Aeration in wastewater treatment is the process of adding oxygen to the wastewater through the use of mechanical aerators or defused aeration. This provides mixing action in the reactor basin to keep the solids in suspension. It also brings the food (BOD5), microorganisms (MLVSS) and oxygen into intimate contact.
This type of treatment system is called a suspended growth system because the bacteria, waste and oxygen are suspended in a water matrix and thoroughly mixed to bring them into intimate contact.
There are different types of aeration systems. Course, fine and micro bubble defusers are supplied air through the use of either compressors or blowers. Vertical or horizontal paddles or wheels are used in oxidation ditches to impart oxygen and velocity to the water and keep the solids in suspension. A major portion of the oxygen added to the system is from the splashing of the wastewater into the atmosphere, and as it falls back, it collects oxygen from the air.
In lagoons, surface aerators, aspirating aerators or defuser systems secured to the bottom of a lagoon are used to supply air and mixing. The wind and wave action on the surface of the lagoon also has a positive effect on the dissolved oxygen level in lagoons.
Another type of aeration is called dissolved air floatation or DAF. The DAF system uses a micro bubbler to produce a curtain of small bubbles in a tank of water with a polymer added to enhance flocculation and to tie the floc particles together with the bubbles to float the solids to the surface. The DAF unit is normally used to remove a large grease load, but it can also be used in conventional wastewater treatment.
The smaller the bubbles, the more power it takes to push them through the defusers. However, more oxygen transfer is accomplished with smaller bubbles, but it can be expensive. Approximately 3 percent of the total electricity produced in the United States is used to treat wastewater, and in most cases, the electricity bill from the treatment plant is the largest bill paid by the city.
According to “Ten States’ Standards,” the requirement for the conventional activated sludge processes is 1.1 lbs. O2 per lb. of BOD5 applied. In the extended aeration processes, the O2 requirement is 1.5 lbs. O2 per lb. of BOD5 applied to include endogenous respiration requirements. If ammonia removal is required, an additional oxygen requirement of 4.6 lbs. O2 per lb. of Total Kjeldahl Nitrogen (TKN) applied is required.
To obtain the specification for the aeration system, go back to the specs of the plant and determine if you can produce the required amount of oxygen to the system. To determine this, use the cubic feet per minute rating of the blower or compressor and multiply it times 0.08 lbs. of air per cubic foot per minute. This number is the pounds of air per minute produced. To determine the pounds of oxygen, multiply the pounds of air per minute by 0.232 or 23.3 percent oxygen content. This will give you the pounds of oxygen per minute added.
Let’s work one for fun.
From the DMR, it was determined that the average BOD load to a 5 MGD extended aeration plant is 15,400 lbs. of BOD per day. They have 1 blower that produces 1,000 cubic feet per minute of air. How many pounds of air do they produce per day?
1,000 ft3 air per min X .08 lbs. air/ft3 X 1,440 min/day
=115,200 lbs. air/day
How many pounds of oxygen are added daily?
To determine the pounds of oxygen, multiply the pounds of air per day by 0.232 lbs. of oxygen per pound of air, so
115,200 lbs. air/day X .232 lbs. O2 per lbs. of air
=26,726.4 lbs. O2 per day
To determine how many pounds of oxygen per pounds of BOD, use the following formula:
Lbs. of O2
Lbs. of BOD
=26,726.4 lbs. O2
15,400 lbs. BOD
=1.735 lbs. O2 per lb. of BOD