In Heemskerk the world's largest dual membrane municipal water treatment works ( WTW) “Jan-Lagrand” (55 Ml/d or 2,250 m3/h or approximately 15 MGD) was commissioned at the end of 1999. At Heemskerk WTW, pretreated surface water is subsequently treated by ultrafiltration (UF) and reverse osmosis (RO). The RO permeate is directly blended into drinking water without chemical disinfection.

Before Heemskerk the dual membrane concept had not been considered on this scale, and during project preparation numerous developments and innovations needed to be flexibly accommodated into the ongoing design. Conceptual system development, pilot research and design and engineering were successfully conducted “in parallel”.

This paper deals with the innovations that were materialized in Heemskerk, e.g. UF-membrane pretreatment, on line membrane integrity testing (UF and RO) for disinfection approval, and a dual floor building concept for membrane installations. This paper explains how these innovations could be accommodated into an ongoing design, while innovative testing was still proceeding.