Stop Short on Trenches?

More research is needed to determine whether new distribution media products require smaller drainfields

Question:

Some manufacturers of newer types of distribution media say the size of a drainfield trench can be reduced over traditional installations using rock because the distribution media is more efficient. I have been doing that and the trenches seem to be working OK. Do you think it’s OK to reduce the length of trenches from what would be required for rock?

Answer:

I think the jury is still out on your question. Unfortunately, there was not a great deal of research available to establish the original numbers we now use for sizing soil absorption systems. However, those numbers have been used for many years and are still generally accepted for a rock-filled system. This system would use 3/4-inch-plus rock as the distribution medium.

There are a number of manufacturers selling distribution media other than rock for drainfield trenches and other soil absorption systems. Most manufacturers offer various technical reasons why the size of the soil absorption system can be reduced when using their distribution medium. However, I am not aware of any physical information on the actual performance of these various systems.

Monitor the system

It is simple to monitor the performance of any drainfield trench system. If the new systems were monitored, the manufacturer of the distribution medium would have performance field data valid for the sizing of future systems. The procedure would be to first carefully analyze the soil texture where the trench system was to be installed.

The total length of trenches designed for the system should be divided into a number of shorter trenches. The shorter the individual trench length, the more accurately the performance could be evaluated. Effluent should be fed into the trenches using sequential distribution with a drop box at the head end of each trench.

The first trench in the system will be filled to maximum treatment capacity before any effluent will flow to the second trench. A drop box at the head of the first trench is required for this to happen. When the first trench is full, the extra effluent flows to the second trench, and so on. Each trench must have an inspection pipe at the far end so the effluent depth can be measured and recorded.

Calculate flow

When the onsite sewage treatment system reaches equilibrium, the total amount of trench length being used can be measured and recorded. Trench length is only part of the information required. The other essential information needed is the amount of sewage flowing into the system. The amount of sewage flow will vary from day to day, so an average daily rate of flow will need to be calculated.

The sewage flow rate can easily be measured with a simple water meter. The meter is connected into the water system piping to measure only the water used in the home. The piping for outside faucets should bypass the water meter. On a new house, such piping can easily be arranged. It may be a little more difficult on existing water system piping.

The water meter does not need to be of the quality used by cities. A water meter having an accuracy of 1 percent is adequate. Such water meters are likely available for less than a hundred dollars.

The object is to get the average daily rate of flow into the sewage treatment system. The other information is the amount of drainfield trench being used.

Water meters required?

The average daily flow rate, together with the length of trench being used will give an important design value. This will be the number of gallons of sewage per day per foot of trench. And this is the value the manufacturer of distribution media should have for their product.

The same procedure should be used for drainfield trenches using rock. Unfortunately, again there is not much valid data on trench performance using drainfield rock.

Most of the distribution media manufacturers use the design table for rock-filled trenches. They reduce the area required by an amount that makes their product price competitive with rock.

I understand some states now require water meters be used with new types of distribution media. I don’t know if there is a reporting system set up to collect valuable data for the onsite industry. It would be a good idea to involve the local permitting agency when collecting performance data on any onsite system. This would make the information public and not subject to question sabout accuracy.

You stated the systems you installed with reduced area were working “OK.” To me this simply means the amount of sewage flow is not greater than the amount the system will treat.

When an onsite sewage treatment system is designed, the average daily sewage flow rate must be established. Most onsite codes use the number of bedrooms to predict the sewage flow rate.

It is typical to use an estimated sewage flow rate of 150 gpd per bedroom. If a four-bedroom house is built, the estimated daily sewage flow rate will be 600 gpd. If the house is occupied by a couple who are both employed and have no children, they will not use an average of 600 gallons of sewage per day.

And, let’s say a reduced area soil absorption system is installed for this house. Of course the system will work “OK” since the sewage flow rate is considerably reduced from the design value. But this doesn’t necessarily mean the reduction in area from rock sizing is valid. It simply means the reduced area drainfield is working because the sewage flow is reduced from the value used to size the system.

Better performance evaluation

Research I was involved with at the University of Minnesota measured sewage flow rates from 10 homes. As I recall, the average flow rate from those homes was about 60 percent of the amount used to size the trench system. Reduced area systems would have worked well for those homes because the sewage flow rate was reduced.

In summary, the onsite sewage treatment industry needs to do a much better job of monitoring and evaluating the performance of all onsite systems. Accurate performance data is needed for the design of future innovative systems.



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