Pocket Power! : increasing the potential of anaerobic digestion at farm-scale

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Stability of thermophilic pig manure mono-digestion : effect of thermal pre-treatment and separation

Anaerobic pig manure digestion holds potential to contribute to a bio-based economy. This work assesses the stability of the thermophilic mono-digestion process. Thermophilic mono-digestion experiments with (i) fresh liquid pig manure and (ii) the fresh fecal fraction from source separation by a pig housing construction were conducted in semi pilot-scale continuous stirred tank reactors. The effect of separation on the digestion stability was studied by comparing thermophilic mono-digestion of fresh liquid (unseparated) and fresh source separated pig manure. Influencing factors and inhibitors were identified during the experiments. An unstable thermophilic mono-digestion process was observed for fresh liquid pig manure at a digester retention time of 60 days, due to high levels of ammonia and sulfur-containing components. Thermophilic mono-digestion of the fresh fecal fraction was more promising in terms of stability, provided enough time for digestion. In addition, the effect of low temperature (70 degrees C) thermal pre-treatment of manure on the digestion stability was investigated. In the case of liquid pig manure, no improvement in the digestion stability was noted upon thermal pre-treatment. For the fecal fraction, the stability of the thermophilic mono-digestion process did improve. Moreover, thermal treatment and subsequent thermophilic mono-digestion of the fresh fecal fractions from two different farms with a similar pig housing construction suggested an effect of the (organic) dry matter content on the process stability

Nutrient recovery from digestate: case study report

Due to the historic presence of intensive livestock production and the limited amount of arable land for manure disposal, nitrate pollution in certain European areas is considerable. The European Nitrates Directive, implemented in 1991, intended to improve water quality in Europe by preventing pollution of ground- and surface water by nitrate leaching from agriculture. This has forced local administrators and government to introduce stringent regulations regarding the use of manure (and later on digestate), resulting in national action plans and the defining of vulnerable zones. The Flemish action plan, for example, consists of an obligation to process manure in such a way that the nitrogen present is not returned on Flemish agricultural soil after treatment, but is either exported, used on non-agricultural land (e.g. in gardens or parks) or converted to nitrogen gas or to a mineral fertiliser. These restrictions, combined with the presence of intensive livestock, imply that anaerobic digestion plants in Flanders (and other nutrient rich areas), may not, or only sparingly, return digestate as a fertilizer in its crude, unprocessed form and have to invest in digestate processing techniques. More recently, the focus in digestate processing techniques has switched from mere processing towards valorization techniques that recover a maximal amount of nutrients (N, P, K). This development is triggered by an increasing worldwide awareness of the depletion of mineral resources (such as phosphorus) and the volatile price of fossil-based mineral fertilizers. Mineral fertilizer use in Europe is high, even in regions with local nutrient surpluses where farmers pay to export or destroy nitrogen in their farmyard slurry. The reason for this is twofold, first of all animal manure spreading is limited to 170 kg of N/ha according to the Nitrates Directive (in vulnerable zones) and second of all nutrient availability and composition in manure can differ from crop requirements. Estimates of the current phosphorus and potassium reserves are highly uncertain, but based on population growth and future nutrient demand, it is predicted that depletion will occur within 93 to 291 years for P and 235 to 510 years for K (Fixen and Johnston, 2012; Van Vuuren et al., 2010; Villalba et al., 2008; Smit et al., 2009). Geopolitical moves can however shift this date forward, making nutrient scarcity an imminent threat which is exemplified by the fact that phosphorus was recently added to the EU ‘critical raw materials’ list. A momentum has been created in which upcycling of digestate derivatives towards high quality fertilizers is aspired. The current challenges are to achieve optimal recovery and recycling of nutrients from digestate in a sustainable way enabling marketing and and valorisation in an adjusted legal framework

Small-scale anaerobic digestion : case studies in Western Europe

This brochure is intended to meet farmers’ demand for more information concerning the current market situation of the small-scale anaerobic digestion technology. The authors aim to quickly guide those who are interested in the technology so that they can adopt a targeted approach in their search for information. This brochure elaborates on how the implementation of small-scale anaerobic digestion can take place at the company level, based on five practical examples. A broad market study in a second part of this brochure shows that there are already many European providers that focus on small-scale digestion. Some are still in a pilot phase, while others have already realized full-scale operational installations