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Effluent and Vent Filters

Figure 1. Placement of filter in tank

Effluent filters are devices that can be affixed to outlets of septic tank and grease trap as pictured at right (Figure 1). The filter is a primary screening barrier designed to reduce the volume of solids passing out of the tank and through to the soil absorption system (SAS). If you were to pour unfiltered effluent from a septic tank into a clear glass (yuk!) and hold it up to the light, you would see that there are many fine particles of organic matter (and some inorganic material like fine grit) floating around. These particles, some barely visible to the human eye (and some that aren’t) are referred to as suspended solids. The measure of their abundance is referred to as Total Suspended Solids or TSS. Average TSS values from residences is 60-120 PPM. When these particles pass out of the septic tank into the leachfield, they settle in the small spaces between the soil, reducing the capacity of the soil to drain away the effluent. Given enough time, and aerobic (free exchange of oxygen) conditions, many of these organic particles break down into the basic components of water, carbon dioxide, and other simpler compounds. If too much of this organic matter is deposited on the soil interface however, the soil spaces clog and ponding of the effluent in the leachfield occurs. This results in anaerobic (no oxygen) conditions which further impedes the complete breakdown of wastes.

By retaining more of the suspended solids in the septic tank and reducing the amount of organic material that “demands” oxygen to breakdown (technically this is called reducing the Biochemical Oxygen Demand or BOD) that passes into the leachfield, the performance of the leachfield in breaking down waste can be improved. This results in a longer leachfield “life”. The goal of a good effluent filter is to do exactly this – prevent the passage of suspended organic and inorganic materials into the leachfield, while not impeding the flow of effluent to the point where it backs up into the building. Effluent filters perform this function by providing either screens or directing the flow across areas where the suspended material becomes trapped or settles out.

Figure 2. Effluent filter located outside the septic tank or grease trap in a separate filter chamber.

The effluent filter is most commonly a simple device that fits into the discharge tee of a septic tank as pictured here. For a household, a 4-inch diameter filter is used. New tanks can easily accommodate the filter installation, while previously installed tanks can often be retrofitted. In some cases, where an effluent filter is desired but a precast baffle is in place (as opposed to a PVC sanitary tee) it may be necessary to install a filter chamber along the outlet pipe between the tank and D-box. This is illustrated in Figure 2 . In addition to acting as a filter, the effluent screen acts as a substrate on which organisms can grow and digest the trapped waste. The mass of organisms and trapped waste eventually grows on the filter to the point where the weight causes it to slough off into the tank below and undergo subsequent anaerobic digestion.

There are a variety of filter sizes to accommodate facilities’ daily design flows. Large systems may require multiple filters going through a manifold arrangement in order to meet the daily flow rate as shown in top view in Figure 3 By utilizing a manifold configuration with the appropriate filter size(s) any tank or grease trap’s daily design flow can be accommodated.

Figure 3. Illustration of a manifold arrangement of effluent filters used to accommodate large daily flows (illustration modified from promotional material from Zabel Environmental Technology).


Until recently, most of the information on the effectiveness of effluent filters has been intuitive and anecdotal. Promotional literature from one of the approved effluent filter companies, Orenco Systems Incorporated (OSI) reported that the average TSS from their filter is less than 30 PPM. TSS levels in unfiltered effluent range from 60-120 PPM. Zabel Environmental Technology’s model A100 (a larger residential unit) in tests performed by Tennessee Technological University averaged a 49% reduction in TSS and a 32% reduction in BOD. The actual performance in any particular situation will depend on a number of factors, the most important of which is daily flow. Both Zabel and OSI provide sizing-criteria charts so that you can size your filter appropriately.

State Approvals

As you might guess, if it has to do with the septic system, it needs a state approval. All three companies shown in Figure 4 have approvals for various models of effluent filters that they market. In general, the approvals have the following conditions:

1) Prior to sale of the product, the owner of the system shall be provided with a copy of the approval and its conditions by the distributor of the filter.

2) Prior to installation in an existing system, the owner shall obtain approval from the Board of Health for the proposed modification of the system.

3) All septic tanks in which the effluent filter is to be installed must have a 18″ or 20″ manhole over the outlet tee (approval letters between the three manufacturers differ because of the date and code under which they were approved). In addition, both the inlet and outlet manhole covers must clearly note the system is equipped with the filter.

4) The filter outlet tee must extend below the flowline in accordance with a provided table (See Title 5 Section 15.227).

5) A Septage Handler, licensed by the local board of health must service the filter and pump the septic tank regularly – at least once every two years. Boards of Health should have copies of the approval letters for those models of filters that have approvals.

The popularity of effluent filters will undoubtedly increase in coming years. Since many of the upgrades to “old code” systems will allow the continuance of the 1000 gallon tank, many installers may rightfully advise owners regarding this relatively inexpensive accessory that can prolong the life of a leachfield. There are at least two towns in Massachusetts that require effluent filters for new construction and repairs (Pembroke and Duxbury). Their reasoning for the requirement is the belief that the filters help to save the SAS.

The servicing of effluent filters is relatively simple. The filter should be removed from its basket and rinsed down while being held over the tank opening. Care should be taken not to spray the filter growth onto surfaces that might be contacted by the unprotected person. In addition, the person servicing the filter should protect themselves from backspray. Washing filters off may become problematic in winter months when garden hoses are usually not connected. Servicing of the filter should only be performed by a licensed septic pumper familiar with the cleaning precautions and procedures.


Effluent filters are not totally restricted to gravity-fed septic systems. Filtered or screened pump vaults are fairly common nationwide and are gaining popularity in Massachusetts. Screened pump vaults (Figure 5.) as their name implies, are simply protective screened cages that surround the effluent pumps used in pumped-dosed or pressure-dosed systems. They are extremely important in situations where effluent is pumped directly from the septic tank (however this is a rarer situation in Massachusetts), as opposed to the more common situation where the effluent is pumped from a separate pump chamber. The screen type used in screened pump vaults is very similar to that used in effluent filters.

Figure 4. Popular models of effluent filters from each of the three manufacturers having approval in Massachusetts. In order Zabel Model A1800, Orenco Systems Incorporated various models, Flowlink Mfg. Company Model EFS 12.

Zabel Environmental Technology 10409 Watterson Trail Jeffersontown, KY 40299 Phone: 800-221-5742

Orenco Systems Inc. 814 Airway Avenue Sutherlin, Oregon 97479 Phone 503-459-4449

Flowlink Mfg. Company 7225 Pacific Ave., SE Olympia, Washington 98503 Phone 1-800-982-5393

In addition to screening at the pump, the installation of an effluent filter between septic tank and the pump chamber of dosed or pressure-dosed systems is certainly a prudent measure that can prevent passage of solids to the pump chamber (and ultimately the leachfield). This is particularly true for larger or commercial systems. In commercial installations, the filter may act as one more line of defense against the introduction of grease into the leachfield.

Figure 5. Simple schemata of a screened pump vault


The restriction on placing leaching systems deeper than 3 feet below grade or beneath driveways has caused a number of requests for variances from these provisions. This is particularly true in repair situations where the elevation of existing plumbing is difficult and expensive to change. To compensate for this deficiency, a number of system designers propose venting the leachfield. This is generally quite simple if you have enough room between the vent and a susceptible nose. Sometimes however, that telltale odor arrives just in time to cause complaint and ruin the appetite. To address this issue, charcoal filters that fit on the end of the vent are often used. These filters can be homemade or manufactured. The key ingredient is activated charcoal. Below is illustrated a commercially produced filter. Figure 6 is taken from promotional literature of OSI, but, as you can see, the principle is pretty simple. The essential characteristics of a good vent filter are that it does not restrict the airflow (hence the charcoal should not be compacted in the filter), it should be secure from weather, and it should be reasonably accessible for maintenance. As far as we know, no special permits are required for this accessory, nor are charcoal filters required on any venting system. They are really for that rare case when odor becomes a problem.

Final Note on Vent Filters

Our program has also received information on a product called SweetStack II®. This device apparently fits on the roof vent of your septic system. The manufacturer describes the “operating technologies” as “thermal absorption, conduction and convection; venturi and a three stage tirbulator”. A description on how the system works is presented in the promotional literature that can be obtained by contacting the address below. Again, in the rare event that vent odor from a roof vent becomes problematic, this product purportedly helps. It is marketed by Cape Cod Envirotech, P.O. Box 821, Denmark, Maine 04022 (Tel. 207-452-2842).

Figure 6. Charcoal filter for the end of a septic vent pipe.

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