Sources and Types of Water Pollution

Collected here are articles on filtration and water.

Sources and Types of Water Pollution

Postby Fred Goodall » Wed Sep 13, 2006 6:38 pm

Sources and Types of Water Pollution
koi are effective sewage-making machines
Written by: Frank Prince-Iles


Even though the title is Koi, the science applies directly to discus and other tropical fish.

Koi pond filtration and fish health?

A biological filter extends the capabilities of what is an unnatural and otherwise inadequate system by continuously treating the water and removing some toxic compounds - ammonia and nitrite being the most common. However, as we all know, there are many different types of filter, some more efficient and effective than others. So which are best? This is a deceptively simple question without a simple answer. What we can do, though, is to look a bit more closely at how a filter works and what sort of load is placed on it and thereby arrive at some basic guidelines.

Do we really need a filter?

If we consider a range of ponds, from the standard goldfish pond with just a few relatively small fish and lots of plants, up to the other extreme - an overstocked koi pond containing several dozen large fish - the weight or biomass of fish per unit volume can vary dramatically. The goldfish pond may have only a few grams of fish while the koi pond may contain several kilograms worth.

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The filtration or purifying requirements for these two very different extremes will be completely different. Indeed, the goldfish pond may manage quite adequately without any additional filtration to that naturally available within the pond itself. The plants in the pond would remove the small amount of nitrogenous waste produced by the fish and there would be adequate surface area in the pond for the relatively small numbers of nitrifying bacteria. Many owners of small ponds like to have a waterfall and it is then no problem to run water through a small filter, mainly in an attempt to produce clearer water. In a low stocking situation a filter may be an optional extra and the amount of beneficial biological filtration produced is liable to be small.

Koi are efficient sewage making machines

In contrast, in a typical koi pond the amount of waste produced by more and larger fish means that the water is continuously being polluted to higher levels, which in turn will have a serious affect on fish health. Indeed, with no filtration the fish would soon generate sufficiently high levels of pollution to kill themselves. In this situation filtration is not an optional extra, but a vital necessity. If the filter also helps produce clearer water that is an added bonus.

I view koi as extremely efficient sewage-making machines. We throw in high quality, expensive food and the koi convert it to high quality water-polluting sewage. If we are to maintain good water quality, we need to remove or neutralise the sewage as fast as the fish produce it, otherwise there will be a steady increase in unwanted pollutants. This is an important point that I will return to later.


What is pollution?

At this stage we should clarify what we mean by pollutants and at the same time widen our expectations of an adequate filter system. A simplistic view of filtration is the conversion of toxic ammonia into less harmful compounds. While this may reduce or remove any potentially harmful toxins, it doesn't necessarily result in unpolluted water.

Ultimately, what we are trying to achieve and maintain are optimum water conditions that are as near as possible to those that the koi would find in their natural surroundings. Any chemical or substance present at higher than normal levels, even if it is not directly toxic, should be considered a potential pollutant. So our filtration system, together with our routine pond maintenance schedule, should be designed to remove not only toxins such as ammonia and nitrite but also the products that result and often accumulate when these initial metabolites are degraded. If we are going to extend our view of filtration to include the control of non-toxic waste products, it is obviously going to mean a completely different approach to our previously simplistic view of water quality, filters and routine pond husbandry.


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Background, non-toxic pollution affects koi health and encourages disease

Why should we make life difficult for ourselves in bothering about these non-toxic pollutants and what are they anyway? A short answer as to why we should be concerned about the level of this type of 'background pollution' is that it can indirectly encourage increased levels of both bacteria and parasites and it has also been implicated in lowering resistance to infection. It also encourages algal growth, which in turn can affect dissolved oxygen levels and pH stability.

And as we all know, the presence of unsightly amounts of blanketweed can encourage the koi-keeper to start loading the pond with yet more chemicals in an attempt to eradicate the problem. So although many of these pollutants are themselves not directly toxic, they can be indirectly involved in many of the more common health problems.

If we again make a comparison between the lightly stocked goldfish pond and the often overstocked koi pond, and ask which system is more prone to health problems, the answer must surely be the koi pond. The main difference between the two, apart from stocking levels, is the background level of non-toxic pollutants. A better understanding of these pollutants requires a change in the often over-simplified view of water quality. The conventional and popular view is that the fish produce metabolic ammonia and all of the fish waste and mulm also breaks down, in a single step, to ammonia. In the filter, these copious amounts of ammonia are converted to harmless nitrate - end of story. But that is only the beginning.

Firstly, we have to realise that fish food is concentrated, containing high levels of protein and other nutrients. This means that a relatively small amount will have a large polluting effect. This could be demonstrated quite convincingly by simply placing a couple of pellets of fish food into a small container of pond water for a few days and then testing the water sample for a range of parameters. One of the major changes is a two-to-threefold increase in the level of dissolved organic carbon (DOC), which indicates an increase in pollution. (It is referred to as carbon because the basic structure of all organic molecules such as proteins, fats and carbohydrates, is based on carbon atoms). The other noticeable effect of this little experiment is a dramatic increase in phosphate, an ideal plant food.


Food in, sewage out

The effectiveness of a fish's digestive system is directly related to the availability of food. Therefore, when food is scarce or of low nutritional value, as it would be in the wild, the digestive tract extracts the maximum amount of nutrition from the food. The resulting excreta will therefore be fairly well degraded, bearing in mind that, in the wild, the food was liable to have been of relatively low nutritional value in the first place. However, when food is plentiful or rich, the digestive tract gets lazy and tends to absorb only enough for it's immediate requirements, so the faeces are likely to contain quite high amounts of undigested nutrients, or pollutants as they have now become. So you can perhaps accept my analogy between koi and sewage machines - food in one end sewage out the other.

It is important to appreciate that fish waste is in many respects similar to solid food, inasmuch as it still contains proteins, nucleic acids, fats and carbohy­drates. Because it still has a high organic content, this type of semi-solid organic matter is called particulate organic carbon (POC). When this waste is further degraded by decomposer organisms part of the protein content will be ultimately converted to ammonia and other nutrients will be progressively broken down into sugars, organic acids and a whole range of simpler organic compounds.


Ammonia - a toxic by product.

Perhaps at this stage I should clarify what is meant by metabolic ammonia. This is ammonia produced in the fish's body as a result of breaking down amino acids for use as an energy source. This involves a process called deamination, taking place in the liver, during which the amine chemical is removed from amino acids. All animals produce metabolic ammonia but, as it is such a toxic compound, virtually all other animals, including humans, immediately convert it into a less toxic substance before it is excreted. Humans convert ammonia into urea, which is passed out of the body as urine.

Fish 'don't bother' to convert ammonia, they simply excrete it continuously from their gills into the surrounding water. In a natural environment the immediate dilution by thousands of gallons of water would render it harmless. However, no-one told Mother Nature about koi-keepers and their ponds so it's not quite the case in koi ponds, where ammonia can build up to a dangerous level because of the large number of fish in a small volume of water.

So we can see that pollution in a pond has at least three sources: metabolic ammonia (the quantity being determined by the number of fish and how active they are), inorganic compounds (such as phosphate) and various dissolved and particulate organic carbon compounds. Understanding the chemistry is not important here, it is enough simply to realise that these processes are taking place and if we want to maintain optimal water quality we need to consider the removal or neutralisation of all of these pollutants, not just ammonia.


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Postby Fred Goodall » Thu Sep 14, 2006 6:02 am

Organic Pollution
a common koi pond pollutant
Written by: Frank Prince-Iles
FishDoc


Koi ponds and organic wastes

If we ignore the fate of metabolic ammonia for a moment and concentrate on the organic matter, it will be easier to understand how these pollutants can affect the long term health of our koi if we have a clear idea of what happens to them in the pond. To do this we have to understand a little about fungi and bacteria, Nature's rubbish disposers. We tend to think of microorganisms such as fungi and bacteria as being disease-causing agents but, in truth, relatively few of them are pathogenic (disease-causing). Indeed, without the continued decomposing actions of these micro­organisms, the planet Earth would by now be covered with a layer of sewage several miles deep!

Heterotrophs & autotrophs

We can divide microorganisms into two basic types. First are those that need a supply of ready-made organic carbon to provide them with energy and the building blocks for other molecules such as amino acids. Organisms that require organic carbon, which includes humans, are called heterotrophs. Some micro­organisms and all plants can extract carbon from inorganic carbon dioxide. These are called autotrophs. This distinction between the modes of nutrition of various micro-organisms is important because an environment that encourages one type of micro-organism -for example, autotrophic nitrifying bacteria - may be unsuitable for heterotrophs such as Aeromonas bacteria and vice versa.

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Mineralisation

Solid organic matter, such as fish waste, is broken down in a series of steps by heterotrophs into progressively simpler compounds. Most of the initial decomposition is done by fungi, which use enzymes to break down the larger, complex organic molecules into simpler, soluble nutrients that the fungi can reabsorb. This process of using enzymes to break organic matter into smaller molecules is carried out by all micro­organisms, each time producing a different organic compound, until finally the original matter is converted into basic non-organic components such as nitrogen, potassium and phosphorous. This whole process of converting organic materials into non-organic matter is called mineralisation.

The ultimate fate of these inorganic elements is to be converted back into organic matter, usually by plants and other autotrophic organisms, and the whole cycle starts all over again. While the biology of decomposition may be mildly interesting, the important point is that it is carried out in many stages, requiring large numbers of different species of microorganisms that produce a wide range of different organic compounds in the process.


Ideally there would be little in the way of 'free' organic compounds

In an ideal situation, the rate of mineralisation would be matched by the production of organic materials and there would be little in the way of free organic carbon compounds (or other pollutants) in the surrounding water. But, unless our filtration and pond husbandry is designed to remove these organics at the same rate as they are produced, there will be a small but often significant level of free organics. As a direct consequence of this mild form of pollution a rising DOC level - the following problems may be encountered:

excessive algal growth leading to either green water from free-floating algae or dreaded blanketweed. These aquatic plants (or weeds, depending on your point of view) will thrive on the non-organic products of minerali­sation, such as nitrate and phosphates


high levels of heterotrophic bacteria, which is as good a time as any to point out that many of the common pathogenic bacteria (such as Aeromonas and Pseudomonas) are actually opportunistic heterotrophs. This means they are usually present in the pond feeding as decomposer organisms but can, if their numbers are high enough or conditions are right, turn their attention to the fish. The classic example is when fish are stressed and the normal defence systems are weakened. While it would be impossible and undesirable to eliminate these opportunists entirely, I think that most people would agree there is no point in encouraging them either!


  • high levels of ectoparasites such as flukes and protozoa. These parasites thrive in waters with a high organic load and, because some of them feed on bacteria, the presence of high levels of bacteria will encourage an increase in parasites
  • there is some evidence that water with a high organic load can depress the immune system. However, this may be a result of the increase in parasite and bacteria levels, or it could be that some organic compounds produced during mineralisation are stressful to fish.
  • organic matter consumes a lot of oxygen while it is being oxidised or decomposed which could, under certain conditions, be detrimental to the well-being of the fish
  • high organic loads are also implicated in environmental gill disease, a serious and relatively common koi health problem
  • raised levels of organic compounds can make the water look mucky, often resulting in foam being produced at water returns, e.g. filter outlets and waterfalls.

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Article placed here with permission from the author,
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Postby Fred Goodall » Thu Sep 14, 2006 7:08 am

Solid Wastes
a major source of pollution
Written by: Frank Prince-Iles
FishDoc


Dealing with pollution

In an active koi pond we have two types of pollution; dissolved and solid. If we could remove the solid wastes from the system before they had chance to dissolve and pollute the water we would have better water quality, less dissolved pollutants and fewer health problems.

Solid wastes and koi health

If we summarize the situation so far, we can see that if we are to maintain the status quo as far as water quality is concerned, we need to remove the pollutants at approximately the same rate as they are produced. We have also seen that the pollution is basically in three forms: dissolved compounds, such as ammonia, inorganic pollutants such as phosphate and DOC, and solid particulate waste.

Unseen but there!

Solid waste will ultimately be broken down by decomposer microorganisms into a wide range of dissolved pollutants, adding to those already in the system. It makes no difference where in the system these solids decompose - the end result will always be the same, that is, further pollution. This is an important point as many koi-keepers think that once solid waste is out of sight (in the filter) it is no longer a problem.

With the rapid throughput of most filters, the dissolved pollutants produced as these solids break down are quickly pumped back into the pond. What we really need is two filtration systems - one that enables us to remove the solid wastes from the system before it has time to pollute the water and the other to deal with the dissolved pollutants. After all, if we could remove waste solids from the system, we would prevent most of the sources of pollution.

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So perhaps, we should look on our filter as a system of two parts, one part dedicated to removing solid waste matter from the system (not just the pond) and the other removing dissolved pollutants.

Remember, too, that the pond is also part of the filtration system, as a significant amount of mineralisation and nitrification will take place on surfaces within the pond. The pond will also act as a settlement chamber for solid wastes, which will need regular removal to prevent them polluting the water. It is my opinion that the regular removal of accumulating solid wastes presents the koi-keeper with his or her biggest challenge and a great many problems will be avoided if this can be done effectively. For regular disposal of solid waste there are essentially two practical options: settlement and entrapment.

Settlement areas or chambers have to be fed by gravity-flow systems, ideally via a bottom drain. This way, solids are moved gently to a collection area, ready to be flushed out of the system. The traditional method required a large settlement chamber and these can be very effective provided that the chamber is large enough and the flow rate is low enough to give lighter solids time to settle. The retention time for water in the chamber is important. The retention time is simply the filter volume divided by the flow rate, thus:

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It has to be said that, in the above example, the short retention time of 6 minutes is unlikely to be satisfactory, whereas a chamber volume or capacity of 300 gallons would give a retention time of 9 minutes, allowing much better settlement.

Having collected solid wastes, it is important that they are flushed out of the system regularly, before they have time to decompose. During summer this could be as often as twice a day, and obviously less frequently in winter. This means that the settlement chamber will need to have a drain to facilitate easy flushing to waste.

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A better way of collecting solids

An increasingly popular settlement option nowadays is the cylindrical chamber with conical base that promotes a slow swirl of water. The cylindro-conical shape encourages settlement of wastes into the bottom of the cone, where they collect together, making removal to waste simple and efficient. Again, retention time is important and a slow throughput will be more successful than a chamber that resembles a vigorous whirlpool. One should be guided by the manufacturer as to the right size for your systems but, if in doubt, err on the large side.

When set up correctly these chambers work well and are probably superior in practice to rectangular settlement chambers of similar dimensions. Both types need additional cleaning if solids are not readily flushed to waste since solids may cling to the sides and there may be areas of poor water flow or 'dead spots'.

Last but not least is the pond itself. Even the best-designed pond seems to have dead spots, where mulm and fish waste collect. Any waste that isn't drawn through the bottom drain will need to be removed from the pond before it pollutes the water and there are several options, depending on the pond design. It could be gently pushed towards the drain with a soft broom; the waste could be carefully removed with a fine net; or the pond could be vacuumed. In most cases it is probably a question of combining all three actions, with most ponds benefiting from a regular vacuum during summer.


Entrapment

The most common entrapment systems consist of filter brushes or sheets of foam. We should consider what type of bacteria likely to be attached to the brushes or foam, which are heavily loaded with trapped solids? Common sense tells us that it is going to be heterotrophic bacteria, and do we really want to encourage high levels of these bacteria in any part of the system? You will recall that many heterotrophs are also opportunistic pathogens and are quite happy to lunch on our koi - given half a chance! Obviously the answer is No. So the important thing with entrapment is that the entrapment media are kept clean, otherwise they themselves become a source of pond pollution.

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[size=14]This reminds me of a case last year, when several fish in a quarantine tank became ill. The tank was spotless yet the fish had parasites and were suffering from the onset of bacterial problems. Further investigation showed that while the tank was exceptionally clean, the filter wasn't. When we took the media out for cleaning, the smell was overpowering. The media were covered in a yellow slime which, of course, was all solid fish waste slowly rotting down. In this case, the filters were slowly poisoning the fish. Following a good clean-out of the filters, the fish were soon back to normal

By using a combination of settlement and entrapment it is possible to remove a lot of solids from your pond before they rot down provided, of course, that these areas are cleaned regularly. If we are successful in removing solids from the pond before they pollute the water we are part way to achieving the ideal of unpolluted water.

Article placed here with permission from the author,
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Postby Fred Goodall » Thu Sep 14, 2006 9:32 am

Biological filtration and filter media
what goes on inside the filter?
Written by: Frank Prince-Iles
FishDoc


At the heart of the koi pond

At the heart of a koi pond is the filtration system. To encourage a vigorous growth of nitrifying bacteria in the filter we need to keep the filter media fairly clean, which is often easier said than done! Efficient biological filtration also depends on the media of choice having an adequate specific surface area (SSA), adequate voiding and water retention time.

Biological filtration

For good filtration and good water quality very little solid waste should enter the 'biological' section of the filter. Although, for simplification, I make a clear distinction between the settlement / entrapment areas of the filtration system and the biological section, it is worth remembering that the two are linked, so some nitrification occurs in the settlement area, and some settlement and mineralisation occurs in the biological section.

However, the aim is to ensure that conditions in the biological section are such that vigorous growth of important nitrifying bacteria is encouraged, while conditions that would promote growth of heterotrophic bacteria are avoided. Most likely, heterotrophic bacteria will predominate at the start of the biological section, where the smaller particles of remaining solid organic matter become trapped. So, in general, when we refer to the biological section we mean the area where dissolved pollutants and particulate matter are converted by microbes into less harmful substances.

To re-state the point made on other pages; the more effective the settlement area of the filter at removing solid waste, the lighter the load on the following biological section - provided, of course, that trapped waste is removed before it decomposes.


More than nitrification

I made the point that a filter does more than simply convert ammonia to nitrate. Although ammonia is often the most toxic pollutant in a pond, we should not forget that normal biological, metabolic and chemical activity produces a wide range of pollutants. Even with the most effective settlement and entrapment system many organic wastes, particularly fish faeces, will start to decompose, producing various dissolved organic carbon compounds (DOC). We expect the filtration system to deal with these pollutants also, not just with metabolic ammonia.

The diverse range of biochemical processes occurring in the filter are due to many species of bacteria, fungi, protozoa and various worms and possibly snails present- not just nitrifying bacteria. However, our pond husbandry routine should be designed to maintain conditions which encourage nitrifying bacteria, and this is achieved by:

regular maintenance to keep the biological area clean and free of mulm,
reducing the level of dissolved organic compounds by effective settlement/entrapment, together with regular cleaning of the settlement area.

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Filtration how & why

Understanding the fundamentals of biological filtration is helpful in diagnosing how common fish health problems occur. However, before we look closer at what goes on in the biological section of a filter, it is worth considering other important aspects such as filter size, efficiency and overall design. There are many rough guides to determine required filter size and flow-rate.

One such suggests that the filter surface area should be approximately one tenth that of the pond; another, that there should be a pond turnover rate of once every 2 to 3 hours. While a rough guide is helpful, the huge variety of different filter media and designs necessitates a more specific approach. When it comes to water quality there are three main reasons for why things go wrong:

  • indiscriminate use of chemical treatments, which can damage filter function
  • poor maintenance of pond or filter
  • poor filter design

It is the latter problem that we need to address first, by considering exactly what we expect from a filter and what are the basic parameter values needed for its optimal performance.

Probably one of the most discussed subjects in the hobby of koi-keeping is the merits of various filter media - and there are sometimes quite incredible claims made for the various types. But the first thing to be clear about is that bacteria will thrive on almost any surface and the particular choice of medium has very little influence on their growth.


How much surface area?

Nearly all types of filtration system rely on attached-growth processes in which a bacterial slime layer or biofilm -comprising bacteria, algae and often larger invertebrates - forms on the media. Microorganisms present in the biofilm 'feed' from water that flows past. So, as a first approximation, the amount of biological activity will be determined by the amount of available surface area for bacterial colonisation. However, in practice this available specific surface area (SSA), as it's called, is rarely a limiting factor since most filtration systems are large.

Obviously, if you had just a square piece of material measuring say I m x 1 m this would give a total area of two square metres (with both sides being available for bacterial colonisation and assuming almost zero thickness). Even this small area could support millions of microorganisms, attached in a slimy biofilm. But typical filter media have a far greater SSA. For instance, gravel has an available surface area of about 100 to 200 square metres per cubic metre (100-200 m2/m3). And other, more specialist media can have significantly more surface area; for example:

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So we can see that even a small amount of filter medium provides a potentially vast SSA for bacterial colonisation. Each square metre of biologically active surface can metabolise nearly one gram of ammonia per day, dependent on temperature, and given that most ponds will usually be producing fewer than 10g of ammonia per day, the amount of SSA required is really small - and not a lot of people know that, as Michael Caine might say. If we based filter sizing on the basis of SSA alone, filters could be incredibly small -perhaps the size of a shoebox! However, there are other factors to consider ....

Is void important?

The void size or empty space within a filter medium is important in determining the right filter size and efficiency. Void size is a measure of how much of the medium consists of empty space. If we consider sand, for instance, each particle has a large surface area in relation to its volume and the total SSA per cubic metre of sand works out at thousands of square metres. Despite this enormous SSA, sand would make a poor filter medium because the small particle size would soon lead to blockages and subsequent 'tracking' as water found the 'easy routes' round the medium. And, of course, because of the dense packing, any flow through the sand would be very slow. So, despite its massive surface area, once compacted and blocked the amount of surface area exposed and the volume of water that could be treated per hour, would actually be quite small.

There is another important disadvantage of a medium like sand - retention time, or the amount of time the water spends in contact with the biofilm. It is obvious that if we wish to avoid blockages and tracking, some void space in the filter medium or media is desirable. If we consider a medium such as gravel, although its larger size yields less SSA it is less prone to tracking and blocking. And specialist media such as filter matting, plastic or sintered glass, have both a large SSA and a generous void space. In fact, many of them are more than 90% void or empty space! This makes tracking and blockage almost impossible.


What about cleaning?

Another important consideration - which becomes more important the longer you keep koi!- is ease of cleaning. In the early days of the hobby, part of the novelty lies in spending weekends cleaning and vacuuming. But after a while, strangely, it seems that there are more pleasurable ways to spend a sunny Sunday. And with gravel and other granular media, it really isn't much fun trying to clean several tons of the stuff! Compared to gravel, cleaning light-weight media is a delight. Obviously, regular maintenance is somewhat easier if each filter chamber has its own bottom drain but, even so, ease of maintenance has to be a major consideration in the choice of filter medium.
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Article placed here with permission from the author,
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Postby Fred Goodall » Fri Sep 15, 2006 2:32 am

Filter flow rates and retention times
calculating filter requirements
Written by: Frank Prince-Iles
FishDoc



Filter requirements and calculations

Calculating ideal flow rates and filter retention times for koi pond filtration systems can sometimes be contradictory and for the average koi keeper with modest stocking levels and a reasonable filter there shouldn't be a problem. But there are a lot of over-stocked ponds with pretty poor filtration systems - find out why.

Let's get complicated

When it comes to filter sizing, life can get complex. As I've said, if we only wanted simple nitrification, it is probable that filter sizes would be small. However, as well as nitrification koi-keepers want:

  • gin-clear' water
  • breakdown & removal of DOC,
  • conditions which discourage filamentous algae (blanketweed)
  • generally optimal water conditions for fish.

In trying to meet these wide-ranging demands filters are built far larger than they would be if based on the required SSA of filter media alone.


The longer the better

Broadly speaking, the effectiveness of biological filtration is improved the longer the 'polluted' water is held in the filter - i.e. the longer the retention time. The most time-consuming process in filtration is the breakdown of dissolved organic carbon compounds into simple inorganic compounds. These compounds are ultimately incorporated back into living organisms. This complex chain of processes is not instantaneous and will, even under ideal circumstances, take some time. If insufficient filtration time is available, intermediate products will be pumped out of the filter back into the pond. This is clearly undesirable and rather defeats the object of having a filtration system. Indeed, this may well be the reason why excessive algal growth occurs in some ponds, with the filter merely producing an endless supply of plant nutrients!

So for how long should water be retained in the biological section? This depends on how polluted the water is in the first place. Certainly, industrial water treatment plants - which handle much higher levels of pollution from sewage etc. - would retain water in the plant for many hours before it was deemed sufficiently clean to return to the nearest water-course. Given that pond water is likely to be only mildly polluted, a retention time of ten minutes, possibly longer, will usually suffice.


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So how do you calculate the retention time of your filter? This is determined by the flow rate and the volume of water in the filter. If water output from the filter is 2,000 gallons/hour and the filter contains 500 gallons (when full of media) of water then:

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In the above, the filter capacity represents the amount of water in the filter - not the physical size of the filter, which will be greater. The retention time or the size of the filter will depend to a very large extend on the type of filtration medium used. A solid medium with low void space such as gravel will occupy much more filter space than large-pored, lightly packed media and therefore leads to a lower retention time.

More calculations! Using our same example of a 500 gallon filter. If we now nearly fill it with gravel, the volume of water it will hold will be reduced substantially - maybe to as little as 150 to 200 gallons. Using the above example, the retention time of such a filter would now become;

200/2000 = 0.1 hours (6 minutes) or less


This compares the original estimate of a retention time of 15 minutes

In comparison, if the same filter was filled instead with matting or plastic, there would be hardly any displacement and the filter will probably still hold in excess of 450 gallons, giving a retention time over double that of gravel. So a filter with a dense, low-void medium, such as gravel, will need to be substantially larger than one based on light-weight media, in order to achieve the same retention time, which explains why koi filters were traditionally so large.

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The quicker the better?

Just when everything starts to make sense, along comes a complication. While a longer filter retention time will produce better water quality we also have to consider pond turnover times. Why? Because polluted water is produced in the pond and, if there was a slow turnover at the filter, it would take longer for pond water to get processed by the filter.

To make sense of pond turnover rates it is helpful to return to the original analogy of koi being sewage-making machines: expensive food in one end and sewage out the other. Our seemingly impossible aim should be to remove this pollution as fast as it is produced. If we can manage that then we would have perfect water conditions most of the time.

When we are considering pollution the primary concern is not so much the volume of water, but rather the number of fish and the amount of food we feed - because this is what determines both the amount of metabolic ammonia and the quantity and quality of solid waste. There are several ways to calculate ammonia production in a koi pond. A rough and ready estimate can be made based on the amount of food fed each day.

Each kilogram of fish food will result, on average, in 37 grams of ammonia being produced, together with copious faeces. And there is other organic waste, such as that from decomposing algae and micro­organisms. The important point is that as the stocking, and thereby feeding level, is increased the water will have to be treated at an ever quicker rate if water quality is to be maintained.

  • If, for instance, we had a pond of 20,000 litres (4,500 gallons) and the fish were fed 200 grams of food per day, this would produce approximately 7.5 grams (7,500mg) of ammonia per day, an average of say 300 mg per hour. (In reality the ammonia level would fluctuate throughout the day, being highest shortly after feeding).
  • At this feeding rate, if no ammonia was removed, at the end of a day the ammonia content of the water would be 24 x 300 mg ammonia = 7 200 mg in 20,000 litres of pond water, giving an ammonia concentration of 0.37 mg/litre, which is too high.
  • Conversely, if it was possible to remove the ammonia at the same rate as it is produced - namely, 300 mg per hour - the steady state ammonia level would be zero. To remove ammonia this quickly we would have to pass the entire contents of the pond through the filter every hour, giving a flow-rate of 20,000 litre/hour, otherwise there will always be some residual ammonia present.
  • Deep breath! - If, instead of a flow-rate of 20,000 litre/hour, we had a flow rate of the pond volume every two hours - or half the pond volume every hour (same thing), an oversimplified calculation would give:
  • 300 mg ammonia / 20 000 litres (pond volume) x 10000 (flow rate litre/hour) = 150 mg ammonia removed per hour, leaving 150mg in the pond, or a steady state of >0.01 mg / litre. (This makes the simplifying assumption that there is no nitrification occurring in the pond.)

We can see the effects of increased stocking and / or feeding levels if we take an exaggerated example in which we treble the feeding rate to 600 mgs of food per day

  • 600 grams of food per day would produce around 900 mg ammonia per hour. With the same flow rate we would remove 900 mg ammonia / 20,000 litres (pond volume) x 10 000 (flow rate litres /hour) = 450 mg ammonia removed per hour leaving 450 mg in the pond, or a steady state of 0.02 mg /litre, an increasingly unacceptable level.

Clearly the only way to balance the increased ammonia production would be to 'feed' the ammonia to the filter at an ever increasing rate.

I should stress that the above examples are an over-simplification of what actually happens since other factors, such as nitrification in the pond rather than in the filter, also have to be taken into account. Indeed, where the flow rates or filter retention times are less than optimum, an increasing proportion of the ammonia nitrification will take place in the pond rather than the filter. While it is not immediately important where in the system nitrification takes place – it does help to explain why some ponds are more upset as a consequence of disease treatments than others. However, if flow-rates are kept constant and the feeding rate is increased, there will be a steady increase in the background level of ammonia.

It is not necessary to get any further involved in calculations, the important point is that when high feeding/stocking levels are involved, the flow-rate is an important factor in determining the ammonia removal rate.


Adequate flow-rate

So what is an adequate flow­rate? As explained, it depends on the feeding rate. The most commonly quoted advice is: turn over the volume of the pond between 8 and 12 times a day. But it is important to remember that this is a rule of thumb and flow-rates may well need to be increased for higher feeding and/or stocking rates. Certainly, koi-keepers who feed in excess of 0.25 kg of food per day may have to consider increasing flow rates, particularly if there is a periodic ammonia problem. Conversely, it may be possible to have a slower rate when feeding levels drop, as they do in winter.

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Filter size

Taking retention times and flow rates into consideration, when it comes to choosing the right filter size, there are two important but conflicting factors:

  • the right filter retention time, which ensures all the required biological activity occurs,
  • brisk water flow to prevent a high pond ammonia level.

If we decide that a flow-rate of say 10,000 litres per hour (2,200 gal/hour) and a filter retention time of 10 minutes are required then the volume of water in contact with the filter media at any time will need to be;

10,000/60 (minutes) x 10 (minutes retention time) = 1666 litres or 1.6m3.


This means that the filter should be able to hold 1.6 m3 of water after it is filled with media. This is in addition to settlement and spaces below the media trays. The required size of filter will then depend on the media used. Using a high-void medium, such as matting or plastic, we would need a little over 1.6 m3 of media to compensate for the small amount of water displacement, whereas, with a solid medium, we might need at least 3m3 to ensure the same volume of water in contact with the media after displacement.

Although this may seem complex, these are the factors which need to be considered to avoid some of the most common filtration problems which often beset koi-keepers - namely, fluctuating water quality, high levels of opportunistic micro-organisms and excessive algal growth.

The size of a filtration system becomes more critical as stocking level, and thereby feeding rates, increase. Even when no new fish are added, the continued growth of the existing pond occupants will gradually increase the demand on filter performance.


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Other considerations

After all this discussion on retention times, flow-rates and filter media, it is worth considering some other salient aspects of filter design. Most purpose-made, retail filter units are practical and well designed but I have to say that some are pretty poor, for the following reasons.

  • Apart from overall filter size, which we have already discussed, another important aspect is shape and water transfer between the chambers. There is little point in having several cubic metres of expensive filter medium if it is not properly utilised. The design of a filter system should be such that water passes evenly through all of the media and not just at one end or through the centre.
  • Ideally, transfer ports should be the full width of the chamber; otherwise there will be a tendency to create a narrow channel of water flowing into the next chamber, leading to 'dead' spots within the chamber. Square chambers are not the most efficient, giving little water flow in the comers. This drawback has been overcome in some cases by the used of curved or circular chambers, giving a more effective 'working' area within the chamber. With careful design it is also possible to create a swirling motion as water is transferred from one chamber to the next. This helps avoid dead spots, giving an even flow through the media and, to a lesser degree, will help settle some of the finer solids.
  • Just as important in filter design is ease and efficiency of maintenance. The best design is for each filter chamber to have a bottom-drain for easy cleaning, and the base should be benched or sloped towards the drain. Regular flushing of the bottom drain in each chamber will help clear away fine solids; and periodic cleaning of chambers by emptying them and flushing the media with pond water will prevent a build-up of unwanted mulm and other organic debris.

So there we have it - the basic requirements for good filter design and performance. All filters will comply with these guide lines to a great extent. At the end of the day the proof of the pudding is in the eating and if your filters provide consistent good water quality (to the five point standard), nice clear water and you don't have to spend half the week end cleaning it - then you probably have things about right. If however, you are constantly having niggling water quality or fish health problems..................

Article placed here with permission from the author,
Frank Prince-Iles
FishDoc
http://www.fishdoc.co.uk/
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