developed and maintained in the aeration basins. During the oxidation process, a
certain amount of the organic material is synthesized into new cells, some of
which then undergoes auto-oxidation (self-oxidation or endogenous respiration)
in the aeration basins, the remainder forming net growth or excess sludge.
Oxygen is required in the process to support the oxidation and synthesis reactions. In the conventional activated sludge plant, the wastewater is commonly
aerated for a period of 4–8 hours (based on average daily flow) in a plug flow
hydraulic mode. Either surface or submerged aeration systems can be employed
to transfer oxygen from air to wastewater.
Aerobic digestion: The process equipment of aerobic digestion is similar to that of
the activated sludge process. Aerobic digestion is not a suspended-growth biological process in principle and is for sludge treatment (not for wastewater
treatment). It is a suspended-destruction biological process. In the presence of
oxygen, the biodegradable matter in the activated sludge (biosolids) is oxidized,
destroyed, and converted to carbon dioxide and water. Oxygen must be supplied
in direct proportion to the mass of volatile solids (VS) destroyed. Sludge stabilization is not complete until there has been an extended period of primarily
endogenous respiration (15–20 days).
Aerobic: An environmental condition in which free and dissolved oxygen is
available in an aqueous environment (for instance, nitrification is an aerobic
process).
Alkalinity: The capacity of water to neutralize acids by the water’s content of
carbonates, bicarbonates, hydroxide, and other compounds, also known as buffering capacity.
Ammonia nitrogen: Elemental nitrogen present in the form of ammonia (NH 3 ).
Ammonification: A biochemical or chemical process in which soluble organic
nitrogen is converted to ammonia.
Anaerobic: An environmental condition in which free and dissolved oxygen is not
available (dissolved oxygen ¼ 0) in an aqueous environment (for instance,
anaerobic digestion is an anaerobic process).
Anaerobic digestion: This is a biological destruction process for destroying biodegradable organic sludge (biosolids) in the absence of molecular oxygen or
under anaerobic conditions. It is a three-stage sludge stabilization process for
sludge volume reduction: (a) hydrolysis stage, breakdown of particulate matter
and large macromolecules; (b) acid formation stage, fermentation of the soluble
organic matter formed in the first reaction to volatile acids; and
(c) methanogenesis stage, conversion of the volatile acids to the stable end
produces, such as methane gas, carbon dioxide gas, and water. Four anaerobic
digestion operational modes are available: (a) standard-rate digestion mode, or
one stage, unheated, and unmixed; (b) high-rate digestion mode, one stage,
heated, and mixed; (c) two-stage digestion mode; and (d) anaerobic contact
process. Anaerobic digestion can occur in two temperature ranges: mesophilic
(80–110 degree F) and thermophilic (113–149 degree F). Mesophilic range
operation is more stable, while thermophilic range operation offers improved
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L. K. Wang et al.
certain amount of the organic material is synthesized into new cells, some of
which then undergoes auto-oxidation (self-oxidation or endogenous respiration)
in the aeration basins, the remainder forming net growth or excess sludge.
Oxygen is required in the process to support the oxidation and synthesis reactions. In the conventional activated sludge plant, the wastewater is commonly
aerated for a period of 4–8 hours (based on average daily flow) in a plug flow
hydraulic mode. Either surface or submerged aeration systems can be employed
to transfer oxygen from air to wastewater.
Aerobic digestion: The process equipment of aerobic digestion is similar to that of
the activated sludge process. Aerobic digestion is not a suspended-growth biological process in principle and is for sludge treatment (not for wastewater
treatment). It is a suspended-destruction biological process. In the presence of
oxygen, the biodegradable matter in the activated sludge (biosolids) is oxidized,
destroyed, and converted to carbon dioxide and water. Oxygen must be supplied
in direct proportion to the mass of volatile solids (VS) destroyed. Sludge stabilization is not complete until there has been an extended period of primarily
endogenous respiration (15–20 days).
Aerobic: An environmental condition in which free and dissolved oxygen is
available in an aqueous environment (for instance, nitrification is an aerobic
process).
Alkalinity: The capacity of water to neutralize acids by the water’s content of
carbonates, bicarbonates, hydroxide, and other compounds, also known as buffering capacity.
Ammonia nitrogen: Elemental nitrogen present in the form of ammonia (NH 3 ).
Ammonification: A biochemical or chemical process in which soluble organic
nitrogen is converted to ammonia.
Anaerobic: An environmental condition in which free and dissolved oxygen is not
available (dissolved oxygen ¼ 0) in an aqueous environment (for instance,
anaerobic digestion is an anaerobic process).
Anaerobic digestion: This is a biological destruction process for destroying biodegradable organic sludge (biosolids) in the absence of molecular oxygen or
under anaerobic conditions. It is a three-stage sludge stabilization process for
sludge volume reduction: (a) hydrolysis stage, breakdown of particulate matter
and large macromolecules; (b) acid formation stage, fermentation of the soluble
organic matter formed in the first reaction to volatile acids; and
(c) methanogenesis stage, conversion of the volatile acids to the stable end
produces, such as methane gas, carbon dioxide gas, and water. Four anaerobic
digestion operational modes are available: (a) standard-rate digestion mode, or
one stage, unheated, and unmixed; (b) high-rate digestion mode, one stage,
heated, and mixed; (c) two-stage digestion mode; and (d) anaerobic contact
process. Anaerobic digestion can occur in two temperature ranges: mesophilic
(80–110 degree F) and thermophilic (113–149 degree F). Mesophilic range
operation is more stable, while thermophilic range operation offers improved
140
L. K. Wang et al.