In the primary (or mechanical) treatment stage of wastewater (sec the schematic diagram in Figure 14-8), the larger particles—including sand and silt—are removed by allowing the water to flow across screens and slowly along a lagoon or settling basin. A sludge of insoluble particles forms at the bottom of the lagoon, while "liquid grease" (a term which here includes not only fat, oils, and waxes but also the products formed by the reaction of soap with calcium and magnesium ions) forms a lighter-than-water layer at the top and is skimmed off. About 30% of the biochemical oxygen demand (BOD, Chapter 13) of the wastewater is removed by the primary treatment process, even though this stage of the process is entirely mechanical in nature. The treatment and disposal of the sludge is discussed later.
After passing through conventional primary treatment, the sewage water has been much clarified but still has a very high BOD—typically several hundred milligrams per liter (ppm)—and is detrimental to fish life if released at this stage (as occurs in some jurisdictions that discharge into the ocean). The high BOD is due mainly to organic colloidal particles. In the secondary (biological) treatment stage, most of this suspended organic matter as well as water
Tertiary treatment iiF. t o n o „ o o Waste-_a, . ®
• Solid matter
Removal of various chemicals
River or lake that actually dissolved in the water is biologically oxidized by microorganisms
' 7 t t. iii i i common stages in the to carbon dioxide and water or converted to additional sludge, which can treatment of wastewater.
readily be removed from the water. Either the water is sprinkled onto a bed of sand and gravel or of plastic covered with the aerobic bacteria (trickling filters), or it is agitated in an aeration reactor (activated sludge process) in order to effect the microorganism-driven reaction. The system is kept well-aerated to speed the oxidation. In essence, by deliberately maintaining in the system a high concentration of aerobic microorganisms, especially bacteria, the same biological degradation processes that would require weeks to occur in open waters are accomplished in several hours.
The biological oxidation processes of secondary treatment reduce the BOD of the polluted water to less than 100 ppm, which is about 10% of the original concentration in the untreated sewage. For comparison, Canadian wastewater quality standards require that the BOD in treated water be 20 ppm or less. The process of nitrification also occurs to some extent, converting organic nitrogen compounds to nitrate ion and carbon dioxide (see below). In summary, the secondary treatment of wastewater involves biochemical reactions that oxidize much of the oxidizable organic material that was not removed in the first stage. Treated water diluted with a greater amount of natural water can support aquatic life. Normally, municipalities take the water produced by secondary treatment and disinfect it by chlorination or irradiation with UV light before pumping it into a local waterway. Recent research in Japan has shown that chlorination of the effluent before its release produces mutagenic compounds, presumably by interaction of chlorine-containing substances with the organic matter that remains in the water.
A few municipalities employ tertiary (or advanced or chemical) wastewater treatment as well as primary and secondary ones. In the tertiary phase, specific substances are removed from the partially purified water before its final disinfection. In some cases, the water produced by tertiary treatment is of sufficiently high quality to use as drinking water. Alternatively, river water into which the effluent from sewage treatment plants has been deposited is used by municipalities downstream as drinking water. The reuse of water after it has been cleansed is particularly prevalent in Europe, where less fresh water is available than in North America and the population density is high.
Depending upon locale, tertiary treatment can include some or all of the following chemical processes:
• further reduction of BOD by removal of most remaining colloidal material, using an aluminum salt in a process that operates in the same manner as described previously for the purification of drinking water;
• removal of dissolved organic compounds (including chloroform) and some heavy metals by their adsorption onto activated carbon, over which the water is allowed to flow (see Box 14-1);
• phosphate removal (as discussed in the next section; some phosphorus is removed in the secondary treatment stage since microbes incorporate it as a nutrient for their growth);
• heavy metal removal by the addition of hydroxide or sulfide ions, S2~, to form insoluble metal hydroxides or sulfides, respectively (see Chapter 15);
• iron removal by aeration at a high pH to oxidize it to its insoluble Fe3+ state, possibly in combination with use of a strong oxidizing agent to destroy organic ligands bound strongly to the Fe2+ ion, which would otherwise prevent its oxidation; and
• removal of excess inorganic ions, as discussed below.
In some wastewaters, the further removal of nitrogen compounds—usually either ammonia or organic nitrogen compounds—is deemed necessary. Ammonia removal can be achieved by raising the pH to about 11 (with lime) to convert most ammonium ion to its molecular form, ammonia, NH3, followed by bubbling air through the water to air-strip it of its dissolved ammonia gas. This process is relatively expensive, however, because it is energy-intensive. Ammonium ion can also be removed by ion exchange, using certain resins that have their exchange sites initially populated by sodium or calcium ions.
Alternatively, both organic nitrogen and ammonia can be removed by first using nitrifying bacteria to oxidize all the nitrogen to nitrate ion. Then the nitrate is subjected to denitrification by bacteria to produced molecular nitrogen, N2, which bubbles out of the water. Since this reduction step requires a substance to be oxidized, methanol, CH3OH, is added if necessary to the water and is converted to carbon dioxide in the process:
5 CH3OH + 6 NOf + 6 H~-» 5 C02 + 3 N2 + 13 HzO
Continue reading here: Time Dependence of Concentrations in the Two Step Oxidation of Ammonia
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