Thermal pre-treatment of aerobic granular sludge: impact on anaerobic biodegradability

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.

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.

Thermally enhanced solubilization and anaerobic digestion of organic fraction of municipal solid waste

Organic fraction of municipal solid waste (OFMSW) is an ideal substrate for biogas production; however, complex chemical structure and being heterogeneous obstruct its biotransformation in anaerobic digestion (AD) process. Thermal pre-treatment of OFMSW has been suggested to enhance the solubilization and improve the anaerobic digestibility of OFMSW. This paper critically and comprehensively reviews the characterization of OFMSW (physical, chemical, bromatological) and enlightens the valuable properties of OFMSW for waste valorization. In following sections, the advantages and limitations of AD of OFMSW are discussed, followed by the application of temperature phased AD, and various thermal pre-treatments, i.e., conventional thermal, microwave, and thermo-chemical for high rate bioenergy transformation. Effects of pre-treatment on COD, proteins, sugars and VS solubilization, and biogas yield are discussed. Formation of recalcitrant during thermal pre-treatment and the effect on anaerobic digestibility are considered. Full scale application, and techno-economic and environmental feasibility of thermal pre-treatment methods are also revealed. This review concluded that thermophilic (55 °C) and temperature phased anaerobic digestion, temperature phased anaerobic digestion, TPAD (55 + 37 °C) processes shows effective and stable performance at low HRTs and high OLRs and achieved higher methane yield than mesophilic digestion. The thermal pre-treatment at a lower temperature (120 °C) improves the net energy yield. However, high-temperature pre-treatment (>150 °C) result in decreased biogas yield and even lower than the non-pre-treated OFMSW, although a high degree of COD solubilization. The OFMSW solubilization in terms of COD, proteins, and sugars cannot accurately reflect thermal/hybrid pre-treatments' potential. Thus, substrate pre-treatment followed by anaerobic digestibility of pretreated substrate together can evaluate the actual effectiveness of thermal pre-treatment of OFMSW.

Long-term monitoring of a thermal hydrolysis-anaerobic co-digestion plant treating high-strength organic wastes: Process performance and microbial community dynamics

Two parallel anaerobic digesters (8500 m³ capacity each), combined with thermal hydrolysis (TH) pretreatment, co-digesting dewatered sewage sludge, dewatered human feces, and food wastewater were monitored over a 12-month period from start-up to explore the feasibility of field application of the combined process. The waste mixtures before and after pretreatment and the feed and digestate of each digester were taken semimonthly (i.e., 48 samples in total) for analysis of the feed characteristics, process parameters, and digester microbial community structure. The TH pretreatment proved effective in improving the bioavailability of the waste mixture. The solubilization efficiency tended to increase with the particulate organic fraction in the raw mixture. Although fluctuations in the feed characteristics and loading significantly influenced the process and microbial behaviors, the digesters maintained stable performance during the study period. Our results demonstrate that the TH-anaerobic digestion process can achieve an effective and robust treatment of the waste mixture.

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.

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.

Enhancement of methane production and phosphorus recovery with a novel pre-treatment of excess sludge using waste plaster board

A new pre-treatment process for excess sludge is proposed to increase methane production and recover phosphorus by adding waste plaster board as calcium sulfate. The content of calcium sulfate in the plaster granules (PG) used in this study is 99%. When PG and calcium sulfate are added to the excess sludge generated from a municipal wastewater treatment plant, acetate production is enhanced as per sulfate reduction and phosphorus release is reduced via the formation of calcium phosphate. In the continuous pre-treatment experiment performed at 25 °C and for 10 days of sludge retention time (SRT) using calcium sulfate, 1935 ± 395 mg/L of acetate is produced with 1070 ± 255 mg/L of sulfate, which is reduced. Desulfobulbus spp., which can oxidize organic matter to acetate incompletely, have been observed in the pre-treated sludge. The pre-treated sludge has subsequently been used for methophiric anaerobic digestion. The methane yield from the pre-treated sludge is found to be 1.2 times that of the non-pretreated sludge at an SRT of 30 days, indicating that the pre-treatment using PG can improve methane production. Phosphorus is released from the non-pretreated sludge in the digester. Nevertheless, a decrease in phosphorus content has been observed, resulting in the digested sludge containing calcium phosphate that is useful for agriculture.

A study of solubilization of sewage sludge by hydrothermal treatment

The use of activated sludge process for biological treatment of domestic and industrial wastewaters generates large amounts of sewage sludge, which is regarded as problematic biowaste. Conventional waste treatment methods such as landfilling and ocean pumping have been used to dispose the unwanted sludge, but this practice is no longer recommended due to serious secondary pollution and strict environmental regulations. Hydrothermal treatment represents a promising alternative that has attracted attention in recent years. In this study, batch experiments of hydrothermal treatment of domestic sewage sludge were conducted under varying conditions (temperature of 150-300 °C, reaction time of 0.5-3.0 h, and sludge concentration of 5-30 g/L). A statistical study of the responses, including disintegration degree and concentration of dissolved compounds, was performed using a response surface methodology. Optimal conditions for hydrothermal treatment of sewage sludge were obtained through mathematical modeling.

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.

Application and Evaluation of Energy Conservation Technologies in Wastewater Treatment Plants

High energy consumption is an important issue affecting the operation and development of wastewater treatment plants (WWTPs). This paper seeks energy-saving opportunities from three aspects: energy application, process optimization, and performance evaluation. Moreover, effective energy-saving can be achieved from the perspective of energy supply and recovery by using green energy technologies, including wastewater and sludge energy recovery technologies. System optimization and control is used to reduce unnecessary energy consumption in operation. Reasonable indexes and methods can help researchers evaluate the application value of energy-saving technology. Some demonstration WWTPs even can achieve energy self-sufficiency by using these energy conservation technologies. Besides, this paper introduces the challenges faced by the wastewater treatment industry and some emerging energy-saving technologies. The work can give engineers some suggestions about reducing energy consumption from comprehensive perspectives.

Elucidating pyrolysis behaviour of activated sludge in granular and flocculent form: Reaction kinetics and mechanism

The pyrolysis kinetics of sewage sludge was studied to determine the constituent of sludge and explore the feasibility of pyrolytic post-treatment. Both flocculent sludge and granular sludge were pyrolysed in a thermogravimetric analyser under inert atmospheric conditions. The pyrolysis of granular sludge and flocculent sludge were described by three parallel reactions model with three individual pseudo-components. The decomposition activation energy values of the three pseudo-components were determined by iso-conversional methods to be 263.97 kJ/mol, 257.18 kJ/mol and 153.61 kJ/mol in flocculent sludge and 139.89 kJ/mol, 228.78 kJ/mol and 142.78 kJ/mol in granular sludge, respectively. Granular sludge exhibited better thermal stability but lower devolatilisation activation energy than flocculent sludge, which could be attributed by enriched alkali and alkaline metals during granulation. Master plots of experimental data sets suggested that the decomposition of all organic pseudo-components of flocculent sludge followed the nth-order mechanism while the pyrolytic mechanism of the first organic fraction in granular sludge coincided with random nucleation and nuclei growth. By investigating the pyrolytic behaviour, this study sheds light on the composition of granular sludge and the impact of sludge components on granular sludge pyrolysis, and lays the foundation for the treatment of waste granular sludge with potential for resource and energy recovery in the near future.

Novel insight into high solid anaerobic digestion of swine manure after thermal treatment: Kinetics and microbial community properties

Compared to traditional anaerobic digestion (AD), high solid anaerobic digestion (HSAD) had the advantages of small digester, low heating energy and less digestate. However, the methane production was poor. In our previous study, thermal treatment (70 ± 1 °C, 3 days) without any dilution could satisfactorily enhance the methane production rate of HSAD by up to 39.5%. However, effects of solid content on HSAD after thermal treatment were not yet studied. In this study, HSAD was conducted at 11.7-17.6% solid content, and the control experiment was conducted at low solid content (4.4% solid content). Results showed that HSAD's methane production rate was the highest at 11.7% solid content (158 mL CH₄/g VS), and could reach up to 89.2% of that at 4.4% solid content. The utilization of organics was revealed by kinetics analysis that the readily biodegradable organics could be utilized at increasing solid content with decreasing hydrolysis rate. Furthermore, it was notable that methylotrophic methanogens predominated in HSAD with the abundance of 82.6%. This was quite unique from the general belief that AD system was usually dominated by acetoclastic or hydrogenotrophic methanogenic pathways. In this study, the microbial community structure of HSAD after thermal treatment was firstly studied, its unique specific methanogenic pathways was firstly revealed.

Influence of thermal hydrolysis pretreatment on physicochemical properties and anaerobic biodegradability of waste activated sludge with different solids content

The influence of thermal hydrolysis pretreatment (THP) on physicochemical properties (pH, total solids, volatile solids, chemical oxygen demand, total nitrogen, ammonium nitrogen, volatile fatty acids, viscosity, and cell morphology) and anaerobic biodegradability of highly concentrated waste activated sludge (WAS) with TS content ranging from 1 to 7% was evaluated at different temperatures ranging from 100 to 220 °C. The biomethane potential (BMP) of the WAS was systematically analyzed and evaluated. Images of its cellular structure were also analyzed. The results indicated that THP is a useful method for solubilizing volatile solids and enhancing CH₄ production regardless of the TS content of the WAS feed. The ultimate CH₄ production determined from the BMP analysis was 313-348 L CH₄/kg VS (72.6-74.1% CH₄) at the optimum THP temperature of 180 °C. The results showed that THP could improve both the capacity and efficiency of anaerobic digestion, even at a high TS content, and could achieve the dual purpose of sludge reduction and higher energy recovery.

A review of biogas and an assessment of its economic impact and future role as a renewable energy source

Anaerobic digestion (AD) is a technology that is gaining popularity because of the need for more renewable energy sources around the world. AD is a complex series of biochemical reactions that ultimately result in the formation of biogas, which is a mixture of methane and carbon dioxide with other trace elements. From large installations to small personal reactors, the underlying basic process is the same, but through research, pretreatments and substrate co-digestion are becoming more popular to enhance biogas production. Reactor design and substrate selection also vary depending on the installation’s location. Biogas cleaning and upgrading help to increase the usability of the gas for multiple applications. The economic viability depends on the location in the world and the available substrate quality and quantity. AD processes rely heavily on government subsidies to stay profitable. In developing countries, AD profitability is not a concern, as this technology provides a way to better human life in these areas. This review presents a detailed look at the AD technology, provides a discussion on the economics of AD, and suggests future studies to enhance the technology.

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.

Combining microwave irradiation with sodium citrate addition improves the pre-treatment on anaerobic digestion of excess sewage sludge

This study investigated the synergistic effect of sodium citrate (SC; Na₃C₃H₅O(COO)₃) and microwave (MW) treatment on the efficiency of the anaerobic digestion of excess sewage sludge. In terms of the methane yield, an increase of the digestion's efficiency was observed. Taking into account the cost for the MW energy supplied to the system, the optimum treatment conditions were a MW energy input of 20 MJ/kg TS and a SC concentration of 0.11 g/g TS, obtaining a methane yield of 218.88 ml/g VS, i.e., an increase of 147.7% compared to the control. MW treatment was found to break the sludge structure, thereby improving the release of extracellular polymeric substances (EPS) and volatile fatty acids (VFAs). The treatment of sodium citrate further strengthened the breakage of loosely bound extracellular polymeric substances (LB-EPS) and tightly bound extracellular polymeric substances (TB-EPS). The increased VFA content stressed the improved digestion by this pretreatment. Furthermore, the preliminary economic analysis showed that at this point in the research, only operational but no financial gains were achieved.

Enhanced methane production and wastewater sludge stabilization of a continuous full scale thermal pretreatment and thermophilic anaerobic digestion

A continuous full scale thermophilic anaerobic digestion (AD) of wastewater sludge in conjunction with thermal pretreatment was developed for enhanced CH₄ production as well as sludge stabilization and reduction. Continuous thermal sludge treatment obtained 45.5, 51.7, and 26.1% of hydrolysis based on TS, VS, and COD at 160°C for 30min. After AD, TS, VS, and COD removal efficiencies reached 54.7, 60.4, and 59.2%, respectively, at 3.50kgVS/m³·d. Maximum CH₄ yield (0.35m³ CH₄/kg VS_add) was obtained at 2.49kgVS/m³·d and less, and the yield decreased with increasing sludge load while net available energy production increased. Continuous thermal sludge treatment and thermophilic AD enhanced CH₄ production and solids reduction and showed stable performance for full scale application.

Enhanced biogas yield by thermo-alkali solubilization followed by co-digestion of intestine waste from slaughterhouse with food waste

Intestine waste generated from slaughterhouse (IWS) is difficult to degrade in anaerobic process due to the presence of high protein and lipid contents. However, anaerobic co-digestion helps to increase the degradation of IWS by the addition of carbon-rich food waste (FW). To increase the biogas yield, thermo-alkali pretreatment may be more viable method for the anaerobic digestion of protein and lipid rich wastes. In the present study, Thermo-alkali pretreatment of intestine waste from slaughterhouse and food waste alone and mixing of IWS and FW with different ratios (1:1-1:3) on VS basis have been studied. To study the effect of Thermo-alkali pretreatment on solubilization of substrate, the substrate was mixed with alkali solutions (NaOH and KOH) at different concentrations of 1, 2, 3, 4 and 5% solutions. The results revealed that the maximum solubilization was observed to be 94.7% and 90.1% at KOH (1:3 and 5%) and NaOH (1:3 and 5%), respectively. Based on the study, enhancement in biogas yield by 16% (IWS), 11.5% (FW), 12.2% (1:1), 18.11% (1:2) and 22.5% (1:3) in KOH pretreated waste when compared with NaOH pretreated waste.

Recovery of phosphorus via harvesting phosphorus-accumulating granular sludge in sequencing batch airlift reactor

A novel approach was developed for phosphorus recovery from wastewater through thermal treatment of matured phosphorus-accumulating granular sludge cultivated in sequencing batch airlift reactor (SBAR) system. Results showed that SBAR system had stable performances, in which COD, total phosphorus (TP) and total nitrogen (TN) removal efficiencies were stabilized at 80%, 89% and 86%, respectively. The matured granules were gathered from SBAR reactor and heated at relatively low temperature (100°C, 200°C, 300°C). The total P content in thermal treated granular sludge was more than half of total nutrient. Furthermore, the phosphorus release rate for treated granules was negatively correlated with thermal treatment temperature. These results demonstrated that the granules harvested from SBAR system followed with thermal pre-treatment could probably be applied as excellent slow-release phosphorus fertilizer. Hence, low temperature treatment of phosphate-accumulating granules is efficient for phosphorus recovery from wastewater, which is likely to promote the application of granulation technology.

Anaerobic fermentation combined with low-temperature thermal pretreatment for phosphorus-accumulating granular sludge: Release of carbon source and phosphorus as well as hydrogen production potential

Releases of organic compounds and phosphorus from phosphorus-accumulating granular sludge (PGS) and phosphorus-accumulating flocculent sludge (PFS) during low-temperature thermal pretreatment and anaerobic fermentation were investigated. Meanwhile, biogas production potential and microbial community structures were explored. The results indicate that much more soluble chemical oxygen demand (SCOD) and phosphorus were released from PGS than from PFS via low-temperature thermal pretreatment because of the higher extracellular polymeric substances (EPS) content in PGS and higher ratio of phosphorus reserved in EPS. Furthermore, PGS contains more anaerobes and dead cells, resulting in much higher SCOD and volatile fatty acids release from PGS than those from PFS during fermentation. PGS fermentation facilitated the n-butyric acid production, and PGS exhibited the hydrogen production potential during fermentation due to the presence of hydrogen-producing bacteria. Therefore, anaerobic fermentation combined with low-temperature thermal pretreatment can facilitate the recovery of carbon and phosphorus as well as producing hydrogen from PGS.

Towards a metagenomic understanding on enhanced biomethane production from waste activated sludge after pH 10 pretreatment

BACKGROUND

Understanding the effects of pretreatment on anaerobic digestion of sludge waste from wastewater treatment plants is becoming increasingly important, as impetus moves towards the utilization of sludge for renewable energy production. Although the field of sludge pretreatment has progressed significantly over the past decade, critical questions concerning the underlying microbial interactions remain unanswered. In this study, a metagenomic approach was adopted to investigate the microbial composition and gene content contributing to enhanced biogas production from sludge subjected to a novel pretreatment method (maintaining pH at 10 for 8 days) compared to other documented methods (ultrasonic, thermal and thermal-alkaline).

RESULTS

Our results showed that pretreated sludge attained a maximum methane yield approximately 4-fold higher than that of the blank un-pretreated sludge set-up at day 17. Both the microbial and metabolic consortium shifted extensively towards enhanced biodegradation subsequent to pretreatment, providing insight for the enhanced methane yield. The prevalence of Methanosaeta thermophila and Methanothermobacter thermautotrophicus, together with the functional affiliation of enzymes-encoding genes suggested an acetoclastic and hydrogenotrophic methanogenesis pathway. Additionally, an alternative enzymology in Methanosaeta was observed.

CONCLUSIONS

This study is the first to provide a microbiological understanding of improved biogas production subsequent to a novel waste sludge pretreatment method. The knowledge garnered will assist the design of more efficient pretreatment methods for biogas production in the future.

High pressure thermal hydrolysis as pre-treatment to increase the methane yield during anaerobic digestion of microalgae

Anaerobic digestion of algal biomass will be an essential component of algal biofuel production systems, yet the methane yield from digestion of algae is typically much lower than the theoretical potential. In this work, high pressure thermal hydrolysis (HPTH) is shown to enhance methane yield during algae digestion. HPTH pre-treatment was applied to both raw algae and algal residue resulting from lipid extraction. HPTH and even the lipid extraction process itself increased methane yield, by 81% and 33% respectively; in combination they increased yield by 110% over that of the raw algae (18L CH4 gVS(-1) substrate). HPTH had little effect on the rate of anaerobic digestion, however lipid extraction enhanced it by 33% over that for raw algae (0.21day(-1)). Digestion resulted in solubilisation of nitrogen (and phosphorous to a lesser degree) in all cases, showing that there is potential for nutrient recycling for algal growth.

Influence of microwave pre-treatment on sludge solubilization and pilot scale semi-continuous anaerobic digestion

Anaerobic digestion is widely applied for the recovery of energy from waste activated sludge. Pre-treatment methods are of high interest to increase the biodegradability of the sludge and to enhance the digestion efficiency. This paper studies the application of a microwave pre-treatment. An experimental set-up of two pilot scale semi-continuous digesters was used. During a long term experiment, one of the reactors was fed with untreated sludge, while microwave pre-treated sludge (336 kJ/kg sludge) was introduced in the second one. A solid retention time of 20 days was kept during the experiments. (Organic) dry solids, carbohydrates, proteins and volatile fatty acids were monitored during digestion. It was seen that the microwave pre-treatment resulted in an effective solubilization of the organic matter in the sludge. The changes to the sludge composition resulted in an increase in biogas production by 50%, while the methane concentration in both reactors remained stable.

Kinetic model of excess activated sludge thermohydrolysis

Thermal hydrolysis of excess activated sludge suspensions was carried at temperatures ranging from 423 K to 523 K and under pressure 0.2-4.0 MPa. Changes of total organic carbon (TOC) concentration in a solid and liquid phase were measured during these studies. At the temperature 423 K, after 2 h of the process, TOC concentration in the reaction mixture decreased by 15-18% of the initial value. At 473 K total organic carbon removal from activated sludge suspension increased to 30%. It was also found that the solubilisation of particulate organic matter strongly depended on the process temperature. At 423 K the transfer of TOC from solid particles into liquid phase after 1 h of the process reached 25% of the initial value, however, at the temperature of 523 K the conversion degree of 'solid' TOC attained 50% just after 15 min of the process. In the article a lumped kinetic model of the process of activated sludge thermohydrolysis has been proposed. It was assumed that during heating of the activated sludge suspension to a temperature in the range of 423-523 K two parallel reactions occurred. One, connected with thermal destruction of activated sludge particles, caused solubilisation of organic carbon and an increase of dissolved organic carbon concentration in the liquid phase (hydrolysate). The parallel reaction led to a new kind of unsolvable solid phase, which was further decomposed into gaseous products (CO(2)). The collected experimental data were used to identify unknown parameters of the model, i.e. activation energies and pre-exponential factors of elementary reactions. The mathematical model of activated sludge thermohydrolysis appropriately describes the kinetics of reactions occurring in the studied system.