This article dives into a case where a small defect in a multimedia filter had major consequences, affecting the integrity of reverse osmosis membranes and highlighting the importance of having a robust and reliable design.
The photo above was taken from the inlet of a reverse osmosis (RO) pressure vessel, where you can see a multi-orifice protection cover and the membrane polymer sheets rolling onto themselves in a spiral wound configuration.
The notable feature of this photo is that the inlet channels are visibly deformed by incrustations of black particles, which were identified as anthracite (red arrow). How did these particles manage to travel through the system and clog the membranes?
Anthracite is commonly used in multimedia filtration as one of the primary pretreatment systems due to its high filtration capacity. Its main purpose is to capture suspended particles from the water before it reaches the RO membranes. However, in the event of a failure in the multimedia filters, the sand media can carry over into the system, leading to severe operational issues.
In large-scale water plants, multimedia are usually constructed on-site as part of the civil works. The base floor, where the sand media and anthracite lie, consists of prefabricated concrete slabs that are carefully placed together. The joints between the slabs and their contact with the walls are sealed using a silane-terminated polymer for enhanced durability.
However, failure in the sealing of the slab joints allows anthracite particles to escape, passing to the clear water section. This led anthracite traveling through the system, passing through the cartridge filters and eventually embedding themselves in the RO membrane layers, causing significant fouling, damage, and final operation shutdown.
Even though prefabricated slab installation is more cost-effective and quicker installation, considering the potential issues that could arise, it is more reliable to design one continuous concrete slab fabricated at site, with reinforcement bars, embedded into the walls of the filter. This approach would avoid joints, sealings polymers and does ensure a more robust and reliable final design.
When it comes to media filters, even the smallest oversight in construction works, can lead to significant downstream impacts.
In this desalination plant in Southern Europe, resulted in severe fouling of the membranes, causing the entire system to shut down for 30 days, followed by another 20 days of operation at medium load operation. By the time the problem was detected, all the media filtration was spread all over the piping system.
It was very difficult to identify which filters were affected by the cracks. The inspection process was extensive and challenging, involving multiple stages. Each media filter had to be inspected, followed by verification of all cartridge filters and element replacements. Inspections were also carried out at pump suctions, valve seats, and any point where sand could accumulate and cause further damage to inline equipment. Ultimately, each RO pressure vessel cover was opened to inspect membranes individually. This labor-intensive process significantly contributed to the downtime.
Once some systems were restored, the plant operated at reduced output, directly impacting the water supply to meet demand from end users. Operating at medium load was necessary due to limited availability of equipment and RO skids, while overlapping plant operation with ongoing maintenance efforts to meet some of the region’s water demand.
The associated costs included not only the replacement of membranes but also significant operational downtime, specialist labor costs, and financial penalties due to the inability to meet contractual water supply obligations.
This case highlights the importance of ensuring every aspect of the system design is robust, construction as per correct processes, intensive supervision activities, from the quality of materials used to the integrity of the installation process. The failure to properly seal the multimedia filter joints caused a chain reaction of issues that could have been avoided with a more rigorous focus on design and installation standards.
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