RAS - Recirculating Aquaculture System

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When is a RAS NOT a RAS? click here

RAS - Recirculating Aquaculture System - A simple description
I am not an expert on RAS so my comments here are based on lectures I've attended, places I have visited, and a little personal experience I have using simple systems for my fry and brood fish over a few decades.

The performance of any system can very much be affected by the species.
Jade perch are VERY different from other species.
They can be stocked at extraordinarily high densities compared to most other freshwater species, provided your system can cope with the waste products from the fish. Click for short video

Mechanical filtration
Generally meaning a method of separating solids.
This is an important component of a RAS filter system.
Not normally considered a stand alone filtration system.
Solids can be separated with a screen or a swirl filter.
A screen generally needs to be manually cleaned, a swirl filter is generally self cleaning.
Most commercial RAS systems also use a drum filter to separate solids.
Drum filters are also self cleaning.

Here is a link to a video, (Not one of my videos.) that will help you understand. 

Chamber system (Often called a bio-filter, or biological filter.)
The main job of a RAS is to break down organic waste.
This is mostly done by the bacteria in the chamber of a RAS.
This waste is a by-product of the fish which is processed by bacteria in the chamber.
The bacteria breaks down ammonia into nitrite then into nitrate.
Ammonia is deadly to fish as is nitrite. Fish are ok in very high levels of nitrate.
Ammonia can be in two forms, ammonia which will kill fish and ammonium which is pretty much harmless to fish.
The key is to maintain the PH at low levels because ammonia is in the form of ammonium at low PH. More about ammonia here

The bacteria must have plenty of oxygen.
There are two main types of bacteria.
Bacteria that need oxygen and bacteria that doesn't need oxygen.
Anaerobic bacteria does not need oxygen. Aerobic bacteria are bacteria that need oxygen.
RASs rely on aerobic bacteria.
The system must provide plenty of oxygen, constantly, in order to maintain the aerobic bacteria in the system.
A little complicated to explain in greater detail here but I will explain briefly what aerobic bacteria needs.
It needs three main things.
1. food, which is the waste provided by the fish.
2. oxygen
3. somewhere to live, which is all about the surface area of the biomedium, another vital component of a bio filter.
 
Bio medium can be many different types of material.
The bigger the surface area the better.
Also it should allow the water fo flow freely and let air in to supply oxygen to the bacteria.

Bio medium  Different filter medium

Bio balls (Below) are a common filter medium. Frequently used in a fluid bed system.

BioBalls More bioballs Fluid bed filter 800x735

Here is another video, also not one of my videos, but you can see a small fluid bed filter working right at the beginning of the video.

A rotary drum is specifically to reduce solids in a system. I see this being used at most of the facilities I have visited.
It is not an alternative to bio filtration, rather it is a component of the RAS.
What happens is the solids are trapped on a screen which is automatically backwashed as needed.
See the video link above.
 
Is ultra violet sterilization sufficient to maintain water quality?
In short, NO.
Its main job is to kill harmful bacteria.
Not actually "filter" the water.
These is also a hazard to employees and they a difficult to maintain.
I prefer not to use them. (Just a personal preference.)