■The concentration varies depending on the type of drainage. In food factories, the MLSS (Mixed Liquor Suspended Solids) in aeration tanks is generally maintained at 3,000 to 5,000 mg/L. In long-term aeration methods, it ranges from 3,000 to 6,000 mg/L, while in membrane filtration methods (MBR), it is about 8,000 to 15,000 mg/L. These values are higher than the design values for sewage, and the appropriate values vary significantly depending on the method used. ■What is SV30? SV30 is the ratio of the volume occupied by the settled sludge after placing a mixed liquor sample taken from the aeration tank into a graduated cylinder and letting it sit for 30 minutes. ■Assessing settleability with SVI! From SV30 and MLSS, you can calculate the SVI (Sludge Volume Index), which indicates the settleability of the sludge. SVI = SV30(%) ÷ MLSS(mg/L) × 10,000 For example, if SV30 is 30% and MLSS is 2,000 mg/L, then 30 ÷ 2,000 × 10,000 = 150. The SVI represents the volume (mL) occupied by 1g of settled sludge. A larger value indicates that the sludge is bulky and settles poorly. If SV30 increases to 60% while MLSS remains at 2,000 mg/L, the SVI would be 300.

■ How to adjust MLSS? MLSS is primarily adjusted by the amount of excess sludge withdrawn. Increasing the withdrawal will lower the MLSS, while decreasing it will raise the MLSS. Microorganisms proliferate by decomposing organic matter in the wastewater, so if the increased amount is not removed as excess sludge from the system, the concentration will continue to rise. ■ Viewing in conjunction with retention time In standard activated sludge processes for wastewater, the aeration tank is designed for a retention time of about 6 to 8 hours. Even with the same MLSS, if the amount of incoming water or the concentration of pollutants changes, the BOD load per kg of sludge (BOD-MLSS load) will also change. ■ Why does SVI increase?! When bulking occurs, making the sludge difficult to settle, the SVI increases. An increase in SVI is one of the typical manifestations of bulking. ■ Changing concentration with the seasons During winter, when water temperatures drop, reactions slow down, so we compensate by maintaining a higher MLSS. The same applies during periods of increased BOD inflow. Therefore, it is important to organize and analyze operational data to manage concentrations according to seasonal changes. Please feel free to consult with us first.