TECHNICAL ADVISORY COUNCIL
FOR
ONSITE WASTEWATER TREATMENT
ADVISORY STATEMENT
SAND FILTER MEDIA
INTERMITTENT SAND FILTERS (ISF’s)
BACKGROUND
Since sand filters were introduced in their present form in Michigan in early 1994, the industry has struggled somewhat with finding the correct sand media. The M.D.O.T. Specification for 2 NS sand was first thought to be the appropriate material. But early experience with this material proved troublesome. The specifications for 2 NS have been found to be too broad, allowing a gradation with either too many fines, or too high a uniformity coefficient. In either case, the end result may lead to premature plugging of the media in the filter. It has been found necessary to be more specific than the 2 NS specification; and there is no other written standard or specification in Michigan that will work.
The Technical Advisory Council has spent considerable time discussing and researching this issue during the last year. A number of aggregate suppliers were contacted to determine the availability of different media, and a variety of answers were received. Some indicated no problem supplying whatever we wanted, and others could not. Up until now, most sand filter contractors and designers have simply searched to find what was currently already available. The TAC feels that we need to publicize and distribute what we feel to be the ideal media with the hope that suppliers will start processing this material for use.
MEDIA RECOMMENDATION
With the above stated goals in mind, the TAC is recommending that the attached specification be used for sand filter media in all INTERMITTENT SAND FILTERS (also known as SINGLE-PASS SAND FILTERS). This media specification was previously published by Orenco Systems, Inc. for intermittent sand filters with higher loading rates. The TAC is recommending that it be used in all ISF’s.
The TAC recognizes that in some parts of the state there may be difficulty in getting this specific media. It is our hope that with the publication and adoption of this recommendation suppliers will step forward to process this material. However, the raw materials are not always available. For this reason, we are including in this recommendation the following discussion on the factors to consider when you are thinking of using other media gradations.
FACTORS TO CONSIDER
If a sand filter media is chosen that is “too fine”, there is a danger of plugging the media with solids that build up due to slow permeability. If a media is chosen that is “too coarse”, the wastewater will pass through it too rapidly and treatment will not be as good. In the later case, solids may wash through the filter and pass on to the drainfield, plugging the soil interface area. It is therefore important to obtain the correct media.
However, other design choices that are made may have as significant an impact on the filter performance as the choice of media. In fact, these other design choices can be used to compensate for the lack of the ideal media. The designer must become familiar with the relationship between media size, loading rates, method of effluent distribution, timed dosing, dose volumes, and even depth of cover over the filter --- and how each impacts the other --- such that the proper design choices can be made, if needed, to compensate for construction constraints in a particular case.
Researchers have concluded that best treatment in a sand filter is obtained by achieving slow, unsaturated “film flow” of the wastewater downward over the surface of the media particles. This is accomplished with small “micro-doses” that just wet the surface of the media at frequent intervals, but do not flood the pore spaces surrounding the media particles. Programmable timer settings, lateral design, hole size and spacing, etc., as well as media size, are all inter-related in the effort to accomplish this. (For further information on effluent application rates, dosing size and frequency, the reader is referred to an article by Robert W. Emerick, Rebecca M. Test, George Tchobanoglous, Ph.D., P.E., and Jeannie Darby, Ph.D., P.E., entitled “Shallow Intermittent Sand Filtration: Microorganism Removal”, in the Winter, 1997 issue of The Small Flows Journal, Volume 3, Pages 12 - 21)
CLEAN STONE
Stone that is not clean on the top of a sand filter can lead to filter failure through plugging of the stone/sand interface. Dirty stone around the sand filter underdrain and within the drainfield can lead to clogging of the stone/soil interface in the drainfield. IT IS IMPERATIVE THAT CLEAN WASHED STONE BE USED IN THE CONSTRUCTION OF EVERY SAND FILTER. ACCEPT NO SUBSTITUTES. Many times stone is delivered to the site clean, but it is contaminated in handling at the site. Contractors should be advised of this danger, and educated on how to handle stone so that it does not become contaminated during construction. In fact, this same practice should be followed in all drainfield construction.
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NOTE: The media recommendation contained herein is for INTERMITTENT (SINGLE-PASS) SAND FILTERS ONLY. No attempt is made here to address the media size that should be used for recirculating sand filters (RSF’s).
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This recommendation adopted by the Technical Advisory Council (TAC) for Onsite Wastewater Treatment at their meeting on July 15, 1998, and approved for immediate distribution. For further information, contact Chuck Cousino, Chair, TAC, 3121 E. Grand River Ave., Howell, MI 48843; (517) 546-4836.
