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Aerobic Treatment Units

Aerobic Treatment Units

Aerobic Treatment Units (ATUs) are pre-engineered treatment systems that provide a higher level of treatment (secondary) than a septic tank alone (primary). ATUs are required for all developers where estimated wastewater flows on a given parcel exceed 1,800 gpd.

There are a variety of ATU designs but all provide mechanisms to carry out the three functions necessary to secondary treatment:

  • Aeration: To circulate air through the wastewater to provide oxygen to keep the aerobic microorganisms alive.


  • Mixing: To ensure adequate contact between wastewater and microorganisms.
  • Settling: To allow the developed biomass to separate from the treated effluent.

All ATUs shall be of a design certifed (NSF/ANSI Standard 40 or equivalent) as capable of achieving effluent quality standards of 30 mg/L Biochemical Oxygen Demand (BOD5) abd 30 mg/L Total Suspended Solids (TSS).

Proposals for Custom Treatment Systems will only be considered in cases where wastewater flows exceed the hydraulic or organic treatment capacity of Standard ATUs. Custom systems shall be designed by a professional engineer.

Service providers may log in to the Onsite Wastewater Management Programme (OWMP) system  here.