All industrial processes, whether for manufacturing, refining, processing, disposal, or decontamination, generate waste. Agricultural, medical, and mining activities also generate waste but will not be addressed here. Primary wastes are those created along with commercial products. Secondary wastes are those produced during treatment of primary wastes. The factory pictured here is generic and could be the site where any of the waste-generating processes mentioned above occur. The figure addresses only the liquid wastes produced; those potentially amenable to microbial treatment are contained in aqueous milieu.
Factory wastes may be discharged directly to the environment in the form of lagoons (ponds or impoundments) on the soil surface. Both planktonic organisms (including phototrophic bacteria and algae) and subsurface organisms (anaerobic bacteria and archaea contained in sediments) may act upon dissolved contaminants. Microbial waste treatment of organic residues may result in complete mineralization to CO2 and/or CH4.
Biodegradation occurs via fermentation or respiration. Recalcitrant organic compounds may be transformed or partially degraded; in some cases, the biotransformation products may be more toxic than the parent compound. Alternatively, organics may undergo biosorption—a physicochemical process that incorporates adsorption to the cell surface, absorption (uptake) into the cell, and/or association with insoluble cell products such as exopolymers. Inorganic contaminants are not biodegradable but may be biotransformed via microbial oxidation or reduction. Biotransformation typically involves anaerobic respiration of metals or oxides by lithotrophic prokaryotes. This respiratory metabolism can also result in precipitation (either intra- or extracellularly) of contaminants as insoluble salts. Dissolved metal contaminants can also be precipitated indirectly, as sulfides, by sulfate reducers. Inorganic materials may adsorb to cell surfaces by ion exchange at acidic or basic sites or by complexation. The surface groups involved in all of these activities are typical biomolecular components (proteins, lipids, polysaccharides, and possibly nucleic acids) of the microbial surface. That surface may be cell wall, outer membrane, or extracellular structures such as capsules or slime layers.
Wastes are stored either on site (within the fence illustrated here) or off site, and above or underground. The stored wastes are not sterile; biodegradation, biotransformation, and/or biosorption probably occur within the waste container. But microbial activity can also result in acid and gas production in situ, changing the chemistry of the waste mixture. Microbial corrosion of the waste container, especially when coupled with internal pressure changes, can cause leaks. Leaked waste can affect soil and groundwater quality. Organisms in the affected area may carry out relevant biodegradative, biotransformative, or biosorptive activities; or they may be killed by toxic contaminants. Released wastes may have a significant impact on local microbial ecology. Toxic releases, instead of sterilizing soil, sediment, or groundwater, appear to enrich and select for specific microbial ecotypes (acidophiles, alkaliphiles, halophiles, etc.). It may be noted that groundwater and surface water are contiguous. The fence around the factory does not prevent migration of dissolved contaminants into groundwater flows.
Factory wastes are often treated on site, within a “treatment works” consisting of one or more chemical stages (adsorption, oxidation, pH adjustment, etc.). Biological treatment with specific microbial types (such as nitrate-reducing bacteria for treatment of nitrate wastes) is becoming more popular. Microbial treatment can indeed achieve many of the same goals of chemical (nonbiological) treatment and serve as a more environmentally-acceptable substitute. Biological processes do not require use of chemical solvents or operation at extremes of temperature, pressure, or pH, and thus limit capital and operating costs and environmental impact. The insoluble secondary waste, or sludge, produced within the treatment works may be stored on or off site. Concerns about release to the environment are described above with reference to primary waste. Sludge may also be incinerated for ultimate disposal. The aqueous secondary waste generated on the factory site is released to surface water only when it has met specific regulatory standards. But low concentrations of contaminants may still remain and affect microbial populations in natural waters, soil, and sediments.
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