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What to do with manure?

Pigs, cattle and poultry produce around 1,800 million tons of manure in Europe every year. Farmers provide crops with nutrients by spreading manure on their fields. But what can be done with manure when there are not enough fields to use it on? Researchers from the Fraunhofer Institute for Interfacial Engineering and Biotechnology (IGB) and the University of Hohenheim have joined forces with 13 partners from Germany, the Netherlands, Spain and the United Kingdom in the EU project BioEcoSIM that aims to turn valuable manure constituents into soil-improving products.

In Germany, regions with intensive livestock farming such as Lower Saxony and North Rhine-Westphalia have to find ways of dealing with an oversupply of liquid manure. At present, local farmers transport the stinking broth in large tankers to areas where the manure is urgently needed. Manure is used as an agricultural fertiliser because it contains nutrients such as phosphates, nitrogen, potassium or magnesium which plants need for growth. “However, manure is about 90% water, which makes transporting it very expensive,” says Jennifer Bilbao from the Fraunhofer IGB and BioEcoSIM project manager. The project partners hope to solve the problem by withdrawing the nutrients directly at the site where the manure accumulates and processing them into phosphate and nitrogen fertilisers as well as biochar.

“We are able to recover almost all the phosphorus from manure. Germany is currently importing almost all of its phosphorus from abroad,” says Bilbao, adding, “however, natural phosphate reserves in Africa, China and the USA are slowly but surely running out, so alternatives are needed.” In addition, the new method might also help reduce the use of artificial nitrogen fertiliser. Nitrogen fertiliser is usually produced using a method known as the Haber Bosch method, which consumes a great deal of energy. Another product that can be produced from manure is a type of charcoal made from biomass by way of pyrolysis. It is called biochar and is not that well known in Germany, but has long been used by Amazonian Indians to retain nutrients and water in poor soil.

A made-to-measure fertiliser blend

Jennifer Bilbao and her team have developed a concept to produce biochar and recover phosphorus from manure. © Helmine Braitmaier

“Our products can be mixed and adapted to the nutritional requirements of individual plant species and the nutrient composition of the soil where the plants are grown,” says Bilbao. “The nitrogen-phosphorus ratio of manure is quite different from the ratio plants need, so care has to be taken not to over-fertilise plants,” she adds. The new method reduces the risk of over-fertilisation. Manure has the disadvantage that excess phosphorus that plants cannot take up with their roots, is washed out and enters lakes and water courses. These become eutrophic due to an increase in nutrients, which makes life in the water impossible for fish and other organisms. Moreover, excess nitrogen could also enter the groundwater in the form of nitrate and nitrite, and thus pose a threat to human health.

So how does the process work? The researchers from the Fraunhofer IGB start by acidifying the manure; they then remove the solids and dry them with hot steam. The dry biomass is then converted into biochar in a process known as pyrolysis, which is the decomposition of organic material at a temperature of around 300 degrees in the absence of oxygen. The liberated gas is combusted in a block heat and power plant, where heat and power are generated. In another plant, phosphorus is removed from the liquid filtrate in the form of salts such as calcium phosphate and magnesium ammonium phosphate (struvite). The remaining filtrate is then further processed in another plant, where nitrogen is recovered in the form of ammonium.

Fertiliser from manure: good for plants and the environment

The pilot plant in the city of Kupferzell converts 50 l of manure per hour. © Fraunhofer IGB

“The fertilisation effect of our phosphate salts is even better than that of traditional mineral fertilisers,” says Bilbao, who originates from Ecuador. Her colleagues from the University of Hohenheim have applied conventional and new fertiliser to potted plants and measured and compared the dry mass of adult plants. The researchers found that the heavy metal concentrations of fertilisers produced from manure were below the threshold value stipulated by the German Fertiliser Regulation. The researchers from Hohenheim now plan to test the products in the field. A project partner and private research institute called ITAGRA.CT has studied the effect of the fertilisers on earthworms and found that the worms are not adversely affected.

Bilbao reckons that the cost of preparing the nutrients is about half the cost incurred by farmers for the long-distance transportation of manure. “Fertilisers produced from manure are dry and can thus be transported more easily than liquid manure, which has much lower nutrient concentrations than the dried manure mass,” says the 34-year-old. "In general, our new method helps prevent the generation of greenhouse gases,” says Bilbao. The carbon dioxide balance would be quite bad if farmers carried on spreading manure on fields, dried it in order to be able to store it more effectively or fermented it with corn to produce biogas. Another project partner, the University and Science Centre in the Dutch city of Wageningen, has calculated that all these processes would lead to relatively large quantities of greenhouse gases.

The initial laboratory-scale method was tested in a pilot plant on the premises of a company called Agroenergy in Kupferzell in summer 2015. This plant is able to convert 50 l manure per hour. The project partners hope to be able to present the plant to interested parties in the agricultural and industrial sectors in June 2016.

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