The purpose of this work is to introduce the novel notion of synthesizing reverseosmosis networks (RONs) for waste-reduction applications. The RON design task aims at synthesizing a network of reverse-osmosis units, booster pumps and energy recovery devices that can separate a set of waste streams into lean (product) streams and rich (retenate) streams at minimum total annualized cost. A systematic and generally applicable procedure for tackling RON synthesis problems is developed. First, a structural representation is devised to embed all potential network configurations. Then, the problem is formulated as an optimization program whose objective is to minimize the total annualized cost of the network while satisfying all environmental and technical requirements. The solution to this program provides the optimal arrangement, types and sizes of the reverse-osmosis units, the booster pumps, and the energy-recovery turbines to be employed in the network. Furthermore, the solution also provides the optimum stream distribution, operating conditions, and separation levels. The applicability of the proposed synthesis technique is demonstrated by tackling case studies and comparing the optimal solutions with current industrial practices.