A comparison between chemical cleaning efficiency in lab-scale and full-scale reverse osmosis membranes: Role of extracellular polymeric substances (EPS)

M Jafari,A D'Haese,J Zlopasa,E.R Cornelissen,J.S Vrouwenvelder,K Verbeken,A Verliefde,M.C.M Van Loosdrecht,C Picioreanu

doi:10.1016/j.memsci.2020.118189

Abstract

Chemical cleaning is vital for the optimal operation of membrane systems. Membrane chemical cleaning protocols are often developed in the laboratory flow cells (e.g., Membrane Fouling Simulator (MFS)) using synthetic feed water (nutrient excess) and short experimental time of typically days. However, full-scale Reverse Osmosis (RO) membranes are usually fed with nutrient limited feed water (due to extensive pre-treatment) and operated for a long-time of typically years. These operational differences lead to significant differences in the efficiency of chemical Cleaning-In-Place (CIP) carried out on laboratory-scale and on full-scale RO systems. Therefore, we investigated the suitability of lab-scale CIP results for full-scale applications. A lab-scale flow cell (i.e., MFSs) and two full-scale RO modules were analysed to compare CIP efficiency in terms of water flux recovery and biofouling properties (biomass content, Extracellular Polymeric Substances (EPS) composition and EPS adherence) under typical lab-scale and full-scale conditions. We observed a significant difference between the CIP efficiency in lab-scale (~50%) and full-scale (9–20%) RO membranes. Typical biomass analysis such as Total Organic Carbon (TOC) and Adenosine triphosphate (ATP) measurements did not indicate any correlation to the observed trend in the CIP efficiency in the lab-scale and full-scale RO membranes. However, the biofilms formed in the lab-scale contains different EPS than the biofilms in the full-scale RO modules. The biofilms in the lab-scale MFS have polysaccharide-rich EPS (Protein/Polysaccharide ratio = 0.5) as opposed to biofilm developed in full-scale modules which contain protein-rich EPS (Protein/Polysaccharide ratio = 2.2). Moreover, EPS analysis indicates the EPS extracted from full-scale biofilms have a higher affinity and rigidity to the membrane surface compared to EPS from lab-scale biofilm. Thus, we propose that CIP protocols should be optimized in long-term experiments using the realistic feed water.

Highlights

Biofouling is an undesired accumulation of microorganisms on sur faces due to the deposition of organic compounds and/or growth of microorganisms [1,2]
The water flux was measured for the virgin membrane, as well as for the fouled membranes before and after CIP cleaning
The water flux for the virgin membrane was measured to compare the impact of biofouling in water permeability decline

Summary

Introduction

Biofouling is an undesired accumulation of microorganisms on sur faces due to the deposition of organic compounds and/or growth of microorganisms (biofilm formation) [1,2]. Biofouling adversely impacts membrane filtration systems by causing an additional hydraulic resistance [3,4], reduction in apparent membrane permeability and selectivity [5,6], and higher feed channel pressure drop [2,7,8]. Physical and chemical cleaning routines are periodically applied to reduce biofouling impacts on membrane systems. Chemical cleaning protocols include chemically-enhanced backwash and Cleaning-In-Place (CIP) typically using acid and base solutions. The goal of both physical and chemical cleanings is to remove foulants and restore membrane performance as close as possible to virgin membranes. In dense membrane systems (RO, NF) in spiral wound configuration, hydraulic cleaning options are limited because the membranes cannot be backwashed

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