What You Need to Know About Dosing Tanks

Here’s a look at dosing tank requirements for systems using pressure distribution

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The dosing tank is located after the septic tank or other sewage tank and before the lateral system for effluent distribution. If there is an on-demand system the pump is turned on when enough effluent collects in the tank and shut off after the dose is delivered. Dose amounts in this case are generally about one-quarter of the estimated daily sewage flow from the residence.

The pump is controlled by a set of floats with a third separate alarm float to warn if the water level in the tank rises too high, indicating a pump problem. A timer controls the pumping of doses in a timed system but also has a high-level alarm. The tank needs to be large enough to deliver the flow necessary to supply the system and provide enough storage space in timed systems to hold the flow from the house until the time of pumping or to provide some emergency storage in on-demand systems in the event of a pump failure.

The pump should be set off the bottom of the tank to provide storage space for any solids that may make it to the tank and keep the pump above those solids so they do not interfere with the operation. A 4- to 8-inch-high concrete block or blocks works.

Dosing tank construction requirements are the same as for other sewage tanks. They can be made of concrete, fiberglass or polyethylene but must be durable and watertight. Tanks must also be able to withstand soil loads at the depth they will be placed. The environment in the tanks is very corrosive so there should be no metal parts or fittings in the tank. Any electrical connections need to be made outside the tank.

Dosing tanks are emptied or nearly emptied during the day; so it is important to evaluate the buoyancy potential and anchor it against flotation. This is especially important in areas of high permanent or seasonal water tables. Since pressure distribution is often used for shallow or above-ground systems to avoid high water tables this is especially important. It is also why the tank must be watertight. Seals around the pipes coming in and out and the electrical lines to control the pumps may leak. Any additional water is then delivered directly to the system which can hydraulically overload the system very quickly.

Dosing tanks can be round or rectangular. Pumpout depths to determine the amount of effluent are calculated using different formulas. But as long as the tank has the required capacity, shape does not matter.

Risers from the manhole access should be brought to the surface. The pump should be removable through this access without a technician needing to enter the tank. It also provides access so the tank can be cleaned. It’s important, just as with other sewage tanks, that the access point be secure, either through locking mechanisms, weight or unique safety screws in plastic lids.

About the Author
Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program, is an emeritus professor in the university’s Department of Soil Water and Climate, and education coordinator for the National Association of Wastewater Technicians. Send him questions about septic system maintenance and operation by email to kim.peterson@colepublishing.com.

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