Physicochemical properties and biogas productivity of aerobic granular sludge and activated sludge

Case study of aerobic granular sludge and activated sludge-Energy usage, footprint, and nutrient removal

This study demonstrates a comparison of energy usage, land footprint, and volumetric requirements of municipal wastewater treatment with aerobic granular sludge (AGS) and conventional activated sludge (CAS) at a full-scale wastewater treatment plant characterized by large fluctuations in nutrient loadings and temperature. The concentration of organic matter in the influent to the AGS was increased by means of hydrolysis and bypassing the pre-settler. Both treatment lines produced effluent concentrations below 5 mg BOD7 L-1 , 10 mg TN L-1 , and 1 mg TP L-1 , by enhanced biological nitrogen- and phosphorus removal. In this case study, the averages of volumetric energy usage over 1 year were 0.22 ± 0.08 and 0.26 ± 0.07 kWh m-3 for the AGS and CAS, respectively. A larger difference was observed for the energy usage per reduced population equivalents (P.E.), which was on average 0.19 ± 0.08 kWh P.E.-1 for the AGS and 0.30 ± 0.08 kWh P.E.-1 for the CAS. However, both processes had the potential for decreased energy usage. Over 1 year, both processes showed similar fluctuations in energy usage, related to variations in loading, temperature, and DO. The AGS had a lower specific area, 0.3 m2  m-3 d-1 , compared to 0.6 m2  m-3 d-1 of the CAS, and also a lower specific volume, 1.3 m3  m-3 d-1 compared to 2.0 m3  m-3 d-1 . This study confirms that AGS at full-scale can be compact and still have comparable energy usage as CAS. PRACTITIONER POINTS: Full-scale case study comparison of aerobic granular sludge (AGS) and conventional activated sludge (CAS), operated in parallel. AGS had 50 % lower footprint compared to CAS. Energy usage was lower in the AGS, but both processes had potential to improve the energy usage efficiency. Both processes showed low average effluent concentrations.

Anaerobic digestibility of aerobic granular sludge from continuous flow reactors: the role of granule size distribution

There is an increasing interest in integrating aerobic granular sludge (AGS) technology into wastewater industries. Several projects are being performed to cultivate the aerobic granules for continuous flow reactors (AGS-CFR), while there is a scarcity of those projects that investigate the bio-energy recovery from AGS-CFR. This research was designed to examine the digestibility of AGS-CFR. Beyond that, it aimed at defining the role of the granule size on their digestibility. For this purpose, a series of bio-methane potential (BMP) tests have been run at mesophilic conditions. The results showed that AGS-CFR has a lower methane potential (107.43 ± 4.30 NmL/g VS) compared to activated sludge. This may be the result of the high sludge age (30 days) of AGS-CFR. Additionally, the results revealed that the average size of granules is among the main factors that reduce their digestibility, but it does not inhibit it. It was noticed that granules of size >250 μm have a significantly lower methane yield than the smaller ones. Kinetically, it was noticed that the kinetic models with two hydrolysis rates fit well with the methane curve of AGS-CFR. Overall, this work showed that the average size of AGS-CFR characterizes its biodegradability, which in turn defines its methane yield.

Anaerobic fermentation of aerobic granular sludge: Insight into the effect of granule size and sludge structure on hydrolysis and acidification

Aerobic granular sludge (AGS) has different physicochemical properties and microbial communities compared to conventional activated sludge (CAS), which may result in different behaviors during anaerobic fermentation and require further investigation. This study investigated the effect of granule size and sludge structure on the hydrolysis and acidification of AGS. Experimental results show that AGS exhibited significantly higher soluble chemical oxygen demand (SCOD) dissolution and total volatile fatty acids (TVFA) production (330.6-430.3 mg/gVSS and 231.0-312.5 mgCOD/gVSS) compared to conventional activated sludge (CAS) (167.0 mg/gVSS and 133.3 mgCOD/gVSS). This is because AGS (90.6-96.9 mg/gVSS) had higher extracellular polymeric substances (EPS) content than CAS (81.2 mg/gVSS). EPS can not only serve as substrates but also release the trapped hydrolases. Moreover, the relative abundances of hydrolytic/acidogenic bacteria and genes were higher in AGS (0.46%-3.60% and 3.01 × 10^-3%-4.04 × 10^-3%) than in CAS (0.30% and 1.23 × 10^-3%). The optimal granule size for AGS fermentation was found to be 500-1600 μm. The crushing of granule structure promoted the dissolution of small amounts of EPS and the release of some trapped hydrolases, thereby potentially enhancing the enzyme-substrate contacts and bacteria-substrate interactions. Therefore, the highest SCOD dissolution (510.6 mg/gVSS) and TVFA production (352.1 mgCOD/gVSS) from crushed 500-1600 μm AGS were observed. Overall, the findings of this study provide valuable insights into the recovery of organic carbon from AGS via anaerobic fermentation.

Impact of primary treatment methods on sludge characteristics and digestibility, and wastewater treatment plant-wide economics

Biogas production from anaerobic sludge digestion plays a central role for wastewater treatment plants to become more energy-efficient or even energy-neutral. Dedicated configurations have been developed to maximize the diversion of soluble and suspended organic matter to sludge streams for energy production through anaerobic digestion, such as A-stage treatment or chemically enhanced primary treatment (CEPT) instead of primary clarifiers. Still, it remains to be investigated to what extent these different treatment steps affect the sludge characteristics and digestibility, which may also impact the economic feasibility of the integrated systems. In this study, a detailed characterization has been performed for sludge obtained from primary clarification (primary sludge), A-stage treatment (A-sludge) and CEPT. The characteristics of all sludges differed significantly from each other. The organic compounds in primary sludge consisted mainly of 40% of carbohydrates, 23% of lipids, and 21% of proteins. A-sludge was characterized by a high amount of proteins (40%) and a moderate amount of carbohydrates (23%), and lipids (16%), while in CEPT sludge, organic compounds were mainly 26% of proteins, 18% of carbohydrates, 18% of lignin, and 12% of lipids. The highest methane yield was obtained from anaerobic digestion of primary sludge (347 ± 16 mL CH₄/g VS) and A-sludge (333 ± 6 mL CH₄/g VS), while it was lower for CEPT sludge (245 ± 5 mL CH₄/g VS). Furthermore, an economic evaluation has been carried out for the three systems, considering energy consumption and recovery, as well as effluent quality and chemical costs. Energy consumption of A-stage was the highest among the three configurations due to aeration energy demand, while CEPT had the highest operational costs due to chemical use. Energy surplus was the highest by the use of CEPT, resulting from the highest fraction of recovered organic matter. By considering the effluent quality of the three systems, CEPT had the highest benefits, followed by A-stage. Integration of CEPT or A-stage, instead of primary clarification in existing wastewater treatment plants, would potentially improve the effluent quality and energy recovery.

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

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-PO₄^3- 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.

Biohythane Production in Hydrogen-Oriented Dark Fermentation of Aerobic Granular Sludge (AGS) Pretreated with Solidified Carbon Dioxide (SCO2)

Though deemed a prospective method, the bioconversion of organic waste to biohydrogen via dark fermentation (DF) has multiple drawbacks and limitations. Technological difficulties of hydrogen fermentation may, in part, be eliminated by making DF a viable method for biohythane production. Aerobic granular sludge (AGS) is a little-known organic waste spurring a growing interest in the municipal sector; its characteristics indicate the feasibility of its use as a substrate for biohydrogen production. The major goal of the present study was to determine the effect of AGS pretreatment with solidified carbon dioxide (SCO₂) on the yield of H₂ (biohythane) production during anaerobic digestion (AD). It was found that an increasing dose of SCO₂ caused an increase in concentrations of COD, N-NH₄⁺, and P-PO₄^3- in the supernatant at the SCO₂/AGS volume ratios from 0 to 0.3. The AGS pretreatment at SCO₂/AGS ratios within the range of 0.1-0.3 was shown to enable the production of biogas with over 8% H₂ (biohythane) content. The highest yield of biohythane production, reaching 481 ± 23 cm³/gVS, was obtained at the SCO₂/AGS ratio of 0.3. This variant produced 79.0 ± 6% CH₄ and 8.9 ± 2% H₂. The higher SCO₂ doses applied caused a significant decrease in the pH value of AGS, modifying the anaerobic bacterial community to the extent that diminished anaerobic digestion performance.

Valorization of full-scale waste aerobic granular sludge for biogas production and the characteristics of the digestate

Despite the dynamic development of aerobic granular sludge (AGS) technology in wastewater treatment, there is limited data on how the different properties of AGS and activated sludge (AS) translate into differences in waste sludge management. Waste sludge generated in both AGS and AS technology is the biggest waste stream generated in wastewater treatment plants (WWTPs). This study aimed to assess biogas production from waste AGS from a full-scale system. Additionally, the properties of the digestate were investigated in terms of its management in line with the assumptions of a circular economy. Both aspects are important because the characteristics of AGS differ from those of AS. Its dense, extracellular-polymer-rich granule structure makes the susceptibility of AGS to anaerobic stabilization lower than that of AS. Given the advantages of AGS for sustainable wastewater treatment and its increasing popularity, waste AGS management will pose a serious challenge for WWTP operators. Therefore, AGS from a full-scale municipal WWTP was valorized for biogas production by increasing the accessibility of the organics in the sludge by homogenization or ultrasound pretreatment. Ultrasound pretreatment released about an order of magnitude more organics from the biomass than homogenization and significantly improved the production of methane-rich biogas (455 L/kg VS, about 66% of CH₄). The digestion time of pretreated AGS was reduced by 25% in comparison with that of untreated AGS making anaerobic digestion of AGS a feasible solution for sludge management. The AGS digestate was rich in Ca (77.0 g/kg TS), Mg (10.9 g/kg TS), N (35.1 g/kg TS) and P (32.4 g/kg TS), whereas its heavy metal levels and biochemical methane potential were low. AGS digestate is not only environmentally safe, but it can serve as a rich source of organics and elements essential for soil fertility and stability.

Integrated review of resource recovery on aerobic granular sludge systems: Possibilities and challenges for the application of the biorefinery concept

Aerobic Granular Sludge (AGS) is a biological treatment technology that has been extensively studied in the last decade. The possibility of resource recovery has always been highlighted in these systems, but real-scale applications are still scarce. Therefore, this paper aimed to present a systematic review of resources recovery such as water, energy, chemicals, raw materials, and nutrients from AGS systems, also analyzing aspects of engineering and economic viability. In the solid phase, sludge application in agriculture is an interesting possibility. However, the biosolids' metal concentration (the granules have high adsorption capacity due to the high concentration of extracellular polymeric substances, EPS) may be an issue. Another possibility is the recovery of Polyhydroxyalkanoates (PHAs) and Alginate-like exopolymers (bio-ALE) in the solid phase, emphasizing the last one, which has already been made in some Wastewater Treatment Plants (WWTPs), named and patented as Kaumera® process. The Operational Expenditure (OPEX) can be reduced by 50% in the WWTP when recovery of ALE is made. The ALE recovery reduced sludge yield by up to 35%, less CO₂ emissions, and energy saving. Finally, the discharged sludge can also be evaluated to be used for energetic purposes via anaerobic digestion (AD) or combustion. However, the AD route has faced difficulties due to the low biodegradability of aerobic granules.

Thermophilic and mesophilic biogas production from PLA-based materials: Possibilities and limitations

Recently, the use of bio-based products, including biodegradable poly(lactic acid) (PLA), has increased, causing their rapid growth in municipal waste streams. The presence of PLA in biowaste may increase biogas production (BP). However, the rate of PLA biodegradation, which affects the time frame of anaerobic digestion, is a key parameter for an efficient process. In this study, detailed kinetics of BP from PLA were determined at 58 °C and 37 °C. At both temperatures, lag phases were observed: 40 days at 37 °C, and 10 days at 58 °C. After the lag phase BP proceeded in two phases, differed in process rate. At 58 °C, during the 1st phase (up to day 30), the rate of BP (r_B1,58) equaled about 35 L/(kg OM·d). At the end of this phase, the amount of biogas was 710 L/kg OM, which constituted 84% of the maximal BP (831-849 L/kg OM). In the 2nd phase (10 days), only 13% of maximal BP was produced (r_B2,58 of 16.1 L/(kg OM·d)). At 37 °C, maximal BP (obtained after 280 days) was 1.5-fold lower (558-570 L/kg OM) than at 58 °C. In the 1st phase (100 days), r_B1,37 was 1.4 L/(kg OM·d); at the end of this phase, BP constituted merely 14% of the maximal BP. A majority of biogas was produced in the 2nd phase (the next 180 days), and r_B2,37 doubled to 2.6 L/(kg OM·d)). At 58 °C, intensive biogas production took place when PLA pieces were still visible. At 37 °C, in contrast, biogas was mainly produced when the PLA pieces had been disintegrated. Although PLA anaerobically biodegrades and produces a high yield of biogas, the time frame of PLA digestion is much longer than that of biowaste and, in thermophilic conditions requires separate digesters. In mesophilic conditions, however, is unacceptable at technical scale.

Membrane Bioreactor Technology: The Effect of Membrane Filtration on Biogas Potential of the Excess Sludge

Although the membrane bioreactor technology is gaining increasing interest because of high efficiency of wastewater treatment and reuse, data on the anaerobic transformations of retentate are scarce and divergent. The effects of transmembrane pressure (TMP) in microfiltration (MF) and ultrafiltration (UF) on the pollutant rejection, susceptibility of ceramic membrane to fouling, hydraulic parameters of membrane module, and biogas productivity of retentate were determined. Irrespective of the membrane cut-off and TMP (0.2-0.4 MPa), 97.4 ± 0.7% of COD (chemical oxygen demand), 89.0 ± 4.1% of total nitrogen, and 61.4 ± 0.5% of total phosphorus were removed from municipal wastewater and the permeates can be reused for irrigation. Despite smaller pore diameter, UF membrane was more hydraulically efficient. MF membrane had 1.4-4.6 times higher filtration resistances than UF membrane. In MF and UF, an increase in TMP resulted in an increase in permeate flux. Despite complete retention of suspended solids, strong shearing forces in the membrane installation changed the kinetics of biogas production from retentate in comparison to the kinetics obtained when excess sludge from a secondary clarifier was anaerobically processed. MF retentates had 1.15 to 1.28 times lower cumulative biogas production than the excess sludge. Processing of MF and UF retentates resulted in about 60% elongation of period in which 90% of the cumulative biogas production was achieved.

Structural extracellular polymeric substances determine the difference in digestibility between waste activated sludge and aerobic granules

Aerobic granular sludge (AGS) technology is an alternative to conventional activated sludge to reduce the process footprint and energy consumption. Strategies for the efficient management of its produced biomass, that is grown in a granular morphology as well, need further development. Anaerobic digestion (AD) is commonly applied in waste activated sludge (WAS) treatment and is a potential option also for produced AGS treatment. In earlier studies, the biochemical methane potential of AGS was found lower than that of WAS both grown in full-scale municipal wastewater treatment systems. In order to understand this difference, this study aimed to investigate the anaerobic conversion of structural extracellular polymeric substances (SEPS), which is a type of gel-forming biopolymer, being responsible for the aggregation of sludge. Using WAS and AGS as substrates, a comparative AD batch experiment was performed for 44 days during which the SEPS fraction was extracted from both types of sludge. The changes in the SEPS chemical composition was analysed by Fourier transformed infrared spectroscopy and three-dimensional excitation and emission matrix analysis. In addition, the mechanical strength of hydrogels of extracted polymers cross-linked with Ca²⁺ ions was investigated by dynamic mechanical analysis. Results showed that the amount of SEPS was reduced by 26% in AGS (SEPS_AGS) and by 41% in WAS (SEPS_WAS), respectively. Polysaccharides and, to a lesser extent, the proteins in the SEPS_AGS were more refractory compared to those in SEPS_WAS. This resulted in a lower loss of the gel stiffness of SEPS_AGS than that of SEPS_WAS during the AD process. Moreover, the release of SEPS from tightly bound EPS to loosely bound EPS were observed in both types of sludge, but that in AGS exhibited a lower transition rate. The observed properties explain the distinct differences in anaerobic biodegradability, the slower decomposition of the sludge structure, as well as the better dewaterability of AGS as compared to WAS after the AD process.

Effect of bioaugmentation on digestate metal concentrations in anaerobic digestion of sewage sludge

This study examined the influence of bioaugmentation on metal concentrations (aluminum, cadmium, chromium, cobalt, copper, iron, lead, manganese, molybdenum, nickel and zinc) in anaerobically digested sewage sludge. To improve the digestion efficiency, bioaugmentation with a mixture of wild-living Archaea and Bacteria (MAB) from Yellowstone National Park, USA, was used. The total concentration of all metals was higher in the digestate than in the feedstock. During anaerobic digestion, the percent increase in the concentration of most of metals was slightly higher in the bioaugmented runs than in the un-augmented runs, but these differences were not statistically significant. However, the percent increase in cadmium and cobalt concentration was significantly higher in the bioaugmented runs than in the un-augmented runs. At MAB doses of 9 and 13% v/v, cadmium concentration in the digestate was 211 and 308% higher than in the feedstock, respectively, and cobalt concentration was 138 and 165%, respectively. Bioaugmentation increased over 4 times the percentage of Pseudomonas sp. in the biomass that are able to efficiently accumulate metals by both extracellular adsorption and intracellular uptake. Biogas production was not affected by the increased metal concentrations. In conclusion, bioaugmentation increased the concentration of metals in dry sludge, which means that it could potentially have negative effects on the environment.

Digestibility of waste aerobic granular sludge from a full-scale municipal wastewater treatment system

Full-scale aerobic granular sludge technology under the trade name Nereda® has been implemented for municipal, as well as industrial wastewater treatment. Owing to the operational reactor procedures, two types of waste aerobic granular sludge can be clearly distinguished: 1) aerobic granular sludge selection discharge (AGS-SD) and 2) aerobic granular sludge mixture (AGS-RTC). This study systematically compared the anaerobic biodegradability of AGS-SD and AGS-RTC under mesophilic conditions. Results were further compared with the anaerobic conversion of waste activated sludge (WAS) as well as primary sludge (PS) from full-scale municipal wastewater treatment plants. Analysis showed similar chemical characteristics for AGS-SD and PS, which were both characterized by a high carbohydrate content (429 ± 21 and 464 ± 15 mg glucose/g VS sludge, respectively), mainly cellulosic fibres. Concurrently, AGS-RTC exhibited chemical properties close to WAS, both characterized by a relatively high protein content, which were individually 498 ± 14 and 389 ± 15 mg/g VS sludge. AGS-SD was characterized by a high biochemical methane potential (BMP) (296 ± 15 mL CH₄/g VS substrate), which was similar to that of PS, and remarkably higher than that of AGS-RTC and WAS. Strikingly, the BMP of AGS-RTC (194 ± 10 mL CH₄/g VS substrate) was significantly lower than that of WAS (232 ± 11 mL CH₄/g VS substrate). Mechanically destroying the compact structure of AGS-RTC only accelerated the methane production rate but did not significantly affect the BMP value. Results indicated that compared to WAS, the proteins and carbohydrates in AGS-RTC were both more resistant to anaerobic bio-degradation, which might be related to the presence of refractory microbial metabolic products in AGS-RTC.

Thermal pretreatment to enhance biogas production of waste aerobic granular sludge with and without calcium phosphate precipitates

To develop aerobic granules based sustainable wastewater treatment, it is necessary to view wastewater treatment process and excess sludge treatment as a whole to evaluate resource recovery and sustainability. We thus investigated in this study how mineral characteristics of aerobic granules with/without calcium phosphate precipitates for phosphorus removal in treatment process affect the excess sludge digestion for energy recovery. Steam explosion at 170 °C as an effective thermal sludge treatment approach was studied in parallel with normal thermal treatment in an autoclave at 70, 100 and 125 °C, respectively. A liner relationship was found between the thermal treatment temperature in the autoclave and biogas production of aerobic granules. The untreated granules with only 10% mineral content (G1) generated 30% more biogas than the untreated granules with 39% mineral content (G2), but steam explosion is more effective to G2 with high mineral content and relatively poor methane yield potential. In addition, steam explosion improved methane production from G2 more compared with activated sludge although both untreated activated sludge and G2 had comparable methane production, i.e. around 0.235 L CH₄/g VS. Therefore, steam explosion is potential to be used to increase methane production especially when the untreated granular sludge has low methane yield due to high mineral content. This work provides a good basis for a holistic evaluation of resource recovery based on aerobic granular sludge, i.e. combined energy recovery and phosphorus removal and recovery via CaP precipitates, and trade-off between different factors with steam explosion.

A Review on Nanoparticles as Boon for Biogas Producers—Nano Fuels and Biosensing Monitoring

Nanotechnology has an increasingly large impact on a broad scope of biotechnological, pharmacological and pure technological applications. Its current use in bioenergy production from biomass is very restricted. The present study is based on the utilization of nanoparticles as an additive to feed bacteria that break down natural substances. The novel notion of dosing ions using modified nanoparticles can be used to progress up biogas production in oxygen free digestion processes. While minute nanoparticles are unstable, they can be designed to provide ions in a controlled approach, so that the maximum enhancement of biogas production that has been reported can be obtained. Nanoparticles are dissolved in a programmed way in an anaerobic atmosphere and are supplied in a sustainable manner to microbiotic organisms responsible for the degradation of organic material, which is a role that fits them well. Therefore, biogas fabrication can be increased up to 200%, thereby increasing the degradation of organic waste.

Nitrifying aerobic granular sludge fermentation for releases of carbon source and phosphorus: The role of fermentation pH

The effect of fermentation pH (uncontrolled, 4 and 10) on the releases of carbon source and phosphorus from nitrifying aerobic granular sludge (N-AGS) was investigated. Meanwhile, metal ion concentration and microbial community characterization were explored during N-AGS fermentation. The results indicated that N-AGS fermentation at pH 10 significantly promoted the releases of soluble chemical oxygen demand (SCOD) and total volatile fatty acids (TVFAs). However, SCOD and TVFA released from N-AGS were inhibited at pH 4. Moreover, acidic condition promoted phosphorus release (mainly apatite) from N-AGS during anaerobic fermentation. Nevertheless, alkaline condition failed to increase phosphorus concentration due to the formation of chemical-phosphate precipitates. Compared with the previously reported flocculent sludge fermentation, N-AGS fermentation released more SCOD and TVFAs, possibly due to the greater extracellular polymeric substances content and some hydrolytic-acidogenic bacteria in N-AGS. Therefore, N-AGS alkaline fermentation facilitated the carbon source recovery, while N-AGS acidic fermentation benefited the phosphorus recovery.