Effect of Different Pretreatments on Sludge Solubilization and Estimation of Bioenergy Potential

Abstract

Most of the conventional treatments of waste-activated sludge (WAS) are devoted to their minimization and destruction. On the other hand, the biomass contained in WAS can be utilized as a valuable source of renewable carbon. In this study, the influence of different pretreatments (ultrasonication, chemical, thermal, and combined pretreatments) was explored for sludge solubilization. Effects of the pretreatments were investigated as a function of the solubilization of total solids (TS), volatile solids (VS), and chemical oxygen demand (COD). Concentrations of soluble carbohydrates and total nitrogen were also measured. The most effective pretreatment to hydrolyze sludge was found to be the combined alkali–thermal (pH 12, 75 °C) pretreatment method, leading to TS and vs. solubilization of 9.6% and 17.2%, respectively. Soluble COD, carbohydrates, total nitrogen, and proteins estimated in the liquid phase were 5235 mg/L, 732 mg/L, 430 mg/L, and 2688 mg/L, respectively. Thus, the alkali–thermal method could be used for efficient valorization of WAS. Moreover, the solid fraction from all pretreated samples was further subjected to thermogravimetric analysis to estimate its potential for bioenergy from its higher heating value (HHV), which was found to be in the range of 10–11.82 MJ/kg. This study can provide better insight into the efficient valorization of liquid and solid phases of sludge after pretreatment.

A Comprehensive Review on Pretreatment Methods for Enhanced Biogas Production from Sewage Sludge

“…Due to the pivotal role of alkali addition in COD solubilization [43], there are several studies that have focused on coupling mechanical with alkaline pretreatment (AP). In the study of Babu et al [200], it was demonstrated that ultrasonication at pH 12 with NaOH addition presented an improved SCOD, compared to that obtained by the individual ultrasonication. Specifically, it was mentioned that ultrasonic pretreatment aided in the flocs breakage, which led to a more opened structure and promoted the OH − attacks, resulting in a better disintegration.…”

“…Although the pretreatment methods are commonly applied individually as standalone procedures, the sequential or simultaneous application of some different methods has also been scrutinized [9]. The combined pretreatment has been of high popularity owing to better efficiencies in terms of sludge hydrolysis, solubilization of organic constituents, biogas and methane yield, compared to when these methods are applied alone [54,161,[200][201][202]. Additionally, coupling of single pretreatments in some cases, enables the improvement of some limitations that occur during the implementation of individual methods, which are usually associated with increased energy consumption, high capital, investment and operation costs [12].…”

“…For example, when alkaline pretreatment is combined with the thermal one, it provokes a synergistic effect that allows reducing both the energy requirements and alkali reagent quantity, without adversely affecting the pretreatment efficiency [22]. Therefore, considering the beneficial aspects arising from combined pretreatment, several studies have been conducted to evaluate these integrate systems, with the most common (Table 5) to include the mechanical-thermal [200,202,203], mechanical-chemical [123,202,204], and thermo-chemical combinations [51,205].…”

“…Along with the improvements in solubilization, several studies that examined such integrated/combined systems have demonstrated improved biogas quality and thus, increment in the methane content [70]. The combination of ultrasonication with ozonation (UO) [55], Fenton's oxidation (UF) [206], or alkaline pretreatment (UA) [200,207] and the application of microwaves (MP) or high pressure homogenization (HPH) with concomitant base dosing [123,161,204,208] are all examples of mechanical/chemical mechanisms integration that have presented appreciable results of effectiveness.…”

Indexing energy and cost of the pretreatment for economically efficient bioenergy generation

“…Several studies focus on the combination of mechanical and alkaline pretreatment for the enhancement of COD solubilization. It was proved in the study by Babu et al (2021) that sonication at pH 12 shows enhanced solubilization due to the floc breakage, leads to a more open structure, and promotes better disintegration by OH radicals.…”