Moisture effect on the storage, handling and flow properties of poultry litter

Abstract

Poultry litter, a combination of accumulated chicken manure, feathers and bedding materials, is a potential feedstock for bioenergy and other value-added applications. The effect of moisture on the physical properties, equilibrium moisture relations, flow properties and wall friction characteristics of poultry litter were measured. Poultry litter densities (bulk, particle and tap) reduced and compressibility increased with increase in moisture content. The fit of the GAB equation to poultry litter equilibrium moisture isotherm at 25 °C indicated that the monolayer moisture content for poultry litter is 5.5% (wb). Increasing the moisture content of the poultry litter reduced its flowability from easy flowing (flow index of 6.4) at moisture content of 10.3% (wb) to very cohesive (non-flowing) (flow index of 1.9) at moisture content of 30.9% (wb). Flow on a stainless steel surface was improved when the surface was mirror-finished or galvanized coated.

Introduction

Poultry litter is a combination of accumulated chicken manure, feathers and bedding material, which is typically wood shavings, sawdust, wheat straw, peanut hulls or rice hulls (Edwards and Daniels, 1992). The type of bedding material used is highly dependent on the available agro-processing residues in the poultry production area. For example, wood shavings and peanut hulls are, respectively, used in northern and southern Alabama, USA, whereas rice hulls are traditionally used in Arkansas, USA.

Poultry litter is the most valuable of all manures produced by livestock. This is due to the high levels of nitrogen (4.0%), phosphorous (1.6%) and potassium (2.3%), in addition to lesser quantities of Ca, Mg, Mn, Cu, and Zn present in the litter. Close to 11 million tons of poultry litter is produced in United States annually (Gollehon et al., 2001). Due to its excellent NPK (nitrogen:phosphorous:potassium) ratio (Nicholson et al., 1996), and due to the high cost incurred in transporting low density poultry litter, this waste material has been traditionally used to fertilize the lands that are close (<10 miles) to the poultry production facilities. Unfortunately, in poultry production US states such as Alabama, Arkansas and Georgia, poultry houses are locally concentrated. The prolonged application of poultry litter on the neighboring agricultural lands has therefore resulted in a buildup of N and P in the soil on these lands (Kingery et al., 1994). Groundwater and surface water problems have also been created as excess nutrients run off the land or leach into groundwater supplies (Moore et al., 1998, Wood et al., 1999).

Various studies have been conducted to discover other value-added uses for poultry litter such as bioenergy feedstock (Abelha et al., 2002, McMullen et al., 2005), methane generation (Safley et al., 1987) and activated carbon production (Lima and Marshall, 2004). These value-added uses require that appropriate equipment and facilities be designed and selected to store, handle, and transport poultry litter, hence the need to quantify the storage, handling and flow properties of poultry litter (Shamlou, 1988, Colley et al., 2006).

Bulk density (ρ_b) is the ratio of the mass of a bulk material to the bulk volume. Knowledge of the bulk density of a material is essential in the design of storage silos and hoppers and of conveying systems (Woodcock and Mason, 1987). Factors such as size, shape, cohesion and moisture content of the particles have been found to affect the bulk density of biological materials (Newman, 1995). Particle density (ρ_p) is the ratio of the mass of particles to the volume of the individual particles. Compressibility is a measure of the increase in strength of powder-like materials with increase in consolidating pressure. Compressibility values are useful for calculating hopper angles, opening sizes and bin loads. This property is also a direct function of the elasticity of the material under applied load (Schulze, 1996).

Poultry litter is a biological material and therefore will degrade chemically or microbiologically. The extent and rate at which this degradation occurs depend on the storage environment. Knowledge of equilibrium moisture behavior is crucial in determining environmental conditions that will minimize spoilage of poultry litter during storage (Wilhelm et al., 2004).

Flow properties are used to: (a) design new and retrofit existing bins, hoppers and feeders; (b) determine cause of flow problems; and (c) understand differences between various bulk materials or grades of same material (Fitzpatrick et al., 2004). Flow properties of powder-like materials are quantified by means of flow index, cohesion and angle of internal friction. Details of the determination of the flow behavior of materials can be found in Jenike, 1964, Fitzpatrick et al., 2004. The flow index is used to characterize the flowability of a material (Table 1). It is obtained from the inverse of the flow function. The flow function is the slope of the plot of unconfined yield stress versus the major consolidating stress that is obtained when the material is sheared to failure in a shear cell (McGlinchey, 2005). In addition, it is generally assumed that materials with a cohesion of >2 kPa and angle of internal friction <30° are amenable to handling using gravity alone (Puri, 2002). Wall friction is also a critical parameter in the structural design and stability of silos, and in the design and operation of silos for reliable and consistent discharge (Iqbal and Fitzpatrick, 2006). Lower wall friction values result in higher normal loads being transferred onto the wall surfaces of the silo. Several studies have shown that moisture content has a significant effect on the properties (physical, compressibility and flow) that are important to the storage, handling and flow of biological materials (Teunou et al., 1999, Balasubramanian, 2001, McNeill et al., 2004, Barbosa-Canovas et al., 2005, Colley et al., 2006).

To our knowledge, there is no reported study on the effect of moisture content on the properties that affect the storage stability, handling and flow properties of poultry litter. Therefore, the objectives of this study were to: (a) quantify the effect of moisture content on the physical (bulk density, particle density, tap density, compressibility) and flow properties; (b) determine the wall friction characteristics and (c) develop equilibrium moisture relation for poultry litter at a temperature of 25 °C.