Most of us are connected to mains sewerage, and this is usually a very convenient and trouble free way of getting rid of both sewage and 'grey water' (from our sinks, baths, washing machines, etc.) We would recommend that you connect to mains sewerage when available, as it provides an efficient way of disposing of material that would otherwise be difficult to treat on a small scale.
Water conservation is always the best place to start when trying to reduce your 'water footprint', so whatever your situation, have a look at our advice on on water conservation in the home and in the garden.
For those not on the mains, many treatment options can be considered. Some of these could be of interest to people who are on the mains, but want to reclaim some of the nutrients they are currently flushing away. Treating sewage usually involves a combination of different methods to separate out solids, deal with pathogenic (disease carrying) organisms, and remove nutrients.
The first stage (primary treatment) is the separation of most of the solids from the liquid effluent, usually by some kind of settlement chamber. The most common example is the septic tank. These are a robust and economic option if used properly. Most of the solid material will either sink (as sludge) or float to the top (as crust), leaving the effluent between to flow out to a secondary treatment stage.
In a septic tank, some breakdown of solids by bacteria takes place, but they will gradually build up over time and need removing occasionally (a process known as desludging). No additives should be needed to make your septic tank operate properly. People occasionally tell us of tanks that have never been emptied, but this will be either due to a hole in the tank (which could mean the solids are being eaten by rats), or because solids are washing into the leachfield (this can lead to a blockage, a bad smell, and a costly rescue operation). If you are concerned about your septic tank system or have recently moved to a property that has one, then we recommend the booklet Septic Tanks: An Overview, which tells you all you need to know about getting a good performance from your tank.
Cesspools tend to be used when conditions prohibit a secondary treatment system; they have no outlet and so need emptying every few weeks, making them expensive to run.
A common secondary treatment system is a leachfield. This is a system of perforated pipes laid in underground gravel trenches. They are unsuitable for very clay soils or areas with a high water table. The liquid effluent percolates through the gravel, where solids are removed and digested by micro-organisms, leaving the liquid clean enough to filter into groundwater.
Vertical flow reed beds are another kind of secondary treatment. The effluent from a septic tank (or similar) is percolated through a tank containing layers of sand and gravel planted with reeds. The reeds help bacteria to break down the pollutants and make the beds attractive to wildlife. Horizontal flow reed beds are usually a third stage of treatment, used after other systems when a high level of treatment is needed.
Several kinds of package treatment plants are available. These carry out primary and secondary treatment in compact units, and so produce a cleaner effluent than a septic tank. However, these usually require some kind of power supply – if so, they will be more costly to install and run, and not necessarily more environmentally friendly.
Another option is a composting (or dry) toilet. See How can a composting toilet be designed and managed?
For much more detail on these systems and others, see the CAT book Choosing Ecological Sewage Treatment, which covers all aspects of small-scale wet sewage treatment. It looks at typical water and nutrient flows through domestic systems and evaluates treatments including reed or willow beds, septic tanks, leachfields, trickling filters, solar ponds, living machines, and compost toilets. The book also explains how to collect and use urine and grey water, and reduce clean water use.