Sludge Volume Index, Food / Microorganism Ratio & Sludge Retention Time
Sludge Volume Index, Food / Microorganism Ratio, and Sludge Retention Time are used in conjunction to assure the proper function of the activated sludge process.
Sludge Volume Index
The Sludge Volume Index (SVI) is the ratio of volume of sludge to settling quality. SVI is an indication of the density the settled sludge attains in a 30 minute settling test. A low number (100 to 200) indicates good settling and shows the process is working properly. A high number (>500) indicates poor settling and shows the process may need adjustment.
Food / Microorganism Ratio
Food / Microorganism Ratio (F/M) is a measure of the amount of food material (organic material in the wastewater) available to the activated sludge microorganisms. The plant processes function best when the amount of food is balanced with the amount of microorganisms. Either too much food or not enough will cause poor plant performance.
A high rate process would require a high food to microorganism ratio (>0.5). This requires a fairly constant food rate and is more prone to upsets. A conventional rate process requires a midrange F/M ratio (0.25 to 0.5). Conventional rate processes take food rate changes better and are easier to control. Extended aeration processes require a F/M in the range of 0.05 to 0.1. They are relatively simple to operate and even less prone to upsets. The NW Treatment Plant is a modified Extended aeration process. The process is regulated to keep the ratio in the range of 0.09 to 0.15.
Sludge Retention Time
Sludge retention time (SRT) is the average number of days microorganisms are kept in the activated sludge process before they are removed for beneficial reuse. The plant processes work best at the proper SRT. The plant is designed to operate in a range of 10 to16 days.