When Does an ATU Need Cleaning?

When Does an ATU Need Cleaning?

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There are potentially two or three compartments of an ATU that will need cleaning at some point. With some units these compartments may be combined in one tank and separated by a baffle. Never forget that anytime a lid is opened or removed it must be properly secured back in place.

The first compartment or pretreatment tank is generally referred to as the “trash tank” because it settles out materials that microorganisms cannot degrade. Pretreatment tanks are designed to store solids and perform limited biological treatment.

Pumping out the trash tank is mandatory to remove and dispose of these solids before they discharge from the tank. Initially after startup, check solids depth at six-month intervals to determine the amount of solids accumulating in the tank. Typically, this tank will need cleaning on a schedule similar to a septic tank pumping interval of every two to three years, but this is highly dependent on the household practices. The goal is to maintain scum and sludge accumulations to less than 1/3 depth of the liquid level. During the tank cleaning:

  • Remove all materials from the trash tank
  • Check baffles
  • Confirm structural integrity and water tightness
  • Fill back up with water as required by manufacturer

The second compartment is an aeration chamber, where aerobic microbes decompose waste in the water. There are two types of aeration systems: suspended growth and fixed film. These two types of ATUs have different protocols for determining when they need to be cleaned.

Note: Some systems require that equipment be removed prior to pumping to prevent damage (refer to the manufacturer’s instructions). 

Suspended growth systems typically check the mixed liquor suspended solids (MLSS) in the aeration chamber using a settleability test. A sample of the mixed liquor is taken from the aeration compartment (see manufacturer’s instructions on the depth to draw sample from). If the system has a timer to control the aeration system, it is important that the aeration system should operate for a minimum of 10 minutes prior to collecting an aeration sample to obtain a more representative sample. The sample size should be approximately 1 quart. To perform this test any tall, straight-sided, clear glass container, about 1 quart in capacity, will be needed. Divide and mark the container into 10 equal parts using a waterproof marker, each mark should signify approximately 10% of the container. Allow sample to stand for 30 minutes and then measure sludge volume by locating interface between clarified effluent and settled sludge using the marks. The interface should be between the 20% and 60% marks, indicating proper operation. The sludge layer should be a light brown in color and full of small particles resembling pieces of sponge. When the sludge volume in the plant aeration compartment reaches 60% to 80% it is time to pump the aeration chamber.  Where removing the sludge, note that some mixed liquor needs to be left in the aeration chamber to restart the system. Refill the tank with water to assure the tank does not float.

Above are MLSS samples immediately after collection. Below are the same samples after 30 minutes.
Above are MLSS samples immediately after collection. Below are the same samples after 30 minutes.

Fixed film systems more commonly settle out the solids at the bottom of the tank below the media. With these systems a sludge-measuring device is used to find the level of buildup at the bottom of the tank to determine how close this layer is the bottom of the media. You must know the tank and unit dimensions to determine the depth of sludge allowable before pumping is required. If tank cleaning is required, be sure to access the tank as recommended by the manufacturer. Refill the tank with water to cover the media and assure the tank does not float.

Some ATUs could have a separate compartment after the aerobic portion to settle out dead bacteria and other suspended solids. Typically, these tanks have a pump in them to return effluent back to either the septic tank or aerobic tank. This tank should be opened and sludge measured to determine if the sludge return pump is working properly, and pumped if the sludge depth is more than one-third of the liquid depth. The operator or designer should be contacted to determine if adjustments are needed.

Depending on the size of the ATU and the users’ habits, pumping may need to be performed on an annual basis, but frequency is highly dependent upon use. It could be a shorter time in heavily used systems or a longer time on systems lightly used. Pumping aeration too frequently will have a negative impact on performance as when you remove all of the biomass it takes time for the system to recover and resume proper treatment. It is advised to check with the manufacturer for more specific requirements regarding pumping of the ATU. 

About the author
Sara Heger, Ph.D., is a researcher and educator in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota, where she also earned her degrees in agricultural and biosystems engineering and water resource science. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is the President-Elect of the National Onsite Wastewater Recycling Association and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system design, installation, maintenance and operation by sending an email to kim.peterson@colepublishing.com.


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