As the days continue to shorten and the temperature drops, it’s time to think about what changes need to be made to the wastewater treatment plant to continue to be in compliance during the colder months.
As the air temperature drops, the water temperature also goes down, especially in smaller treatment works. A cooler water temperature causes the bacteria to slow down the degradation process because they have an optimum temperature in which they live and grow best.
If the bacteria and other microorganisms slow down when the water temperature drops, how do you continue removing as much biochemical oxygen demand (BOD), total suspended solids (TSS), nitrogen and phosphorus as you do in the warmer months? We could raise the temperature of the influent, but that would take a lot of energy. We could raise the concentration/number of microorganisms in the treatment system, so then we change the concentration of organisms in the process.
How do we do that? We raise the concentration of mixed-liquor suspended solids (MLSS) in the treatment system, specifically the aeration section of the system, by increasing the return-activated sludge (RAS) and decreasing the waste-activated sludge (WAS). This provides more organisms to degrade the pollutants in the wastewater in the cool months as efficiently as they do in the warmer months.
How much do we raise the concentration? Each facility is different because each influent is different. Raise the MLSS concentration in the aeration section of the plantslowly by adjusting the RAS and WAS and allow it to stabilize. Then test to determine how well it degrades the pollutants at that concentration. If it is not removing them efficiently enough, increase the MLSS until it does and chart it. If you didn’t write it down, you didn’t do it!
When you adjust the RAS and WAS to increase the MLSS concentration, it also affects sludge age, food-to-microorganism ratio (F/M), dissolved oxygen (DO), mixed-liquor volatile suspended solids (the living biomass) (MLVSS), clarifier operations and sludge wasting, dewatering and disposal at the plant.
When the MLSS is increased, it also increases the MLVSS and sludge age, but it decreases the F/M ratio and unless adjustments are made to the aeration system, this also decreases the DO. The increase in MLSS also has an effect on the clarifier sludge-bed depth.
How does increasing the RAS rate raise the sludge age but drop the F/M and decrease the DO?
As you increase the concentration of MLSS in the aeration section of the facility, the sludge age equation comes into play.
Lbs. MLSS /aeration basin ÷ Lbs. TSS/ influent = sludge age
If you increase the number of pounds of MLSS in the aeration basin by increasing the RAS rate and the influent TSS stays about the same, the sludge age goes up.
Conversely, by increasing the RAS rate, the F/M ratio decreases due to an increase in MLVSS in the aeration section of the plant. The F/M ratio equation shows this.
Lbs. BOD/influent ÷ Lbs. MLVSS in the aeration section = F/M ratio
The potential drop in DO is caused by the increase of microorganisms in the aeration section of the plant. This increase in oxygen use is directly proportional to the increase in concentration of the microorganisms. It should be easier to maintain an elevated DO in the cooler months due to the oxygen saturation levels being higher in cooler water.
Another factor that is affected by the change in RAS rates is clarifier operations. The sludge-bed depth will change in the secondary clarifiers when the MLSS is more concentrated. Instead of a 3-foot bed depth, you may have a 5-foot bed depth.
When you increase RAS rates normally, the WAS rates are reduced. This slows the process of sludge wasting, dewatering and final disposal.
It is important to anticipate the changes in temperature brought on by the change in seasons and to make the appropriate changes to the treatment process to remain in compliance. Trend charts built with years of data are very helpful in determining when to anticipate these seasonal changes.