The Final Touches To Water

<> Disinfection - water completely free of suspended sediment, is treated with a powerful oxidizing agent usually chlorine, chlorine and ammonia (chloramine), or ozone. A residual disinfectant is left in the water to prevent reinfection. Chlorine can form harmful byproducts and has suspected links to stomach cancer and miscarriages.

<>pH adjustment - so that treated water leaves the plant in the desired range of 6.5 to 8.5pH units.

conditioner, as well as a some fertilizer value. This requires the highest quality "biosolids", free of contamination with toxic metals or industrial organic compounds, and low in pathogens. At a somewhat lower quality, it can be used for similar purposes on non-agricultural land and for land reclamation (e.g., strip mines). Poorer quality sludge can be disposed of by landfilling or incineration. One commonly used method of sludge treatment, called digestion, is biological. Since the material is loaded with bacteria and organic matter; why not let the bacteria eat the biodegradable material? Digestion can be either aerobic or anaerobic. Aerobic digestion requires supplying oxygen to the sludge; it is similar to the activated sludge process, except no external "food" is provided. In anaerobic digestion, the sludge is fed into an air-free vessel; the digestion produces a gas which is mostly a mixture of methane and carbon dioxide. The gas has a fuel value, and can be burned to provide heat to the digester tank and even to run electric generators. Some localities have compressed the gas and used it to power vehicles. Digestion can reduce the amount of organic matter by about 30 to 70 percent, greatly decrease the number of pathogens, and produce a liquid with an inoffensive, "earthy" odor. This makes the sludge safer to dispose of on land, since the odor does not attract as many scavenging pests, such as flies, rodents, gulls, etc., which spread pathogens from the disposal site to other areas- and there are fewer pathogens to be spread.

A liquid sludge, which might contain 3 to 6% dry weight of solids, can be dewatered to form a drier sludge cake of maybe 15 to 25 percent solids, which can be hauled as a solid rather than having to be handled as a liquid. Equipment used to dewater sludge includes centrifuges, vacuum filters, and belt presses or plate-and-frame presses. Chemical coagulants are commonly added to help form larger aggregates of solids and release the water. Further processes such as composting and heat drying can produce a drier product with lower pathogen levels. Another approach involves treatment with lime (calcium oxide), which kills pathogens due to its highly alkaline nature as well as the heat that is generated as it reacts with the water in the sludge; this also results in a drier product. A final disposal method which eliminates all of the pathogens and greatly reduces the volume of the sludge is incineration. This is not considered a beneficial use, however, and is becoming less popular due to public concerns over air emissions. Sludges from physical-chemical treatment of industrial waste streams containing heavy metals

and non-biodegradable toxic organic compounds often must be handled as hazardous wastes. Some of these will end up in hazardous waste landfills, or may be chemically treated for detoxification- or even for recovery of some components for recycling. Recalcitrant organic compounds can be destroyed by carefully controlled high-temperature incineration, or by other innovative processes, such as high-temperature hydrogen reduction.

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