Horse manure consists of feces, urine, and varying amounts of various bedding materials. The management of horse manure causes environmental problems when emissions occur during the decomposition of organic material, in addition to nutrients not being recycled. The interest in horse manure undergoing anaerobic digestion and thereby producing biogas has increased with an increasing interest in biogas as a renewable fuel. This study aims to highlight the environmental impact of different treatment options for horse manure from a system perspective. The treatment methods investigated are: (1) unmanaged composting; (2) managed composting; (3) large-scale incineration in a waste-fired combined heat and power (CHP) plant; (4) drying and small-scale combustion; and (5) liquid anaerobic digestion with thermal pre-treatment. Following significant data uncertainty in the survey, the results are only indicative. No clear conclusions can be drawn regarding any preference in treatment methods, with the exception of their climate impact, for which anaerobic digestion is preferred. The overall conclusion is that more research is needed to ensure the quality of future surveys, thus an overall research effort from horse management to waste management.

Horse keeping is of great economic, social and environmental benefit for society, but causes environmental impacts throughout the whole chain from feed production to manure treatment. According to national statistics, the number of horses in Sweden is continually increasing and is currently approximately 360,000. This in turn leads to increasing amounts of horse manure that have to be managed and treated. Current practices could cause local and global environmental impacts due to poor performance or lack of proper management. Horse manure with its content of nutrients and organic material can however contribute to fertilisation of arable land and recovery of renewable energy following anaerobic digestion. At present anaerobic digestion of horse manure is not a common treatment. In this paper the potential for producing biogas and biofertiliser from horse manure is analysed based on a thorough literature review in combination with mathematical modelling and simulations. Anaerobic digestion was chosen as it has a high degree of resource conservation, both in terms of energy (biogas) and nutrients (digestate). Important factors regarding manure characteristics and operating factors in the biogas plant are identified. Two crucial factors are the type and amount of bedding material used, which has strong implications for feedstock characteristics, and the type of digestion method applied (dry or wet process). Straw and waste paper are identified as the best materials in an energy point of view. While the specific methane yield decreases with a high amount of bedding, the bedding material still makes a positive contribution to the energy balance. Thermophilic digestion increases the methane generation rate and yield, compared with mesophilic digestion, but the total effect is negligible.

Keeping horses causes environmental impacts through the whole chain from feed production to manure. According to national statistics, the number of horses in Sweden is currently 360,000 and is continuing to increase. This result in increasing amounts of horse manure that has to be managed and treated, which is currently done using practices that cause local, regional, and global environmental impacts. However, horse manure and its content of nutrients and organic material could be a useful fertiliser for arable land and a substrate for renewable energy production as biogas. The aim of the paper is to identify and describe potentially critical factors in horse keeping determining the amount (total mass) and characteristics (nutrient content and biodegradability) of horse manure, and thus the potential for anaerobic digestion. A systematic combining approach is used as a structural framework for reviewed relevant literature. All factors identified are expressed as discrete choices available to the horse keeper. In all, 12 different factors were identified: type and amount of feed, type and amount of bedding, mucking out regime, residence time outdoors, storage type and residence time of manure in storage, spreading and soil conditions, and transport distance and type of vehicle fuel used. Managing horses in terms of these factors is of vital importance in reducing the direct environmental impacts from horse keeping and in making horse manure attractive as a substrate for anaerobic digestion. The results are also relevant to environmental systems analysis, where numerical calculations are employed and different biogas system set-ups are compared to current and other treatments. In such assessments, the relevance and importance of the critical factors identified here and corresponding conditions can be examined and the most promising system set-up can be devised.

A transition to renewable energy sources and a circular economy has increased interest in renewable resources not usually considered as energy sources or plant nutrient resources. Horse manure exemplifies this, as it is sometimes recycled but not often used for energy purposes. The purpose of this study was to explore horse manure management in a Swedish municipality and prospects for energy recovery. The case study includes a survey of horse manure practices, environmental assessment of horse manure treatment in a biogas plant, including associated transport, compared to on-site unmanaged composting, and finally a simplified economic analysis. It was found that horse manure management was characterized by indoor collection of manure most of the year and storage on concrete slabs or in containers, followed by direct application on arable land. Softwood was predominantly used as bedding, and bedding accounted for a relatively small proportion (13%) of the total mix. Anaerobic digestion was indicated to reduce potential environmental impact in comparison to unmanaged composting, mainly due to biogas substituting use of fossil fuels. The relative environmental impact from transport of manure from horse facilities to anaerobic digestion plant was small. Results also indicate a relatively high cost for horse keepers to change from composting on site to anaerobic digestion in a centralized plant.

European Union and Swedish national energy policy and energy objectives state an increased interest in transition of energy systems to more efficient use of energy, as well as increased use of renewable sources of energy. Horse manure is a potential resource available for renewable energy. Horse manure is sometimes considered a waste problem, resulting in research of possible energy recovery processes, such as combustion and anaerobic digestion. In this study 13 energy actors’ experiences of horse manure were explored by means of interviews and e-mail. Five related to combustion of horse manure and eight related to anaerobic digestion. The aim was to make a compilation of their knowledge and from the results identify how horse manure could be made more attractive as an energy resource. The challenges the actors face are mainly connected with horse manure being a heterogeneous material, primarily due to its bedding content (straw, wood bedding, etc.), and occasionally to other types of added waste. These unpredictable variations in the substrate as well as impurities like sand make it more difficult for plants to have standard procedures for processing horse manure. The view that bedding material needs to be specifically straw pellets and that all impurities should be kept out of the collected horse manure for anaerobic treatment was also expressed. Horse manure as part of co-combustion processes was perceived as a fuel with capacity to contribute to plant economy as it gives revenue from gate fees and could reduce costs for its NO_x reducing capacity. Another view was that grate furnaces could possibly be more suitable than fluidized beds as incineration technology. However, problems with odour made two plants end their combustion trials. In farm-scale incineration horse manure required a lot of monitoring and co-combustion with pellets in order to maintain an effective process. When compiling all available information this study has a number of suggestions for how horse manure should be treated already at the production stage in order to be a more versatile resource in energy recovery processes. The recommendation is to keep horse manure dry (transports and incineration), avoid initiation of composting processes (AD and incineration), sort the waste = no added other waste (AD), and depending on intended AD-treatment process, use a specific bedding type. Most of these issues may be solved by supplier’s involvement in the supply chains for resource recovery, i.e. closing natural cycles of plant nutrients and energy recovery.