Can Sewage Trenches Provide Temporary Effluent Storage?

Find the appropriate treatment solution for the homeowner who has an annual blow-out holiday party that shocks the septic system.

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A few months ago, I wrote a column addressing when it makes sense to add a flow equalization tank to improve system operation and efficiency. If you recall, a flow equalization tank utilizes timed dosing for uniform delivery of sewage to a component over a period of several days rather than all at once. In the case of a pressure distribution system, this can spread out large flow events preventing periodic hydraulic overloads to the soil treatment and dispersal area. In the case of aerobic treatment units, this means spreading flow out to provide aerobic bacteria in the tank adequate time to do their job. 

All very good ideas and necessary where you as a service provider know the system will be subject to periodic high-flow events. Example areas where flow equalization would be particularly advantageous would be churches, schools, campgrounds, event centers and — in certain situations — residences. We usually do not consider them for residences, but it may be desirable to spread the flow out during the day or week in homes where a lot of entertaining takes place. 

This led a reader to ask the following question: Can a chamber system plus filter eliminate the need for a balancing tank for well-attended events? I take it, based on some other comments, the reader has a gravity-fed series of trenches with chambers as the media and there is an effluent filter at the septic tank outlet. It also appears from the “well-attended events” comment, he hosts events at his residence that would make flow equalization helpful. Whether it is necessary depends on the frequency of events.


Let’s take a look at his question. First, if there are trenches in the system without effluent ponded in a gravity-loaded system — yes, there is storage. In fact, one of the stated advantages of chambers and some other rock-free systems is they provide the opportunity for storage. In the past, I have also mentioned there is a 35% void space in a rock-filled trench, which can act as something of a buffer to large flow events.

To give you an idea of how many gallons could theoretically be stored, consider a 100-foot-long trench 3 feet wide under a chamber. There would be 300 cubic feet of volume available if we used a maximum depth of 1 foot. This area would hold 2,250 gallons under the chambers. In a rock-filled trench, the storage volume available would be 788 gallons. 

Do I recommend or agree loading a gravity fed system with 700 to 2,200 gallons of effluent in one day is a good practice? Absolutely not. As a matter of fact, I recommend with even everyday activities, such as doing laundry, it is much better for our gravity systems to spread the flow out over the week, when at all possible, rather than focusing the activity on one day, like Saturday. Large flows in short periods can interrupt the ability of the septic tank to settle solids due to turbulence. This will quickly plug effluent filters and may push unwanted solids into the drainfield. 

The bottom line is that all systems benefit from controlling or limiting larger flow events either through incorporation of storage into the system or through in-house management or control of flows.


In terms of gravity distribution, the use of dropboxes delivering effluent to each trench is an advantage over parallel gravity distribution. This forces the effluent to move through the system sequentially so only the necessary area is used or impacted by the large flow and the impacted trenches are easily shut down for a period following the event to give them time to recover.

If the chambers were already part of a pressure distribution system with a pump tank and pump, looking at the chambers as a potential storage area would have even more serious consequences. Pressure distribution systems are specifically designed to handle a finite amount of effluent daily. If the system is shock-loaded, the infiltrative surface can be damaged, with infiltration capacity reduced and leading to ponding of effluent at the surface. 

Once ponding occurs in a pressure system, it is broken and headed to ultimate failure. The presence of a pump tank and pump, and adding tank capacity and a timer, will effectively store effluent and distribute it over a longer period of time.  


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