Wastewater Treatment Process

Wasterwater treament is cleaning used water and sewage so it can be returned safely to our environment. 

WWTP BuildingWastewater treatment basically takes place in three stages:

  1. Preliminary & Primary treatment, which removes 40-60% of the solids.
  2. Secondary treatment, which removes about 90% of the pollutants and completes the process for the liquid portion of the separated wastewater.
  3. Sludge (biosolids) treatment and disposal.

Preliminary & Primary Treatment  


Sanitary sewers carry wastewater from homes and businesses to the raw wastewater pumping station at the treatment plant.  The wastewater flows by gravity, rather than pressurized pipe flow, in the pipes. Routine cleaning and closed circuit television inspection of Columbia’s sewer lines helps keep the collection system in good shape.  Each year new construction in the City of Columbia adds about 10 miles of new sewer lines and 250 manholes to the sewer collection system.

 WWTP sign


Bar Screens let water pass, but not trash (such as rags, diapers, etc.).  There are two bar screens located inside the Raw Wastewater Pump Building.  The trash is collected and properly disposed of.  The screened wastewater is then pumped to the Primary Settling Basins.  

Bar Screen



Two Primary Settling Basins allow smaller particles to settle from wastewater by gravity.  This primary wastewater flows out to the next stage of treatment.  Scrapers collect the solid matter that remains (called “primary sludge”).  A surface skimmer collects scum or grease floating on top of the basins. 

settling basin



Secondary Treatment


Two Aeration Basins supply large amounts of air to the mixture of primary wastewater and helpful bacteria and the other microorganisms that consume the harmful organic matter.  The growth of the helpful microorganisms is sped up by vigorous mixing of air (aeration) with the concentrated microorganisms (activated sludge) and the wastewater.  Adequate oxygen is supplied to support the biological process at a very active level.  The ratio of food (organic matter) to organisms to oxygen is continually monitored and adjusted to meet daily variations in the wastewater. 

Aeration Basin




Two Final Settling Basins allow the clumps of biological mass (the microorganisms) to settle from the water by gravity.  90-95 % of this mixture, called “activated sludge,” is returned to the aeration basins to help maintain the needed amount of microorganisms.  The remaining 5-10 % is pumped to the anaerobic digester (described later). 

Secondary Settling Basin



The final effluent (liquid portion from Step 2) travels through a 2.25 mile long, 72 inch diameter pipeline to WTU-4, the first of the City’s four constructed wetland treatment units, in the McBaine Bottoms for further treatment.  Cattails planted in these constructed wetland cells (ponds) consume some of the remaining nutrients in the effluent. The wastewater passes through three more units before treatment is completed.  This “polished” effluent leaves the City of Columbia’s constructed wetlands at the  pump station where it is pumped into the Missouri Department of Conservation’s Eagle Bluffs Conservation Area (EBCA), where it is used as a source of water.   

Wetland Units 1 & 2

Wetland Cell


All effluent returned to natural bodies of water must meet National Pollutant Discharge Elimination System (NPDES) criteria.  The final effluent is monitored daily.  In-house laboratory staff performs sampling and analysis for process control and NPDES compliance. 

Sludge (Biosolids) Treatment  


The “primary sludge” from the Primary Settling Basins is pumped to the Hydrocyclone Grit Separator where it is spun, thereby separating the inorganic solids (grit) from the lighter weight organic solids.  The grit is disposed of in the City landfill. 






The primary sludge continues on to the gravity Sludge Thickener where the solids are concentrated and pumped to the anaerobic digesters.  The liquid overflow is returned to the pump station.  Waste Activated Sludge from the Final Settling Basins is pumped to a Centrifuge (image at right) for further solids processing, then pumped to the anaerobic digesters. 




Primary and activated sludges are anaerobically digested (decomposed by bacteria without the presence of air) in the two-stage digesters.  Stabilized sludge has little odor and conforms to the EPA requirements to further reduce harmful microorganisms. 



Methane gas is produced by this anaerobic digestion and is used as fuel for an engine-generator providing 240 kW of electrical power used in the treatment process.  Waste heat from the engine is recovered for heating the treatment plant buildings and to provide heating to improve the sludge digestion process and produce more gas. 
large red generator



Step 4 is the application of the stabilized sludge (biosolids) onto both City-owned and private farmland.

The biosolids are utilized in an environmentally acceptable manner as a beneficial and valuable fertilizer and soil conditioner.  The biosolids applied to all sites are monitored for nutrients, metals, other compounds and fecal coliform bacteria.  Soil testing is performed at all sites prior to biosolids application. 

Sludge transport