April 07, 2015
Safe drinking water, and the care and treatments needed to make it so, are serious responsibilities for cities. It is also where, unseen by citizens, many nickel-containing stainless steel applications are at work.
Drinking water and recreation
Portland Maine, a city of 66,000 people on the North American eastern coast, provides an example of how stainless steels are ensuring one essential of life—drinking water—even as another important part of quality of life—recreational use of water—continues.
Inland some 30 kilometres from Portland are a number of fresh water lakes, the largest being Sebago Lake. It is a deep lake, with a surface area of some 115 square kilometres (45 square miles) and containing almost 3.78 billion cubic metres of water (approximately one trillion US gallons). In 1869 a connection to the lake was established and the first water service came to Portland. Today the lake water remains vital for the city of Portland but some 90% of the lake remains open to public boating and swimming, secure in the knowledge that the city’s drinking water will arrive in homes clean and healthy.
Increasing demands on materials
The federal Safe Drinking Water Act was amended in 1986 to require that surface water be filtered and established new standards for disinfection. The exceptional quality of water from Sebago Lake meant filtration was not required but the Portland Water District (PWD) decided to switch from chloramines
(a less problematic alternative to the use of chlorine) to ozone for primary disinfection. The Portland facility was the first free-standing ozonation plant in the USA to meet the disinfection requirement with ozone alone.
A project to update the ozone disinfection process and install a new UV water treatment system was completed in 2014. Gaseous oxygen for ozone generation is now obtained from liquid oxygen stored outside the plant and delivered through a stainless steel system. Joel Anderson, Chief Operator of the water treatment plant, notes that two Ozonia ozone generators, one acting as a spare, now generates the ozone used to make the 8 to 10% ozone gas that is dissolved into process water and then added to the reaction tank. The generator shells and associated piping are all stainless, mostly Type 316 (UNS S31600). Once treatment is complete, any remaining ozone is converted back to oxygen in ozone destruction units. The oxygen is then discharged into the atmosphere.
A more recent Federal Environmental Protection Agency regulation focused on concern for the pathogen Cryptosporidium (a diarrhea-causing protozoan). While PWD has never detected Cryptosporidium in a two year monitoring program, UV treatment was incorporated as an additional level of protection against potential contamination. Two Wedeco UV units were installed, and again back-up is provided as only one unit is used at a time.
Until the water leaves the plant, essentially all piping is stainless steel. For corrosion control and to prevent possible contamination in the non-stainless steel potable water distribution system, chemical inhibitors and chloramines are added. But the water that enters the system is as safe and of as high a quality as Sebago Lake can offer, thanks to appropriate treatments enabled by nickel-containing stainless steel.