Attached Growth System Sludge

The microorganisms in attached growth secondary treatment systems such as trickling filters and rotating biological contactors (RBCs) are biochemically similar to microorganisms that predominate in activated sludge systems. Consequently, the biomass production from attached growth systems and activated sludge systems is roughly similar when compared on the basis of kilogram of biomass produced per kilogram of substrate removed. Therefore, equations that predict solids production in an attached growth system are similar in form to those used in predicting solids production in an activated sludge system. However, there are differences between the two systems. The main difference lies in the term used to define the quantity of biomass in the system. Figure 2.6 is a schematic of a trickling filter treatment system. The following equations for predicting solids production in a trickling filter system (the same equations can also be applicable for RBCs) are similar to equations (2.3) and (2.4), but are based on the assumption that the total mass of microorganisms present in the system is proportional to the media surface area:

Primary Clarifier

Trickling Filter

Secondary Clarifier

Screened and Degritted Influent

Primary Clarifier

Trickling Filter

Secondary Clarifier

Screened and Degritted Influent


Secondary Sludge


Secondary Sludge

Sludge Return

Primary Sludge

Sludge Processing

Figure 2.6 Schematic of a typical trickling filter system.



= net growth of biomass (VSS), kg/d or lb/d


= gross yield coefficient, kg/kg or lb/lb


= influent substrate (BOD), kg/d or lb/lb


= effluent substrate (BOD), kg/d or lb/lb


= decay coefficient, d-1


= total media surface area, m2 or ft2


= waste trickling filter solids, kg/d or lb/d


= influent nonvolatile suspended solids, kg/d or lb/d


= effluent suspended solids, kg/d or lb/d

The yield coefficient values for attached growth systems are similar to the values for activated sludge systems. However, the decay coefficient values are higher for attached growth systems because of the apparently long time the solids stay attached to the media (longer effective SRT). Typical values of decay coefficients range from 0.03 to 0.3 d-1.

Nitrification can occur in attached growth systems. The resulting quantity of biomass, similar to that in activated sludge systems, is small: 3 mg/L (25 lb/MG) may be used for design purposes.

Attached growth solids have better settling and thickening properties than those for activated sludge. Consequently, attached growth sludge tends to be of higher concentration, typically 1 to 4%.

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