Technological, Ecological, and Energy-Economic Aspects of Using Solidified Carbon Dioxide for Aerobic Granular Sludge Pre-Treatment Prior to Anaerobic Digestion

Abstract

The technology of aerobic granular sludge (AGS) seems prospective in wastewater bio-treatment. The characteristics as well as compactness and structure of AGS have been proved to significantly affect the effectiveness of thus far deployed methods for sewage sludge processing, including anaerobic digestion (AD). Therefore, it is deemed necessary to extend knowledge on the possibilities of efficient AGS management and to seek viable technological solutions for methane fermentation of sludge of this type, including by means of using the pre-treatment step. Little is known about the pre-treatment method with solidified carbon dioxide (SCO2), which can be recovered in processes of biogas upgrading and enrichment, leading to biomethane production. This study aimed to determine the impact of AGS pre-treatment with SCO2 on the efficiency of its AD. An energy balance and a simplified economic analysis of the process were also carried out. It was found that an increasing dose of SCO2 applied in the pre-treatment increased the concentrations of COD, N-NH4+, and P-PO43− in the supernatant in the range of the SCO2/AGS volume ratios from 0.0 to 0.3. No statistically significant differences were noted above the latter value. The highest unit yields of biogas and methane production, reaching 476 ± 20 cm3/gVS and 341 ± 13 cm3/gVS, respectively, were obtained in the variant with the SCO2/AGS ratio of 0.3. This experimental variant also produced the highest positive net energy gain, reaching 1047.85 ± 20 kWh/ton total solids (TS). The use of the higher than 0.3 SCO2 doses was proved to significantly reduce the pH of AGS (below 6.5), thereby directly diminishing the percentage of methanogenic bacteria in the anaerobic bacterial community, which in turn contributed to a reduced CH4 fraction in the biogas.

Application of hydrodynamic cavitation in the disintegration of aerobic granular sludge - evaluation of pretreatment time on biomass properties, anaerobic digestion efficiency and energy balance

“…The highest biogas and CH4 production yields, 476±20 mL/gVS and 341±13 mL/gVS, respectively, were obtained with a volume ratio of solidified CO2 to AGS of 0.3. The biogas yield from raw AGS was 309±21 mL/gVS, and the CH4 concentration in the produced biogas reached 68.84±2.2% [15]. Anaerobic digestion of raw, non-cavitated AGS (V0) allowed to obtain 398.8±23.4 mL/gVS, and the CH4 content in the biogas was 63.2±1.2% (Table 4).…”

“…A significant increase in biogas production was observed with the extension of the disintegration process [14]. The use of solidified carbon dioxide for the disintegration of AGS was also investigated [15]. The highest biogas and CH4 production yields, 476±20 mL/gVS and 341±13 mL/gVS, respectively, were obtained with a volume ratio of solidified CO2 to AGS of 0.3.…”

“…There are few reports on the effects of AGS disintegration on the technological efficiency and economic profitability of anaerobic digestion. Thermal hydrolysis [13], ultrasonic disintegration [14] and the use of solidified carbon dioxide [15] have been investigated. Therefore, there is a large space for additional knowledge on the evaluation of the impact of other pretreatment methods on the efficiency of anaerobic digestion of AGS and the final technological effects and energy consumption.…”

The Use of Hydrodynamic Cavitation to Improve the Anaerobic Digestion of Waste from Dairy Cattle Farming—From Laboratory Tests to Large-Scale Agricultural Biogas Plants

“…An example of this type of research is the use of solidified CO 2 in the disintegration process of aerobic granular sludge (AGS) prior to AD, where a successive increase in COD in the supernatant was observed with increasing doses of solidified CO 2 from 0.1 to 0.5 in the supernatant from 334 ± 15 to 450 ± 13 mgO 2 /L. For raw sludge, the COD concentration in the overlying liquid was 152 ± 14 mgO 2 /L [33]. Other studies focused on the use of ultrasonic disintegration for the pretreatment of excessive sewage sludge [75].…”

“…To date, many techniques have been analysed to improve the susceptibility of organic substrates to degradation under anaerobic conditions. These include comminution and mechanical destruction [24], treatment with acids and bases [25], advanced oxidation [26], enzymatic hydrolysis [27], disintegration with ultrasound [28,29], hydrothermal depolymerisation by conventional [30] and microwave heating [31], the use of low temperatures in the freezing processes [32] and the injection of solidified carbon dioxide [33,34], as well as the use of high-voltage disintegration [35]. Unfortunately, in many cases, the optimistic results of the preliminary investigations, which were achieved under laboratory conditions, were negatively confirmed in pilot studies on a technical scale [36].…”