In this paper several arrangements of two-module feed forward reverse osmosis (RO) systems
were investigated. The membrane module was modeled as a tube side feed flow tubular module. A
superstructure that incorporates all possible arrangements for two-module feed fomard RO
systems was developed. This superstructure isused to develop the general mathematical model for
the RO system which can be used to s&ulate any of the module arrangements. Mass materials and
energy balances have been applied to each component ii~ the systems. The suggested model has
been constructed and solved numerically to obtain a better understanding of characteristics and
behavior of modules arrangement. This paper also aims to investigate how external forces such as
feed specifications, utility and product costs affect the performance and optimal design of RO
systems. The effect of the following parameters on unit performance is investigated such as; feed
specifications (flow rate and concentration), operating conditions (feed pressure and temperature)
and Membrane dimensions (length and diameter).