Types of Pretreatment Units & Devices

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. The effluent enters the unit, and an equal amount of effluent exits the unit, similar to what happens in a septic tank.

The suspended growth process is also known as "activated sludge" or extended aeration. This 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.

Aerobic Treatment Unit Characteristics

A number of these types of units are approved by the State of Wisconsin and 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 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 waste hauler 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.

Supplemental Treatment Devices

Some ATU’s have supplemental treatment devices installed such as an ultraviolet light to reduce E.coli bacteria colonies.

Installed supplemental treatment devices used with ATU designed POWTS require periodic service and replacement every 12-24 months.

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 
  • Rotating biological contactors.

Types of Filters

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

Sand Filters

There are two general classes of sand filters. The single pass allows effluent to pass through the sand, gravel or peat media once. It is then sent on to final dispersal in the soil.

The multiple-pass or re-circulating filter splits the wastewater flow. It sends some of the effluent onto the final dispersal cell in the soil. The rest gets returned to a chamber upstream of the filter and is blended with the less-treated wastewater. This blended effluent continually gets passed through the filter.

Synthetic Media Filters

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

Fixed Film Aeration Unit Characteristics

General characteristics for fixed film aeration units are as follows:

  • 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
  • All fixed film units require a septic tank upstream to remove settable solids
  • 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 convert organic matter to carbon dioxide and water. Organic nitrogen and ammonia are converted to nitrate
  • Maintenance of the system consists of removing settled biosolids by contracting a licensed waste hauler and reporting the maintenance to Kenosha County
  • Most of the units have effluent applied to the fixed film by a pump on a time clock or on demand
  • Most units have a collection pipe at the bottom of a container or membrane to collect filtered and treated effluent
  • 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 to fifteen years
  • Some synthetic media filters will require replacement of their media approximately every ten years