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Making The Most Of Digestate Applications



Since acidogenesis is early in the process of anaerobic digestion, most of the organic matter has not been fully degraded leaving a digestate that is fibrous and consists of structural plant matter including lignin and cellulose. Thus, it is often referred to as solid digestate. Acidogenic digestate has high moisture retention properties. The digestate may also contain minerals (primarily phosphorus) and remnants of bacteria.




Making the most of digestate applications


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By this point most of the organic matter has broken down leaving behind the Methanogenic digestate known as a sludge (sometimes called a liquor or liquid digestate). The sludge is high in nutrients such as ammoniums and potassium. The other byproduct of this step is methane, which is often collected and used as a fuel source.


The major parameters to assess digestate quality when being used for agricultural applications include pH, nutrients, total solids (TS), volatile solids (VS), and total carbon (TC). This quality depends on feedstock and type of anaerobic digester system.[3] Generally the ammonia content of the digestate accounts for approximately 60-80% of the total nitrogen content, but for a feedstock like kitchen food waste it can be as high as 99%. Digestate has also been reported to have a higher phosphorus and potassium concentration than that of composts. The average P to K ratio is about 1:3. All this together makes digestate a potentially viable source for agricultural soil amendments of certain crops.[4]


Feature papers represent the most advanced research with significant potential for high impact in the field. A FeaturePaper should be a substantial original Article that involves several techniques or approaches, provides an outlook forfuture research directions and describes possible research applications.


Fertiliser wise we use liquid fertiliser in two splits starting early March, and then early April. All the land around the main farm has digestate applied as we've got an on-site AD plant giving us digestate as a by-product. That is applied at approx. 60 cubic meters/hectare, giving us around 110 kilos of nitrogen, plus about 100 kilos of phosphorus and 400 kilos of potassium with sulphur, magnesium, etc as well; so, it's given a good chunk of nutrients allowing us to reduce the bagged synthetic fertiliser for the wheat. For subsequent potato & sugar beet crops, we also cut back on the P and K that we need to put on. So, we are fortunate to be massively reducing the fertiliser we're buying. We recently trialled a couple of fields with only digestate applications which both performed well.


quasar energy group is developing a system at its Wooster, Ohio facility in which digestate is pretreated to remove 75 percent of the water, rendering it clean enough to be piped to a municipal wastewater treatment plant on the same site, says chief operating officer Clemens Halene. Most of what remains, including the phosphorous and any fibrous material, is processed into a dry fertilizer with a solids content of 25 to 30 percent, which is competitive with commercial, petroleum-based products. The nitrogen is processed into a small quantity of liquid ammonium sulfate.The process is working at the Wooster site and quasar plans to expand it to its other digesters, where most digestate is currently land applied. The next step, Halene adds, is to further dry the solids in a special furnace that uses some of the digester biogas, to create biochar.


In this context, a summary of all available scientific knowledge is needed to objectively assess the implications of digestate applications for the agricultural soil biodiversity. We carried out a meta-analysis to elucidate the impact of digestates on the biological quality of agricultural soils by systematically inventorying the international academic literature issued in the last 20 years. Surprisingly, the soil biodiversity has been essentially investigated from the angle of soil microorganisms to date, with only rare studies on nematodes and the soil fauna (according to a search on Web of Science in March 2021). Thus, our meta-analysis was mainly focused on the soil microbiological quality.


In the present study, we first analyzed the evolution of the number of studies over time and localized the geographical origin of the studies. Then, we produced a scientific summary of the questions investigated in the literature and of the experimental approaches set up to answer them. To evaluate the global ecological impact of digestates, we quantified the proportion of studies reporting deleterious, neutral and beneficial effects following digestate application compared to a fertilizer-free control, a mineral fertilizer, and any other organic fertilizer. Finally, we summarized the results concerning the most relevant questions addressed in the literature for the 23 most measured parameters related to microbial abundance, diversity, and activity. As a result of this review, we identified orphan lines of research that need to be investigated to provide most accurate and operational recommendations for stakeholders and environmental policies at the European and national scales.


Bibliometric analysis of the impact of digestates on the soil microbiological quality: a Mapping of the geographical origin of the studies. b Temporal dynamics of the number of articles for 15 years. c Evolution of the scopes of the journals publishing the studies. Large part of studies was conducted in European countries, mostly in Germany and Italy, and in China. The first articles were published in 2008. but 83% of the articles were issued between 2015 and 2021 in journals with two main scopes: Soil and agriculture, and Ecology and Environment. From 2020, generalist journals became interested in the publication of research articles about the impact of digestates on soil biodiversity


Number of scientific articles on the impact of digestates on the soil microbiological quality according to a the type of experiment (experimental strategy developed) and b the time scale of the studies. Two third of studies set up in laboratory and most of them are short-term experiments. The long-term experiments generally take place on the field, in realistic pedoclimatic conditions


Animal manure digestates were applied in one third of the studies, and most of it was cattle manure. Manure from other animals like pigs, poultry or sheep was also used. Plant matter digestates were applied in 30% of the studies. Plant matter came from energetic crops such as maize or grass, or plant residues such as marc, bark or parings. Waste were used in only 14% of the studies, and were food waste in more than half of the cases. Digestates based on a balanced mix of different types of feedstocks were applied in 6% of the studies, and a various range of digestates was tested in 9% of them.


All the results of each of the 6 main questions were reviewed and summarized, and are presented on one radar chart per question. As the most frequent reference was the fertilizer-free control, it was used as a reference on the charts. To contextualize and appreciate the genericity of the results, different data were summarized in addition to the effects, i.e., (i) the number of articles about the same combination [condition x microbiological parameter]; (ii) the temporal scale of the study: less than 6 months was considered as short-term, more than two years was considered as long-term, and 6 to 24 months was considered as mid-term; (iii) the type of digestate concerned by the results: vegetal feedstock, animal feedstock, a mix of vegetal and animal feedstocks, and waste; (iv) the digestate fraction: raw residues, liquid fraction, solid fraction; and (v) the type of organic materials when the digestates where compared with other fertilizers or amendments. All these data were used to plot the generic character of each result on one synthetic figure and return to the specificities of the observed effect if necessary.


Nine articles reported the simple effect of digestate application, without dealing with another question (Table 2). The results are summarized in Fig. 6. As far as their global ecological impact was concerned, most data showed positive or null effects of digestates on the soil microbial parameters compared to the fertilizer-free control, whatever the parameters and the digestate fraction. The digestates used in these studies were mainly based on vegetal or animal feedstocks, and the experiments were set up on a short time scale. As depicted in Fig. 6, only one result indicated that the application of liquid digestates of animal origin induced dehydrogenase activity lower than or equal to the control (Fernández-Delgado Juarez et al. 2015). This result, reported by one article only, was not generic.


The analysis of all these results did not highlight any one type of feedstock proved to be systematically deleterious or favorable to all microbial parameters. Animal feedstocks produced digestates that stimulated fluorescein diacetate hydrolytic activity, urease activity and bacterial diversity, but had a negative impact on fungal abundance compared to mixed or waste feedstocks (Barra Caracciolo et al. 2015; Wentzel & Joergensen 2016; Muscolo et al. 2017; Coelho et al. 2019, 2020; Pagliaccia et al. 2020). The digestates from vegetal feedstocks showed lower fluorescein diacetate hydrolytic activity, acid phosphatase, beta-glucosidase and urease activities than the digestates from animal feedstocks (Muscolo et al. 2017). Conversely, they seemed to stimulate catalase activity more strongly than the digestates based on animal feedstocks did (Muscolo et al. 2017). Concerning the digestates based on waste feedstocks, different digestates provided similar results for most microbial parameters, except the metabolic quotient on which the effects were highly variable according to the waste (Manfredini et al. 2021).


The dose effect of digestates was studied in 11 articles, and the effects were measured on almost all microbial parameters except archaeal communities and aryl-sulfatase activity on the short or mid-term, i.e., up to 18 months. The results highlighted that the dose effect was more frequently studied on the solid fraction than on the liquid fraction or raw digestates. When a dose effect was observed, higher doses generally stimulated the microbial parameters: all the parameters of microbial abundance, fluorescein diacetate hydrolytic activity, dehydrogenase activity, alkaline phosphatase activity. In some cases, the highest dose was less stimulating than the lower doses, suggesting the existence of an optimal dose, at least for bacterial diversity, acid phosphatase activity, beta-glucosidase activity, urease activity and catalase activity (Barra Caracciolo et al. 2015; Muscolo et al. 2017; Telesiński et al. 2017; Różyło and Bohacz 2020). Finally, only two studies showed a negative effect of the digestate dose. Acid phosphatase activity decreased by increasing the dose of a digestate mixing animal and vegetal feedstocks, 56 days after application (Telesinski et al. 2017). A similar effect was observed on urease and betaglucosidase activities 3 months after application of an olive-based digestate (Muscolo et al. 2017). 041b061a72


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