J. Eng. Technol. Sci., Vol. 49, No. 2, 2017, 193-209 193 Received January 13 th , 2017, Revised April 10 th , 2017, Accepted for publication May 3 rd , 2017. Copyright ©2017 Published by ITB Journal Publisher, ISSN: 2337-5779, DOI: 10.5614/j.eng.technol.sci.2017.49.2.3 Case Study of a Small Scale Reverse Osmosis System for Treatment of Mixed Brackish Water and STP Effluent I Nyoman Widiasa* & Retno Dwi Jayanti Department of Chemical Engineering, Diponegoro University, Jl. Prof Sudarto- Tembalang, Semarang 50239, Indonesia *E-mail: widiasa@undip.ac.id Abstract. A case study on utilizing reverse osmosis (RO) technology to fulfill fresh water needs at a mall and a hotel has been done on Bali Island, Indonesia. A mix of brackish water and sewage treatment plant (STP) effluent was used as feed water in the RO system. The system used 36 membrane elements (CSM RE 8040 BLN) arranged into two stages: 8 pressure vessels (PVs) in the first stage and 4 PVs in the second stage, each loaded with 3 membranes. The objectives of this research were to assess the cleaning effectivity in the plant, to evaluate the cleaning of 1 membrane element using a CIP system, and to assess the use of the membrane for filtration in the pre-treatment system. SEM and FTIR analysis indicated that the foulants on the membrane surface were dominated by organic foulants and inorganic deposits. To clean the discarded membrane the proposed method used NaOH solution (pH 12 and pH 13) and citric acid (pH 2 and pH 3). All membranes displayed a dramatic decline in rejection of about 80%. Based on the rejection tests of SO 4 2- , Cl - , turbidity reduction approached 100%. It can be concluded that an RO membrane that has undergone selectivity decline can be re-used as a filtration membrane in the pre-treatment system. Keywords: domestic wastewater; chemical cleaning; ion rejection; reuse; RO membrane. 1 Introduction Reverse osmosis (RO) technology is used to reclaim domestic wastewater. Fouling and scaling are still the major problems in RO application since they can cause membrane performance degradation and flux decline, and influence the product water quality [1-3]. Fouling and scaling depends on several factors, i.e. membrane morphology, feed water composition, and operation conditions [4-8]. Pre-treatment can be done in RO application to control membrane fouling and scaling by removing impurities from the feed water [3,9,10]. Unfortunately, pre-treatment is not capable to perfectly remove all impurities from feed water and even causes fouling of the membrane [9,11]. Chemical membrane cleaning is an alternative way to control fouling on the membrane [12]. This can be done when the membrane fouling and scaling causes the flux decline to approach 5- 10% [13-15]. In RO application, chemical membrane cleaning is more often