J.A. Gonzalez Ortega
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1
The pre-treatment of waste activated sludge (WAS) has become more common since it often results in improved bioconversion to methane, in both rate and extent. However, thorough insights on the possible effects and mechanisms of mild pre-treatment techniques, such as temperatures <100 °C combined with the addition of H2O2, are still limited. This study reports the effects of the addition of 5–30 mgH2O2/g TS and its interaction with thermal pre-treatment at 70 °C on methane production, using WAS as the substrate. It was found that the addition of H2O2 increased the methane production rate, coinciding with a decrease in apparent viscosity of WAS, which probably improved mass transfer under non-ideal mixing conditions. While H2O2 solubilized proteins and carbohydrates and mineralized a small fraction of the humic substances in WAS, these biochemical transformations did not suffice to explain the observed extent and rate of methane production. A decreased particle size, the presence of Fenton's reagent, and the presence of cationic polymers in the WAS were discarded as the reasons for the observed decrease in apparent viscosity. It was concluded that the pre-treatment conditions applied in the present study might be a strategy to enhance mixing conditions in full-scale anaerobic digesters.
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, kCH4 rapid, increased from 0.44 ± 0.01 to 0.47 ± 0.01 d-1 and the low rate, kCH4slow, 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.
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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, kCH4 rapid, increased from 0.44 ± 0.01 to 0.47 ± 0.01 d-1 and the low rate, kCH4slow, 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.
The highly variable characteristics of waste activated sludge (WAS) hinder the comparison of experimental results on WAS bioconversion between the different studies that use excess sludge from different origin. Sludge grown under laboratory conditions with synthetic wastewater as feed showed high resistance to commonly applied pre-treatment techniques, such as thermal pre-treatment. However, a distinctly higher bioconversion of this sludge was recorded compared to WAS from a full-scale wastewater treatment plant (WWTP). The observed results casted concern on the suitability of the experimental laboratory-based data for practice. The physicochemical and biochemical characteristics of both WAS and lab-grown sludge are dependent on the wastewater characteristics or growth media on which the sludges were grown. The objective of this study was to formulate a growth medium that results into a lab-grown sludge which shows high similarity to the WAS coming from a specific full-scale WWTP in response to a pre-treatment technique. More specifically, in this study we targeted the formation of slowly-biodegradable lab-grown sludge that is similarly responsive to mild thermal pre-treatment with H2O2 addition. By comparing real and synthetic wastewaters, we discussed the various wastewater constituents that may lead to a higher degree of recalcitrance of the produced sludge. We then formulated a growth medium, which was fed to a lab-scale activated sludge reactor and evaluated the nutrient removal capacity, as well as the characteristics of the cultivated sludge before and after pre-treatment. Finally, the growth medium was modified to provoke a change in both the bioconversion and in the response to mild thermal pre-treatment. The growth medium proposed in this study resulted in a slowly-biodegradable sludge (195 ± 3.7 NLCH4/kgVSadded) that after thermal pre-treatment resulted in an increase in methane production of 9 %, which was similar to the WAS coming from the full-scale WWTP. It was concluded that not only the bioconversion but also the response to mild thermal pre-treatment of lab-grown sludge was determined by the composition of the growth media.
Pre-treatments to enhance the biodegradability of waste activated sludge
Elucidating the rate limiting step
Pre-treatments for waste activated sludge (WAS) are, in most cases, an attempt to increase the biodegradation and/or improve hydrolysis rate of WAS after anaerobic digestion. This review presents an extensive analysis of WAS pre-treatments effectiveness focusing on increasing the biodegradability. In the first part of the review, WAS is considered as a cluster of organic components: proteins, carbohydrates, humic substances and cells. Based on this breakdown into components, the effect of different pre-treatments on each component (and in combination) is described. Also, possible reasons for the contradictory results frequently found among different studies dealing with the same pre-treatment are included. In the second part, the review describes the effects on volatile solids removal by digestion after pre-treatment and on the dewaterability of the final digestate. The energy balance and potential limiting factors for each pre-treatment are also taken into account. From the published works it is concluded that some pre-treatment techniques, such as thermal hydrolysis, thermal phased anaerobic digestion and low-temperature pre-treatment are effective ways to increase energy production and to improve other sludge properties, such as dewatering. However, these techniques are very energy intensive and require a large capital outlay, so research on milder pre-treatment techniques is valuable.