Maximisation of energy recovery from waste activated sludge via mild-temperature and oxidative pre-treatment

The overall objective of the present study was to investigate the effects of thermal pre-treatment of waste activated sludge (WAS) at 70 °C with addition of H₂O₂ to enhance sludge hydrolysis and subsequent methane production during WAS anaerobic digestion. The research was divided into four parts: Firstly, a bibliographical part, in which literature research revealed that WAS can be considered a mixture of proteins, humic substances, cells (and others).
Subsequently, the effects of several pre-treatment techniques on these constituents and on biochemical and physicochemical properties of WAS, such as methane production and dewatering, were analyzed. This part reviews the response of WAS subjected to pre-treatments of different nature (e.g., thermal, acid-base, oxidative) at different energy intensities. It also compiles the role of pre-treatment techniques on sterilization, dewatering and methane production.
Ultimately, it was made clear that the mechanisms of most of the pre-treatments still remain unknown, hindering a fair comparison of their effects.
In the second part, the effects of low-temperature pre-treatment with the addition of H₂O₂ on WAS were analyzed in both lab- and pilot-scale scenarios to detect and quantify its effects. During lab-scale experiments, it was found that the application of low-temperature thermal pre-treatment combined with H₂O₂ at 70 °C; 30 minutes and 15 mgH₂O₂/g TS increased the methane production rate, which consisted of 2 differently recognizable parts. The high rate, k_{CH4 rapid}, increased from 0.44 ± 0.01 to 0.47 ± 0.01 d^-1 and the low rate, k_CH4slow, from 0.09 ± 0.00 to 0.11± 0.01 d^-1. There were inconclusive results regarding an increase in specific methane production. The lab-scale observations were reproduced during a pilot-scale experiment, although due to methodological restrictions, pre-treatment was applied together with two-staged compartmentalized digestion. It was observed that due to the adoption of pre-treatment and compartmentalized digestion, organic loading rates could be increased from 1.4 to 4.2 kg volatile solids VS/(m³d), which resulted in a solids retention time (SRT) decrease from 23 to 15 days without apparent process impairment. It was considered that most of the observed effects were caused by the pre-treatment, while the influence of compartmentalized digestion remained marginal in this study.
In the third part, further study at lab-scale was conducted to determine the individual contributions of the separate components of pre-treatments, i.e., thermal and oxidative. For instance, thermal pre-treatment solubilized most of the EPS; deactivated catalase and accelerated the reaction rate of H₂O₂, while H₂O₂ decreased the apparent viscosity of WAS by 12-30%, resulting in a synergistic effect on the WAS digestibility. As suggested by other rheological parameters, the addition of H₂O₂ improved the flowability of WAS at 70 °C. The cause of the decrease in viscosity was not determined. However, the presence of hydroxyl radicals via the Fenton’s reagent; the decrease in particle size of WAS, and the combination of H₂O₂ with conditioning agents were discarded. On the other hand, results suggested that the reason behind the decrease in viscosity was the molecular modification of the carbohydrates in WAS as a result of their reaction with of H₂O₂.
The above-described experiments were restricted to the grab samples taken at 3 wastewater treatment plants (WWTPs). However, WAS is a matrix of variable composition, depending on location and season. Therefore, in the fourth and last part, the applicability of the pre-treatment methods to WAS with a different composition was tested, using lab-grown sludge. Based on the results, it was inferred that the concentration of metals embedded in lab-grown sludge was relevant for the effectiveness of pre-treatment in terms of methane production, both rate and extent.
The evidence obtained in this study suggests that the lower viscosity of the pre-treated WAS was reflected in the viscosity of the digestate, which allowed a better mass-transfer during non-ideal mixing and therefore a higher methane production rate. Since full-scale digesters are very often poorly-mixed, the applied pre-treatment conditions might be a possible strategy to improve mixing and increasing the BMP without increasing the mixing energy.