(11d) Direct Integration of RO Desalination and UF Pre-Treatment with RO Concentrate Backwash

A design for a reverse osmosis (RO) desalination system directly integrated with an ultrafiltration (UF) pre-treatment unit was developed.  The integration involves direct RO feed from the UF filtrate and UF backwash using the RO concentrate.  This alignment removes the necessity of intermediate tanks for a more compact design which is vital in areas where space is at a premium.  The use of RO concentrate for UF backwash allows the UF to operate at 100% recovery since all UF filtrate is fed directly to the RO.  Also, due to the constant production of RO concentrate, the length of backwash is not constrained and the system is capable to carrying out a variable UF backwash strategy where the backwash duration is dynamically adjusted to adapt to temporal variability of feed water quality.  Since the UF pre-treatment system is a “slave” of the RO desalination system due to its task of constantly providing the RO feed, a control system is necessary to ensure the RO inlet retains a specified level of flow and pressure.  The UF pre-treatment system is equipped with UF membrane modules capable of operating independently of each other so that the UF pre-treatment system can still produce RO feed if some of its membrane modules are being backwashed.  This design was used in the construction of an 18,000 gallons/day seawater desalination plant.  Through field testing, the advantages of direct integration of UF and RO, and RO concentrate adaptive (in terms of both frequency and duration) backwash of the UF system, were demonstrated.