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 sludge. More information can be found in the Office of Water Programs, California State University, Sacramento textbook: Operation of Wastewater Treatment Plants, Volume 1.
DRINKING WATER - By Drew Hoelscher
At a surface water treatment plant, what point in the treatment process would NaOH be introduced for corrosion-control purposes?
A. At the beginning to prevent corrosion on treatment plant equipment
B. Toward the end after all other treatment has been accomplished
C. In the distribution system where water has aged
D. At any point where the pH of the water is below 7
Answer: B. In regard to quality, water sources are not equal. The appropriate level of treatment and the chemical feed point are specific to each source. A surface water source containing particulate matter will most likely require conventional treatment followed by disinfection. The coagulation and disinfection processes are typically more successful at lower pH levels. That means applying chemicals to raise the pH for corrosion-control purposes is more beneficial after all other treatment processes are complete.
