When primary clarifier sludge and secondary clarifier sludge are mixed, digestion time tends to be longer or shorter?

• A. Shorter
• B. Unchanged
• C. Variable
• D. Longer

Answer: (D)

Mixing primary clarifier sludge with secondary clarifier sludge changes what the digester has to break down. Primary sludge is relatively easy to digest, but secondary sludge adds a lot of microbial biomass and extracellular polymers that are slower to hydrolyze. In anaerobic digestion, hydrolysis is the rate-limiting step, so this tougher, more complex material slows the overall breakdown and the higher solids concentration can limit mass transfer within the digester. As a result, more time is needed to reach the same level of stabilization, so digestion time tends to be longer when both sludges are combined.