Here are the answers to the Dec. 31, 2020 edition of TPO's Exam Study Guide:

In fixed film processes, what must be reduced before nitrification can occur?
The answer to the question is A, organic loading rate. Higher organic loading rates favor the growth of heterotrophic bacteria over nitrifying bacteria. In most fixed film systems, the higher OLRs cause rapid growth of heterotrophic bacteria, forcing the weaker nitrifiers to go deeper into the biomass. This process slows their growth rates. By reducing the OLR, the heterotrophic bacteria will slow, preventing the nitrifiers from being overtaken. Operating the system in a series mode where the OLR is reduced after the first stage will allow nitrification in later stages of the process. Understanding the different ways nutrient removal is accomplished helps the operator further understand wastewater operations.

2. Using the table below, what finished water quality parameter(s) should the operator try to improve?
The correct answer is C, iron is above the SMCL. Iron is listed on the National Secondary Maximum Contaminant Level list at 0.3 mg/L. At or above this level, iron may cause discolored water complaints at the point of use and/or staining on plumbing fixtures. The SMCL list of contaminants are non-enforceable and are only set as guidelines for public water supplies. More information regarding National Primary Maximum Contaminant Levels and National Secondary Maximum Contaminant Levels can be obtained at


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