Private Onsite Wastewater Treatment Systems



Conventional In-ground Treatment

Aeration Treatment

Aerobic Treatment Units

Packed Bed Filters

Do's and Don'ts

Advantages and Disadvantages




Aerobic Treatment Units

Aerobic Treatment Units treat wastewater using aerobic bacteria that live in oxygen-rich environments.  The bacteria break down and digest the wastewater inside of the aeration unit.  Most aeration treatment units require some primary treatment, or settling, which is done inside a septic tank, trash tank, or some sort of compartment before entering the aeration chamber.  The solids and scum must separate from the wastewater before it can be processed in most aeration chamber, to prevent clogging.

Effluent leaving the aeration units is unsafe to return directly to the environment.  The effluent must go through a final treatment process.  Final treatment methods include: a soil absorption field, a sand filter, or an evapo-transpiration bed.  The most commonly used final treatment method is the soil absorption field.  The amount of dissolve oxygen helps the microorganisms in the soil treat the effluent. 

 Water flow plays an important role in the effectiveness of the system.  Some designs have devices to control the incoming flow, such as surge chambers, baffles, and pretreatment tanks.  Pumps and siphons are used to control the amount of wastewater or effluent in the aeration chamber.  When there is too much wastewater being introduced to the system all at once, the treatment process suffers.  The wastewater can be pushed through the system without proper treatment, which can end up clogging the distribution pipes to the drainfield.

All aerobic treatment units are required to have alarms programmed to sound when system malfunctions.  The alarms will sound when the water level gets too high or when the pump or other components fail.  The alarms are located both inside and outside of the home. 

The first two years of maintenance is usually provided by the NSF (National Sanitation Foundation) if the unit is NSF approved.  After the initial two-year maintenance, it is mandatory to find a service provider to continue maintenance of the aeration unit. 



There are three types of aeration treatment units.  There are suspended growth units, attached growth units, or a combination of the two.

Suspended Growth Units

Suspended growth units are the most used aerobic units.  Suspended growth units consist of an aeration chamber where air is mixed with the wastewater.  The aeration chamber contains a pump or a compressor to bring into the wastewater so it can be used by the bacteria.  The bacteria are kept in suspension in the aeration chamber, and air is mixed with the effluent.  The bacteria digest the solids in the wastewater or effluent and turn it into new bacteria cells, carbon dioxide, and water.


Multi-Flo Waste Treatment Unit

 The Multi-Flo Waste Treatment Unit is NSF Class 1 rated.  The Multi- Flo unit comes in many sizes from 500 gpd to 1500 gpd depending on the amount of bedrooms in the household.  The unit is usually preceded by a settling tank or trash tank.  The effluent from the settling tank flows into the inlet of the system, and a timer introduces small amounts into the aeration chamber throughout the day to keep a uniform flow into the system. 

The aerator pump pulls in air and releases small air bubbles into the effluent for the bacteria to utilize.  The effluent moves through fabric filter tubes located in the chamber.  The fabric filters keep the organic solids in the basin of the chamber.  The filter surface also provides a bacteria growth media.  The unit also has sensors located in the top of the chamber to detect high water and pump failure. 

The Multi-Flo unit removes some nitrogen but it is not part of the treatment process.'aeration%20treatment%20onsite%20domestic%20wastewater'



Photos: Waukesha County Health Department


Delta Whitewater

The Delta Whitewater is also NSF Class 1 rated.  The whitewater unit consists of a single tank with an Imhoff cone in the center, and ranges in size from 400 gpd to 1500 gpd. Wastewater enters the tank from the top, and moves down around the Imhoff cone where the air is introduces by multiple blowers.  The effluent moves up inside the cone where settling occurs.  The solids then fall back into the bottom of the tank.  Solids must be removed from the tank periodically.  The effluent then travels through the outlet pipe to the distribution field. 

The Delta Whitewater unit also removes some nitrogen from the effluent, but it is not necessarily part of the treatment process.




Photo: Waukesha County Health Department


Attached Growth Units

Attached Growth Units treat wastewater by providing a surface for bacteria growth. The bacteria-covered surface is introduced to the wastewater and the air alternately.  These units sometimes contain fixed or floating cylinders or spheres that move around in the wastewater.  Pretreatment is required for attached growth units.  An advantage to attached growth units is that there is no mixing of air in the wastewater.


Bio-Microbics – FAST Unit

The Bio-microbics unit is a NSF Class 1 rated system.  This unit consists of a two-compartment tank.  In the first compartment the solids are settled out, and then the effluent flows into the second compartment through a space at the top of the divider.  The FAST media chamber is located at the top of the second chamber, with an opening at the bottom.  An air lift tube is located inside the media chamber.  The air is forced down the inner tube and flows up between the larger tube creating air bubbles that force the wastewater up into the media chamber.  This is the surface for bacterial growth where the organics are removed, and turned into carbon dioxide, water, and new bacterial cells. 

Organic nitrogen and ammonia are converted into nitates.  There is also a trough at the top of the media chamber (not shown) that directs the wastewater to the second compartment where the nitrates are denitrified into nitrogen gas.  The nitrification/denitrification process was designed into the Bio-microbics system.







Created 12/08/04