The SEDs of each unit process for the example given in this section are summarized in Table 1. Neglecting all other plant energy consumptions that are not directly related with wastewater treatment, 45% of energy is consumed for biological wastewater treatment and 55% for membrane filtration for the given case. Permeate suction and mixed liquor recycle are relatively minor at <10% each.
The input values of the following table can be modified to see the impact of each variable on the overall energy breakdown.
Although it was assumed that oxygen consumption occurs only in aeration tank in above calculation, it also occurs in membrane tank. One study shows that in membrane tank 33% of the total oxygen dissolution and 23% of the dissolved oxygen consumption occurs, where 10% of dissolved oxygen is transferred to aeration tank through the internal mixed liquor recycle. Details are here.
Fig. 2 shows a survey result from six different MBR plants, where two MBRs were equipped with flat sheet (FS) membranes and four were with hollow fiber (HF) membranes. As shown in the above example, aeration causes majority of the energy consumption. For flat sheet, membrane aeration requires more than twice of the activated sludge aeration (or bio-air). On the other hand, for hollow fiber, membrane aeration and bio-air are approximately equal. Overall, FS membrane consumes much more membrane scouring air than HF (see Fig. 2 here).
Fig. 2. Distribution of energy consumption of MBR units (biological step only) for six MBRs (Barillon, 2012).
© Seong Hoon Yoon