Preliminary or pretreatment is the very first stage, it is the removal of larger materials and grit that if not removed could hinder subsequent treatment processes. It is accomplished through the use of equipment such as bar screens, macerators, comminutors, racks and grit removal systems.

Primary treatment is usually comprised of preliminary treatment followed by primary clarifiers which remove approximately 50% suspended solids and 35-40% B.O.D.5. This is accomplished by channeling flows through large tanks with residence times of 2-4 hours, thus allowing suspended solids to settle.  Post disinfection and a biosolids treatment process are normally included. Enhanced Primary Treatment can be performed by the addition of a coagulant such as ferric chloride along with a polymer, improving the degree of S.S. and B.O.D. removal to 80% and 60% respectively.

Secondary treatment involves a primary process and a biological treatment stage. There are many biological processes in the treatment operation with the majority being variations of fixed film and mixed culture applications. The activated sludge process (mixed) is achieved by establishing large diversified cultures of bacteria. The bacteria metabolizes and provides the enzymatic breakdown of organic components, ie., liquids, carbohydrates, proteins and cellulose, in the wastewater. 90% removal rates of B.O.D. and suspended solids are typical of secondary treatment.

Tertiary treatment is comprised of a three stage process which further improves effluent quality past the primary and secondary treatment phases (see above examples). Usually 95-98% removal rates are achieved. Often, nutrient removal is included with both secondary and tertiary systems.

BOD5: Biochemical Oxygen Demand is a biological laboratory procedure that measures the rate of oxygen use while stabilizing decomposable organic mater under controlled conditions of time (5 days) and temperature (20ºC). It is an important regulatory parameter used to indicate organic strength of the wastewater. Typical BOD5 of raw domestic sewage:240 mg/L.

Suspended Solids: Particles in wastewater, usually 75% organic and averaging 200 mg/L in raw sewage.

Fecal Coliform: A type of coliform bacteria which originate from the feces of various warm blooded animals (humans inc.). Usually a strong indicator of pollution or possible pathogenic bacterial contamination.

Nutrients: Substances which are required to support living plants and organisms. An overabundance of phosphorus and nitrogen can stimulate excessive plant and algae growth under certain conditions. In fresh water, phosphorus is the limiting nutrient while nitrogen is the limiting nutrient in salt water.

Disinfection: The process designed to kill most microorganisms in wastewater, including pathogenic bacteria. There are several ways to disinfect, namely, chlorination, ultra violet light, ozone, chlorine dioxide, etc.