Algal biomass pretreatment in Seawater Reverse Osmosis.

Abstract

Harmful algal blooms (HABs) can result in a substantial increase in the organic and solids load in the seawater feed to be treated at a desalination plant. In this chapter, the removal of this material is addressed in the context of the multi-barrier treatment process for seawater reverse osmosis (SWRO) as presented in Chapter 8 on risk management for HAB events. While this chapter covers removal of non-toxic material, Chapter 10 builds upon these principles and discusses the mechanisms and effectiveness for each barrier with respect to toxin removal. This chapter covers only the main barriers used in the SWRO desalination plants for HAB bloom risk mitigation, though the authors acknowledge that other niche treatment barriers exist in SWRO systems. The treatment processes discussed here are chlorination and dechlorination, dissolved air flotation (DAF), granular media filtration (GMF), microscreens for microfiltration/ultrafiltration (MF/UF), MF/UF itself, cartridge filtration and SWRO. Coagulation is discussed in general terms and then more specifically for DAF, GMF, and MF/UF pretreatments. Each treatment process is broken down into a discussion of how the process works and then how HAB cells affect the process operation. Importantly, the chapter deals with how upstream actions can detrimentally affect downstream treatment processes with respect to algal blooms. In particular, this chapter discusses removal mechanisms for algal organic matter (AOM) and how operational actions can prevent detrimental effects of AOM. As discussed in Chapter 2, the chemical composition of AOM usually includes proteins, polysaccharides, nucleic acids, lipids, and other dissolved organic substances. AOM compounds typically cover a wide size spectrum, ranging from less than 1 nm to more than 1 mm. Based on their size cut-off, GMF and MF/UF are expected to remove only part of high molecular weight AOM (as shown in Chapter 2, Figure 2.2). SWRO is expected to achieve complete removal of AOM, but will suffer from fouling issues if AOM is not removed upstream