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English Issue Biogas Journal | Spring_2021 Overview of the NuTriSep process Biogas plant of Agro Energie Hohenlohe GmbH & Co. KG. The process starts with the separation of organic solids: The phosphate and the ammonium nitrogen dissolve when sulphuric acid is added; the solid matter is also freed of it. The (gaseous) ammonia converts to liquid ammonium when the pH value is lowered, thus preventing it from escaping. The sulphuric acid that is used does not endanger plant growth. That is important, as after the hygienization, the solid matter that accrues should be able to be used as a peat substitute in horticulture. This material is valued at more than 10 Euros per cubic meter and more than 40 Euros per tonne. “Phosphate is precipitated from the remaining liquid. The phosphate salts that result are of mineral quality and can either act as fertilizer instead of apatite fluoride or be used in the chemical industry,” says Geltz describing the next process step. The phosphate salt costs about 80 Euros per tonne. The liquid that is now free of phosphate is stripped of ammonia in a closed stripping unit – by raising the pH value and the temperature. According to Geltz, “in this process, the ammonium is changed back to ammonia and advances to the gas phase in the stripping unit. It is freed of ammonia in an acid gas scrubber, which produces an ammonium sulphate solution, briefly referred to as ASL.” With a nitrogen content of 8 percent, ASL is a common commercial product. Value: about 25 Euros per tonne. After this process step, the residual water is free of solids, phosphate and ammonium. It can be used to irrigate the fields. WRA Irrigation on arable land Separation of organic matter Phosphate precipitation Ammonia stripping Physical separation Direct discharge Organic solids Phosphate salts Nitrogen solution out a fertilization effect to create valuable, marketable products.” Development of digestate The digestate is pumped over from the biogas plant via a pipeline. A new supply shaft was constructed specially for that. On its way through the units in the former pigsty, the liquid always has to flow through increasingly finer filters and a number of chemicals are added during the process. At the beginning of the process chain, there are two identical storage tanks, T1 and T2. “The addition of sulfuric acid dissolves phosphorus from the solid and it passes into the liquid phase. The coarse organic solids are separated,” says Maier. Thomas Karle describes the solids and fibres that are filtered out as a “loose product that is very similar to peat”. Agro Energie sells this peat substitute “to a certain industrial customer, who uses it to make composites for riding arenas or for the production of turf.” The liquids essentially go through three steps: A screw press sorts the fibres. Then even smaller solids are sifted out in a vacuum separator, also called a vacuum spiral filter. The final micro-filtration then removes the entire suspended matter residues that are above 0.2 micro-meters (µm). “After that, the digestate looks like apple juice,” says Isabelle Maier, holding up a glass of that liquid. The salts and other nutrients that are still dissolved in it are extracted later. Recovery of phosphate salts and ammonium sulphate “Phosphate is taken out by adding caustic soda. The phosphate salts become phosphate sludge and in the end, after the drying, there are phosphate plates that can be granulated,” Maier and the plant operator Karle explain together. The nitrogen in turn is separated from the liquid as “ammonium sulphate”. During the so-called “ammonia stripping”, the liquid is heated to 50 degrees Celsius, the pH value rises, the ammonia becomes gaseous and escapes. Energy is later extracted again from the heated liquid through heat recovery. “The packing of the stripping unit enlarges the Dosage of sulphuric acid: When sulphuric acid is added, the phosphorous is dissolved from the solid matter and goes into the liquid phase. 36
Biogas Journal | Spring_2021 English Issue surface of the liquid through which the ammonia is emitted,” Maier explains pointing to an enclosure covered by Plexiglas with two redundant systems. That is where the ammonia gas is filtered in an “acid scrubber” and combined with sulphuric acid to form the ammonium sulphate solution ASL. “The procedure is finished when the pH value is 7, meaning that it is neutral,” says Maier. An ASL produces about 200 liters per hour. The fourth separation step is in the testing phase But the development of NuTriSep is still continuing: Geltz Umwelttechnologie is currently trying to treat residual waters by means of reverse osmosis so that it can be discharged into watercourses. “After the process preceding it, the reverse osmosis is extremely efficient. The concentrate of the reverse osmosis is rich in potassium and can be used as a potash fertilizer.” So, will the digestate from the biogas plant of Agro Energie Hohenlohe, that is often considered a “taboo issue” (Thomas Karle), soon be producing a fourth viable product besides peat substitute, phosphate and an ammonium sulphate solution? Solid content is separated by the vacuum separator. Author Dipl.-Ing. Heinz Wraneschitz Feld-am-See-Ring 15a 91452 Wilhermsdorf 00 49 91 02/31 81 62 heinz.wraneschitz@t-online.de www.bildtext.de · www.wran.de Original image at the gas leak detection at the biogas plant Hands free and best light in the danger area with ATEX lights from AccuLux – Made in Germany No compromise on safety! HL 12 EX with ca. 200 Lumen www.acculux.de 37
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