Land Based Systems

Fish production produces waste materials in the form of solids, dissolved chemicals and gases that are all accounted for and reduced/ removed. Fish production also utilises water parameters which must be replaced and a levels of bacteria and pathogens can proliferate to adverse levels which have to be managed.

The most pertinent factors accounted for are;
  1. Solids (Wasted feed and faeces).
  2. - Ammonia, NH3 (Excreted from fish gills and derived from feed and faeces).Follow to next line
    - Nitrite, NO2- (by-product of Nitrosomonas bacteria breaking down NH3).Follow to next line
    - Nitrate, NO3- (by-product of Nitrobacter bacteria breaking down NO2-).
  3. Bacteria, viruses and parasites.
  4. Carbon Dioxide, CO2 (respiration from fish and bacteria).

The main aspects of water quality used during culturing are;

  1. Alkalinity (used by bacteria during the nitrification process).
  2. Oxygen, O2(used by the fish for and bacteria for metabolism).

The factors described under "Principles" are controlled by a number of standard processes carried out within and after the filtration units.

There are 3 main filtration processes to ameliorate the wastes produced and to control bacterial and pathogenic populations. After ancillary processes address physical water parameters;

  1. Mechanical filtration - addressing solid wastes produced.
  2. Biological filtration - addressing the waste derivatives; NH3, NO2-, NO3-
  3. Chemical filtration - addressing the proliferation of bacteria and pathogens.
Ancillary processes are used to control other important water parameters
  1. CO2 is removed through a process of "de-gassing"
  2. Alkalinity is controlled by pH adjustments through addition of a strong "base" such as Sodium Hydroxide (NaOH)
  3. O2 is added to the water through aeration or direct oxygen injection.
Systems Process Configuration
All processes are enabled through the implementation of systems in sequence.
  1. a) Heavy solid wastes are removed from the water as early as possible so as to reduce load on latter filtration processes and reduce the "chemical oxygen demand" (COD) of the water. This is achieved through appropriate hydraulic design of the fish tanks and mechanical filtration using drum filters with a fine micron mesh to remove heavy solids.

    b) Smaller "suspended solids" are also important to remove to maintain overall water quality and to deliver high quality fish. This is achieved through fine mechanical filtration and/or foam fractionation, a process using "Van der Waals" forces of bubbles to attract fine particles from the water then collected and ejected from the systems.
  2. 2. Water filtered of heavy solids, is allowed to pass into a bio-filter that contains bio-media to promote the growth of aerobic bacteria required to break down; Ammonia (NH3) and Nitrite (NO2-) into their less toxic form, Nitrate (NO3-). Nitrate can also become toxic and inhibit growth above certain concentrations and so is either diluted or removed through anaerobic bio-filtration.
  3. 3. Chemical filtration is carried out by oxidative agents that will not bio-accumulate within the system. The 2 processes routinely employed are injection of Ozone (O3) and/or Ultra violet (U.V.) treatment. These treatments are performed after mechanical filtration. Ozone is often used in conjunction with foam fractionation as it promotes the bubble forming process required for fine mechanical filtration of solids.
  4. 4. Carbon dioxide (CO2) is removed through a process of "de-gassing" which essentially exposes the water to ambient air containing a comparatively lower concentration of CO2 and so allowing it to dissipate from the water. This can be done through heavy aeration within the bio-filter (a process required to supply aerobic bacteria with oxygen) and/or trickling water over a large surface area to expose it to the air.
  5. 5. Alkalinity used during the bio-filtration process and is controlled via supplement of a alkalis such as Sodium hydroxide (NaOH) to maintain the optimal pH. This is injected into the water sump after the bio-filter.
  6. 6. Oxygen (O2) is replenished through aeration and de-gassing processes to "normal" levels but within "intensive" systems higher levels of oxygen are required.. This is achieved through conditioning the water with pure O2, before it returns to the fish tank.
The "principles, processes and systems" have been described in simple terms but in reality there are many more parameters than described and chemical and physical interaction dynamics to control them which AES  take incorporate during planning, design and engineering.
Custom designed R.A.S. filtration units
Above: Custom designed R.A.S. filtration units