The True Measure of Performance…Sludge Reduction

The basic process of biological treatment of wastewater involves the conversion of soluble organics to carbon dioxide, water and bacterial cells. These bacterial cells produced are the main component of sludge that must be collected and removed (wasted) for the plant to fully recycle the biological treatment process.

The amount of biomass produced in the form of sludge, which must be disposed of, is dependent on the Food/Mass (F/M) Ratio since this determines the amount of energy that is available, above what is required for the bacteria cell maintenance and cell synthesis. Studies have shown that certain bacteria species that are more efficient (i.e. have a higher metabolic rate) can be used to augment the populations in the mixed liquor resulting in an Improved (lower )biomass conversion efficiency, which we express as a yield coefficient, Y.

The yield coefficient is calculated by dividing the kilograms of solids removed from the system by the kilograms of BOD removed in the process. The calculation of solids generation must take into account the solids coming into the system in the influent, the solids being wasted (removed) from the system and the solids going out in the effluent.

The BOD removal calculation must take into account soluble and particulate BOD coming into the system as well as soluble and particulate BOD exiting the system.

The formula for this reaction is:

Yield Coefficient = BOD inf (mg) – BOD eff (mg) x F or Q inf (MG) x 8.34 lbs/mg-MG

(TSS was (mg) x F or Q was (MG)) – (TSS eff x F or Q eff (MG))x 8.34 lbs/mg-MG

AquaClean and Sludge Reduction in Biological Treatment Systems

The basic function of a biological treatment system is to convert soluble organics to carbon dioxide, water and bacterial cells. The cells can then be separated from the purified water and disposed of in a concentrated form called "sludge". The advantage of biological treatment is that it can reduce soluble organics in large volumes of water far more economically than any other type of chemical or physical / chemical process.

This bulletin will deal with aspect of sludge reduction. As mentioned earlier, the basic process of biological treatment is the conversion of soluble organics to carbon dioxide, water and bacterial cells. This reaction is defined by following equation:

BOD + N + P + O₂ CO₂ + H₂O + Cells

The soluble organics are represented by BOD. The N is nitrogen, required for protein synthesis and P is for phosphorous which is required for energy transfer. Depending on the F/M or food / mass ratio in the system there is another process that goes on simultaneously. This process, digestion, occurs when there is not enough food for the bacteria to continue growing and is represented as follows:

Cells + O₂CO₂ + H₂O + N + P

In a batch system where cells are grown and allowed to go into endogenous respiration, eventually all of the carbon would go off as CO2 and the only thing left in the water would be minerals. Hence, the process is called "mineralization".

The amount of sludge produced per kg of organics removed across the system is defined by the yield coefficient, Y_c. The yield coefficient is dictated by the equilibrium F/M ratio in the system. Normally the F/M ratio is defined as the total kg of BOD per day / total . kg of biological solids in the system (not including inventory in the claryfier). However, in this case, the equilibrium (F/M) is defined as the concentration of Mixed Liquor Suspended Solids MLSS/concentration of BOD in the mixed liquor.

In a completely mixed activated sludge system, it is assumed that the concentration F organic is uniform throughout the aeration basin and equal to the effluent concentration of organics. (Effluent is just mixed liquor with the solids settled out.) The equilibrium concentration of organics is the amount of organics the bacteria have to work within their immediate environment. As the effluent organic concentration approaches 1 ppm, the cells have less and less energy to work with in maintaining all vital cell functions. The minimum level of energy required to keep a cell intact is called the cell maintenance energy. The cell maintenance energy is the energy the cell needs to overcome entropy, the tendency of things to precede to a state of maximum randomness. With no energy the cell could not stay intact and would lyse, as in the digestion process. When there is more energy available that what is required for cell maintenance, the cell can synthesize new building blocks and multiply, producing sludge.

In waste water treatment plants (sewage works), there is a direct relationship between the equilibrium concentration of organics and the yield coefficient. The lower the equilibrium F/M, the less sludge production. This concept is applied to extended aeration systems that tout lower production as an advantage.

Two inherent drawbacks to biological treatment that AquaClean can alleviate through improved biological performance.

1. The fact that is a living system and treatment efficiency can be affected by temperature and the presence of toxic or inhibitory compounds which can kill the system.

2. The fact that a biological sludge is produced which must be removed for disposal. In areas where space is at a premium, sludge disposal costs can be significant.

In many AquaClean programs conducted to improve BOD removal efficiency of the system a secondary benefit has been a reduction of the amount of waste activated sludge generation from 15 to 40%. What occurs when the AquaClean ACF-32 is added is the establishment of a more dynamic microbial population. This population not only responds faster to changes in loading on the system, but also reduces the effluent concentration of BOD. This then lowers the equilibrium F/M, reducing the amount of available energy closer to cell maintenance energy of the cell. While AquaClean ACF-32 is an important tool in sludge reduction, to optimize this benefits requires careful control of the critical operating parameters to optimize the sludge reduction.