WASTEWATER - By Rick Lallish
What type of settled sludge digests the most efficiently in an anaerobic digester?
A. Grit and manually removed screenings
B. Primary clarifier settled sludge
C. Scum and mechanical screenings
D. Secondary clarifier settled sludge
Answer: B. Anaerobic digesters are more efficient with predominately sludge removed from a primary clarifier. They may be supplemented with secondary clarifier sludge. Primary sludge has a much higher organic composition and is better able to combine with acid-forming bacteria to form organic acids, one of the two types of bacteria to achieve anaerobic digestion (the other being methane-forming bacteria). Aerobic digesters use almost exclusively secondary clarifier
