Ultrafiltration (Uf) and Biological Oxygen-Dosed Activated Carbon (Bodac) Filtration to Prevent Fouling in Reversed Osmosis (Ro): A Mass Balance Analysis

Abstract

Numerous attempts to prevent fouling of membranes used in water treatment are reported in the literature. Still, none of these found a way to completely omit regeneration, backwashing, or periodic membrane replacement. And exactly this was achieved in practice in an Ultrapure Water Plant (UWP) (Nieuw-Amsterdam, the Netherlands), in a reverse osmosis (RO) system running for ten years as the final part of treating WWTP effluent. The present work aims to elucidate the reasons for this absence of fouling. The UWP has unit operations placed before the RO units:  ultrafiltration (UF) and biological oxygen dosing activated carbon (BODAC) filters (together UF+BODAC) to which a mass balance approach was applied. Organic and inorganic parameters related to fouling precursors (COD, TOC, tN, PO 4 3- , Mn, Fe, Ca, K, Mg, Si, TIC) were included in a sampling and analysis campaign on all inlet and outlet streams of the separate unit operations. The results show that UF+BODAC reduces organic carbon significantly, using different filtering and conversion steps in the UF and BODAC units. Mn and Fe were effectively removed in the BODAC filters.Regarding nitrogen, UF+BODAC proved to be a nitrification system, effectively converting all the ammonium and nitrite to nitrate with a closed mass balance for total nitrogen. For the other elements assessed, Si was slightly removed, and Ca was removed and released, while phosphate, K, and Mg were not retained. In conclusion, the combined removal of organic, Fe, and Mn species by UF+BODAC appear to be causing the absence of fouling in the downstream RO units, calling for further investigations into the mechanisms involved.

Keywords: Ultrafiltration, Biological Oxygen-Dosed Activated Carbon, mass balance, fouling prevention, fouling precursors