Design fluxes of membrane is given by membrane manufacturers for typical municipal wastewaters with a set of conditions that need to be met to obtain the flux including MLSS ranges, temperatures, scouring air flow rate, F/M ratio, etc. Design fluxes again split depending on the limit in duration, e.g. monthly or weekly average flux, daily or hourly peak fluxes, etc. The number of membrane modules required to treat influent is determined in order not to violate any of the constraints given by the manufacture in terms of the flux and the durations at the water temperature in any given moment. But, determining required membrane area is not straightforward in real world due to the following reasons.
- Actual wastewater flow rate pattern is quite complex because it varies depending on the time of the day, temporary weather conditions, season, types of the residence around the plant, existence of industrial sources, etc. In addition, the size and the operation strategy of the equalization tank also affect the actual flow rate to MBR. It is not only laborious to obtain rigorous information on the flow rate pattern, but also it is time consuming. As a consequence, MBR plant often has to be designed without having all required hydraulic information.
- The rate of membrane fouling increases gradually at below the sustainable flux. But, it exponentially increases as it exceeds the sustainable flux. The sustainable flux itself is a rather fuzzy concept, which is dependant on biological condition. Since biological conditions tend to change depending on time, running membrane system under the manufacturers’ guideline does not guarantee the successful operation. Therefore, the stability of the membrane system increases as the design flux is lowered below the guideline.
- As a consequence of the varying flow rate and the varying sustainable flux, it is not completely certain how well membrane can perform during the peak time. Therefore, determining the membrane area is a matter of how much risk we would like to or must take due to the budget constraint.
- Fig. 1 shows an example, where daily average flux varies between 15 LMH and 73 LMH while yearly average is 25 LMH. Assuming hourly flow variation is handled by equalization tanks, membrane system must be operated at above 40 LMH for more than 10 days in a row in April. If 50% more membrane modules are installed to handle the peak flow, yearly average flux and peak daily flux will decrease to 17 LMH and 50 LMH, respectively, but significant capital and operating costs will be required.
- As discussed above, overlapping the membrane manufacturers’ flux guidelines with the hydraulic profile and finding out the required membrane area in order not to violate the flux guideline is the standard procedure. But, given the many uncontrollable natural conditions, budget constraint, and the uncertainties in hydraulic data, design engineers also rely on the experiences obtained from the prior sites to estimate the number of membrane modules required.
Fig. 1. Daily average flux profile depending on daily average flow rate in a municipal WWTP, where total membrane area is 50,000 m2.
© Seong Hoon Yoon