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Filter flow rates and retention times

calculating filter requirements

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:

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. 

the more polluted the water is, the longer it needs to be retained in the filter. Most koi ponds will require a retention time of at least a few minutes

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:

filter retention time = filter size/pump rate,

so, in our example:

retention time = 500 (litres) / 2000 (litres / hour flow rate) = 0.25 hours (which is 15 minutes).

so a given sample of water will take 15 minutes to pass through the filter and back to the pond

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;

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.

the retention time and therefore the filter size will depend on the filter media used. Cheaper, dense media such as gravel will need larger filters to achieve the same efficiency as lightweight media

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.

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.

the pond flow rate is dependent on the total ammonia produced within the system, With higher stocking densities there has to be a corresponding increase in flow rate. In an average koi pond, a flow rate of 1/2 to 1/3 of pond volume per hour should suffice.

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:

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.

Ideally, what we want is a fairly brisk flow-rate, turning over the pond volume every 1 to 3 hours (depending on feeding and stocking rate) but at the same time a slow, almost imperceptible flow through the filter, allowing sufficient time for the various important biological processes to occur. Water passing through the filter should be in contact with the filter media, and therefore the biofilm, for at least ten minutes, possible longer.

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.

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..................