Scalability of algae scrubbers in food-fish applications

[perlboy]

Hello at AlgaeScrubber.net,

I would like to ask a question about the scalability of this technology.

I am and have been for several years involved in several projects to raise food fish in RAS (recirculating aquaculture systems,) first tilapia and now Pacific white shrimp. To make a long story short, the killer problem with both of those cultures is nitrate accumulation. Both tilapia and various varieties of shrimp are raised in many parts of the world in pond cultures where the nitrate problem is dealt with by dilution. This is environmentally unsound and is not sustainable for the long term. In many coastal areas where shrimp are cultured the ocean is suffering from the artificial nitrate load. Increasingly, there is political resistance to such practices. For example, both California and Alaska have permanently banned the construction of pond aquacultures from their coastal environments. And more intensive fishing is not the answer. From satellite images you can see the damage done to coral in the Gulf of Mexico by bottom trawlers, not to mention the collateral damage of the bycatch, the unwanted species caught and killed as a function of taking the targeted species. By value worldwide shrimp production, fishing and farming, is the leading source of seafood for human consumption, a multi-billion dollar industry.

At the University of the Virgin Islands wastewater from tilapia is successfully used to fertilize leaf lettuce, basil and similar non-fruiting plants in a floating raft system. In Israel promising research in zero-exchange, aerobic, heterothropic systems (ZEAH or by its other name, Biofloc) is being done. Unfortunately, neither approach responds well to downscaling. D.O. in biofloc systems is problematic, especially in smaller (relatively) tanks; balancing the simultaneous management of both aerobic and anaerobic bacteria colonies is challenging and not yet well understood. ZEAH also has a marketing problem: try selling shrimp to consumers after you’ve first explained that the water the shrimp have been living in, and thriving in, resembles a septic tank.

None of the people I’m working with have any confidence in using algae as a nitrate filter. My friend at Cornell recently wrote: “If you want a system that makes a profit, I’d avoid anything to do with algae other than what might occur without intention in a bioflock (sic) system.”

That may be because they have so much intellectual capital invested in dilution and ZEAH but I have yet to see a successful implementation of ZEAH in round tanks. At Cornell and Texas A&M the current thinking is something called a mixed-cell raceway, which is claimed to have the best characteristics of circular tanks and linear raceways. Using air circular adjacent counter rotating currents are created within the raceway to cause solids to settle out at center drains, as they do in round tanks. The remaining waste products are managed by the bacteria in the water, a green soup. Unfortunately, the smallest one of these I have seen, a prototype, is 16.3 m x 5.44 m, built inside a greenhouse (sunlight for photosynthesis,) a long way from the four 2,060 g tilapia tanks at UVI (the folks there scaled their system down to four 380 g tanks but to my knowledge, none of that size has been built. And of course, this does not work with salt water.

I am intrigued by what I have seen and read here. I lurked and read many of the threads before amwassil’s post made me want to register and join the discussion. After learning what he has done with an MBBR/scrubber combo, I’ve spent all my time since designing on paper various down flow and up flow examples in an attempt to see how one might scale them. I can solvent weld acrylic and since light boxes are mostly simple right angles, it seems relatively easy to pack a lot of filtering power into a small space. One back-of-the-envelope up flow design I came up with is four banks of 9 Expressions LED light bars mounted vertically 3 in o.c. in a 35 g Roughneck tote (32 in x 20 in x 19.6 in.) That’s 4x say, 12 x 27 in² of algae substrate and 324 watts, according to Expressions data, not bad if it does the job, except that’s 18 transformers. Is that really practical? Even if I had to scrape the algae daily because it was assimilating so much NO₃^-, if the culture tank was free of nitrate it would be a big win.

But what could such a design accomplish?

What could a smaller scrubber, say one half that size accomplish?

What if such a system was too small?

I need to get a better handle on scrubber properties and their scalability before I can proceed to build even a small pilot, since the cost is several thousands of dollars. So, here is my question and the assumptions inherent in the system I want to build:

The smallest round tank in which Litopenaeus vannamei, Pacific white shrimp can be raised to market size, profitably, is 210 g (794 l). This is done with 5 tanks stocked serially every 14 days with 10 g individuals and raised to three cohorts (22.7, 30.2 and 45.4 g avg wgt). Twenty-five % are harvested at 22.7 g, another 25% at 30.2 and final harvest when the remainder reach 45.4 g. There is an 8-week lag before the first tank is ready to harvest but after that there are mature shrimp available weekly. Such a system has a theoretical yield of 3,400 lbs of shrimp/year.

Remember, this is a pilot. If it is successful, the next one might be 5x 500 g tanks, or 800 g, or even 2,000 g, all readily available round tanks, using the pilot system to raise shrimp larvae to stocking size. Picture this: suppose you had a spare fifty grand lying around. You could lease an empty warehouse in Chicago and put in 5x tanks to raise shrimp and sell them locally. Do you see how the scalability of the nitrate filter, whatever type it is becomes the pacing item? I’ll name my first grandchild after whoever helps me permanently solve the nitrate problem, that is, if there is a solution.

Now please consider one tank from the POV of sizing an algae scrubber?

210 g (794 l).

Water volume: 0.794 m³

A minimum of 2x tank volume passed through the filters/hour

Biomass at maturity: 56 kg/m³ (based on experience with tilapia, UVI actually got >60)

Daily feed: 3% of body weight at 35% protein; that’s 0.794 x 56 x .03 = 1.3 kg/day at maturity

FCR (feed conversion ratio): 1.5 – 1.8

Salinity: 32 – 35 ppt

pH: 7.8 – 8.2

Water temp: 28 – 30°C

The scrubber’s only job is the removal of nitrate. A settling tank is typically used to remove suspended solids and an MBBR for nitrification.

So, given feeding 1.3 kg of protein rich food/day during late-stage maturation, what size scrubber would be needed? (I’m assuming the answer to this question is the surface area of the substrate. Please correct me if that is wrong?)

Thanks in advance to all who read, consider and comment.

[SantaMonica]

Yes they are a good combination, the shrimp and algae (scrubbers). In addition to removing nitrate, the scrubbers could also handle the ammonia and nitrite, in one nice package. And to top it off, the shrimp can eat the algae and thus reduce feeding costs (waste nutrients are converted back into food).

The Expression lights are way too small; the larger plant-grow hydroponics lights are preferred at that size.

Do the shrimp prefer shade or sun, and will sunlight be available during the day?

[perlboy]

Shrimp seem to do equally well indoors or out. The mixed-cell raceways that have been built so far have all been inside greenhouse bays. The light is for the algae/bacteria in the water rather than for the shrimp, although nauplii (first larval stage) are attracted to light. That is how they are collected in hatcheries. Pacific white shrimp are egg scatterers. After a pair spawn they are removed. After the eggs hatch and the nauplii are motile the room lights are turned off and a light is shined on the surface where they are collected. My project will be indoors but well lit.

I again searched through algaescrubber.net archives after I posted my question and I found several references to allowing 1 in² of substrate/gallon of tank water. However, I've also seen references to using the amount of food fed/day as the real criteria for sizing a scrubber's substrate. I don't know how to convert cubes/day to the way we feed shrimp, which can involve kilograms/day fed on a continuous basis with a spring powered belt feeder. Do you think that 1 in² parameter is valid for food loads of the size I've mentioned?

I've searched the Internet for LED grow lights and those targeted at hydroponic growers are massive compared to light bars. The fixture builders seem to have embraced larger is better, 3w and 5w chips. Hanging the light two feet above a couple of MJ plants makes sense but 1-2 inches from an algae substrate seems like overkill. Those I've found don't seem suitable for the sort of up flow scrubbers I've read about here. Can you be specific with a recommendation of a light or a light supplier that has LEDs that are suitable for scrubbers?

[atoll]

Re lighting, could this thread be of assistance .
http://algaescrubber.net/forums/show...-Tank-Lighitng

[perlboy]

Interesting... Thanks for the vector. Way too much $$$ for small pilot but that algae assoc is another useful source. I'm thinking I need to build an example and run some tests of the type performed to evaluate filters. For example, there is a protocol aquaculture researchers use to measure the effectiveness of different types of filters. It involves introducing ammonia into a culture tank to simulate biomass respiration and then measuring the amount of nitrate produced over time. I need to review the protocol with an interested colleague at Cornell to verify my tests correctly mimic the biomass in the tank and the feeding schedule. What I'm interested in discovering is the relationship between the size of the substrate and the amount of food fed to a given size of biomass. That's the critical parameter. The biomass is being encouraged to grow by continuous feeding and this produces waste. There is some predictable relationship (to be determined) between feed by weight and algae production that will determine the commercial scalability of algae scrubbers. Interested minds want to know...

I have an idea for a rectangular acrylic box similar to the Expressions box but designed for up flow rather than down flow. The box will have three chambers, two for lights, one for substrate. The lights will be mounted vertically rather than horizontally, as done in the Expressions box. I have a source for extruded acrylic tubing. This tubing can be cut in half lengthwise to yield U channels. These U channels then can be solvent welded to the partitions so that light bars and substrate can be easily inserted from the top. At the bottom of the substrate partition will be a ceramic airstone, 1.5" by the horizontal length of the box, say 9" (0.75 cfm). I can visualize a 10" x 12" substrate lit from both sides by 6x 9w light bars and two of these boxes will fit inside a 5 gal square bucket. That's 4x 120 in² (two sides). This would represent a modest investment and would permit the type of testing referred to above. If this proves workable, the width of the box can be changed as a function of the size of the light-tight container, as in the example in my original post, a 35 g Roughneck tote.

I've also found those Expressions light bars in 11w size, 2 blues and 9 reds, from pcgrowcase.com. I'm discussing with them now some sort of bus to connect multiple light bars to a single transformer. I know these puppies exists, it's just a matter of finding one of the right size.

[SantaMonica]

Screens that are 10 x 12 = 120 square inches, so two of them would be 240 square inches. This would need 240 real watts of CFL, or 120 watts of LED. For LED, this would be 30 watts per side. If those light strips are 11 watts:

http://pcgrowcase.com/grow-box-acces...light-bar.html

then you would need 3 of them per side of screen, for a total of 12 lights.

[atoll]

http://www.ebay.com/itm/10W-20W-30W-...item339ee11b2e

[SantaMonica]

Yes but those will not fit into his sliced-tube idea.

[perlboy]

Not only that but there is too much surround frame housing which would obviate butting several side by side if and when I attempted to up scale. I am greatly attracted to the notion of a sandwich, lights on the outside very close to the substrate, the entire package inside a light-tight box. However, there is no "right size" for what I am envisioning. I see the sandwich box as a module in an expandable filter system: one or two if that is enough filtering capacity, or multiples of them if larger is required. It seems like there are only two physical limitations: the ratio of light bars or light packages to transformers and the size of the light-tight box. Remember, this is a commercial application, to raise lots of shrimp for human consumption.

[atoll]

i am using 2 of the 20w versions on my waterfall scrubber and I have them within 3" of the screen I could put them closer, the spread is very wide as there are no lenses.