Ask
pond constructors or koi keepers to state the 'must have' things on a koi pond and one item will invariably appear on most
lists - the discharge box. There appears to be a fallacy in the koi industry that a pond MUST have a discharge box. Few have
questioned this. As a result many discharge boxes are not successful because the reasons why a discharge box can be used on
a pond are rarely questioned.
There are some fine examples of ponds
with absolutely no discharge boxes that are working far better than the many ponds with discharge boxes. Observation and common
sense will demonstrate exactly why.
I do not advocate that ponds
must have or ponds must not have a discharge boxes. Individual circumstances dictate the course of action. Success for the
koi keeper or pond keeper, no matter which route he or she chooses, is the object of the exercise. If you do not have a discharge
box then there are certain things that must be done in the pond design. If you do have a discharge box or intend to attach
or make one on the new pond then there are certain design features needed to make this accessory successful.
What is the function of a discharge box?
We know what the function
of a biological filter is. We know what the function of a pump is. We know the function of a mechanical filter. But, what
IS the function of a discharge box on a koi pond? Understand the function and the reason then the design will be successful.
It is safe to say, and generally
accepted, that the discharge box has two functions i) to settle out some of the solids before the pump or filter (depending
on the push or pull design of your system) and ii) to collect water from various points in the pond. If the only function
of a discharge box was to collect water from the various bottom drains and skimmer, then the system would work far better
if the pipes were connected directly to the pump. A few valves to control the flow from various collection points and you
could fine tune the flow through the system. Q,E.D. The pump would then have to be checked regularly for leaves and algae
in the basket.
The addition of a discharge
box onto a pond and filter system must be considered the first line of defence before the pumps and/ or filter media. The
time between opening pumps and/or cleaning filter chambers is then extended to some degree, but can never be eliminated altogether.
This is the theory behind discharge boxes. In practice…………?
The discharge box is, in many cases, literally a box placed next to the pond. This can be a brick made design
or a plastic barrel or container. Water is gravity fed via pipes to the discharge box from the bottom drains and possibly
the surface skimmer as well. The pump then sucks from the discharge box and pushes the water through the filters. Alternatively
the pump is situated after the filters and sucks through all the chambers. The first chamber acting as the "discharge box"
or settlement chamber. Simple, logical - yes and no!?
Gravity fed pipes
Gravity fed pipe work will only allow a limited amount of
water through it. Pipe-work under pressure (water being pushed or sucked) has much more water pouring through it. It is astounding
the number of ponds that have been constructed without the slightest regard for differences in flow rate capabilities between
gravity fed pipes and pressure fed pipes. There are many frustrating ponds with a 50 mm bottom drain (50 mm-maximum capacity
of about 3,600 litres per hour when gravity fed) leading to a discharge box. The water is sucked out of the discharge box
with a swimming pool pump (capacity of about 16,000 litres per hour for the smallest pump). The result is the discharge box
runs dry and the pump burns out. The easy fix solution: throttle the pump back. This negates the flow rate calculations for
the pond and filter. The pond keeper has paid for the pump to deliver a certain amount of water. He now has to contend with
the same monthly running cost, but a greatly decreased water flow.
A 50 mm bottom drain would
have to be linked directly to the pump in order to be effective.
Swimming pool type surface
skimmers simply do not work very well when linked to the discharge box instead of the pump. Water takes the path of least
resistance. This will be the large bottom drains. Occasionally 110 mm piping is used as a 'surface skimmer’. This system
is incorrectly referred to as a 'skimmer: We discuss surface skimmers and their application in another Filter Facts article.
A clear distinction has to be made between a true surface skimmer and placing a pipe (often a 110 mm pipe) at the surface
that draws top water to a discharge box.
Sizing discharge boxes
The size of the discharge
box is critical to it's success. We calculate the size of the filter chambers. the size of the pump, the size of the pond-why
not the discharge box? When the discharge box is small the water moves rapidly through this mini chamber to the pump. Fast
movement of water does not allow the settlement of solids. If the discharge box is small why bother having one in the first
place? What purpose does it serve? Enlarging the discharge box and placing some form of screening, such as brushes, between
the inlet pipes from the bottom drains and the suction will greatly enhance the efficiency of the design. A fair amount of
solids will then be trapped.
Therefore, the first design
consideration of a discharge box should be to slow the water down. The only way this can be achieved is by increasing the
size of the discharge box. Longer, broader and relatively shallow discharge boxes appear to work better. Deep, narrow discharge
boxes have the water welling up from the bottom swirling the solids around without much settlement. Placing brushes in the
discharge box greatly improves settlement and trapping of solids.
In many discharge box things
work well in the beginning. However, after a season or two the trapped solids build up to the point that they simply by-pass
the discharge box and flow through to the filters or pump.
The owner is confronted
by a hole in the ground next to his pond full of smelly decomposing solids. A great number of discharge boxes have been built
and are still being built that do not have any capacity to drain what-so-ever! In this case solids are being moved from one
hole in the ground to another with out any regard to what happens next. Pond constructor’s imagination often stops at
this point. The only way a discharge box (or filter chambers for that matter) can be cleaned properly is when they can be
completely drained.
The discharge box is the
front line of defence which is designed to collect solids before the pump or other chambers. The second design consideration
of discharge box design is therefore, ease of maintenance. It is critical that the discharge box have some method of easily
flushing the accumulated solids to waste. It makes no logical sense what-so-ever to collect solid waste in this area and then
make it impossible to flush or clean! The owner will be forced to purchase another pump that can pump sludge out or he will
have to physically drain the discharge box by hand from time to time.
An outlet directly to the
storm water drain or garden should be installed in the discharge box. Valves/ stand pipes to close off the pond and the filter
boxes or chambers are vitally necessary. The discharge box should be totally isolated, completely drained and thoroughly flushed
when necessary and as often as is necessary.
In order to be effective,
discharge boxes must have various mechanisms such as valves or stand pipes to totally isolate them from the pond and other
chambers. This is the area of the filter system which will require the most maintenance. Discharge boxes that cannot be isolated
cannot be cleaned as any attempt to clean the discharge box will result in water gushing through from the pond or back from
the next chamber.
If basic design ideas of
settlement and collection are executed properly the discharge box will then do its job of removing some solids from the pond
water before the pump or filter media as well as collecting the water from various points in the pond.
Placement of bottom drains
An interesting
observation regarding the placement of bottom drains and discharge boxes. You will notice that when a pond has many - bottom
drains going to discharge box sediment collects in the pipe work under the pond. This can be a breeding ground for parasites
and a site for the sediment of become stagnant thus producing hydrogen sulphide. Hydrogen sulphide is toxic to koi in extremely
low levels. Be careful of any place in the pond that may become anaerobic.
Bottom drains that do accumulate
sediment will need a good flushing from time to time. Simply block of all but one bottom drain in the discharge box. Turn
on the pump so that the water is drawn quickly through the single stand pipe. Flush each drain in turn. Another method is
to block off all bottom drains and drain the discharge box. Then lift a stand pipe to let the water gush through to the discharge
box. Drain the discharge box again and do the next bottom drain.
There are some interesting observations
regarding the placement of bottom drains on a koi pond - but that is for another Filter Facts article.
Article
by
Chris Neaves
