I recently addressed biomat development and its importance in treating septic tank effluent. In gravity distribution systems, the biomat distributes effluent across the soil treatment area. Until the biomat is fully developed, treatment efficiency is reduced due to preferential flow patterns in natural soil. To avoid this time lag in treatment and to better manage and control the application of effluent, pressure distribution is used.

By adding a pump and pressure distribution network to the system, sewage effluent is applied over the entire infiltrative surface of the soil treatment area from the outset without relying on development of a biomat for distribution. By spreading effluent across the entire system (space) and time (number of applications per day), treatment can be enhanced by ensuring unsaturated flow through the soil and maintenance of aerobic conditions without appreciable biomat development.

FOUR DOSING STAGES

A typical dosing cycle in a pressure distribution system has four stages (how effluent is applied to the infiltrative surface). 1) The pump turns on and the piping is filled with effluent (pressurization), 2) when pressurized effluent is delivered out the holes or orifices, 3) when the pump shuts off effluent drains out of the piping, 4) a resting stage when the applied effluent moves into and through the soil by gravity. The resting stage promotes unsaturated flow through the soil and the presence of oxygen in the system. Just as under a biomat, presence of aerated (with oxygen) soil is a key to treatment.

In any system – gravity or pressure distribution – designers should use both hydraulic and organic loading rates. For most residential systems, an assumed level of treatment in the septic tank from an organic load standpoint is assumed within the hydraulic loading rate. However, the actual organic load can affect how well any system performs. If actual organic loads are consistently higher than estimated, the impact on pressurized soil treatment systems and media filters becomes obvious. A biomat begins to form and effluent will begin to pond on the surface and not go away during resting periods.

This happens because our pressurized distribution systems are imperfect. Hydraulic and organic loading rates assume effluent is applied evenly over the infiltrative surface. We even say this:  Use pressure distribution to evenly distribute the effluent. What really happens is effluent is delivered to a localized area around the orifice. The more orifices, the better distribution. But there are obviously physical limitations to the number of orifices in the piping. A typical layout for a mound will have one orifice covering 6 to 10 square feet of area. Is distribution more uniform than gravity? Yes. But is it totally uniform? No.

PONDING PROBLEMS

Effluent is delivered to this more localized area. If it is too large a load hydraulically, you can have some localized saturated flow moving effluent deeper into the soil, perhaps resulting in less treatment. If the organic loading is too large in the localized area, a biomat can begin to develop. In fact, there is often some development evident even in well-designed systems.

In a previous column, I said the author of a paper maintained that a biomat provided no benefit to a septic system. I indicated that he was confusing and mixing two entirely different distribution systems that operate differently and need to be looked at differently. In the case of pressure distribution, presence of any extensive biomat development should be viewed as a problem.

A pressure distribution system uses a piping network with a specific configuration, with a set number of orifices to provide distribution across the area. The dosing and resting cycles spread the application out over the day. Design of the system should include not only the hydraulic and organic loading rates, but also the dosing frequency to spread the flow as evenly as possible. This will optimize system hydraulic and treatment performance.

When pressure distribution is used in a media filter, the same parameters are important. Here the system is in some type of box or tank with access. The distribution system within the filter is designed on the basis of hydraulic and organic loading with certain expected performance standards in terms of treatment.

When a biomat begins to form or has formed (usually evidenced by brown or black color on the surface) at the infiltrative surface, both acceptance and treatment are affected. In media filters, this means the media needs to be cleaned or replaced. In soil trenches or beds with pressure distribution – if ponding is occurring between dosing –something is wrong. In both cases, it is time to reevaluate design assumptions in terms of the loading rates and make necessary adjustments or additions to the system. Unlike in a gravity trench, any ponding in a pressure distribution system indicates the system is broken and needs to be fixed!