Because of the increase of urban infrastructure of roads and parking lots, rainwater is being prevented from natural drainage. Run-off from these tarred surfaces cause problems to combined stormwater and wastewater plants, in some cases causing the release of raw sewage into rivers, lakes, or sea.
Gate Valves in Stormwater and Wastewater Drainage Systems
Stormwater and wastewater (sewage) systems can be built as separate or combined facilities. The more modern houses and businesses now have separate piping systems for sewage and stormwater, while some cities are upgrading their present combined systems due to the occurrences of spillage of sewage into the environment following a rainstorm. This spillage can be from overflow at the wastewater plant or from regulators in the piping system, but both measures involve dumping raw sewage into the local environment.
This happens because of a combined plant’s inability to process the additional stormwater and sewage at the same time, preventing a back-flow of sewage to the local houses.
This is an article on flow control of drainage from wastewater and stormwater systems, in particular from combined sewage systems. We begin with a look at the some of the problems with a combined sewage plant and then move on to the control of outflow.
Combined Wastewater and Stormwater Plants (Combined Sewage)
As we have seen, combined wastewater and stormwater plants are being phased out in favor of a separate plant for each process, however many major cities of the developed world do still use this system. This is mainly because sewage and stormwater collection in the same piping system was acceptable in the early 1900’s. However, as the city populations increased, the sewage plants could not handle both sewage and stormwater drainage, and thus we have the current problem of controlling the inflow, processing, and outflow from a combined sewage processing plant.
To overcome the problem of sewage overflow from current plants, some plants install large underground storage tanks to hold the influx of stormwater and sewage, which is then processed later. Tunnels at the facility have also been used to store and treat drain water and sewage influx following rainstorms.
In order to have separate processing facilities, the stormwater and wastewater must be collected by two separate systems; one for sewage and grey water, and one for rainwater drains. In most cities with combined sewer systems, it is difficult to convert to separate systems, but many cities in the US and Europe are converting a little at a time.
In the developing countries, there is little effort to treat sewage. 80-90% of sewage is pumped directly into rivers, lakes, or the sea; the majority of Sub-Sahara countries are without any sewage processing facilities.
Combined Wastewater and Stormwater Drainage/Overflow Control Valves and Components
To prevent local flooding during a rainstorm, the excess rainwater from run-off must be stored then processed at a later date.
However during a particularly heavy and prolonged rainstorm even the largest of storage tanks can fill, so the flow of water/sewage from the plant must be controlled.
Because of the nature of the effluent, gate valves should not be used as control valves in a wastewater system.
Listed below are some of the control valves used along with other components to control the flow of effluent into the environment.
Weighted Hinged Flapvalve
This consists of a heavy bronze hinged flap that is fitted to the pipe discharge. It is weighted, usually with lead bars, and has PTFE seals on the flap valve faces. As the water builds up behind the flap, the pressure builds up until it is sufficient to open the valve, discharging the effluent. Once the flow recedes, the pressure reduces and the valve swings shut, closing off the flow.
This is a very simple method, but it is not without problems due to debris lodging in the valve/pipe seal, which has to be cleared regularly.
Hydro-Valve Vortex Control
This is a relatively new concept, and as it has no moving parts, is quite reliable. It operates on the principle of a vortex, which creates a different pressure at the inlet to the outlet, when there is normal flow the valve allows the processed water to discharge. The vortex is produced by high flow from a prolonged rainstorm entering a vortex chamber; this restricts the flow to a predetermined controlled rate.
Hydro-chamber Underground Storage
The hydro-chamber consists of an arched corrugated unit manufactured from high density polyethylene. It is buried under car-parks or other non permeable surfaces and the run-off is fed to the chamber. Here the water is allowed to filter through the soil into the ground naturally. The chambers can be joined together to provide a variety of sizes. Although this is not strictly controlling the overflow from a wastewater/stormwater plant, it is stopping the excess water from entering the plant.
Hydrocell
This consists of a polypropylene pipe which can be joined together to form an underground storage facility at the combined sewage plant.
Sanitary Sewer Overflow (SSO)
The combined sewer overflow is a flow control system which operates when there is excess water entering the system during and following rainstorms. The SSO is installed into the pipelines supplying the wastewater and stormwater to a combined sewage facility. When shut, the effluent is allowed to flow along the piping to the plant, opening when required and diverting the effluent from the plant directly to another outflow into a local water-body.
SSO’s are regarded as regulators and incorporate a plug valve to achieve this. A plug valve consists of a tapered plug having a slot that the liquid flows through and has has only two operating positions – full open or tightly shut.
Plug valves used in SSO are normally manufactured from nickel and can be actuated by a motor through a gearbox or manually through an extended T-wrench, with a quarter turn opening or closing the valve.
