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Related FAQs:  Up-flow Pond Filtration,

Related Articles: Filtration Overview, Pond Filter Media, Biological Filters, Up-flow Filters, In-Pond Filters, Ultraviolet Sterilizers for Ponds, Pond Components/Gear,

/Products for Garden Ponds

Up-flow Pond Filters, Filtration 

by Robert Fenner

 Outside, in-line biological filter... "reverse flow"

Aquatic Gardens

Ponds, Streams, Waterfalls & Fountains:

Volume 1. Design & Construction
Volume 2. Maintenance, Stocking, Examples

V. 1 Print and eBook on Amazon
V. 2 Print and eBook on Amazon

by Robert (Bob) Fenner

Rather than a specific type of filtration, as the name implies, upflow refers to the path of water through a particular filter media. This direction is often contrasted with "gravity", "natural" or "top to bottom" flow. This article describes the benefits, design, construction and utilization of upflow filters and offers specifications and instructions for a production model our old corporation used to manufacture and distribute.

Optimized reverse or upflow path filters incorporate the follow design pro forma: a suitable chemically-inert container of correct size and shape, remoted from the main holding system (as opposed to "In-Pond" filters) for ease of operation and maintenance.

For biological ponds and multi-tank systems these filters cost less to fabricate, install and operate and are more efficient per dollar in terms of electrical, labor and down-time costs than any other filter-type. (a more definitive, persuasive argument for these filters is offered in the last article in this series.)

Biological Filters in General:

Have two sets of specific purposes:

1) Functionally they provide sufficient habitat, oxygen and nutrient via circulation for conversion of metabolites- ammonia, nitrites, albumen, and other dissolved organic compounds (DOC's) to less noxious chemicals by beneficial bacteria and other microbes.

2) Aesthetically: they help keep the system "clean", clearer by

limiting nutrient availability to algae and micro-organisms in other parts of the system. To some extent, they accomplish these ends by trapping particles like mechanical filters. Depending on media type, surface area, flow rates, and characteristics of the water, they also act as chemical filters in some ways. Biological filters are best understood as living entities, affecting and being affected by the rest of the environment.

Bio-filters accomplish these feats by having enough, but not too much flow of the system's water pass through proper sized and placed media where aerobic and anaerobic microbes work on it. The products and by-products of their metabolism are dissipated as gases and absorbed by physical, chemical and biological processes and diluted by frequent partial water change. Some folks aid and abet these processes with ultra-violet sterilizers, protein skimmers, and/or chemical filtrants... for the most part these are expensive and unnecessary when the rest of the system is designed and built properly, provided with adequate circulation and filtration, and not thrown out of whack by over-population, feeding or fertilization.

Circulation path and rates are important as it is intended that solids and free-floating algae be trapped in the filter media, and not vigorously "pushed" through the media without due processing. Empirically, the rule of thumb value is two to four gallons per minute flow per each square- and/or cubic-foot of a given type of filter media.

Other contributing factors aiding or detracting from a

system's intended function(s) will be discussed in other Sections in this book in terms of design, construction and maintenance. Some examples are the effects of lighting/shading, appropriate foods and feeding, adequate depth, size, shape and dark color of basins.

Placement Of The Filter:

May be either above or below ground level, depending on whether you desire to pump the water to the filter or from the filter after it has been fed by gravity-overflow from your system.

I urge you to engineer your filter system above grade due to the following advantages:

1) The ability to hide or disguise the filter(s) behind falls, landscape, etc.

2) Ease of backwash and other manipulation as necessary.

3) Not having to rely on transit volume to keep your filter filled with enough water.

Construction: The Filter Container

Should be as large as possible; at least ten percent of the volume of the rest of the system. Many folks use multiples of less expensive smaller containers hooked up in series and/or parallel, rather than building or buying just one large filter sump.

The shape of the filter sump(s) is important as well; they must be deep enough to provide adequate space for 1) sedimentation of solid wastes on their bottom 2) usually twelve to eighteen inches of media depth and 3) overflow. See illustration.

Filter construction, as the rest of the feature should be made of chemically unreactive materials. Ours were made of polyethylene, the same plastic used in wrapping food (ours came from soda concentrate shippers). This compound is heat, cold and jar-resistant, and will not rot, crack, or delaminate like fiberglass and polyurethane resins.

Filter basins can be made from part of a multi-tiered system as detailed in the last Section; or made like a pond itself with block, cast concrete with or without a liner, sealed with a cementatious coating. These are typically more time and money consuming and less serviceable than plastic drums.


The filter system is fitted with lines as depicted below in specifications. Various guidelines are presented for plumbing water features in a later piece. To re-cap: The intake from the system should be screened to keep large "gunk" (a scientific term) out of the pump and biofilter. This water should be drawn from the mid-water with some possibly skimmed from the surface.

The filtered water is best returned to the system at an opposite end, being agitated (e.g. by a waterfall) to aid in de-gassing.

The discharge line from the pump to the biofilters aeration tower should be arranged to maximize mixing of air with the water. Some people fit their towers with "wet-dry", "reef" media used in saltwater aquariums.

The overflow and drain lines should be as large as possible.

We provided one and one-half inch on our production models. The intake line should be no larger in diameter than the discharge plumbing size from the pump. Which brings us to...


And pumping. As I stated in the Pumps artilce, these bare careful consideration in terms of section and engineering. In the not-too-long-run, they cost more than the rest of the system combined in electrical costs. Get the right kind and size pump, and install and maintain it properly.

Enough pump is a balance between size, shape, composition of basins and water, and processing rates of livestock/nutrient and filter, plumbing and desired degree of "cleanliness" and escalating electrical cost. Whew! let's see; did I leave anything important out? Oh yes; I should mention that these, like all good biofilters, operate continuously. That's right; the pumps should stay on twenty four hours a day.

So, match your filter (ten percent size and volume or better) to your system, to your pump, per your perceived desire and ability to pay for utilities. Small systems may be turned over twice or more per hour; larger, deeper, "more-balanced" systems may re-circulate much more slowly. Don't "over-drive" your filter system! If you want to "make a big splash", divert all the other flow, bypassing the filter system's other than the two to four gallon per square, cubic foot of media rule of thumb.

Filter Media:

Various types of materials, from volcanic to metamorphic rock, to "mesh-like" substances to advanced wet-dry/sewer-technology are available and endorsed by various schools. All are better or worse depending on considerations of availability/cost, flow rate, backwash and replacement frequency, loading, feeding rates....

I encourage you to use crushed rock of one or two sizes (grades) depending on media depth, flow rate & backwash frequency. Think about it; the smaller the grade, the more surface area per given volume of media, therefore requiring more frequent cleaning. As can be read in our specifications below, we call out a layering of larger (three-quarter inch) under a thinner layer of smaller (one eighth by one quarter inch (pea), or three-eighths) gravel. Some authorities suggest a few inches of silica sand of most any grade on top of the gravel, but I caution against this. There is little to be gained in filtering, greater likelihood of anaerobiosis and quite a disadvantage in back-washing ability.

Multiple Filter Basins; Series and Parallel:

It is better to have two or more filter basins rather than one larger one. These can be arranged in such a way that one flows to another (in series) or split for flow individually to seperate filters and back to the system (in parallel). Rationale? In series filter basins may be arranged to have successively smaller and smaller graded media, facilitating cleaning and reducing down-time. Further chemical filtration;, zeolite, carbon, may be placed in latter units, thus protected from premature "sliming" and "clogging".

Multiple series and parallel filtration is far superior to a single larger filter in that all the units' microbial populations need not be disturbed should one part of the system need to be thoroughly disrupted (thorough cleaning, backwash, replacement of media).

Thorough Cleaning:

In contrast to periodic back-washing, can involve digging out and replacing, or acid/bleach washing and/or air-drying media. Some, more sophisticated designs spell out more or less elaborate air and/or water jet systems to aid in time to time cleaning and back-washing operations. These are very appropriate technology for large systems with large filters.

& now, Ta Da, for some PR from the old direct-sales Division of our defunct corporation, Aquatic Gardens: This is a circular we provided with a production upflow biofilter we manufactured and distributed.




Illustration Showing A Single and Multiple Installation


ALS 600 (sixty gallon barrel-filter) BIOFILTER


This pond filter represents proven, easy to install biofilter technology that really works. In fact, this system is guaranteed to keep your pond clear when installed and maintain according to these directions. Aquatic Life Services, one of Southern California's most experienced and knowledgeable aquatics maintenance companies has developed this design over many years of field testing.

Features and Advantages of the ALS 600 Biofilter:

* Keeps pond clear

* Reverse flow

* Aeration Tower built in

* Back-washable- does not have to removed from pond to clean

* Over seven cubic feet of filter volume for aerobic bacteria growth

* Can be remoted from pond- does not take up valuable pond space

* Filter can be run by small, low electrical consumption pump

* System can be incorporated into waterfall if desired

* Great value for the $$$



Follow these simple step by step instructions (please read through once before installation)

1) Locate an appropriate spot to place your ALS 600 biofilter; ideally in a hidden location within 10-20 feet of the pond (e.g. behind landscaping, around the corner of the house). This location must allow for at least two feet of height difference between overflow "B" and the pond's water level.

2) Make a level site at the chosen location and put the filter in place with the plumbing parts (see Diagram, parts B,C,E) situated to allow pump , return to pond and backwash connections.

3) Fill the filter basin (F) to level "D" with clean, well-rinsed, crushed rock; three bags of 3/4" rock on the bottom, three bags of 3/8" rock on top. Do not allow gravel down aeration tower!

4) Using 3/4" flexible tubing and with pump in it's normal operating position, hook up the discharge of the pump (where the water exits the pump) to barbed fitting "C". PVC pipe may also be used in conjunction with or in place of flexible tubing for long runs, to conceal underground, etc.

5) Using 1 1/2" PVC pipe, parts and solvent, run the return "B" back to the pond so that the filtered water will flow by gravity over the pond edge into the pond or into a waterfall. Again, this pipe can be hidden by burying, painting or otherwise concealing.

6) To assure optimal operation of your new filter, it's best to start with a clean pond. If your pond is dirty, clean it out; if new, go ahead and fill it (in both cases, don't forget to treat the new water for chloramines). With the pond filled, all plumbing in place and the backwash valve "E" closed, turn on the pump and start filter operation. You will note the filter tub "F" starting to fill and eventually overflowing out of "B" to return to the pond. Check for any leakage in your plumbing work at this time.

7) With fish in your pond, add eight ounces of bacteria culture (available from your neighborhood fish shop) into the aeration tower to initially inoculate your biofilter. Also at this time, place one Pond Block per 250 gallons of pond water onto the top of the filter gravel. Continue to add bacteria to the filter at the rate of two ounces per every other day until a total of two quarts have been used. At this point, your filter bacteria should have developed a healthy population. Run your filter pump 24 hours per day.

8) Maintenance:

After one month of operation, backwash your filter on a once or twice monthly basis, depending on fish load and amount of debris/detritus that enters the pond. To backwash, turn the pump off, hook up a 1 1/2 pool-hose to a desired drainage location, and open the backwash valve. To thoroughly clean the filter you may alternately fill the tub with the pump

and drain to backwash, and even leave the pump running with the backwash valve open. Change 10-20 percent of your water monthly. Keep your pH between 6.5 and 7.5 and do not allow fertilizer into your pond.

Further Specifications:

Recommended flow rate at four feet of head pressure is 300-450 gph. Install one ALS 600 filter (a fifty five, sixty gallon barrel-filter) per 600 gallons of pond water. For larger ponds, multiple filters may be installed in parallel or series, maintaining appropriate flow/filter.


In most application, pond and multi-tank filter systems can and should be remoted for ease of manipulation and to alleviate stock stress. If possible and practical these systems should be plumbed as up-flows, incorporating the features shown here: Air\mixing tower, sediment trap, large backwash valve, large relative filter size with appropriate grade media in concert with adequate flow.

Reverse flow filters channel less, and are more easily backwashed, with consequent fewer and less severe related problems of anaerobiosis and clogging. Reverse flow is definitely not backwards.

Aquatic Gardens

Ponds, Streams, Waterfalls & Fountains:
Volume 1. Design & Construction
Volume 2. Maintenance, Stocking, Examples

V. 1 Print and eBook on Amazon
V. 2 Print and eBook on Amazon

by Robert (Bob) Fenner
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