preferably three monolithic sub - assemblies — preferably molded from plastic , mounted together as one single assembly and packaged as a kit — permit the retrofit of diverse existing non - zero - waste reverse osmosis systems by unskilled labor to become zero waste . the retrofit preferably transpires by simple screwed connection of , most typically , six fittings so as to connect , most typically , some eight ro system components present in three total major flow paths . by use of the monolithic sub - assemblies , new zero - waste ro systems are similarly modularly constructed at a high degree of integration , thereby simplifying system installation , improving system reliability , and extending system longevity .