Abstract
Depending on the quality of the input substrates, process parameters, and postfermentation treatments, digestates may contain a broad spectrum of potentially toxic elements. We suspected that these contents may vary on a broad scale even under seemingly stable process conditions at the biogas plant. Digestates from four biogas plants were therefore continuously analyzed for their contents of phosphorus, nitrogen, cadmium, copper, lead, and zinc over a period of six years. The input substrates varied between the plants (e.g., cattle and pig slurry and rye and maize silage), but were the same for each plant over the whole period. The N : P ratio of the digestates ranged from 2 to 24, with the digestate coming from cofermentation of pig slurry and energy crops (“DG Pig”) having the widest range of N : P ratio over the years. Heavy metal loads of all digestates and during all evaluations did not exceed the limits set by European or German legislation, but as previously expected, showed a large variability especially if cattle or pig manure were used as substrates. Copper content of Cattle slurry before digestion was 897.7 mg kg−1 DM in one case, and zinc content of DG Pig reached 590.2 mg kg−1 DM also once during the investigation. As a result, we strongly recommend to monitor especially phosphorus, copper, and zinc contents in digestates very closely and in short intervals.
Highlights
In regions with intensive livestock production, there is often a surplus of plant nutrients when organic fertilizers exceed the nutrient requirements of the crops, leading to a discharge or to emission to the environment [1]. e focus often lies on the amount of nitrogen applied with liquid or solid manure [2], but the problem includes other nutrients as well
For example, the N : P ratio ranges from 1 : 1 to 2 : 1, while the required N : P ratio of most crops is between 3 : 1 and 15 : 1. If these fertilizers are applied in an amount that satisfies the nitrogen demand of the crop, P is usually applied in excess [5, 6]. e majority of this phosphorus is rapidly adsorbed and absorbed by soil particles [7]
Conclusions e N : P ratio of digestates meets on average the requirements for most of the crops
Summary
In regions with intensive livestock production, there is often a surplus of plant nutrients when organic fertilizers exceed the nutrient requirements of the crops, leading to a discharge or to emission to the environment [1]. e focus often lies on the amount of nitrogen applied with liquid or solid manure [2], but the problem includes other nutrients as well. For example, the N : P ratio ranges from 1 : 1 to 2 : 1, while the required N : P ratio of most crops is between 3 : 1 and 15 : 1. If these fertilizers are applied in an amount that satisfies the nitrogen demand of the crop, P is usually applied in excess [5, 6]. It is known that some organic fertilizers such as dairy manure improve plant availability of phosphorus through enhanced enzyme and soil microbial activities [9]. Since digestates usually lead to an increase in soil microbial activity [10], they may likewise lead to an increase in phosphorus plant availability.
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