Trickle Filters, Pond Filter Flows & Static Bed Filters

I've outdone myself this Easter. 4 articles in 4 days is a record for me. Maybe I should have gone away for the weekend.

In regard to these articles which have covered the most important aspects of good pondkeeping I have received a number of good comments and questions. I'll address these briefly below. Some of the comments have been quite technical in content and very correct. However for the sake of writing for an extremely broad audience I put away my Chemical Engineer's hat and don my "common sense" hat. I do however welcome any input and am grateful to see that my articles are read.

DON'T FORGET This is the last article in which you'll see the 60% discount off "The Complete Pond Solver"... I've given the discount because at this time of the year in the northern hemisphere the content is invaluable. Read more below...

Question 1...

Is it better to have a high flow rate or low flow rate through a static bed biofilter? The process of biological conversion of ammonia to nitrate in the presence of dissolved oxygen is favoured by a turbulent environment. While it is difficult to achieve turbulence in a static bed biofilter it is certainly better to maximise the flow rate while at the same time ensuring a good even distribution of water flow.

Question 2...

Is it better to have water flow up through a static biofilter bed or for water to flow down the bed? In my view it is better to have an upflow arrangement especially if there is a drain valve on the bottom of the biofilter casing. Upflow is better because the biomedia bed is then always submerged. A second reason is that any solids in the flow to the filter will tend to settle at the bottom of the filter and in the first section of the biomedia. By taking a bucket of pond water is becomes quite easy to "backwash" the biomedia bed. Make life easier by encasing the biomedia in a string bag. This way it becomes easy to remove the whole bed of biomedia in one go. Rinse the bag of biomedia in pond water and return.

Question 3...

Should I remove the old biomedia when replacing with new?

NO! The new biomedia should be mixed with the old until biological growth on the new media has been achieved. Typically allow 6 weeks.

Question 4...

Why do some filters have a spray bar at the top (inlet) of the filter? An alternative and efficient type of biofiltration is what is known as a trickle filter.

Such filters distribute fine water sprays across a bed of biomedium. This kind of trickle filter is employed on a large scale in sewage works (you will have seen a rotating bar above large cylindrical chambers filled with stones). In pond filters the presence of a spray bar does not mean the filter is acting as a trickle filter and in general the action of the single spray bar is such as to reduce the efficiency of the biofilter though channeling and only partial use of the biomedia bed.

Question 5...

What is the absolute best type of biofilter?

Any biofilter in which an extremely turbulent water flow can be made around the biomedia is best. In this case it's important to create or augment the turbulence with air sparging. Classical examples of such systems are Japanese matting in Vortex chambers, Kaldnes type suspended bed filters and bubble bead filters. For the vast majority of garden ponds these systems are overkill. It is impossible to "wash off" the bacteria contrary to popular belief. Well that's it for now. Hope you found the articles useful