Abstract
The aim of this work was to study the effect of ultrasonic pretreatment on sludge digestion, particle size distribution (PSD), and dewaterability of digestates (measured by capillary suction time (CST) and specific resistance to filtration (SRF)). Sludge was pretreated with ultrasound (24 kHz) at an energy dosage of 4300 kJ/kg TS. Digestibility of sludge was increased by ultrasound pretreatment resulting in a higher specific methane production (SMP). The digestate of pretreated waste activated sludge (WAS) obtained under batch conditions presented a better dewatering performance. Digestion under semi-continuous conditions was evaluated using sewage sludge (mixture of primary sludge and WAS). In this case, digestates presented a much higher mean particle size for both cases evaluated (pretreated and non-pretreated) than that obtained under batch conditions. A wide PSD was a characteristic of these digestate samples. Flow dynamics inside the reactor resulted in the presence of high-diameter flocs, thus significantly affecting the mean particle size and specific surface area (SSA) values.
Highlights
The management of sewage sludge in wastewater treatment plants (WWTPs) is currently an issue of great concern
The benefits of ultrasound pretreatment are indicated by the increase of the cumulative methane production and biogas production rate during the initial stage of the digestion experiments
The digestion of sewage sludge was significantly enhanced by ultrasound pretreatment around 14%
Summary
The management of sewage sludge in wastewater treatment plants (WWTPs) is currently an issue of great concern. The treatment and disposal of sludge may represent up to 50% of plant operating costs. Anaerobic digestion (AD) is a well-known technique that is used for the valorization of sludge. This process is usually the preferred stabilization method due to low operating costs and biogas production. Sludge dewatering, which is highly affected by the treatment option of sludge, is an important issue for decreasing sludge volume and, reducing the operating cost in WWTPs. Dewatering is a difficult and costly process in wastewater treatment. The process is mainly carried out by physical separation, and presents a variable efficiency based on the nature of the sludge
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