Pump to Gravity: Does It Make Sense?

Once a pump has been added to the onsite system design, why not use it to take advantage of the benefits of pressure distribution?

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In the more than 30 years I have worked with onsite wastewater treatment systems, I’ve seen many things that don’t seem to make sense: drainfields with six feet of soil cover, tanks buried more than 10 feet to the top of the lid, a system pumped under a driveway through a garden hose.

Each time I ask myself: Why take half measures? Why not design and install the best system for the money? I offer here some opinions about one type of drainfield design that doesn’t make sense to me, once we step back and evaluate what is being done against the goal of every drainfield design; long-term functionality. To me, functionality means all parts of the system perform as designed over at least the system’s full life expectancy.

In my opinion, pump-to-gravity drainfields fall into the category of not making sense. In looking at the reasoning behind what makes a drainfield work well, my understanding is that we want even distribution of effluent over the entire field to reduce the volume of effluent being treated per unit soil area. We also want the field to be as shallow as practicable to ensure good air exchange. While pump-to-gravity can facilitate the latter, it rarely accomplishes the former.

Once we have run through all the engineering decision-making steps to avoid a pump, with an understanding of good system function in mind, and once we have decided that a pump is necessary, for whatever reason, we have crossed a line, and there is no returning.

The low cost of gravity discharge to the field is behind us. We have incurred the cost of a pump, a level-sensing system and a control panel to operate the pump properly, so a gravity drainfield will not offset that cost.

Since the pumping system is there, why not use it to its full advantage? Why not discharge to small-diameter laterals that can provide uniform application of a calculated effluent dose over the entire field? A pump-to-gravity system most likely will not evenly distribute the dose of effluent over the entire drainfield (Machmaier & Anderson, 1987).

This means that instead of the entire drainfield area working to treat the effluent, we have a portion of the field doing that job. That translates into a less-effective system, because our actual application rate of effluent to media is greater than we have calculated and designed for.

What is the difference between a pumped system with pressurized laterals, and a pump to gravity system? What is it that makes designers not want to follow the logical continuum of design to pressurize the laterals in the field once a pump has been included?

The only thing I can see is that a pump-to-gravity lateral system is easy. We avoid the work of selecting lateral diameters, orifice diameters and orifice spacing and matching a pump to allow effluent to cover the entire drainfield. We just need to install a distribution box, allow the effluent to be pumped in, and let gravity take care of the rest.

So, in essence, pump-to-gravity designers choose less work on their part over better distribution for the long-term functionality of the system. The pressure-dosed system also allows beneficial alternatives to field distribution that are not available with a gravity-flow system.

Timed dosing is now possible, where the effluent can be metered out to the drainfield over the full 24 hours in a day, rather than as the wastewater is produced. This results in a minor ($500) increase in panel cost, while providing the benefit of smaller doses to the field over a greater span of time. That can translate into better treatment.

For me, the design of laterals in a pressure-distributed drainfield is an iterative process. I can’t simply select pipe size, orifice diameter, orifice spacing and pump and move on. I need to check those selections against pump curve software in my computer to verify that what I have selected works.

More than half the time, my first stab at these parameters does not work well. I have to change one or more of the variables to make the laterals function, distributing effluent evenly over the field within the capacity of the pump. There are probably dozens of software programs out there to help us do this.

Orenco Systems has complimentary downloadable software. Tom Kallenbach with Eliminite in Montana has written a powerful program that has an annual fee, so most designers would need to weigh the fee against the number of pressure systems they design in a year. Computers provide us with spreadsheet software if we want to set up the calculations ourselves.

The bottom line here is that there are tools available to allow each designer to responsibly design pressure distribution systems that achieve what the experts indicate we are looking for if we want to promote good soil treatment.

Two closing thoughts. First, I can think of only one reason to use a pump-to-gravity soil treatment area, and that is if we are designing a repair drainfield and maintaining the existing field in the new system, and the existing field uses pump-to-gravity. Then we design a pressure distribution system for the new field and discharge to the existing field through a valved connection, which doses to the existing distribution box and allows alternation of fields.

Second, when we design pressure distribution to a large drainfield, where all the laterals will require more gallons than a reasonably priced, high-head pump can handle, there are alternating distribution valves that dose to only a portion of the field per cycle. This reduces the number of orifices the pump sees, allowing a smaller pump to be used, and increases time between doses to each section of the drainfield, effectively enhancing treatment.

In my opinion, when we move away from “the way we have always done it” and take a look at what we are doing, and why, we often find that methodologies have developed for the convenience of the practitioner, rather than the best onsite system functionality for the customer. It seems we should be striving for the best system functionality, even if it means a little more work for us as practitioners.


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