Jim Anderson

Over the past few months, I have received a few questions and comments that pertain to pressure distribution systems. I have discussed what makes a good pressure distribution system and the advantages to using these systems. By spreading the effluent out during the day (time) and across the entire treatment area (space), treatment efficiency can be increased and provide a system that will last indefinitely.

The downside of using pressure distribution is there are more design and installation factors to consider. If the system is not designed or installed properly, it will likely fail prematurely and be relatively more expensive to repair or replace. To use pressure distribution requires not only the usual understanding of site and soil conditions, but also an understanding of pumps and flow through the supply and lateral lines. If mistakes are made in pump sizing and design flow rates through the piping, problems will usually show up very quickly.

This is the description of a problem site during a real estate time-of-sale inspection: “I was recently contacted by a homeowner selling his home. His potential buyer had contracted another septic company to perform an inspection of the current system, which is original to the dwelling and installed sometime in the early 1990s. The absorption area consists of four in-ground pressure-dosed trenches, and the inspection showed the uppermost (highest elevation) trench had 6 inches of effluent in the aggregate, the next lowest had fully saturated aggregate, and the lowest two each had dry aggregate.”

UNEVEN DISTRIBUTION

For purposes of a real estate transfer, it’s my opinion this system would be unacceptable because it is not working the way a pressure distribution system should work. The flow is ending up in only two of the trenches and more is ending up in the second downslope trench. Since effluent is not distributed equally — and anytime effluent is ponded in a pressure trench or bed — the system is broken. The conclusion of a broken or nonworking system should be straightforward. What is not straightforward is determining the problem and a solution so the property can be sold.

There are several possible reasons for the condition found by the installer. Determining the reason for the condition requires additional research and exploration. A good place to start is to obtain the initial design. If the person selling the house was the owner at the time of system installation, they should have a copy of the initial design and the as-installed diagram. The homeowners probably don’t know, but they should be asked.

Next, the county or permitting authority office should have a copy of the completed system design and installation. Since this system was installed almost 30 years ago, they may not have one either. In that case further on-site investigation should be conducted to see if there are additional pieces to the system, such as a pressure manifold vault or use of flow-switching devices.   

Designing and installing pressure distribution systems where distribution laterals are at different elevations is more complicated than designing a pressure bed (such as in a mound system) or series of trenches where the laterals are at the same elevation. In my opinion, it means designing laterals with different perforation diameters to compensate for the pressure differences due to the elevation changes. The goal is to deliver the same amount of flow within a 10% variation to each of the trenches; this is considered even distribution.

Since the system was installed in the 1990s, it is likely this was not the design; instead, it was probably designed with flow to the trenches controlled by a series of valves to be adjusted to supply the proper flow. In one potential scenario, flow in the trenches is not under pressure at all! In this situation, effluent is pumped to a manifold and delivered to the trenches either in parallel or sequentially controlled by valves in a manifold vault.

I have often heard people say they have a pressure system because they have a pump. They really have a pump-to-gravity situation. The trenches will act like any other set of gravity trenches where a biomat will form and effluent will pond in the trench.

SOMEONE SHUT OFF VALVES

The installer indicates the dose tank was located uphill from the manifold, which was in the center of the trenches. It appears the goal here was not to deliver by gravity but to pressurize the entire lateral system; and for some reason, in either the gravity or pressurized scenario, effluent is not being delivered to all trenches.

A common scenario I have seen is where two of the valves were shut off. This may have been by design to rest one-half the system and switch to the other half on a set schedule. They may have been closed by a previous service provider during service and not turned back on. Another possibility is that the homeowner performed system management and did not follow through on the switching schedule. In any case, the result would be similar — more effluent would end up in the lower trench, leading to ponding and eventually surfacing.

Whatever the case, the correct call here is the system is not operating the way it should. The system needs to be fixed to operate the way it was designed or replaced. The buyer and seller should agree about how the system is going to be fixed and who is going to pay for it before the property changes hands.