Posted 20 January 2011 - 08:46 AM

Based on studies from industrial sized constructed wetlands for wastewater treatment it occurred to me a downscaled version might be useful as a HOB aquarium filter. Recent discussions here prompted me to actually run some numbers. To run these numbers I chose Water Hyacinth (Eichhornia crassipes), which has the disadvantage of being an invasive and restricted in some states. The idea is to numerically demonstrate the feasibility though, rather than restrict the concept to a particular plant species or monoculture.The idea of a HOB plant filter for the aquarium in principle has many advantages.1: They can be precycled (matured), before being placed on the back of an aquarium.2: They are not dependent on dissolved CO2 levels in the water, allowing additional aquarium plants without interference.3: The lighting requirements are somewhat reduced for exposed plants.4: It provides a habitat for scud colonies or similar food stock.5: The increased surface area and oxygen in the root system provides for better gas absorption in the water.6: The root system provides mechanical filtration.7: The symbiotic nitrosomonas and nitrobacter bacteria in the root system provides standard biofiltration.8: The plants metabolize nitrates not achieved through standard biofilters.9: It greatly reduces the need for water changes.10: It can provide for the phytoremediation of heavy metals.The primary concern is whether an effective filter can be produced in a sufficiently small area behind the aquarium. For area requirements I will assume a 12" inch extension on the back of various standardized aquarium sizes the length of the aquarium. On www.oas.org it gives the effluent treatment rate for hyacinth as "approximately 1 m2/m3/day of water to be treated". Converting this to a ft^2, for the overhang behind the aquarium, we get 1 ft^2 cleans 80.5 gallons/day. The effectiveness of treatment, using four different unspecified larger scale models, is given in the above link as:Source Reduction --- BOD ---- COD - TSS - N --- PRaw wastewater ----- 97% ---- n/a - 75% - 92% - 60%Secondary effluent - 83% ---- 61% - 83% - 72% - 31%Secondary effluent - 35% ---- n/a - n/a - 44% - 74%Secondary effluent - 60-79% - n/a - 71% - 47% - 11%Source: U.S. Environmental Protection Agency, Innovative and Alternative Technology Assessment Manual, Washington, D.C., 1976, ( Report No. EPA-430/9-78-009 ). As provided on oas.org link.Aquarium water is best described as a "secondary effluent". Here BOD is Biochemical Oxygen Demand, COD is Chemical Oxygen Demand, and TSS is Total Suspended Solids, N is Nitrogen, and P is Phosphorus. Reduction effectiveness is variable, and depends a lot on initial concentrations. So my idealized reduction rates, to be calculated, at various averages is only an indicator. Of course aquarium filters only need to keep up with a specific bioload, rather than improve a particularly bad source effluent.Since the HOB plant filter is always considered the length of the aquarium the gallons of a 12 inch length section of a particular relative to a 12^2 inch plant ray is all that is needed to determine cleaning cycles/day. For a standard 10 gallon the width and height is:Standard 10 gallon aquarium;10" x 12" x (12") = 6.23 gallonsThat same 12" x 12" hyacinth tray will process 80.5 gallons/day with one cleaning cycle/day.Thus a hyacinth tray 12" inches x width of 10 gallon aquarium will provide 12.9 cleaning cycles/day.With a 50% average reduction per cleaning cycle times 12 cycles is a 99.98% relative reduction/day.With a 10% average reduction per cleaning cycle times 12 cycles is a 71.76% relative reduction/day.The indication is, based on various sources, is that reduction efficiency goes down as the target effluent decreases. But once the target levels are reached there is no need for reduction. An aquarium filter only needs to maintain enough filtering capacity for the bioload provided. As the width and height of an aquarium increases the cleaning cycles per day is reduced. This would give shallower breeder tanks an advantage. Hence a 55 gallon aquarium, (48") x 13" x 21", comes to:Standard 55 gallon aquarium;13" x 21" x (12") = 14.18 gallonsThus a hyacinth tray 12" inches x width of aquarium will provide 5.67 cleaning cycles/day.With a 50% average reduction per cleaning cycle times 5 cycles is a 96.9% relative reduction/day.With a 10% average reduction per cleaning cycle times 5 cycles is a 41% relative reduction/day.This shows that such filters are, at least in principle, feasible. Obviously test need to be done with actual aquariums involving more than just monoculture plant species. In principles the controllability of variables is superior to larger outdoor systems, offering the potential for greater efficiencies. Experimenting with plant mixtures is also warranted. Mixing irises and other bog plants that doesn't significantly interfere with the hyacinth growing space or enhance effectiveness could add significant utility and beauty. Some other things that could be done is fertilizer ports added such that the fertilizer goes directly to the bottom roots, preventing it access to the main aquarium. This would increase overall efficiency, as efficiency tends to go up as nutrients are increased. Three sided models would also greatly increase the total filtration capacity.The phytoremediation of heavy metals and rates is documented by many sources. Water hyacinth is not particularly unique in this respect, but here are some basic references showing this effect with hyacinth.Cadmium, lead, copper, zinc, and nickel remediation experiments (pdf); http://www.apms.org/...ol42/v42p60.pdf Mercury remediation; http://www.informawo...96~frm=abslink" Zinc and Chromium remediation (pdf); http://www.bscw.ihe....001/Gakwavu.pdf It may even be possible to dump contaminated water (sewage) in the aquarium and given enough cycling time be well suited for fish. I think this is a worthwhile filtering approach to pursue. What do you think of growing a swamp around your aquarium instead of in it, or in addition to internal plants?

Edited by mywan, 20 January 2011 - 08:48 AM.