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Filter-speak:
What
is a sand filteration system
and what are the risks to the habitat?
Cutting to the chase: These were the only hints of the filtration
system contemplated for project NOAH: "usage of huge side-stream
tidal filters to change water clarity" in the NOAH press
release and "create a simple sand filtration system to eliminate
most of the sediment from the water flowing into the lagoon"
in the media.
The best guess from this limited info is that consideration is being
given to a pressurised sand filter in a recirculation system.
According to the technical explanation below, there are limitations
to such a system. These limitations are overcome by adding disinfectants
to the water!
To maintain such systems, the filter must be backflushed very regularly
to flush out gunk that is blocking up the filters. Where does the
gunk go?
Filteration systems are highly expensive to maintain, forcing a
compromise between performance and cost. Will the habitat be compromised
to save cost?
Filtered systems can also crash producing toxins,
reducing oxygen that may kill everything that uses the system.
Full definitions and explanations
from Aquatex: The Free
Online Aquaculture Dictionary
Mechanical Filtration
Term used to describe a physical process (i.e. one not reliant on
chemicals or biological organisms) to remove solid particles from
the water. Mechanical filters fall into three categories - settlement,
screen filtration and bed filtration.
Settlement involves passing water through a tank which has a residence
time and slow enough current sufficient to allow the particles to
sink to the bottom of the tank, from where they can be removed. Modifications
to improve settlement tank design include the use of directed water
flow, either through a series of baffles, which ensure that the water
stays in the tank for the maximum amount of time, or by using a swirl
of water to draw the particles to the centre of a round tank, from
where they can be removed. (This works in the same way as when you
put some sand in a glass and stir it with a spoon. The sand will ed
up in a pile in the centre of the glass.) Settlement is a good method
to remove large particles from the water column, but can be an expensive
method of removing fine solids, due to the large tank size required.
Screen filtration involves passing the water through mesh or bars.
Such a process inevitably results in the screen becoming blocked with
removed particles. For large solids such as leaves and weed, the screens
can be manually cleaned, but for smaller particles, including fish
waste, self-cleaning systems are necessary. Careful design of manually
cleaned screens can reduce management time, examples are given in
the diagrams (buttons left).
Automatically cleaned filters usually either use a rake / brush system
or water jets to clean the screen. In general, rake/brush systems
are used for large solids and water jets for smaller solids. Water
jet cleaned filters follow either a drum, disc or belt method, where
the screen material is constantly moving and cleaned as it rotates.
Such filters are capable of removing solids as small as 6 microns.
These filters have the advantage that they have a very low head loss
(typically 10-70mm) which reduces the pumping costs in recirculation.
they can also, compared to bed filters handle very large flows at
comparatively low capital cost. See also drumfilters, discfilter,
conveyor filters
Bed filters use a volume of sand or other media through which the
water percolates. The filters then reverse their flow and backwash.
The backwash cycle can be either automatic or manually started. These
types of filters are ideal for small hatcheries, but the water pressure
required to force the water through the bed results in high operational
costs and limits their use in larger systems.
Sand filters in recirculation systems pose two challenges. Firstly
they are energy expensive due to the pumping power required, and secondly
the sand acts a a suitable media for biofiltration which results in
the filters blocking more often due to the bacterial growth. There
are sand medias available which have been specially treated to reduce
the bacterial growth. See sand filters
Sand Filters
Used for the mechanical filtration of solids, not to be confused with
fluidised sand filters, which are used for biological filtration.
Although the size of particle that the sand filters remove is dependant
on factors such as the size of the sand particles, the depth of the
bed of sand and the flow rate through the bed, sand filters are usually
regarded to filter water to a nominal 10 microns.
Sand filters are designed in two distinct ways; the first is a simple
box structure which operates with a very low pressure across the filter.
The water flow rate : cross sectional area ratio of such filters is
very low, and the filters tend to rapidly block in the first few centimetres,
with the rest of the filter staying clean. Such filters are only of
use in applications where the use of pressure filters is impossible
or the water is generally very clean and there are only a few particles
that need removing. Such an example may be a ground water supply which
is thought to be contaminated with pathogens through seepage into
the spring / borehole. Very large filters of this design are very
difficult to clean effectively, usually resulting in the bed being
periodically dug out and replaced by fresh sand.
Pressurised sand filters are in very common use in many aquaculture
applications. They consist of an enclosed vessel which is typically
half to two thirds full with sand. See
diagram. Water is pumped into the top of the filter under a pressure
of approximately 1-2 bar and is forced through the sand to a water
collecting device at the bottom which allows the water through, but
not the sand particles. The flow is then reversed to back flush the
filters.
Pressurised sand filters are very expensive to use for high flows
due to the cost of pumping the water through them. They are however
used extensively in hatcheries and also some recirculation systems,
where they are either plumbed in for all the water or as a side stream,
where only a percentage of the water flows through.
Their limitations in recirculation systems
is that, in addition to the operational costs, they use a
lot of water for backflushing (a typical sand filter in a recirculation
system will require back flushing 4-6 times a day for 5 minutes each
time. The water flow rate whilst backflushing is the same as the flow
rate when filtering).
This is exacerbated by the fact that sand filters in recirculated
water will also act as biological filters, and a layer of heterotrophic
and nitrification bacteria will build up on the sand, causing channeling
and increase back washing frequency. The back wash process is insufficient
to eliminate all the bacteria which soon multiply and block the filter
again.
A way round this is to add ozone or other disinfectant chemicals
to the water when back flushing (the advantage of ozone here is that
any residual amounts after backflushing will quickly be neutralised
by the organic compounds in the water).
Now that self cleaning mechanical screen filters are available with
screens of less than 10 microns, the use of sand filters is becoming
less common.
Fluidised bed
These are flooded vessels which are partially filled with a random
packed media. Water flows up through the media and the velocity of
the water pushes the particles up into the vessel, causing them to
swirl around. As the water velocity increases, the particles swirl
in higher up the column and become more fluidised.
Fluidised beds are sometimes used with ion exchange resins, types
of lime and activated carbon as they have the advantage that solids
particles can pass through them, whereas they would get caught up
in a static bed.
They are more commonly used for biological filtration , where sand
or small plastic particles are used for the substrate for bacteria
to adhere to and grow on. The constant movement of water and particles
in the vessel ensures that there are no dead spots, and the velocity
of the water is controlled so that as the particles collide, they
do so with just enough force to knock off any excessive or dead bacterial
floc.
The layer of floc is therefore maintained at an ideal thickness. This
occurs when the filter is approximately 100% expanded. That is, when
the media is occupying a volume twice as great as when the water is
switched off. Air is also sometimes used to assist in the fluidising
process, especially where water velocities are too low to obtain sufficient
fluidising.
The use of sand in large biofilters has sometimes been problematical,
both in achieving an even bed fluidisation and also from the fact
that if the water flow stops, the sand packs down, suffocating the
bacteria rapidly.
Plastic media filters are easier to fluidise, especially if plastics
such as polypropylene or polyethylene are used, due to to their density,
which is very close to that of water. Air diffusion is often used
to fluidise these filters rather than water.
Biological Filtration
The growing of bacteria colonies on a media surface over which the
water passes to remove nutrients form the water. Used as an essential
part of most water recirculation systems and also sometimes for treatment
of outlet water from a farm to reduce waste loadings entering a river
or stream to comply with regulations.
Although the process of biological filtration handles many different
types of waste, the main ones that we are concerned with in aquaculture
are BOD, ammonia and nitrite.
The BOD is oxidised by a group of bacteria called heterotrophic bacteria.
These are fast growing, dominant bacteria which often comprise a high
percentage of the "sewage fungus" found in tanks, pipes, sumps, channels
etc. The ammonia is converted to nitrite by a group of bacteria called
nitrosomonas bacteria, the nitrite is converted to relatively harmless
nitrate by a group of bacteria called nitrobacter. The process of
conversion of ammonia to nitrite and then nitrate is commonly known
ad nitrification.
Because the heterotrophic bacteria are more dominant than the nitrification
bacteria, the fist 25% of biofilters often comprises a high percentage
of heterotrophic bacteria and a small percentage of nitrification
bacteria, the rest of the filter being mainly nitrification bacteria.
Because of the ratios of BOD to ammonia in fish waste, and the way
that a biological filter functions, it is generally held that if ammonia
levels are kept in check by biological filtration, then the BOD will
also be kept in check. The ammonia is therefore used as the main indicator
that full biological filtration is taking place.
There are three main types of biological filter ; trickle, submerged
and fluidised. The sizing of biological filters is calculated by knowing
the surface area of the media which is being used, for example pre-formed,
plastic media typically has a surface area between 100 and 800 m2m3
(depending on the design).
Once the amount of ammonia that the fish are producing has been calculated,
the media surface area required can be calculated by using the rule
of thumb 0.4 - 1.0 g ammonia removed per day per m2 of media surface
area. This figure equates to temperature operation of between 10oC
- 30oC respectively.
Although the process of biological filtration involves changing the
water quality, sudden changes in the incoming water quality to
the biological filter can "stress" the bacteria lead to poor performance.
Such changes may include sudden changes in pH, flow rates, temperature
etc.
The processes of oxidation of BOD and nitrification are aerobic processes.
Another type of biological filtration which is sometimes used in aquaculture,
especially in high rate recirculation systems is denitrification.
Denitrification filters convert nitrate to nitrogen gas, the bacteria
in such filters are anaerobic.
Recirculation
The process of taking water from a holding system which would otherwise
be discarded from the system and reintroducing it to the same system.
Prior to being reintroduced, the water is often treated to remove
some of the wastes produced by the fish so that the water quality
is maintained at a sufficient high level that it remains suitable
for fish culture.
The amount that is recirculated is often called the percent recirculation
although this can be a misleading figure as it only takes account
of the hydraulics of the system, rather than the biological processes
which are occurring in it. A more accurate way of describing the amount
that water is recirculated or reused, is to refer to the amount of
water brought into a system per kg of feed given to the system.
The recirculation of water increases the operating costs of a farm
through the process of pumping the water round and also the additional
costs associated with any water treatment (e.g. power for filters,
cost of oxygen etc.).
In most recirculation systems a compromise is reached between the
operational costs, the capital costs and the amount of make up water
required.
Water is usually recirculated for one of the following reasons; limitations
in the water quantity or quality available, restrictions in discharge
of wastes into the environment, maintenance of stable water condition
(such as year round raised temperatures). See also biological filtration,
mechanical filtration, ozone. pH control, oxygenation.
Side Stream
General term used to describe how a percentage of the main flow is
taken away and then reintroduced to the main flow. This is usually
to allow a process to take place on a percentage of the main flow.
Some processes (such as measurement, oxygen injection, pH buffering)
do not require the entire flow of water, and to pump the whole flow
of water through is usually more expensive than operating on a side
stream. Side streams also allow individual pieces of equipment to
be isolated from the main flow for maintenance. Where two different
pieces of equipment are used on a side stream, by passes are usually
built in to allow maintenance of one without interrupting the other.
Filter Media
A particle or structure which is used for either the mechanical filtration
of solid matter (by restricting its ability to pass between particles)
or the substrate on which bacterial and/or algal colonies are formed,
which provide water treatment as the water passes the colonies.
Filter media can be divided into two categories, that which is a solid
structure and is packed into a filter in a structured method, and
that which is random packed into a filter (i.e. by filling it up without
caring how the items of media lie against each other). The latter
of these two is the more common type found in aquaculture and includes
naturally occurring particles such as sand, gravel, and man made items
such as plastic rings, tubes and beads.
A general criteria for aquaculture media is that it is non-toxic to
fish, and will not breakdown as a result of the water quality that
it is immersed in. For the other required specification of media for
a specific task see the relevant section (e.g. biological filtration,
degassing etc.).
Channeling
The term used to describe the concentration of flow through some areas
of a filter, whilst other areas become blocked. This particularly
applies to sand filters and some biological filters and degassers.
The media in the filter gradually becomes blocked with solids and
bacterial floc, eventually forming clumps of bound up media. Unable
to pass between the individual pieces of media in the clump, the water
flows around the clump.
Channeling can dramatically reduce the ability of a piece of equipment
to perform to it's design specification. Once channeling has started,
the only way to remedy the situation, is to backflush or wash
the filter media to remove excess floc/particulate matter.
Back flushing
The general term used to describe the process where filters are cleaned.
Also sometimes called back washing. The washing can be either from
flows in filters a being reversed, filter beds being agitated by air
or some mechanical means, or the jetting of water/air onto screens
to clean them. Although many of these operations are not technically
correct under this heading, they all tend to be grouped into the back
washing/flushing term
Crashing - (1of 2)
Term used to describe the failure of a biological filter.
Commonly regarded to be caused by the death of the bacteria in
the filter due to changes in their environment (either due to
stress caused by changes in the water quality in excess of their normal,
acclimated range, or through a lack of available nutrients).
Often however, especially with regard water quality, the bacteria
are not killed off but cease to perform due to there normal levels
whilst they try to acclimate to the new conditions.
Changes in water quality which are likely to actually kill the bacteria
will probably kill the fish first. Once stressed, the bacteria cease
to perform as efficiently which leads to a build up of metabolites
in recirculated water systems. This in turn exacerbates the problem
as the stress to the bacteria is compounded by increased levels of
ammonia and BOD. This can then result in a downward spiral, where
the bacteria become more and more stressed by the ever worsening water
quality, sometimes to a point where they barely function at all.
All the farmer sees is the water quality worsening and his fish suffering.
Crashing - (2 of 2)
Term used to describe the death of an algal bloom, usually due to
the exhaustion of nutrients or the lack of light penetration, usually
caused by the bloom itself.
The death of a bloom can lead to fish kills, either through the
release of toxins into the water (from some species of algae) or more
commonly, through the reduction in dissolved oxygen concentrations
as, as the dead algae decomposes. In green water systems, this makes
the management of a stable algae bloom, as important as the management
of the fish.
links
Vancouver
Aquarium: issues in water management in a really BIG aquarium
(easy version for the public).
On Jaap's Marine Mammal Pages: seriously technical issues in
water management in a dolphinarium
A biological approach to dolphinarium purification: I. Theoretical
aspects. (1987) Originally published in: Aquatic Mammals 13.3:
83-92
A biological approach to dolphinarium water purification: II. A practical
application: The Delfinaario in Tampere, Finland. (1988) Originally
published in: Aquatic Mammals 14.3: 92-106
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