Energy usage of MBR can be optimized by performing proportional scouring aeration, partial shut down of membrane trains at low flow, etc. Since there is no absolute correlation between the flow rate and the required scouring aeration, it is not clear how much benefit can be obtained with the completely proportional aeration that requires variable speed blower system. Therefore, scoring air flow is adjusted stepwise according to the flow rate, where air flow rate can be adjusted at 2-5 different levels by using multiple blowers at a fixed speed or less number of blowers with variable frequency drive (VFD). High speed, direct-driven blowers with VFD with magnetic bearings that do not need heavy foundation are known to conume less energy than multistage or integrally-geared single-stage blowers (Levesque, 2010). Fine bubble diffusers are also used to save bio-aeration.
Based on the diurnal flow shown in Fig. 1, flow pattern can be characterized as Fig. 2 considering the capacity of equalization tank. After dividing the flows to low, medium, and high, proper scouring air flows are assigned as shown in Fig. 2. All the examples shown here is based on Ovivo’s design parameters.
As summarized in Table 1, overall plant operation strategy differs based on flow rate. At no flow condition, aeration is performed only occasionally in the membrane tank to prevent solids settling. At low flow (<0.5Q), two of the three membrane trains are used while the standby train has aeration occasionally. If the net flow rate to each train is below 0.5Q, air flow rate to each cartridge maintains at 3.5 LPM. Otherwise, it can be 5.25 LPM per cartridge. At medium flow (0.5Q-1.5Q), PC based software, e.g. Energy ProTM, decides whether using two trains at high air flow is better or using three trains at medium air flow is better in terms of energy consumption. At peak flow at 2.0Q, all trains are used at high air flow rate (7.0 LPM per cartridge).
Fig. 1. Daily flow rate pattern, where Q indicates maximum monthly average flow (Codianne, 2012)
Fig. 2. Characterized flow pattern and the duration of low, medium and high flows (Codianne, 2012).
Table 1. Illustration of Energy Pro system operation (Codianne, 2012)
© Seong Hoon Yoon