Ever wonder what happens after you flush that toilet? How about industrial waste? It all goes down the drain into the municipal or area sewage treatment plant. So how do they turn bacterial and viral effluent chock full of trace chemicals and heavy metals into potable (drinkable) water? Well it is a process. The goal of waste treatment is to remove the contaminates from the wastewater. The wastewater contains sewage, just teaming with bacterial and viral infectious organisms, chemicals and heavy metals depending on local industrial concerns. The sewage is treated to allow the cleaned effluent (treated water) to be safe for release into the environment. The by-product is a semi-solid waste or slurry referred to as sewage sludge. The sludge needs further treatment for disposal or land applications.
Pre Treatment – Screening Municipal Solid Waste
First stage is screening. A surprising amount of solid waste is included in the sewage arriving at the plant. Plastic and metal containers are common. Since most treatment plants receive storm run off as well, the sewer systems ingest a variety of solid objects. Gravity feeds the flow where possible but can be aided by pump stations if necessary that need protection against large solids. These larger objects then are transported to landfills. Another item removed early in the process is fats, oils and grease. This is done by passage through a small tank with skimmers collect the floating oil, grease and fats. Pretreatment removes all the easily collected stuff such as trash, tree limbs, leaves, branches and other large objects. The bar screen then removes all the big stuff like cans, rags, sticks and plastic objects that find their way into the sewage system. This stuff goes to the landfill or is incinerated. If not removed they can damage pumps and piping causing damage and clogs.
Grit Removal in Wastewater Treatment
True Grit – not the movie, but it includes all the sand, dirt, gravel, cinders, and other heavy material. Not to mention things like eggshells, bone chips, seeds and not least those prodigious amounts of coffee grounds. This is necessary to reduce formation of heavy deposits in aeration tanks, aerobic digestors, pipelines and channels and conduits. Maintenance of all those mysterious items just mentioned is reduced by the removal of grit by the reduction of accumulated mater and abrasion to machinery.
Primary, Secondary & Tertiary Treatment of Wastewater
Primary stage is in a basin or pond where the heavy solids can settle to the bottom of the basin. Lighter constituents float on top, like oils, grease and lighter solids. The settle solids and the floaters are removed during this stage.
Secondary treatment removes suspended biological matter. This stage utilized indigenous water-borne micro-organisms that are held in a managed habitat. These ‘digest’ the bioburden and clean up the water. Flow is equalized to promote the efficiency of clarifiers and other secondary stages as they work more efficiently if the flow is uniform.
Tertiary treatment may include the inclusion of chemical disinfectants or admission to lagoons or use micro-filtration before discharge into fragile habitats like coral reefs, low flow rivers or estuaries.
Some communities use this water for irrigation of golf courses, parks or agriculture if sufficiently cleaned of harmful biological, chemical or heavy metals.
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