Corrosion and Fouling Control in Desalination Industry

In the next decade, about 70% of the world’s population is likely to face water scarcity. This is rapidly becoming critical in coastal areas, arid/semi-arid regions and island countries. This chapter presents an overview of the desalination concept and describes the basic aspects of desalination system design, including desalination techniques, energy demand and supply, and environmental issues pertinent to desalination. The availability of vast seawater and brackish water resources and evolving desalination techniques and system design appear to provide ample opportunities to address global water scarcity. Furthermore, the rapidly changing energy conservation and renewable energy technologies will also support the emergence of environmentally sustainable small-scale and decentralized desalination infrastructure networks for densely populated urban areas, as well as rural and remote areas. However, there remains an urgent need to implement desalination plant permitting and monitoring systems to balance the cost-effectiveness of evolving technologies with environmental sustainability.

The two major thermal processes which are employed in large scale desalination plants are MSF (multi-stage flash) and MED-TVC (multi-effect distillation coupled with thermal vapor compression). This chapter provides a brief description on these processes, their performances and challenges. The operational and design developments which have been associated with the thermal desalination processes are explained. Salient features of conventional power water cogeneration cycles in which the MSF/MED-TVC distillation plant operates are highlighted. Challenges that have to be addressed to enhance developments of thermal desalination processes such as introduction of innovative methods to reduce specific energy consumption are also discussed.

Fresh water production by desalination of seawater is an expensive affair. During the last few years, seawater reverse osmosis (SWRO) desalination technology has gone through a remarkable transformation and gained widespread acceptance, which is evident from the increased share of SWRO. Typical SWRO desalination consists of four major components: an intake; a pre-treatment system; a high-pressure pumping system; and a membrane module. The performance of the entire system is dependent upon the proper design and operation of each component and assessed by its ability to produce required quantity of water with acceptable quality at lowest possible cost. Recent advances in seawater reverse osmosis (RO) that has allowed a drastic reduction in the cost of desalinated water include the application of highly efficient energy recovery devices (ERDs) and the utilization of advanced RO membranes. However, fouling and membrane degradation are some of the major challenges faced by the RO systems, which need to be carefully addressed. The chapter discusses various parameters, which are typically used to assess RO desalination plant performance. It will also cover the challenges faced by SWRO plants and measures adopted to address these challenges to maintain the best plant performance.

Desalination has been used for thousands of years. Greek sailors boiled water to evaporate fresh water away from the salt and Romans used clay filters to trap salt. The most matured and commercially implemented technologies include multi stage flash (MSF), multi effect distillation (MED), vapour compression distillation (VC), reverse osmosis (RO), ion exchange (IX), and electrodialysis (ED). These technologies, however, have their own drawbacks and limitations. For example, MSF requires high capital cost and consumes more energy per cubic meter of product water than other technologies. RO, on the other hand, is susceptible to membrane fouling, has high maintenance and operation cost, and is unable to effectively desalinate brine water (i.e. TDS > 70,000 mg/L) due to elevated applied pressure requirement. To overcome the shortcomings of the readily available commercial technologies, researchers and engineers around the world have been working on new and unconventional desalination technologies. These technologies include novel membrane based processes such as forward osmosis (FO) and membrane distillation (MD). Additionally, adsorption and freezing are among other unconventional desalination technologies that have shown promise in recent years. This chapter discusses these emerging technologies and highlights their recent advancements and their potentiality for scale up and commercialisation.

When talking about seawater thermal desalination, continuous operation in harsh environments comes directly to our mind. Thus, materials integrity and plant reliability is a major concern for plant managers to satisfy the increasing water demand in arid countries. Either in Multi-Stage-Flashing (MSF) or Multi-Effet-Desalination (MED) process, hot seawater and brine are the main threats to both static and rotating equipment. In the present chapter, different expected forms of corrosion with their mitigation practices will be described. This will include two main sections: pretreatment and the desalting system where three media will be considered i.e. seawater, brine and vapors. The mitigation practices will include material selection, environment control (chemical and physical) and monitoring (direct and indirect) activity.

Membrane-based desalination is dominated by the reverse osmosis (RO) technology since the late 1990s. This technology is progressing very fast to replace most of the thermal-based desalination technologies. Since this process is dealing with ambient seawater with intensive pretreatment, corrosion and materials integrity in RO plants has many common practices with oil and gas industry, particularly in offshore installations. In the present chapter, crevice and microbial corrosion are described as the main corrosion forms in RO systems based on practical desalination experience. The corrosion-resistant alloys (CRAs) used in RO plants, and major research efforts and case studies in this area are discussed. Material selection criteria and future trends and opportunities are highlighted.

Stainless steels and desalination industry is a long story centered on life-cycle cost principle. Indeed, their use and development affected the choice of designers and plant end-users. Although, many learned lessons have been acquired on the performance of these ferrous materials, each stainless steel grade has a defined role and record within the desalination process. Depending on process conditions and equipment design, the involvement of mechanical stresses induces what is called “Environmental Assisted Cracking” for some susceptible stainless steel grades. In the present chapter, we will discuss the pre-requisite and practical solutions for this type of damage and the concerned alloys with available case studies covering static and rotating equipment.

Corrosion monitoring is an essential element in the overall operation of desalination plants. Monitoring involves the application of different mechanical, electrical, or even electrochemical devices, which can evaluate and quantify corrosion characteristics. The monitoring techniques can be used on-line or off-line, directly or indirectly, and can be intrusive or non-intrusive, all merely depending on the level of service necessary. Corrosion monitoring provides us with a general idea of the corrosive environment and even a direct estimate of corrosion rates. A monitoring scheme is selected to improve the economy of plant operations, and it correlates to the likely mechanism(s) of corrosion and their implications of the failure on a plant component. In this chapter, we have provided detailed insight on different corrosion monitoring methods that are currently employed in desalination plants.

The addition of chemical additives has been considered as a standard operation in water treatment systems. This chapter discusses the chemical additives used for the control of corrosion in desalination systems. Specifically, corrosion inhibitors for various metallurgies, biocides, and oxygen scavengers are covered. The pros and cons of the additive chemicals have been highlighted. The need to utilize green corrosion inhibitors based on plants and ionic liquids materials have been emphasized. This class of materials are environmentally friendly, cheap, and readily available.

Heat exchangers are vital equipment for thermal desalination industry for desalting system or heat recovery purpose to increase thermal efficiency. Most of the existing heat exchangers are in contact with seawater cooling medium which induces a time-dependent phenomenon: the “scaling”. The necessity to shut-down the equipment for chemical cleaning which is most of the time based on acids requires a proper procedure with strict control on performance indicators challenged by a variety of metallic alloys. The present chapter discusses the acid cleaning methods used for descaling and the associated base metal protection. The related aspects such as the acid used, acid concentration, flowing conditions and the selection of corrosion inhibitor are described.

Research and development in advanced corrosion prevention strategies for potential industrial applications are of paramount importance. This chapter aims to provide recent advancements in two prospective corrosion prevention strategies falling under surface coating and cathodic protection. Primarily, the impending technologies such as smart coating and photoelectrochemical cathodic protection are described. On-demand release smart nano/microcontainers are discussed with reference to supramolecular interactions under four subheads, namely, polymer-based, host-guest chemistry-based, inorganic clay-based, and polyelectrolyte-based. The basics of photoelectrochemical cathodic protection that uses solar energy to generate electricity using a semiconductor photoelectrode are described along with the mostly used titania photoanodes. Different approaches to augment the photoactivity and electron storage ability of the photoanodes are briefed.

Inorganic scaling is a major concern for thermal and reverse osmosis (RO) seawater desalination plants, which are the key technologies for meeting the increasing freshwater demand. Membrane scaling is a detrimental effect in both thermal and RO process that reduces its performance particularly by reducing the water flux and causing membrane wetting. In this chapter, the formation mechanisms of inorganic scaling in thermal and reverse osmosis plants are discussed. The factors contributing to scale formation such as temperature, pH, and ionic strength of the treated water and the scale indices frequently used to predict the scale formation are explained. Finally, prevention methodologies developed to mitigate inorganic scaling (pre-treatment strategies, cleaning processes, advanced materials, etc.) are enumerated and discussed. Overall, this chapter provides a fundamental perspective of the inorganic scaling in thermal and reverse osmosis seawater desalination plants.

Biofouling is referred to as the unwanted deposition and growth of biofilms, which leads to increase of operating pressure, more frequent chemical cleaning, and shorter membrane lifespan. This chapter aims to provide a comprehensive overview of reverse osmosis (RO) membrane biofouling in desalination applications. Firstly, the necessity of implementation of RO technique for water treatment and reclamation to combat water scarcity is briefly introduced along with the basic transport mechanisms, membrane types, and major challenges (fouling) hindering practical application. Afterwards, the phenomenon of biofouling is discussed describing the formation of biofilms, the role of extracellular polymeric substances (EPS), and crucial factors affecting biofilms. This is followed by a section related to the impact of biofouling on membrane processes with emphasises on permeate water flux and salt rejection. The performance degradation mechanism and enhanced energy consumption are also discussed in this part. Finally, the chapter concludes with a summary with the significance of better understanding of membrane biofouling.

Scale control is essential for the economic operation of desalination plants. Formation of scale depends on the feed characteristics, desalination methodology and operating conditions. The approach could be the removal of scaling components or preventing them from forming the scale by binding them in some form. This chapter provides a comprehensive overview of preventive scale control approaches in desalination. The preventive strategies are briefed under three sections, namely chemical, physical and physicochemical. In each section, several important methods discussed. Recent developments of scale control based on nanotechnology and magnetic and electrochemical methods briefed. A short account on scaling in non-conventional desalination systems such as forward osmosis and membrane distillation is also provided.

This chapter focuses on chemical methods for scaling control. We attempt to effectively present all available methods for chemical scale control, placing certain emphasis on selected case studies. Focus is given to small molecule scale inhibitors (mainly polyphosphonic acids for mineral scale inhibition), anionic polymers (mainly derivatives of polyacrylic acid and the likes for mineral scale inhibition) and neutral and cationic polymers (for silica/silicates scale inhibition). Underlying scale inhibition mechanisms, as well as various problems occurring during and due to the scale inhibition, are presented and discussed. The chapter is a concise overview of the chemical control strategies for scaling/fouling, rather than an exhaustive review of the literature. However, appropriate references are given that direct the reader to additional information, as needed.

In seawater desalination plants, a wide variety of marine biofouling organisms enter the intake and distribution system and settled and grow as it provides optimal conditions for the buildup of biofouling communities. This settlement results in an increased wall roughness of the material and reduces the pipe diameter resulting in the hydraulic head loss. To prevent biofouling settlement, generally, biocides are dosed at the seawater intake. Chlorination has shown to be a proven practice in the past decades to be efficient in avoiding biofouling in seawater intakes and culverts. However, the dosing strategy is often “standard” and not tailored to local requirements or potential adverse side effects. This chapter provides a detailed account of the fundamentals of biofouling and their impacts on seawater desalination. Both physical and chemical methods of biofouling control that are currently in use in industrial desalination plants are explained. An account of biofouling monitoring is also provided.

Membrane fouling results from the interaction between the membrane surface and foulants via adsorption, accumulation, or precipitation of organic and inorganic constituents. This bottleneck significantly impedes the efficient application of membrane technology by reducing productivity and permeates quality and leads to higher operating cost. In recent years, major advancements have been made in elucidating the antifouling mechanisms of membrane processes through numerous approaches. Those strategies involved the surface modification and development of novel desalination membranes. Various innovative membrane modifications have been developed to alter and customize the membrane structure and surface properties. The development of next-generation antifouling membrane is focused on tailoring the surface hydrophilicity, reducing surface roughness, introducing specially structured polymers through either grafting or coating as well as introducing new surface functionalities. This chapter overviews the current developments and the prospects of the novel strategies/preparation techniques in designing antifouling membranes for innovation toward water sustainability.