Types of Pretreatment Units

Pretreatment units can be placed in one of two categories, namely: suspended growth and fixed film processes.

Suspended Growth Process

Microorganisms are kept in suspension within an aeration tank where air is mixed with the wastewater. The bacteria convert the organic matter to bacterial cells, carbon dioxide and water. Cell recycling and wasting is required along with settling and/or filtering. Some units operate on a batch dose where specified amounts of wastewater are pumped into the aerobic unit at time intervals. Other units operate on a flow-through basis where influent enters the unit and an equal amount of effluent exits the unit, similar to what happens in a septic tank.

Suspended growth process is also known as “activated sludge” or extended aeration, which is typically the same process used in a municipal wastewater treatment plant. Individual home wastewater treatment units of this type are more commonly known as Aerobic Treatment Units or ATU’s.

There are a number of these types of units that are approved by the State of Wisconsin – Department of Commerce that are currently being used in Kenosha County. Typically these ATU’s have the following characteristics:

  • As part of a complete POWTS, there is a septic tank to settle the large solids and scum. Some systems may have a pump on a timer sending effluent from the septic tank to the ATU. Effluent from the ATU discharges to some sort of dispersal cell in or on the soil.
  • All systems have a method of incorporating air into the wastewater to maintain a dissolved oxygen in the wastewater either continuously or intermittently.
  • All systems incorporate some method of solids separation by settling and/or filtering through a fabric or screen.
  • All systems require maintenance pumping by a licensed wastehauler who removes settled-out biosolids. They also require periodic maintenance by a licensed POWTS maintainer and reporting to Kenosha County.
  • ATU’s can be quite sensitive and can easily be upset by the addition of toxic chemicals (cleaning products) and certain medications. Rapid and large changes in organic and hydraulic loading will also cause problems. As a result, ATU’s may foam and froth with increased BOD and suspended solids in the effluent.
  • Most ATU’s have an alarm system to alert the homeowner of a system malfunction.


Fixed Film Process

Fixed film aeration consists of a unit in which wastewater that has gone through primary settling such that occurs in a septic tank and passes through a porous media such as fine or coarse aggregate, peat or synthetic media. The bacteria attach themselves to the media and extract food and nutrients from the wastewater as it passes through the media. Examples of fixed film aeration include sand filters, gravel filters, trickling filters, peat filters, synthetic media filters and rotating biological contactors.

The State of Wisconsin – Department of Commerce has approved a number of these fixed film aeration systems. Probably the most common of these systems to be used currently in Kenosha County are sand filters which are constructed on-site and the pre-fabricated synthetic media filters.

There are two general classes of sand filters, namely: the single pass which allows effluent to pass through a sand, gravel or peat media one time and it is then sent on to final dispersal in the soil; and, the multiple-pass or re-circulating filter, which splits the wastewater flow sending some of the effluent onto the final dispersal cell in the soil and some gets returned to a chamber upstream of the filter and is blended with the less treated wastewater. Then this blended effluent continually gets passed through the filter.

Synthetic media filters are typically made in the form of geotextile fabric on a frame, plastic spheres, permeable foam blocks or poly-ethylene disks. As stated in the above paragraph, these units always rely on some sort of re-circulation mechanism to blend aerobically treated effluent with untreated effluent.

General characteristics for fixed film aeration units are as follows:

  • All fixed film units require a septic tank upstream to remove settable solids.
  • Most units have a collection pipe at the bottom of a container or membrane to collect filtered and treated effluent.
  • All units have an extensive system of effluent distribution to spread the effluent as uniformly as possible over the fixed film
  • As effluent passes through the filter, various physical, chemical and biological reactions take place. Suspended solids are filtered out, bacteria converts organic matter to carbon dioxide and water. Organic nitrogen and ammonia are converted to nitrate.
  • Most of the units have effluent applied to the fixed film by a pump on a time clock or on demand.
  • Aerobic conditions must exist on the filter or synthetic media. Air is drawn into the filter after dosing by the downward movement of the effluent.
  • Maintenance of the system consists of removing settled biosolids by contracting a licensed wastehauler and reporting the maintenance to Kenosha County.
  • Porous media filters such as sand, gravel and peat filters eventually clog with organic carbon and will need to be reconstructed. Projected maintenance is every ten (10) to fifteen (15) years.
  • Some synthetic media filters will require replacement of their media approximately every ten (10) years.