Effect of ultrasonic pretreatment on the semicontinuous anaerobic digestion of waste activated sludge with increasing loading rates
Introduction
Anaerobic digestion (AD) is the most common technology applied to treat waste activated sludge (WAS) (Campo et al., 2017). This biological waste contains organic matter, heavy metals, pathogens and emerging contaminants (pharmaceuticals or drugs) that are not currently covered by existing environmental and water quality regulations (Boix et al., 2016; Zhen et al., 2017). Inadequate treatment, use and disposal of WAS causes severe environmental impacts and risks to human health. The water and wastewater industry are facing unprecedented economic and environmental constraints because of increasingly stringent regulations and the large amounts of WAS produced. Proper management of waste sludge is a big challenge to wastewater treatment industries, because sludge handling and disposal account for up to 60% of the total operating costs of wastewater treatment plants (Uma Rani et al., 2013; Xie et al., 2016).
In light of the challenges described above, AD is an excellent alternative for the successful treatment of WAS to reduce negative environmental and health impacts. However, due to its low biodegradability and complexity, as well as the presence of micro-contaminants and inhibitors of the anaerobic process, the resulting biogas and methane yields and the reduction in organic matter and pathogens is limited. The low biogas yields are due to the complexity of the cell walls of the bacteria that make up the sludge flocs (Zhang et al., 2017). However, mesophilic AD can be improved by adding a pretreatment step (Carrère et al., 2010; Carlsson et al., 2012).
Ultrasonic pretreatment (USp) is the most effective sludge disintegration technology and has been applied in lab, pilot and full-scale systems (Pilli et al., 2011; Appels et al., 2012; Tyagi et al., 2014). USp has been successful because it generates a high level of solubilization, is an easy and simple pretreatment, increases the biogas and methane yields, improves digestate quality, and can be used together with other pretreatments (Gong et al., 2015; Zhao et al., 2015). However, there are strong disagreements regarding the effects of ultrasonic pretreatment studies, especially with respect to the energy applied, their energy expression, the equipment used, the total solids concentration, and the effects on solubilization and anaerobic biodegradability (Tyagi et al., 2014; Le et al., 2015). Most of the research on the effects of pretreatments of the AD of sludge only investigated the process using single stage batch digestion experiments (BMP-tests) (Ruiz-Hernando et al., 2014; Kim et al., 2015; Pilli et al., 2016). Therefore, there is still a lack of information on the behavior of the process in semicontinuous systems, particularly with respect to the optimum ultrasonic specific energy (SE), the maximum production of biogas and the maximum organic loading rate (OLR) at which the reactor can be operated.
In the first part of this work, USp was performed with a wide range of specific energies (5000–35000 kJ/kg TS) to evaluate its effects on solubilization, sanitization, and anaerobic biodegradability. The most effective energies were evaluated by BMP tests, with solubilization as a discriminating variable (Lizama et al., 2017). Nevertheless, is necessary to carry out semicontinuous studies to assess the performance under continuous disturbances to the reactor to evaluate the stability over long periods and with various organic load increments. The information provided by investigating this operational mode is more useful and representative of real-life scenarios, because the majority of high-scale anaerobic plants operate under continuous or semicontinuous conditions.
Therefore, the objective of this study was to evaluate the influence of USp at three SE levels on the semicontinuous AD of WAS with increasing organic loads during long duration experiments. Special attention was given to the performance and stability of the digesters, biogas production, organic matter reduction, volatile fatty acid production and composition, and the kinetic evaluation of biogas production, providing a full perspective of the influence of USp on the semicontinuous anaerobic digestion of WAS.
Section snippets
Waste activated sludge samples
Waste activated sludge samples were collected from the municipal “Pensiones II” WWTP, which operates using an extended aeration system. This plant has the capacity to treat 12 L/s and is one of the 26 plants serving the city of Merida, Yucatan, Mexico. Sludge total solids (TS) were concentrated by mechanical filtration to obtain concentrated sludge with approximately 3% TS, which was stored at 4–6 °C prior to pretreatment and analysis. On average, the concentrated raw WAS samples used in this
The impact of ultrasonic pretreatment on biogas production and organic loading rate
The different USps evaluated in this work allowed digesters to operate at low, medium and high organic loading rates, while the reactor with raw sludge was not able to withstand the increases in organic load. The behaviors in the production of biogas due to the USps during the semicontinuous AD using different OLRs are shown in Fig. 1. The biogas production in the reactor with untreated WAS decreased as the OLR increased from 0.040 to 0.033 L/d at 1 and 3 kgVS/m3·d, respectively, a reduction of
Conclusions
USp is a suitable technology to increase the semicontinuous AD efficiency of WAS. In this study, the best USp was at 25000 kJ/kg TS, which displayed the highest cumulative biogas production (13.9 L) and allowed for operation at a high OLR without acidification inhibition and with results similar to those obtained at 35000 kJ/kg TS, but with lower energy requirements. The anaerobic digester fed with ultrasonic pretreated WAS at 25000 kJ/kg TS increased the daily biogas production by 560%
Acknowledgements
We would like to thank the Consejo Nacional de Ciencia y Tecnología of Mexico (CONACyT) for the financial support provided to Mr. Alfredo Córdova Lizama as part of his PhD Scholarship (support number: 429358).
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2022, Journal of Environmental ManagementCitation Excerpt :VS reduction in all systems was in agreement with the biogas yield as discussed formerly. The efficient organic removal from the pretreated sludge was attributed to the solubilization and fragmentation of sludge particles, which became more vulnerable for enzymatic attacks (Lizama et al., 2018). Moreover, the particle size of sewage sludge decreased and surface area increased after the pretreatment, improving efficient organic biodegradation by microorganisms and organic removal.