# Physical Properties of Dairy Manure Pre- and Post-Anerobic Digestion
## 1 Introduction  
Manure management is a critical activity for the economic and environmental sustainability of dairy farms. Manure can improve soil health and provide valuable nutrients for crop production when land applied [1]. However, the handling of manure can also be costly and lead to negative environmental impacts [2; 3]. The most common utilization method for manure is land application, which costs approximately $306 per cow per year to store, haul, and land apply [4]. In addition, manure is responsible for nearly 7% of both agricultural methane (CH\({}_{4}\)) and nitrous oxide (N\({}_{2}\)O) emissions [5]. As a result, there has been increased attention to developing and implementing processing technologies that enhance the economic value of manure and reduce the related environmental impacts [6; 7]. Anaerobic digestion (AD) is a common manure processing technology that can achieve both goals [8]. AD systems can use animal manure to produce biogas, which typically contains 50% to 65% CH\({}_{4}\) and can be collected and used for direct burn applications, converted to electricity, or compressed for use as a transportation fuel [9]. In addition, AD has environmental benefits as it reduces odors, greenhouse gas (GHG) emissions, and pathogens [10; 11]. The physical properties of manure, such as total solids (TS), volatile solids (VS), and density, are important in designing and operating manure handling and processing systems [12]. For example, the TS content will determine transportation options to move manure through the collection, processing, storage, and application cycle. The VS loading rate is a key design parameter for biogas production in AD systems [13]. Maximizing biogas production is important for increasing the availability of energy to use on-farm or sell as an additional source of income to the farmer. Finally, knowing the manure density is important to estimate operational volumes. Farmers can use this knowledge to make technology selections and operational decisions to increase efficiency and decrease costs. These dataare also important for researchers and engineers to determine design parameters, such as storage volume, digester volume, and power capacity, among others. Moreover, these design parameters are important for models predicting biogas production [14] and environmental impacts from manure management [13]. This study goes one step further and develops an equation relating the physical characteristics of manure to facilitate representative data generation to be used by process-based, economic, and life cycle assessment (LCA) models. Determining the relation between manure TS and density has been studied by researchers in the past, but many of these studies have focused on unprocessed manure, solid manure, and compost [15; 16; 17]. [PERSON] et al. [18] evaluated thermal conductivity, specific heat, and bulk density of fresh beef cattle manure in relation to moisture content to estimate thermal diffusivity. The authors sampled manure at TS contents ranging from 15% to 75% and developed a polynomial relation to predict density of manure according to moisture content. [PERSON] [19] presented engineering properties of beef cattle manure including density, rheological properties, specific heat, and thermal conductivity. The author found that manure density increases gradually for TS from 0% to 50% and has a sharp drop when TS increases above 50%. This drop might be attributed to the large void space in manure at these TS levels. [PERSON] and [PERSON] [20] analyzed the bulk density, specific heat, and thermal conductivity of Moroccan dairy manure with respect to TS content. The authors analyzed samples with TS contents between 2.5% to 12.0% at temperatures between 20 and 60 \({}^{\circ}\)C. Findings showed that density of manure increased when TS content increased from 0% to 14%. The authors concluded that bulk density of cattle manure is a function of TS content and void spaces. However, when manure has less than 16% TS, it contains liquid and solid phases with no void spaces.