Sludge Bulking | Activated Sludge Process | Wastewater Treatment Plant

 Sludge Bulking

Sludge bulking is a condition in which activated sludge is poorly settled and incompatible with the environment. The sludge blanket in the secondary clarifiers rises as a result, allowing solids to escape and exit the plant..

Causes

• Sludge bulking is primarily caused by the presence of filamentous organisms. These organisms form long filaments that connect floc masses during the settling process, preventing effective floc mass compacting.
• Excess water in activated sludge bacteria cells, also known as bound water, reduces the density of the sludge.
• Low pH, low DO, and low nutrient levels were associated with sludge bulking, but the main reason for repeat bulking are high F/M ratios (and low MCRTs). Microorganisms grow more quickly in a dispersed state than in clumps or flocs in an environment with a high F / M ratio. When this happens, activated sludge has poor settling characteristics, making it difficult to keep it in the secondary clarifier without using a chemical flocculant or another method to improve sludge settleability.

Solutions

• Increase the MCRT

Maintain a close eye on plant records to ensure that the desired MCRT is met. In general, increasing the MCRT may help to reduce sludge bulking. Decreasing the F/M ratio has the same effect as increasing the MCRT. Low F/M ratios (high MCRTs) can encourage filamentous organism growth, so compare your plant's F/M ratio or MCRT to the "typical" values published in the literature for properly functioning plants.

• Increase the DO

Low DO levels in the aeration tank may promote the growth of filamentous organisms. As long as you have enough aeration capacity, you should be able to keep DO concentrations between 2 and 4 mg/L.

• Increase the Aeration Basin's Hydraulic Detention Time

Sludge bulking occurs in the aeration basin as a result of short hydraulic detention times due to a design defect or an excessive pumping rate of return sludges. To extend the hydraulic detention time, reduce the return sludge pumping rate. Add a chemical flocculant to the secondary clarifier to thicken the return sludge and maintain the same mass of activated sludge under aeration at a lower return sludge pumping rate.

• Chlorinate the Return Sludge

In an emergency or as a temporary solution, chlorination of the return sludge can alleviate sludge bulking caused by filamentous organisms. Chlorination will not resolve the underlying cause of the filamentous bulking. Chlorine is typically applied at a rate of 2 to 3 mg/L of Cl2 per 1,000 mg/L of MLVSS. The death of nitrifying bacteria may cause an increase in secondary clarifier effluent turbidity during chlorination. 

• Add Flocculant

The addition of a chemical flocculant to the secondary clarifier can improve the settling characteristics of a bulking sludge.

• Control sulfide ions entering the aeration tanks

If there are sludge bulking, check the sulphide ion concentrations in the aeration tanks and all streams entering into the aeration tanks. Thiothrix, a sulfur-reducing filamentous bacteria, may be responsible for the bulking. The Thiothrix will be killed if the sulphide ions are removed.