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07 May '15

Pond Design Conderations

Posted by Mark Pearson
Pond Design Considerations
  • Pond Location
  • Shade
  • Pond Shape
The location of a pond on your property can make a big difference in long term management. Try to keep deciduous trees away from the pond. Although they may look nice and might shade the pond, the continuous supply of fallen leaves is a nuisance. Leaves will plug up drains quickly no matter how good your filter design is. The increased organic load from decaying leaves spoils water quality and puts a burden on your filter system. Try evergreens or palm trees around the pond for aesthetics and some shade.

Shade is critical for a low maintenance pond. Keep the sunlight out and your algae problems will be minimal. Use 70% to 80% shade cloth or wooden lattice type overhead structures for best results. Daily temperature fluctuations will be better controlled with good shade. Koi do not appreciate direct sun all day and it is possible to sunburn your koi. They will seek the shady spots in the pond but do like to bask in sunshine at times. Moderation is best. Morning sun or late afternoon sun is OK. Overhead shade also deters bird predation since the pond is less visible from the sky with all those colorful and yummy morsels swimming so innocently.

The steepness of the pond walls is important for a number of reasons. Vertical walls do not accumulate debris. Vertical walls expose less surface area to the sun so algae growth is inhibited. Predators cannot walk into a pond with deep vertical sides. Three foot deep should be considered a good minimum. A deep pond will maintain a more stable temperature.

The Shape of the pond itself should be designed with self-cleaning in mind. Very free form ponds will need lots of jets to debris from accumulating in dead spots. Simple pond shapes work best. Bottom drains should be evenly spaced and the jets action will create a huge toilet bowl effect in essence. Mid water drains and surface skimmers both increase solids removal and lighten the load on the bottom drains.

Filter systems

  • Pump First
  • Filter First
  • Settling Tanks
  • Mechanical Filters
  • Biological Filters
Koi ponds can accumulate and produce a lot of debris. Leaves, food, fish waste and algae particles all contribute to the organic waste load in the pond. These large particles create the bulk of our maintenance problems. This solid waste plugs up the drain lines, pumps and filters; thereby effectively choking the water flow through the system. Furthermore, this solid waste decomposes into dissolved waste which contributes to higher ammonia and nitrite levels. A pond with a lot of solid waste will tend to have algae problems and koi health problems.

Pump First Scenario (Poor pump efficiency)

This design places the pump between the pond and the filter. The water is pumped to the filter. All the debris within the pond must pass through the pump first. Obviously, this can be troublesome. First we choke off the pond by throttling the flow of debris through a small inlet at the pump. The leaf trap at the pump can plug very quickly in some cases. Secondly, the pump acts like a blender so any small solids which do pass through are turned into juice. Efficient settling of solids after it is turned to juice is improbable. This juice is more difficult to filter than a large solid. Another consideration is the function of circulation jets within the pond. Bypassing the filter to induce strong jet action will simply inject organic juice back into the pond. This juice only creates more algae problems. The pump first scenario may require a larger pump motor to overcome many of these problems. (Hint : electricity $).

The pump first filter system is fairly easy to install and many ponds are plumbed this way and do work. Ponds with sand filters or bubble bead filters are usually plumbed this way. Draining the pond may be very easy; simply open a valve and you can pump the water out to sewer. The filter will in most cases be above grade and drainage or backwashing may be easy. The overall characteristics of your pond must be considered. The number of fish, the amount of sunshine, the shape and size of the pond, even the type of soil and yard topography, all play an important role in determining the efficiency of a pump first system.

Filter First Scenario (Better pump efficiency)

The filter first scenario is a better design for most koi ponds and water gardens. This concept in design utilizes settling or trapping of solids before they reach the pump. Solids are transferred through large diameter drain pipes from the pond by gravity flow to the filter. The filters are built at the same water level as the pond water level. Water fills the filter by seeking it's own level. The pump pulls water through the filter. The large diameter drain pipes should be 3 inches diameter minimum with 4 inches diameter being preferable. Large diameter pipes (3" to 4") do not restrict the flow of water and debris.

The water is simply flowing by gravity to the filter and if the pipes plug up with debris, the pump can drain the filter faster than the pipes can fill the filter. A good recommendation for the number of drain pipes is: 2 bottom drains (3" to 4"), 1 midwater drain (3" to 4"), and 1 surface skimmer drain (2" to 3"). The midwater drain is a back up in case heavy debris plugs off the flow to the bottom drains. The surface skimmer will collect floating debris and oils. Of course, pump horsepower and pond size also dictate the number and diameter of pond drain lines.

Settling Tanks

Heavy solids, flowing by gravity, can settle out from the water flow in a large settling tank. The quantity of solids settling is dependent on the flow rate and contact time within the settling tank. Large settling tanks with slow flow rates work best. Centrifugal force cone tanks can settle more solids in a smaller space if flow rate is correct. I must mention at this point that the drain pipes themselves can act as settling area if the flow rate is slow enough. Settling tanks and the drain pipes must be flushed frequently to rid the system of heavy waste. Drain pipes can be flushed after the settling tank is drained and a pressure differential exists between the pond water level and the drain pipes.

Mechanical Filters

Not all solids will settle out; usually a secondary form of mechanical entrapment must be used. This secondary mechanical entrapment can be filter brushes or polymat type material. These materials are positioned in a way which the water flow cannot bypass. Water flow and debris will find the path of least resistance and flow around the mechanical filter if possible. The material used in mechanical entrapment must not have a tendency to plug up quickly. A course grade material (brushes or enkamat) will trap medium large solids but not restrict the flow of water. Finer particles which pass through should be removed in sequence by a finer media (polymat or foam).

The entrapment of solids is effectively spread out gradually. This reduces the tendency of the filter to plug up to quickly. All mechanical filters should be flushed frequently to rid the pond or organic waste. Essentially, most of the solid waste should be removed before it reaches the biofilter.

Biological Filters

The biofilter's job is to detoxify waste products such as ammonia, nitrite and some phosphate. These invisible waste products can weaken or kill your koi and also act as fertilizer which creates algae problems. Good bacteria, called nitrifying bacteria, attach themselves to the biofilter material (media) and break down these waste products. Good flow through the biofilter must be maintained. The bacteria require good flow of oxygenated water to function properly. Bacteria are slow growers and if left undisturbed will colonize more completely and quickly. Many species of bacteria are responsible for complete biofiltration. Biofiltration also includes the decomposition of organic wastes like starches and fats. Clear water is dependent on a complete population of many types of bacteria. Constant cleaning of a biofilter sill simply wash away the beneficial populations of bacteria. The biofilter will function extremely well if most of the solids have already been removed by a good mechanical filter. Backwashing will be minimized due to lack of solids.

Bacteria will grow the fastest in warm water. The summer is a good time of year to clean the biofilter because the bacteria can grow back quickly. When temperatures are below 60 degrees F, cleaning should be minimal if at all.

An important design aspect to the biofilter is the manner in which the media is contained within the filter tank. The media should be supported on a strong grate above an intake manifold to ensure adequate flow of water through the media. 6" to 12" of water above and below the media will help in the even distribution of water flow.

Bacteria will grow on almost anything inert. The hobbyist can choose from a wide range of medias to use in the biofilter. Gravel is cheap and does provide a fair growing site for nitrifying bacteria. Gravel also provides the correct environment for the "algae lysing" type of bacteria. I have used this phrase for lack of a better word. In theory these bacteria are responsible for a pond's ability to clear itself of "pea soup" algae. However, gravel can plug up quickly because it is heavy. Gravel should be evenly graded and sized so that small gravel grains do not plug up water passage ways. One big drawback to gravel is it's tendency to trickle around the support grate and end up at the bottom of your filter, clogging pipes and creating stagnant piles of sludge. The support grate and any screening should be made to fit very tightly inside the filter tank to prevent this problem.

Synthetic medias don't have a lot of the problems inherent to gravel. Being lightweight and less clogging makes synthetic medias easier to handle and clean. Most of the synthetics have a large surface area per cubic foot for growing bacteria. This attribute gives a bigger biofilter in a smaller space. However synthetic medias may not grow the "algae lysing" type of bacteria very easily. The "algae lysing" bacteria grow better in slightly stagnant and low oxygen zones. Synthetics are usually very free flowing. Be careful when backwashing synthetic medias because they can clean to easily and might wash away too much bacteria. Examples of proven useful synthetic bio-medias are: open cell foam, polymat, bioballs, bio bale, and filter brushes. A new synthetic media with an extreme internal porosity and surface area. "Nu-Balance" is lightweight gravel type media with an extreme internal porosity and surface area. "Nu-Balance" has many of the beneficial qualities of both gravel and synthetic media. There is another break through synthetic filter material that is essentially layers of lightweight self supporting matting that can be stepped down in porosity to make for high internal surface area, ease in cleaning and slow clogging. This material is called "Matala", and is sure to start a revolution in synthetic filter media.

Ultraviolet Sterilizers

U/V sterilizers are quickly becoming a standard piece of filter equipment. Planktonic algae (pea soup) is easily killed by a U/V light. Pea soup has a tendency to dominate in new ponds and ponds mismanaged. The U/V really makes life easy. A pond can balance more quickly with a U/V sterilizer. Your enjoyment will be greater. Do not expect a U/V to kill disease pathogens like parasites or Aeromonas bacteria. The flow rate and contact time through the U/V required to kill these bugs is not practical for koi ponds. A very large and expensive unit on a small pond would be required. Essentially, the parasites can grow faster than the U/V can kill them. Pea soup algae on the other hand is light sensitive and is controlled with relatively fast flow rates through the U/V. The U/V will have a minimal impact on the growth of stringy algae attached to the pond walls. This algae does not come into contact with the light. However, some stringy algae spores may be killed as they pass through the U/V sterilizers to avoid problems with defective equipment. Choose a unit specific for your ponds gallonage, number or fish and amount of sunlight. Flow rates through the U/V are determined by the wattage rating of the unit. Consult a knowledgeable dealer for your specific needs.

Koi keeping is a lot of fun, gives some great lessons in nature and brings people together in shared interest. Koi just seem to bring out the friendly side in people. I hope your understanding of koi keeping grows quickly and smoothly.