37 GeoScience Engineering Volume LXIV (2018), No. 4 http://gse.vsb.cz p. 37 – 40, ISSN 1802-5420 RECYCLING OF PHOSPHORUS AND AMMONIA NITROGEN FROM DIGESTATE Katrin Calábková, Petra Malíková, Silvie Heviánková, Michaela Červenková 1 VŠB-Technical University Ostrava, Faculty of Mining and Geology, 17. listopadu 15, Ostrava, Czech Republic ABSTRACT Digestate from biogas plants, formed by dewatering anaerobically stabilized sludge, is characteristic of high concentrations of phosphates and ammonia nitrogen suitable for further use. Phosphorus is an element widely used to produce fertilizers, and because of its continually shortening natural supplies, recycling of phosphorus is gaining on significance. Both phosphorus and nitrogen are important elements and their presence affect the quality of water resources. Both elements can contribute to eutrophication. At the same time, both phosphorus and ammonia nitrogen, are important elements for agricultural production, and therefore greater demands are being made on the effort to connect sewage treatment processes and the process of recycling of these nutrients. A suitable product of phosphorus and ammonia nitrogen are phosphates in the form of a structurally-poorly soluble precipitate of magnesium ammonium phosphate (struvite). This form of slowly decomposing fertilizer is distinguished by its fertilizing abilities. Compared to direct use of digestate as a fertilizer, struvite is more stable and can gradually release ammonia nitrogen for a long time without unnecessary losses. In the reported experiments, the precipitation efficiency of the recycling of ammonia nitrogen and phosphorus from the digestate liqour (liquid discharge from digestate) was, at a stoichiometric ratio of Mg 2+ : NH4 + : PO4 3- (3.2: 1: 0.8) and a stirring time of 15 minutes, 87 % for ammonia nitrogen ions. Keywords: digestate, biogas plants, ammonia recovery, precipitation 1 INTRODUCTION Digestate, a nutrient-rich material produced by anaerobic digestion at biogas plants, can be applied as a fertilizer. All the nitrogen, phosphorous and potassium present in the feedstock remain in the digestate as none is present in the biogas. However, the nutrients are extensively more available in digestate, meaning it is easier for plants to utilize them. The digestate handling options mainly depend on the resulting quality of the product generated after anaerobic digestion. This means that the way of disposal depends mainly on the properties of input raw materials entering the biogas plants [1]. Agricultural biogas plants are biogas plants where materials of plant nature and livestock manure, or bedding, are processed. The processing of wastes listed in Czech Waste Act 185/2001 Coll and other materials stated by Regulation (EC) 1774/2002 of the European Parliament and of the Councilis is forbidden in agricultural biogas plants. On the authority of the Czech and European legislation, digestates are considered organic fertilizers in the most cases. The digestate´s nutrient content and its nutrient subsequent availability for plants is affected by an input substrate. However, a common feature for digestate is low dry matter content, which for agricultural biogas plant, is between 2 and 9 %. The digestate´s chemical composition is similar to the mineral fertilizer rather than organic one and almost all easily degradable organic materials decomposed during anaerobic digestion. This resulted in a presence of minimum beneficial substances in digestate which may be utilized by plants. Nitrogen content in fresh digestate ranges from 0.25 to 0.75 %, of which 25 to 75 % is in the form of ammoniacal nitrogen (i.e. N-NH4 + ) making up a weak alkalinity of digestate (pH = 7-8). Therefore, land spreading of digestates can cause a discharge of large quantities of nutrients into the environment, which participate in eutrophication and reduction of dissolved oxygen in water bodies [2]. In practice, there are two views of the use of digestate. One party believes it is necessary to modify the digestate in some way and then get rid of it. However, a more modern approach seeks to make the most of this raw material. Firstly, it aims to recycle all nutrients (nitrogen, phosphorus, potassium), use them as fertilizers and use the separated water to irrigate or dilute fermentor contents [3]. With the increase in population on Earth, the demand for raw materials and the associated need for fertilizers increased. Higher consumption of fertilizers results in a rapid decline in natural phosphates. Currently, 80 % of the extracted phosphorus is used to produce fertilizers. Phosphorus is mainly mined in South Africa, China, and Morocco [4]. If the phosphorus consumption remains the same, the natural phosphates supply will be