WASTER WATER SYSTEMS: MATERIALS ENCYcLOPEDIA
HOW THE SYSTEM WORKS
A building’s drain system empties into a publicly maintained sewer system, a network of subterranean pipes that flow (or is pumped) to a centralized wastewater treatment facility. These facilities are typically located close to a natural body of water, as a natural low-elevation point and a place to discharge treated water.
The operations and processes at wastewater treatment facilities vary depending on local practices, regulations and environmental conditions, but all follow a similar overall procedure:
Primary treatment — Wastewater is collected in holding tanks or settling ponds so that “scum” (grease, oil, soaps) can rise to the surface and solids settle to the bottom. Scum is removed for separate treatment or landfill disposal. Solids, or “sludge,” is typically sent to a digester, where anaerobic activity helps to break down dangerous pathogens.
Secondary treatment — En route from primary to secondary treatment water is aerated. Secondary treatment processes vary widely between facilities. In some cases, additional settling and mechanical filtration are the only remaining steps before the water is discharged into waterways. Sometimes it is given adequate conditions to allow microorganisms to treat dissolved and suspended biological matter.
Tertiary treatment — There are a variety of possible tertiary treatments, including biological processes, chemical treatment and microfiltration. After tertiary treatment, water is released to the environment, or may be directed toward other uses such as irrigation or industry.
Sludge treatment — In every cubic meter of wastewater, there is between 80 to 220 grams of solids. This aspect of treatment facilities is often overlooked when volumes of treated water are discussed. Sludge is subjected to different treatment processes, from landfill disposal to digestion, drying and finishing. It is often highly contaminated and only partially treated before being used as fertilizer, resulting in contaminated surface water and soil.
Any discussion of public wastewater treatment must be preceded by a reminder that many municipalities provide little or no treatment of wastewater before it is ejected into waterways. According to Environment Canada, in 2009, the water used by 18 percent of the population has only primary treatment or less. The Clean Water Act (1972) and supporting grants in the U.S. eliminated raw sewage discharge by the 1980s, but according to the EPA ten million people are still served by systems that provide only primary treatment. In both countries, the number of people served by systems that include some form of tertiary treatment prior to discharge of wastewater is remarkably small.
TYPES OF WASTE HANDLED
- Black water
- Grey water
Environmental impacts: very High
Residential use of surface water does not typically have negative impacts on surface water sources. Large municipal systems can draw quantities significant enough to change overall water levels in some surface sources and create changes to ecosystems.
Material costs: very high
There are two elements to the costs of municipal sewer systems. Initial hookup costs will vary by municipality, but are usually lower than the cost of installing most on-site treatment systems.
The larger costs include the public maintenance and capital investment required to operate the system. A high percentage of most municipalities’ budget, all citizens bear the high cost of these systems through local taxation.
Labor input: very 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
Maintenance — Easy
Hookup to a municipal sewage system is an acceptable solution in all codes. Where the service is available, hookup may be a requirement and other options may be disallowed.
Municipal sewage facilities are available in the majority of towns and cities.
Durability: low to moderate
The portion of the system for which the homeowner is responsible is very durable. The publicly owned balance of the system requires constant maintenance and updating.
Pressure is growing on all levels of government to address wastewater effluent issues. A growing number of municipalities are investing in better secondary and tertiary treatment systems, and many of these are using technologies pioneered by sustainable builders, including constructed wetlands and lagoons, in which biological processes are managed to add a significant degree of cleaning to effluent.
Current improvements in municipal wastewater treatment are motivated by concern about the pollution of waterways. A movement toward seeing the nutrients in “waste” water as valuable resources could provide further motivation for improvement and help turn a revenue-losing activity into a generative process.
Wastewater treatment infrastructure depends entirely on fossil fuel energy. From pumping stations to move effluent to the plant-to-plant operations and hauling of treatment products and departing solids, there is no part of the system that can operate with little or no energy.
Tips for a successful installation
Connecting a home to an available city sewer service is not complicated. Be sure to determine the location and depth of the service, and follow local regulations for applying for hookup.