Modeling of Sewage Sludge Flow in a Continuous Paddle Dryer

Abstract

International audienceA model based on the theory of Markov chains has been developed to represent the residence time distribution (RTD) of municipal sewage sludge in a continuous paddle dryer. The flow of dry solids is described by a chain of n perfectly mixed cells, n corresponding to the number of paddles attached to the shaft. The transition probabilities between the cells are governed by two parameters: the parameter of internal recirculation, R, and the solids hold-up, Hu. In the absence of available correlation, both parameters are identified by fitting the model to experimental RTD data. The model demonstrates its ability to describe the sludge flow in a continuous lab-scale paddle dryer. A sensitivity analysis highlights that R is critical for the treatment uniformity while Hu controls the mean residence time and thus the final moisture content

Modeling of a continuous sewage sludge paddle dryer by coupling Markov chains with penetration theory

“…The Markov chain modeling of sludge flow has been described in another publication [18], but its main features are also recalled here. In a Markov chain with discrete time and discrete space of state, a property is distributed between n different states; the evolution of this distribution relies on an n-by-n matrix, which represents all the possible transitions from any state to every other.…”

“…Once a steady state is reached and maintained for at least two hours, feeding and stirring are simultaneously stopped and sludge is sampled at nine different locations inside the dryer in order to obtain a water content profile along the dryer. The experimental data used for the model validation were obtained in the framework of a design of experiments, in which overflow height, dryer slope, SS feeding flow rate and stirring speed were varied [28]. SS used in this experimental study was produced in the Albi municipal WWTP (France), operating for 60,000 population equivalent.…”

“…However, there is yet no theory that could a priori lead to the estimation of R based on sludge water content. Hence, while in Tazaki's work, R was assumed to vary between the eleven zones studied and represented as much adjustable parameters, it was considered constant in Milhé's work [18]. The differences in scale and stirrer design of both experimental devices do not allow to judge of the respective pertinence of one or the other approach, all the more since all R values are not available in Tazaki's article, but rather four smoothed values.…”

“…This study showed that water and dry solids have the same RTD, that is to say SS acts as a single phase during drying. Moreover the same authors showed that this RTD can be described by a Markov chain, where each paddle represents a continuous stirred tank reactor (CSTR) [18]. These CSTRs exchange sludge flows in both directions, resulting in recirculation between the reactors: this is characterized by a recirculation coefficient R representing the ratio of recirculated material to the overall throughput in the reactor calculated on a dry basis, as in…”

Comparative Study on Drying Characteristics of Sewage Sludge in Two Kinds of Indirect Heat Drying Equipment

“…According to the principle of reduction, harmlessness and resource utilization of sludge treatment and disposal, resource utilization after heat drying is an ideal way for sludge attracting the attentions of scholars. However, the viscosity of sludge is a problem [1][2][3][4][5] during the drying process which needs to be solved for the development of sludge drying equipment. The operation conditions also have a great influence on the drying characteristics of sludge.…”