When we think of pressure distribution, the first thing that comes to mind is the use of a series of pressurized laterals to distribute effluent over a rock bed and into the clean sand layer to distribute over the original soil surface. Use of these pressure distribution networks in mounds has been continuous since the early 1970s.

Today there are many more applications of pressure distribution networks – constructed directly in the soil in shallow drainfields, in the use of drip irrigation methods and in various media filters to provide pretreatment. Initially there was a lot of resistance to the use of pressure systems due to unfamiliarity with pumps and how they operate, and how to match the pump with the distribution network.

Now if systems have media filters, there is often a pump supplying effluent to a pressure distribution system in the filter and another to the pressurized laterals in the final treatment and dispersal area. So there is a much higher comfort level using this technology, not to mention better pumps and other equipment. It is always good, though, to stop and think about why we’re using pressure distribution and what is involved to avoid needless problems.

BUILDS FLEXIBILITY

Pressure is used to distribute effluent over the entire infiltrative surface, which can improve efficiencies of treatment in the soil, avoid being dependent on development of the biomat to provide treatment from day one of the operation, and give more flexibility in terms of when and in what quantities effluent is applied.

If the desire is to equalize the flow over both time and the area, a timer type of system is needed versus an on-demand system where the pump runs whenever a certain amount of effluent is generated. Using a timer eliminates variations in flow during the day, such as in the morning when everyone in the house is getting ready for school or work, and spreads it out more evenly. Of course, this requires some additional storage capacity, but eliminating large peaks and stresses on the soil treatment area helps systems last longer.

As mentioned earlier, dosing effluent can also increase treatment. A dosing cycle has four parts: when the pipes are filling; when they are full and effluent is discharging from all of the orifices; when the pump shuts off and the effluent drains out; and finally, the resting period between the doses where the effluent moves into and through the soil under unsaturated conditions to provide good treatment.

So think of the pump as acting like the biomat in a gravity system: It distributes the effluent over the entire area, and it provides unsaturated flow to allow oxygen to be in the soil to assist the aerobic soil organisms in treatment as well as breaking down the organic matter.

CHOOSE PUMPS WISELY

Discharge through the perforations is not equal until the entire network is pressurized. So the time spent under pressurized conditions should be longer than the filling and draining stages to make sure effluent is delivered evenly. A design figure often used and one that can be debated relative to treatment efficiencies is that the minimum dose volume should equal five times the volume of the distribution piping. Bottom line is the pump needs to be matched to the distribution system. Usually this is not a problem in new construction but something to be aware of when the pump needs replacement. Not just any old pump will do; it needs to deliver what is necessary for equal distribution.

In mound systems and most media filters, the pressure distribution network is laid out so the manifold and the laterals are level, which means they can be loaded from either end or in the middle depending on the type of system, the site, the elevation difference between the pump and the manifold, and other factors. In sloping sites, where the laterals are at different elevations, a shorter manifold is generally used, and it should be loaded from the top down to avoid excess water coming out of the orifices in the lower distribution laterals.

From an installation standpoint, a major decision is where the supply line from the pump will connect to the distribution manifold and laterals and how the line will be installed to avoid damaging the drainfield area.

UNIFORM DISTRIBUTION

One last general comment: Design of pressure systems is based on hydraulic and organic loading rates over the entire surface. But in practice, effluent spraying from the orifices will not cover the area uniformly. Typical low-pressure pipe configurations in mounds or shallow trenches have an orifice for every 6 to 10 square feet of surface area. The larger the surface area the more likely that there will be localized areas of saturated flow and even some biomat development, so numbers and spacing tied with the pump requirements becomes an important part of the design and performance of the system.

Pressure distribution uses a network of pipes, manifold and laterals loaded from a certain point with a specific set of orifices of a specific size to control the uniformity of distribution with respect to the area of the system. Dosing and resting cycles control the distribution of effluent over time. So that is why we say pressure distribution provides equal distribution over both time and space. How all these parts of the system match up with the hydraulic and organic loading rates will go a long way to determining system longevity and performance from a treatment standpoint.