Sewer Systems and Sewage Treatment
Sewer Systems and Sewage Treatment (English)
An understanding of the basic concepts of storm water and wastewater collection and wastewater treatment is necessary in order to help Surfrider activists and other concerned citizens address water pollution problems that result in beach closures, adverse human health effects, and a stressed aquatic environment.
The first important concept to understand is that in most locations, there are two separate sewer systems - a stormwater sewer system and a sanitary sewer collection and treatment system. These systems have distinctly different purposes.
Stormwater systems, consisting of gutter drains, open storm water drainage channels and underground storm sewers, are designed to prevent flooding problems. There are typically no pumps in a stormwater drainage system. Water flows by gravity to a low spot, generally the ocean. A basic design principal of stormwater systems is to get the rainwater and other urban runoff away from residential, commercial, and industrial areas as fast as possible, and discharge it to a creek, river, or directly into the ocean. Civil engineers design these systems to be water superhighways. Except in areas with Combined Sewer Systems, the water receives no treatment before it enters the receiving water body, which might be your favorite beach. So any water running off the streets and gutters, off business properties, off of your lawn and driveway, carrying with it animal waste, pesticides, fertilizer, leaked automotive fluids, brake lining residues, and general litter, will end up at the beach.
During dry weather, any water flowing in a gutter or surface drain should be looked at with suspicion. Before humans arrived on the scene with our cities, associated infrastructure, and miles and miles of pavement, there was no dry weather Urban Runoff. Hence, there was no flow to flush pollutants down to the beach. Even in wet weather, undeveloped land and Wetlands allowed infiltration and natural cleansing of runoff. Now, drainage channels have become ugly pseudo rivers that nearly always have water.
What can you do? The most important message for us is to simply turn off the tap. Use as little water as possible. Don't over-water your lawn. Adjust your sprinklers to avoid watering the pavement (it won't grow). Consider saving more water and further reducing runoff pollution by installing an Ocean Friendly Garden. Commercial car washes generally contain and recycle their water. If you do wash your own car, use as little water as possible and try to divert it to non-paved areas. Fix fluid leaks from your car. Pick up your pet waste. Don't over-fertilize your lawn and avoid the use of insecticides and herbicides (weed killers). Use a broom instead of a hose. Encourage these practices by your friends. Be on the lookout for water flowing down the gutter and if you find the source, see if you can educate the perpetrator to use more environmentally-friendly practices.
Sanitary Sewer Collection and Treatment Systems
Sewage collection and treatment systems, also sometimes referred to as sanitary sewer systems, collect and treat wastewater flowing from drains (toilets, sinks, showers, washing machines, and dishwashers) inside residences and businesses. Like stormwater systems, they make use of gravity (as much as possible), to convey wastewater to a treatment facility. Because of the gravity flow concept, treatment plants are frequently located in low areas next to rivers or the ocean. If wastewater does need to be pumped, the water flows into a concrete pit in the ground called a "lift station" or "pump station" where pumps are used to transfer it through a "force main" to the treatment plant. Sanitary sewers are always closed pipes, with vertical connections to sewer manholes situated along the pipe. If there is a blockage in the main sewer line (from grease, tree roots, or large objects flushed down the sewer) raw (untreated) sewage will flow out of the sewer manhole, down the street or nearby storm drain, and then directly into the ocean. This situation (called a sanitary sewer overflow) is one of the main causes of beach closures.
Once the sewage gets to the treatment plant, it is typically treated using both physical and biological treatment methods. The different stages of treatment are referred to as primary, secondary, and tertiary treatment. Primary treatment consists of separation of liquids from solids. Typically, the raw wastewater first passes through some grates and screens to remove large pieces of trash and debris. The wastewater then flows to large basins called clarifiers where both floatable and settleable solids are removed. Floatable solids are skimmed off the top of the water and settleable solids are removed from the bottom of the clarifier. The settleable solids are called primary treatment sludge. The basic principle at work here is to use gravity and density differences to allow water and solids to separate by slowing the flow down. Sometimes chemicals such as ferric chloride and/or anionic polymers are added to the clarifier to promote the clumping together (agglomeration) and settling of fine particles. This may be referred to as advanced primary treatment.
After primary treatment, the wastewater still contains substantial concentrations of suspended organic waste solids, and both the solids and the water contain very high concentrations of fecal coliform and other types of bacteria. Further solids and bacteria removal occurs during secondary treatment. Secondary treatment utilizes special strains of aerobic bacteria (bacteria that need oxygen to grow) to break down the organic waste left after primary treatment. The two most common processes that use aerobic bacteria are the trickling filter and activated sludge processes. In a trickling filter, wastewater is sprayed over a bed of rocks or other media that are coated with a slimy layer of bacteria that eats organic waste. The activated sludge process is more efficient but also more complicated and energy intensive. Wastewater is mixed with a bacteria-containing sludge and air and is then allowed to settle in a secondary treatment clarifier. As in the trickling filter, the bacteria eat a large percentage of the fine organic waste. Wastewater overflows from the secondary treatment clarifier to the outfall pipe or to tanks where it is stored for further treatment and use as reclaimed water.
Settled solids from the secondary treatment clarifier are combined with solids from the primary clarifier and are sent to large, closed tanks called sludge digesters. In the digesters, anaerobic bacteria eat organic matter in the sludge and produce methane gas, which can be used in heating or to generate power for the treatment plant. The digested solids are then dewatered (water is removed from the solids by filters) and the resulting biosolids may then potentially be used as fertilizer, as a soil amendment, or as fuel.
Reclaimed water is wastewater that has gone through primary and secondary treatment and typically additional filtration and/or chlorination/dechlorination. It is often used for irrigation of parks, golf courses, and general landscaping. It is not suitable for drinking. The Federal Clean Water Act requires that all wastewater treatment plants use primary and secondary treatment before discharging their water. In some cases, however, sewer agencies have been able to use a process known as a 301(h) waiver to get permission to discharge their wastewater with less than full secondary treatment. The Orange County Sanitation District (OCSD) in Orange County, California is an example of an agency that historically received such a waiver. OCSD treated approximately half of their wastewater using secondary treatment and the other half using only advanced primary treatment. The two wastewater streams were then combined and pumped out a 4-1/2 mile-long outfall pipe into the ocean. In 2002, pressure from Surfrider Foundation, other environmental groups and the general public convinced the OCSD Board of Directors to drop their application for a continued 301(h) waiver. They then embarked on a multi-year, multi-million dollar upgrade to their facilities to achieve full secondary treatment. The new facilities came online during 2011/2012. The City of San Diego also has a 301(h) waiver for their Pt. Loma treatment plant that employs only advanced primary treatment. Rather than upgrade their level of treatment at Point Loma, the city is concentrating on increasing the generation and use of reclaimed wastewater produced at other wastewater treatment facilities (potentially including Indirect Potable Reuse) to lessen the discharge of partially treated wastewater from the Pt. Loma facility.
Wastewater can also be treated to a higher standard than secondary treatment. A number of processes, called tertiary treatment, can be used to convert secondary treated wastewater into water that is good enough to drink. The processes used in tertiary treatment include reverse osmosis and microfiltration. Both of these processes pump water through membranes to remove very small particles, including bacteria and viruses. The water may then be disinfected using ultraviolet light, ozone, or chlorine. A combination of such processes is now being used in Orange County, CA to produce about 70 million gallons per day of potable-quality water in their Groundwater Replenishment System. This system has now produced over 100 billion gallons of high purity water and has greatly reduced the flow of partially treated wastewater into the ocean.
In theory, a municipal wastewater treatment plant could operate with zero discharge. Secondary treated wastewater can be reused for irrigation and other non-potable uses, while tertiary treatment can be used to produce water for potable uses. Surfrider members should all push their local sewer agency to work towards the goal of 100% recycle of their treated wastewater with no ocean discharge. It's all part of Knowing Your H2O and stopping the Cycle of Insanity.
See the 301(h) Waivers page.
Also see the Natural Resources Defense Council report titled Swimming in Sewage: The Growing Problem of Sewage Pollution and How the Bush Administration Is Putting Our Health and Environment at Risk.