Manure treatment with superheated steam as part of integrated resource management
Land application of animal manure supplies agricultural soil with valuable and essential nutrients and organic matters, which help to meet crop nutrient requirements and to maintain soil fertility. Nevertheless, the livestock production has experienced significant changes in the last decades like intensification and regional conglomeration generating significant amounts of surplus manure in regions where it probably cannot be efficiently used. The consequence is that manure, which has been recognized long time as a resource, nowadays is perceived in some regions as a waste to be treated and disposed of, losing the benefit of its fertilizer and soil conditioner value and consuming additional energy e.g. for transport. In order to minimize the increasing environmental impacts caused by industrialized livestock production, integrated resource management strategies have to be developed to make use of the manure value instead of regarding it as a problem. Superheated steam drying (SHSD) has emerged as a viable technology with immense potential for industrial applications with regard to its manifold advantages in energy saving, emission reduction, fire and explosion prevention and product quality over conventional convective drying. Drying of manure might be an alternative option to reduce its volume and weight, in a short period of time, decreasing significantly storage requirements and transportation cost. In this study, the treatment of cattle manure, from a local organic farm located in Baden Württemberg, Germany, by means of the SHSD is investigated. Dairy manure was collected after anaerobic digestion. The digested manure was centrifuged and the liquid fraction was decanted while the separated solid fraction was used for the drying experiments. It was observed that the ammonia nitrogen (and consequently the total nitrogen) and the potassium content of the liquid decreases considerably after the solid-liquid separation of the digested manure. This can be explained by the fact that ammonia and potassium become trapped or absorbed in the matrix of the high biomass content (obtained during the anaerobic digestion). The separated solid fraction was dried with superheated steam at temperatures between 130°C and 160°C at different exposure time periods (between 25 and 45 minutes). After solid-liquid separation a reduction of around 54% in weight due to water removal is obtained while a drastically reduction of around 90% of the manure amount is obtained when the SHSD is employed. The influence of the exposure time on the total solid (TS) of the dried manure is higher at the temperature in the range between 130 and 150°C while similar TS results are obtained for temperatures around 160°C at the different defined exposure times. The dried samples have been stored for 2 months in glass bottles at room temperature without any sign of decomposition. However, additional experiments to analyze in detail the influence of the temperature range (at different exposure times) on contained pathogens has to be carried out in further research. In addition, ammonia is mostly removed by the steam during the drying process but it could be recovered by integrating an additional ammonia recovery unit. The aim of this study was not ammonia recovery, but based on the obtained results, this issue will be considered for further investigation. The final dried solid obtained after the SHSD can be employed as a source of several nutrients (organic nitrogen, phosphorus, potassium, calcium) and/or as a soil conditioner due to the high content of organic matter (around 80%). The SHSD is a flexible technology which could be integrated in both centralized and decentralized farm systems for the treatment of surplus manure directly after solid-liquid separation of original or digested manure according to the existing farm situation. In addition, the SHSD units can be operated using alternative energy sources available at the point of use e.g. heat from solar energy, heat from firing of dried biomass. Due to the compact design of SHSD units, the integration of this technology can be very attractive for farms with limited storage capacities. Besides, mobile SHSD units can be considered as an alternative to high amounts of manure transportation within collective farms, enabling reasonable throughput capacities and sharing the investment cost. However, an economic analysis has to be carried out considering the respective farms conditions in order to obtain an adequate manure management.