Submarine outfalls
Introduction
An underwater outfall is a closed conduit for the transport of partially treated wastewater from the treatment area located on land to the discharge area generally located offshore at a certain distance and depth to facilitate a rapid, high and efficient initial dilution so that said discharge does not pose a health risk or have harmful effects on the local ecosystem.
The previous definition can be extended to pipelines whose discharge areas are located in rivers and lakes, although it is true that these are less common and efficient than the previous ones.
Within the group wastewater we can include those waters derived from an industrial process such as cooling water, brine, etc., and those derived from an urban or domestic process such as sewage, rainwater, etc.
The most common applications of underwater outfalls and outfalls are:
· In desalination plants for water intake and brine discharge.
· In purified water collectors for the discharge of treated water.
· In fish farms, emissaries are made to collect water with nutrients for breeding and emissaries for discharge.
· For water consumption only underwater intakes are carried out.
· As bottom drains in dams to maintain ecological flows and as emergency elements.
· In cooling systems, both intake and discharge systems are used.
· Submarine outfalls for oil pipelines or gas pipelines are the land-sea connection section between underwater and land conduction. Therefore, the direction of the fluid can be intake and/or discharge.
The construction of an underwater outfall generally involves the combination of two or more construction processes that vary depending on the type of installation to be carried out, which may be, through installation in a trench, laying the pipeline directly on the sea, lake or river bed and/or, more recently used, through trenchless technologies.
Within trenchless technologies, four installation methods stand out, the choice of which will depend on the type of coating and the characteristics of the pipeline to be installed:
· PIPE-JACKING/MICROTUNNELING – Pipe jacking or microtunnel. The tunnel boring machines associated with pipe/microtunnel driving are usually of the hydroshield type. The installation is carried out by pushing the pipe from the attack or launch well using a hydraulic pushing system until reaching the reception well or rescue point of the TBM. It is the most versatile methodology since the range of diameters is much wider than in the rest of the installation methods, from small diameters (600-800 mm) to large diameters (3,000-3,500 mm).