In November we talked about pressure distribution systems having five parts: lateral pipes with small holes, manifold and main connected to the laterals, dosing tank to collect septic tank effluent, pump to pressurize the system, and controls to operate the pump. This month we take a closer look at pump tanks and pumping situations.

The pump tank is placed between the sewage tank and the lateral system. The pump can operate in either on-demand or timed configurations. On-demand pump operation is controlled by float switches suspended in the pump tank. The pump turns on when enough effluent collects in the tank and turns off when the dose has been delivered.

In timed pumping, a timer controls when the pump turns on and off to deliver a specific amount (or dose) of effluent.

Tank attributes

Pump tank construction and installation requirements are the same as for any other type of sewage tank. The tank must be durable and watertight and must withstand the soil loads that tend to push in on the walls. The environment in the tanks is very corrosive, so no metal parts or fittings should be used. The major difference between a septic tank and a pump tank is that the pump tank is emptied on a daily basis.

Since the tank is emptied every day, anchoring it against flotation is critical in areas with a high seasonal or permanent water table. If the pressure distribution system being pumped to is in a mound treatment system, this is a common situation.

Flotation is not as much of a problem with concrete tanks as with fiberglass or polyethylene, but all tanks should be anchored according to manufacturer specifications. One common method is to use concrete curbs on either side of the tank with ropes or chains across the tank to hold it down when the pump runs or the tank is maintained.

A compartmented tank can help reduce the buoyancy problem. Since one side of the tank is always filled with water, this helps keep the tank from becoming buoyant. However, the strength of the inside wall is critical. Since there will be constant water pressure on one side of the wall, the tank construction should be such that the wall can withstand that pressure when the compartment is emptied.

Add a riser

Pump tanks can be round or rectangular. A riser to the ground surface should provide access to the pump through the maintenance hole. It is important from a maintenance standpoint that the pump, pump controls, and pump discharge line be accessible through the maintenance hole without entering the tank.

The riser cover should be made secure to prevent children or other unauthorized individuals from gaining access to the tank. In the last decade, a number of plastic risers and lids have come on the market that require different numbers and kinds of screws that limit access for safety reasons. A chain and padlock still work for tanks with concrete risers and lids. After you have installed the tank, make sure the tank can be secured.

We recommend that the access be at least 20 inches in the least dimension and preferably 24 inches. Never enter a pump tank or other sewage tank. Any work to replace pump switches or connections should be performed from the outside. Sewage gases produced in the tank can kill a person in a matter of minutes.

For residential applications, we recommend that the pump tank be a minimum of 500 gallons, or 100 percent of the estimated daily sewage flow, whichever is greater. When installing, always check that the pump tank has the capacity to collect and store the amount of effluent you need to pressurize the system and deliver the required dose. Often this will require a larger tank.

Nothing is worse than getting to the point of setting the floats, only to find you do not have enough tank capacity! One item of note: the inlet to a pump tank can be higher than a normal septic tank inlet to provide for additional reserve capacity in the event of a pump failure or malfunction.

Placing pumps

The pump should be set in the tank so that the intake is at least 4 inches above the bottom of the tank to prevent pumping solids from the bottom of the tank. A common way of doing this is to place a 6- to 8-inch concrete block at the bottom of the tank and setting the pump on the block. We actually recommend using two blocks side-by-side, as that makes for easier replacement if you need to pull the pump.

Electrical installations must comply with all laws and ordinances, including the latest codes, rules and regulations of public authorities having jurisdiction, and with the National Electrical Code. Make no electrical connections inside the pump tank. This includes plug-ins, screw type, twisted wire, boxes, relays or any other type of connection that requires movement to connect or operate.

When connections are made, they should be located in a watertight, corrosion-resistant junction box with watertight, corrosion-resistant fittings and a cover sealed by a gasket. Watertight boxes seal against water coming from any direction.

Individual junction boxes, switch boxes and receptacle boxes designed for outside installation will usually be watertight. They are designed to withstand temporary immersion or spray from any direction. They are made from cast aluminum, zinc-dipped iron, bronze or heavy plastic. They have threaded entries for watertight fittings and covers sealed by gaskets.

One common method to run wires is through sealed conduit. This provides physical, water and corrosion protection. Several kinds of conduit are acceptable for outdoor use. Rigid metal conduit made from aluminum or steel provides equivalent wire protection. How-ever, aluminum should not be used in contact with the soil.

Rigid PVC conduit can be used above ground. High-density polyethylene conduit is suitable for underground installation. Do not use thin-wall (EMT) conduit for underground or outdoor installations.