Wednesday, September 27, 2017

28/09/2017: Defining the extraordinary: Designing recirculating aquaculture systems

by Ben Green MA (Cantab) CEO The Supreme Salmon Co, UK

Recirculating Aquaculture System design is a fascinating subject; it’s been one of my life’s great projects

There are many ways to build a system that works, but there are different designs that work better than others, and that’s the challenge; what’s a very good system and what’s not?

RAS systems should be shaped by cost, durability, efficiency and suitability for the species cultured.

This article is all about the layout of RAS system that I’ve reached after 25 years of designing and redesigning RAS for salmon.

The main holding tank
Salmon are pelagic fish and love to swim, if they can’t swim in a straight line they have to swim in a circle, and if there’s a flow of water against them so much the better. They don’t do lengths like a human swimmer so a raceway design is out and that includes concentric annular ring designs with a large filter in the middle, which are effectively curved raceways. 

Image credit: The Supreme Salmon Co
A circular tank with outlets in the middle takes advantage of the conservation of angular momentum, so a relatively small flow of water can provide both a comfortable environment for a salmon and will clean itself sending all the solid wastes to the middle.

This is where the fish live but our elegance of design principle states that the most functions must be completed with the least effort in the simplest, safest way. The main tank is the best place to introduce supplementary oxygenation; it should be deep enough to efficiently dissolve from diffusers and if it’s supplied from a liquid oxygen cryotank and all else fails, there is some independent life support for the precious stock.

I’ve recently started introducing ozone along with the oxygen (something marine aquarists have been doing for years) so no need for a separate external power consuming apparatus for this and it has the wonderful side effect of removing the earthy taste which can make the product unsaleable, so no need for a separate depuration set up.

Most of the systems I’ve seen have a single outlet but why not make use of the fact that nearly the entire solid wastes sink to the bottom of the tank? My tanks have a double circulation, the main outlet in the middle, high in the water and a smaller outlet at the bottom to take away the solid waste.

A double circulation builds in more safety if an outlet gets blocked or either one of them fails for any reason. It’s taken many years before I had the nerve to remove the outlet screen to the bottom drain but salmon like open water and hate dark pipes so now any mortalities are carried straight out of the tank, inefficient mort removal can be a massive problem in RAS.

The main tank is also a good place to skim any froth from the surface of the water; it can be a third outlet at the middle at the surface. I haven’t thought of a good use for a third circulation so it’s a good place for a small overflow pipe to discharge any changeover water as well.

So we already have several functions operating before we’ve even left the main fish holding tank and not really consuming any power either. 

Read the full article, HERE.

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