What is density wasting (DWAS)?

What you will learn:

• What density wasting is.
• How to calculate wastewater density.
• How to remove sludge and grit from wastewater.
• Why density wasting is important in wastewater plants.

In wastewater treatment, wasting refers to the removal of excessive buildup such as foam, solids, sludge and grit. It is an important practice to ensure that the wastewater treatment process is running effectively and to prevent damage to the wastewater treatment facility caused by excessive sludge and filamentous overgrowth. 

There are different types of wasting applications in wastewater treatment, specifically surface wasting and density wasting. Density wasting (DWAS) is discussed here.

What is density wasting?

As the name implies, density wasting refers to the removal of dense material from the wastewater, specifically sludge and grit.

What is the density of wastewater sludge?

The numerical value for the density of the wastewater sludge varies for wastewater treatment plants because it depends on several factors including:

• The average amount of sludge that enters the plant
• The physical and biological treatment processes occurring in the plant
• The dry solid content in the sludge
• Processes employed in the wastewater treatment process such as aerobic granular sludge treatment

Research studies have reported a wide range for the density of wastewater sludge as:

• 1.020 to 10.030 kg/m3 in a sludge treatment plant
• 1400 kg/m³ for sludges of greater than 10% solid content

Sludge density index vs. density of wastewater sludge

Sludge density index is not to be confused with the density of wastewater sludge because the two terminologies are different. By definition, sludge density index is a calculation to show the settleability of the sludge in a secondary clarifier and it is related to sludge volume index by the formula:

Sludge density index = 100/sludge volume index

Techniques for measuring the density of wastewater sludge

Density gradient centrifugation

In density gradient centrifugation, the solid particles are allowed to settle until the particles are stationary. A study used this technique to measure the density of activated sludge solids at several wastewater treatment plants.

While the density of the activated sludge solids remained relatively constant at a given wastewater treatment plant, there was variation in the density of the activated sludge solids at different wastewater treatment plants attributed to plant operational practices.

Sludge probes

With this technique, the probe is put in a tank and is used to detect and measure the weight of the solids that can then be used to calculate the density of the wastewater sludge.

What is the density of wastewater grit?

Several factors affect the density of the wastewater grit including:

• The organic composition of the incoming influent
• The size range of the solid particles
• The design of the wastewater treatment system including the use of primary clarifiers

The numerical values for the density of the grit in wastewater treatment varies depending on the above factors. Some of the reported values are:

• Above 1800 kg/m³ when inorganic solid particles in the 0.2-4 mm size range were present
• Around 2650 kg/m³ for a wastewater effluent treatment plant

Why conduct density wasting?

Density wasting is an integral practice in wastewater treatment to remove excess sludge and grit to maintain process efficiency and mechanical issues. This section discusses sludge and grit wasting.

What is sludge wasting and why is it necessary?

Sludge wasting is a process for removing excess sludge from the wastewater treatment plant. This process is necessary because sludge contains microorganisms involved in the biodegradation process.

By removing excessive sludge, sludge wasting prevents the overgrowth of microorganism. The overgrowth can lead to solid separation problems, slowing down of the biodegradation process, and sludge bulking.

The consequences of sludge bulking

Microbial imbalance, due to the overgrowth of the microorganisms in the activated sludge, leads to the growth of filamentous bacteria.

 While filamentous bacteria are important to stabilize the floc structure which in turn improves the efficiency of the separation of the solids from the liquid in the settling tank, the overgrowth of filamentous bacteria is an issue because the filamentous structures trap air and water and prevent the sludge from settling properly. This causes sludge bulking and thus the need for the removal of excess sludge, a process known as sludge wasting.

What is grit wasting and why is it necessary?

Grit includes sand, gravel, and large organic particles such as food waste. Wastewater grit can also include material with different specific gravities such as limestone.

Grit wasting refers to the removal of grit to prevent unnecessary abrasion and wear of the mechanical equipment and its deposition in the digesters and other parts of the wastewater treatment set-up. Excessive grit can accumulate in a wastewater treatment plant depending on the configurations. For example, without primary clarifiers, excessive grit can get into the aeration basin and digesters. 

How does the density wasting process work?

Density wasting works by removing the excess sludge and grit in the wastewater treatment system. These involve mechanical and chemical technologies. 

What technologies are used for density wasting?

Various technologies, listed in this section, can be used for density wasting, specifically for sludge and grit wasting in wastewater treatment facilities. The selection of a technology depends on various factors such as the concentration and composition of the sludge and the grit. 

Technologies for sludge wasting

• Volume reduction technologies: These basically involve dewatering to decrease the sludge liquid volume so that the sludge can be removed.
• Conditioning technologies: These use chemicals including polymer chemicals in the dewatering process.

Technologies for grit wasting

• Aerated grit chambers
• Gravitational systems
• Centrifugal systems
• Vortex-type grit removal systems including mechanically induced vortex tanks and hydraulically induced vortex
• Detritus tanks
• Horizontal flow grit chambers

Density wasting is a concern in wastewater treatment plants because excessive sludge and grit can impair the wastewater treatment process and can also cause damage to the wastewater treatment plant. Although there are several types of technologies for sludge and grit wasting, the selection of suitable technologies depend on several factors including on the sludge settleability and the organic content of the grit.