Dual-Membrane Filtration Plant Will Treat Dutch Surface Waters

Written by Fred de Bruijn and David Eddy

The Heemskerk Water Treatment Plant, when completed in 1998, will be one of the world's largest dual membrane filtration plants. The facility will take pretreated surface water and process it through ultrafiltration and reverse osmosis, to produce 50 Ml/d (13,2 mgd) of softened water, with reduced sodium levels free of micro-pollutants and disinfection by-products.

Provincial Water Company North Holland (PWN) has focused on reverse osmosis as a treatment process to meet its commitment to provide softened water to its customers. Reverse osmosis, as a treatment concept, was selected because of its dual function of providing desalination capability while also providing an effective physical disinfection barrier. This approach ensures PWN is able to overcome the latest drinking water challenges without the complications of disinfection by-products that are associated with current chemical oxidation processes.

Dutch use US protocol
Following extensive pilot testing that identified the potential of membrane technology as an appropriate disinfection process that overcomes the health risks associated with the byproducts of chemical oxidation processes, it was necessary to establish a protocol of appropriate disinfection accreditation for the membrane process. In the absence of Dutch guidelines for the removal of viruses and cysts of giardia and cryptosporidium, the U.S. Environmental Protection Agency's (EPA) surface water treatment rule (SWTR) was used.

Pre-design studies
From testing by the PWN, it was evident that a single membrane step, whether ultrafiltration or reverse osmosis, would be sufficient to achieve water quality targets. A single membrane barrier however would result in compromised disinfection properties at any time when membrane integrity would be infringed. For this reason, as well as for reasons of complicated membrane integrity monitoring, it was decided that a double barrier should be developed.

Following extensive pilot testing, a pre-design study was commissioned to evaluate three double-barrier pretreatment alternatives:

• Rapid gravity filtration + cartridge filtration + reverse osmosis (RO) + ozonation/advanced oxidation + GAC
• Ultrafiltration (UF) + reverse osmosis
• Microfiltration (MF) + reverse osmosis + ultraviolet (UV) disinfection

Pre-design studies evaluated the economic and operational benefits of each of these options. The report concluded that the various options were similar in overall cost, though microfiltration and ultrafiltration were more effective pretreatment processes, minimizing the fouling potential upstream of the fouling-sensitive RO membranes. Since UF performed extremely well in pilot trials and provides in excess of 5 log units removal of micro organisms, this was the logical process combination with composite polyamide RO membranes.

PWN chose Montgomery Watson and Witteveen + Bos as engineering consultants and Plan Architekten as architect to prepare detailed designs for the entire project.

Membrane design
The dual-membrane system at Heemskerk comprises hollow fiber ultrafiltration modules followed by spiral-wound thin film composite polyamide reverse osmosis membranes. Both membrane filtration systems apply similar plant configurations using multiple 1.0-meter-long membrane elements housed within 200-mm-diameter pressure vessels supported horizontally. Designers used this consistent feature in conjunction with UF membrane supplier, X-Flow, to develop similar block designs for both UF and RO trains.

Membrane blocks are sited in two staggered rows separated by a wide central gallery. This layout was preferred since it provides sufficient space for straightforward pressure vessel replacement and also gives comfortable maintenance access. This solution also ensures a clear relationship of pumps with the treatment train.

Each membrane train is provided with a dedicated feed pump, rather than common feed pumps connected to a manifold. Main advantages of dedicated pumping are:

• Independent control of each membrane train
• Simplicity of installation and operation
• Avoids inevitable energy losses if feed flow is regulated to each block
• Higher overall pumping efficiency because of smaller range for the operating point.

These advantages are offset against the disadvantage of higher capital investment for the extra pumps.

Reverse osmosis
Reverse osmosis uses the thin-film composite spiral wound polyamide membranes operated in a cross-flow mode. The initial design selected for the Heemskerk installation was a three-stage 28 + 14 + 7-block array, with six elements per vessel and operated at average feed pressures of around 18 bar and at 80 percent recovery rates.

However, a new generation of ultra-low pressure RO membranes became available while design of the three-stage configuration was being finalized and the new design comprises a two-stage 24 + 12-block array with seven elements per vessel. These new membranes operate at substantially lower feed pressures, providing savings in energy consumption of over 30 percent. Following successful trials, it was decided to amend the design of the RO block to exploit the energy savings offered by these new low-pressure RO membranes.

Ultrafiltration
The ultrafiltration configuration is modeled on the typical RO layout -- multiple 1.0-meter-long elements housed within a horizontally mounted pressure vessel. Engineering efforts concentrated on achieving similarity of the two systems.

The UF block comprises two symmetrical sets of 12 pressure vessels, each set being considered as a rack. This gives the impression of there being the identical number of blocks as the RO process. The block has a single feed system that bifurcates into each rack. The racks also share a common filtrate manifold. This means that when one rack is initiating a backwash cycle, the other rack remains out of operation.

In the design of the UF system, the head loss within the system and the hydraulic distribution within the pressure vessel demand a larger feed and permeate port than is needed with conventional RO installations. Fifty-millimeter diameter feed and permeate ports are required to minimize energy loss in feed and backwash operations.

Air poses significant problems in the commissioning and operation of the X-Flow UF system. Unlike conventional UF units, it is mounted horizontally and operates at too low feed pressures to achieve air saturation in the feed water as is the case with an RO system. To counter these problems, the side port is rotated towards the vertical to assist with the relief of air from the system.

The selected ultrafiltration element to be installed at Heemskerk is an X-Flow S-150 PVC UFC M5 element with 0.8-mm-diameter hollow fibers. This membrane has been the subject of lengthy pilot testing. The feed flow enters from within the element's hollow fiber with permeate passing through the hollow fiber walls where it is collected in a central permeate collector pipe, using the same principles as with conventional spiral wound RO elements.

The plant must still prove itself once it goes into operation in 1998. However, it is evident that in pursuing a comprehensive approach to the engineering of such a complex treatment plant through pilot testing, pre-design studies, and detailed engineering, we can conclude the following:

• A plant of this complexity must focus on logic and simplicity
• A full design effort resulted in optimizing the arrangement of components
• A flexible design mentality enabled the design team to incorporate ongoing developments in membrane technology
• Advancing technology has proved that membrane applications are competitive in economic and technical terms.

The Heemskerk plant will demonstrate that water sources which were previously considered less than ideal can be treated to potable levels with existing and emerging technologies while satisfying regulatory and the public's concerns about water quality.

About the Authors: Fred de Bruijn Ir., CEng, MICE, is with Witteveen + Bos in the Netherlands. David Eddy, CEng, MICE, is with Montgomery Watson in England.

Editor's note: This article was published originally in the 1997 Yearbook of the International Water Supply Association and is reproduced here with permission. The IWSA is headquartered in London in the United Kingdom.