Lesson Learned: Bottom-up Aerator to Treat Washwater in Settling Tanks

Not every solution implemented results in success. Those various mistakes and failures are discussed here as well as the lessons learned from them.

Goal: Decrease Total Suspended Solids (TSS) while increasing Dissolved Oxygen (DO) concentrations in a settling tank

Solution: Installation of a Bottom-up Aerator.
A Bottom-up Aerator works by pumping compressed air down to a diffuser (Figure 1) situated on the bottom of the tank. The air is released through the diffuser as bubbles which work their way to the surface. As the bubbles rise, the oxygen is dissolving into the water.

Figure 1: A Bottom-up Aerator diffuser (left) and the surface disturbance caused by the system (right)

Problem: The primary goal of the settling tank is to remove suspended particles by sedimentation and removal of clear water at the surface. The bubbles surfacing from the aerator interrupted this process by keeping the particles in suspension (Figure 2). Also, water testing results showed that there was no significant positive impact on the DO (Figure 3). 

Figure 2: Total Suspended Solids in a settling tank over time before and after Bottom-up Aerator installation, as shown by the green line

Figure 3: Dissolved Oxygen concentration in a settling tank over time before and after Bottom-up Aerator installation, as shown by the green line, with a target level of 7-10 mg/L (CCME, 1999)

Discussion: The system failed to increase the DO in the water and had a negative impact on the TSS. While a Bottom-up Aerator is suitable in other situations where particle settling is not a concern, it was not appropriate for resolving this problem. It was removed and replaced with a Surface Aerator.

References

• Canadian Council of Ministers of the Environment (CCME). 1999. Canadian water quality guidelines for the protection of aquatic life: Dissolved oxygen (freshwater). In: Canadian environmental quality guidelines, Canadian Council of Ministers of the Environment, Winnipeg, MB.