In the initial design, all the permeate streams from 11 UF stages are recycled by 11 NF stages (except of the flows to the retentate stream in the final NF stage). This design was modelled to maximise acid recycling. Most of the acid is filtered in the first few UF stages, as the acid becomes more dilute with the increase in the number of UF stages. Figure shows that the acid is diluted approximately by half in each UF stage, thus the acid recovery becomes negligible after the 6 th UF stage. The acid concentration starting from the 4 th UF stage can be too dilute (< 1 wt.%), therefore acid recycling after the fourth UF stage might not suit industrial practice. Further, the first three UF stages already contain 80% of the acid. Hence, we refined our design to partial acid recycling with only three UF stages. The effect of acid recycling adjustment is summarized in Table . Acid recovery can linearly reduce the consumption of H2SO4 or NaOH. This is to say that 80% (or 99%) acid recovery reduces 80% (or 99%) H2SO4 or NaOH, compared to the design without acid recycling.