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I've been raising Daphnia magna cultures over the last few months as feedstock for freshwater aquarium fish. From my reading, Daphnia feeds on bacteria and single-celled algae, as well as other small crud suspended in the water column.

Practically, yeast or soy flour can be suspended in water and poured into a tank containing a population of Daphnia, and the critters usually clear the water in a few days. The clarity of the resulting water is quite impressive. Which leads me to...

My question: How effective are these types of organisms at removing harmful bacteria and substances from a volume of water? Lets say you only needed a few gallons of clean water each day, could you have a tap that drew from a tank containing a dense culture of Daphnia, and use that water as drinking water?

I've not found information that studies the quantities of bacteria in the water column before and after introduction of Daphnia or other filter feeders.

P.S. I thought about posting this question in the Sustainability SE but figured I'd get more accurate information here.

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  • $\begingroup$ Great question. +1. I slightly edited your post. It was great, but I thought to add a link to wikipedia and do some formatting. I removed the cooking part, because boiling water to cook typically kills all microorganisms in the water anyway. It's definitely on topic here. $\endgroup$ – AliceD Sep 23 '15 at 3:53
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My answer from the top of my head is that they will not be very effective at cleaning the water. One reason is that if the substances and organisms are harmful to the filter feeder as well, they will avoid eating them or they will die from them or have their health reduced. In either case they will be filtering less.

Another reason is that if you use the tap and replenish the water with new contaminated water regularly, the filter feeders will probably not filter all water in the tank, and the outflow from the tap will still contain the contaminant. This is certainly the case if the tank is also the culture vessel for your filter feeders. In this case, the tank has come close to being a continuous culture. There are decades of science (including R* theory) which show that resource levels in such systems settle at a positive level (and you propose that the harmful micro-organisms are used as resource or at least eaten in proportion to the resource if the filter feeder cannot be selective). This positive level will then thus also be present in the effluent from the tank (your tap water).

An alternative is to grow the filter feeders in a dedicated tank, and than add them later in high concentrations to your drinking water. This would not be efficient because you need to feed your filter feeders with something.

Finally, the harmful substances may be absorbed by the filter feeders, but also be excreted, in which case the will end up in your drinking water anyway.

For getting rid of the harmful micro-organisms, it is probably more efficient to boil your water!

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  • $\begingroup$ Great links and information, thanks. I clearly have more reading to do. I suppose a common example of a functioning chemostatic continuous culture system would be a biofilter for a common aquarium, where ammonia-enriched water is fed into an environment full of bacteria that metabolize the ammonia into nitrites and nitrates? $\endgroup$ – cathode Sep 23 '15 at 20:07
  • $\begingroup$ Yes. Continuous cultures (e.g. chemostats) have a continuous supply of resources (nutrients/food), as opposed to batch cultures, in which resources are added in batches. Because continuous cultures are easier to model, lots of theory is derived from them. Batch cultures come close to continuous cultures if resources are added often in small batches, as opposed to rarely but in large batches. So chemostat theory may apply to batch cultures as well. $\endgroup$ – Daan Sep 24 '15 at 8:33

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