Well thank you for basing the calculations on my home town! One genuine question on the phosphate and nitrate piece in the system that is used in this model on farms. We have in the Poole Harbour catchment issues with these in chemical form polluting the water. Is there a difference here or would there need to be a measure to deal with that in the design?

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May 17·edited May 17Author

the calculations were based on the inhabitants of Poole, UK because the people there are particularly regular with their bowel movements. Nobody is sure why. Lots of fish and chips maybe?

The P and N in the harbour is a problem, made worse probably by the shape of the harbour which is wide but with a narrow entrance. This means that the waters get less mixed even with the tides. The sources of the P and N would be the usual suspects, sewage discharge, surface run-off, industrial discharge. Stop them and the harbour water will clean up.

It's a ridiculous situation as nitrogen is produced through a very energy (oil/gas) intensive process. The Haber process requires high pressures (around 200 atm) and high temperatures (at least 400 °C), and a lot just runs off or is otherwise wasted.

Billions of kg of phosphate rock are mined annually, but the size and quality of the remaining ore is decreasing and the process requires huge quantities of sulphuric acid. Much of the P applied to soil as mineral fertilizers or organic manure is bound to soil, creating a pool of residual P, or is lost via leaching, runoff, and/or erosion and may reach waterbodies. Agricultural practices have to change to reduce runoff and to recycle these minerals. Return of sewage sludge can help but biogas digestate would be better.

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