By the time sewage has arrived at a wastewater treatment plant, it is in the form of a thin liquid which contains food materials, human waste, detergents, fats, oils and greases, sand clay, paper fibres as well as other waste chemicals and debris. Within the plant, it undergoes a process of screening, settlement and oxidation of polluting matter until it is sufficiently clean to be discharged back into the natural environment.
Traditional alloys used in the construction of wastewater treatment plants tend to be ferrous based materials such as carbon steel, galvanised steel and concrete lined ductile iron. These metal systems corrode and corrosion allowances are taken into consideration at the time of design. Stainless steels by contrast have the attributes of being able to handle a wide range of effluent streams experienced at wastewater treatment plants in various locations and also the treated waters, gases and sludges within each plant as they progress through the treatment processes. Typical applications are shown in Table 7.
Table 7. Typical Stainless Steel Applications in Wastewater Treatment Plants
||Travelling Bridges |
||Pumps and Valves |
|Sludge Transfer Piping
|Digester Gas Piping
|Ozone Generators and Piping
||Ultraviolet Equipment |
|Chemical Treatment Lines
Unlike carbon steels, high levels of aeration in activated sludge processes are beneficial to the performance of stainless steels in maintaining their protective surface film and keeping the surface of the metal clean. Overall ease of handling and installation enhance versatility and cost effectiveness.
Applications for Stainless Steel in the Water Industry, IGN 4-25-02, WRc. UK
Guidelines for the Use of Stainless Steels in Municipal Wastewater Treatment Plants, NI Publication 10076
L. Fassina and C.Powell, Waste Water Stainless Steel Equipment in Italy and Abroad; Applications, Guidelines and Life Cycle Cost Analysis. Stainless Steels for the Municipal Companies, Cermet/CentroInox. Bologna, Italy, June 2001
SCI P245 – Steel Package Water and Waste Water Treatment Units (2000)
Stainless Steel Water Gates (Nickel Magazine, July 2004)
A total of 520 tonnes of stainless steel were used in this wastewater project in Alabama.
A New Material for an Old Technology (Nickel Magazine, Oct. 2003)
Ten huge Archimedes screws are slated for a sewage treatment plant in Los Angeles.
Stainless Steel Sludge Scraper (Nickel Magazine, Feb. 2003)
Corrosion-resistant nickel stainless steels help improve the service life of equipment in waste water treatment plants.
China Opts for Fine Screening (Nickel Magazine, Feb. 2003)
S31600 stainless steel is used for fine screening drums in a waste water treatment plant that's designed to last 25 years.
New York City Waste Water Plant Chooses Stainless (Nickel Magazine, Oct. 2002)
A major, US $2.2-billion waste water treatment plant in New York City uses nickel stainless steel equipment for long service life, low maintenance.