# Study on Low Temperature Drying Characteristics of Chicken Manure in a Humid Environment
## 1 Introduction  
The problem of chicken manure treatment has attracted more attention with the expansion of the poultry industry because improper treatment of chicken manure can cause environmental problems [1; 2]. Resource utilization of chicken manure is the main method for treating chicken manure, such as making organic fertilizer. Thermal convective drying was proven to be an effective method to remove moisture from chicken manure and an effective pretreatment technology for resource utilization of chicken manure [3; 4; 5]. There are many applications and studies on chicken manure drying. A and many studies can be retrieved from the literature on the drying of chicken manure. [PERSON] and [PERSON] found that the dehydration rate of chicken manure was directly related to drying temperature and wind speed [6]. [PERSON] and [PERSON] studied the effects of drying temperature on the drying characteristics of chicken manure [7]. [PERSON] reported that the dehydration rate of chicken manure was also related to the thickness of the manure layer [8]. However, the above research and applications are based on traditional energy with a drying temperature of 60-120 \({}^{\circ}\)C, which requires significant energy consumption. A new method has been developed in recent years to dry chicken manure using exhaust air from the chicken house, which shows a remarkable effect on reducing energy consumption [9]. The drying systemis usually installed at the end of the chicken house and is kept at an appropriate distance from the gable fan. Waste air from the chicken house is transported to the drying system by fan pressure. The airflow organization of the drying system is shown in Figure 1 (blue arrow). As the heat source of the drying system is mainly derived from the exhaust air of the chicken house, the drying temperature in the system is between 15 and 35 \({}^{\circ}\)C for the whole year.  
However, few studies have been conducted on the chicken manure drying process in this low-temperature environment. In 2015, Wageningen University developed a system to dry chicken manure by using the exhaust air from the chicken house. It was found that the system could dry chicken manure with a 75% moisture content to less than 30% within 48 h [10]. [PERSON] tested the drying performance of several types of chicken manure drying equipment using waste heat from chicken houses in the Netherlands and studied the relationship between drying rate, ventilation rate, and dry matter content over time [11]. This method has been widely used in China in recent years, but it has also encountered some problems because of the climate. Particularly when the drying process is met with continuous rain or when the wet curtain in the chicken house is turned on, the relative humidity of the exhaust air from the chicken house will rise to about 85% and the dehydration rate of the chicken manure will decrease greatly [12; 13; 14]. As a result, the drying time exceeds the predetermined period, causing the retention of chicken manure. The chicken manure retention will further increase ammonia emissions and cause air pollution [15; 16; 17]. So, it is necessary to study the drying characteristics and drying behavior of low-temperature drying of chicken manure in a humid environment and select reasonable drying parameters to ensure that chicken manure can be dehydrated within a predetermined drying period in a high-humidity environment. The aim of this research was: (1) to observe the effect of process parameters such as drying temperature, air velocity, and manure layer depth on the low-temperature drying characteristics of chicken manure in a humid environment; (2) to establish a mathematical model and estimate the constants of selected model equations as well as a diffusion coefficient for the low-temperature drying process in a humid environment; and (3) to provide a theoretical basis for the application of a chicken manure drying system using the residual heat from the chicken house.