Case histories for Harmful Algal Blooms in desalination.

Abstract

Algae have long been an issue impacting desalination plant operation in areas prone to algal blooms or where macroalgae (seaweeds) and detritus became dislodged from the seabed. Previously and still today, operators and designers may elect to turn down production or shut down SWRO plants, if contract obligations allow, when blooms are infrequent or of short duration. Alternatively, in areas subject to frequent and prolonged blooms, additional pretreatment such as conventional dissolved air flotation (DAF), hitherto designed for brackish water applications, began to be employed as early as 1995. The unprecedented 2008/2009 bloom of Cochlodinium polykrikoides in the Gulf of Oman and the Gulf1, brought algal blooms to the fore in the desalination industry. SWRO plant shutdowns were up to four months long as pretreatment processes struggled to remove the increased biomass and produce the required RO feedwater quality. Apart from a few exceptions, thermal desalination plants continued to operate without major issue throughout the bloom, as phytoplankton blooms generally pass through intake screens and thermal processes are very forgiving of source water quality. This was demonstrated at the Fujairah 1 hybrid desalination plant where the multi-stage flash (MSF) plant operated throughout the bloom while the adjacent SWRO plant was shut down. Globally, harmful algal blooms (HABs) similar to the 2008 bloom of Cochlodinium polykrikoides are increasing in frequency and severity (Anderson et al. 2012). Coupled with the increasing use of RO as the desalination technology of choice, HABs have become one of the major challenges facing the industry as RO membranes are extremely vulnerable to feedwater quality, making pre-treatment exceptionally important. Smooth operation is contingent on the selection of appropriate pretreatment processes upstream to remove organics, solids, colloids and other foulants from the RO feedwater. The 2008 Gulf HAB highlighted the limitations of conventional pretreatment based on ferric chloride coagulation and single stage dual media filtration (DMF) in removing algal biomass and organics. Ongoing research efforts to identify the algal organic matter (AOM) constituents responsible for membrane fouling and measurement of their removal in pretreatment intensified. To this end, the spike in AOM occurring during a bloom was found to comprise mainly of high molecular weight biopolymers (polysaccharides and proteins), which include sticky transparent exopolymer particles (TEP) (Myklestad 1995; Villacorte 2014). TEP have been shown to form microgels with a high hydraulic resistance and are increasingly recognized to promote biofouling of RO membranes (Villacorte 2014; Berman and Holenberg 2005; Li et al. 2015). With the increasing adoption of low pressure microfiltration (MF) and ultrafiltration (UF) membrane pretreatment, questions were raised as to their performance during algal bloom events and how they compared to conventional pretreatment in removal of AOM. In preparing the Manual and to address some of the above questions, operators, researchers, and plant owners in the desalination industry were contacted as part of an informal survey and invited to contribute case studies related to their experience with algal blooms. As expected, it became clear that algal bloom issues were predominantly encountered in SWRO plants rather than those using thermal desalination. Twelve SWRO plants (Figure 11.1.1) at eleven different sites were in a position to share their experiences from a shortlist of 30 sites that may have experienced HAB issues. Algal blooms, primarily phytoplankton, were reported in almost all geographic locations, in cold and warm seas over a range of salinities affecting municipal and industrial desalination plants. Notable areas affected include the warmer waters of the Gulf of Oman and the Gulf in the Middle East. Case studies include Sohar and Barka 1 in Oman, Fujairah 2 in UAE and the Shuwaikh plant located close to Kuwait’s most important commercial port in the upper reaches of the Gulf where seawater quality is at its poorest. HABs are also commonly found in the cooler waters off the coast of Antofagasta in Northern Chile supplying industry and drinking water for towns in one of the driest areas of the world. Key insights from the 12 case studies are summarized below in terms of impacts experienced, if any, in both conventional and advanced MF/UF membrane pretreatment plants during algal blooms. Commonly recommended measures implemented in the industry to combat algal blooms are discussed in relation to the case studies mitigation strategies, and lessons learned. This encompasses measures adopted during design and/or during plant operation, e.g. deepwater intakes (Gold Coast), and DAF (Fujairah 2, Shuwaikh) and/or direct MF/UF filtration (Jacobahaven, Sohar), or subsequently enacted in response to HAB events (La Chimba).