It’s Time For Tough Talk About Gravity Distribution

Systems are sized assuming even distribution along drainfield lines. How often do the results match the assumption?

It’s Time For Tough Talk About Gravity Distribution

Tree roots are clogging a distribution box.

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With most conventional onsite systems, flow from the septic tank is via gravity with the goal of even distribution across the soil treatment area. Flows can be split through perforated piping and rock, synthetic media or hollowed-out chambers, and other mechanisms. Because soil drainfield size is partly flow-volume dependent, equal distribution across the soil drainfield is crucial to properly treat wastewater and preserve the life span of the soil. “Even distribution” includes both even flow volumes split between trenches/beds and even longitudinal dispersal across the length of each trench/bed.

Importantly, systems are sized with the assumption that even distribution is achieved. In other words, the long-term acceptance rate of a system only holds if effluent is distributed evenly.

Uneven distribution poses several threats to the health of a septic system, including ineffective secondary treatment and potential groundwater contamination. Soil pore clogging and impacts on soil longevity are other potential risks.


In some areas, header pipes directly distribute effluent from the primary treatment device to receiving drainfield pipes. A header pipe is the simplest method of effluent distribution, but there is no research available on the evenness of distribution with this technique, and once installed adjustments are not typically available.

A splitter tee is another option for parallel distribution drainfields. It operates very simply by splitting effluent into two flows with a baffle system inside a T-shaped pipe junction. A splitter tee can only split flows 50/50 and no research is available to document evenness over time. Certainly, with biofilm and sludge buildup, header pipes and splitter tees may be less effective at even distribution. 

Gravity-fed distribution devices are advertised to distribute flow equally across drainfields and trenches and beds. In practice, equal distribution is nearly impossible to achieve with current gravity distribution devices. Studies conducted by Gill published in 2009 reveal that distribution boxes and splitter tees are unreliable.

Even when installed perfectly level, the distribution boxes performed poorly, preferencing two of four outlets. The tee splitters performed better than distribution boxes under perfect laboratory conditions. However, in the field both distribution devices performed poorly — over time, uneven solids deposition and biofilm growth made even distribution impossible.

A V-notch weir is often added or required with a traditional distribution box but provided marginal improvement in distribution efficacy. Distribution box service is essential to adjust the weir and clean the box of biofilm, sludge and scum, which can build up over time and effect the evenness of distribution.


Realistically, distribution devices are not installed perfectly level. Settling and, in colder areas, freeze/thaw cycles undoubtedly unlevel even the best of installations. Many septic systems also only operate under low-flow conditions (< 0.53 gpm) that contribute to preferential drainage. Under typical low-flow velocities, a distribution box only off zero slope by a few degrees results in drainage to only one or two outlets.

Inserts in distribution boxes that are designed to be re-leveled are a good concept, but they build up in biofilm over time and require frequent maintenance and monitoring — not the general practice of gravity septic systems where typical maintenance is only cleaning the septic tank.

With some applications, the septic tank effluent is pumped into a distribution box. It is important that the pump not be too large and the energy from the pump dissipated to minimize short circuiting and uneven distribution.

Another distribution option is a drop box. Drop boxes differ from other gravity distribution devices because they dose trenches in serial. They are designed to load one bed before moving on to the next bed. The “evenness” of distribution is called into question for drop boxes when considering the completeness of trench loading and the contour loading rate.

In a study we performed at the University of Minnesota, more than 100 drop box trench systems five to 10 years old were evaluated for loading. We found 60% of the systems did not observe ponding at the end of the first trench in the serial systems. The study suggests drop box systems do not guarantee even distribution along even a single trench because biomat layers were unable to form at the end of the trenches with a minimum of five years growth time. 

This lack of biomat development calls into question our general understanding of biomat formation timelines, identifies our conservative design approach, and highlights that shorter trench installation may be helpful in the performance of systems. Incomplete trench loading in parallel distribution is more likely due to the dosing of all thelines from the start, however studies have not been conducted to test this.


Siphons can be used to dose flows and still align with the passive management of gravity systems. However, older research has found that siphons commonly trickle without frequent maintenance, reducing the efficiency of these devices. Pumps are the active alternative to siphon dosing and have been utilized in pressurized systems. However, once a pump is added, the system is no longer passive, which is often a goal of gravity-fed systems.

Many component manufacturers recognize on their websites that completely level installations are not possible and they have designed inserts such as V-notch weirs, baffles or internal leveling devices to improve distribution. In Contrast to a traditional distribution box, there are distribution devices containing a dosing manifold inside a distribution box. When enough volume builds up, the device triggers and releases effluent to the distribution box’s outlets.

Manufacturer tests on the SeptiSurge fluid manifold device showed even flows under lab conditions even when a 0.12-inch tilt was applied. The velocity of effluent induced by the “surge” likely overcomes some limitations of gravity distribution. Further distribution along the length of drainfield piping and forceful removal of biofilm buildup could result in more equal effluent loading.

The septic system industry — installers, regulators, product manufacturers — anecdotally realize that traditional gravity-fed distribution devices are unlikely to provide even-flow distribution. However, apart from research conducted by Gill, few studies exist. Many gravity-fed drainfields are likely receiving uneven distribution either laterally, between trenches or longitudinally because of poor installation, natural settling, biofilm/sludge buildup and low flow rates. There is no evidence to show current distribution devices are capable of even distribution.


Uneven distribution can easily fly under the radar and produce consequences such as a shorter soil treatment life span, pore clogging and most detrimentally, groundwater contamination. Despite recognizing the limitations of distribution technologies, many rules and regulations assume even distribution across gravity drainfields. There is no scientific evidence to support current distribution devices that can elicit even distribution both laterally and longitudinally.

Distribution systems that store doses or siphons could improve lateral and longitudinal distribution and overcome some off-level field conditions. More frequent and rigorous inspections of distribution boxes and other accessible technologies could also contribute to better distribution. However, the more effective alternative is replacing gravity distribution with pressure distribution, which can better guarantee flow across the entire soil treatment area.


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