Aerobic Lagoons | Activated Sludge Process | Wastewater Treatment Plant

 Aerobic Lagoons

Aerobic lagoons are operated with an aeration system installed in the basin to maintain a high dissolved oxygen concentration throughout the water column. These lagoons are also typically divided into two or three zones with baffles to provide what amounts to serial treatment. The effluent from the first compartment flows to the second and the effluent from the second flows to the third, and so forth. Often, several smaller lagoons are constructed in series, rather than baffling the main lagoon, to accomplish the same end result. These lagoons and ponds are typically used to cut high strength waste ahead of a facultative lagoon or secondary system or to treat industrial wastewater to a condition compatible with municipal systems. They are also commonly used in rural areas as polishing lagoons following mechanical primary treatment options.

Aeration systems may consist of plastic tubes with a series of carefully placed and evenly sized holes drilled into them that are placed across the bottom of the lagoon and anchored in place. They are often set up on small pedestals to provide space beneath them for sludge accumulation without clogging the pores. The plastic feeder lines are connected to a header along the bank. The header is connected to a blower inside a blower house and control building. Sometimes separate headers are installed on each side of the lagoon feeding intermediate tubes so that if one or more tubes clog, or there is a failure in the header for any reason, there is still an opportunity to provide good aeration. All the cross tubing is connected to both headers with shut-off valves at each end of each tube. Alternate tubes are then fed from opposite headers unless there is a problem detected. Problems are generally rather obvious because the bubble pattern on the lagoon surface will change as flow rates from the bubble tubes change.

A second method is to install aerators on the bottom of the basin, generally on risers that allow for sludge storage beneath the bubblers. These are more often rigid galvanized pipes, but may be plastic, as well. The air comes from headers, as described earlier, and is squeezed out through tiny pores in the surface of the aerator.

A debate has evolved between those who favor larger bubbles because of the reduced pressure needed to produce them and those who favor fine bubble diffusers because of the better oxygen transfer characteristics of the fine bubbles. Indeed, fine bubble enthusiasts frequently point out that large bubbles must first disintegrate into fine bubbles before any significant oxygen transfer can occur. Therefore, it is better to put the energy into creating the smaller bubbles in the first place than to push much larger quantities of air through the system at a lower pressure. Coarse bubble diffusers are typically used where a more aggressive rolling action in the reactor is desired or required for any reason.

Large bubble diffusers tend to be round caps placed on top of the riser pipe with strategically placed holes through which the air is forced by the blower. Fine bubble diffusers use a similar approach, but with much smaller openings. Sandstone bubblers, as well as artificial sandstone bubblers similar to those found in a home fish tank, have been used successfully in lagoons.

Surface aerators are often employed in lagoons. The main issue with these devices is that they tend to keep the sludge in suspension until it is essentially oxidized to a nonorganic ash. This may be a good thing in that the sludge is more easily oxidized in suspension, but it also leads to suspended solids carry-over from the lagoon effluent. A sedimentation basin is generally required following such lagoons and it is noted that the fine ash particles seldom settle well without some form of coagulant aide.