What is surface wasting (SWAS)?

What you will learn:

• The differences between sludge and surface wasting.
• What surface wasting is and what it removes.
• Why is surface wasting is needed.
• How surface wasting works.

What is wasting in wastewater treatment?

Wasting in wastewater treatment is the removal of foam and solid buildup from the wastewater treatment plant to maintain the microbial population in the system responsible for the biodegradation of chemicals and for the overall health of the wastewater treatment plant. 

There are different types of wasting in wastewater treatment and each is a different application. Sludge wasting, surface wasting, and density wasting are some examples. Surface wasting is discussed in this article. 

What is the difference between sludge wasting and surface wasting?

They are different processes, as their names imply. Sludge wasting is the process of disposing excessive sludge, based on calculations for the amount of sludge present in the wastewater treatment set-up. The sludge contains microorganisms involved in chemical biodegradation and when the excessive sludge is removed, some of these microorganisms are also removed. 

This removal is important because older sludge can have reduced biodegradation activity leading to the excessive growth of filamentous bacteria. This in turn can lead to sludge bulking. Removing the older sludge allows the growth of newer microorganisms so that the biodegradation process can continue at a steady pace.

Surface wasting involves the disposal of surface material, usually foam, from the surface of the wastewater in the wastewater treatment set-up. This is discussed further in the next sections.

What is surface wasting? 

In surface wasting (SWAS), also known as surface skimming, foam and other floatable substances from the surface are collected and removed from the wastewater treatment system. 

What is removed in surface wasting in wastewater treatment?

Froth-forming bacteria: As the name implies, these bacteria form froth. They accumulate in the foam/solid matrix. 

Floating substances: Floating substances include oil and grease among oleophilic material. These can accumulate on the surface and form scum. 

The froth can mix with the scum and so it is not unusual to find a mixture of substances on the surface in wastewater treatment processes. 

Why is the surface wasting process needed?

The advantages of the surface wasting process is that it reduces trapping and clogging issues that can occur due to the excessive growth of filamentous bacteria. Without surface wasting, the F/M ratio (food to mass ratio) decreases and this leads to the growth of filamentous bacteria. Filamentous bacteria can cause clogging issues, which can disrupt process efficiency. 

The presence of a decreased F/M ratio and excessive growth of filamentous bacteria slow down the microbial degradation of chemicals. The chemicals accumulate in the wastewater treatment plant and they can also become toxic due to chemical reactions in the water.

How does the surface wasting process work?

The surface wasting process generally works by skimming floating substances at the surface. It can occur at locations in the wastewater treatment plant where floating substances including foam can accumulate and can be collected 

Three examples of how this process is employed include the use of froth removal pumps, surface skimmers, and baffle walls.

1. Froth removal pump

The wastewater from the aeration tank can collect into a froth collection box equipped with a spray bar. The resulting froth can be removed by the froth removal pumps. 

Oxidizing agents such as chlorine can be added to the froth for disinfection. In some wastewater treatment plants, this is applied as a spray in an enclosed froth hood. While this is an effective strategy to control the formation of filamentous bacteria, the dosing amount is critical to minimize the impact of the oxidizing agent on the microbial population.

2. Surface skimmer

Surface skimmers improve the outcome of the wastewater treatment process by removing froth and other floating contaminants. There are various types of skimmers with their own functionalities. Examples include weir and disc skimmers. 

Weir skimmers capture floating substances as the water and the froth flows over their edge. They can be adjusted to accommodate varying water levels. Disc skimmers attract oleophilic material, which can be collected. They can also be used with a moving belt. 

3. Baffle wall

A baffle wall is essentially a chamber where froth and other floating contaminants accumulate. Flocculants can be added to help the floating material bind together and therefore make them easier to remove. 

In what application is surface wasting common?

Surface wasting is a common application in digesters and in the biological nutrient removal (BNR) process. It is also applied in aerated lagoons.

The sludge in the digester contains microorganisms that carry out chemical degradation, resulting in the formation of gases and acids. These gases and acid rise to the surface and can accumulate as foam. These need to be removed and surface wasting is commonly used to remove that foam. 

The BNR process requires relatively higher solid retention time. At this relatively higher retention time, filamentous bacterial growth occurs and higher amount of foam is produced, and thus the need for surface wasting to remove the foam. 

Baffle walls are typically used in aerated lagoons. Algae is a concern in aerated lagoons. Excessive algae growth can disrupt the growth of microorganisms involved in the biodegradation of chemicals in the water. Baffle walls increases aeration of the wastewater, which in turn can minimize algal growth. 

Surface wasting is a simple yet a necessary process for removing floating substances including foam and scum in wastewater treatment to maintain the health of the microorganisms involved in chemical biodegradation. It has wide applications – from digesters and BNR processes to aerated lagoons.