I think there may be some confusion here. What exactly is a septic tank?
A concrete or plastic tank receives wastes and household, ( but not rain), water. It is sized and dimensioned, (volume & length /width/height ratios) to create non-turbulent conditions, such that solid wastes sink to the bottom, & of sufficient volume that a practically clear effluent gently emerges. Weirs are placed at the outfall so that floating debris cannot pass over, and in fact a leather-like layer forms on the surface which acts as a seal against smell. The tank is usually high-level vented. The action in the septic tank is by anaerobic bacteria; these digest a % of the solids in the absence of oxygen, creating methane as a by-product. While in 'septic' conditions, ( ie.denied oxygen ), the sewage remains relatively inoffensive. The effluent, while clear, is still very active and must be subjected to further treatment to reduce its biological oxygen demand ( BOD ). Treatment using aerobic bacteria is required, and these require abundent oxygen to do their job. There are many ways to achieve oxygenation;
* By draining the effluent away through a suitable area of the soil, via perforated pipes. The countless millions of naturally occurring soil bacteria render the effluent harmless. On heavy clay or where shallow aquifers exist close to the surface, or on fissured chalk/limestone/ or where over-porous sandy soils pertain, the solution is sealed quartz sand or other artificial media (coke breeze, zeolite, pozzellani etc.) filters arranged to do the same function.
* By agitation, entraining air into the mix, either with paddles or by air-injection. Aerobes attach themseves to debris in the effluent and circulate constantly. The problem with this, the so called activated sludge method, is the vast amount of solids produced, (ie the bodies of all the bugs); far more than the original solids before settlement, leading to frequent and expensive emptyings.
* The fixed media method, of which several types exist. In the most used type clarified waste effluent is treated by rotating discs, usually of polypropylene, which dip into the liquor. The bacteria live on the discs, and are constantly exposed to fresh liquor and oxygen. This method is called a Bio-disc, ( TRADE NAME).
Solids produced are less, and as the liquor is transmitted across banks of discs by wipers into separate compartments, produces very clean water at outfall.
All systems require periodic emptying, every 3-6 months for activated sludge, 1 year for fixed media, 4 years plain septic settlement.
This 'sludge' consists principally of Glucose, Fats and oils, vegetable fibre and inert earthy solids.
Now, whether you live in the middle of LA or the middle of nowhere, the basic method is the same- settle solids-aerobic treatment-release. While a city-system is fine, the simple home-plant has advantages for us country dwellers. First it can be built to operate soley on gravity, no electical power, and we can use standard WCs and fitments. Or, home-size Bio-disc /activated sludge tanks are avaiable where soil-type or small garden size dictates, and these typically consume perhaps 100W, with the advantage that exit water can be put into a watercourse or ditch. And, when excessive rain falls and a city system can't cope with the flow- they put raw sewage into the river system as a matter of policy. A home based system rarely if ever floods your yard.
Oh, and Mike, you say you wouldn't drink the water out of a treatment system. Mate, you already do. In London, they reckon the water has been through 6 people before it comes out of the faucets! Of course it gets further treatment- storage/ filtration/ chlorination.
Sorry if all this is boring, but it's my pet subject just now.
Alan
[This message has been edited by Alan Belson (edited 10-16-2005).]
