SKETCH of a typical aerobic treatment unit tank, aerator, chamber AEROBIC Septic Systems Final Treatment Stage

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Aerobic treatment unit final treatment stage details: here we explain the final treatment stage of aerobic septic systems or aerobic treatment units, and we suggest sources for designs and products for aerobic septic treatment units (ATUs) for onsite waste disposal, also called fine bubble aeration systems. We address aerobic septic system design, features, inspection, repair, and maintenance. Product sources are also listed.

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Post Processing or Final Treatment of Aerobic or Aeration Septic System Effluent

Septic effluent, after having been processed through the aerobic treatment unit (ATU), may require further polishing, such as by a private wetland or greenhouse system, and may also require disinfection such as by chlorination or UV light. Other common final ATU effluent treatment systems include sand bed filters, raised bed septic fields, drip irrigation systems, evaporation/transpiration effluent disposal systems, or even a conventional septic drainfield.

When all of this processing has been completed, the effluent may be sufficiently sanitary to permit discharge into a nearby body of water or directly onto the ground surface. In other circumstances the effluent is discharged into a subsurface system such as a drainfield. In this last case, the drainfield or soil absorption system will not need to meet the same treatment requirements as a conventional septic drain field, because of the pre-processing and sanitizing that have already taken place.

Aerobic or aeration septic systems should have a second settlement chamber at or past the septic tank

Because the treatment tank is continually agitated the level of suspended solids remains high.

Therefore an aerobic treatment unit will also include a second settling chamber to make sure that solids are not being sent out to the soil absorption system. Some designs may include a filter at the tank outlet for added protection of the absorption system. Sludge from the settling chamber is returned to the primary tank for further treatment.

Chlorine or Calcium Hypochlorite Disinfection for Aerobic or Aeration Septic Systems

Disinfection methods for treated effluent include chlorination or UV light treatment. Where chlorine tablets are used it is essential that the proper type of tablets are placed into the system (see the two links below about chlorine tablets, swimming pool chlorine tablets, and calcium hypochlorite tablets (the proper stuff to use).

OPINION DF: Although they may be approved for this application, I have never been completely confident in UV-light disinfection systems for several reasons:

  • The "install and forget it" image of a UV light treatment system means that owners may never notice that the system has stopped working
  • Sludge and debris may coat the UV light treatment bulb, shortening its effective life
  • Any UV light water or wastewater treatment system is limited by the flow-rate of water past the light. If water flows faster than the design point, treatment will be ineffective. Many systems I've seen installed had no method for limiting or even monitoring the flow rate past the light.


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ATU FINAL TREATMENT at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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