A typical onsite wastewater treatment system (OWTS) consists of a pretreatment tank or system, some method to distribute the treated effluent, and a soil treatment system. Pretreatment can range from simple primary treatment that takes place in a septic tank, where solids are separated from the liquid effluent and a limited amount of anaerobic treatment occurs, to more advanced aerobic treatment units and bio-filtration treatment systems. The soil treatment component can range from gravity lateral trenches, to pressurized networks of pipe in lateral trenches, to drip irrigation dispersal systems. Which soil dispersal system is needed for a particular site is often dictated by the suitability of the soils on that site, as determined by an onsite site/soil evaluation. Similarly, the best effluent distribution method for a site will depend on the soils, as well as the soil dispersal system and pretreatment selected. Except for flat-lot systems, where all lines are at the same elevation and connected at both ends, ‘gravity manifolds’ should never be used.

Methods that distribute effluent by gravity are relatively inefficient, especially when distributing flows that trickle from a sewage tank. Their use is limited to sites with deep permeable soils, which can provide acceptable final treatment and safely disperse the water back into the environment. Serial distribution systems use a step-down, or overflow line, to restrict flow out of each trench in the series until it accepts as much flow as possible before effluent flows to the next trench. In a serial distribution system all of the effluent is discharged to one lateral trench and for a period of time, part of that trench will be loaded at a very high rate. As bio-mat develops at the trench/soil surface, it reduces the rate at which effluent can move into the soil. As the bio-mat thickens, eventually that one trench cannot disperse all of the wastewater and some begins to flow to the next trench in the series. Unless clogged trenches can be taken out of service to rest, and to allow time for the bio-mat to degrade before putting the trench back into service, the entire trench system can be expected to eventually fail. A drop box can be used to manage a sequential distribution system and allows a trench (or trenches) to be rested.

Another method of gravity distribution uses a distribution box (d-box) to divide the flow between lateral trenches. However, distribution boxes are not always effective in dividing flow when it trickles out of a sewage treatment tank. As the number of trenches increases, their effectiveness decreases. There are some devices that can be used to help manage distribution through a d-box, and other devices can be used in place of a simple d-box, to improve distribution. These include insert devices for each outlet pipe, to help adjust flow, and d-boxes with a tipper bucket that collects the trickling flows before tipping and discharging about 1.5 gallons at a time. Regardless of the type of d-box used, system management can help insure the most equal distribution possible. An accessible watertight lid is needed to allow flow to be adjusted and outlets to be cleaned when necessary.

Dosing to a d-box and gravity trenches can also improve distribution. A large volume of effluent delivered in a short time ensures that each trench receives wastewater and that more of the trench/soil surface receives effluent. Additionally, resting time between doses allows the soil around trenches to re-aerate. Since the dosed effluent is distributed over more trench bottom area, instead of being concentrated in a smaller area, the soil can provide more complete treatment before the effluent reaches groundwater or surface waters. Another advantage that pumped dosing may offer, is the ability to lift effluent to the d-box and allow the d-box and lateral trenches to be installed at a shallower depth where soil properties and treatment is often better. However, distribution by dosing to a d-box has some limitations. The flow rate and velocity into the box must be controlled and a baffle may be needed in the d-box to reduce turbulence that would result in unequal distribution. Even if the flow is divided more equally by the d-box, distribution within a trench is still accomplished by gravity and the bio-mat.

Pressure manifold systems can distribute effluent more uniformly than gravity systems and even improve distribution compared to dosed d-boxes. A pressure manifold web page discusses level and sloped pressure manifolds in detail, as well as a calculation tool that can be used to assist with sloped manifold system design.

Alternative pressure distribution systems include low-pressure pipe (LPP) and subsurface drip dispersal systems. These systems use networks of pressurized lateral lines to distribute effluent more uniformly over the whole absorption area. In some cases, better distribution alone can overcome site/soil limitations such as shallow depth to limiting condition; in other cases, advanced pretreatment will also be needed due to a reduced capacity for soil treatment. It is essential that the onsite system selected, including the method of effluent distribution, is appropriate for the soil conditions at the site.