Wastewater mining

The nutrient crunch is not a headline grabber, but it ought to be and it is something we are going to learn to get pretty concerned about.

Wastewater used to be seen simply as a waste, something to be treated and disposed of. It is nothing of the sort – it is a vital and under-appreciated resource. Apart from the water that can be recovered, it is a source of increasingly scarce nutrients and contains significant amounts of embedded energy.

From 1450 to 1850, London’s dung was carted to sewage farms, where it was spread, untreated on the ground. It was a brutal job (the carters were well paid, but tended to die young) which provided fertiliser for farms supplying the city. In the 1850s the Guano trade blasted the sewage farms out of use, which was probably a good thing at the time. It meant on the one side a safe source of nutrients and that sewage treatment was taken seriously as an alternative to dumping raw effluent on farmland.

At its peak, the Guano trade saw 100,000 dry tonnes of fertiliser being imported to Britain every year. It has been taken over in turn by other sources of phosphorous, nitrogen and potassium, but none of these sources are likely to last for much longer given the rising demand for food worldwide.

The real problem is that when nutrients are washed out into rivers and seas not only do they harm these habitats but they cannot be beneficially recovered. The nutrient cycle is different to the water cycle – there is no vast sea of nutrients that can be mobilised each year and replaced on the ground. For example, the natural rate of airborne nutrient deposition on a cleared site such as a former clay pit is such that it takes approximately 100 years before there is a nutrient build up enough to support ‘non-leguminous woody shrubs’ as I was taught as a young Environmental Biologist at university thirty years ago. Clearly, it is a fact that has stuck in my mind and it shows just how dependent we are on securing nutrient supplies to optimise agricultural productivity.

The more intensive the agriculture is the more nutrients are needed, even is genetically modified crops become more broadly socially and politically acceptable. Whichever way agriculture evolves, it faces the task not only of feeding the currently hungry, and meeting the challenges of affluence and dietary change, there will also be a further two billion mouths to feed by 2050.

So it is time to take a modern look at nutrient recovery from wastewater. This means going as long way beyond applying post treatment sludge to land, let along the sewage farms of old.

Properly applied, recovering nutrients from sewage can account for a significant amount of what is needed for crop growing. Most of the nutrients are in fact in urine, rather than the sewage sludge so you need to be able to get all of those nutrients out of the combined stream in a recoverable form.

Bluewater Bio’s Hybacs plays its part in realising the potential benefits wastewater has to offer us. As part of its high BOD removal rate, the SMART system is geared towards nitrogen and / or phosphorous removal, creating a post treatment sludge which is well suited for recovering these nutrients as well as ensuring more efficient water and energy recovery.

Written by Dr David Lloyd Owen – Senior Advisor at Bluewater Bio