Pocock J, Septien S, Makununika B S N, Velkushanova K V, Buckley C A
Chemical Engineering Discipline, University of KwaZulu-Natal, 4041, Durban, South Africa.
WASH R&D Centre (formerly known as the Pollution Research Group), University of KwaZulu-Natal, 4041, Durban, South Africa.
Heliyon. 2022 Mar 29;8(4):e09221. doi: 10.1016/j.heliyon.2022.e09221. eCollection 2022 Apr.
In striving to achieve sustainable sanitation, one challenge is to ensure hygienic treatment of faecal sludge from on-site sanitation. Thermal drying is an important treatment step for moisture removal and disinfection. Improved understanding of the drying process is crucial for the proper design of treatment technologies for faecal sludge. In this study, faecal sludge from ventilated improved pit (VIP) latrines from Durban, South Africa, were dried in a convective drying thermobalance by varying the temperature from 40 to 80 °C, the relative humidity from 0 to 25% and the air velocity from 0.3 to 1.2 mm/s. The faecal sludge samples were in the form of a thin layer and pellets with different diameters from 8 to 14 mm. Kinetic parameters were determined from the experimental data, were compared to classical drying models in literature and were then used to develop a correlative drying model. Drying rates ranged between 1 and 40 g/min/m, leading to drying times comprised between 100 and 300 min. The drying kinetics increased as temperature was higher, and pellet diameter and relative humidity were lower. Temperature had the greatest influence on the drying kinetics (in both the constant and falling rate periods), followed by the effect of pellet diameter. The drying kinetics were affected in a moderate way by the relative humidity in the constant rate period and part of the falling rate period. The air velocity had a slight effect of drying kinetics during the constant rate period, but this becomes insignificant during the falling rate period. The effective diffusivities increased from 7.81 × 10 to 1.97 × 10 m/s by increasing the temperature from 40° to 80 °C, leading to an activation energy of 23 kJ/mol. These values are typical from those found for wastewater sludge. The sludge exhibited a critical moisture content varying between 2.4 and 3.2 g/g db during drying without a clear trend as a function of the operating conditions and suggested that sludge was composed of considerably more bound moisture than unbound. The experimental data fitted the most closely to the Page model and, based on this, a new model was proposed for the prediction of drying times across the range of explored temperatures and pellet diameters in this study. The results of the proposed model fitted the experimental data with acceptable accuracy, so that the developed model could be employed as an analytical tool for the design, operation and optimisation of drying equipment.
在努力实现可持续卫生设施的过程中，一个挑战是确保对现场卫生设施产生的粪便污泥进行卫生处理。热干燥是去除水分和消毒的重要处理步骤。更好地理解干燥过程对于粪便污泥处理技术的合理设计至关重要。在本研究中，通过将温度从40℃变化到80℃、相对湿度从0%变化到25%以及风速从0.3mm/s变化到1.2mm/s，在对流干燥热重天平中对来自南非德班的通风改良坑式（VIP）厕所的粪便污泥进行干燥。粪便污泥样品呈薄层和直径为8至14mm的不同直径颗粒的形式。从实验数据确定动力学参数，与文献中的经典干燥模型进行比较，然后用于建立相关干燥模型。干燥速率在1至40g/(min·m)之间，导致干燥时间在100至300分钟之间。干燥动力学随着温度升高、颗粒直径和相对湿度降低而增加。温度对干燥动力学影响最大（在恒速期和降速期均如此），其次是颗粒直径的影响。在恒速期和部分降速期，相对湿度对干燥动力学有中等程度的影响。风速在恒速期对干燥动力学有轻微影响，但在降速期变得不显著。通过将温度从40℃提高到80℃，有效扩散率从7.81×10增加到1.97×10m²/s，导致活化能为23kJ/mol。这些值与废水污泥的典型值相符。污泥在干燥过程中表现出临界水分含量在2.4至3.2g/g干基之间变化，且没有明显的趋势作为操作条件的函数，这表明污泥中结合水比自由水多得多。实验数据与Page模型拟合得最紧密，并在此基础上提出了一个新模型，用于预测本研究中所探索温度和颗粒直径范围内的干燥时间。所提出模型的结果以可接受的精度拟合实验数据，因此所开发的模型可作为干燥设备设计、操作和优化的分析工具。
