Experimental Determination of Moisture Sorption Isotherm of Fecal Sludge

Abstract

Dewatering and drying of fecal sludge (FS) is a key treatment objective in fecal sludge management as it reduces volume (thereby reducing emptying frequency and associated transportation costs), inactivates pathogens, and is beneficial and/or necessary to resource recovery activities such as composting and combustion as fuel. However, studies on dewatering performances of FS are limited. The physical water distribution of such matrices is not fully understood, limiting the progress in the development and optimization of FS dewatering technologies. The objective of this study is to present a gravimetric method intended to assess the dewatering characteristics and associated modelling of FS through moisture sorption isotherms. Samples were placed in airtight jars containing different saturated salt (NaOH, CaCl 2 , NaCl, KCl, K 2 SO 4 ) solutions to reproduce a range of relative humidity values (6 to 97%). Results confirmed the achievement of characteristic sigma-shaped moisture sorption isotherms with increasing moisture adsorption at higher values of relative humidity. Furthermore, experimental data best fit the three-parameter Guggenheim-Anderson-de Boer (GAB) model. This method can be replicated to contribute critical data about the characterization of fecal sludge, a seriously under-researched matrix.

Measurement and Modelling of Moisture Sorption Isotherm and Heat of Sorption of Fresh Feces

“…Results from the initial COD and moisture content characterization analysis are summarized in Table 2 and compared with literature values. [21]. The values reported by Rose et al [15] represent a literature survey of 47 samples.…”

“…For determination of pH and electrical conductivity of feces, 1:400 m/v suspension with deionized water was made. The typical approach to measuring the pH and conductivity of fresh feces is to measure a homogenized sample rather than a sample in suspension [22][23][24][25], but the 1:400 suspension method was followed for consistency with exploratory work performed by Bourgault et al [21].…”

“…Equilibrium moisture contents of fresh feces were determined experimentally in the Public Health & Environmental Engineering (PH2E) Lab of UVic using the static gravimetric analysis proposed by Bourgault et al [21] in the study of fecal sludge. This method has been validated as a technique to measure water distribution within waste activated sludge [8].…”

“…It is necessary to test several models because no one equation is accurate for all materials for the complete range of water activities, because the way water is distributed within a solid is influenced by different mechanisms in different water activity regions [16]. Models were selected based on their use in describing biological materials with a higher moisture content [8,13,21]. Different models are most effective in describing the sorption isotherm at different a w levels as shown in the table below [13,30,31].…”

“…This work builds on exploratory research performed by Bourgault et al [21] that justified the use of MSI to derive fundamental characteristics for fecal sludge. In addition to validating the approach explored by Bourgault et al [21], this work evaluates the moisture sorption isotherm of fresh feces at three different temperatures with the objective of calculating the isoteric heat of sorption, a value that can be used to predict the energy requirements of drying.…”