Lanzarote is the furthest east of the seven major Canary Islands, situated about 100 km off the coast of Africa. This popular tourist destination relies largely on reverse osmosis (RO) conversion of seawater to support its tourist, domestic, and agricultural activities.
The RO operations on Lanzarote are owned and operated by Insular de Aguas de Lanzarote S.A. (Inalsa). Inalsa provides water production, distribution, and recycling services as well as wastewater treatment to more than 40,000 customers. Bottled water has been produced by Inalsa since 1990 and is marketed under the label Agua Chafariz.
An ongoing problem with Inalsa's RO elements involves leakage at the o-rings of element interconnectors, resulting in an average failure rate of three vessels per train per month at the Lanzarote III plant. With four trains in operation at the plant, the time dedicated to correcting o-ring leaks is approximately 18 man-hours per month.
Filmtec elements with iLEC interlocking endcaps are designed to eliminate this problem by replacing the current sliding couplers with a single, stationary seal. A field test of twelve elements with iLEC endcaps was conducted in the Lanzarote III plant to determine the effect of this new element design on permeate quality, energy consumption, and ease of installation and removal.
Leakage problems
Daily measurement of the permeate conductivity from each vessel in the plant is used to identify leakage problems as they occur. Vessels with particularly high conductivity are opened and unloaded. If no interconnector problems are found, the elements are tested individually. Between one and five vessels per month per train are found to have leaking interconnectors. Vessels with elevated but acceptable conductivity, which may have small or developing leaks, are monitored for signs of further deterioration.
Scheduled shutdowns are used to correct interconnector problems, so there is currently no lost production.
However, exchanging elements in a vessel with interconnector problems is typically a three-person job that requires approximately 30 minutes. Given an average failure rate of three vessels per month, and considering all four trains at Lanzarote III, the time dedicated to correcting o-ring leaks is approximately 18 man-hours per month.
The five-year net present value (NPV) of this expenditure, given a $25 per hour labor cost and a 10% discount rate, is $21,000. If the plant were operating at capacity and subject to monetary losses during shutdown, the penalty would be far higher. Cost of materials represents another added expense because problem interconnectors are replaced with a custom-machined Inalsa sliding coupler that incorporates three o-rings at each end.
Benefits of iLEC Interlocking Endcaps
The endcaps eliminate the need for multiple sliding seals between adjacent membrane elements, reducing the number of sealing surfaces per connection to a single, axially compressed o-ring. The result is a seal that is lubricant- and maintenance-free for the life of the element.
Long-term permeate quality is improved through prevention of o-ring leaks that occur as o-rings gradually become worn and abraded. In contrast to standard interconnectors, the possibility that o-rings will be pinched or damaged during installation, requiring the "debugging" of leaks upon start-up, is also eliminated.
The interlocking endcaps also remove the permeate-side flow restriction associated with the small flow diameter of conventional interconnectors, reducing energy costs.
Elements using iLEC interlocking endcaps are installed in minutes with a level of effort comparable to that associated with standard interconnectors. Special snap and alignment features provide tactile, audible, and visual feedback to inform the installer that a positive connection has been made between adjacent elements. Optional tools are available to make the handling of elements even easier.
Plant design
Filmtec SW30HR-380 elements with iLEC interlocking endcaps were installed into two vessels at Lanzarote III, a plant that produces 20,000 m3/d of permeate at a concentration of 430 ppm from four reverse osmosis trains. The vessels for this trial were located in the first and second stages of Train 4, a two-stage, 60 by 48 array of 8-inch pressure vessels.
Each vessel holds six elements, which currently range in age from 4 to 12 years. The seawater feed is extracted from beach wells and pretreated using sand followed by cartridge filtration. The feed water contains 38,500 ppm TDS (total dissolved solids) and is adjusted to pH 7 using bisulfite.
Upon start-up of this evaluation, the train operated with a Stage 1 recovery of 34% and an overall recovery of 46%. The train produced approximately 255 m3/h of permeate. The feed pressures for Stages 1 and 2 were 64.0 bar and 62.8 bar, respectively. The temperature was 21°C .
In both of the test vessels, interlocking vessel adapters, replaced the standard adapters provided by the vessel manufacturer. These eliminated two more sliding connections, just inside the leading and trailing elements in each vessel, and reduced the permeate flow restriction at the vessel permeate port.
Plant performance
The Filmtec SW30HR-380 elements with iLEC endcaps commenced operation on March 12, 2003. Upon start-up, the vessel using iLEC endcaps in Stage 1 produced permeate containing 95 ppm TDS at an average flux of 20 Lmh, while the vessel using iLEC endcaps in Stage 2 produced a permeate with 270 ppm TDS at an average flux of 7.8 Lmh. Sustained performance at this level requires standard-test rejection of 99.85% and perfect sealing between elements.
To verify leak-free operation, a conductivity probing of elements with and without iLEC endcaps at Lanzarote III was conducted. The profiles shown in Figure 4 contrast a leak-free vessel using elements with iLEC endcaps with a neighboring vessel containing standard elements. A leaking o-ring between elements 1 and 2 in the standard vessel produced an obvious spike in conductivity and a degradation in permeate quality for that vessel.
Eliminating the standard couplers and vessel adapters provided a small boost in energy efficiency. The flow restriction of the couplers and adapters was measured inside a vessel in Stage 1. The backpressure imposed by the couplers and adapter, measured at the end of the vessel opposite the permeate port, was approximately 0.2 bar. The corresponding result for the vessel using iLEC endcaps in Stage 1 was just 0.09 bar.
Operating personnel at Lanzarote III were observed and questioned as they handled elements with iLEC endcaps for the first time. The time required for installation of the elements with iLEC was just 3 minutes per vessel, consistent with that required for loading standard elements. Regardless of the element type, a far greater period of up to 30 minutes was required to open and later close the same vessels.
Operating personnel rated the iLEC elements easier to install and "less work" than standard elements, in part because there was no need to handle interconnectors. Other comments from operating personnel included:
- Elements were easy to install, even without the strap wrench and element pulling tool;
- There should be no increase in the number of individuals, typically two to four, required for element installation and removal;
- Interlocking vessel adapters were readily attached to the first and last elements in both vessels using iLEC endcaps; and
- The snapping action of the iLEC endcaps was easily detected, giving a good indication of a successful connection even in the noisy plant environment. Operators responded with "no doubt" when asked whether elements just installed were properly coupled together.
The SW30HR-380 elements with iLEC interlocking endcaps were successfully installed in the Inalsa Lanzarote III reverse osmosis plant. Installation was easily accomplished with no need to search for leaks upon start-up. More than one year after installation, the vessels using iLEC are still leak-free, overall quality of the product water is improved, and permeate backpressure is reduced.
