ENVIRONMENTAL IMPACT OF WASTE BASED BIOGAS DIGESTATES AS INFLUENCED BY POST-TREATMENT METHODS Ross, C.-L.; Wragge, V.; Nielsen, K.; Sensel-Gunke, K.; Bechstein, F. Institute of Agricultural and Urban Ecological Projects affiliated to Berlin Humboldt University (IASP) Philippstraße 13, 10115 Berlin, Germany roschris@agrar.hu-berlin.de, verena.wragge@agrar.hu-berlin.de, kerstin.nielsen@agrar.hu-berlin.de, karen.sensel@agrar.hu- berlin.de, felicitas.bechstein@agrar.hu-berlin.de ABSTRACT: Biowastes contain valuable plant nutrients. After being processed, they could be used as fertilizers. Yet therefore among other things their environmental impacts should be known. The impacts of biowaste digestates - composted after fermentation and processed in three different ways - were tested using two bio-tests with earthworms and two with plants. None of the tests with earthworms led to negative effects within the tested ranges. The tests with plants showed a slightly negative response especially of monocotyledonous plants (Avena sativa in the test) towards the pure compost but not towards the further worked up products. For dicotyledonous plants (here represented by Brassica rapa) no negative effects were found within the tested ranges. The results of the conducted tests indicate that bio-wastes from households, if treated in an appropriate way, could be used as fertilizers without negative impacts to plants and earthworms. Keywords: organic waste, digestate, environmental impact, earthworms, biomaterial 1 INTRODUCTION Since January 2015 the separate collection and recovery of organic wastes from households (“biowaste”) is a basic statutory obligation in Germany. Biowaste is particularly suited as substrate for biogas plants, since it can be digested effectively under both mesophilic and thermophilic conditions [9]. Consequently more biogas plants using organic household wastes as feedstock are being built. Under the aspect of nutrient recycling an agricultural use of the resulting digestates as fertilizers on arable land is desired [2]. However so far not enough knowledge exists about the ecological aspects of biowaste application. High amounts of impurities like heavy metals or pieces of plastic and glass could cause environmental problems and make the products unpopular for farm use. Two ecotox-tests (one with earthworms and one with plants) were therefore carried out to estimate environmental impacts of biowaste application. 2 MATERIAL AND METHODS 2.1 Production of Digestate Products The tested digestate products are obtained by anaerobic digestion, done with a batch high solids anaerobic digestion process of the company GICON (Germany) with separated hydrolysis and methanization utilizing percolation, and subsequent composting of organic wastes. In a second step the thus obtained solid digestate is further processed, resulting in three different products (Fig. 1): 1. “Compost fine”: sieved mechanically (20 mm) and thus ridded of rough impurities like bigger plastic particles 2. “Pellet”: Compost fine pressed into Pellets of 20 mm in diameter and differing in length 3. “Agglomerate”: moistened Compost fine formed into small round agglomerates through treatment with a mixer of the company Eirich (Germany) Figure 1: The three tested digestate products, a) Compost fine, b) Agglomerate, c) Pellet 2.2 Avoidance-Response Tests with Earthworms Earthworms feed of the organic matter in the soil. This and the fact that they are in permanent contact with the soil, its water content and therein dissolved chemicals as well as their sensitivity against chemicals, makes them an ideal organism for testing potentially toxic soil conditions [4, 8]. The bio-tests with earthworms were conducted as an Avoidance-Response Test in accordance with the International Standard Norm ISO 17512-1 (2008). The composts and two reference fertilizers, cow manure and calcium ammonium nitrate (CAN), were tested in two concentrations according to a nitrogen application of 200 and 400 kg ha -1 . The tests were conducted with two-section vessels (1 liter volume) with Eisenia fetida and lasted 48 hours. In each vessel ten worms had the choice between an untreated control soil and soil mixed with compost or reference fertilizer. Each treatment was tested in four replications. Two tests were carried out with individual batches of the composted bio- wastes, which differed only slightly in their composition. In test 1 the soil was completely untreated, whereas in test 2 the soil in the control and in the treated variants was mixed with a small amount of dried lucerne green meal. The soil was adjusted to 45 % (1 st test) and 50 % (2 nd test) of the water holding capacity. After 48 hours the number of worms in each section was counted. On the assumption of a 50:50-distribution the avoidance of the treated soil by the earthworms was calculated. An avoidance of > 80 % of the treated soil would be evaluated as a risk potential for earthworms [6].