photo of a septic effluent filter system-photo adapted from US EPA

Signs of a Clogged Septic or Graywater Filter

This document discusses the diagnosis and repair of clogging of various types of septic effluent filter systems and graywater filter systems to protect the septic drainfield or drywell system from damage. These simple septic filters, such as greywater filters, washing machine filters, and filters at the effluent outlet of a conventional septic tank are all designed to protect the septic effluent absorption field or leach field.

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Signs of a Clogged Septic Tank Filter or Graywater Filter

Septic filters are placed either in the septic tank at the outlet tee or in a separate chamber outside of and close to the septic tank. Graywater filters may be placed at the inlet to a drywell or graywater disposal system. Washing machine filters are often installed at the clothes washer outlet hose. Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted.

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Before digging up a septic field or drywell or ordering a costly repair or septic system treatment, if your septic tank or greywater system use a filter, check for the following signs that the septic filter is clogged and needs cleaning:

  • Sluggish drainage in the building; if there is an access port to inspect the sewer line between the building and the septic tank, open it and flush a toilet. Does the wastewater run normally (in a rush, not a trickle) through the line to the septic tank?
  • High effluent levels in the septic tank: open the septic tank at an inspection or service port. Is the effluent level abnormally high, at the top of the tank or over the baffles?
  • Dirty septic filter: open the septic tank at the outlet end where the filter is probably installed; (Some systems may install an add-on filter in a small chamber just downstream from the septic tank, or upstream from a drywell which receives graywater. Inspect the septic filter for clogging and clean it anyway before returning it to the tank. If you see a sudden out rush of effluent out of the septic tank when you pull out the septic filter, then it's pretty likely that the problem was a clogged septic filter.
  • Drainfield abnormally dry: if the septic filter at the septic tank is clogged, effluent is being discharged abnormally slowly through it to the drainfield - which gives the drainfield a relief but is abnormal. If the septic filter is very clogged effluent may be leaving the septic tank underground at one of its service ports or covers - look for wet areas around the septic tank. If the drainfield becomes saturated and appears to fail after cleaning the septic filter (because you've now suddenly released a large volume of wastewater into the drainfield), further investigation of the drainfield condition and its piping is needed.
  • Septic filter monitors are available from some effluent filter manufacturers to help determine when it's necessary to clean the filter.


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