Encyclopedia of Environmental Science and Engineering, Volume I and II

532 INDUSTRIAL WASTE MANAGEMENT

TABLE 3

Major physical, chemical, and biological treatment unit operations and processes used for liquid treatment

Unit Operations and Processes Principal Applications

Screening Racks or bar screens are the first step in wastewater treatment. They are used to remove large objects.

Flow equilization Used to dampen the flow rate and mass loading.

Skimming Used for oil separation where free oil is floated to the surface of a tank and then skimmed off. The oil separator

specified by American Petroleum Institute is commonly used.

Dissolved Air Flotation (DAF) Process used to remove suspended solids, and oil and grease from the waste stream. Uses dissolved air to

produce line bubble that float the solids.

Foam separation Process used to remove minute concentrations or refractory organics and heavy metal ions. Contaminants are

carried up by rising bubbles to the pool surface where they are deposited as the bubbles exit.

Sedimentation Use to remove settleable solids.

Neutralization Process used to neutralize acidic or alkaline waste streams prior to chemical and/or biological treatment.

Precipitation Dissolved solids in solution are chemically transformed into insoluble form. Process is extensively used to

precipitate phosphorous and heavy metals. Heavy metals are generally precipitated as hydroxide through the

addition of time or caustic to a pH of minimum solubility.

Chemical Oxidation Used to oxidize pollutants to terminal end products. Common oxidants are Cl 2 , O 3 , H 2 O 2 , and KMnO 4. Oxidation

of iron and manganese is used for precipitation. Cyanides, sulfides and many organics arc destroyed by

oxidation.

Chemical Reduction Used to precipitate certain ions from solutions, e.g., hexavalent chromium. Also many oxidizing agents are

destroyed by reduction. Common reducing agents are ferrous sulfate, sodium metabisulfite, and sulfur

dioxide.

Coagulation Used to agglomerate suspended materials so that efficient settling can take place. Commonly used chemicals are

alum, iron salts, and polymers. The process consists of chemical feed, flash mix and flocculation basins.

Suspended growth biological reactor Biological process using suspended biomass to remove dissolved organics. Principal variation is an activated

sludge process.

Attached growth biological reactor Biological process using attached biomass to remove dissolved organics. Principal variations are a trickling

filter and a rotating biological contactor (RBC).

Anaerobic Processes Attached or suspended microbial process operated in the absence of oxygen. Normally used to treat high strength

organic wastes. Methane is produced as an energy source. Sludge (biomass) production is small.

Oxidation Pond Large basins (normally earthern) used for waste storage and treatment. Treatment is achieved by natural

processes of settling and biological decomposition.

Aerated Lagoon Basins (normally earthern and frequently with plastic liners) with aeration equipment where waste are aerated

over long periods of time. The waste characteristics are altered by biological oxidation.

Land Treatment Organic waste is applied over land for treatment. Common application methods are slowrate irrigation, rapid

infiltration, and overland flow.

Nitrification Process is used to convert ammonia nitrogen to nitrate nitrogen. It can be achieved in suspended or attached

growth biological reactors.

Denitrification Nitrite nitrogen and nitrate nitrogen are reduced to nitrogen gas by microorganisms. Denitrification is achieved

under anaerobic condition in suspended or attached growth reactors. An organic source such as methanol is

needed.

Disinfection Process used to reduce the number of water borne pathogens in water. Normally applied to effluents of

wastewater treatment plants containing microbes. Chlorination and ozonation are the most common

methods.

Ammonia stripping Ammonia gas is air stripped from the wastewater, normally by using stripping tower. High pH values are required.

Breakpoint Chlorination Ammonia nitrogen is oxidized to nitrogen gas by breakpoint chlorination in a mixing basin.

Filtration Used to polish the effluent by removing total suspended solids and turbidity. Biological activity in the filter bed

may reduce some of the biochemical oxygen demand (BOD).

Carbon adsorption Used to remove soluble refractory organics from wastewater effluent.

Ion Exchange A demineralization process in which the cations and anions in wastewater are selectively exchanged for the ions

in an insoluble resin bed. When the resin capacity is used up it is regenerated by using high concentrations of

the original ions that are exchanged for the polluted ions attached to the resin. Flow from regeneration is

composed of highly concentrated brine.

Reverse osmosis or ultrafiltration A demineralization process applicable to production of high-quality water from effluent. The water permeates

through semipermeable membrane at high pressure, producing high-quality water in one stream and a high

concentration of mineral ions in another.

Electrodialysis A demineralization process where electrical potential is used to transfer the ions through ion-selective

membranes. Produces two streams; one high in mineral content and the other free of minerals.

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