Anaerobic digestion is clearly a good way to get energy back from our organic waste, but that doesn't tell the whole story. The digestate, the solid waste left behind, will contain lots of nutrients that can be returned to the ground as fertiliser.

But if this only happens close to the digesters themselves then we'll end up with a nutrient imbalance. Some areas will have too much and others will have too little.

I spoke to Cynthia Carliell-Marquet from the University of Birmingham to find out what the implications of these could be.

Cynthia - There is a lot of emphasis on anaerobic digestion in terms of the energy that we can recover from organic waste and the government has identified 20-30 million tonnes of food waste, 100 million tonnes worth of agricultural waste and there's about 2 million tonnes worth of sewage sludge waste, all off which can be anaerobically digested if we can construct enough anaerobic digesters.

The issue that hasn't been considered as fully as energy is the fact that in those wastes, there is specifically nitrogen and phosphorus which are more difficult to extract and recycle that energy.

Ben - Surely, if we are using a technique that allows us to keep hold of the nitrogen and the phosphorus, and then put it back on the land then surely, this is a good thing rather than just locking it away in a landfill somewhere?

Cynthia - It is a good thing. Returning the sludge of the digestate from anaerobic digester back to the land is a good thing. The problem or the challenge comes in from the fact that the digestate, even when it's dried, has quite a high concentration of water which makes it very bulky to deal with and to move around, and to store.

Generally, it would be quite difficult to transport it economically, more than about say, 20 miles in a radius from your anaerobic digester. So if you've got enough land to accommodate that digestate then yes, putting it back to the land with this nutrient potential is a really good use of it. The challenge comes in getting it back to where you need it to be, so getting it back from livestock areas, back to arable crop production for example.

Ben - So we're going to end up with a very uneven distribution of nutrients and particularly, you've mentioned nitrogen and phosphorus. What is it about these two that we need to be cautious about?

Cynthia - They both have completely different issues. Nitrogen is quite energy hungry. It's freely available. We make nitrogen fertiliser from nitrogen in the atmosphere, but it's a reductive process and it uses a lot of energy. Phosphorus is a completely different issue. It's a finite resource. All the phosphate that we introduce into our agricultural system comes from phosphate rock mines. It's a non-renewable resource and there was some talk about 2 years ago that we were going to run out of phosphorus. That's been explored quite fully now and it seems that we're not going to run out ofphosphorus - at least in the short term. There isn't the urgency that surrounds some of the other minerals that we feel we're going to run out of much more quickly, maybe even within a decade. With phosphorus they're now looking at say, 370 years before we run out.

But the phosphate rock mining industry is, in itself, quite a polluting industry. It's not particularly sustainable to say "right well because there are quite a lot of reserves, we can just keep using this and then go and dig up some more, and re-use that".

So, it's a question of sustainability and trying to close resource loops, and trying to be more economical about the way that we use resources. The way that we use it is just not sustainable. I think we lose about 80% between Field and Fork. So about 80% of what we put on you know, does not end up in our food. So we mine an awful lot of this stuff. We put out on the fields, we do produce crops at the yields that we want, but it's just a very wasteful system.

Ben - What would be the impact on a given area if it ends up with not enough phosphorus? So we're just going to see farms unable to grow food anymore or is it a bit more substantial on the whole ecosystem?

Cynthia - Phosphorus is really an issue for farming, for food production. The yields that we manage to attain today that help to feed the world's population are, to quite a large extent, dependent on phosphorus fertiliser. If you don't have enough phosphorus and you don't have enough phosphorus in your soil then the yields will decline until it gets to a point that it will not be possible to produce enough food.

Ben - And is there a problem if we have too much of it, if too little will lead to reducing food crop? What will happen if we've got too much?

Cynthia - If you've got the right amount of phosphorus on your field then it'll be immobilised within the soil through precipitation reactions. But if you have too much phosphorus then it can be washed off the field into the water bodies where it performs pretty much the same job as it does in agriculture. Generally, phosphorus is a rate limiting nutrient. When you don't have enough phosphorus, it keeps plant growth in check. This is the same within rivers and all freshwater bodies. So when you get more phosphorus in the river then you get a big increase in plant growth or in algal growth, and this causes a problem to the whole river system, to the biodiversity of the river system, and it also causes a problem to people who use the water. Algal overgrowth for example is problematic in terms of extracting water, drinking water. Obviously, for farmers as well, it can be problematic if it changes the water course such that they can't use the water for irrigating their animals.

Ben - It sounds like we're in something of a Catch 22 situation really because we can't regain it from our waste products without things like anaerobic digesters but that will then be inefficient and expensive to transport it around the country. What solutions do you think we should be looking at?

Cynthia - The solutions have got to be done on a case by case basis. So, if the local situation is suitable for putting that manure or that digastate back to the land, that sounds like a very good solution. If the local situation is such that the area already has a phosphorus surface, in that case, there is a real need to get the phosphorus away from that area and one of the things you can look at is trying to extract the phosphorus from the digestate or from the animal manure and there's various chemical processes that you can use to extract the phosphate. Then you can try to re-crystallise it in some kind of mineral phosphate form that would be very similar to a mineral fertiliser. So forms of, for example, Struvite which is magnesium ammonium phosphate or calcium phosphate would mean that the phosphorus was in a much more manageable form which we could then store and transport pretty similarly to mineral phosphate.

Ben - Cynthia Carliell-Marquet from the University of Birmingham.