Following a column about sequential distribution a few months ago, I have received some comments and questions about installation and operation of drop boxes for effluent distribution to sewage treatment trenches. I thought it would be a good idea to focus specifically on the boxes themselves and reiterate their use in sequential distribution at the same time.

A drop box has an inlet for the supply line pipe from the septic tank or previous trench, two potential outlets at the bottom of the box which connect to the supply pipe to the trench and an outlet that will connect to the next lower trench in sequence. Inside the box it is important to have a minimum 1-inch drop between the inlet and outlet. This ensures the effluent drops to the bottom of the box and move out of the pipes connected to the trench. The boxes should be installed so the invert of the outlet is at the elevation of the top of the trench it is supplying.

Some contractors and designers have suggested drop boxes be installed with the outlet invert an inch or two above the top of the trench elevation to provide a little head to help push effluent from the trench into the soil. It would give a little more infiltration; whether this is worthwhile depends on the designer’s preference and perceived benefits. An advantage of drop box distribution is that each trench operates independently. 

SOIL DETERMINES DISTRIBUTION RATE

The trenches are not directly connected so there is no hydraulic head from trench to trench. Effluent does not move more quickly into or through succeeding trenches because they are at lower elevations. This means each trench will accept effluent at the amount and rate determined by the soil. This is what allows sequential distribution to have trenches of different lengths. The soil determines what each trench will accept, and what it does not flows on to the next trench or trenches in sequence.

This means that with two trenches of equal length, the one with less permeable soil will accept less effluent and the one with more permeable soil will accept more. If because of system orientation one trench has more evapotranspiration in the summer, it will automatically accept more effluent than one of similar soils in the shade. Since they operate independently, trenches can be “turned on and off” based on their performance. It can be part of a regular management plan where differences are detected. It also allows an easy rotation between trenches over time if desired.

During installation, drop boxes must be properly bedded to remain stable during backfill and over time. Proper bedding can be achieved with a thin layer of washed rock on a level foundation or by compacting suitable native soil. The bottom line is the box needs to remain where it is intended over the life of the system. To achieve the distribution described above, the box can’t shift or settle. If it does, effluent may not be distributed properly.

Similarly, piping into and out of the box must be properly bedded to avoid settling and movement. Both inlet and outlet supply pipes from other parts of the system should have at least a 1% slope. In typical installations where the trenches are placed 8-10 feet apart, this is a 1-inch drop. This is to make sure the pipes are empty between water-use events to avoid problems with freezing. Properly bedding piping in the vicinity of the boxes will probably require some hand-work to avoid damaging either the pipe or the box.

Drop boxes and the piping must be watertight. This obviously prevents leaks, but of equal importance it prevents root intrusion. Drop boxes are constructed of concrete or poly materials. Both types come standard with pipe-penetration seals. Some seals accommodate pipes of different diameters. It is important to cut these carefully and according to the manufacturer’s recommendation so the box remains watertight.

DESIGN FLEXIBILITY

Where a pump delivers effluent to the first drop box in sequence, the pump discharge rate should be a minimum of 10 times greater than the water supply rate and lower than the rate effluent will leave the box into the trenches. In general, this means a flow rate between 10 and 45 gallons per minute. Flow into the box should be directed either to the side of the box without an outlet or against a deflection wall, baffle or other energy dissipater.

Upon completion of system backfill, the drop box should be covered with a minimum of six inches of soil and have an inspection port installed providing easy access to determine water levels in the box and trenches.

Use of drop boxes provides flexibility. If necessary due to increased flows, additional trenches are easily added if soil and site conditions allow. The system can be constructed on steeper slopes than other distribution methods. This flexibility allows systems to be installed in the best soils on the lot and at an ideal distance from other lot improvements or significant trees or other features the homeowner wants to maintain. Easy access allows quick inspection and the opportunity to manage individual trenches over time through periodic resting.