Seawater intake and pre-treatment/brine discharge — environmental issues

Abstract

In some cases the only remaining option to satisfy the increasing demand for potable water in good quality for the growing population, is the desalination of seawater, being used more and more the membrane process reverse osmosis. But whereas for the design and the manufacturing of the desalination units itself meanwhile are available well known and approved technical solutions, the intake and the pre-treatment of the seawater, as well as, the discharge of the brine have to be adapted to the specific conditions at the construction site of each plant. These conditions can differ in a wide range, as beside the influences determining the raw water quality, also items like the geological situation and details related with infrastructure or logistic are usually very different and have to be considered consequently during design, construction and operation of a desalination plant. The same applies for the aspects concerning the environment, that are becoming more and more important due to changing local legislation, increasing public awareness, growing construction density at coastal areas and also due to findings regarding the impact on the marine environment based on long-term operation of desalination plants. Thereby one of the most important prerequisites for reliable long-term successful operation of reverse osmosis seawater desalination plants is a consistent high quality pre-treatment of the feed water. One possibility to fulfil this requirement is the use of directed drilled horizontal drains below the seabed as seawater intake and partial pretreatment system according to the patented Neodren technology. With these horizontal drains, that are based on an improved horizontal directional drilling (HDD) technology and high efficient patented filtering devices, the raw and fine screening, as well as, the conventional filtration of seawater and the dosage of different pre-treatment agents will not be necessary. Due to extremely low filtering velocities, this is achieved without impingement and entrainment of aquatic organism. This system, developed by Catalana de Perforacions with the first unit in operation since 1996, can be operated in sandy and karstic seabeds as ecological and economical alternative for conventional open seawater intake systems, and has even advantages in comparison with indirect seawater intake systems like beach well installations. Due to the modular design there is no limitation regarding the production of pre-filtered water. Also based on the HDD technology have been developed systems for a controlled brine discharge that consider the environmental conditions on site. Details of these different intake and discharge systems and their environmental impact will be compared.