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<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

4.2 Collection and transfer (Topic b)

Sewage is generally conveyed to a treatment facility by gravity piping, and/or pressure force mains. The exceptions are small on-site systems such as pit privies, composting toilets and incinerating toilets in which there is no need for sewage conveyance.

4.2.1 Wastewater

Transport from on-site systems

On-site treatment systems typically serve single dwellings, or small clusters of dwellings. They are used when central collections systems are not available or are not economically practical due to disperse population, difficult construction (shallow rock), or other practical constraints. On-site treatment systems typically dispose of treated effluent directly into the ground, and rarely into surface waters such as creeks, rivers, and lakes, although marine discharges may be permitted in certain jurisdictions

In Canada, 20-25% of the population lives in rural surroundings, much of which is unlikely to be provided with municipal piped services. In the United States approximately 35 percent (about 87 million) of the population is served by non-centralized treatment and disposal systems, and this percentage is expected to increase over the next ten years.

Waste sludge generated by these systems may be in the form of septage from septic tanks, waste activated sludge from small activated sludge plants, or chemical and biological sludge from small industrial pre-treatment plants. The sludge is usually taken from the site by vacuum trucks, and transported to a nearby central facility for co-treatment and/or disposal. The central facility may be a wastewater treatment plant with a special septage receiving station, or it may be a composting or lime stabilization operation. Landfilling, while prohibited in many locations, is still practiced in various parts of North America.

On-site systems can also be incorporated into central collection systems such as the septic tank effluent pumping (STEP) process, small diameter variable slope piping, or vacuum piping systems.

Individual household septic tanks provide pre-treatment (grease, oil, and solids removal) in STEP systems. Septic tank effluent is then pumped into small diameter (25-50 mm) plastic pipes using a small (1-2 horse power) grinder pump, and the effluent is transported to a common secondary treatment facility through a low pressure or gravity-flow collector sewer. A typical application location would be housing located along a shoreline, or areas where conventional deep trench sewer construction is economically impractical due to the presence of rock near the surface.

Advantages of the STEP system include the use of low cost small diameter pipes (from 2 inches (50mm) to 6 inches (150mm)); pipe layout which is independent of topography; reduced excavation, infiltration, and manholes compared to gravity sewers; less power requirement than vacuum systems, and typically lower construction costs than other alternatives. Disadvantages include the need for effluent pumps or grinder pumps at each residence; electrical power is required; air release valves are needed; flushing connections are required for periodic pumping of septic tank; and power outages disturb service.

Small diameter variable slope systems are also used with septic tanks, but the tanks are fitted with a positive method for solids removal such as an effluent filter vault, internal deflection baffles, and inclined clarifier tubes. A net positive slope from inlet to outlet is required, but pipe is laid at a constant depth regardless of grade. The resulting flow regime includes surges, delays, transitions from full to partial pipe, and continually full sections of pipe. Air release valves are required. Cleanout ports are installed instead of manholes, and piping is cleaned by pigging. This technology was developed by Rural Housing Research Unit of USDA-ARS, Tuskegee Institute, and Farmer's Home Administration.

Vacuum collection systems combine gravity flow from the wastewater source to a holding tank upstream of a vacuum ejector valve, which seals the vacuum from the main pipe, and opens automatically on tank level allowing a plug of liquid to enter the main. A central vacuum pump station maintains the vacuum main at approximately 400 mm Hg. This type of system is not generally cost effective compared to small diameter variable flow or STEP systems. This type of system is typically used in areas with limited water sources, small low-lying areas such as marinas, or ski resort facilities on the tops of mountains (e.g. Whistler Mountain, B.C. Canada).

Advantages of vacuum systems include: electrical power is not needed at each residence; reduced excavation compared to gravity sewer systems; grade and depth of trenching is not critical; exfiltration is virtually impossible; and manholes are not needed. The disadvantages include: electrical power is required at central vacuum station; vacuum valves have a shorter life than pumps; the system has higher energy consumption than other alternatives; there can be major odor problems at the central vacuum station; and vacuum loss (system break) requires immediate repair.

Gravity systems

Gravity collection and transport systems for wastewater are the most common in North America. They are usually the most economic system if the topography is favourable. Their performance is well-established and documented, and their design, construction and operation are well understood. The systems include a combination of gravity sewers, pump stations and forcemains.

Piping for new gravity systems is plastic, with PVC being the most likely choice. Bell and spigot joints with elastomer gaskets are usually used for diameters less than 200 mm. In larger sizes, common choices include PVC, ribbed polyethylene piping, and reinforced concrete piping, all with elastomer gaskets. The choice is usually dictated by both economics and design conditions. A consideration however is sulphate corrosion of reinforced concrete piping - although sulphate resistant cements are available.

Where a collection area is lower than the overall system, wastewater must be pumped. Stations handling smaller flows generally use submersible pumps installed in a wet well, while larger stations will often use separate pumps in a dry well for ease of maintenance. Underground forcemains are usually made of plastic, but piping in dry and wet wells is usually made of coated steel for durability. Odour control for pump stations and forcemain outlets can be an issue, and biofilters and carbon filters have been used successfully for odour control.

Infiltration minimization for wastewater management cost control is a concern in many communities, especially those with older sewers. In-situ lining of old piping often is an economic solution, and is often referred to as no-dig or no-trench sewers. There are a number of methods in which smooth liners are installed in pre-existing old sewer pipe to increase the capacity of the pipe and reduce or eliminate the infiltration of groundwater into the pipe.

4.2.2 Stormwater

The high percentage of paved land in urban North America has increased the amount of stormwater runoff from streets and parking areas, and reduced retention time before release to surface water resulting in sharper and higher flow peaks. Paved areas are sloped to catchbasins, which collect refuse before stormwater enters piping. Piping materials are generally the same as for sanitary sewers, with the exception that sulphate resistance is typically not an issue. The pipe is usually sized based on a design year storm event, with the specific recurrence period specified by local or regional codes.

Small areas such as parking lots, or industrial sites such as mill yards with specially contaminated runoff, may treat the stormwater runoff prior to discharge to a central stormwater or combined sewer system. Green space around individual homes, in parks, and in agricultural areas within urban catchments is often used to provide a passive form of stormwater treatment and transportation/retention, linked by ditches and culverts.

Combined sewer systems (storm and sanitary combined) often exist in older large urban communities; a result of common practice in the early 20th century. Over 45 percent of communities in the US with populations greater than 100,000 have combined sewers (Metcalf and Eddy Inc. 1991). Many communities are separating them as they replace the older infrastructure. In new developments, stormwater piping is now separate from sanitary sewage piping.

Stormwater control

Overland flow, or runoff, is sometimes directed into control structures to flatten peak flows, especially in combined systems which discharge to a treatment plant. Swales (flat grassed ditches) are used to direct flow around structures. Ponds, which may be dry grassy areas and part of parkland in the dry season, are allowed to fill and drain naturally. Peak flows may also be diverted into reservoirs, which are drained later when stormwater flows abate.

Technologies for stormwater diversion and flow control range from simple side weirs, to automatic gates which respond to water level, tipping gate regulators whose motion is controlled by water level upstream and downstream of the gate, and Hydro-Brake regulators. The latter, a patented device, causes centrifugal motion in the entering liquid, which decreases the amount of water passing through the outlet and maximizes flow storage in the upstream sewer.

 

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