Wastewater New York State Practice Exam

What is the primary reason for increased MCRT during winter?

1. Lower temperature in the digester

2. Increased microbial activity

3. Higher organic load

4. Decreased sludge wasting rates

The correct answer is: Decreased sludge wasting rates

In wastewater treatment, Mean Cell Residence Time (MCRT) refers to the average time that a microorganism spends in the treatment system. During winter months, many wastewater treatment facilities experience a decrease in temperature, which can have a significant impact on the biological processes occurring within the system. Typically, colder temperatures can slow down the metabolic rates of microorganisms, leading to reduced microbial activity. Given this context, decreased sludge wasting rates become a primary factor affecting MCRT. In winter, operators may reduce the amount of sludge wasted to maintain enough microorganisms in the system to ensure adequate treatment, particularly given the slowed metabolism of the existing microbes due to lower temperatures. This retention of sludge increases the MCRT, as the same population of microorganisms remains in the system for a more extended period to continue the treatment process effectively. Additionally, while lower temperatures influence the microbial activity and organic load, the action of manually decreasing the sludge wasting rates stands out as the primary operational strategy involved in managing MCRT during winter. In this sense, keeping more biomass in the system becomes crucial to compensate for any decreases in microbial efficiency brought on by the colder conditions.