microbial cells. If the detention time is long enough in the stabilization tank,
endogenous respiration will occur, along with a concomitant decrease in excess
biological sludge production. Following stabilization, the re-aerated sludge is
mixed with incoming wastewater in the contact tank, and the cycle starts anew.
Volatile compounds are driven off to a certain extent by aeration in the contact
and stabilization tanks. Metals will also be partially removed, with accumulation
in the sludge. This process requires smaller total aeration volume than the
conventional activated sludge process. It also can handle greater organic shock
and toxic loadings because of the biological buffering capacity of the stabilization
tank and the fact that at any given time the majority of the activated sludge is
isolated from the main stream of the plant flow. Generally, the total aeration basin
volume (contact basin plus stabilization basin) is only 50–75% of that required in
in the conventional activated sludge system. An equalization basin is generally
recommended for equalizing the wastewater influent flow.
Conventional biological wastewater treatment system: It normally includes
(a) preliminary treatment units (i.e., screen, comminutor, grit chamber, etc. for
the removal of sand, gravel, cinders, coffee grounds, small stones, cigarette filter
tips, logs, cans, and other large-sized unwanted materials from raw wastewater),
(b) primary sedimentation clarification for removing mainly total suspended
solids from preliminary effluent, (c) secondary biological treatment units (such
as activated sludge aeration or equivalent plus secondary sedimentation clarification) for removing dissolved organic/inorganic pollutants from primary effluent, and (d) tertiary treatment plant units (i.e., filtration, granular activated carbon
adsorption, ion exchange, oxidation, nitrification, denitrification, and/or disinfection) for final polishing the secondary effluent in order to meet the effluent
discharge standards.
Dechlorination: It is a process for the removal of excess amount of residual
chlorine in a chlorination disinfection process. Dechlorination is accomplished
by using a reducing chemical such as sulfur dioxide (gas), sodium bisulfate
(liquid), or sodium metabisulfite (liquid).
Denitrification filter: It is an attached-growth biological process filter using wood,
plastic, rock, granular activated carbon, or sand as filtration media for denitrification (conversion of nitrate to nitrogen gas) of wastewater under anoxic/anaerobic condition.
Denitrification followed by flotation clarification: The process involves the
reduction of nitrates and nitrites to nitrogen gas through the action of facultative
heterotrophic bacteria. In suspended-growth separate-stage denitrification processes, nitrified wastewater containing primarily nitrates is passed through a
mixed anaerobic vessel containing denitrifying bacteria. Since the nitrified feedwater contains very little carbonaceous materials, a supplemental source of
carbon (such as methanol, sugar, acetic acid, ethanol, or other compounds) is
required to maintain the denitrifying biomass. This supplemental energy is
provided by feeding methanol, for instance, to the biological reactor along with
the nitrified wastewater. Mixing in the anaerobic denitrification reaction vessel
3 Biological Processes
143
endogenous respiration will occur, along with a concomitant decrease in excess
biological sludge production. Following stabilization, the re-aerated sludge is
mixed with incoming wastewater in the contact tank, and the cycle starts anew.
Volatile compounds are driven off to a certain extent by aeration in the contact
and stabilization tanks. Metals will also be partially removed, with accumulation
in the sludge. This process requires smaller total aeration volume than the
conventional activated sludge process. It also can handle greater organic shock
and toxic loadings because of the biological buffering capacity of the stabilization
tank and the fact that at any given time the majority of the activated sludge is
isolated from the main stream of the plant flow. Generally, the total aeration basin
volume (contact basin plus stabilization basin) is only 50–75% of that required in
in the conventional activated sludge system. An equalization basin is generally
recommended for equalizing the wastewater influent flow.
Conventional biological wastewater treatment system: It normally includes
(a) preliminary treatment units (i.e., screen, comminutor, grit chamber, etc. for
the removal of sand, gravel, cinders, coffee grounds, small stones, cigarette filter
tips, logs, cans, and other large-sized unwanted materials from raw wastewater),
(b) primary sedimentation clarification for removing mainly total suspended
solids from preliminary effluent, (c) secondary biological treatment units (such
as activated sludge aeration or equivalent plus secondary sedimentation clarification) for removing dissolved organic/inorganic pollutants from primary effluent, and (d) tertiary treatment plant units (i.e., filtration, granular activated carbon
adsorption, ion exchange, oxidation, nitrification, denitrification, and/or disinfection) for final polishing the secondary effluent in order to meet the effluent
discharge standards.
Dechlorination: It is a process for the removal of excess amount of residual
chlorine in a chlorination disinfection process. Dechlorination is accomplished
by using a reducing chemical such as sulfur dioxide (gas), sodium bisulfate
(liquid), or sodium metabisulfite (liquid).
Denitrification filter: It is an attached-growth biological process filter using wood,
plastic, rock, granular activated carbon, or sand as filtration media for denitrification (conversion of nitrate to nitrogen gas) of wastewater under anoxic/anaerobic condition.
Denitrification followed by flotation clarification: The process involves the
reduction of nitrates and nitrites to nitrogen gas through the action of facultative
heterotrophic bacteria. In suspended-growth separate-stage denitrification processes, nitrified wastewater containing primarily nitrates is passed through a
mixed anaerobic vessel containing denitrifying bacteria. Since the nitrified feedwater contains very little carbonaceous materials, a supplemental source of
carbon (such as methanol, sugar, acetic acid, ethanol, or other compounds) is
required to maintain the denitrifying biomass. This supplemental energy is
provided by feeding methanol, for instance, to the biological reactor along with
the nitrified wastewater. Mixing in the anaerobic denitrification reaction vessel
3 Biological Processes
143