Several inorganic components are common to both wastewater and natural waters and are important in establishing and controlling water quality. Inorganic load in water is the result of discharges of treated and untreated wastewater, various geologic formations, and inorganic substances left in the water after evaporation. Natural waters dissolve rocks and minerals with which they come in contact. As mentioned, many of the inorganic constituents found in natural waters are also found in wastewater. Many of these constituents are added via human use. These inorganic constituents include pH, chlorides, alkalinity, nitrogen, phosphorus, sulfur, toxic inorganic compounds, and heavy metals.
When the pH of a water or wastewater is considered, we are simply referring to the hydrogen ion concentration. Acidity, or the concentration of hydrogen ions, drives many chemical reactions in living organisms. A pH value of 7 represents a neutral condition. A low pH value (less than 5) indicates acidic conditions; a high pH (greater than 9) indicates alkaline conditions. Many biological processes, such as reproduction, cannot function in acidic or alkaline waters. Acidic conditions also aggravate toxic contamination problems because sediments release toxicants in acidic waters.
Many of the important properties of wastewater are due to the presence of weak acids and bases and their salts. The wastewater treatment process is made up of several different unit processes (these are discussed later). It can be safely stated that one of the most important unit processes in the overall wastewater treatment process is disinfection. pH has an effect on disinfection. This is particularly the case with regard to disinfection using chlorine; for example, with increases in pH, the amount of contact time needed for disinfection using chlorine increases. Common sources of acidity include mine drainage, runoff from mine tailings, and atmospheric deposition.
Chloride, in the form of the Cl- ion, is one of the major inorganic constituents in water and wastewater. Sources of chlorides in natural waters include: (1) leaching of chloride from rocks and soils; (2) in coastal areas, saltwater intrusion; (3) agricultural, industrial, domestic, and human wastewater; and (4) infiltration of groundwater into sewers adjacent to saltwater. The salty taste produced by the chloride concentration in potable water is variable and depends on the chemical composition of the water. In wastewater, the chloride concentration is higher than in raw water because sodium chloride (salt) is a common part of the diet and passes unchanged through the digestive system. Because conventional methods of waste treatment do not remove chloride to any significant extent, higher than usual chloride concentrations can be taken as an indication that the body of water is being used for waste disposal (Metcalf & Eddy, 2003).
As mentioned earlier, alkalinity is a measure of the buffering capacity of water and in wastewater helps to resist changes in pH caused by the addition of acids. Alkalinity is caused by chemical compounds dissolved from soil and geologic formations and is mainly due to the presence of hydroxyl and bicarbonate ions. These compounds are mostly the carbonates and bicarbonates of calcium, potassium, magnesium, and sodium. Wastewater is usually alkaline. Alkalinity is important in wastewater treatment because anaerobic digestion requires sufficient alkalinity to ensure that the pH will not drop below 6.2; if alkalinity does drop below this level, the methane bacteria cannot function. For the digestion process to operate successfully, the alkalinity must range from about 1000 to 5000 mg/L as calcium carbonate. Alkalinity in waste-water is also important when chemical treatment is used, in biological nutrient removal, and whenever ammonia is removed by air stripping.
In domestic wastewater, "nitrogen compounds result from the biological decomposition of proteins and from urea discharged in body waste" (Peavy et al. 1987). In wastewater treatment, biological treatment cannot proceed unless nitrogen, in some form, is present. Nitrogen must be present in the form of organic nitrogen (N), ammonia (NH3), nitrite (NO2), or nitrate (NO3). Organic nitrogen includes such natural constituents as peptides, proteins, urea, nucleic acids, and numerous synthetic organic materials. Ammonia is present naturally in wastewaters. It is produced primarily by deaeration of organic nitrogen-containing compounds and by hydrolysis of urea. Nitrite, an intermediate oxidation state of nitrogen, can enter a water system through its use as a corrosion inhibitor in industrial applications. Nitrate is derived from the oxidation of ammonia.
Nitrogen data are essential in evaluating the treatability of waste-water by biological processes. If nitrogen is not present in sufficient amounts, it may be necessary to add it to the waste to make it treatable. When the treatment process is complete, it is important to determine how much nitrogen is in the effluent. This is important because the discharge of nitrogen into receiving waters may stimulate algal and aquatic plant growth. These, of course, exert a high oxygen demand at nighttime, which adversely affects aquatic life and has a negative impact on the beneficial use of water resources.
Phosphorus (P) is a macronutrient that is necessary to all living cells and is a ubiquitous constituent of wastewater. It is primarily present in the form of phosphates, the salts of phosphoric acid. Municipal wastewaters may contain 10 to 20 mg/L phosphorus as P, much of which comes from phosphate builders in detergents. Because of noxious algae blooms that occur in surface waters, there is much interest in controlling the amount of phosphorus compounds that enter surface waters in domestic and industrial waste discharges and natural runoff. This is particularly the case in the United States, where approximately 15% of the population contributes wastewater effluents to lakes, resulting in eutrophication of these water bodies. Eutrophication leads to significant changes in water quality. Reducing phosphorus inputs to receiving waters can control this problem.
Sulfur (S) is required for the synthesis of proteins and is released in their degradation. The sulfate ion occurs naturally in most water supplies and is present in wastewater as well. Sulfate is reduced biologically to sulfide, which in turn can combine with hydrogen to form hydrogen sulfide (H2S). H2S is toxic to animals and plants; moreover, in certain concentrations, H2S is a deadly toxin. H2S in interceptor systems can cause severe corrosion to pipes and appurtenances
Toxic inorganic compounds such as copper, lead, silver, arsenic, boron, and chromium are classified as priority pollutants and are toxic to microorganisms. Thus, they must be taken into consideration in the design and operation of a biological treatment process. When introduced into a treatment process, these contaminants can kill off the microorganisms needed for treatment and thus stop the treatment process.
Heavy metals are major toxicants found in industrial wastewa-ters; they may adversely affect the biological treatment of wastewater. Mercury, lead, cadmium, zinc, chromium, and plutonium are among the so-called heavy metals, which have a high atomic mass. (It should be noted that the term heavy metals is rather loose and is taken by some to include arsenic, beryllium, and selenium, which are not really metals and are better termed toxic metals.) The presence of any of these metals in excessive quantities will interfere with many beneficial uses of water because of their toxicity. Urban runoff is a major source of lead and zinc in many water bodies. (Note: Lead is a toxic metal that is harmful to human health; there is no safe level for lead exposure. It is estimated that up to 20% of the total lead exposure in children can be attributed to a waterborne route, consuming contaminated water.) The lead comes from the exhaust of automobiles using leaded gasoline, whereas zinc comes from tire wear.
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