Wastewater New York State Practice Exam

If mixed liquor appears black and has very low dissolved oxygen (DO) levels, what is this indicative of?

1. High organic loading

2. Septic conditions

3. Nutrient deficiency

4. Presence of toxic substances

The correct answer is: Septic conditions

When mixed liquor in a wastewater treatment plant appears black and has low dissolved oxygen (DO) levels, this situation typically indicates septic conditions. Septic conditions arise when there is a significant breakdown of organic matter Anaerobic bacteria dominate the decomposition process in environments lacking sufficient oxygen, leading to the release of various gases and compounds that can darken the coloration of the mixed liquor. The absence of sufficient oxygen means that aerobic processes are impaired, leading to the accumulation of organic material and potentially harmful byproducts, further evidenced by the black coloration. This situation can disrupt the biological processes necessary for effective treatment and can adversely affect the overall health of the treatment system. Although high organic loading can also contribute to the depletion of dissolved oxygen, it typically wouldn't result in a distinct black coloration of the mixed liquor as seen in septic conditions. Nutrient deficiencies and the presence of toxic substances may lead to issues within the treatment process but are not directly correlated with the specific combination of black color and very low DO levels.