HOW TO MANUALS FOR AQUACULTURE

Part I. Understanding Recirculating Aquaculture Systems

Biofilters

Fish and other aquatic organisms excrete ammonia, a toxic by-product of protein metabolism, into the culture water. Ammonia is also released into the culture water as the result of decomposition of uneaten food. Under normal conditions, around 30 grams of ammonia is produced in the culture system per 1 kilogram of feed fed.

In recirculationg systems, biofilters are used to remove ammonia and another harmful nitrogenous compound, nitrite, from the culture water in a process called nitrification. Biofilters use a media, often plastic, that provides a suitable substrate for the growth of nitrifying bacteria (e.g., Nitrosomas spp. and Nitrobacter spp.). Nitrifying bacteria convert ammonia, first, into nitrite (toxic) and, then, into nitrate (non-toxic at normal concentrations) (Figure 3).


Figure 3. (Reference: The Council's Biotechnology Applications in Aquaculture)


 Biofilter media are rated according to the surface area they have available for bacterial colonization (measured in feet squared) per volume of space the media takes up (measured in feet cubed). A well-designed biofilter should be able to handle between 300 to 1000 mg of Total Ammonia - Nitrogen (TAN) per day per square meter of surface area.

There are many different types of biofilter media available commercially. A very common and versatile "fixed film" biofiltration medium is known by several names: "bio-deck", "bio-strata" or "bio-block". Bio-deck is commonly used in waste water treatment and may be available locally. Bio-deck is a honey-combed black plastic medium that can be used in a "trickle down" filter unit (i.e., water is trickled down through the media) or in an "up-flow" filter unit (i.e. filter is submerged so that the water moves up through the media). Bio-deck is made with different ratings, with 68 ft2/ft3 and 110 ft2/ft3 used frequently for biofilters.

Other types of commercially available biofilter media are: Bio-Fill, rated at: 160, 250 ft2/ft3; Bio-Ball, rated at: 98 ft2/ft3 ; Bio-Barrel, rated at: 26, 33, 44, 64 ft2/ft3 ; and Plastic Scrub Pads, rated at: 370 ft2/ft3 . For small recirculating systems, suitable, cheap, biofilter media can be made from recycled materials such as: empty film canisters, cut-up plastic milk containers, plastic hair curlers, plastic packing peanuts, etc.

Many types of biofilters are being used in recirculating systems and each has advantages and disadvantages for the culturist. In determining what type of biofilter to use, it's good to talk with people who are experienced with using several different types. Some of the basic types of biofilters are:

Packed Tower Filters (e.g., down flow trickling, up flow or down flow submerged) - These filters, generally: are compact in size (+); are easy for students to design, construct and see in operation (+); aerate (adds oxygen) and degasses (removes carbon dioxide) the culture water (+); require a pump to move water (-); tend to plug with bacterial growth (-).

Expandable Media Filters (bead filter) - These filters, generally: are compact in size (+); have low head loss (+); need regular backwashing (-); can be fairly expensive (-).

Expandable Media Filters ("upflow" sand, gravel, crushed shell filter) - These filters, generally: are compact size (+); function effectively with changing ammonia levels (+); have a high nitrification capacity (+); are expensive (-); have high energy requirements for fluidization and backwashing (-); use a lot of water for backwashing (-); use a lot of oxygen (-).

Rotating Biological Contactor (RBC) - These filters, generally: are compact in size (+); have low head loss (+); use air or water to move (+); aerate (adds oxygen) and degasses (removes carbon dioxide) the culture water (+); are expensive (-); demand constant mechanical maintenance and operational problems can occur (growth of bacteria increases weight of filter significantly and uneven distribution of bacteria can stop rotation of filter) (-).

Clarifiers | Culture Tanks | Biofilters | Aeration Systems | Home