Biotechnology and Bioinformatics (1+1)

6.2.1.Introduction

Biofilters are devices to culture microorganisms that will perform a given task. Different types of organisms will perform different tasks. Designing and using biofilters is to create an environment that will promote the growth of the organisms that are needed.

Biofilters are used

• to maintain water quality in recirculating or closed systems and
• to improve water quality before water is discharged from a facility.
• It is however, a very important and essential component especially for recirculating aquaculture or aquarium systems.

Biofilters perform the following functions. The first three functions are performed by biological means and the last four are done by physical processes that do not depend on living organisms.

Biological functions

1. Remove ammonia

2. Remove nitrites

3. Remove dissolved organic solids

Physical functions

4. Add oxygen

5. Remove carbon dioxide

6. Remove excess nitrogen and other dissolved gasses

7. Remove suspended solids

In gene ral, there are three types of aerobic microorganisms that colonize biofilters for aquaculture.

a) Heterotrophic bacteria utilize the dissolved carbonaceous material as their food source.

b) Nitrosomonos sp. bacteria utilize ammonia as a food source and produce nitrite as a waste product.

c) Nitrospira sp. utilize nitrite as a food source and produce nitrates as a waste product.

Nitrosomonos and Nitrospira which grow relatively slow will grow and colonize the biofilter as long as there is a food source available. Heterotrophic bacteria grow about 5 times faster and will out compete the other two types for space if food is available. Since most aquaculture biofiltration systems are designed for the purpose of converting and removing ammonia from the water this presents a problem.

There are three ways to deal with this problem.

i) The first is to remove most of the carbonaceous BOD ( biological oxygen demand ) before the water enters the biofilter.

ii) The second method is to provide sufficient extra capacity (surface area) in the biofilter to allow all of the various bacteria to grow.

iii) Another method is to have a very long plug flow path through the biofilter. This allows different zones of bacteria to establish themselves in different parts of the biofilter.

There are 4 main types of aerobic biofilters and several subcategories of each.

I. Recirculated suspended solids (Activated sludge)

II. Aquatic Plant Filters

i) Unicellular (Microscopic)

ii) Multicellular (Macroscopic)

III. Fluidized Bed Filters

i) Sand Filters

ii) Bead Filters

IV. Fixed film

i) Rotating Biological Contactors (RBC)

ii) Trickling Filters

iii) Submerged Filters (with or without aeration)

a) Up flow

b) Down flow

c) Horizontal flow

d) Moving Bed

Anaerobic filters

Anaerobic filters can also be defined as biofilters but they are never the main biofilter used for maintaining water quality in the culture system.

Disadvantages

i) They are not capable of effectively cleaning the water to the level required.

ii) They operate too slowly.

Advantages

i) Convert the nitrates into N₂.

ii) Best suited for processing high strength waste.

• The sludge produced by the physical filter system is an example of a high strength waste.
• Processing plant wastes are another candidate for anaerobic digestion.
• In an integrated production/processing plant these two streams could be combined.
• The best feature of anaerobic systems is the production of methane.
• There are specially designed engines that can burn this gas to produce electricity. Using the gas to heat water is another obvious possibility. However, the capital cost of these systems generally limits their use to large operations.