Kim, G.-Y.; Jeong, J.-W.; Kim, M.-G.; Kim, H.-S.; Kim, J.-H., and Kim, H.-S., 2017. A study on scale determination of FO-RO hybrid process based on FO recovery rate. In: Lee, J.L.; Griffiths, T.; Lotan, A.; Suh, K.-S., and Lee, J. (eds.), The 2nd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 79, pp. 75–79. Coconut Creek (Florida), ISSN 0749-0208.Recent studies have been actively conducted on forward osmosis-reverse osmosis hybrid process combining seawater desalination and wastewater reuse technology. It uses seawater as draw solution and reclaimed wastewater as feed solution in the FO process. Although it is possible to save operating costs in forward osmosis-reverse osmosis hybrid process, organic matter contained in treated water of forward osmosis may have a negative effect on the coastal environment. Therefore, studies are needed to evaluate the removal efficiency of organic matter in forward osmosis process and to enable the commercialization of forward osmosis process. Furthermore, the forward osmosis-reverse osmosis hybrid process is a region-specific technology because most wastewater treatment plants and seawater desalination facilities are not constructed together yet. Mass balance has not been considered in full scale plants. In this study, we determined material balance of forward osmosis-reverse osmosis hybrid process based on recovery rate of forward osmosis membranes, and estimated the scale. The results indicated that the recovery rate of the reverse osmosis process could be increased to 69.44% in case of 3 vessels connected in a series. The amount of wastewater that must be treated by the forward osmosis process was approximately 16,735 m3/day, indicating that mass balance can be set. In addition, the reverse osmosis and forward osmosis processes can be configured through simulations.