Florida Wastewater Practice Test 2025 - Free Wastewater Certification Practice Questions and Study Guide

In an aerobic digester, the introduction of air supports aerobic bacteria, which consume organic matter and utilize oxygen for respiration. When air is turned off periodically, the conditions in the digester become anaerobic, leading to a change in the types of microbial processes occurring.

In this anaerobic environment, the nitrate that was previously present is used by bacteria in a process called denitrification. This is where nitrates are reduced to nitrogen gas, effectively decreasing the concentration of nitrates in the system. As for the pH, the anaerobic conditions tend to promote the production of acids as organic matter is broken down without oxygen. The buildup of these acids typically leads to a decrease in pH.

So, when the air is turned off periodically, it results in decreased nitrates due to their conversion to nitrogen gas by anaerobic bacteria, and a decrease in pH due to the accumulation of acid from anaerobic digestion. This is why the correct choice signifies that nitrates decrease while pH increases under these specific conditions.