Last month we discussed how, in gravity systems, effluent distribution over the area of a sewage treatment trench is accomplished by the biomat. Full treatment efficiency is not obtained until the biomat has developed over the infiltrative surface of the trench; the entire area is used throughout the day. If pressure distribution is used to deliver effluent to the trench or bed, effluent is spread across the entire system area from Day 1 of system operation.

Pressure distribution involves the use of a pump to pressurize the distribution network and ensure equal amounts of effluent are delivered throughout the network. The pump controls the amount and time effluent is applied to the soil infiltrative surface. There are two ways effluent can be applied under pressure: on-demand or time dose.

On-demand dosing occurs periodically during the day and is determined to a large extent on when water is used in the house. The pump runs and delivers a set amount of effluent to the system whenever effluent fills the pump tank to a level typically based by some fraction of the estimated daily sewage flow.

The amount is often set at one-quarter the estimated daily sewage flow. For a four-bedroom house with an estimated sewage flow of 600 gpd, each dose would be set to deliver 150 gallons. Demand-dose floats are used to set the amount of dose to be delivered. The reasoning behind this is the pump would run on average four times a day and the flow would be spread out during the day. In between each dose, effluent would have time to run through the soil by unsaturated flow across the entire system, providing both acceptance and treatment.

This does provide better distribution of effluent through the day than gravity flow, where effluent is delivered every time water is used in the house; it does not spread flow out during the day. Each time the pump runs it delivers 150 gallons regardless of the time of day. We know water is not used evenly during the day; with higher use in the morning before everyone goes to school or work and then again later in the day for dinner and baths.

Look for overload

There is potential to overload the ability of the soil to accept and treat effluent in vicinity the orifices where effluent is delivered, which can result over time in excess development of a biomat and reduced treatment. In a worst-case scenario, the system could be consistently overloaded with more than 600 gpd.

The only way the homeowner would know this was happening is when the service provider would check a cycle counter to determine how many times the pump ran, or more likely, when the system begins to have hydraulic problems evidenced by surfacing of effluent in the soil treatment area.

To improve distribution of effluent over time and to have some additional controls and alarms built in, a timer can be used in conjunction with a high-water alarm to spread the flow out more evenly during the day. In time-dose, an adjustable timer is used to control pump run time (pump delivers a certain amount per minute), pump-rest interval and specific dosing amounts based on overall flow. Water until the specified time to deliver effluent. This eliminates variations in flow by applying equal amounts over time.

If additional water is being added to the system, either intentionally (family using more water than estimated) or unintentionally (leaky fixtures, water infiltration), this system that requires storage of water would trip the high-water alarm consistently. This would cause the homeowner to recognize more water is going out for some reason and hopefully adjust overall use or fix any leaks before damage was done to the soil part of the system.

The right pump matters

Additional comments about pressure distribution: Orifices do not deliver equal amounts of effluent unless the entire piping system is fully pressurized. Until the pump fully pressurizes, the system flow will not be even across the distribution system. Similarly, when the pump shuts off it takes time for the flow to stop, which provides potential for uneven distribution.

After flow out of the pipe stops, effluent moves by gravity through the soil under unsaturated flow. Time between doses is important to allow effluent to move into and through the soil to provide treatment. Matching the pump to the pressure distribution network is critical. The pump needs to fill the piping quickly, then deliver the dose under pressure and shut off quickly, allowing the pipe to empty. Not any old pump will do when the pump needs to be replaced; it needs to be compatible with system design.

A final word about pressure distribution: If the trench or bed in a pressure distribution system has effluent ponded, it is broken! For it to work properly requires effluent to have moved into and through the soil before the next dose. If there is standing water, the cause needs to be determined and rectified.