Sewage: the answer to our prayers?

Poo as fertiliser - why we're flushing away a valuable source of phosphorus...
06 May 2016

Interview with 

Pete Vale, Severn Trent Water


When we flush the toilet, we are disgarding a valuable source of phosphorus. Could we use our poo as fertilister? Pete Vale from Severn Trent water thinks so, as he explained to Graihagh Jackson...

Graihagh - Just over 15 years ago, a professor of civil engineering in the US, Don Mavernick, was trying to find a way to get rid of a pesky white mineral that clogged pipes in sewage treatment facilities. This pesky white mineral was struvite, a combination of magnesium, ammonia and, yes, you guessed it - phosphate. He thought, "well, what if we could choose where this struvite formed, i.e. not in pipes but in a convenient collection pot? Then we could stop pipes being blocked and sell this struvite as fertiliser - win, win!" But, somehow, the idea of soiling my sorrel with a derivative of human excrement seems unpalatable at best, abhorrent at worst. Before we make any judgments though, let's see how you get from poop to plantpot at Severn Trent Water...

Pete - There's three main stages to sewage treatment. The preliminary treatment stage where we take out core solids and that's what you can see over in that corner - the big steel structure. Then we have the primary treatment where we settle out the settleable solids - the faecal matter. It might sound quite unpleasant but it's actually full of energy and it's a really useful resource for us because we take that sludge and we digest it in anaerobic digesters, which are those huge concrete covered tanks that you can see a couple of hundred yards away.

Graihagh - And those concrete tanks were massive!

Pete - They are very big tanks. There's an awful lot of sludge in those tanks.

Graihagh - Like five stories high full of sludgy, sludgy, poo. All cooped up in an oxygen free, or anaerobic tank.

Pete - Anaerobic bacteria break down the organic content in the sludge and produce methane, and that methane we take and burn in a gas engine. The gas engine produces electricity, which we can then use on site, and export to the grid, and heat. And the heat we recycle back to the anaerobic digesters to keep them warm because they like to operate between 35 and 40oC.

Graihagh - And after this 15 days, what happens to all that sludge in there?

Pete - The sludge then is stabalised, so that means all the pathogens in it have been killed. We then dewater the sludge, so we use Stoke Bardolph centrifuges. So we wizz the sludge around to form what we call a "sludge cake."

Graihagh - This delicious sounding sludge cake is one of the end products of all our poo. It's the mother of all confectionery in the plant world, plied with important ingredients needed for life, and who wouldn't want to see that...

That's the sound of a fresh sludge cake dropping out of the centrifuges and, on closer inspection... Oh it's steaming! It's steaming! It's very um... yes they're basically like mountains of black steaming soil but this is really good for crops, really nutritious, rich in, I guess, phosphorous but also other nutrients as well?

Pete - That's right. So rich in nitrogen and rich in phosphorous.

Graihagh - Now, remember how the centrifuge syphons off the liquid from the solid sludge cake. Well this liquid is called "the liquor" and it's extremely rich in phosphorous. But Pete still needs to make it into struvite - this white mineral, so it can easily be sold and transported as fertilizer...

So, we left the sludge cakes in search of the phosphorus recovery plant. A big concrete barn decorated with numerous coloured pipes...

Pete - We've taken the sludge liquor that's come from the centrifuges and we're dosing in some magnesium to get the struvite to form. And the other thing that helps the struvite form is to raise the ph a little bit so, by blowing a bit of air into the tank, you strip carbon dioxide out, which raises the ph a bit and then the struvite forms crystals. What happens to this is it's sold to a fertiliser blender, who makes a liquid suspension fertiliser out of it. So they add in nitrogen and potassium to make a balanced fertiliser.

Graihagh - And if every sewage plant in the country was making this struvite, how much could we make?  I mean, would it be enough to cover the sort of stuff that we're importing at the moment?

Pete - It's a bit hard to calculate but, as a rough guide, I think if we were able to recover all the phosphorus from sewage, and that's not an easy ask I have to admit, but it would account for about 50% of the total mass of phosphate we import into the country. So, a significant contribution.

Graihagh - But is it commercially viable - what's the sort of price range of this sort of stuff versus the stuff we're importing from rock phosphates, for instance?

Pete - A very interesting question and, at this stage, the revenue we get from the struvite offsets the cost to make it but doesn't provide much revenue. Now that may well change over time as phosphate rock reserves are beginning to deplete, so the likelihood is phosphate rock prices will rise.

Graihagh - I mean, values one thing, but I think the perception of turning what is essentially sewage into something that we then put on our crops and use to grow and then eat again, is somehow - I know it's treated - but it's somehow unpalatable. I mean, would you be comfortable eating crops from this?

Pete - Absolutely! It's a purer form of phosphate than mined phosphate rock. It doesn't have contaminates that phosphate rock has that can be fairly high in cadmium and all sorts of things.

Graihagh - So, whilst sewage is seen as  - well sewage, today actually this will be a resource. This is something that you could actually marketably sel. I mean, there's something quite amazing about that.

Pete - Yes, absolutely right! I think that's very much the future of sewage treatment because there's an awful lot of useful stuff in sewage.


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