A Ro-TAC Case Study – Prefiltration (TDS reduction) for Reverse Osmosis Boiler Makeup Water System


A major electrical utility in the Lake Charles Louisiana area required high quality water for boiler feed makeup water.  A ground water pumping station provided 5700 l/m water flow to a clarifier.  The treated water with high Total Dissolved Solids to two parallel carbon steel pressure filters filled with sand/anthracite media.

5,700 l/m of filtrate water with low TDS was required to feed a large Reverse Osmosis system, where the product water was to be used for boiler makeup water at the electricity generating plant.  Two problems were encountered with the multi-media filters:

  1. The design flux rate could not be achieved without the delivery pressure increasing forcing frequent backwash cycles.
  2. The filtrate TDS target for the RO feed water quality could not be achieved, Hence the RO equipment could not be operated at the design specifications to provide the 4500 l/m of RO product water for the boiler

Unable to operate the RO system due to the poor performance of the multimedia filter system the energy plant engineer contacted a NEXT treatment engineer and arranged to undertake a pilot trial of NEXT-SAND.  After a 10 day trial, the utility plant actioned replacement of the multimedia filters with NEXT-SAND.

The two pressure vessels were loaded with NEXT-SAND and went on line in early 2002.  The filters operated at the net 5.700 l/m design capacity for the following 2 years consistently producing a filtrate below the 50 mg/l TDS limit thus ensuring a reliable supply of water for the RO.  Table 1 shows representative water quality data for the NEXT-SAND filter application.

Table 1. Operating conditions and next-Sand performance
System Flow 5,700 l/m
Surface loading 50l/0.1m2
Filter Performance Feed TDS Filtrate TDS
Initial 350 < 50
Average over 24 months 350 < 50


The NEXT-SAND media allowed economic operation of a high volume Reverse Osmosis system on a water supply that was unusable based on conventional filtration methods