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.
Highlights
The European Union with a population of 500 million people is estimated to have a total sludge production of about 13 million tons of dry matter as of 2020 from wastewater treatment plants (WWTPs) [1,2]
waste-activated sludge (WAS) solubilization improved when chemical pretreatment was combined with ultrasonication (TS 7.2%, vs. 12.7%) and the values increased greatly for the combined alkali–thermal pretreatment to total solids (TS) 9.6% and vs. 17.2%
This paper evaluated the effect of different pretreatments on WAS solubilization, which is the first step for a biorefinery exploiting WAS as a source
Summary
The European Union with a population of 500 million people is estimated to have a total sludge production of about 13 million tons of dry matter as of 2020 from wastewater treatment plants (WWTPs) [1,2]. Population and economic factors have led to an increase in the number of WWTPs, and, with stricter effluent discharge laws enforced on them, the rate of biosolid production is expected to increase [3,4,5]. Sludge management involves the disposal of sludge using conventional methods such as incineration, landfilling, and agricultural use [6]. It does not include the valorization of sludge as feedstock for the production of fine chemicals and bioproducts [7]. To facilitate valorization of sludge, it is important to firstly solubilize the organic content present
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