Removal of algal toxins and taste and odor compounds during desalination.

Abstract

A major challenge in desalination is the removal of harmful algal bloom (HAB) toxins and taste and odor compounds (hereafter referred to as algal metabolites) using common treatment techniques. Removal of other compounds such as polysaccharides, proteins or transparent exopolymer particles (TEP) are discussed in Chapter 2. Taste and odor compounds are materials produced during a HAB that are not detrimental to human health, but cause customer dissatisfaction and often a misconception that the drinking water is not suitable for consumption. Toxins are detrimental to human health and are discussed in Chapter 2. Here the objective is to assess each process unit in a common desalination treatment train, both for SWRO and thermal desalination, and address how each is best optimized to act as a barrier to these specific algal metabolites. Where treatment techniques in seawater applications exist, these have been referenced and used as examples. As little documentation exists on removal of algal metabolites from seawater blooms, fresh water algal species are referred to whenever needed. This information is relevant in understanding the removal mechanisms that are possible. For clarity, these are denoted for each example. Algal metabolites can be either intracellular or extracellular. Many algal species have high percentages of intracellular metabolites, such as Microcystis (freshwater) in which the toxin microcystin can be up to 98% intracellular (Chow et al. 1997). Lefebvre et al. (2008) showed an approximate 81% intracellular saxitoxin (STX)-equivalent concentration for an Alexandrium (seawater) bloom, although further data are needed to confirm this observation. STX-eq (or STX-equivalents) is a measure of total toxicity due to all saxitoxin analogues in a particular solution. In contrast, Smith et al. (2012) report that 60% of the okadaic acid produced by Dinophysis cultures was extracellular, while Kudela (pers. comm.) reported total and extracellular concentrations of 100 and 50 μg/L domoic acid respectively during a massive bloom of Pseudo-nitzschia along the US west coast in 2014. Extracellular metabolites are inefficiently removed by pretreatment processes, and this is discussed below in more detail. The nutritional status of HAB cells will affect the percentage of extracellular metabolites in a bloom. At the outset of a bloom, HAB cells will be more robust than toward the end of the bloom period when stresses from nutrient limitation, grazing, or other factors can lead to the leakage of metabolites into the seawater. Smith et al. (2012) noted that, in general, the concentration of extracellular toxin in a lab culture of Dinophysis acuminata (seawater) significantly increased upon culture aging and decline; cells did not appear to be actively or passively releasing toxin during the stationary phase (see Chapter 1, Figure 1.3), but rather extracellular release was likely a result of cell death.