Reducing the Risk Associated to Desalination Brine Disposal on the Coastal Areas of Red Sea

Abstract

By the end of 2005 the global installed capacity for desalination of seawater was about 24.5·106 m³/day. The geographical distribution of the desalination plants was as follows: 77 % in the Middle East and North Africa, 10 % in Europe, 7 % in the Americas and 6 % in the Asia-Pacific region. The volume of brine discharged in the Red Sea increased from 6.4 million m³/day (in 1996) to 6.8 million m³/day (in 2008) and is still increasing due to the observed tendency to improve the average recovery ratio from 30 to 50 %. This will make the environmental concerns much more important in the future. There are two main sources of problems, i.e. the concentrate and chemical discharges and the cooling water effluent discharges. The salinity is expected to increase in the long term if larger and larger amounts of desalinated water are removed from the water bodies. A proper solution for the desalination waste brine disposal process requires a good balance between technical constraints (i.e. placing a pipe on the sea-bottom), environmental conditions (i.e., finding an optimum distance from the seashore where high salinity brine should be discharged without significant environmental impact; also, the availability for long run of brine disposal placement should be considered) and as low as possible overall economic costs.

Since the potential cumulative impacts of desalination activity on the marine environment is expected to be more significant in case of regional seas, in this chapter we focus on the desalination plants in Red Sea. The objective is to discuss a modeling framework for the environmental-hydraulic design of the outfall system for desalination plants. The chapter presents an interdisciplinary combination of environmental issues with physical processes and discharge modeling. We assess the technical viability of disposal the brine effluent produced by desalination plants into Red Sea coastal regions via submarine pipes.

There are several approaches to mitigate the environmental effects of the brine discharges. By tradition, the brine is discharged back to the sea in open channels. Impacts from high salinity may be avoided by pre-dilution of the desalination plant rejected stream with other waste streams, such as power plant cooling water. Impacts from high temperature may be avoided by ensuring heat dissipation from the waste stream to the atmosphere before entering the water body. However, simulations models for the brine plume dispersion from desalination power plants reveal the inadequacy of using surface discharging outfalls in order to brine discharging. Large capacity plants require submerged discharges which ensure a high dilution, reducing the harmful impacts on the marine environment. Mixing and dispersal of the discharge plume can be enhanced by installing a diffuser system, and by locating the discharge in a favorable oceanographic site which dissipates the heat and salinity load quickly.

The central concept of the brine disposal by submerged pipes is the available head at the discharge point. Higher values of the available head ensure larger jet dispersion lengths and better conditions for ejected brine dilution. We have shown that the quality of the dilution process is well quantified by the Froude number of the brine discharge jet, whose optimum values range between 20 and 25. Using the Froude number allowed us to find the optimum pipe length and the optimum depth of the discharge point.

About half of the Red Sea desalination plants are based on reverse osmosis. The percentage of RO plants is expected to increase, taking into account the lower production costs and the favorable technological evolution. The most representative RO desalination plants around the Red Sea coast are considered both by country and by capacity points of view. In order to provide relevant conclusions for the study reduced capacity desalination plants were selected due to their large number. For not available/existing desalination plants some hypothetic “case studies” were considered.

Optimum brine dispersion may be obtained by using underwater pipes for any sort of high capacity and low capacity RO desalination plants operating on the Red Sea coasts. In case of large capacities, a pipe length around lX = 1,000 m allows optimum operation. A pipe length about lX = 500 m is needed for low capacity RO desalination plants.