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Sewage System - Component Parts
Details on the various parts I chose for my septic system.
Tank size
Your local health department will have specs on what size of tank is needed for a septic system. In my area it needs to be 1500 gallons or more and have two compartments. The first compartment (the largest chamber) collects most of the solids and the second compartment has mainly just effluent.
Note that whatever tank size you choose, you don't want the vertical height of the tank to be too high because that all adds to the pumping height. Better to get a short fat tank. You can get a 1500 gallon tank that only has a height of about 51" although a typical concrete tank is a bit taller.
A big issue when buying tanks is the shipping cost, so see what you can buy locally. It is probably best to just get a concrete septic tank from a local supplier. A local supplier is also more likely to be approved in your local jurisdiction.
I my case I opted for a 1750 gallon two compartment concrete septic tank because the incremental cost between 1500 and 1750 gallon was not too bad. A particularly nice feature of the bigger tank is that it uses thicker concrete (4") for the walls than the smaller tank (which has 3" thick walls). The price with shipping is about $1,700. It is supplied by...
Berg Vault Company
2308 Cedardale Road, Mount Vernon, WA 98274
360-424-4999
http://www.bergvault.com
Peggy@BergVaultInc.com
Note that the output hole is 3" lower than the input hole. This must be allowed for when deciding on how much lower in the ground the pump tank needs to be compared with the septic tank.
Effluent Pump to sand filter
The good news is that when you have a full dual compartment septic tank and a separate pump tank down by the house then it is only "effluent" (ie dirty water) that you are pumping rather than sewage that has solids. That reduces the cost of the pump. The bad news is that you need some extra pressure to force the water through the sand filter and that increases the cost of the pump.
My pump is a "SHEF 100".
Details here .
As it is 230V, it connects between L1 and L1 via a special plug. The float switch also connects using that plug type.
Details here .
Check valve
A check valve goes in the 2" PVC exit pipe from the pump at the pump end of the pipe and is used to prevent sewage in the pipe flowing back into the tank when the pump is not on. They cost about $25.
Details
here .
The design also calls for a second check valve on the free end of the pipe in the tank.
Alarm system
In addition to the float switch used to turn the pump on and off there needs to also be an alarm float switch to warn if the tank gets to nearly full. This tells you if the pump that empties the tank has stopped working.
I decided to use a float switch operating at 110V and route a wire back to the house to light a bulb to indicate that the level is too high.
Details
here .
It is however a requirement that there must at minimum be an external light visible when standing on the pump tank and there must be an audible alarm.
Dose Timer
You need an "Off period timer" that activated an "On period timer".
The "off period timer" will apply power to the an "on period timer" once per hour (or whatever it is set to) and the "On period timer" will then turn on power to the pump for a specified time such as 30 seconds (finely adjustable). To ensure good reliability use a 40 amp external contactor relay on the output of the "On period timer" timer.
Rather than be pumping to the sand filter continuously when the float level is high, you need to provide "doses" to the sand filter. You typically need about 30 seconds on and 1 hour off.
Use the normally open contacts on the contactor relay in series with the float switch. If the float is high (and remains high) then the pump will come on for 30 seconds and then be off for an hour. That cycle will repeat throughout the 24 hour day.
Implementation notes
The 2" schedule 40 PVC pipe that goes from the pump tank to the sand filter carries high pressure effluent. It is important to use high pressure water PVC connectors on this pipe. Do not use PVC DWV type connectors. You will probably not be able to find Wye connectors in high pressure fittings so if as in my case you need a wye connection (with the two shutoff valves that for a switch between the old and new tank set) then you would need to use a T piece and a 45 degree piece. Even better is to separately tap the two pipes into the vertical pipe that goes up the side of the sand filter and this is what I chose to do.