Nutrient recovery from digestates : techniques and end-products

In nitrate vulnerable zones application of animal manure to land is limited. Digestate from anaerobic digestion plants competes with manure for nutrient disposal on arable land, which forms a serious hinder for the biogas sector to develop in these regions. Hence, one of its biggest challenges is to find cost-effective and sustainable ways for digestate processing or disposal. Furthermore, primary phosphorus resources are becoming scarce and expensive and will be depleted within a certain time. This urges the need to recycle P from secondary sources, like digestate or manure. From a sustainability point of view, it seems therefore no more than logical that digestate processing techniques switched their focus to nutrient recovery rather than nutrient removal. This paper gives an overview of digestate processing techniques, with a special focus on nutrient recovery techniques. In this paper nutrient recovery techniques are delineated as techniques that (1) create an end-product with higher nutrient concentrations than the raw digestate or (2) separate the envisaged nutrients from organic compounds that are undesirable in the end-product, with the aim to produce an end-product that is fit for use in chemical or fertiliser industry or as a mineral fertiliser replacement. Various nutrient recovery techniques are described, with attention for some technical bottlenecks and the current state of development. Where possible, physicochemical characteristics of the endproducts are given

Effect of injection depth of digestate liquid fraction on soil carbon dioxide emission and maize biomass production

The aim of this study was to evaluate, in open field conditions, the effect of injection depth of digestate liquid fraction (10 cm, 25 cm and 35 cm) in clay loam soil, on CO2 emission. An un-amended soil was considered as control. The study was performed in 2014 on a farm located in Terrasa Padovana, Veneto region (Italy) distributing digestate before maize sowing. Digestate injection determined a high soil CO2 emission in the first hour after application, followed by a progressive reduction in as early as 24 h, reaching significantly lower values, similar to those measured in the un-amended control, after 48 h. Gas emissions measured 1 h after digestate application decreased as injection depth increased with significantly higher emission values in the 10 cm treatment (median value 23.7 g CO2 m\u20132 h\u20131) than in the 35 cm one (median value 2.5 g CO2 m\u20132 h\u20131). In the 3 days between digestate distribution and maize sowing, soil CO2 emission was significantly higher in the amended treatments than un-amended one, with median values of 1.53 g CO2 m\u20132 h\u20131 and 0.46 g CO2 m\u20132 h\u20131 respectively. During maize growing season, no significant soil CO2 emission difference was monitored among treatments, with a median value of 0.33 g CO2 m\u20132 h\u20131. Digestate application significantly improved maize aboveground dry biomass with an average yield of 22.0 Mg ha\u20131 and 16.2 Mg ha\u20131 in amended and un-amended plots, respectively, due to the different amount of nutrients supplied

Contribution to the study of flocculation of digestate

The paper deals with the intensification of separating the solid phase of digestate using flocculants only. The separated solid phase should subsequently be used in agriculture for fertilising. Flocculants (polyacrylamides) are difficult to biodegrade. In this respect, they should not deteriorate the properties of the solid phase and the flocculant dose must be as low as possible. The research aimed to identify the optimal cationic flocculant and its application procedure which would enable a dosage that would be both economically and ecologically acceptable. We tested 21 cationic flocculants of different charge density and molecular weight and 1 mixture of two selected flocculants (Sokoflok 53 and Sokoflok 54) with the aim to discover the lowest possible dose of flocculating agent to achieve the effective aggregation of digestate particles. The lowest flocculant doses were obtained using the mixture of flocculants labelled Sokoflok 53 and Sokoflok 54 in 4:1 proportion, both of a low charge density and medium molecular weight, namely 14.54 g/kg of total solids for a digestate from the biogas plant Stonava and namely 11.80 g/kg of total solids for a digestate from the biogas plant Vrahovice. The findings also reveal that flocculation is most effective during two-stage flocculant dosing at different mixing time and intensity

Production of the freshwater microalgae Scenedesmus dimorphus and Arthrospira platensis by using cattle digestate

Microalgae are considered one of the most promising feedstocks for biofuels; these microorganisms are also able to enhance the nutritional content of conventional food preparations, or can be converted into other fuel products, such as hydrogen, ethanol, long-chain hydrocarbons resembling crude oil, or biogas. Scendesmus dimorphus 1237 is an oleaginous eukaryotic microalga, able to produce and accumulate lipids up to a fraction around 43%. In condition of nitrogen starvation this percent grow sup to 50% of dry weight. Therefore this microalga is considered a promising feedstocks for biofuels. Arthrospira platensis is a cyanobacterium with a considerable potential as a source of high biologic value proteins (“superfood”), pigments (phycocyanin and beta-carotene) and poly-unsaturated fatty acids (PUFA) which have been shown to have therapeutic effects on humans. Anaerobic digestion liquid effluents feature the presence of nutrients, such as nitrogen and phosphorous, which makes them interesting for a potential application in microalgal biomass production. Aim of this work is investigating the use of liquid anaerobic cattle manure digestate for the photosynthetic growth of these microalgae

Integration of on-farm biodiesel production with anaerobic digestion to maximise energy yield and greenhouse gas savings from process and farm residues

Anaerobic co-digestion of residues from the cold pressing and trans-esterification of oilseed rape (OSR) with other farm wastes was considered as a means of enhancing the sustainability of on-farm biodiesel production. The study verified the process energy yields using biochemical methane potential (BMP) tests and semi-continuous digestion trials. The results indicated that high proportions of OSR cake in the feedstock led to a decrease in volatile solids destruction and instability of the digestion process. Co-digestion with cattle slurry or with vegetable waste led to acceptable specific and volumetric methane productions, and a digestate low in potentially toxic elements (PTE). The results were used to evaluate energy balances and greenhouse gas emissions of the integrated process compared with biodiesel production alone. Co-digestion was shown to provide energy self-sufficiency and security of supply to farms, with sufficient surplus for export as fuel and electricit

Effects of compost and digestate on environment and plant production – results of two research projects

A yearly amount of 9.3x106t compost and digestate derived from separately collected organic waste is produced in the 25 European Union member states. The improvement of soil properties is a major benefit of compost application. However, little is known about the occurrence of organic pollutants in compost. In order to estimate the potential of Swiss composts and digestates to influence soil fertility and plant health, one hundred products representative for the different composting systems and qualities available on the Swiss market were analyzed in two research projects. In the first study, polycyclic aromatic hydrocarbons (PAHs), ortho substituted and dioxin-like polychlorinated biphenyls (PCBs, DL PCBs), polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F), polybrominated diphenyl ethers (PBDEs), hexabromocyclododecane (HBCD), tetrabromobisphenol A (TBBPA), perfluorinated alkyl substances (PFAS), pesticides, chlorinated paraffins (CPs), phthalates and nonylphenol (NP) were analyzed. All compound classes were detected except for NP. PFAS, HBCD, TBBPA, some compounds out of PBDEs and pesticides were found in compost and digestate for the first time. Concentrations of most compounds were in the low ppb range. Contents of PAHs were between 600 and 12473 μg/kg dry weight (dw) and contents of HBCD and CPs between 17 and 384 μg/kg dw. Tests with springtails (Folsomia candida) have been shown to be a versatile tool for ecotoxicological assessment. Within these tests, inhibiting and stimulating effects due to compost application were observed. Except for high PAHs contents, no major problem with regard to contamination of compost and digestate was identified. In the second study, the physical, chemical and biological properties of the composts and digestats, and their influence on soil fertility and plant growth, were characterized. The organic substance and the nutrient content of the composts varied largely between the composts with the feedstock materials as major influencing factors. The respiration rate and enzyme activities exhibited large variations as well, particularly in the youngest composts. These differences decreased when the composts became more mature. Maturity, the degradation stage of the organic matter, depended not only on the age of the compost, but also on the management of the process. The N-mineralization potential of compost added to soil showed that a high proportion of young composts immobilized the nitrogen in the soil. Two compost parameters allowed to predict the risk of nitrogen immobilization in soil: the NO3- and the humic acids contents. The phytotoxicity of the composts varied largely even in mature composts, showing that the storage of the compost plays a decisive role. While the majority of composts protected cucumber plants against Pythium ultimum, only a few composts suppressed Rhizoctonia solani in basil. With respect to disease suppression, the management of the maturation process seems to play a major role. In field experiments, some biologically immature composts immobilized nitrogen in soil and reduced growth of maize. With additional fertilization, however, it was possible to compensate this effect. Digestates and composts increased the pH-value and the biological activity of soil. These effects were observable also one maize season after compost application. In conclusion, the management of the composting process seems to influence the biological quality of the composts and digestats to a higher extent than the feedstock materials or the composting system. More attention should be paid to this biological quality, in order to produce composts with more beneficial effects on crops

Assessment of Biofertilizer Quality and Health Implications of Anaerobic Digestion Effluent of Cow Dung and Chicken Droppings

Anaerobic digestate have been identified as a rich source of essential plant nutrients. Nevertheless, its safety measured by the concentration of pathogen present is of great concern to end users. This research explored the efficiency of the mesophilic biodigestion process in the stabilization and sanitization of cow dung and chicken droppings. Six (6) kg each of cow dung and chicken droppings were collected fresh and free from impurities, pre-fermented, mixed with water in the ratio 1:1 w/v to form slurry, fed into the respective reactors and digested for 30 days at an average ambient temperature of 30 � 2 �C. The pH of the medium fluctuated between 6.5 and 8.0. The analysis of the feedstock and effluent of the digesters showed that a total solids reduction of 75.3% and 60.1% were recorded for cow dung and chicken droppings while the reduction in total coliforms was 95% and 70% respectively for the dung and droppings. Microbial analysis of the biofertilizer produced reveals both aerobic and anaerobic organisms which include species of Pseudomonas, Klebsiella, Clostridium, Bacillus, Bacteroides, Salmonella, Penicillum and Aspergillus. Escherichia coli and Shigella spp. were removed while species of Salmonella and Klebsiella were still present in the digestate. Notwithstanding these results, the digestate still requires further treatment for it to be suitable for application on unrestricted crops either as fertilizer; otherwise a health problem would be created as attempt is made to improve soil fertilit

Co-digestion of the mechanically recovered organic fraction of municipal solid waste with slaughterhouse wastes

The current work aimed to resolve some long-standing questions about the potential benefits and limitations of co-digestion of slaughterhouse wastes. To achieve this, a laboratory-scale trial was carried out using the mechanically recovered organic fraction of municipal solid waste mixed with either sheep blood or a mixture of pig intestines with flotation fat. Both of these co-substrates are difficult to digest in isolation because of their high nitrogen and lipid concentrations, and are regulated as Category 3 materials under the Animal By-Products Regulations (EC 1069/2009). The results showed that at an organic loading rate of 2 kg VS m?3 day?1 with the slaughterhouse material making up 20% of the load on a volatile solids basis the process could operate successfully. As the loading was increased to 4 kg VS m?3 day?1 signs of inhibition appeared with both co-substrates, however, and volumetric methane production was reduced to a point where co-digestion gave no process advantage. The main operational problem encountered was an increase in the concentration of volatile fatty acids in the digestate, particularly propionic acid: this was thought to be a result of ammonia toxicity. The concentration of potentially toxic elements in the digestate made it unsuitable for agricultural application for food production, although the increased nitrogen content made it more valuable as a fertiliser for non-food crop use