Fish waste in algae scrubber

[perlboy]

Hi amwassil. Saw you name in this thread and thought I'd offer a solution.

Fish poop, otherwise known as solids, is bad because it decays to ammonia. Ammonia must be removed via the nitrification process by some means, as it can be toxic. Timmons in Recirculating Aquaculture, 2007, recommends keeping NH₃ < 3.0 ppm. I know algae scrubbers prefer to do that job with algae scrubbers (pun intended) but there are many types of biofilters that do that job IMO better (I’ll defend that point in a moment, but first…). My preferred biofilter is an MBBR.

Full disclosure: I have never maintained a marine display aquarium. My experience is with commercial aquaculture, tilapia (a fresh water species) and Pacific white shrimp (a salt-water species.)

It is standard procedure in aquaculture applications to remove solids with a settling tank. Some call this device a clarifier. It is a simple device, easy and cheap to make and relatively easy to maintain. An effective example can be made from a 5-gal plastic bucket. Purpose, to reduce the load on the biofilter by removing suspended solids (fish poop and uneaten food.)

A settling tank introduces water containing solids (we call this a slurry) at the top but directed to the bottom. This is done through a baffle. Suppose your input pipe is 1 in. A 3” or 4” piece of PVC, its top level with the top of the clarifier, makes a good baffle. I hang mine by screwing ½” PVC to the outside of the baffle (stainless steel screws) and drilling holes in the bucket (slightly higher than the output port) to hold these support tubes. Solids are denser than water so they settle to the bottom. Clear water (that is directed to the biofilter) flows out of the clarifier near the top. It is useful to suspend some sort of flexible netting at the midpoint of the clarifier to keep the solids from rising back to the top. There should be as little turbulence as possible inside a clarifier so this is not the place to put an airstone or diffuser. What we call sludge is dumped from the clarifier through a bottom drain. A gravity blast of 1-2 gals from a 5-gal bucket is sufficient. The sludge can be dried in the sun and then used as fertilizer, since it is rich in organic matter. A useful rule of thumb, Van Gorder, Small Scale Aquaculture, 2000, when sizing a clarifier is to allow 5.5% of tank volume. How often one dumps a clarifier is a function of how fast they fill. In aquaculture we prefer translucent tanks so one can see how much solids is building up. Translucent cone or dome bottom tanks are ideal but considerably more expensive than plastic buckets, typically 4x more expensive in 5-gal size. A 5-gal bucket should be good for up to a 90 gal aquarium, and of course, there are 6 and 7-gal buckets readily available, and you can run two or more in parallel.

You want the flow through a clarifier as slow as possible to provide the maximum time for the solids to settle. Timmons has a bunch of mass balance equations for calculating optimal flows but for our purposes, a simple airlift siphon provides all the flow needed.

In aquaculture tanks often a center bottom drain is used to eject the slurry. This is because in round tanks the circular hydraulic flows incorporated in round aquaculture tanks by design cause the solids to settle at the center. However, as long as there is a circular flow, an airlift pickup tube will do the job almost as well. It is easy to create a circular flow in a round tank. It can be done with air or water simply by ejecting the medium through an oriented orifice.

It’s only a little harder in a rectangular tank. Placing injector pipes in the corners and orienting the orifices in the same direction does it.

Now here is where it gets controversial. Believe me, I am not trying to pick a fight with scrubbers since I am one too although I disagree completely with the notion that a scrubber should be used to remove NH₃ and NO₂⁻. I believe what algae does best is remove NO₃⁻ (and phosphate, if that is a problem.) Here’s why.

Ammonia, NH₃, is oxidized by nitrosomonas bacteria, not by algae. There are at least a dozen different species but what they all have in common is that they are photophobic. That means they avoid light. In the presence of light they form an opaque slime as a shield. That may be what scrubbers are seeing when they start up a new scrubber filter, that yellow or brown slime that is not algae but some judge to be algae.

Nitrosomonas bacteria metabolize NH₃ to NO₂⁻ where the nitrobacter bacteria take over. I have not seen any reports that nitrobacter bacteria are photophobic. Now I do not dispute that algae scrubbers can perform nitrification but what I think happens is that colonies of both types of bacteria build on the substrate and green algae only grows after significant amounts of NO₃⁻ accumulate. This is the bane of RAS aquaculturists who desire to reuse as much water as possible. Throwing away expensive synthetic seawater in order to get rid of NO₃⁻ is both expensive and ecologically unsound. For this reason, and that nitrosomonas bacteria are photophobic, I prefer a separate biofilter upstream from the algae scrubber. That is exactly how I intend to implement mine once my scrubber is built.

There are two chemical pathways for the reduction of NO₃⁻: assimilatory and dissimilatory. Assimilatory is regulated by NH₄⁺ and uses plants, fungi, algae and bacteria. Dissimilatory is regulated by O₂ and C/N (carbon/nitrogen ratio) and uses anaerobic and heterotrophic bacteria. The former is what algae scrubbers do; the later is what bioflocs do.

Now think about that for moment. Nitrosomonas and nitrobacter fellows are aerobic while heterothrophs are anaerobic. Imagine trying to manage colonies of all three in the same soup. The concept comes from waste management and the eggheads are trying to apply it to aquaculture. It obviously can be done but getting it wrong outside of an academic research facility could cause one to go bust.

Currently there appears to be no academic interest in the former but lots of effort and research dollars are being spent on the latter. I think this is a mistake and have said so at Cornell and Texas A&M but since mixed-cell raceways are what are being promoted, bioflocs are preferred to algae scrubbers.

There is an interesting graph that shows the typical startup characteristics of bringing a new biofilter up to full capacity. It shows that ammonia concentration peaks at 14 days, nitrite peaks at 28 days and nitrate begins to accumulate after 21 days. This chart may explain the time lags folks implementing scrubbers see when they add one to their system. In short, first you get ammonia, then nitrosomonas critters, then nitrite, then nitrobacters, then nitrate, then green algae, in that order.

https://ag.arizona.edu/azaqua/ista/I...20Overview.pdf

See page 8 for the chart.

[amwassil]

Wow! Thanks very much for that detailed explanation. I'm dealing with fresh water, although I think many others on this forum are into salt and reefs.

I am clear on the de/nitrification process. My 80 gallon rectangular fish tank with about 75 gallons of water currently has a Marineland C360 canister, 2 Santa Monica Drop 1.2x scrubbers and a 5-gallon home brewed MBBR, soon to be swapped out for a 10-gallon version. My intention is to retire the C360 as soon as the MBBR can take over completely. It's not quite there yet as the bacterial colonization seems to be taking a long time to get established (it's been online for about 5 months). The algae scrubbers are there to remove nitrates, whatever else they do is of little concern to me. I intend to build a single, larger upflow algae scrubber and put the two drops into other smaller tanks that also need nitrate reduction.

Although the water quality in this large tank is good (except for high nitrate) quite a bit of debris accumulates on the bottom. In addition to a pretty heavy fish load, I also have a couple dozen large Apple snails. I currently clean off the bottom with a water vacuum I've made using an Eheim Classic 150 canister. I use this same device to clean the bottom of my turtle tank as well. I am hoping to eliminate the manual cleaning of the bottom somehow. Santa Monica suggested either to leave it alone and let bacteria consume it or to get some triops and see what they can do. It's not like there's an inch or two so I suspect that bacteria are already converting some of it into liquid that can be processed in the various filters. I've also decided to try the triops but have not yet started breeding them. Once I do get the triops started I'll report my adventures in another thread.

I have a question for you, though. I already have functioning MBBR filters on both my turtle and main fish tanks and I use sponge filters covered with 400 micron nylon bags as pre-filters. Still I notice that some debris gets into the MBBR tanks and settles out to the bottom. It's relatively easy to clean that out simply by removing the Hel-x and syphoning the bottom. Or during a semi-annual overhaul by rinsing it out after taking the whole system offline. What I'm wondering is if I remove the pre-filters do you think that would cause any issues with clogging the Hel-x? I suspect the MBBR water tanks are too small to get a really efficient settling action or install a baffle and that most of the debris sucked in would end up in the Hel-x or just recycled back into the fish/turtle tanks via the outflow. What do you think? Thanks.

[SantaMonica]

algae scrubbers can perform nitrification

A simple correction, you probably did not mean that algae can convert ammonia to nitrate.

Dissimilatory is regulated by O2 and C/N (carbon/nitrogen ratio) and uses anaerobic and heterotrophic bacteria

Are you saying that there are two different types of bacterial action? Because anaerobic would not be regulated by O2, correct?

[perlboy]

SantaMonica: You are quoting me correctly but you are not paraphrasing me correctly. I did not say algae can perform nitrification. They can’t. They can perform denitrification because they can assimilate nitrate. What I said and what I meant is that algae scrubbers can perform nitrification, because in addition to hosting algae, they can host nitrifying bacteria. Otherwise, why would anyone recommend an algae scrubber as the only filter in an aquarium?

amwassil: I think your sponge would filter some solids without the 400-micron sock. The best reticulated foam I can find is rated 20 ppi (pores per inch.) You will have to be the judge of whether yours is this porous or more or less. That’s 0.05”. Converting to SI:

0.05 * 25.4 * 1 x 10⁶ = 1,270,000 microns. That’s porous enough to trap lots of solids assuming there is hydraulic flow within the tank to keep solids in suspension long enough to be captured by the filter. Since you are getting lots of settling, the flow isn’t fast enough. With the sock you would only capture fines in suspension. When you clean the sponge what do you see?

All biofilter media have a souring action when agitated. I think the debris you are seeing in your MBBR may be the result of this scouring action rather than stuff passing through that 400-micron sock. Since you report high nitrate concentration your MBBR may be trying to grow some algae, perhaps not the type one would want but nevertheless, some non-bacterial biofilm, and it is being scoured off and if it is not being returned to your main tank, will settle out when you turn off the air. Rather than remove the pre-filters I’d first put a sock on the outflow of your MBBR, to make sure you aren’t passing any of this detritus back to your tank and then remove the socks from the sponges, and very closely inspect what the sponges are trapping. I’d also consider increasing the hydraulic flow in your tank to get less settling and let the pre-filters do their jobs.

SantaMonica: From Timmons, Recirculating Aquaculture, 2007; re your quote:

9.2 Factors Controlling Denitrification

When considering the various factors known to control denitrification, it is important to realize that reduction of nitrate to nitrogen gas proceeds via various intermediates, among them nitrite, a compound extremely toxic to aquatic animals. In aquaculture systems, it is important to identify not only the factors that inhibit or enhance denitrification, but also those that cause intermediate nitrite accumulation by denitrifiers.

Oxygen. Denitrifiers are essentially aerobic organisms (here he is talking about dissimilatory nitrate reduction) with the capability of nitrate respiration in the absence of oxygen. Therefore, under aerobic conditions, denitrification is not expected to take place. However, in aquaculture systems as well as other water treatment systems, nitrate removal in apparently aerobic environments such as nitrifying filters is not uncommon. The observed nitrate removal under these conditions is due to the heterogeneity of the environment. Accumulation of organic matter in an aerobic environment may lead to anoxic conditions within the organic layer or biofilm and may provide suitable conditions for proliferation and activity of denitrifiers. Furthermore, short anaerobic periods of normally aerobic environments may lead to considerable nitrate losses. This is not surprising when considering the fact that denitrifiers form an intrinsic part of microbial communities developing under aerobic conditions. Low oxygen concentrations in the environment may lead to nitrite accumulation as a result of the differential repression of nitrite reductase synthesis (what he is saying here is that the denitrifying heterotrophs are more efficient than aerobic nitrifiers) and activity as compared to nitrate reductase (Coyne and Tiedje, 1990; Korner and Zumft, 1989).

perlboy again: What I believe he is saying is that anaerobic bacteria are always present in a denitrifying community and they replicate during aerobic conditions, in fact, they out-replicate nitrifiers, but only work their magic when conditions are suitable (during anaerobic periods.) This is one of the reasons bioflocs are so difficult to manage since a large volume of water (the mixed-cell raceway they modeled this behavior in was 16.3 m x 5.44 m) can have both aerobic and anaerobic regions and it is virtually impossible to make one or the other uniform throughout.

I’ll say once more, the academic community is focused on dissimilatory (bacterial) processes rather than assimilatory (algae) processes. I can give you Michael Timmons' email address if you'd like to ask him for clarification.

[perlboy]

It's relatively easy to clean that out simply by removing the Hel-x and syphoning the bottom. Or during a semi-annual overhaul by rinsing it out after taking the whole system offline.

amwassil: Most commercial MBBRs have a bottom drain, not necessarily through the floor of the vessel, usually in the sidewall close to the bottom. Since all MBBRs exfoliate (scour) their media, more or less, the bottom drain conceptually serves exactly the same purpose as the bottom drain in a settling tank/clarifier. To use it, turn off the in flow and the air and let the suspended matter settle. How long? Go have a cup of coffee (Canadians drink tea, right?) and when you return, open the bottom drain's valve and purge 10-20% of the water volume. The media floats so it won't be lost. If a few pieces are lost, they are probably at the end of their useful life anyway. Or, put the media in a net bag, loose enough not to interfere with agitation but fine enough to retain it.

[amwassil]

I think your sponge would filter some solids without the 400-micron sock. The best reticulated foam I can find is rated 20 ppi (pores per inch.)

I'm using Ista Round Large Bio-Foam sponges. I can't find any specs for these things but the pore size looks a lot smaller than 20 ppi. My guess would be min 50 and possibly as much as 75-100 ppi. They filter quite satisfactorily by themselves. I added the fabric to protect the sponges. Originally, I had an Eheim 2075 canister on the turtle tank. The filter has an internal 'pre-filter'. The turtles dump a lot of sloughed off skin into the water which clogged the pre-filter quickly and thus required cleaning on a bi-weekly basis. Too much work! So I added bare sponges to the intake screens in both the turtle and fish tanks to keep the skin out of the turtle canister and to enable fewer filter cleanings with the Marineland C360 on the fish tank. Sponges worked great and less trouble to clean than opening the canisters. However, I found that getting turtle skin off and out of the sponge required a toothbrush and elbow grease. Even the sponge in the fish tank absorbed a heck of a load of debris. Thus, the sponges didn't last very long. That prompted me to add the 400 micron bags around the sponges. The bags rinse easily and I can bleach them without killing the bacteria in the sponges. The sponges clean quickly by rinsing/squeezing and now last a lot longer.

Most commercial MBBRs have a bottom drain, not necessarily through the floor of the vessel, usually in the sidewall close to the bottom.

That makes sense for a large container. In my 20-gallon combo filter attached to the turtle tank I've got about 10-12 gallons of water; and in the 5-gallon filter attached to the fish tank about 4-4 1/2 gallons of water. Scooping out the Hel-x to clean the bottom is not much work; plus, it enables me to inspect/juggle the Hel-x to clear any impacted debris. Whenever I stop running the MBBR filters to any reason, I put a nylon sock over the end of the return pipes and leave it there for two or three hours to prevent any debris getting back into the aquaria. If I were to make a bigger sized filter, I would build it around a drainable bottom and I have a couple of ideas of how to do it.

[amwassil]

If I were to make a bigger sized filter, I would build it around a drainable bottom and I have a couple of ideas of how to do it

Actually, I realized after responding with the above, that I can already drain the water out of the container. This because I put unions in the hoses both immediately before and after the water pump to facilitate taking the pump offline for cleaning. The input through the bottom of the container has a three inch ABS nipple screwed into the top of the bulkhead. After removing the pump, I would only need to unscrew the nipple and open the valve to drain the water out of the container. If there was a lot of debris sitting on the bottom I could stir that up to make sure it went out with the water. I hadn't thought about doing that previously. Amazing!

I also just realized that in your previous post "Purging an MBBR" you were likely referring to doing just this: "Go have a cup of coffee (Canadians drink tea, right?) and when you return, open the bottom drain's valve and purge 10-20% of the water volume." So maybe it just took a while for my aging mind to 'get it'. Thanks. And, yes, some Canadians drink tea, the rest of us drink beer.

[perlboy]

amwassil: Good idea, making maintenance drain relatively easy and using gravity whenever possible.

You and I seem to be on the same page wrt our interest in MBBRs, so I’d like to get your take on another idea I’m pursuing. It’s a DIY Packed Bead Filter. Most PBFs are tasked to remove solids but they also can perform nitrification. There are lots of commercial examples (used in swimming pools, spas and Koi ponds) but they are all hideously expensive, as in: http://www.beadfilters.com/bead-filt...ters/assembly/

and

http://www.aquadyne-filters.com/Aqua...stems-c-2.html

BTW: that first link also contains design sketches for simple-to-build MBBRs.

The propeller washed variety is particularly interesting because by not using air to rinse the beads, it allows the vessel to remain anoxic. Why would one desire an anoxic MBBR? Here is a link to a study that explains the notion known as partial nitrification-denitrification: http://www.bioline.org.br/pdf?se10041

The point of this configuration is the complete elimination of all nitrogen compounds, not just ammonia. It exploits the different rates of growth of aerobic and heterotrophic bacteria by using a 3:1 internal recirculation design, from anoxic to aerobic and back.

One reason the cost of a propeller driven PBF is so dear is the mixer motor, shaft and thrust bearings. The idea I have to agitate the beads during the rinse cycle is to use one or more Koralia directional water pumps – very cheap and clearly, very effective moving water. Another high-cost piece of equipment is the multi-port valves used to control the different flows through the PBF. The same can be accomplished with ball valves and the intake and uptake screens can be made from 0.04 in slotted PVC well screen; all through-tank connections via Uniseals. I’m thinking of making a prototype from a 7-gal bucket with a Gamma-seal cover: http://www.usplastic.com/catalog/item.aspx?sku=1862

Thoughts?

My favorite Vancouver Island Brewery offering is Sea Dog Amber Ale.

[amwassil]

A single filtration unit that transforms toxic nitrogen compounds to oxygen and nitrogen is attractive, which is exactly why I added the algae scrubber to my MBBR. Technically, the PBF filter sounds like an interesting project. Can you make one that does not require water under pressure? Also, I'm leery of anaerobic/anoxic environments, especially in a sealed container.

[perlboy]

Although most PBFs, either propeller washed or blower washed, use pressurized vessels, I’ve found one sand based design that originated at LSU that uses an unpressurized open-top vessel. I will follow up with the company (Dolphin Fiberglass Products, aquaculturetanks.com.) I don’t see why a PBF must be pressurized. It seems to me the only difference between PBFs and MBBRs is one has a static bed while the other is fluidized. They both flow bottom to top. The beads, typically 3-5 mm plastic spheres aren’t technically different other than size and shape from other bio-media. By this I mean they facilitate bacterial colonization. Certainly a spherical shape packs more closely than does the complex cylinders we see in bio-media. It is that packed bed that filters solids, the primary reason PBFs are used in aquaculture.

I was attracted to algae scrubbers for the same reason, I think, you were; removing nitrate compounds that are left behind by MBBRs. But I can’t ignore the advice I’m getting from my friend at Cornell, Michael Timmons. I now have a detailed design of a mixed cell raceway, and surprise, surprise, it employs both an aerobic MBBR and an anoxic PBF. What’s even more interesting, the flow rates through the two filters are different and the output of the PBF is not sent to the MBBR. The MCR design he sent me incorporates what is known as a Cornell dual-drain system. In this design 15-20% of the flow is removed from the tank’s bottom center drain while 80-85% is removed from the tank’s side drain. Picture a side drain as a screened orifice at water level. Typically, it is a side sump with a bottom drain. In the MCR (rectangular, length 3x width) Timmons sent me there are three counter-rotating cells each with a center drain and a side sump. (Could be length 2x width, 4x, 5x, 6x, etc.) The center drain flows, which contain the bulk of the solids are sent to the anoxic PBF. The output of the PBF is passed through an oxygenation cone and then returned to the culture tank. Why? Because a propeller-driven PBF is anoxic, by design.

The side drain flows, which contain both fine suspended waste and waste in solution are sent to the MBBR. Each filter has it’s own pump. The flows are typically 2:1 or 3:1 MBBR:PBF, which is strikingly similar to the 3:1 internal recirculation in that partial nitrification-denitrification study I included. Water is fungible and nitrogen ions at various stages of the nitrification process are very likely well mixed, especially if the tank volume is turned over the recommended 2x/hour.

I was thinking I would build a non-pressurized PBF and an MBBR (small, pilot system to study nitrification) and connect them in series with 3:1 internal recirculation, but now that I’ve seen Timmons’ MCR design, I’ll connect them in parallel with separate flow rates. I’m not sure how much of any of this applies to maintaining reef tanks or a few marine fish but the issue for me is the efficient elimination of nitrates. An algae scrubber is not the only way to do that.