Pressurised Sewers

Sewers are a means of collecting wastewater from multiple sources and delivering the wastewater to an existing collection sewer, and/or to semi-centralised secondary treatment system such as constructed wetlands (free-surface, horizontal or vertical), or waste stabilisation ponds. Conventional sewers transport wastewater and sludge by gravity and therefore require a slope and pumping station. This can lead to high construction cost in hilly or flat terrain. In some cases, a conventional system is almost impossible to construct, e.g. where the groundwater table is high or the topography is extremely flat.

Pressurised sewers (and also vacuum sewers) are not dependent on gravity to move wastewater; thus there is less concern about the local topography. Moreover the system can be built with shallow trenches only and relatively small-diameter pipes. Thus pressurised sewers are an effective solution where conventional systems are impractical, e.g. in very hilly or flat areas, densely populated areas or where the groundwater table is high. However, pressurised sewers require a lot of pumps relying on electricity supply and make the system more vulnerable to failure. Thus if the topography, climate, country do/does not require pressurised sewers other systems such as simplified and condominal sewers, solids free sewers or separate sewers or conventional sewers may be preferable.

A typical arrangement for a network of pressurised sewers is for each connection (or small cluster of connections) to have a tank that receives wastewater. When the tank fills to a set point, a pump within the basin injects the wastewater further into the sewer. This transfer of wastewater pressurises the sewer. As various pumps along the length of the sewer inject sewage into the line, the wastewater is progressively moved to the treatment facility (WERF 2010).

The storage tank at the entrance of the system is the heart of the system. It is generally a prefabricated plastic pit that provides wastewater storage, grinding (this allows small diameter piping after the pressure sewer unit) and pumping all in one in a single self-contained unit (see picture below). This all-in-one tank is called a pressure system unit. The wastewater flows first by gravity downhill into the pressure system unit from where it is grinded and then pumped into the sewer, generating the pressure. The unit is installed in a property in consultation with the owner. A small diameter discharge pipe goes from the unit to the pressure sewer pipe into the street. A small box (boundary kit) is installed just inside the property on the discharge pipe. A non-return valve (to prevent backflow from the pressure sewer) and isolation valve is housed in this kit. The unit is wired to the household power supply and controlled by a small panel located near the unit (control panel), either on a wall, fence or pole (SHOALHAVEN n.y.).

An alternative to the all in one pressure sewer unit described above is to first install a septic tank situated outside in the ground or in the basement of the building to settle the solids. The septic tank effluent then flows in an underground holding tank containing a pump and control devices from where it is then pumped into the pressurised sewer system and transferred for treatment. Retrofitting existing septic tanks in areas served by septic tank/leach field systems would seem to present an opportunity for cost savings, but a large number (often a majority) must be replaced or expanded over the life of the system because of insufficient capacity, deterioration of concrete tanks, or leaks. The pump in this system does not have to be as powerful as grinder pump, since it just pumps liquid (U.S. EPA 2002).

The cost of a pressure sewer system can be divided into two major components: The on-lot costs and the collection network costs. On-lot costs include the pump, basin, controls, building sewer, lateral piping, electrical service, and installation. The collection network includes all the piping in the utility easements that directs the sewage to the treatment facility. Depending on the style of pump and basin selected by the managing utility, on-lot costs are estimated to be US$ 4,800 to US$ 7,200 (prices are estimated for the United States of America, 2009 dollars) for an existing single-family home. Typical solids-handling pumps will use less than 1 kW-hr of power per day and the electrical cost would be about 50 dollars per year depending upon local electrical rates (prices are estimated for the United States of America, 2009 dollars). Using many low power-consuming pumps reduces installation cost as compared to a conventional gravity system (e.g. conventional sewers) that may require one or more large-capacity lift stations (sewer pumping stations) as conventional large scale systems barely can be operated by gravity only. A pressurised system also allows more flexibility in choosing locations for and routes to treatment facilities (adapted from WERF 2010).

Regular service is important for all system components to ensure best long term performance to protect public health and the environment. This also protects the investment. Frequency of operation and maintenance is dependent upon wastewater volume, relative risk to public health and the environment as well as the complexity of components used (Adapted from WERF 2010). Electricity needs to be available all the time, the pumps should be checked regularly and the pipe connections should be controlled for leakages.