waste water systems: MATERIALS ENCYcLOPEDIA
HOW THE SYSTEM WORKS
Sewage discharge from the home is carried to a septic tank equipped with one or two chambers. The inlet to the tank is equipped with a baffle that forces incoming solids to the bottom of the tank. Further baffles keep floating scum from clogging the exit pipe. After initially filling, every incoming quantity of sewage forces an equal amount of effluent out the exit pipe and into a series of perforated pipes known as the weeping bed. Here the effluent is discharged into the ground, where it percolates in the soil and is remediated to whatever degree the biological conditions in the soil can provide.
Tanks and weeping beds are sized to meet standard flow rates for the number of bedrooms in the home. The soil around the weeping bed area needs to be tested regularly to ensure adequate percolation rates.
Some anaerobic secondary treatment takes place within the tank, but the majority of treatment occurs in the biolayer of the soil in the weeping bed. Solids are not broken down very much in the tank and slowly accumulate, requiring removal. These solids are often taken to municipal wastewater treatment facilities or to landfill.
Many alternative wastewater treatment systems — including constructed wetland, peat beds, aerobic tanks, soil air injection and biofilters, among others — are extensions of this basic septic system. They use the same type of tank and weeping bed setup, but introduce a tertiary treatment element between the tank and the bed, helping to deposit cleaner water into the soil.
TYPES OF WASTE HANDLED
- Black water
- Grey water
Environmental impacts: very High
The largest impact from most septic systems is not the effluent discharge but the high volume of untreated solids that must be removed from the system and treated off-site. Even systems with high-performance tertiary treatment, such as constructed wetlands and biofilters, produce the same volume of untreated solids. Options for dealing with large quantities of these solids are limited, and few are environmentally sound.
As very little effective treatment of effluent takes place within the septic tank, the discharge from the home winds up in the soil directly outside the home. A properly maintained system draining into a well-graded and biologically active soil is unlikely to cause soil or groundwater contamination. However, any conditions that can disrupt the ecological balance in the soil can cause the system to fail, allowing contaminated water into the ground. Harsh chemical cleaners (including many off-the-shelf products) in the system, too large a volume of flow, too little flow, deep frost penetration, high volumes of grease or oil and saturation of the weeping bed due to high rains or flooding are examples of conditions that cause failure.
Impacts from septic systems are multiplied if numerous systems are in close proximity to one another, such as happens in subdivisions where municipal services are not available.
Material costs: moderate to very high
The components for a conventional septic system are relatively expensive, but are easy to price. Fully installed costs can vary dramatically depending on soil conditions in the weeping bed area. High clay content, shallow bedrock, a lot of large rock or soils that are too sandy can all create poor percolation conditions, which can only be addressed by excavating and removing large amounts of soil and replacing it with better imported soil. This can raise costs dramatically, to the point where, in areas with poor soil conditions for a conventional weeping bed, the costs of alternative systems may become favorable.
Costs for alternative systems can also vary dramatically. Some are commercially produced systems that require professional installation, while others can be built and installed by homeowners.
Recurrent costs for pumping of solids should be taken into account when considering the life cycle costs of the system.
Labor input: low to high
The labor required to make an initial hookup to a municipal sewer system is quite low. Digging to the required hookup point and a small amount of pipe fitting are all that is required.
On the municipal side, many employees are needed to build, operate and maintain a wastewater treatment system, constituting a relatively high percentage of municipal or utility company employees.
Skill level required for the homeowner
The equipment and knowledge required to install a septic system are substantial. Where it is legal for a homeowner to perform the installation, adequate training and/or research should be sought.
Regular usage requires no effort.
Maintenance — moderate to difficult
Annual septic inspections should be performed, requiring the opening of the lid on the tank (an unpleasant task due to high levels of hydrogen sulfide). A professional with the proper equipment must pump solids from the tank and clean weeping tiles as required. Solids removal is usually scheduled on a biannual basis.
Code Compliance: Easy
The components and expertise to install a conventional septic system will be widely available in any region where such systems are an accepted solution.
Alternative tertiary treatment systems will require research to find qualified designers/installers.
Twenty to thirty years is an average lifespan for a septic system, at which point the soil in the weeping field will no longer be able to process effluent and/or the weeping tiles will become clogged with solids and/or the roots of plants.
There are no moving parts in a septic system (unless a sewage pump is required due to elevation issues), but cleaning issues can shorten the lifespan if the tank overfills with solids and/or the weeping tiles become clogged with grease or solids.
Tertiary treatment systems for septic systems have been the subject of a lot of research and development and commercializing, with many more types of systems available. Many of these have been approved as accepted solutions in certain codes, though few systems are universally accepted at this point. More movement in this direction is likely to happen as wastewater treatment issues are taken more seriously.
Rapid changes in regulations dealing with solids removed from septic systems have raised the costs of the process, as waste in many jurisdictions must now be taken to approved facilities and not spread directly on the land.
Stricter regulations regarding the design and installation of septic systems are likely to come into effect as policy makers take water quality issues more seriously.
An owner can build a basic septic system, or a septic system with tertiary treatment components, with little or no energy input. The legality and functionality of homemade systems can be questionable, but there is no reason a well-made system cannot perform on par with commercial systems. However, a composting toilet system will take less effort to create and be more effective.
Tips for a successful installation
- Have the system designed by a qualified professional, working from proper soil tests performed in the weeping bed area.
- Consider using a tertiary treatment element in the system. Though this will add cost, it will also greatly improve the cleanliness of the effluent and reduce environmental impacts.