lagoon can be a facultative stabilization pond (or facultative lagoon), a sedimentation clarifier, or a dissolved air flotation clarifier, each incorporating the recycle
of biological solids (or biosolids). The oxygen required by the aerated lagoon
process is supplied by surface or diffused aerators. As with other suspendedgrowth systems, the turbulence created is used to maintain the contents of the
basin in suspension.
Lagoon, anaerobic: Anaerobic lagoons are capable of treating high-strength
wastewater. They are relatively deep (8–20 ft depth; 20–50 days DT) ponds
with steep sidewalls in which anaerobic conditions are maintained by keeping
wastewater organic loading so high that complete deoxygenation is prevalent.
Although some oxygenation is possible in a shallow surface zone, once grease
form an impervious surface layer, complete anaerobic conditions develop. Waste
treatment or stabilization results from thermophilic anaerobic digestion of organic
wastes. The treatment process is analogous to that occurring in single-stage
untreated anaerobic digestion of sludge in which acid-forming bacteria break
down organics. The resultant acids are then converted to carbon dioxide methane,
cells, and other products. In a typical anaerobic lagoon, raw wastewater enters
near the bottom of the pond (often at the center) and mixes with the active
microbial mass in the sludge blanket which is usually about 6 ft. deep. The
effluent discharge is located near one of the sides of the pond, submerged
below the liquid surface. Excess undigested grease floats to the top, forming a
heat-retaining and relatively air-tight cover. Wastewater flow equalization and
heating are generally not practiced. Excess sludge is washed out with the lagoon
effluent. Recirculation of waste sludge is not required.
Lagoon, facultative: Facultative lagoons are intermediate-depth (3–8 ft depth;
20–180 days DT) ponds in which the wastewater is stratified into three zones.
These zones consist of an anaerobic bottom layer, an aerobic surface layer, and an
intermediate zone. Stratification is a result of solids settling and temperaturewater density variations. Oxygen in the surface stabilization zone is provided by
reaeration and photosynthesis. This is in contrast to aerated lagoons in which
mechanical aeration is used to create aerobic surface conditions. In general the
aerobic surface layer serves to reduce odors while providing treatment of soluble
organic by-products of the anaerobic processes operating at the bottom. Sludge at
the bottom of facultative lagoons will undergo anaerobic digestion producing
carbon dioxide, methane, and cells. The photosynthetic activity at the lagoon
surface produces oxygen diurnally, increasing the dissolved oxygen during
daylight hours, while surface oxygen is depleted at night. It is used for treating
raw, screened wastewater, or primary effluent, secondary effluent, or weak
biodegradable industrial wastewater.
Membrane bioreactor (MBR): Currently it is one of the suspended-growth biological processes (such as activated sludge process) involving the use of a
bioreactor (such as an aeration tank) for carbonaceous oxidation, nitrification,
and/or denitrification and the use of membrane filtration for subsequent bioreactor
effluent treatment. The membrane filter discharges the membrane filter effluent
3 Biological Processes
149
of biological solids (or biosolids). The oxygen required by the aerated lagoon
process is supplied by surface or diffused aerators. As with other suspendedgrowth systems, the turbulence created is used to maintain the contents of the
basin in suspension.
Lagoon, anaerobic: Anaerobic lagoons are capable of treating high-strength
wastewater. They are relatively deep (8–20 ft depth; 20–50 days DT) ponds
with steep sidewalls in which anaerobic conditions are maintained by keeping
wastewater organic loading so high that complete deoxygenation is prevalent.
Although some oxygenation is possible in a shallow surface zone, once grease
form an impervious surface layer, complete anaerobic conditions develop. Waste
treatment or stabilization results from thermophilic anaerobic digestion of organic
wastes. The treatment process is analogous to that occurring in single-stage
untreated anaerobic digestion of sludge in which acid-forming bacteria break
down organics. The resultant acids are then converted to carbon dioxide methane,
cells, and other products. In a typical anaerobic lagoon, raw wastewater enters
near the bottom of the pond (often at the center) and mixes with the active
microbial mass in the sludge blanket which is usually about 6 ft. deep. The
effluent discharge is located near one of the sides of the pond, submerged
below the liquid surface. Excess undigested grease floats to the top, forming a
heat-retaining and relatively air-tight cover. Wastewater flow equalization and
heating are generally not practiced. Excess sludge is washed out with the lagoon
effluent. Recirculation of waste sludge is not required.
Lagoon, facultative: Facultative lagoons are intermediate-depth (3–8 ft depth;
20–180 days DT) ponds in which the wastewater is stratified into three zones.
These zones consist of an anaerobic bottom layer, an aerobic surface layer, and an
intermediate zone. Stratification is a result of solids settling and temperaturewater density variations. Oxygen in the surface stabilization zone is provided by
reaeration and photosynthesis. This is in contrast to aerated lagoons in which
mechanical aeration is used to create aerobic surface conditions. In general the
aerobic surface layer serves to reduce odors while providing treatment of soluble
organic by-products of the anaerobic processes operating at the bottom. Sludge at
the bottom of facultative lagoons will undergo anaerobic digestion producing
carbon dioxide, methane, and cells. The photosynthetic activity at the lagoon
surface produces oxygen diurnally, increasing the dissolved oxygen during
daylight hours, while surface oxygen is depleted at night. It is used for treating
raw, screened wastewater, or primary effluent, secondary effluent, or weak
biodegradable industrial wastewater.
Membrane bioreactor (MBR): Currently it is one of the suspended-growth biological processes (such as activated sludge process) involving the use of a
bioreactor (such as an aeration tank) for carbonaceous oxidation, nitrification,
and/or denitrification and the use of membrane filtration for subsequent bioreactor
effluent treatment. The membrane filter discharges the membrane filter effluent
3 Biological Processes
149