septic tanks 4

[cef2003]

I have a couple of septic tank/system questions.

Does a 1500 gal tank and a 1000 gal tank (both compartmentalized) connected in series do the same job as a 2000 gal tank (baffle wall)?

When does a septic tank need to be vented?

 

[Ussuri]

The main design aspect of septic tanks is volume, which dictates the retention time. Having two smaller tanks in series is perfectly acceptable and would do the same job as one larger one, it is just a two stage settlement system. You will find that the distribution of Sludge within the tank however varies.

Septic tanks treat the effluent in two ways. Firstly they provide settlement (Baffles are installed to prevent surges of flow disturbing the sediments), secondly they provide limited digestion. The process of anaerobic digestion breaks the organics down into a combination of CO2, CH4 and H2S. The vent is provide to allow these gasses to escape to atmosphere and prevent the tank pressuring.

 

[cef2003]

Our septic tank is part of a system that consist of the two septic tanks that then flow into Ecopods that then flows into an existing septic tank where it is then pumped into a field. In our case, would the septic tanks themselves have to be vented or does the vents on the buildings get the job done?

 

[Ussuri]

cef2003

I am not exactly clear how your system is working. You have effluent from a building discharging to two septic tanks in series. The discharge from these tanks goes to an Ecopod (?? you will need explain what that is as a google search throws up something which looks like a greenhouse) which in turn discharges to a third (existing) septic tank.

At the top of the sewer line at your building you should have a soil vent pipe (SVP). This allows any gas build up in the sewer to vent to atmosphere. And because your pipe invert level increases the closer you are to the building, all gas in the pipe should vent there. That said, a lot of sewer gases are heavier than air.

However, the invert of the pipe entering the tank is not at the top of the tank, so any gas above the crown of the pipe needs to escape somewhere (as it cant go up your inlet pipe).

Also you dont want the situation occuring that when someone opens the cover they get gases escaping in their face, not nice (CO2 displaces oxygen but is heavier than air, H2S does the same and is also heavier than air, and CH4 is flammable and lighter than air).

But, in practice this is rarely an issue. Most septic tanks come with light duty plastic covers which will vent any gas to atmosphere. It is only a concern if you are replacing these with heavy duty sealed covers.

 

[Ussuri]

Just thought of another reason for venting. If you have a large influx of flow (outflow is usually restricted by baffle walls and compartments) the water level will rise and if the tank was sealed the air inside will become pressurised.

 

[cef2003]

Ecopod refers to a wastewater treatment unit manufactured by

http://www.deltaenvironmental.com

 

[Ussuri]

OK, makes sense now. They are a biological fixed film secondary treatment system. I did think a greenhouse was a bit odd.

 

[bimr]

You do not need a vent on the septic tank. The septic tank will vent through the inlet and outlet pipes since the pipes are gravity flow and should not be full of fluid.

The septic tank usually does have a manhole that is used to periodically clean the septic tank.

If you septic tank is farther than say 30 ft or more from the building cleanout, you probably want to add a cleanout between the building and the septic tank.

Septic tanks do not purify sewage, eliminate odors, or destroy all solid matter. The septic tank simply conditions the sewage so that it can be disposed of to a subsurface leaching system without prematurely clogging the system. The septic tank is designed to slow down the movement of raw sewage and wastes passing through so that solids can separate or settle out an be broken down by liquefaction and bacterial action.

The outlet of the septic tank usually has a baffle or submerged outlet pipe to prevent floating material from leaving.

Don't quite understand why you have a septic tank in front of a treatment device. The septic tank is little more than a wide spot in the line.

 

[Ussuri]

We often use a similar arrangement in my neck of the woods (Scotland).

For small works we will use a septic tank, feeding a secondary treatment unit (such as a SAF or RBC) then feeding to a clarifier or humus tank.

The treatment process is the same as a typical WWTP design. The septic tanks are used as a cheap, easy to install alternative to the standard radial (or older horizontal) flow primary tanks. So in that regard it is not functioning as a typical septic tank. The other items of plant are scondary and tertiary treatment as per usual.

While I do agree you can vent up the pipes, there will potentially be occasions when the pipe may be blocked or running full bore (assuming the system takes surface water as well). I would always ensure an alternative vent as its a cheap method of removing the risk.

In certain cases (septic tanks in parallel) the invert of the inlet is below the invert of the outlet, and a flooded splitter chamber is provided upstream to ensure a even split of flow. The invert entering the splitter is higher than the outlet of the tanks (to maintain the hydraulic gradient), but the chamber has a bottom exit. This ensure the head above the outlets is the same, and solids do not settle out during the split. Soin this arranngement you cannot vent using the inlet pipe.

 

[bimr]

http://www.nesc.wvu.edu/nsfc/pdf/homeowner_guide_long.pdf

 

[Ussuri]

Details of a smaller onion type (only goes up 9000l = 2400 gallons) bigger tanks are cylindrical (up to 275000l = 73000 gallons)

http://www.klargester.com/pdf/TDS0006-5OperationofKlargesterSepticTanks.pdf