What is densified activated sludge?

What is densification in wastewater treatment?

Densification in wastewater treatment is done using activated granular sludge. It is a process to improve the settling characteristics of activated sludge particles. Specifically, densification improves the settleability and density of activated sludge particles in wastewater treatment.

What is densified activated sludge?

Densified activated sludge (DAS) is aggregated floc with an SVI₃₀ (sludge volume index 30) of less than 100 mL/g. And because of its low SVI₃₀, it settles relatively faster than sludge that has not been treated for densification.

Sludge volume index (SVI)

Sludge volume index (SVI) is a measure of how well the mixed liquor suspended solids are settling. SVI₃₀ is the SVI measured after the sludge is allowed to settle for 30 minutes. 

A typical SVI for sludge in wastewater is between 50-150 mg/L. If the SVI is less than 80 mg/L, the sludge will be very dense and will settle rapidly.

What is the difference between densified activated sludge, conventional activated sludge, and activated granular sludge?

The main difference between densified activated sludge, conventional activated sludge, and activated granular sludge is the particle size distribution, characteristics, and sludge volume index. The comparison of particle size distribution and the characteristics of conventional activated sludge, densified activated sludge, and activated granular sludge are summarized in Table 1. As seen in Table 1, the particle size in densified activated sludge is in the range of 100 µm to 600 µm and microscopy images show these particles as appearing compact with small granules.

A scholarly study showed that at close to 10% biomass, the maximum particle size (nominal particle diameter) is:

• 200 µm for conventional activated sludge
• 375 µm for densified sludge
• 2 mm for aerobic granular sludge

What is the purpose of densification?

The purpose of densification is to:

• reduce the sludge volume index (SVI) and improve the settleability of sludge particles. Sludge resulting from the densification process settles faster leading to increase in the allowable loading rate to the clarifiers. This allows the clarifiers to accommodate higher concentrations of mixed liquor and thus to treat a higher volume of influent. 
• increase nitrification and increase nutrient removal: the small granules in the densified activated sludge enhance biological nutrient removal.

Factors critical for achieving the densification of activated sludge

Food to microorganism ratio (F/M) and shear force pressure are some of the factors that are critical for achieving the densification of activated sludge. Controlling the F/M ratio allows for the selection of microorganisms that can grow and produce extracellular polysaccharide (EPS). Shear forces are known to influence the production of EPS in biological systems. Controlling the shear force pressure can yield compact and dense particles that have the characteristics to increase the rate of particle settling in activated sludge.

How much energy is used for wastewater treatment plants using activated sludge compared to using densified activated sludge?

Typically, the energy requirement for a wastewater treatment plant using conventional activated sludge is between 0.30 kWh/m³ to 0.65 kWh/m³. This value is lower for wastewater treatment plants using densified activated sludge because densified activated sludge process produces compact floc of sizes between 200 µm and 400 µm. Because of the compact nature of densified activated sludge and the presence of higher concentration of mixed liquor in the densified activated sludge, less energy is required for the aeration process in wastewater treatment. 

How much energy is used for the densified activated sludge processes?

Literature has not mentioned the exact value for the energy used in the densified activated sludge processes because the energy demand for the densified activated sludge processes, besides depending on the size of the wastewater treatment plant, varies depending on the chemicals and the equipment used to achieve the densification. 

What chemical are needed for densified activated sludge?

Chemicals such as chlorine, ozone, hydrogen peroxide, iron, and aluminum salts are required in some cases to prevent sludge bulking due to filamentous microorganisms in the flocs. Preventing sludge bulking is critical to achieve densification of activated sludge.

Biocidal agents such as cetyltrimethyl ammonium bromide have been shown to have biocidal effects on filamentous bacteria and the application of these biocidal agents significantly improves sludge settleability. Cetyltrimethyl ammonium bromide eradicates chlorine-resistance microorganisms that are involved in sludge bulking by damaging their cell wall.

What equipment and processes are used for densified activated sludge?

Some of the equipment used for densified activated sludge includes the use of a hydrocyclone. Sludge dewatering equipment such as centrifuge and filter press remove water from the sludge and thus enhance the densification of activated sludge. The selection of sludge dewatering equipment to use depends on the chemical characteristics of the sludge and on the design of the wastewater treatment plant.

The processes for sludge densification includes the Simultaneous Nitrification-Denitrification and Phosphorus Removal (SNDPR) process and gravity densification. Both processes, also discussed in this section, improve sludge settleability. 

Hydrocyclone

A study showed that the use of a hydrocyclone as a sludge densification technology improved total nitrogen and total phosphorus removal efficiency by 16.9% and 29.4% respectively than without the use of the hydrocyclone. The use of the hydrocyclone also improved sludge settleability after densification as shown by a reduced sludge volume index by 7.6 mL/g and an increased settling velocity by 2 m/h compared to the control line. The anoxic zone formed within the densified activated sludge allowed the microorganisms to utilize EPS for nutrient removal. 

Processes for sludge densification

The SNDPR process which operates under alternate anaerobic/aerobic conditions in mostly sequencing batch mode promotes nutrient removal and sludge settleability. The success of the SNDPR process relies on the co-existence of microorganisms involved in the nitrification and denitrification process.

A study showed that introducing the “shift work mode” into an SNDPR system which means temporarily transferring about 50% of the sludge to “rest” in a side-stream reactor improved the removal of inorganic nitrogen and also sludge settleability.

Gravity densification

Gravity can help separate out activated sludge from the treated water. This can be done using an upstream process or a settling tank in the wastewater treatment plant.

A scholarly study showed that the use of sludge treatment via gravity followed by centrifuge for sludge dewatering produced a densified sludge of sludge volume index less than 100 mL/g for four months and thereafter a steady-state year-round sludge volume index below 50 mL/g including during the winter period. The densified sludge had stable settleability and doubled the surface loading rate in the wastewater treatment plant.