Water, without it, we die and without a clean drinking water supply, we are likely to become sick. Therefore, the water processing plants around the world play an important part in people’s lives.
A water processing plant abstracts water either from a surface source such as a river or an underground aquifer. The degree of processing required to produce safe potable water depends on the original water supply. Water abstracted from an underground source generally requires less processing than that taken from a surface source.
The first process for water pumped into a waterworks is that of screening. Pumping through a metal mesh screen at the intake point removes any foreign objects such as leaves or twigs. Water from underground sources does not require screening. Sometimes water requires a pH adjustment using sodium hydroxide prior to any further processing.
Raw water frequently contains a lot of suspended matter. A process known as clarification removes this suspended matter along with any humic acids in the water. Chemicals such as alum and iron salts or sometimes synthetic polymers act as coagulating agents forming ‘flocs’ binding suspended matter in large clumps. The ‘flocs’ are removed from the water as sludge by either sedimentation or flotation. Dewatering of the sludge occurs in sludge pools with the reclaimed water returning to the waterworks’ intake for reprocessing. Alternative names for the clarification process include coagulation and flocculation.
Following clarification filtration removes any further suspended material. Traditional slow sand filtration using a bed of sand uses bacteria and other microbial living in the sand to metabolize and hence remove some organic materials. Granular activated carbon (GAC) filtration also removes organic materials as well as some odor and unwanted color producing chemicals. Filter beds require regular cleaning using a backwash of air and water to remove trapped debris. The backwash water is returned to the intake point for reprocessing. GAC filters require reactivation on an annual basis.
Before water reaches drinking quality standard, microbes such as bacteria and viruses must be eliminated. This requires a disinfection step. The most commonly used disinfectant within the water is industry chlorine. Chlorine containing compounds such as chloramines or chlorine dioxide are also of use for disinfection. Chlorine and its compounds also maintain their disinfectant action within the distribution system. Alternative methods of disinfection include ozone and ultra-violet light. Ozone disinfection also breaks down any remaining micro pollutants so improving the taste of the final product.
Water heavily contaminated with organic chemicals frequently requires an extra adsorption step using activated carbon. The addition of powered activated charcoal at the intake point prior to any processing allows the removal of most of such pollutants at the clarification step so preventing overloading the filtration process.
Some raw waters carry a high load of inorganic elements and compounds such as chromium, uranium, arsenic and nitrates either from passing though rocks rich in such elements or through industrial pollution. To remove such chemicals a further step is required. This is the use of ion exchange chromatography.
Raw water with a low oxygen content benefits from an aeration step at the start of the processing. This helps lower the bacterial load of the water and oxidizes chemicals such as hydrogen sulfide.
In the United States, testing for over eighty different potential contaminants is mandatory for public water distribution systems. Potable water must reach the standards required. In addition to microbial content, the industry measures a variety of organic and inorganic compounds within the finished water product. Because each step in the treatment process adds to the cost of the final product, water companies choose the best treatment for the raw water supply rather than use all available treatment methods.
Reference sources:
Thames Water
EPA Drinking Water Treatment December 1999
EPA Drinking Water and Health October 1999
Lenntech Water Treatment and purification Holding BV
