NAC444.8328. Dosing tank: Design criteria.

     1. A dosing tank must have sufficient volume to provide for the volume desired for dosing and a reserve volume. The reserve volume, which is equal to the volume of the tank between the alarm switch for high levels of effluent and the bottom of the invert of the inlet pipe, must be of sufficient size to allow the owner of the system to respond to a high-level alarm before the level of effluent in the dosing tank reaches the invert of the inlet pipe.

     2. If an electric pump is used in a dosing tank:

     (a) The size of the pump must be determined according to the performance curves provided by the manufacturer, the flow rate needed and the size of the pumping head as calculated by an engineer. The engineer shall calculate the size of the pumping head by adding the difference in elevation between the highest elevation of the discharge pipe and the level for low effluent in the dosing chamber to the friction losses incurred in the discharge pipe. The engineer may make the calculations pursuant to this paragraph without considering velocity head.

     (b) The control system for the dosing tank must include a switch to turn on the pump, a switch to turn off the pump and an alarm switch for high levels of effluent. The alarm switch must emit a visual and audible alarm and must be placed not less than 2 inches or more than 3 inches above the switch that turns off the pump to alert the owner that there is a high level of effluent or malfunction of the pump that will likely cause a high level of effluent. The alarm switch must be on a circuit that is separate from the circuit for the switches which turn the pump on and off. A switch used in a pumping chamber must be able to withstand the humid and corrosive atmosphere inside the tank. The owner shall include in the design plans an information sheet provided by the manufacturer for each pump, switch and alarm to be used in the dosing tank.

     (c) All electrical contacts and relays must be mounted on the outside of the dosing tank to protect the contacts and relays from corrosion. The owner of the individual sewage disposal system shall take such actions as are necessary to prevent sewer gases from traveling through the electrical conduit into the control box.

     3. The following is a diagram of a typical dosing tank with pump:

     4. A siphon may be used in a dosing tank in lieu of an electric pump if the point of discharge is at a lower elevation than the elevation of the primary treatment unit. The size of the siphon must be determined by the average flow rate desired. The drawing depth, which is the distance from the bottom of the siphon bell to the high water level that is necessary to activate the siphon, must be determined by the manufacturer of the siphon. The volume of the dosing tank may be determined by adding the drawing depth to the length and width of the dosing tank. The following is a diagram of a typical dosing tank with a siphon:

     5. A dosing tank must be vented. The vent must be located as far away from the electrical control box as practical, but in no case may the vent be closer than 3 feet from the electrical control box.

     6. The frequency of dosing varies depending on the texture of the soil at the interface of the sand and native soil at the base of the mound as follows:

SOIL TEXTURE	DOSING FREQUENCY
Sand	4 doses per day
Sandy Loam	1-2 doses per day
Silty Clay Loam	1 dose per day

     7. The dosing volume must be of sufficient capacity to distribute effluent evenly to all parts of the distribution system. The dosing volume must be approximately 10 times the volume of the distribution piping in a pressure distribution system and not less than 60 percent or more than 75 percent of the volume of the distribution piping for a system which does not use a pressure distribution system.

     (Added to NAC by Bd. of Health by R129-98, eff. 3-25-99)