Plant filtration and Nitrates

[Pomatomus]

Plants certainly reduce nitrogenous waste, but exactly how much is debatable and depends on species.

It kinda bugs me how blindly people believe in plants for nitrate reduction. My coworker told me he shouldn't have nitrates because his tank was full (literally) of plants. I tested it and found >160ppm nitrate.

I attended a dissertation presentation that featured a mass balance in a saltwater aquaponics setup. She found that although the plants assimilated some nitrogen, the greatest reduction came from the anoxic zones in the system, where denitrifying bacteria flourish. The roots (rhizosphere) were an excellent site for these bacteria to colonize. For the life of me I don't understand why more aquarists don't look into dentrification. We use it at work and there is up to a 90% reduction in nitrate after it goes through the reactor. It took 8 weeks in to do this in the water hyacinth study posted above! All you need is a media reactor (can be made from PVC) and a small pump. In aquariums you can also create anoxic zones by having deep substrate. These are the techniques used in reef tanks to keep nitrate at nearly undetectable levels. Just look up "deep sand beds", "plenums", or vodka dosing. These are all commonly used by reefers but are nearly absent in the freshwater community because fresh water is cheap. But if you want to reduce your nitrate without water changes, this will do it very effectively.

Plants never hurt though.

 

[hart24601]

I found a few pics. Early March when I added plants to the overflow. There is a peace lilly, and couple small varieties of pothos. The lilly was larger than the pothos mass wise. Lighting is 2 kessil 160we plus some indirect sun - lots of light! (I recently took one kessil offline and movies to a dedicated water lettuce refugium). Look at that water lettuce on the surface!



View attachment 1219441

Now here is the same system 9.5 months later. Some big growth! Not as much from the peace lilly. These lights are strong enough the leaves burn directly under them (not the lettuce of course), but other than that no trimming. A lot of growth! Clearly I made some other changes too such as removing plants from the inside.

View attachment 1219443

However this photo is how much water lettuce I would remove from that tanks surface every 7-10 days!! No joke - just taking the smaller plants. The biomass export is insane. Keep on mind this system get a 15% daily RO/DI drip too!

View attachment 1219444

Now I have moved to a dedicated WL refugium under the 180g tank next to this one (plumbed together) but you can see how plants can grow crazy fast and be a wonderful export mechanism even on a system that gets fairly large daily water changes with pure RO/DI water.

I found an a photo of the day I put the pothos and PL in. You can see the lettuce and how it grew in relation to the 1st picture above with the pothos and PL despite being harvested.

 

[hart24601]

Plants certainly reduce nitrogenous waste, but exactly how much is debatable and depends on species.

It kinda bugs me how blindly people believe in plants for nitrate reduction. My coworker told me he shouldn't have nitrates because his tank was full (literally) of plants. I tested it and found >160ppm nitrate.

I attended a dissertation presentation that featured a mass balance in a saltwater aquaponics setup. She found that although the plants assimilated some nitrogen, the greatest reduction came from the anoxic zones in the system, where denitrifying bacteria flourish. The roots (rhizosphere) were an excellent site for these bacteria to colonize. For the life of me I don't understand why more aquarists don't look into dentrification. We use it at work and there is up to a 90% reduction in nitrate after it goes through the reactor. It took 8 weeks in to do this in the water hyacinth study posted above! All you need is a media reactor (can be made from PVC) and a small pump. In aquariums you can also create anoxic zones by having deep substrate. These are the techniques used in reef tanks to keep nitrate at nearly undetectable levels. Just look up "deep sand beds", "plenums", or vodka dosing. These are all commonly used by reefers but are nearly absent in the freshwater community because fresh water is cheap. But if you want to reduce your nitrate without water changes, this will do it very effectively.

Plants never hurt though.

Finally someone talking about SW! Reefkeeping my main focus in the hobby. Here is a photo of my system so you don't think I am just an armchair quarterback, haha:



I do want to add a couple points, because what works great in aquaponics systems doesn't always transfer well to aquaria. Systems like plenums and deep sand beds and even denitrifying column were used extensively a couple decades ago (crazy how 2000 is almost a couple decades...) however there were issues with those systems and while some still run them and are happy with the results in the past number of years a great deal of saltwater systems have moved to chaeto macro algae based systems to help with filtration (nitrates and such). Algae scrubbers have also become quite popular in the past few years and work on the same concept of plant based exports. I used to run chaeto but in the past couple of years have moved to carbon dosing (vodka, vinegar and sometimes biopellets) but those are not the same as denitrification with deep sand beds or plenums where the nitrate is converted to nitrogen gas by obligate anaerobes.

Vodka and carbon dosing can be thought of as a bridge between the two ideas (plant and anoxic systems) because while bacteria do uptake the carbon with phosphorus and nitrogen they do so aerobically and then are physically exported (either the bacteria or bacteria consumers) from the system by a skimmer. So the exportation and the oxygen requirement are more similar to plant based export in that respect.


Even with carbon dosing many people, I would say the majority of reefkeepers, are running some sort of plant based export system either in tandem with carbon dosing or as the sole export mechanism.

As for that study, I don't know which one you are referencing but most are in a week to 10 day timeframe. Here is a quote from one about growth of water lettuce:

"Fonkou et al.[21] stated that lettuce doubles its biomass in just over 5 days; triples it in 10 days, quadruples in 20 days and has its original biomass multiplied by a factor of 9 in less than one month. This evolution indicates that 25 days is the maximum period to allow the plant in the system."

From another article:

"During the 10-day retention period, the sand- based constructed wetland set up with Water Lettuce had improved the wastewater quality significantly as it had reduced 75.66% of Turbidity, BOD5 by 83.43%, NO-3 by over 50%, 90% of SO-4 , 46.2% of Cl- and Conductivity by 46.2% and Dissolve Oxygen by 58%."

Here is the table and pics from the above article, note nitrate effluent started at 20ppm and 1 day reduced to 10ppm and by day 9 was 0!



View attachment 1219463

 

[Pomatomus]

Finally someone talking about SW! Reefkeeping my main focus in the hobby. Here is a photo of my system so you don't think I am just an armchair quarterback, haha:

I do want to add a couple points, because what works great in aquaponics systems doesn't always transfer well to aquaria. Systems like plenums and deep sand beds and even denitrifying column were used extensively a couple decades ago (crazy how 2000 is almost a couple decades...) however there were issues with those systems and while some still run them and are happy with the results in the past number of years a great deal of saltwater systems have moved to chaeto macro algae based systems to help with filtration (nitrates and such). Algae scrubbers have also become quite popular in the past few years and work on the same concept of plant based exports. I used to run chaeto but in the past couple of years have moved to carbon dosing (vodka, vinegar and sometimes biopellets) but those are not the same as denitrification with deep sand beds or plenums where the nitrate is converted to nitrogen gas by obligate anaerobes.

Vodka and carbon dosing can be thought of as a bridge between the two ideas (plant and anoxic systems) because while bacteria do uptake the carbon with phosphorus and nitrogen they do so aerobically and then are physically exported (either the bacteria or bacteria consumers) from the system by a skimmer. So the exportation and the oxygen requirement are more similar to plant based export in that respect.


Even with carbon dosing many people, I would say the majority of reefkeepers, are running some sort of plant based export system either in tandem with carbon dosing or as the sole export mechanism.

As for that study, I don't know which one you are referencing but most are in a week to 10 day timeframe. Here is a quote from one about growth of water lettuce:

"Fonkou et al.[21] stated that lettuce doubles its biomass in just over 5 days; triples it in 10 days, quadruples in 20 days and has its original biomass multiplied by a factor of 9 in less than one month. This evolution indicates that 25 days is the maximum period to allow the plant in the system."

From another article:

"During the 10-day retention period, the sand- based constructed wetland set up with Water Lettuce had improved the wastewater quality significantly as it had reduced 75.66% of Turbidity, BOD5 by 83.43%, NO-3 by over 50%, 90% of SO-4 , 46.2% of Cl- and Conductivity by 46.2% and Dissolve Oxygen by 58%."

Here is the table and pics from the above article, note nitrate effluent started at 20ppm and 1 day reduced to 10ppm and by day 9 was 0!

View attachment 1219463

Beautiful tank! I'm jealous of your SPS. Yes, vodka/carbon dosing can also be used with aerobic bacteria and a skimmer. However, conventional skimming is less effective in freshwater. But yeah dosing the tank will promote growth of aerobic and anaerobic microbes, depending on the environments your system provides. I know the Brightwell product "Reef BioFuel" is sold for aerobic removal of nitrate.

The article I referenced was one posted earlier in the thread. The one you posted first doesn't mention the nitrogen removal, but the second one may solidify my point if I'm reading it right. They saw a reduction in nitrate in a system with water lettuce AND sand (anoxic zones). A mass balance would determine how much was assimilated into the plants and how much was removed through denitrification. As I see it, they tested the effects of a wetland setup on nitrate.

I have used deep sand beds but have no experience with plenums. I know there was a big debate between the two for years but I don't see plenums much now. I also don't understand why they would be better than deep sand. Personally, my live rock provides enough anoxic zones for me to keep my nitrates at 0 with only 2" of sand. I'd say I have a "medium" sand bed lol.

 

[hart24601]

While might be getting a bit off topic haha, but I would caution the importance of the sand in the above article for for two reasons, 1st being the results of the article show similar reductions to many other articles using various floating plants that did not use sand and secondly that time frame if very short for anaerobic bacteria growth. While it's possible they did contribute, the growth times of most anaerobic bacteria are so slow it takes a considerable amount of time for them to make any impact. I am a microbiologist and work developing silage inoculants and habe worked and studed some these bacteria quite a bit. Well I was a microbiologist, just switched careers to an analytic consultant after 8 years at DuPont, so don't listen to me anymore!!

 

[duanes]

I have also found plenums to be helpful in reducing nitrates.
I built the frame from PVC and and wrapped weed barrier cloth around it, although with cichlids it was a regular battle trying to keep enough substrate on the plenum, because of there constant digging. After a while I found it easier to put the plenum on the botton of large, deep sumps.

 

[Pomatomus]

While might be getting a bit off topic haha, but I would caution the importance of the sand in the above article for for two reasons, 1st being the results of the article show similar reductions to many other articles using various floating plants that did not use sand and secondly that time frame if very short for anaerobic bacteria growth. While it's possible they did contribute, the growth times of most anaerobic bacteria are so slow it takes a considerable amount of time for them to make any impact. I am a microbiologist and work developing silage inoculants and habe worked and studed some these bacteria quite a bit. Well I was a microbiologist, just switched careers to an analytic consultant after 8 years at DuPont, so don't listen to me anymore!!

Good point! I'd love to discuss denitrification with you in depth sometime. I just finished my microbiology course as I am preparing for a thesis on probiotics. I suppose the methods of the study are what will determine the culprit. If they took sand from a natural environment, or possibly the water, then it is likely that these bacteria were present. They are actually facultative anaerobes, which will only denitrify when oxygen is not present. However, they can persist and survive even in an oxygenated environment. We clean our denitrifiers by bubbling air through them and denitrification resumes as soon as the chamber becomes anoxic again (less than an hour). Even when we bleach them we see denitrification the next day (haven't tested earlier than that). So if they used clean sand and water that are relatively absent of DN bacteria, then it's probably the plants. If they used seeded sand/water, then denitrification could have occurred. Does the article mention this in their methods?

I appreciate your input on this. I'm learning!

 

[Coryloach]

I tested it and found >160ppm nitrate.

You seem like a very informed guy but then you should know that you can't test accurately for nitrates even in lab conditions......The 160ppm you tested could have been far away from the real truth and what your mate was saying was probably true..

 

[Coryloach]

Now here is the same system 9.5 months later. Some big growth! Not as much from the peace lilly.

In 9.5 months if this peace lily was not limited by something, it would have been huge, as in my previous picture.....Mine grow on average 1.5 leaf weekly. 9.5 months is roughly 41 weeks, that's 61 new leaves....Peace lilies also multiply from the crown so one ends up with several peace lilies in the one pot....When they grow huge, they start flowering. They may need a bit of time to adapt to water conditions first when initially planted....I think your water lettuce may have scavenged all the ferts and the lily couldn't outcompete it as it looks like a small plant.....all the more proof about the scavenging abilities of plants because yours doesn't seem to be limited by light and obviously not co2...Try the lily in a tank that has no lettuce or a larger tank with more fish stock...unless you want to dose additional ferts...

 

[Coryloach]