Recirculating Aquaculture System Fundamentals (Part 2 of 6)

Recirculating Aquaculture System Fundamentals (Part 2 of 6)

Solids represent a huge amount of waste that needs to be removed from the system as soon as possible.

Solid removal

Solids represent a huge amount of waste that needs to be removed from the system as soon as possible. Potentially 35% of the entire bio load can be removed. If these solids are left in the system it will breakdown and convert to ammonia leaving the biological filters to deal with this. This would be an unnecessary and costly process, both in electricity and extra filtration. Large volumes of solids accumulating within a system can also create anoxic zones, with the production of hydrogen sulphide, which is highly toxic to fish

Drum filters and settlement chambers are the most common equipment to deal with solids and are very effective. Drums offer benefits of low floor space and labour, however suit high flow rates better and are expensive.

The most important part of solid removal is that it must not be broken up into smaller particles. Either by gravity or suction the solids need to be gently moved to the filter in its whole state as much as possible. Remove as many bends and different velocities that would break up the solids. If this is not done, the solids will become suspended solids, which are much harder to remove.

Subsequently, the process of oxidation is so much faster and bio filtration would now have to deal with the waste. Using a pump of any kind to move water with solids to a filter from the culture tank would be missing one of the most important fundamental steps. The solids entering a pump would be exposed to an extremely violent action dissolving solids to the point where settlement chambers and drum filters are no longer effective.

Dealing with solids correctly can remove a huge amount of biological load allowing water to be cleaned more effectively.

Suspended solids

Suspended solids if not dealt with, hold a huge amount of bio load and will contribute to ammonia and low dissolved oxygen. Both are enemies of the system and of fish health through smothering gill surfaces and harbouring pathogens.

  1. Sand filters are the easiest means of catching suspended solids because of the ease of backwashing and removing solids from the system. Backwashing can be scheduled daily or weekly depending on the load of solids in the system. It also acts as a bio filter. Most other mechanical systems are difficult to clean and anything difficult is normally neglected and not maintained effectively.
  1. Down-flow sand filters are not appropriate for RAS that are even moderately loaded due to high total suspended solids (TSS) and biochemical oxygen demand (BODs) in a RAS system.
  1. A pressurised sand filter can be an option but only for very low loaded systems.
  1. Up-flowing sand filters offer biofltration benefits and can capture fine solids at low hydraulic loading rates.
  1. Bead filters are multifunctional offering both biofltration and solid removal

So there are many options that need to be matched to the system design and loading.

There are some new sponge filters with backwash capabilities that can deal with small amounts of solids.

Dissolved solids

Protein skimmers have been around for a very long time but not used much in freshwater aquaculture. The benefit of using a foam fractionator to remove dissolved solids is that it removes organic materials directly to waste and performs other filtering actions the sand filter cannot do. However, it is not a substitute for effective primary TSS removal.

In the next part of this article I will be looking at Ammonia, Nitrite, Nitrate and the ever important role of Dissolved Oxygen.

Written by:  - 21 Oct, 2020  
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