What is the goal for SOUR (Specific Oxygen Uptake Rate) in sludge?

• A. Less than 1.5 mg/g VSS-hr.
• B. Greater than 5 mg/g VSS-hr.
• C. Exactly 2 mg/g VSS-hr.
• D. Not defined.

Answer: (A)

Specific Oxygen Uptake Rate shows how fast the sludge microbes use oxygen per hour per unit mass of volatile solids. It reflects how active the sludge is and how much readily biodegradable material remains. The goal is to keep this rate low, typically under 1.5 mg O2 per g VSS per hour. A low SOUR indicates stable, mature sludge with less easily biodegradable substrate, which means lower oxygen demand during aeration and better process stability. If SOUR is much higher, such as above 5 mg/g VSS-hr, it signals more readily biodegradable matter or unstable sludge and higher aeration costs. A single exact value like 2 mg/g VSS-hr isn’t a general target, and saying it’s not defined ignores the established guideline.