Seawater Intake

Seawater desalination facilities require an intake system capable of providing a reliable quantity of clean seawater with a minimum ecological impact. To meet these objectives, it is essential that a thorough assessment of site conditions be conducted. Physical characteristics, meteorological and oceanographic data, marine biology, and the potential effects of fouling, pollution, and navigation must be evaluated, and an appropriate intake design employed. As the first step in the pretreatment process, the intake will affect a range of feed water quality parameters and determine the performance of downstream process systems.

Intake designs are highly site specific, possibly more so then any other aspect of the desalination facility. The design, modeling, monitoring, and permitting activities that surround them, may represent as much as 20% of the capital cost of the entire facility, and it is possible that intake-related issues may ultimately determine the feasibility and performance of the desalination plant itself.

Environmental impacts associated with concentrate discharge have historically been considered the greatest single ecological impediment when siting a seawater desalination facility. However, recent analyses have noted that marine life impingement and entrainment associated with intake designs were greater, harder-to-quantify concerns and may represent the most significant direct adverse environmental impact of seawater desalination.

Introduction

Access to a reliable supply of consistent-quality seawater is one of the most fundamental issues to be addressed when evaluating potential desalination plant sites; however, the supply of seawater to a coastal desalination plant often seems to be relatively straightforward consideration. Because of the apparent simplicity, an intake’s importance on the location, design, and performance of a desalination plant is often underestimated.

The ocean is a dynamic entity with a constantly changing shoreline and bottom profile. Powerful waves and changing currents can damage structures, affect water depths, and dramatically alter water quality. Operational problems are com pounded by seawater’s corrosiveness and the marine organisms that can attack and foul equipment and systems.

The distance of the intake to the desalination plant site affects plant economics, and the available feedwater quality and quantity will directly affect pretreatment process decisions. As the first step in the pretreatment process, an intake’s operation can have far-reaching effects on the overall plant operation and its impact on the marine environment. In fact, the ability to permit a new desalination facility may hinge on the methods used to mitigate environmental concerns associated with the seawater intake. Consideration must also be given to potential loss of recreational uses in the intake area and the visual impact that may result from certain intake arrangements.

Most of the world’s experience with seawater intakes is a result of their use in the electric power generation industry where seawater is commonly used for cooling purposes in large surface condensers. (Figure 1)

Although thermal desalination processes such as multistage flash evaporation (MSF) and multiple effect distillation (MED) have intake water quality requirements virtually identical to power plant condensers, seawater reverse osmosis (SWRO) systems can benefit greatly from a finer level of screening. Since SWRO is expected to be the predominate Desalination technology employed in Texas, this review will focus on its requirements.

Seawater intakes can be broadly categorized as surface intakes where water is collected above the seabed, and subsurface intakes where water is collected via beach wells, infiltration galleries, or other locations beneath the seabed. The most appropriate location and type of the intake can only be determined after a thorough site assessment and careful environmental evaluation.

A good intake design will not only protect downstream equipment and reduce environmental impact on marine life, it will improve the performance and reduce the operating cost of the pretreatment equipment.

Surface Water Intakes

Large seawater desalination plants have traditionally employed open sea, surface water intake arrangements. Such arrangements are the type through which most electric power generation plants obtain condenser cooling water where water is pre-screened using traveling water screens, mechanically cleaned bar screens, or passive “well screens.” In many instances, the screening chamber is located on or near shore and the intake pipe may extend out hundreds of meters into the sea. Each of these arrangements will be individually considered.

Subsurface Intakes

Subsurface intakes may consist of horizontal or vertical beach wells, infiltration galleries, or seabed filtration systems. In each of these designs, the open seawater is separated from the point of intake by a geologic unit. A subsurface intake can be used where geologic conditions beneath a surface water are relatively impermeable or of sufficient thickness and depth to support water extraction. In addition to providing some natural filtration, this arrangement has the advantage of separating most of the marine organisms from the water intake. In some cases, subsurface intakes may be evaluated and regulated as groundwater sources.

The use of subsurface intakes offers a distinct environmental advantage because the ecological impact associated with impingement and entrainment of marine life is virtually eliminated. However, subsurface designs should consider their potential negative impact on nearby fresh groundwater aquifers.