We Remain Bullish on Sequential Distribution in Gravity Systems

If you’re looking to avoid uneven flows between trenches or overloading of a single drainfield line, use this layout to your advantage

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A few months ago, we discussed the importance of managing the biomat so systems are able to accept the amount of effluent delivered and provide treatment before release into the environment. As a part of the discussion, we indicated one way to effectively manage biomat formation in systems with gravity distribution was to use drop boxes and sequential distribution. We have had a few comments and questions about this so, we thought we should revisit gravity distribution methods and look at some of the advantages and disadvantages.

There are three types of gravity distribution between trenches: parallel, sequential and serial. Parallel is probably the most common and the one many people think of when looking at gravity system layouts. It consists of a distribution box with multiple outlets, one solid pipe to the head of each trench. Sequential has a series of drop boxes, one at the head of each trench and connected by solid piping between boxes. Serial distribution can be done using distribution or drop boxes, or by simply piping and elbows or even overflow lines without pipes.


Serial distribution operates just as the name implies; the trenches and piping are connected continually so effluent flows into the first trench and as the first trench fills up, flows move without interruption to the next trench. Most permitting authorities no longer allow this type of distribution. This is due to a couple of major disadvantages with this method. 

In this configuration every drop of effluent ever delivered passes through the first trench with no opportunity for resting or reduction in suspended solids applied to the first trench. This overloads the trench hydraulically and organically, resulting in a thicker and more resistant biomat, which reduces acceptance of effluent. Over time this results in system failure. 

There is little to opportunity to manage the system flow, especially if it set up using piping and elbows or simply rock overflow areas to deliver effluent between trenches. Finally, adding additional necessary trenches due to higher flows from the residence is difficult and involves excavating at least part of the system.


Parallel distribution is designed for use on level sites with each trench being the same length and receiving equal amounts of effluent. A distribution box is set level and as indicated above piping is generally laid from the box to the head of each trench. Theoretically on a level site, effluent is delivered by gravity somewhat equally to each trench. So every trench is receiving effluent every time sewage is delivered from the tank to the distribution box. 

There are a couple of problems with this theory. It only operates this way if the distribution box is absolutely level and it stays that way through time. In actual practice, a slight difference in elevation due to the box settling or being knocked out of position during backfill will result in all the effluent flowing out the line in the box that has the outlet at the lowest level. 

After initial start-up, effluent is just flowing to one trench. To reach the other trenches once that trench is accepting all the effluent it can, effluent must back up into the distribution box. This creates two potential problems: In cold weather areas, water standing in a pipe can be subject to freezing if it is not insulated properly or installed below frost line. Thawing lines and distribution boxes in winter can be daunting and expensive for the homeowner. 

When a system is installed on a sloping site — so the trenches are not all at the same elevation — if all the effluent flows to the trench with the lowest elevation, it may not be able to backup into the box. This means the rest of the system does not receive any effluent and effluent eventually comes to the surface, indicating failure. Flow to trenches in sequence can be accomplished using levelers in the distribution box and there are some boxes manufactured to deliver equal amounts of effluent. But they require additional monitoring to ensure they are functioning.

From our perspective, the thought that all trenches will be able to accept equal amounts of effluent is also flawed. Except in the most uniform of soil conditions, trenches will most likely not accept the same amounts over time. We have recognized that working with a site that is not level leads to some less-than-favorable decisions during installation, such as grading a site or cutting and filling. These activities violate one of our important installation principles, KIN, or keep it natural. This means using natural soil, which will generally be able to accept more effluent than disturbed or manipulated soil. 


In sequential distribution, effluent is delivered to the trenches through a series of drop boxes. No effluent flows out of the first trench to the next until it is being used to its maximum capacity. Effluent then passes on to the next trench in sequence until it is used to capacity and so on. This is ideal for sloping sites, as it ensures each trench is used in sequence and the piping between boxes does not have standing effluent, avoiding potential freezing problems in cold climates.

Use of the trenches can be easily managed. If only one or two trenches are being used on a regular basis, they can be rested by diverting flow to the other trenches in sequence by closing off outlets in the boxes. With a drop box for each trench, this can easily be done when the boxes are installed with inspection ports and accessible to the surface. 

Trenches do not have to be same length; each trench will use what if can and then flow moves on to the next. While with parallel distribution each trench is expected to be the same length, this can be varied as the necessary area is provided based on the long-term acceptance rate (LTAR).

Lastly, if the soils downslope from the last trench in sequence are suitable, adding additional capacity is easily accomplished by connecting into the last drop box. 

It is for these reasons that we are so positive about using sequential distribution.


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