Collection of pipes and mains, treatment works, and discharge lines (sewers) for the wastewater of a community. Early civilizations often built drainage systems in urban areas to handle storm runoff. The Romans constructed elaborate systems that also drained wastewater from the public baths. In the Middle Ages these systems fell into disrepair. As the populations of cities grew, disastrous epidemics of cholera and typhoid fever broke out, the result of ineffective separation of sewage and drinking water. In the mid-19th century the first steps were taken to treat wastewater. The concentration of population and the addition to sewage of manufacturing waste that occurred during the Industrial Revolution increased the need for effective sewage treatment. Sewer pipe is laid following street patterns, and access holes with metal covers allow periodic inspection and cleaning. Catch basins at street corners and along street gutters collect surface runoff of storm water and direct it to the storm sewers. Civil engineers determine the volume of sewage likely, the route of the system, and the slope of the pipe to ensure an even flow by gravity that will not leave solids behind. In flat regions, pumping stations are sometimes needed. Modern sewage systems include domestic and industrial sewers and storm sewers. Sewage treatment plants remove organic matter from waste water through a series of steps. As sewage enters the plant, large objects (such as wood and gravel) are screened out; grit and sand are then removed by settling or screening with finer mesh. The remaining sewage passes into primary sedimentation tanks where suspended solids (sludge) settle out. The remaining sewage is aerated and mixed with microorganisms to decompose organic matter. A secondary sedimentation tank allows any remaining solids to settle out; the remaining liquid effluent is discharged into a body of water. Sludge from the sedimentation tanks may be disposed of in landfills, dumped at sea, used as fertilizer, or decomposed further in heated tanks (digestion tanks) to produce methane gas to power the treatment plant.
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Other technologies: - mechanical aerators (low or high speed mixers, submersible and surface aerators), - jet aerators (Venturi) - paddle aerators
Typical efficiency of a full floor coverage diffused aeration system in clean water is 2%/ft or 6.6%/m. When converted to mass transfer into process or dirty water, it is typically closer to about half of those figures.
Both retrievable and fixed grid type diffused aeration systems are made and serve different purposes. In case a plant has a single tank, a retrievable system is desirable, in order to avoid stopping operation of the plant when maintenance is required on the aeration system. Fixed systems, on the other hand, are typically less costly, and are often more efficient because it is easier to make full use of the floor.
A developments in recent years has been surface coatings of PTFE on EPDM membranes. This provides a buffer between the EPDM substrate and wastewater, hence reducing the likelihood of chemical attack and oxidation, and also providing better resistance to biological fouling and calcium scaling.
Automated software is available on the web to assist with drafting of aeration systems in CAD, as well as calculation software to help determine diffuser requirements for a given wastewater.