Water. Desalination + reuse
Issue link: https://fhpublishing.uberflip.com/i/513849
| 36 | Desalination & Water Reuse | May-June 2015 The HABs event was caused by fne, suspended green algae. Analysis showed a large presence of the dinofagellate, Gonyaulax polygramma, which has the potential to cause fsh and shellfsh mortality. At 5.6 million cells/litre, the levels of Gonyaulax polygramma were four times higher than previously reported elsewhere (South Africa 2007). And the concentration of Gonyaulax polygramma in the backwash water was almost ten times higher than that in the feed water which was also unprecedented (table 1). The subsequent biofouling of the RO membranes in 2013 meant the engineering procurement and construction contractor was unable to commission the plant. However, despite the near-overwhelming challenges, Majis completed constructing its RO1 facility – a 20 Ml/d seawater RO plant – in September 2013 following its implementation of a number of measures to counter the jellyfsh and algal assaults. Pretreatment – start at the beginning In its response to mitigate the HAB event and to prepare for subsequent ones, Majis focused its attention on its pretreatment facilities. The heavy assault from algae in 2009 made it impossible to maintain the required free and combined chlorine residuals at 0.4-0.8 parts per million. Despite increasing the frequency of screen cleaning from once in 12 hours to once in four hours the Sohar Refnery RO plant was shutdown for four months and all of its RO membranes had to be replaced. Following the 2013 event 100-micron mesh SCF was replaced by a 300 micron equivalent. While this measure helped to address the pressure drop caused by fsh paste congestion it naturally transferred a greater solids load to the microfltration (MF) pretreatment. Majis dealt with the subsequent increase in MF fouling with the help of the MF vendor, Pall. Pall came up with a number of measures that included: • to use of all seven skids, • to lower fux, and • to changing the cleaning regime. The amended cleaning regime deployed hydrochloric acid and citric acid. With this combination of changes the MF plant was able to produce silt density indices (SDIs) of less than three despite teChnOLOgY Figure 1. A modifed cleaning regime led to immediate improvement in permeability trends. the raw water having an average SDI [2,5] of 33. rO sYstem issues While the plant's pretreatment systems were the principal focus of Majis' attentions, there were signifcant issues to address at the RO membranes. The differential pressure started to increase on the RO trains in the middle of December when green algal bloom started. This increased the level of total organic carbon (TOC). The attendant biofouling was the result of a combination of factors. Since at least 50% of TOC was in a dissolved state, it passed through the MF and ended up at the RO stage. Furthermore, the seawater intake was continuously chlorinated. And the MF was treated frequently with high chlorine concentration disinfection. The subsequent Slippery customers: through sheer weight of numbers, jellyfsh breached the bar screens.