Fish food from abattoir waste

Can the waste water from abattoirs be put to better use than going in the bin?
21 January 2020

Interview with 

Navid Moheimani, Murdoch University




However much progress is made towards reducing food waste, arguably some waste is inevitable. But what if it could be re-purposed? Scientists in Western Australia have found a way to make better use of the waste water from abattoirs. And, if you’re eating right now, you might want to put down your knife and fork for a few minutes! The group have developed a process to use the nutrient-rich blood and other waste material left over after animal slaughter and turn it into a food for other valuable foodstuffs like prawns and shellfish. They first pass the waste water through an anaerobic digester to convert the organic molecules to methane (which can be used as an electricity source), leaving a liquid rich in ammonia and phosphate. These compounds are fed large pools containing algae, which are aquatic plants. The algae use them, together with light, to grow. Chris spoke to Murdoch University’s Navid Moheimani. First up, he asked, quite how much waste water are we talking about that abattoirs need to get rid of?

Navid - I've done just a little calculation this morning and from a single abattoir in WA, we can generate something around 400 tons of algae per year.

Chris - So it's literally, Olympic swimming pools' worth of water regularly coming out of these industries?

Navid - Yeah, something in that line.

Chris - How do you use it then? Have you've got a swimming pool that you actively fill and turn into an algal bath?

Navid - So the algae grows in there, they take up all of these in-organics, which are nitrogen and phosphorous, and they convert it to the goodies - protein, lipids, and carbohydrates. Those are now very valuable for the aquaculture, for instance, and we can use them as a prawn feed.

Chris - Algae need a lot of light. Are these pools that you put the water in outside? They're just effectively a pond? Or have you got some other clever setup for growing the algae?

Navid - So we try to keep everything as cheap as possible. So these ponds - we're not introducing any other systems - we use a technology called raceway ponds. The shallow Raceway with a paddle wheel on it, which would cause the mixing. So mixing is very important because that mixing would allow the algae to receive the light.

Chris - Is this a continuous process or is it a batch process? Do you fill a pond up, stir it up, let the algae grow, get the algae, chunk the water away? Or do you feed a bit of water in steadily and let the algae continue to grow?

Navid - We call it semi continuous because algae would only receive the light 12 hours a day. We cannot continuously harvest it, but we can semi-continuously harvest it during the day and add the new effluent to it.

Chris - And how do you get the algae out then?

Navid - That's potentially the hardest part of the whole process. The algae, even in the most concentrated, algal pond is still something around one to two grams of algae per litre. So concentrating that is one of the challenges that we face. However, we've been looking at very novel technologies for taking the algae out of the water.

Chris - Cause people sometimes use things like ultraviolet light, which makes the algae die and get charged, so they all clump up and then you can filter out the clumps.

Navid - That can be done. We don't necessarily like to kill them in the water because if we kill the algae in the water, that means that they would release some of the stuff that inside of the cell out of which we don't want to do that. We want to collect them as healthy as possible because then that we can feed it back to the aquaculture organisms.

Chris - And once you've got the algae, what do you do with it then?

Navid - If you are using it as a fertilizer in agriculture for instance, we can use it wet. If we are using it as an aquaculture feed, we need to dry it, we need to palletize it, and then we can directly use it as a feed for aquaculture organisms such as prawn, marin, crayfish, etc. It depends where you are.

Chris - And is this environmentally sound? We're very worried about feeding animals to animals because of the risk of cycling various toxins, other possible infections and so on. Is there anything that might be in there that could be picked up by the algae, concentrated by the algae and then introduced back into the food chain? Because at the end of the day, these things are going to be fed to things that could be eaten by humans, or even eaten directly by humans, so it's got to be safe.

Navid - That's a very good question. So far in none of the animal waste that we are dealing with, we picked up any heavy metals. We also didn't pick up anything toxic. However, if you are dealing with the waste, which can potentially contain a huge amount of heavy metals, you're right, algae may be able to accumulate those. So far we dealt with the piggeries waste and also abattoir waste. We could not detect any heavy metals or any toxic in the process.

Chris - Antibiotics that are given to farm animals? There's also questions about some drugs, some hormone treatments. Could these things be concentrated? And also some algae make toxins in their own right, don't they? Blue green algae. Not that you're going to grow any of those, but they exist. And things like ciguatera toxin, which you get from the ocean.

Navid - Normally the cyanobacteria or dino-flagellates are the one which cause toxic algal bloom or they are producing toxins. The species that we're using, the green algae. These guys don't generate any toxins. Now, antibiotics, and you're right, somebody may actually feed antibiotics to their animals. However, the process we're going through is not directly using the effluent. The effluent goes primarily to anaerobic digestion. It's already treated. So if there is any antibiotics at that point, it's already been gone. If not, the process of anaerobic digestion wouldn't work, so I'm fairly confident that there would be no hormones and no antibiotics by the time that we get the effluent to grow the algae.

Chris - Say I'm an abattoir owner. How much revenue could I see coming back from this? What would previously have been a complete waste stream that I have to pay money to get rid of?

Navid - I don't have all of the numbers, but I can just summarize it this way. That right now, as you rightly said, this effluent is not being used anywhere in the world. So now we can potentially produce 20 grams per square metre per day of algae generated from this. We think that the cost of producing of the algae is around $1 to $1 50 cents a kilogram. Even if you're not putting any value on the water which has been cleaned, if we can sell the algae anything higher than $1 50 cents a kilogram, we generating revenue for the abattoirs.

Chris - You're very lucky here in WA that the sun shines a lot more than it does, say, in Britain. To what extent is that going to determine the viability of doing this? Because abattoirs are everywhere, but the sun doesn't shine everywhere.

Navid - This process would work in every place that you've got light. Now, if you don't have access to the light in some places such as a Scandinavia, they've got access to a huge amount of geothermal that would generate very cheap, at some points, even free electricity. You can use those electricity to generate the light, to grow the algae in more contained conditions. In some places, some other places, I have to admit, this may not work, so it's not a unique process that can work everywhere in the world. You must give them light. Light is a must for the algae.


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