Membranes for estuary water
Faced with insufficient water for their planned expansion, Dow Benelux1, operating from its Terneuzen site in the South West corner of The Netherlands, awarded a Build-Own-Operate contract to a joint venture comprising USF Benelux2 and Delta Nutsbedrijven3. The contract covers the design, construction, supply, installation, operation and maintenance of a water treatment system providing high purity demineralised water for on-site high pressure boilers and make-up water for cooling towers.USF Benelux has overall responsibility for the design, construction, installation and start-up of the plant whilst Delta Nutsbedrijven will then operate and maintain it.
Sourcing water supply
Due to the scarcity of sweet surface water, a cost effective source had to be found in order that the site, where two crackers process napths into products such as ethylene, butadiene, benzene and propylene, could be expanded. Part of the evaluation procedure that led to the placement of the BOO contract (in June 1999) centred around where the bidders were going to obtain their water from.
The winning tenderer proposed the use of seawater abstracted from the estuary along which the Dow Benelux complex is built. The fact that not many high quality sources were available locally for use as feed water in the project meant that pre-treatment was always going to be an essential and key element.
As with any Build-Own-Operate project, a reduction of total lifecycle cost results in the lowest cost price per cubic metre of demineralised water. Economic considerations (confirmed by pilot research) have shown that membrane plant is more competitive than conventional pre-treatment (coagulation, flocculation, sedimentation, filtration) due to several factors that may be specific for the Netherlands.
These factors include, but are not necessarily limited to, the backwash water from the membrane pre-treatment that may be returned to the surface water provided that no chemicals, especially sludge forming additives, have been added. This results in considerable savings both for equipment, maintenance & repair and sludge dumping costs. Therefore membrane filtration sludge treatment is highly desirable.
Couple these factors with a feedwater of variable quality, and it became clear that a membrane pre-treatment process must be used.
The Schelde River source has a peak total suspended solids (TSS) of approximately 80 mg/l, a maximum total dissolved solids (TDS) of 28,800 mg/l, turbidity of 40 NTU and also suffers from tidal and seasonal variations.
In order to select the most effective pretreatment and thus allow the sea water reverse osmosis to operate at its optimum performance, extensive tests were performed using 2 different types of membrane pre-treatment.
These site pilot plants were arranged such that the proposed configuration of the full-scale plant could be imitated to provide accurate information on flux rates, cleaning intervals and the running costs that could be expected.
Best of two choices
Once the initial period of site tests was completed, an evaluation as to which of the two technologies offered the best overall solution in terms of performace and cost took place. The final selection of pre-treatment technology was made for Memcor's Continuous Microfiltration® (CMF) system. Following the CMF stage, the filtrate is then fed to seawater then brackish water RO plants. The selection was made ahead of ultrafiltration with an inside to out capillary membrane.
The plant that Memcor will deliver comprises 8 off 78M10C microfiltration units fitted with 78 membrane modules. Each unit can easily be extended to accommodate up to 90 modules should either further expansion be required or if there is degradation of the feed water quality. Ancillary items such as an air system, clean-in-place process, installation and commissioning were also included in the supply.
The pre-treatment plant will be treating raw water by January 2001 with the final handover to the client scheduled for May 2001.
The principle parameters which made CMF so attractive include its ability to cope with the wide ranging feed water quality during the seasons without compromising the filtrate quality whilst achieving the lowest life cycle costs.
Other factors helping the decision were:
- proven system technology
- in-built and automatic membrane integrity testing
- vast installation base
- simplicity of operation
- no preconditioning of the feed-water (other than a 500 micron security screen)
- no chemicals added to the backwash discharge
- backwash process does not use any filtrate
Whilst the CMF plant is based on proprietary designs, special consideration has been given to the aggressive nature of the feedwater and the subsequent effect this may have on the life of the plant. Protective coatings and corrosion resistant finishes have been applied on exposed parts.
Memcor CMF technology is available for water treatment, waste water treatment, reuse and RO pretreatment. Flows range from 100 - 400,000 m3d-1.
Both pressurised and submerged membrane configurations are available with the choice of chlorine resistant or polypropylene membrane material.
Memcor will be at Aquatech 2000, Stand E98.
1 Dow is a trademark of the Dow Chemical Company. Dow Benelux is a wholly owned subsidiary of the Dow Chemical Company. The Terneuzen site operates some 30 different chemical plants.
2 USF Benelux is a wholly owned subsidiary of Vivendi Water.
3 Delta Nutsbedrijven is a multi-utilities company operating in the South West part of the Netherlands.
4 USF Memcor and Continuous Microfiltration are trademarks of USF Memcor, a wholly owned subsidiary of Vivendi Water.