Synergetic effect of combined pretreatment for energy efficient biogas generation
Graphical abstract
Introduction
Excess amount of waste activated biosolids (WAB) is the unavoidable derivative of biological wastewater treatment (Kavitha et al., 2013). Biosolids are enriched in organic matter which creates numerous environmental issues without proper treatment (Ak et al., 2013). Anaerobic digestion process is most effective in stabilization of biosolids and reduces the economic constraint of treatment facility through energy recovery in the form of methane (Kavitha et al., 2014a, Gayathri et al., 2015). On the other hand, hydrolysis of WAB is the rate-limiting step in AD process due to prolonged retention time of about 25–30 days (Kavitha et al., 2015, Kavitha et al., 2016a, Kavitha et al., 2016b, Kavitha et al., 2016c). To increase the working efficiency of anaerobic digestion and hydrolysis, pretreatment of biosolids is essential. Consequently, numerous sole and combinative pretreatment techniques has been adopted to solubilise or destruct the cell wall of microorganism (sludge lysis) such as thermal, ultrasonic, high pressure homogeniser, chemical, microwave, ozonation and mechanical, thermochemical pretreatments (He and Zhang, 2011, Saha et al., 2011, Ariunbaatar et al., 2014, Pei et al., 2016). Several researchers have proposed the advantageous outcome of the combined pretreatment methods on subsequent anaerobic digestibility. Among those methods, thermochemical pretreatment of biosolids have wide application and known for its efficient disintegration of sludge (Uma et al., 2012). Although it has many advantages, it has some limitations; For instance, the efficient sludge disintegration index for biodegradation process is 30% solubilization (Parvathy et al., 2016). Achieving similar solubilization through thermochemical pretreatment demands high energy input, cost and extended treatment time. This comprehensively limits the overall profitability of the process and resulted in loss of organic matter which in turn limits the methane production in degradability process. In addition, pretreatment of sludge through ozone was one of the potent technologies with the highest disintegration capability (Salsabil et al., 2010). However from an economical point of view, employing ozone for biosolids pretreatment is expensive and this is the major limitation for it to be used in pilot scale plants. Employing higher dosage of ozone also resulted in loss of organics. Therefore to address these issues, in the present study, it was planned to combine the thermochemical pretreatment with ozonation in order to minimize the energy requirement, organic loss and cost of the disintegration process with limited loss of organics. This combinative pretreatment is considered to be novel due to the above mentioned advantages and it has not been documented in literature so far. Therefore, the main objectives of the present study was i) to optimize the pretreatment conditions for effective Thermochemical (TC) disintegration ii) to optimize the operational conditions of combined Thermo chemo ozone (TCO3) pretreatment for cost effective performance iii) to investigate the synergistic effect of this combinative pretreatment on effective solubilization in a cost effective manner iv) to evaluate the effectiveness of this combined pretreatment on biodegrdability process and methane production v) to assess the economic viability of TCO3 pretreatment based on energetic analysis and economic assessment.
Section snippets
Waste activated biosolids and inoculum
Municipal Waste Activated Biosolids (WAB) sample used in the present study was collected from a secondary sedimentation tank of the local MWWTPat Chennai, India. The physiochemical characteristics of WAB were analyzed and indicated in the (Table 1). Anaerobically Digested Sludge collected from the existing biogas plant of waste water treatment facility was used as inoculum in the anaerobic batch test. The collected inoculum was in semi solids state and it was filtered using 0.1 mm filter mesh or
Impact of thermochemical pretreatment on biosolids disintegration
To evaluate the effect of Thermochemical (TC) pretreatment, experiments were executed with six different temperature (50 °C–100 °C) and strong alkali, NaOH. The probable outcome of the waste activated biosolids (WAB) disintegration process was investigated with consideration focussed on COD solubilization (Liquification) and SS reduction (solids changes).
Conclusion
Combined TCO3 pretreatment appreciably improved the COD solubilization to a greater extent of about 30.4% when compared to TC pretreatment of about 22%. Through this combined process, lesser specific energy input of about 141.012 kJ/kg TS was utilized to achieve the anticipated solubilization. The result of biodegradation studies exposes a higher methane production potential for TCO3 pretreatment. Based on economic assessment, TCO3 pretreatment of WAB achieved a positive net profit of about 35.49
Acknowledgement
Authors are thankful to DST, India for affording financial aid for this work (SR/FTP/ETA-0021/2010) on behalf of Young Scientist scheme.
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