Figure 14. Styles of dosing tank: (a) round, (b) rectangular.
Figure 15. Cross section of dosing tank showing riser and pedestal for pump.
The dosing tank is placed between the septic tank and the mound to accumulate septic tank effluent to be periodically dosed to the mound (Figure 1). Once the accumulated effluent reaches a predetermined volume, the effluent is pumped to the laterals in the mound by an electrical control system. Proper dosing tank construction, placement and sizing must be considered to ensure reliable system operation.
The dosing tank construction requirements are the same as for septic tanks. The tank must be durable and watertight and must withstand the soil loads, which tend to push in on the walls. Since the environment in the tanks is very corrosive, no metal parts or fittings should be used. The major difference between a septic tank and a dosing tank is that the dosing tank volume will be lowered on a daily basis. Therefore, anchoring it against floatation is critical in areas with a high seasonal or permanent water table, which is where mound systems are often used. Common methods for anchoring can be used, such as a concrete anchor pad or deadmen anchors for fiberglass and plastic tanks. Placing 1 to 3 feet of soil cover over concrete tanks helps to anchor them against floatation.
Figure 14. Styles of dosing tank: (a) round, (b) rectangular.
Figure 15. Cross section of dosing tank showing riser and pedestal for pump.
Ensuring that the dosing tank is watertight is critical. In areas with a high seasonal or permanent water table, groundwater may leak into the dosing tank and overload the mound system. The seals around the pipes that enter and exit the dosing tank are especially vulnerable to leaks. If the pump is running more than the few minutes a day it takes to pump out the accumulated septic tank effluent, groundwater may be leaking into the septic tank or dosing tank. If groundwater infiltration is suspected, an event counter can document the number of times the pump comes on.
Dosing tanks can be round or rectangular as shown in Figure 14. A riser to the ground surface is needed for access to the pump. Make sure the riser is also watertight. Joints in risers can be a source of ground or surface water infiltration.
Never enter a dosing tank. Any work to replace pumps, switches or connections should be made from the outside. The sewage gases produced in the tank can kill a person in a matter of minutes. When working on a tank, make sure the area is well-ventilated and someone is standing by. Never go into a dosing tank to retrieve someone who accidentally fell in without a self-contained breathing apparatus. While waiting for help, the best thing to do is to put a fan at the top of the tank to blow in fresh air.
The pump in a dosing tank should be set several inches off the tank bottom to provide storage space for solids that may have carried over from the septic tank, as shown in Figure 15. A 6-inch concrete block makes a good pedestal for the pump. Whenever the septic tank is pumped to remove solids, the dosing tank should be pumped.
The minimum size of the dosing tank is the sum of the dosing volume, the volume of the delivery pipes, volume needed to keep pump submerged, and emergency storage volume in case of pump failure. The dose volume equals the delivery pipes plus 5 times the volume that will drain through the holes. A minimum of one-day emergency storage above the high water alarm should be provided. Appropriate dosing tank sizes are listed in Table 4. Avoid using a large tank as a dose tank because it is difficult to achieve adequate switch separation for pump control.
Table 4. Minimum dosing tank size for different sized homes (after Converse, 1978).
| Home size, No. bedrooms | Minimum dosing tank size, gallons |
| 1 | 250 |
| 2 | 250 |
| 3 | 500 |
| 4 | 500 |
| 5 | 750 |
Determine volume of pipe: laterals that will drain through the holes and the volume of the delivery pipes, manifold and main that will drain back into the dose tank. Remember, this is determined by the style of manifold/lateral connections (Figure 9). In tee-to-tee connections, the manifold drains back to the dosing tank. Table 5 lists the pipe volume per foot of pipe for several diameters of pipe.
Design Example
For this tee-to-tee connections, the lateral volume drains through the holes and the manifold and main volumes drain back to the dosing tank.
Lateral diameter and total length = 1.5 inch and 180 feet (45 feet/lateral × 4 laterals)
Manifold and main diameter and total length = 4 inch diameter and 42 feet long
Volume drains through the holes (for laterals) = [Total laterals length (feet) × 0.1 gal/ft]
= 180 ft × 0.1 gal/ft = 18 gal
Volumes drain back to the dosing tank (laterals + manifold + main)
= 18 gal (lateral volume) + [manifold and main length(feet) × 0.65 gal/ft]
= 18 gal + (42 ft × 0.65 gal/ft) = 45 gal
Table 5. Pipe volumes in gallons per foot of pipe (after Clemons, 1991).
| Per size, inches | Volume in gallons per foot of pipe |
| 1 | 0.04 |
| 1 ¼ | 0.07 |
| 1 ½ | 0.10 |
| 2 | 0.17 |
| 2 ½ | 0.24 |
| 3 | 0.38 |
| 4 | 0.65 |
Selected Parameter
(25) Volume drains through the holes (for laterals): 18 gal
(26) Volumes drain back to the dosing tank (laterals + manifold + main): 45 gal
Determine dose volume. Dose volume is 5 times the volume that will drain through the holes plus volumes drain back to the dosing tank calculated by volume of the delivery pipes.
Design Example
Dose volume = [volume that drains through holes (gal) × 5] + [volume that drains back to dosing tank(gal)] = [18 gal × 5 ] + 45 gal = 135 gal
Selected Parameter
(27) Dose volume: 135 gal
Check the size of the dosing tank. The minimum size of the dosing tank may be calculated by storage volume resulting from placement of the pump with concrete block, sum of the dose volume, and one-day emergency storage volume in case of pump failure.
Design Example
Size of dosing chamber = storage space volume resulting from placement of the pump
with concrete block and pipes (0.5 ft height)
+ sum of the dose volume
+ one-day emergency storage volume (360 gal)
= (based on tank dimension)
+ 135 gal
+ 360 gal
+ (based on tank dimension)
>> 500 gal is enough
Selected Parameter
(28) Size of dosing chamber: 500 gal