PH of the Biological Reactor

Changes in pH of the biological reactor (aeration tank or anaerobic digester) change the chemical form of several wastes and can make them more or less toxic or nontoxic. For example, the toxic effects of ammonia, chlorine, cyanide, and hydrogen sulfide are influenced by the pH of the biological treatment unit. All of these compounds are more toxic in their undissociated (non-ionized) molecular state (Table 19.22).

The degree of ionization of wastes in biological reactors is influenced by pH. With increasing pH, ammonia (NH3) is produced in large quantities (Equation 19.6).With

Influent Complete mix

Influent

FIGURE 19.10 Complete mix and plug-flow modes of operation.

TABLE 19.22 Non-ionized and Ionized Molecular States of Some Toxic Wastes

Non-ionized Molecular State Ionized Molecular State of of Toxic Waste Toxic Waste

Name Formula Name Formula

Ammonia NH3 Ammonium ion NH4+

Hypochlorous acid HOCl Hypochlorous ion OCl-

Hydrogen cyanide HCN Cyanide CN-

Hydrogen sulfide H2S Sulfide HS-

decreasing pH, hypochlorous acid (HOCl), hydrogen cyanide (HCN), and hydrogen sulfide (H2S) are produced in large quantities (Equations 19.7,19.8, and 19.9).

HCNoH+ +CN

Examples of common, non-metal toxic, inorganic wastes include ammonia (NH3), hydrogen cyanide (HCN), hydrogen sulfide (H2S), and chlorine in the form of hypochlorous acid (HOCl). Although ammonia, hydrogen cyanide, and hydrogen sulfide often are associated with industrial wastewater and the anaerobic decomposition of domestic wastewater that contains proteins, chlorine in the form of hypochlorous acid is often used in wastewater treatment plants to control malodors and undesired filamentous growth (Equation 19.10). Here, hypochlorous acid comes in contact with the active biomass.

Reduced nitrogen (i.e., nitrogen bonded to hydrogen) exists in two forms in wastewater treatment plants. These forms are ammonia (NH3) and ionized ammonia (NH4+). Ammonia is discharged to wastewater treatment plants from domestic and industrial sources (Table 19.23) and is released during the aerobic and anaerobic degradation of organic-nitrogen compounds, especially proteins.

The ionized ammonia is not toxic and serves two positive roles in wastewater treatment plants. First, ionized ammonia is used as the primary bacterial nutrient for nitrogen. Second, ionized ammonia is the energy substrate for the nitrifying bacteria in the genera Nitrosomonas and Nitrosospira. The pH of the wastewater or sludge determines the quantity of reduced nitrogen that is presence as ammonia or ionized ammonia. At pH values greater than 9, most of the reduced nitrogen in wastewater and sludge is in the form of ammonia.

The cyanide ion (CN-) has a strong affinity for many metal ions and is used in industrial applications, especially metal cleaning, electroplating, and mineral processing. Cyanide is dissociated or ionized, while hydrogen cyanide (HCN) is undis-

TABLE 19.23 Significant Industrial Sources of Nitrogenous Wastes

Automotive facilities Chemical manufacturing Coal gasification Fertilizer manufacturing Food processing facilities Landfills

Livestock maintenance Meat processing Ordnance sites Petrochemical

Pharmaceutical manufacturing Primary metal industries Refineries

Steel manufacturing Tanneries sociated. The undissociated form is highly toxic. In water, hydrogen cyanide exists as a weak acid and dissociates. With decreasing pH, less dissociation of hydrogen cyanide occurs, and more of the toxic hydrogen cyanide exists.

Dissociation of hydrogen cyanide results in the production of cyanide. Cyanide has a strong affinity for metals, especially iron, and quickly forms ferrocyanide (Fe(CN)64-) in the presence of iron. Ferrocyanide as well as other cyanide-containing (cyano-) compounds are toxic.

Hydrogen sulfide (H2S) is produced under septic conditions through the anaerobic degradation of organic compounds that contain sulfur such as proteins. It also is produced when sulfate (SO42-) is used by sulfate-reducing bacteria when they degrade organic compounds. Hydrogen sulfide also may be found in wastewater from chemical plants, paper mills, tanneries, and textile mills. The presence of hydrogen sulfide in wastewater is easily detected by its characteristic rotten-egg odor.

Reduced sulfur (i.e., sulfur bonded to hydrogen) exists in two forms. These forms are hydrogen sulfide (H2S) and sulfide (HS-). In the undissoicated form, hydrogen sulfide is highly toxic. In water, hydrogen sulfide is a weak acid and dissociates to form the sulfide ion (HS-).The pH of the wastewater or sludge determines the quantity of H2S and HS-. At pH values less than 6, nearly all of the reduced sulfur is present as hydrogen sulfide. Hydrogen sulfide is sparingly soluble in water. Therefore, it will partition between water and gas phases.

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