|
Back to Aquarium
products |
|
How to use AFM |
|
Pressure sand filters provide a
simple very effective means of mechanically cleaning aquarium recycle
water, however in many cases inappropriate equipment is used, or
the filters are not operated under the correct conditions.
Sand also acts as a excellent
substrate for the growth of bacteria. These bacteria excrete alginates
which glue the sand grains together and cause channeling of water
through the bed. In chlorinated system bacteria will also grow on the
sand, the acid biofilm layer reacts with chlorine to produce
trichloramines. Indeed it is trichloramines and not chlorine,
that give chlorinated systems their characteristic smell, it is also
trichloramine that is responsible for causing eye, and tissue damage
to the lungs of aquatic mammals which leads to pneumonia (click
here for report).
Simply by changing the sand
to AFM, bacterial coagulation of the sand is eliminated or
largely prevent, the filters do not suffer from channeling and as such
the performance is dramatically improved. In many cases the
visibility through the water will be doubled. In chlorinated systems
the elimination of the biofilm also eliminates trichloramine
production, which in turn eliminates the eye and tissue damage
experienced by aquatic mammals.
AFM is more expensive than
filter sand, however operational costs savings can provide a
return in the capital after a period of 24 months.
|
|
Flowrate
& how to operate a filter |
The performance of a filter bed be it sand or AFM is inversely
proportional to the water flowrate. This fact has been well documented by the water industry where sand filters have been used for many decades.
Rapid sand filtration of drinking water normally ranges between 5 to 10 m3m2hr-1.
In commercial aquaculture the maximum water flowrate is 10 to 15 m3m2hr-1,
for general purpose applications and down 5 m3m2hr-1
for marine hatcheries.
During the early 1970`s sand filter manufacturers started to manufacture GRP and polyester filters in mass quantities for the
swimming pool industry. The new filters were called "High Rate" operating at 25 to 30 cubm/hr. The main reason for the change in
specification was to win contracts over the competition. However high rate filters have become the norm, indeed in some cases we
have seen sand filters operating at flowrates over 40 cubm/hr. With regards to public aquariums and aquaculture you should ignore the water
flowrate recommended by the manufacture, they are not biologists and probably not experts in water quality. There are of course
considerations such as space and cost to take into account, however the aquarium industry can learn a great deal from commercial aquaculture and the
water industry.
In an aquaculture or public aquarium situation, we are dealing with soft fragile organic solids that are easily broken down or
pushed through a filter. Sand filters can actually increase the turbidity of water when operated at high flowrates in the region of 30 m3m2hr-1
. At water flowrate in the region of 25 m3m2hr-1
. a filter will remove approximately 20% of sub 20 micron solids per pass. At a flowrate of 10 to 15 cubm/hr, the
solids removal efficiency increase to around 80%. It is therefore a false economy contrary to good water quality to operate sand filters at a
high rate, and in many cases water quality can be improved simply by reducing the water flowrate.
|
Optimum conditions
for pressure filters |
|
Operational water flowrate (do not allow
differential pressure to exceed 0.5 bar) |
10 to 20 m3m2hr--1 |
|
Air scour prior to back-wash |
70 to 90v m3m2hr--1 |
|
Back wash flowrate |
40 to 50 m3m2hr--1 |
| |
|
|
 10m3m2hr--1
= 4 gals ft2 min-1 |
|
How to inspect a filter |
|
Mechanical filtration by sand filters is the main form of treating recycle aquarium water, however how many times is the sand inspected, and what
are you looking for when you inspect a filter bed. ?
It is absolutely essential that sand filters are installed on a perfectly level base. If the filters are at an angle, there will be a variable bed
depth and pressure differential of sand across the filter. This will lead to variable flowrates, bed channelling and filter failure
When you inspect a filter bed there are certain key points;
1. The sand media bed must be level, if the media is pushed to one end or up the sides or pilled up in the middle of the filter, take the filter out of
service as it will be adversely affecting water quality.
2. Bacterial growth in a sand filter will, after a period of a few months to a year start to cause a differential flow pattern through the filter
bed resulting in channelling. Signs of channelling include a low pressure differential across the filter, and the water runs clear very quickly after
a back-flush. Internally, there may be a sand hole or worm holes running straight down through the sand bed. Take the filter out of
service and change the sand.
3. If a poor quality sand filters with weak laterals are used, a lateral can often break, this will rapidly lead to bed channelling.
Pea gravel or 2 to 6mm gravel must be used on the base of the filter. If the filter does not contain pea gravel, it will fail or will be
adversely affect water quality.
4. Dig down 100mm into the sand bed at the side, and middle of the filter bed. The sand should be free flowing. Pseudomonas bacteria produce
copious quantities of extracellular alginate that binds the sand grains together. When this starts to happen the pressure differential across the bed will
start to increase as a prelude to bed channelling and filter failure.
|
|
Filter design |
|
Horizontal and vertical sand filters are used as pressure filters.
Horizontal filters are often used because they have a higher surface
area per unit volume, and can be stacked two or three units high.
However there is a differential pressure gradient across horizontal
filter bed and as such there is a differential water flowrate through
the bed. This often leads to dead areas and coagulation of the
sand at the edges. AFM is much less prone to biofouling than
sand, the use of AFM in
horizontal filters mitigates the hydraulic and biofouling issues of these filters.
The best type of filter to use is a vertical arrangement with false bottom nozzle plate collector systems. These filters are generally much
more expensive than a horizontal filter with laterals. However the bulk of the water treatment of a facility will be accomplished by the
filters. I fail to understand why architects and designers often end up selecting the most inappropriate lowest cost filters for the application. |
 |
|
Sand filter as a biofilter ? |
Bacteria are essential for maintaining
water quality in closed systems, but their presence in sand filters will result in a deterioration of filter performance due to coagulation of the bed
from bacterial extracellular polysaccharide alginates.
It has been suggested that one of the functions of a sand filter in public aquarium is to act as a biofilter. However our
findings show that the mineralization of protein in the sand filters will contribute the ammonium and/or nitrate levels. The bacterial cell biomass
produced will also increase the turbidity of the water and thereby reduce clarity, indeed sand filters can generate huge quantities of bacteria and
cell wall debris that ends up in the aquarium water.
It is a better approach to physically remove the solids, prevent nitrate production and maintain the mechanical filtration
performance of the filters. The requirement
for biological filtration is greatly reduced, and a properly designed
biofilter using a structured biological filter media will meet the
requirements and take up minimal space.
|
Sand after a few weeks of use, note the high level of surface bacterial contamination. Used AFM, note there is no surface
bacterial contamination on the AFM.
Bacterial fouling promotes filter bed worm holes, which allows a proportion of the water to by pass the filter bed. In
many cases the sand filters can actually contribute to the solids load in the tank by acting as a biofilter and discharging bacteria debris and cell
biomass back into the water.
|
|
Properties of AFM filter media |
|
AFM acts like a physical filter in exactly the same manner as sand, however the surface is negatively charged, which allows it to remove, by attraction,
small particles, bacteria and even organic molecules. The AFM media is therefore doing a great deal more than sand. In addition to these properties, AFM
exerts surface catalytic properties in which the oxygen molecule is dissociated to form free radicals. In many aquariums ozone is used as an oxidising agent
for the cracking of organics and disinfection. With AFM the localised high free radical concentration on the surface also cracks organics. These unique
properties explain the performance of AFM, and allow it to achieve 100% removal efficiency on
back-wash. |
|
How to use AFM |
|
AFM ( Active Filter Media) is a direct replacement for sand in your sand filters. AFM is used in exactly the same water as sand.
AFM does not support bacteria, which means the media grains are not bound together by bacteria alginates, filter performance is therefore
maintained indefinitely. If a good quality sand filter is used with AFM, it is unlikely that the filter media will ever need to be changed.
AFM carries a high surface zeta potential which attracts micron and sub-micron particles directly from the water. AFM will also adsorb organic
molecules, and during the back-flush all of the retained solids and organics will be discharged. 50% less back-flush water will be required to keep
the filter media clean
AFM has a high surface oxidation potential. It is this property that helps to keep the surface of AFM free from bacteria. In fact AFM acts
almost like an ozonation systems. In the presence of oxygen, AFM will dissociate the oxygen molecule to produce a range of free radicals which oxidise
heavy metals, crack organics, decolourise the water and prevent bacterial proliferation.
In commercial aquaculture marine hatcheries, ozone is often used in combination with AFM filtration. A small dose of ozone is used before the
AFM filter. In this context the ozone flocculates solids and improves the performance of the system. The performance of ozone flocculation is
very poor above a flowrate of 20 cubm/hr, however at slow flowrates ( 10 to 15) performance is excellent. After AFM filtration the ozone demand
of the water has been dramatically reduced, and only a small amount of ozone is required to affect disinfection. When changing from sand to AFM,
the ozone demand of the water often decreases by up to 90%.
|
|
Filter bed make up and basic operating parameters |
|
2 to 6mm pea gravel or equivalent must
always be used on the base of the filter to a depth of approx 160mm above
the nozzle plate or laterals. A 1mm to 2mm (Grade 2 AFM) of a
depth of approx. 200mm on top of the pea gravel, and a 0.5mm to 1.0mm
(grade 1 AFM) on top of the grade 2
- recommended operating conditions 10 to 15
cubm/hr water flowrate
- back-flush rate 40 to 45 cubm.hr
|
|
