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Saturday, February 14, 2009

Water Treatment Facilities and the Wastes Removed

There are three general types of water treatment plants which can be classified by their sources. Water treatment plants get their water from either surface or groundwater sources. Wastewater treatment plants get their water from sewage and storm drainage collection systems. Each one of these systems has a unique form of water source impurities to treat, unique methods for treating the waters, a unique set of wastes that are generated, and finally they each have a unique method of handling and disposing of their wastes.

In surface water, the sources of impurities come from contamination in un-maintained watersheds, agricultural runoff, industrial discharge into rivers, lake tourists and campers, and boat fuel. If a lake gets loaded with nutrients then there is a risk of a biological algal bloom which, when treated with copper citrate as an algaecide, can cause bacteria to consume the dead algae and thereby deplete the lake of its dissolved oxygen content, or the lake could become stratified during cooler or warmer seasons, which prevents mixing and leads to the depletion of dissolved oxygen as well. This oxygen depletion not only causes the lake to become more acidified, but it can also cause the ferric and manganic ions to become reduced to more a more polar form of ferrous and manganous hydroxides causing them to dissolve in the water, ultimately causing taste and odor problems, water discoloration, and clothes staining. Also, sulfates will be converted to a more polar hydrogen sulfide structure which really smells bad and presents a health hazard. Of course, there are also many different things, large and small, which can get into the surface water reservoirs and that need to be removed from the water prior to distribution.

The water travels through a series of water purifying treatment elements, each of which need to be periodically cleaned when the waste buildup reaches a certain level. Bar screens are used to remove relatively larger things that are bigger than about an eighth of an inch. The stuff that doesn’t make it through the bar screens is then scraped, washed, dried, compacted, and then hauled to a dump. Next, the water it is pumped through a pipe where it is mixed with a chemical coagulant, which also is a contributor to waste, and then the water is stirred or flocculated so that the suspended particles become globbulated and settle out as sludge in the bottom of the clarifier. The sludge that settles on the bottom is periodically pumped out of the bottom of the clarifier, centrifuged or pressed to thicken it, and then shipped by tanker trucks to drying beds or lagoons for disposal.

Groundwater treatment facilities usually have the job of demineralizing or softening waters that have a very high concentration of total dissolved solids. They generally use reverse osmosis, ion exchange resins, de-ionization resins, electrodialysis, micro- and nano-filtration, oxidation/filtration, and/or distillation for purifying the water to different degrees of quality depending on the necessary localized application. All of these softening elements need to be periodically backwashed or regenerated, and so the cleaning cycles for these elements tend to produce high TDS or brine levels that need to be disposed of properly. In some cases it is okay to flush the wastewater down a sanitary sewer for the wastewater plant to handle, but in most other cases the solids need to be pressed dry or centrifuged and then hauled out to a solar lagoon or sludge disposal site where it is dried and sedimentated.

In wastewater treatment facilities, the waste can come from residential facilities, businesses, chemicals which enter in through storm drains, from cesspools and septic tanks, and from chemical additives that are dosed into the water at the facility. Waste is periodically collected from trash racks, grit collectors, sludge settling basins, scum scrapers, secondary clarifier basins, and from filter backwash water. Waste from a wastewater treatment plant is thickened in centrifuges and then it is digested in a heated solids digester in order to eliminate some of the biohazard before disposal or land application. The methane gas from the digester can be stored and used as energy, or it can be burned off, but it is important to combust it because methane is a worse greenhouse gas then carbon dioxide is. Wastes from the trash screens get washed, compacted and then sent off to a landfill.

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Sources:

1) Wastewater Facilities

2) Water Treatment Facilities

3) Montana Ground Water Manual

4) “Water Treatment Plant Operation” volume 1 and 2, Kennith Kerry

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