QUESTION:

What can cause a media filter to go anaerobic?

 

ANSWER:

The person asking this question explained he installs and services generic sand filters, and that one has been a particular problem. After about six months of operation, the water “just quits going through the sand and ponds on top.” He further explained that while the sand was somewhat discolored, there was no biomat evident.

As with most troubleshooting, there are a number of possible causes and you have to work through them to reach a solution.

A generic or public domain single pass sand filter is typically confined in a watertight container or liner. Effluent from the septic tank is distributed through a pressure distribution network over the sand. This moves the effluent uniformly across the filter surface; maintaining unsaturated flow through the media and promoting aerobic treatment.

Single pass sand filters are effective at removing BOD-5, Total Suspended Solids (TSS), bacteria, and nitrifying the effluent. After effluent flows through 2 feet of sand, it is collected in an under-drain system and delivered to the soil for final dispersal, either through gravity or pressure distribution.

 

START THE INVESTIGATION

Several areas should be investigated when problems arise with sand filters or media filters in general. Start by evaluating the amount of effluent being delivered to the filter to determine if the filter is simply being hydraulically overloaded. Single pass sand filters are usually designed to accept 1.0 gpd per square foot of filter area. To put this another way, to handle the estimated sewage flow from a four bedroom house (600 gpd) requires a sand filter 10 feet by 60 feet to provide the required 600 square feet of treatment area.

The problem may simply be the result of using too much water. Since a pump is involved and the most common method of delivery is through use of a timer, check the run time to determine how many gallons are delivered to the system.

Another common failure is that the system is being organically overloaded: The strength of the septic tank effluent is too high, causing the pores to become plugged as a biomat develops at the filter surface. If the BOD-5 of the effluent exceeds about 150 mg/l, and the TSS is greater than 60mg/l, or fats, oils and grease (FOG) exceed 30 mg/l, the filter may not be able to adequately treat the waste. This could indicate that the septic tank needs to be cleaned more frequently and/or water use and waste generation need to be changed.

Next is the quality of the sand being used as filter media: If the sand does not meet design specifications, the pore size between the sand grains may limit wastewater movement and air diffusion through the sand. Troubleshooting this requires taking sand samples; having a laboratory determine the particle or grain size distribution, and comparing those values with the sand specified in the design.

Two key specifications should be compared. They are effective size (ES) and the uniformity coefficient (UC). ES is the grain diameter at which 10 percent of the sample by weight is finer (ES=D10). Uniformity coefficient is the diameter at which 60 percent of the sample is finer (D60) divided by the D10 number. This provides a measure of the uniformity of sand grain size. If D60 and D10 are the same number, the UC is 1. As the UC moves toward 5, the sand has a variety of grain sizes.

 

WATCH FOR DRAINAGE

A larger value indicates less uniformity and less pore space. The grain size curves obtained for the samples should be compared to the values for the sand specification in the design. If they don’t match, this could be the source of the problem.

Drainage from the bottom of the sand filter is another problem area. Typically there is a gradation of rock placed around the drainage pipe: 4-inch Schedule 40 PVC with 1/4 inch slots spaced 4 inches apart and facing up at the 12 o’clock position. Around and 2 inches over the pipe the rock should be 0.5-1.0 inch in diameter, clean and washed. Over the top of this, 2-3 inches of pea rock 3/8 inch in diameter should be placed to prevent sand from entering and plugging the drainage pipe.

Filter fabric should not be placed over the rock. This can create a place for biological growth, plugging and limiting flow out of the sand causing it to go anaerobic. The outlet piping should be checked to make sure that flow out of the filter is not limited through a blockage, poorly laid pipe or other obstruction.

On single pass sand filters, the system cover presents another common installation mistake that can lead to an anaerobic filter. The idea of the sand filter (any media filter) is to provide aerobic treatment. Anything that limits oxygen exchange will cause the filter to work less efficiently.

Backfill over the system should not be tight clay soil, but rather a sandy loam type of topsoil that will allow for oxygen exchange. Since media filters are often used to help overcome soil problems – and tight clay soils being a common problem – one mistake to avoid is the use of clay material from the excavation as the finishing material.

One final note of caution: Prevent surface runoff into the sand filter. Any excess water entering the filter can contribute to hydraulic overload, causing the system to go anaerobic. Pay attention to the landscaping: Observe if there is runoff from roofs, driveways or other impermeable surfaces.