity: the cloudy or muddy appearance of a naturally clear liquid caused by the suspension of particulate matter that interferes with the transmission of light waves.

unit operation: each separate part of a treatment process.

wastewater: water that has been used for domestic or industrial purposes and contains substances that may interfere with water uses.

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Virginia Department of
Environmental Quality
P.O. Box 1105
Richmond, VA 23218
(804) 698-4000

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