Sewage Treatment Systems

  • Preliminary treatment (removes materials that can cause operational problems, equalization basins are optional)
  • Primary treatment (remove ~60% of solids and ~35% of BOD)
  • Secondary treatment (remove ~85% of BOD and solids)
  • Advanced treatment (varies: 95+ % of BOD and solids, N, P)
  • Final Treatment (disinfection) • Solids Processing (sludge management)


Preliminary treatment

Upon arrival via the sewer system, the wastewater is sent through a bar screen, which removes large solid objects such as sticks and rags.


Leaving the bar screen, the wastewater flow is slowed down entering the grit tank, to allow sand, gravel and other heavy material that was small enough not to be caught by the bar screen to settle to the bottom. All the collected debris from the grit tank and bar screen is disposed of at a sanitary landfill.


Pre-Treatment of Industrial Wastewaters

  • Industrial wastewaters must be pretreated prior to being discharged to municipal sewer system
  • Pretreatment requirements set by regulatory agencies
  • Why: remove materials that will not be treated by municipal system, remove materials that inhibit the biological processes in secondary treatment
  • For example: silver ions are toxic to bacteria which might affect biological process. Thus, silver ions are removed at pre-treatment before biological process so that bacterial performance does not get affected.


Secondary treatment

biological treatment process that removes dissolved organic material from wastewater. The partially treated wastewater from the settling tank flows by gravity into an aeration tank.

mixing of water to solids containing that use oxygen to consume the remaining organic matter in the wastewater as their food supply (use of air bubble for mixing and oxygen supply)


liquid mixture (i.e., solids with micro-organisms and water) is sent to the final clarifier.


In clarifier, solids settle out to the bottom where some of the material is sent to the solids handling process and some is recycled back to replenish the population of micro-organisms in the aeration tank to treat incoming wastewater.


Final treatment

Treated water is disinfected and then it is sent out for wastewater reuse activities or for discharging in river/streams. mostly chlorination and/or ultra violet irradiation is used for disinfection purposes.


Solids processing

The primary solids from the primary settling tank and the secondary solids from the clarifier are sent to a digester.

Micro-organisms use the organic material present in the solids as a food source and convert it to by-products such as methane gas and water.

Digestion results in a 90% reduction in pathogens and the production of a wet soil-like material called “biosolids” that contain 95-97% water.

In order to remove some of this water, mechanical equipment such as filter presses or centrifuges are used to squeeze water from the biosolids to reduce the volume prior to being sent to landfill, incinerated or beneficially used as a fertilizer or soil amendment.