Recent questions about pressure distribution systems include: What are some of the operation and maintenance issues? And, what should we look for and how do we assess these systems?

Pressure distribution systems are more complicated than gravity-fed systems when it comes to inspections and determining operation and maintenance requirements. This is due to the addition to the system at a minimum of a pump tank and a pump to deliver effluent to the pressure distribution layout.

The pump and distribution laterals are designed to operate as a unit. This means the pump must be capable of delivering the amount of wastewater necessary to pressurize the system at a given dynamic head based on pipe size, lateral length and number and size of perforations in the laterals. The pump tank needs to be large enough to accommodate the necessary amount of effluent and if the pump wears out and needs to be replaced, it cannot be replaced by any old pump.

Adequately servicing or inspecting the system requires the provider to either obtain this information from the original design and installation documents or determine them in the field. This is one reason I and others in the industry have suggested homeowners are provided with an operations manual that provides system design and operation information. A few health departments currently require this kind of information be provided with the intent it be transferred with ownership changes.

INSPECTION TIPS

The following are some things to consider when evaluating a pressure distribution system. Since each system is unique in terms of quality of effluent delivered, elevation differences across systems and lateral layout, each component needs to be well documented.

Determine if the effluent is septic tank effluent or if there is an additional pretreatment component (ATU or media filter). This should impact the size of perforations in the lateral and how often laterals may need to be cleaned. Where septic tank effluent is delivered, 1/4-inch perforations are generally used because it delivers more effluent through the perforation 

to avoid plugging. With aerobic effluent, 1/8-inch perforations are common. With smaller perforations the laterals extend longer or the size of the pump is reduced.

As with any system, use your nose and eyes to evaluate around the drainfield area for signs of odors or surfacing effluent. Check for the presence of caps on any inspection ports or access ports to the distribution laterals. Missing caps are a potential source of odors and/or areas where water or animals may enter and interfere with system operation.

In the pump station, check that the supply line drains back freely when the pump shuts off. Usually there is not a check valve in the system, although there may be one where duplex pumps are used. Measure the amount of effluent delivered when the pump runs. Keep a record of this number for future visits.

Some systems have switching valves. They are used to automatically or manually divert flow to different parts of the system. Make sure these valves are operating correctly and keep a record of which laterals are in operation and determine the sequencing interval.

Locate the end of each lateral line and measure the distal head (squirt height) at the end of each lateral. The height should be the same at the end of each lateral for equal distribution. The height should be recorded and changes over time may indicate plugging (height increases) or the pump is wearing out (height decreases).

If there are inspection ports to the infiltrative surface of the trenches or bed, evaluate whether there is any ponding. If there are no inspection ports, they should be installed. In pressure distribution there should not be any ponding other than temporary ponding when the system is running. If there is continuous ponding and total biomat development at the infiltrative surface, something is wrong and a cause must be determined before the system is in total failure.

ACCESS IS IMPORTANT

One advance in the industry is providing maintenance access ports to the distribution laterals. They are now required in most areas for pressure systems above or below ground. It is how you determine squirt height and any system changes. It is also the access point to periodically flush and clean the laterals (done on a frequency of one to three years). Laterals with smaller perforations need to be cleaned more often.

Cleaning lines can be accomplished several different ways: Jet the lines with water; use a plumber’s snake with a bottle brush attached; vacuum the lines with a vacuum truck. Do this procedure one lateral at a time.

What if there is no access to the laterals? To properly take care of the system, this access should be added. A reader who asked the question indicated sometimes he encounters systems under blacktop. Most regulatory agencies require this access. Check your local and state regulations as always. In my part of the world, we would not put systems under blacktop for a variety of reasons. One is the freezing potential and the other is cutting off oxygen exchange with the system.