Pre-treatment mechanisms during thermophilic-mesophilic temperature phased anaerobic digestion of primary sludge

New insights into the enhanced performance of high solid anaerobic digestion with dewatered sludge by thermal hydrolysis: Organic matter degradation and methanogenic pathways

Two lab-scale high solid anaerobic digesters fed with untreated sludge (R1) and thermally hydrolyzed sludge (R2) were operated to investigate the influence of thermal hydrolysis pretreatment (THP) on the degradation of individual macromolecular organic components (MOCs), as well as the functional and metabolic responses of microbes during anaerobic digestion (AD). The degradation of MOCs was improved by THP at different rates, in which improved degradation of proteins (by 49.0%) and hemicelluloses (by 25.0%) were the main factors contributing to the increase in volatile solids (VS) reduction. However, no enhancement of final degradation extent of MOCs was observed. With a more densified microbial population, R2 was also enriched in genes involved in amino acid and carbohydrate metabolism, reflected in the enhanced degradation of proteins and carbohydrates. After THP, the methanogenic pathway shifted from strict acetoclastic methanogenesis to acetoclastic/hydrogenotrophic methanogenesis, consistent with the enhanced methane production and the increase of methane content.

Long solid retention time (SRT) has minor role in promoting methane production in a 65°C single-stage anaerobic sludge digester

In this study, a thermophilic (65°C) single-stage wasted activated sludge (WAS) digester was established and the effects of solid retention time (SRT) on the reactor performance were investigated. The result showed that the optimum SRT was 6days with methane yield of 186.16mL/g VS. It was found that SRT had little effect on the hydrolysis and volatile solids (VS) destruction, and the high temperature employed seemed sufficient to achieve maximum hydrolysis and VS destruction performance. Longer SRT, however, promoted the release of recalcitrant compounds and impaired acidification, leading to the low methane yield. The microbial community analysis revealed that the dominant pathway for methane production was through syntrophic activity of acetate oxidizing bacteria and hydrogenotrophic methanogens while acetoclastic methanogens were absent in the system.

In-situ sludge pretreatment in a single-stage anaerobic digester

This study aimed to develop an in-situ sludge pretreatment method by increasing the temperature from thermophilic to extreme thermophilic condition in a single-stage anaerobic digester. The results revealed that a stable performance was obtained within the temperature range of 55-65°C, and the maximum methane yield of 208.51±13.66mL/g VS was obtained at 65°C. Moreover, the maximum extent of hydrolysis (33%) and acidification (27.1%) was also observed at 65°C. However, further increase of temperature to 70°C did not improve the organic conversion efficiency. Microbial community analysis revealed that Coprothermobacter, highly related to acetate oxidisers, appeared to be the abundant bacterial group at higher temperature. A progressive shift in methanogenic members from Methanosarcina to Methanothermobacter was observed upon increasing the temperature. This work demonstrated single-stage sludge digestion system can be successfully established at high temperature (65°C) with stable performance, which can eliminate the need of conventional thermophilic pretreatment step.

Biomethane potential of food waste: modeling the effects of mild thermal pretreatment and digestion temperature

In order to enhance anaerobic biodegradability of food waste (FW), thermal pretreatment was applied. The effectiveness in terms of biodegradability extent and process rate improvement was investigated. To this aim, Biomethane Potential tests were carried out under mesophilic and thermophilic conditions. The IWA anaerobic digestion Model 1 (ADM1), a powerful tool for modeling the anaerobic digestion (AD) of different substrates, was implemented to predict the methane production. Disintegration constant (k_dis) and maximum acetate uptake rate (km_ac) were identified as the most sensitive parameters and were calibrated over the observed methane production. Pretreatment improvement was more evident in enhancing parameters related to the process rate, such as solubilization extent (+153%) and disintegration constant (+18%), rather than increasing substrate biodegradability. Thermophilic conditions proved to be effective in speeding up the whole AD process, since all the kinetics were significantly improved (disintegration rate increased up to fivefold). Furthermore, it was demonstrated that, after k_dis and km_ac calibration, default thermophilic ADM1 parameters can be suitable to model FW digestion.

Nitrous Oxide Production in a Granule-based Partial Nitritation Reactor: A Model-based Evaluation

Sustainable wastewater treatment has been attracting increasing attentions over the past decades. However, the production of nitrous oxide (N₂O), a potent GHG, from the energy-efficient granule-based autotrophic nitrogen removal is largely unknown. This study applied a previously established N₂O model, which incorporated two N₂O production pathways by ammonia-oxidizing bacteria (AOB) (AOB denitrification and the hydroxylamine (NH₂OH) oxidation). The two-pathway model was used to describe N₂O production from a granule-based partial nitritation (PN) reactor and provide insights into the N₂O distribution inside granules. The model was evaluated by comparing simulation results with N₂O monitoring profiles as well as isotopic measurement data from the PN reactor. The model demonstrated its good predictive ability against N₂O dynamics and provided useful information about the shift of N₂O production pathways inside granules for the first time. The simulation results indicated that the increase of oxygen concentration and granule size would significantly enhance N₂O production. The results further revealed a linear relationship between N₂O production and ammonia oxidation rate (AOR) (R² = 0.99) under the conditions of varying oxygen levels and granule diameters, suggesting that bulk oxygen and granule size may exert an indirect effect on N₂O production by causing a change in AOR.

Resource Recovery from Wastewater by Biological Technologies: Opportunities, Challenges, and Prospects

Limits in resource availability are driving a change in current societal production systems, changing the focus from residues treatment, such as wastewater treatment, toward resource recovery. Biotechnological processes offer an economic and versatile way to concentrate and transform resources from waste/wastewater into valuable products, which is a prerequisite for the technological development of a cradle-to-cradle bio-based economy. This review identifies emerging technologies that enable resource recovery across the wastewater treatment cycle. As such, bioenergy in the form of biohydrogen (by photo and dark fermentation processes) and biogas (during anaerobic digestion processes) have been classic targets, whereby, direct transformation of lipidic biomass into biodiesel also gained attention. This concept is similar to previous biofuel concepts, but more sustainable, as third generation biofuels and other resources can be produced from waste biomass. The production of high value biopolymers (e.g., for bioplastics manufacturing) from organic acids, hydrogen, and methane is another option for carbon recovery. The recovery of carbon and nutrients can be achieved by organic fertilizer production, or single cell protein generation (depending on the source) which may be utilized as feed, feed additives, next generation fertilizers, or even as probiotics. Additionlly, chemical oxidation-reduction and bioelectrochemical systems can recover inorganics or synthesize organic products beyond the natural microbial metabolism. Anticipating the next generation of wastewater treatment plants driven by biological recovery technologies, this review is focused on the generation and re-synthesis of energetic resources and key resources to be recycled as raw materials in a cradle-to-cradle economy concept.

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.

Mesophilic and thermophilic alkaline fermentation of waste activated sludge for hydrogen production: Focusing on homoacetogenesis

The present study compared the mesophilic and thermophilic alkaline fermentation of waste activated sludge (WAS) for hydrogen production with focus on homoacetogenesis, which mediated the consumption of H2 and CO2 for acetate production. Batch experiments showed that hydrogen yield of WAS increased from 19.2 mL H2/gVSS at 37 °C and pH 10-80.1 mL H2/gVSS at 55 °C and pH 10. However, the production of volatile fatty acids (mainly acetate) was higher at 37 °C and pH 10 by comparison with 55 °C and pH 10. Hydrogen consumption due to homoacetogenesis was observed at 37 °C and pH 10 but not 55 °C and pH 10. Higher expression levels of genes relating with homoacetogenesis and lower expression levels of genes relating with hydrogen production were found at 37 °C and pH 10 compared to 55 °C and pH 10. The continuous experiment demonstrated the steady-state hydrogen yield of WAS was comparable to that obtained from batch experiments at 55 °C and pH 10, and homoacetogenesis was still inhibited. However, the steady-state hydrogen yield of WAS (6.5 mL H2/gVSS) was much lower than that (19.2 mL H2/gVSS) obtained from batch experiments at 37 °C and pH 10 due to the gradual enrichment of homoacetogens as demonstrated by qPCR analysis. The high-throughput sequencing analysis of 16S rRNA genes showed that the abundance of genus Clostridium, containing several homoacetogens, was 5 times higher at 37 °C and pH 10 than 55 °C and pH 10.

Thermophilic sludge digestion improves energy balance and nutrient recovery potential in full-scale municipal wastewater treatment plants

The conventional treatment of municipal wastewater by means of activated sludge is typically energy demanding. Here, the potential benefits of: (1) the optimization of mesophilic digestion; and (2) transitioning to thermophilic sludge digestion in three wastewater treatment plants (Tilburg-Noord, Land van Cuijk and Bath) in the Netherlands is evaluated, including a full-scale trial validation in Bath. In Tilburg-Noord, thermophilic sludge digestion covered the energy requirements of the plant (102%), whereas 111% of sludge operational treatment costs could be covered in Bath. Thermophilic sludge digestion also resulted in a strong increase in nutrient release. The potential for nutrient recovery was evaluated via: (1) stripping/absorption of ammonium; (2) autotrophic removal of ammonium via partial nitritation/anammox; and (3) struvite precipitation. This research shows that optimization of sludge digestion may lead to a strong increase in energy recovery, sludge treatment costs reduction, and the potential for advanced nutrient management in full-scale sewage treatment plants.

Lab-scale demonstration of recuperative thickening technology for enhanced biogas production and dewaterability in anaerobic digestion processes

There is growing interest in the use of high performance anaerobic digestion (AD) processes for the production of biogas at wastewater treatment facilities to offset the energy demands associated with wastewater treatment. Recuperative thickening (RT) is a promising technique which involves recycling a portion of the digested solids back to the incoming feed. In general there exists a significant number of knowledge gaps in the field of RT because the studies that have been conducted to date have almost exclusively occurred in pilot plant or full scale trials; this approach greatly limits the amount of process optimization that can be done in a given trial. In this work, a detailed and comprehensive study of RT was conducted at the lab scale; two custom designed digesters (capacity = 1.5 L) were operated in parallel with one acting as a 'control' digester and the other operating under a semi-batch RT mode. There was no significant change in biogas methane composition for the two digesters, however the RT digester had an average biogas productivity over two times higher than the control one. It was found that the recycling of the polymer flocculant back into the RT digester resulted in a significant improvement in dewatering performance. At the highest polymer concentration tested, the capillary suction time (CST) values for flocculated samples for the RT digester were over 6 times lower than the corresponding values for the control digester. Thus, there exists an opportunity to decrease the overall consumption of polymer flocculants through judicious selection of the dose of polymer flocculant that is used both for the thickening and end-stage dewatering steps in RT processes.

Recovery of energy and nutrient resources from cattle paunch waste using temperature phased anaerobic digestion

Cattle paunch is comprised of partially digested cattle feed, containing mainly grass and grain and is a major waste produced at cattle slaughterhouses contributing 20-30% of organic matter and 40-50% of P waste produced on-site. In this work, Temperature Phased Anaerobic Digestion (TPAD) and struvite crystallization processes were developed at pilot-scale to recover methane energy and nutrients from paunch solid waste. The TPAD plant achieved a maximum sustainable organic loading rate of 1-1.5kgCODm(-3)day(-1) using a feed solids concentration of approximately 3%; this loading rate was limited by plant engineering and not the biology of the process. Organic solids destruction (60%) and methane production (230LCH4kg(-1) VSfed) achieved in the plant were similar to levels predicted from laboratory biochemical methane potential (BMP) testing. Model based analysis identified no significant difference in batch laboratory parameters vs pilot-scale continuous parameters, and no change in speed or extent of degradation. However the TPAD process did result in a degree of process intensification with a high level of solids destruction at an average treatment time of 21days. Results from the pilot plant show that an integrated process enabled resource recovery at 7.8GJ/dry tonne paunch, 1.8kgP/dry tonne paunch and 1.0kgN/dry tonne paunch.

Evaluation on the Nanoscale Zero Valent Iron Based Microbial Denitrification for Nitrate Removal from Groundwater

Nanoscale zero valent iron (NZVI) based microbial denitrification has been demonstrated to be a promising technology for nitrate removal from groundwater. In this work, a mathematical model is developed to evaluate the performance of this new technology and to provide insights into the chemical and microbial interactions in the system in terms of nitrate reduction, ammonium accumulation and hydrogen turnover. The developed model integrates NZVI-based abiotic reduction of nitrate, NZVI corrosion for hydrogen production and hydrogen-based microbial denitrification and satisfactorily describes all of the nitrate and ammonium dynamics from two systems with highly different conditions. The high NZVI corrosion rate revealed by the model indicates the high reaction rate of NZVI with water due to their large specific surface area and high surface reactivity, leading to an effective microbial nitrate reduction by utilizing the produced hydrogen. The simulation results further suggest a NZVI dosing strategy (3–6 mmol/L in temperature range of 30–40 °C, 6–10 mmol/L in temperature range of 15–30 °C and 10–14 mmol/L in temperature range of 5–15 °C) during groundwater remediation to make sure a low ammonium yield and a high nitrogen removal efficiency.

Innovative two-stage mesophilic/thermophilic anaerobic degradation of sonicated sludge: performances and energy balance

This study investigates for the first time, on laboratory scale, the possible application of an innovative enhanced stabilization process based on sequential mesophilic/thermophilic anaerobic digestion of waste-activated sludge, with low-energy sonication pretreatment. The first mesophilic digestion step was conducted at short hydraulic retention time (3-5 days), in order to favor volatile fatty acid production, followed by a longer thermophilic step of 10 days to enhance the bioconversion kinetics, assuring a complete pathogen removal. The high volatile solid removals, up to 55%, noticeably higher compared to the performances of a single-stage process carried out in same conditions, can guarantee the stability of the final digestate for land application. The ultrasonic pretreatment influenced significantly the fatty acid formation and composition during the first mesophilic step, improving consequently the thermophilic conversion of these compounds into methane. Methane yield from sonicated sludge digestion reached values up to 0.2 Nm(3)/kgVSfed. Positive energy balances highlighted the possible exploitation of this innovative two-stage digestion in place of conventional single-stage processes.

Microbial diversity in innovative mesophilic/thermophilic temperature-phased anaerobic digestion of sludge

Anaerobic digestion (AD) is one of the few sustainable technologies that both produce energy and treat waste streams. Driven by a complex and diverse community of microbes, AD may be affected by different factors, many of which also influence the composition and activity of the microbial community. In this study, the biodiversity of microbial populations in innovative mesophilic/thermophilic temperature-phased AD of sludge was evaluated by means of fluorescence in situ hybridization (FISH). The increase of digestion temperature drastically affected the microbial composition and selected specialized biomass. Hydrogenotrophic Methanobacteriales and the protein fermentative bacterium Coprothermobacter spp. were identified in the thermophilic anaerobic biomass. Shannon-Weaver diversity (H') and evenness (E) indices were calculated using FISH data. Species richness was lower under thermophilic conditions compared with the values estimated in mesophilic samples, and it was flanked by similar trend of the evenness indicating that thermophilic communities may be therefore more susceptible to sudden changes and less prompt to adapting to operative variations.

Hygienization performances of innovative sludge treatment solutions to assure safe land spreading

The present research aims at the evaluation of the hygienization performances of innovative sludge treatment processes applied for the separated treatment of secondary sludge. Namely, two digestion pretreatments (sonication and thermal hydrolysis) and two sequential biological processes (mesophilic/thermophilic and anaerobic/aerobic digestion) were compared to the mesophilic (MAD) and thermophilic anaerobic digestion (TAD). Microbial indicators (Escherichia coli, somatic coliphages and Clostridium perfringens spores) and pathogens (Salmonella and enteroviruses), which show different resistances to treatment processes, were monitored in untreated and treated sludge. Overall, microbial load in secondary sludge was shown to be similar or lower than previously reported in literature for mixed sludge. Notably, the anaerobic/aerobic digestion process increased the removal of E. coli and somatic coliphages compared to the simple MAD and always achieved the hygienization requirement (2-log-unit removal of E. coli) proposed by EU Commission in the 3rd Working Document on sludge (April 2000) for the use of treated sludges in agriculture with restriction on their application. The microbial quality limits for the unrestricted use of sludge in agriculture (no Salmonella in 50 g wet weight (WW) and E. coli <500 CFU/g) were always met when thermal digestion or pretreatment was applied; however, the required removal level (6-log-unit removal of E. coli) could not be assessed due to the low level of this microorganism in raw sludge. Observed levels of indicator removal showed a higher resistance of viral particles to thermal treatment compared with bacterial cells and confirmed the suitability of somatic coliphages as indicators in thermal treatment processes.

Changes in glucose fermentation pathways by an enriched bacterial culture in response to regulated dissolved H2 concentrations

It is well established that metabolic pathways in the fermentation of organic waste are primarily controlled by dissolved H2 concentrations, but there is no reported study that compares observed and predicted shifts in fermentation pathways induced by manipulating the dissolved H2 concentration. A perfusion system is presented that was developed to control dissolved H2 concentrations in the continuous fermentation of glucose by a culture highly enriched towards Thermoanaerobacterium thermosaccharolyticum (86 ± 9% relative abundance) from an originally diverse consortia in the leachate of a laboratory digester fed with municipal solid waste. Media from a 2.5 L CSTR was drawn through sintered steel membrane filters to retain biomass, allowing vigorous sparging in a separate chamber without cellular disruption. Through a combination of sparging and variations in glucose feeding rate from 0.8 to 0.2 g/L/d, a range of steady state fermentations were performed with dissolved H2 concentrations as low as an equivalent equilibrated H2 partial pressure of 3 kPa. Trends in product formation rates were simulated using a H2 regulation partitioning model. The model correctly predicted the direction of products redistribution in response to H2 concentration changes and the acetate and butyrate formation rates when H2 concentrations were less than 6 kPa. However, the model over-estimated acetate, ethanol and butanol productions at the expense of butyrate production at higher H2 concentrations. The H2 yield at the lowest dissolved H2 concentration was 2.67 ± 0.08 mol H2 /mol glucose, over 300% higher than the yield achieved in a CSTR operated without sparging.

The combined effect of dissolved oxygen and nitrite on N2O production by ammonia oxidizing bacteria in an enriched nitrifying sludge

Both nitrite [Formula: see text] and dissolved oxygen (DO) play important roles in nitrous oxide (N2O) production by ammonia oxidizing bacteria (AOB). However, few studies focused on the combined effect of them on N2O production by AOB as well as the corresponding mechanisms. In this study, N2O production by an enriched nitrifying sludge, consisting of both AOB and nitrite-oxidizing bacteria (NOB), was investigated under various [Formula: see text] and DO concentrations. At each investigated DO level, both the biomass specific N2O production rate and the N2O emission factor (the ratio between N2O nitrogen emitted and the ammonium nitrogen converted) increased as [Formula: see text] concentration increased from 3 mg N/L to 50 mg N/L. However, at each investigated [Formula: see text] level, the maximum biomass specific N2O production rate occurred at DO of 0.85 mg O2/L, while the N2O emission factor decreased as DO increased from 0.35 to 3.5 mg O2/L. The analysis of the process data using a mathematical N2O model incorporating both the AOB denitrification and hydroxylamine (NH2OH) oxidation pathways indicated that the contribution of AOB denitrification pathway increased as [Formula: see text] concentration increased, but decreased as DO concentration increased, accompanied by a corresponding change in the contribution of NH2OH oxidation pathway to N2O production. The AOB denitrification pathway was predominant in most cases, with the NH2OH oxidation pathway making a comparable contribution only at high DO level (e.g. 3.5 mg O2/L).

Combined ultrasonication and thermal pre-treatment of sewage sludge for increasing methane production

This article focuses on the combination of ultrasonic and thermal treatment of sewage sludge (SS). The combination involved ultrasonicating a fraction of the sludge and thermal treatment at various temperatures and this resulted in solubilization of proteins and carbohydrates, and so contributing to increased COD solubilization. During the treatment, SCOD, soluble proteins and carbohydrates increased from 760 mg L(-1) to 10,200 mg L(-1), 110 mg L(-1) to 2,900 mg L(-1) and 60 mg L(-1) to 630 mg L(-1), respectively. It was found ultrasonication of only a fraction of the sludge (>20%) followed by thermal treatment led to significant improvement compared to thermal and ULS treatments applied on their own. At 65°C, the kinetic of solubilization was improved and the hyper-thermophilic treatment time could be reduced to a few hours when ultrasonication was used first. A linear correlation (R(2) = 95%) was found between the SCOD obtained after ultrasonication pre-treatment and anaerobic biodegradability. The combined treatment resulted in 20% increase in biogas production during the anaerobic digestion of the pre-treated sludge.

The effect of dissolved oxygen on N2O production by ammonia-oxidizing bacteria in an enriched nitrifying sludge

Dissolved oxygen (DO) is commonly recognized as an important factor influencing nitrous oxide (N2O) production by ammonia-oxidizing bacteria (AOB). However, it has been difficult to separate the true effect of DO from that of nitrite, as DO variation often affects nitrite accumulation. The effect of DO on N2O production by an enriched nitrifying sludge, consisting of both AOB and nitrite-oxidizing bacteria (NOB), was investigated in this study. Nitrite accumulation was minimised by augmenting nitrite oxidation through the addition of an enriched NOB sludge. It was demonstrated that the specific N2O production rate increased from 0 to 1.9 ± 0.09 (n = 3) mg N2O-N/hr/g VSS with an increase of DO concentration from 0 to 3.0 mg O2/L, whereas N2O emission factor (the ratio between N2O nitrogen emitted and the ammonium nitrogen converted) decreased from 10.6 ± 1.7% (n = 3) at DO = 0.2 mg O2/L to 2.4 ± 0.1% (n = 3) at DO = 3.0 mg O2/L. The site preference measurements indicated that both the AOB denitrification and hydroxylamine (NH2OH) oxidation pathways contributed to N2O production, and DO had an important effect on the relative contributions of the two pathways. This finding is supported by analysis of the process data using an N2O model describing both pathways. As DO increased from 0.2 to 3.0 mg O2/L, the contribution of AOB denitrification decreased from 92% - 95%-66% - 73%, accompanied by a corresponding increase in the contribution by the NH2OH oxidation pathway.

Modeling of simultaneous anaerobic methane and ammonium oxidation in a membrane biofilm reactor

Nitrogen removal by using the synergy of denitrifying anaerobic methane oxidation (DAMO) and anaerobic ammonium oxidation (Anammox) microorganisms in a membrane biofilm reactor (MBfR) has previously been demonstrated experimentally. In this work, a mathematical model is developed to describe the simultaneous anaerobic methane and ammonium oxidation by DAMO and Anammox microorganisms in an MBfR for the first time. In this model, DAMO archaea convert nitrate, both externally fed and/or produced by Anammox, to nitrite, with methane as the electron donor. Anammox and DAMO bacteria jointly remove the nitrite fed/produced, with ammonium and methane as the electron donor, respectively. The model is successfully calibrated and validated using the long-term (over 400 days) dynamic experimental data from the MBfR, as well as two independent batch tests at different operational stages of the MBfR. The model satisfactorily describes the methane oxidation and nitrogen conversion data from the system. Modeling results show the concentration gradients of methane and nitrogen would cause stratification of the biofilm, where Anammox bacteria mainly grow in the biofilm layer close to the bulk liquid and DAMO organisms attach close to the membrane surface. The low surface methane loadings result in a low fraction of DAMO microorganisms, but the high surface methane loadings would lead to overgrowth of DAMO bacteria, which would compete with Anammox for nitrite and decrease the fraction of Anammox bacteria. The results suggest an optimal methane supply under the given condition should be applied not only to benefit the nitrogen removal but also to avoid potential methane emissions.

Fate of selected emerging micropollutants during mesophilic, thermophilic and temperature co-phased anaerobic digestion of sewage sludge

The removal of endocrine disrupting compounds (EDCs) and non-steroidal anti-inflammatory drugs (NSAIDs) was studied in three lab-scale anaerobic digestion (AD) systems; a single-stage mesophilic, a single-stage thermophilic and a two-stage thermophilic/mesophilic. All micropollutants underwent microbial degradation. High removal efficiency (>80%) was calculated for diclofenac, ibuprofen, naproxen and ketoprofen; whereas triclosan, bisphenol A and the sum of nonylphenol (NP), nonylphenol monoethoxylate (NP1EO) and nonylphenol diethoxylate were moderately removed (40-80%). NSAIDs removal was not affected by the type of AD system used; whereas slightly higher EDCs removal was observed in two-stage system. In this system, most microcontaminants were removed in thermophilic digester. Biotransformation of NP1EO and NP was affected by the temperature applied to bioreactors. Under mesophilic conditions, higher removal of NP1EO and accumulation of NP was noticed; whereas the opposite was observed under thermophilic conditions. For most analytes, higher specific removal rates were calculated under thermophilic conditions and 20 days SRT.

Modeling of nitrous oxide production by autotrophic ammonia-oxidizing bacteria with multiple production pathways

Autotrophic ammonia oxidizing bacteria (AOB) have been recognized as a major contributor to N2O production in wastewater treatment systems. However, so far N2O models have been proposed based on a single N2O production pathway by AOB, and there is still a lack of effective approach for the integration of these models. In this work, an integrated mathematical model that considers multiple production pathways is developed to describe N2O production by AOB. The pathways considered include the nitrifier denitrification pathway (N2O as the final product of AOB denitrification with NO2(-) as the terminal electron acceptor) and the hydroxylamine (NH2OH) pathway (N2O as a byproduct of incomplete oxidation of NH2OH to NO2(-)). In this model, the oxidation and reduction processes are modeled separately, with intracellular electron carriers introduced to link the two types of processes. The model is calibrated and validated using experimental data obtained with two independent nitrifying cultures. The model satisfactorily describes the N2O data from both systems. The model also predicts shifts of the dominating pathway at various dissolved oxygen (DO) and nitrite levels, consistent with previous hypotheses. This unified model is expected to enhance our ability to predict N2O production by AOB in wastewater treatment systems under varying operational conditions.

Effect of microwave hydrolysis on transformation of steroidal hormones during anaerobic digestion of municipal sludge cake

Fate and removal of 16 steroidal (estrogenic, androgenic and progestogenic) hormones were studied during advanced anaerobic digestion of sludge cake using microwave (MW) pretreatment. Effect of pretreatment temperature (80, 120, 160 °C), operating temperature (mesophilic at 35 ± 2 °C, thermophilic at 55 ± 2 °C) and sludge retention time (SRT: 20, 10, 5 days) were studied employing eight lab-scale semi-continuously fed digesters. To determine the potential effect of MW hydrolysis, hormones were quantified in total (sorbed + soluble) and supernatant (soluble) phases of the digester influent and effluent streams. Seven of 16 hormones were above the method reporting limit (RL) in one or more of the samples. Hormone concentrations in total phase of un-pretreated (control) and pretreated digester feeds ranged in <157-2491 ng/L and <157-749 ng/L, respectively. The three studied factors were found to be statistically significant (95% confidence level) in removal of one or more hormones from soluble and/or total phase. MW hydrolysis of the influent resulted in both release (from sludge matrix) and attenuation of hormones in the soluble phase. Accumulation of estrone (E1) as well as progesterone (Pr) and androstenedione (Ad) in most of the digesters indicated possible microbial transformations among the hormones. Compared to controls, all pretreated digesters had lower total hormone concentrations in their influent streams. At 20 days SRT, highest total removal (E1+E2+Ad +Pr) was observed for the thermophilic control digester (56%), followed by pretreated mesophilic digesters at 120 °C and 160 °C with around 48% efficiency. In terms of conventional performance parameters, relative (to control) improvements of MW pretreated digesters at a 5-d SRT ranged in 98-163% and 57-121%, for volatile solids removal and methane production, respectively.

Mathematical modeling of nitrous oxide (N2O) emissions from full-scale wastewater treatment plants

Mathematical modeling of N2O emissions is of great importance toward understanding the whole environmental impact of wastewater treatment systems. However, information on modeling of N2O emissions from full-scale wastewater treatment plants (WWTP) is still sparse. In this work, a mathematical model based on currently known or hypothesized metabolic pathways for N2O productions by heterotrophic denitrifiers and ammonia-oxidizing bacteria (AOB) is developed and calibrated to describe the N2O emissions from full-scale WWTPs. The model described well the dynamic ammonium, nitrite, nitrate, dissolved oxygen (DO) and N2O data collected from both an open oxidation ditch (OD) system with surface aerators and a sequencing batch reactor (SBR) system with bubbling aeration. The obtained kinetic parameters for N2O production are found to be reasonable as the 95% confidence regions of the estimates are all small with mean values approximately at the center. The model is further validated with independent data sets collected from the same two WWTPs. This is the first time that mathematical modeling of N2O emissions is conducted successfully for full-scale WWTPs. While clearly showing that the NH2OH related pathways could well explain N2O production and emission in the two full-scale plants studied, the modeling results do not prove the dominance of the NH2OH pathways in these plants, nor rule out the possibility of AOB denitrification being a potentially dominating pathway in other WWTPs that are designed or operated differently.

Modelling of the temperature-phased batch anaerobic digestion of raw sludge from an urban wastewater treatment plant

The disposal of excess sludge from wastewater treatment plants is a serious problem that needs to be addressed. Temperature-phased anaerobic digestion (TPAD) which combines thermophilic and mesophilic processes in one, brings together the advantages of both systems. The aim of the present work was to develop a simple kinetic model to describe the TPAD of sewage sludge in batch completely stirred tank reactors (CSTRs) and to determine the kinetic parameters of both thermophilic and mesophilic stages. A zero-order kinetic equation described the thermophilic step after 2, 4 and 6 days of digestion time (experiment 1, 2 and 3, respectively), yet a first-order equation was found to be adequate to correlate the methane gas accumulated with time in the mesophilic step, the kinetic constant being 0.21 days(-1). The methane yield coefficient obtained was found to be almost proportional to the digestion time used in the thermophilic step with values of 0.067, 0.132 and 0.193 L CH(4) STP/g VS(added) for experiments 1, 2 and 3, respectively. By contrast, the kinetic constant of the mesophilic stage was not influenced by the digestion time used in the thermophilic phase.

Net energy production associated with pathogen inactivation during mesophilic and thermophilic anaerobic digestion of sewage sludge

The potential for anaerobic digester energy production must be balanced with the sustainability of reusing the resultant biosolids for land application. Mesophilic, thermophilic, temperature-phased, and high temperature (60 or 70 °C) batch pre-treatment digester configurations have been systematically evaluated for net energy production and pathogen inactivation potential. Energy input requirements and net energy production were modeled for each digester scheme. First-order inactivation rate coefficients for Escherichia coli, Enterococcus faecalis and bacteriophage MS-2 were measured at each digester temperature and full-scale pathogen inactivation performance was estimated for each indicator organism and each digester configuration. Inactivation rates were found to increase dramatically at temperatures above 55 °C. Modeling full-scale performance using retention times based on U.S. EPA time and temperature constraints predicts a 1-2 log inactivation in mesophilic treatment, and a 2-5 log inactivation in 50-55 °C thermophilic and temperature-phased treatments. Incorporating a 60 or 70 °C batch pre-treatment phase resulted in dramatically higher potency, achieving MS-2 inactivation of 14 and 16 logs respectively, and complete inactivation (over 100 log reduction) of E. coli and E. faecalis. For temperatures less than 70 °C, viability staining of thermally-treated E. coli showed significantly reduced inactivation relative to standard culture enumeration. Due to shorter residence times in thermophilic reactors, the net energy production for all digesters was similar (less than 20% difference) with the 60 or 70 °C batch treatment configurations producing the most net energy and the mesophilic treatment producing the least. Incorporating a 60 or 70 °C pre-treatment phase can dramatically increase pathogen inactivation performance without decreasing net energy capture from anaerobic digestion. Energy consumption is not a significant barrier against improving the pathogen quality of biosolids.

A comparison study on the high-rate co-digestion of sewage sludge and food waste using a temperature-phased anaerobic sequencing batch reactor system

Assessing contemporary anaerobic biotechnologies requires proofs on reliable performance in terms of renewable bioenergy recovery such as methane (CH(4)) production rate, CH(4) yield while removing volatile solid (VS) effectively. This study, therefore, aims to evaluate temperature-phased anaerobic sequencing batch reactor (TPASBR) system that is a promising approach for the sustainable treatment of organic fraction of municipal solid wastes (OFMSW). TPASBR system is compared with a conventional system, mesophilic two-stage anaerobic sequencing batch reactor system, which differs in operating temperature of 1st-stage. Results demonstrate that TPASBR system can obtain 44% VS removal from co-substrate of sewage sludge and food waste while producing 1.2m(3)CH(4)/m(3)(system)/d (0.2m(3)CH(4)/kgVS(added)) at organic loading rate of 6.1gVS/L/d through the synergy of sequencing-batch operation, co-digestion, and temperature-phasing. Consequently, the rapid and balanced anaerobic metabolism at thermophilic stage makes TPASBR system to afford high organic loading rate showing superior performance on OFMSW stabilization.

Increased temperature in the thermophilic stage in temperature phased anaerobic digestion (TPAD) improves degradability of waste activated sludge

Two-stage temperature phased anaerobic digestion (TPAD) is an increasingly popular method to improve stabilisation of sewage waste activated sludge, which normally has inherently poor and slow degradation. However, there has been limited systematic analysis of the impact of the initial thermophilic stage (temperature, pH and retention time) on performance in the main mesophilic stage. In this study, we demonstrate a novel two-stage batch test method for TPAD processes, and use it to optimize operating conditions of the thermophilic stage in terms of degradation extent and methane production. The method determines overall degradability and apparent hydrolysis coefficient in both stages. The overall process was more effective with short pre-treatment retention times (1-2 days) and neutral pH compared to longer retention time (4 days) and low pH (4-5). Degradabilities and apparent hydrolysis coefficients were 0.3-0.5 (fraction degradable) and 0.1-0.4d(-1), respectively, with a margin of error in each measurement of approximately 20% relative (95% confidence). Pre-treatment temperature had a strong impact on the whole process, increasing overall degradability from 0.3 to 0.5 as temperature increased from 50 to 65 °C, with apparent hydrolysis coefficient increasing from 0.1 to 0.4d(-1).

Temperature phased anaerobic digestion increases apparent hydrolysis rate for waste activated sludge

It is well established that waste activated sludge with an extended sludge age is inherently slow to degrade with a low extent of degradation. Pre-treatment methods can be used prior to anaerobic digestion to improve the efficiency of activated sludge digestion. Among these pre-treatment methods, temperature phased anaerobic digestion (TPAD) is one promising method with a relatively low energy input and capital cost. In this study, an experimental thermophilic (50-70 °C)-mesophilic system was compared against a control mesophilic-mesophilic system. The thermophilic-mesophilic system achieved 41% and 48% volatile solids (VS) destruction during pre-treatment of 60 °C and 65 °C (or 70 °C) respectively, compared to 37% in the mesophilic-mesophilic TPAD system. Solubilisation in the first stage was enhanced during thermophilic pre-treatment (15% at 50 °C and 27% at 60 °C, 65 °C and 70 °C) over mesophilic pre-treatment (7%) according to a COD balance. This was supported by ammonia-nitrogen measurements. Model based analysis indicated that the mechanism for increased performance was due to an increase in hydrolysis coefficient under thermophilic pre-treatment of 60 °C (0.5 ± 0.1 d(-1)), 65 °C (0.7 ± 0.2 d(-1)) and 70 °C (0.8 ± 0.2 d(-1)) over mesophilic pre-treatment (0.2 ± 0.1 d(-1)), and thermophilic pre-treatment at 50 °C (0.12 ± 0.06 d(-1)).

Pretreatment methods to improve sludge anaerobic degradability: a review

This paper presents a review of the main sludge treatment techniques used as a pretreatment to anaerobic digestion. These processes include biological (largely thermal phased anaerobic), thermal hydrolysis, mechanical (such as ultrasound, high pressure and lysis), chemical with oxidation (mainly ozonation), and alkali treatments. The first three are the most widespread. Emphasis is put on their impact on the resulting sludge properties, on the potential biogas (renewable energy) production and on their application at industrial scale. Thermal biological provides a moderate performance increase over mesophilic digestion, with moderate energetic input. Mechanical treatment methods are comparable, and provide moderate performance improvements with moderate electrical input. Thermal hydrolysis provides substantial performance increases, with a substantial consumption of thermal energy. It is likely that low impact pretreatment methods such as mechanical and thermal phased improve speed of degradation, while high impact methods such as thermal hydrolysis or oxidation improve both speed and extent of degradation. While increased nutrient release can be a substantial cost in enhanced sludge destruction, it also offers opportunities to recover nutrients from a concentrated water stream as mineral fertiliser.