What is sludge mixing?

Sludge mixing is a vital process in wastewater treatment operation. However, sludge mixing is affected by physical factors and chemical properties of the wastewater. And the use of chemicals presents challenges.

What is considered sludge?

Sludge is the settled solids in the wastewater treatment tanks. Due to gravity, the solids, that are chemicals, settle at the bottom of the tanks.

Different terminologies are associated with sludge indicating the origin of the wastewater. For example, the terms “sewage sludge” and “municipal sludge” refer to sludge formed from wastewater originating from sewage and municipal wastewater, respectively. 

Sludge consists of metabolically active microorganisms involved in the treatment of chemicals in the wastewater. The types of microorganisms and the chemical composition of the wastewater depend on the origin of the wastewater. 

What is the difference between biosolids and sludge?

Sludge is settled solids in a wastewater treatment plant. Biosolids is a type of sludge. Biosolids is sewage sludge produced by biological processes during which the sludge has been treated to reduce the concentration of pathogens in it.  

What happens in sludge treatment?

The sludge in the tanks of the wastewater treatment plant contains microorganisms that metabolize specific chemical compositions present in the wastewater. The type of microbial metabolic processes that occurs in the tanks depends on the oxygen concentration in the wastewater. Mixing of the sludge basically enhances this metabolism. 

In anoxic denitrification, nitrogen is removed from the wastewater. By mixing the sludge containing the suspended microorganisms in the anoxic tank reactors, the microbial actions are enhanced. The bacteria reduce nitrate to nitrite and then to nitrogen which is released as a gas.

Mixing the sludge in anoxic conditions is a sensitive application because it requires low shear mixing and preventing foaming which is achieved by minimizing unwanted air in the process. Mixers are used for mixing the sludge and these mixers are designed to minimize the introduction of atmospheric oxygen. Specifically these mixers operate at low speed. 

In aerobic digestion, the aerobic bacteria metabolize organic matter and convert it into carbon dioxide. This results in low concentration of solids. Mixing the sludge enhances this microbial metabolic process by providing oxygen transfer and better contact between the organic matter in the sludge and the microorganisms. 

In anaerobic digestion, the chemicals in the sludge are metabolized by the anaerobic microorganisms into carbon dioxide and methane. Mixing the sludge provides more contact between the microorganisms and the chemicals, allowing more efficiency for the anaerobic digestion process. 

Why is sludge mixing important?

Sludge mixing is important to prevent operational issues in wastewater treatment and to promote the metabolic activities of the microorganisms. 

Sludge mixing also:

• Disperses chemicals and minimizes toxicity. When the chemicals in the wastewater are dispersed due to sludge mixing, the chemicals are no longer concentrated in one place and thus, there is less toxicity of the chemicals particularly to the microorganisms involved in the digestion process. 

• Increased gas production. Increased metabolism of chemicals (i.e., digestion) by the microorganisms in the sludge leads to increased production of gas. The gases are produced from the microbial metabolism processes.

• Decreased mass of the sludge blanket. Due to the mixing of the sludge, there is less settled solids and so the mass of the sludge blanket decreases.

Sludge mixing is affected by the physical conditions and the chemical properties in the wastewater treatment tanks. 

Physical conditions in sludge mixing

Inactive mixing zones

Inactive mixing zones in the digesters can lead to digester issues. The mixing of the sludge promotes the movement of thermal fluid and reaction kinetics in anaerobic digestion. 

Mixing and temperature

A study on the effects of temperature on mixing in waste activated sludge in a wastewater treatment plant showed that higher temperatures in the reactor along with mixing led to higher concentrations of volatile fatty acid-chemical oxygen demand than in reactors without mixing. This indicates that the lack of mixing reduces the amount of volatile fatty acid-chemical oxygen demand generated and the process would not be feasible at low temperature. 

Density and viscosity of the sludge

A study was done on the effects of sludge density and viscosity on continuous stirred tank reactor. It showed that optimizing the density and viscosity of the sludge enhances the mixing efficiency of the continuous stirred tank reactor.

The viscosity of the sludge affects pump operations, mass transfer rates, and settling and filtration in the sludge-water separation. The viscosity is correlated with the concentration of total suspended solids. The higher the concentration of the total suspended solids, the higher the viscosity of the sludge.

Chemical properties of sludge mixing

The chemical properties of the sludge affect sludge handling operations and therefore, sludge mixing is important to maintain and in some cases, to improve the process efficiency of wastewater treatment operations. 

What chemicals are used in sludge mixing?

Chemicals are sometimes necessary to mix into the sludge for sludge conditioning. For example, chemicals might be used to thicken the sludge in a process called dewatering. 

Chemicals used in sludge mixing include:

• Ferric chloride and aluminum chloride: These promote the aggregation of sludge flocs and enhance sludge dewaterability.
• Lime such as calcium hydroxide: Lime raises the pH of the sludge to resolve issues associated with odor. 
• Cationic polyacrylamides: Cationic polyacrylamides are used for sludges containing high organic matter content. These chemicals produce shear-resistant flocs.
• Anionic polyelectrolytes: Anionic polyelectrolytes are used along with metals such as ferric chloride. 

However, there are challenges associated with the use of chemicals for sludge mixing. Using the wrong concentration and mixing parameters can be costly to fix for the wastewater treatment system. These chemicals also require proper agitation for uniform suspension, to prevent coagulation to different material in the tanks, and to avoid accumulation that can cause clogging in the wastewater treatment system. 

Sludge disposal

Sludge accounts for a large portion of the operation and capital costs for wastewater treatment plants. The sludge present at the end of the wastewater treatment processes is rich in nitrogen and phosphorus and is also pathogen-free, as required by regulations. And thus, it can be refined into fertilizers and compost. 

To prepare sludge for disposal, the sludge collected from the primary and secondary clarifiers is treated and eventually dewatered to dry it out. Then it can be disposed on land as fertilizer following regulatory guidelines, in landfills or through incineration.