Seawater intake considerations to mitigate HAB impacts.

Abstract

Seawater intakes are a key element in the design, construction and success of desalination plants. Various intake options exist and are generally classified based on their abstraction depth. Surface ocean intakes abstract seawater from the top of the water column or at depth, while subsurface1 intakes are embedded in the seabed or beach, thereby pre-filtering the abstracted seawater. Location, intake type and depth are important determinants of water quality. Intakes are also the first point of control in minimizing the ingress of algae into a plant or where algal impacts first manifest. Originally the more robust thermal desalination processes dominated the desalination market where feedwater quality was not the primary driver in determining intake type or location. Instead, feedwater supply was critical, as thermal plants were configured as cogeneration power/desalination plants with common intakes with large volume requirements to generate both power and water. Intake and screening systems were often limited to shallow nearshore intakes with screens sized to meet the necessary seawater quality for power plant, multi-stage flash (MSF) and multi-effect distillation (MED) condenser tubes (Pankratz 2015). Macroalgal seaweed species were initially a significant issue in thermal desalination plants, completely blinding intake screens or clogging settling basins (Figure 6.1). In the mid-1970s, the availability of MSF thermal plants in Libya was dramatically reduced to 100 days/year, with seaweed blockage of the intake pipes the third leading cause for plant outages. At the Zuara plant intake, up to 800 m3 of seaweed was removed every second day during winter when seaweed became dislodged from the seabed at the end of summer and during storms (Kreshman 1985; 2001). Due to advances in the design of intake systems, the extent of macro-algal intake blocking has been greatly reduced at thermal desalination plants and now mainly results in short term outages. Nowadays with seawater reverse osmosis (SWRO) dominating the desalination market, microscopic algal species (phytoplankton) have been more problematic. Occasionally issues have occurred at plant intakes when a high suspended solids load of phytoplankton and debris have overloaded trash racks and/or clogged intake screens (Figure 6.2). In some cases, these impacts have been severe. The notorious 2008/2009 bloom of Cochlodinium polykrikoides in the Gulf of Oman resulted in the frequency of cleaning seawater intake screens at Sohar increasing to every 4 hours (Sohar Case Study, Chapter 11). More often adverse impacts are observed in downstream SWRO pretreatment processes or through the promotion of (bio)fouling on membranes as microscopic algae and algal organic matter (AOM) pass through conventional open intakes and screens. The potential for phytoplankton and AOM to be entrained into SWRO plant intakes, the focus of this chapter, varies greatly. In addition to the intake system design characteristics, prevailing marine conditions, nutrient concentrations at the site, the type, motility and concentration of the algal bloom species play a role. Intake characteristics are recognized to have a significant effect on raw seawater quality and therefore the pretreatment processes required, as well as limiting marine environment impacts which can be a major concern in some projects. Consequently, more attention is given to the selection and location of intake systems in SWRO feasibility studies and during design. In areas prone to algal blooms, subsurface or open intakes abstracting seawater at depth are often considered a solution to reduce the ingress of floating or surface-concentrated algal blooms into desalination plant intakes. Subsurface intakes offer the advantage that they serve both as a water intake and as pretreatment for a SWRO plant. The seawater is filtered during passage through the strata of the subsurface intake, removing algae and natural organic matter, including components of AOM by both physical and biochemical processes, providing a high-quality feedwater, thereby potentially reducing or replacing conventional pretreatment processes (Missimer et al. 2013; Rachman et al. 2014; Dehwah et al. 2015; Dehwah and Missimer 2016). The effectiveness of these strategies for reducing the entrainment of algae and associated AOM into an intake is discussed below. It should be noted this is a little–studied area in the desalination industry, especially during algal bloom events. Therefore, research on the removal of fractions of natural organic matter (NOM) such as biopolymers produced by both bacteria and algae are examined here, as results may be indicative of what can occur during an algal bloom. Finally, other factors such as engineering constraints, environmental concerns, costs, construction time, and operability may ultimately drive the selection and siting of an intake. A brief overview of approaches to determine the seawater intake for a project is therefore provided in the last part of this chapter.