Recently I had a question from a reader about a shallow trench pressure distribution system. He is a landlord and his tenants inadvertently turned the pump switch off. When they realized what they had done, they turned the switch back on, delivering an estimated 1,200 gallons of effluent in one dose to a system designed to handle 450 gpd. Almost three days of sewage flowed in the time it took the pump to deliver the effluent.

The reader says the area at the end of the pressurized trenches became soggy and some effluent bubbled to the surface. He wondered if there was anything he could do to help the system recover. My suggestions were to give the system some resting time by pumping the tanks and having the renters reduce their water use for a few days to give it some “drying” time. Then he should keep a close watch on the system to see if it was handling the daily flow without effluent ponding in the trenches.

REVIEW FLOW DISTRIBUTION

This situation led me to think it was time to address pressure distribution and the dosing methods that can be used to manage flow of effluent to the system. Pressure distribution systems are designed to deliver effluent to final soil treatment area over time and space, whether the area is in-ground trenches, aboveground mound, or at-grade systems.

Delivering effluent over the entire soil treatment area (space) and at prescribed doses during the day (time) provides more efficient distribution and treatment from the first moment effluent is delivered to soil. This is opposed to gravity distribution where maximum treatment does not occur until the biomat has developed to slow the flow throughout the treatment area. In pressure distribution, the pump controls the amount of effluent delivered to the soil infiltrative surface, ensuring unsaturated flow to provide the treatment necessary.

Two dosing methods can be followed to control the pump: on demand and timed.

Over the years, a lot of the design work I did with pressure systems was for sewage treatment mounds and we specified a demand dose regime. A set of floats (or other water level sensors such as transducers) in the tank turn the pump on when the liquid in the tank reaches a certain volume and turns the pump off when the desired amount of effluent is delivered to the system. In the past, the pump plug was piggybacked with the float plug. This provides little control and no record of usage. When working on these, the service provider should recommend upgrading to a control panel.

Typically, in an on-demand system, the pump floats are set to provide up to 25 percent of the estimated daily sewage flow each time the pump starts. This serves two functions: It helps to spread the flow out during the day, and it does not allow all the effluent to be delivered in one continuous dose, maintaining unsaturated soil conditions. A high-water alarm float is included in this system to alert the homeowner and service provider when the pump has not run and the water level in the tank exceeds the normal operating level.

This method has been used extensively in the past, and it provides some control over delivery of the effluent. But there is nothing that ensures the pump runs at evenly spaced intervals during the day because pump operation depends on when water is used in the home to the degree that it fills up to the required pump volume.

In many homes, larger volumes of water are used in the morning before work and school and then again when everyone is home in the evening. In between, no effluent is delivered to the system. If one of the goals of pressure distribution is to spread flow out over time, this provides uneven performance at best. In addition, unless a cycle counter or water meter is built into the system, there is no history of how much water is delivered. This makes troubleshooting a problem more difficult for the service provider.

IMPROVE MANAGEMENT

With today’s advanced pretreatment units and the recognition that system longevity can be improved through proper management, a timed-dosing method is more desirable. By storing peak flows and specifying pump runtimes for the desired volume, effluent can be delivered to the treatment components evenly throughout the day. This is also done using floats (or other sensors), but through a control panel to build in more controls and options.

There is the high-water alarm; but also the possibility to enable an unexpected peak flow to be delivered that shortens the time between the pump running. Controls are set to avoid the reader’s problem where nearly three days’ volume of flow was delivered in one continuous pumping. They will not allow more than the design flow for the system. The pump is shutdown and water continuing to rise in the tank will trigger the high-water alarm.

Redundant off floats are often provided. This is a single float where if the water level in the tank drops to the level of the pump, it will not let the pump run according to the timer schedule. This protects the pump during times of little or no flow. Although a timer override float can be installed to allow the system to operate as an on-demand system at times of high flow, this should not be done. Having this built into the system defeats the purpose of the timed dose and can lead to hydraulic overloading just as the renters did in our reader’s situation.