Optimization of biochemical sulfide potential (BSP) assay for anaerobic biodegradability assessment.

The anaerobic biodegradability assessment (biodegradation extent and kinetics) of organic wastes is critical for optimum design and evaluating treatment efficiencies for anaerobic treatment technologies. The biochemical sulfide potential (BSP) assay has previously demonstrated the advantages of its time efficiency and measurement accuracy for biologically assessing substrate degradability, while its application is limited by undefined operational parameters. In this study, the BSP assay was further optimized through a systematic investigation of a critical parameter, inoculum-to-substrate ratio (ISR), and the applicable kinetic model to unravel the potential use of BSP assays for anaerobic waste treatment. Under two series of experimental scenarios, the common ISR ranges of 0.5-4.0 (based on the traditional BMP assay) and extreme ISRs (as low as 0.1) were studied, in which the advantage of a BSP assay on extreme ISRs was highlighted. Meanwhile, the underlying cause and mechanism for biodegradability discrepancies under different ISRs (0.1-6.0) were further investigated. The extracellular polymeric substance (EPS) characterization of residual organics and the two-substrate first-order hydrolysis model analyses revealed that the hydrolysis process of slowly-biodegradable organics fraction was hindered under improper ISR conditions. Furthermore, the Cone model was evaluated as more appropriate for biodegradation kinetics analysis in BSP assays among the five common kinetic models (i.e., Exponential, Fitzhugh, Cone, Transference, and modified Gompertz models). Overall, the results provide fundamental guidance on designing consistent BSP assays and put a step forward in standardizing the BSP assay for anaerobic biodegradability assessments.

Integrated food waste management with wastewater treatment in Hong Kong: Transformation, energy balance and economic analysis.

Diversion of food waste (FW) away from the solid waste stream into the wastewater stream is proved viable through the use of food waste disposers (FWDs). However, this may cause unwanted influences on the wastewater treatment system. In this context, this study has comprehensively evaluated integrated food waste and wastewater management on a city scale for the first time. A plant-wide COD-based transformation model was first established to assess the impacts of the use of FWDs on the networks of biological wastewater treatment plants (WWTPs) in Hong Kong. The biological WWTPs can remove about 78% of solids and 58% of chemical oxygen demand (COD) in FW. Moreover, the diversion of FW poses limited impacts on treatment capacity and effluent quality in WWTPs with the FWDs penetration rate up to 30%. The increases in energy consumption and operational cost are highly dependent on the treatment processes and the FWDs penetration rates, while municipal solid waste treatment can benefit from the diversion of FW. This study suggests that upgrading treatment processes (e.g., with less aeration) and optimizing the operation of WWTPs (e.g., reduce sludge retention time) may be required with the use of FWDs to achieve an energy-efficient and cost-effective goal. More importantly, this study not only provides a methodology for effectively evaluating the impacts of diverting FW into wastewater treatment in Hong Kong but also facilitates FW management in similar metropolises.

Potential for co-disposal and treatment of food waste with sewage: A plant-wide steady-state model evaluation.

The water, food and energy nexus is a vital subject to achieve sustainable development goals worldwide. Wastewater (WW) and food waste (FW) from municipal sources are the primary contributors of organic waste from cities. Along with the loss of these valuable natural resources, their treatment systems also consume a considerable amount of abiotic energy and resource input and make a perceptible contribution to global warming. Hence, the global paradigm has evolved from simple pollution mitigation to resource recovery systems. In this study, the prospects of FW co-disposal into the sewer system and treatment with municipal sewage were quantitatively investigated for Hong Kong's largest biological WW treatment plant (WWTP) by integrated plant-wide steady-state modelling (PWSSM) and lifecycle assessment (LCA) approaches. The investigation assessed the impacts on the design and operational capacity of the WWTP, effluent quality, sludge output, and its net energy and carbon footprint. The results revealed that even at a higher than normal FW to sewage ratio, the WWTP's organic load capacity and performance in terms of organics and nitrogen removal was not significantly degraded, in fact the denitrification efficiency was improved by the FW organics with low N/C ratio. The net energy balance was improved by 80-400%, the net carbon footprint was lowered by 37-63% (without biogenic emissions), while the sludge production was increased by ∼33%. The results are very sensitive and improved with greater influent FW concentration and solids capture in the primary settling unit of the WWTP. The differences in the results have to be seen in relation to uncontrolled methane emission and a faster filling rate if the FW were disposed to landfill. The study provides valuable insights and policy guidelines for the decision makers locally and a generic methodological template.

A novel approach for rapidly measuring volatile fatty acids in anaerobic process.

Volatile fatty acids (VFAs), the intermediate of the anaerobic process, are considered to be the critical, high-sensitive and reliable indicators of the process stability. Close monitoring and control of VFAs are paramount for the efficient operation of the anaerobic reactors. In this study, a buffer intensity-based mathematical model was developed, and the least square method was integrated into the model to solve the issue of non-linear fitting of the titration curve. An automatic analyzer embedded with the developed model was designed and implemented for measuring VFAs and alkalinity. Through model optimization, the pH range of 3.5-5.6 was found to be suitable for VFAs analysis. The developed approach was validated by different VFAs (up to 500 mg/L as acetic acid) and carbonate alkalinity concentrations (up to 1500 mg/L as CaCO3) with high recovery rates (>0.9). Optimal ratios of carbonate alkalinity to VFAs are identified in the range of 2.4-7.5 for accuracy. Owing to the non-linear fitting of the titration curve, the impact of other weak acid subsystems (e.g., phosphate, ammonium and sulfide) can be negligible. The one-year real-time monitoring of environmental samples by using the automatic analyzer indicates a high consistency and stability compared with the 5 pH point titration. This approach proves to be rapid (<3 min/sample), accurate, reliable and can be applied for real-time automatic monitoring of the anaerobic process.

Ground food waste discharge to sewer enhances methane gas emission: A lab-scale investigation.

Emission of sulfide and methane from sewerage system has been a major concern for a long time. Sewers are now facing emerging challenges, such as receiving food waste (FW) to relieve the burdens on solid waste treatment. However, the knowledge of the direct impact of FW addition on sulfide and methane production in and emission from sewers is still lacking. In this study, two lab-scale sewer reactors, one without and one with FW addition, were continuously operated to investigate the production of sulfide and methane and microbial communities arising from FW discharge to freshwater sewerage system. The 190-day long-term monitoring and the batch tests on days 69 and 124 suggest that the FW addition has little impact on sulfide production possibly due to the limited sulfate concentration (40 mg S/L) but enhanced methane production by up to 60%. Moreover, cryosection-fluorescence in situ hybridization (FISH) revealed that the FW addition significantly stimulated the accumulation of methanogenic archaea (MA) in sewer biofilms and altered the spatial distributions of sulfate-reducing bacteria (SRB) and MA. Moreover, the relative abundance of MA in biofilms with FW addition was higher than that without FW addition, whereas the relative abundance of SRB was similar. Metabolic pathway analysis for sulfidogenesis and methanogenesis indicates that sufficient substrates derived from the FW addition were biodegraded during fermentation to produce acetate and hydrogen, and consequently facilitate methanogenesis. These findings shed light on the impacts of changes in wastewater compositions (e.g., FW addition) on sulfide and methane production in the freshwater sewerage system for improved policy-making on sewer management.

Coupling of sulfur(thiosulfate)-driven denitratation and anammox process to treat nitrate and ammonium contained wastewater.

This study investigated the feasibility of a new biological nitrogen removal process that integrates sulfur-driven autotrophic denitratation (NO3-→NO2-) and anaerobic ammonium oxidation (Anammox) for simultaneous removal of nitrate and ammonium from industrial wastewater. The proposed sulfur(thiosulfate)-driven denitratation and Anammox process was developed in two phases: First, the thiosulfate-driven denitratation was established in the UASB inoculated with activated sludge and fed with ammonium, nitrate and thiosulfate for 52 days until the nitrite level in the effluent reached 32.1 mg N/L. Second, enriched Anammox biomass was introduced to the UASB to develop the integrated thiosulfate-driven denitratation and Anammox (TDDA) bioprocess (53-212 d). Results showed that nitrate and ammonium could be efficiently removed from synthetic wastewater by the integrated TDDA system at a total nitrogen (TN) removal efficiency of 82.5 ±â€¯1.8% with an influent NH4+-N of 101.2 ±â€¯2.2 mgN/L, NO3--N of 101.1 ±â€¯1.5 mgN/L and thiosulfate of 202.5 ±â€¯3.2 mg S/L. It was estimated that Anammox and autotrophic denitritation (NO2-→N2) contributed to about 90% and 10% of the TN removal respectively at stable operation. The established TDDA system was further supported by high-throughput sequencing analysis that sulfur-oxidizing bacteria (e.g., Thiobacillus and Sulfurimonas) coexisted with Anammox bacteria (e.g., Ca. Kuenenia and Ca. Anammoxoglobus) in this syntrophic biocenosis. Additionally, batch experiments were conducted to reveal the kinetic rates and to reconcile the stoichiometry of the electron donor/acceptor couples of the TDDA process. The results unraveled the mechanisms in the new bioprocess: i) sulfite and elemental sulfur (S0) were initially generated from branched thiosulfate; ii) oxidation of sulfite and elemental sulfur coupled with fast and slow denitratation; iii) nitrite produced from denitratation together with ammonium were effectively converted to dinitrogen gas via Anammox.

Elucidating the biofilm properties and biokinetics of a sulfur-oxidizing moving-bed biofilm for mainstream nitrogen removal.

The sulfide-oxidizing autotrophic denitrification (SOAD) process offers a feasible alternative to mainstream heterotrophic denitrification in treating domestic sewage with insufficient organics. Previously SOAD has been successfully applied in a moving-bed biofilm reactor (MBBR). However, the biofilm properties and biokinetics are still not thoroughly understood. The present study was therefore designed to investigate these features of sulfur-oxidizing biofilms (SOBfs) cultivated in a lab-scale MBBR under stable operation for over a year. The biofilms developed were 160 µm thick, had an uneven and porous surface on which elemental sulfur (S0) accumulated, and the SOB biomass was highly diverse. The bioprocess kinetics were evaluated through 12 batch experiments. The results were interpreted by adopting a two-step sulfide oxidation model (sulfide→S0 and S0→ sulfate) with all specific rates having a linear regression coefficient of R2 > 0.9. Moreover, the inhibitory kinetic analysis revealed that 1) the maximum treatment capacity (about 480 mg S/(m2·h) and 80 mg N/(m2·h)) was observed at low sulfide level (40 mg S/L), while higher sulfide level (60-150 mg S/L) showed increasing inhibition on the oxidation of both sulfide and sulfur and denitrification. 2) The denitritation activity decreased by up to 43% when free nitrous acid reached a maximum of 8.6 µg N/L, whereas the oxidation of sulfide and sulfur did not have any significant effect. Interestingly, two physiologically diverse SOB groups were found in this special biofilm. The mechanisms of the cooperation and competition for electron donors and acceptors between these two SOB clades are proposed. The results of this study greatly enhance our understanding of the design and optimization of SOAD-MBBR for mainstream nitrogen removal.

A new sulfidogenic oxic-settling anaerobic (SOSA) process: The effects of sulfur-cycle bioaugmentation on the operational performance, sludge properties and microbial communities.

In-situ sludge reduction can be achieved by inserting an anaerobic side-stream reactor in the sludge return line of the conventional activated sludge (CAS) process. This modified oxic-settling-anaerobic (OSA) process can reduce sludge production by 30-50% through feast-fast alternating conditions. This paper proposes a new bioprocess called the sulfidogenic oxic-settling anaerobic (SOSA) process with OSA configuration and the addition of sulfate in side-stream reactor. The new bioprocess augments the conventional anaerobic/anoxic/aerobic feast-fast bioconversions with sulfur biochemical transformations (i.e. sulfate reduction and sulfur-oxidizing autotrophic denitrification). A lab-scale SOSA process was operated for 260 days in parallel with the anoxic/oxic (AO) CAS process and the conventional OSA process as control systems. Based on the experimental results, the feasibility of the new SOSA process was evaluated, and the effects of sulfur bioaugmentation on the effluent quality, sludge reduction, sludge physico-chemical properties and microbial communities were examined. The SOSA process i) removed 98% of the organics (chemical oxygen demand, COD) and 99% of the ammonia present with a lower observed sludge yield (0.204 g TSS/g CODremoved) than those of the OSA and AO processes (0.292 and 0.473 g TSS/g CODremoved respectively), ii) denitrified 18% and 6% more nitrogen to dinitrogen gas than did the CAS and OSA processes respectively, iii) produced sludge with improved settleability and dewaterability, iv) encouraged sludge decomposition with greater destruction of extracellular polymeric substances and v) enriched sulfur-cycle related and hydrolytic/fermentative bacteria. The possible mechanisms of sulfur augmentation and limitations of the present study are also discussed.

Achieving methane production enhancement from waste activated sludge with sulfite pretreatment: Feasibility, kinetics and mechanism study.

Sulfite has been widely employed as a key agent in many industrial processes, leading to a large amount of sulfite-laden wastes generated. Given its antimicrobial function and destructive ability on cell walls, detailed mechanisms for impacts of sulfite on waste activated sludge (WAS) and outcomes of methane production after the sulfite-pretreatment have not been clear so far. In this study, the feasibility of methane production from sulfite pretreated WAS was verified and investigated. Biochemical methane potential tests demonstrated that methane production from WAS after the pretreatment at 800 mg S/L of sulfite (a typical level in sulfite-laden wastes) increased by up to 25%. Kinetic analysis of the test results indicated that sulfite pretreatment increased the sludge hydrolysis rate (khyd) by 1.71 times while the ultimate biochemical methane potential (Bu) by 1.20 times. Further study investigated the effects of sulfite on WAS from the macro-scale (i.e. sludge physicochemical properties) to the micro-scale (i.e. bacterial viability, microbial community). Sulfite concentrations of up to 800 mg S/L substantially enhanced WAS disintegration and solubilization, reducing the particle size by up to 39%, boosting substrate release by 87% and improving cell lysis by 43% through the direct destruction of gram-positive microorganisms (e.g., norank_p_Saccharibacteria) in WAS. Adverse impact on anaerobic digestion by introduction of sulfite was not observed in this study, though a long-term evaluation is needed in the future work. Based on the findings of the present study, sulfite-laden by-products or wastes from industrial processes may be co-treated with WAS when overall cost-effectiveness is concerned.

Advances in sulfur conversion-associated enhanced biological phosphorus removal in sulfate-rich wastewater treatment: A review.

Recently an innovative sulfur conversion-associated enhanced biological phosphorus removal (S-EBPR) process has been developed for treating sulfate-rich wastewater. This process has successfully integrated sulfur (S), carbon (C), nitrogen (N) and P cycles for simultaneous metabolism or removal of C, N and P; moreover this new process relies on the synergy among the slow-growing sulfate-reducing bacteria and sulfur-oxidizing bacteria, hence generating little excess sludge. To elucidate this new process, researchers have investigated the microorganisms proliferated in the system, identified the biochemical pathways and assessed the impact of operational and environmental factors on process performance as well as trials on process optimization. This paper for the first time reviews the recent advances that have been achieved, particularly relating to the areas of S-EBPR microbiology and biochemistry, as well as the effects of environmental factors (e.g., electron donors/acceptors, pH, temperature, etc.). Moreover, future directions for researches and applications are proposed.

Investigation of multiple polymers in a denitrifying sulfur conversion-EBPR system: The structural dynamics and storage states.

Polyhydroxyalkanoates (PHAs), polyphosphate (poly-P) and polysulfide or elemental sulfur (poly-S) are the key functionally relevant polymers involved in the recently reported Denitrifying Sulfur conversion-associated Enhanced Biological Phosphorus Removal (DS-EBPR) process. However, little is known about the structural dynamics and storage states of these polymers. In particular, investigating the poly-S generated in this process is quite a superior challenge. This study was thus aimed at simultaneously qualitative-quantitative investigating poly-S and associated poly-P and PHAs through the integrated chemical analysis and Raman micro-spectroscopy coupled with multiple microscopic methods (i.e. optical microscopy, confocal laser scanning microscopy, and differential interference contrast microscopy). The chemical analytical results displayed a stable DS-EBPR phenotype in terms of sulfur conversion, P release/uptake and the dynamics of relevant polymers. The multiple microscopic images and Raman spectrum profiles further clearly demonstrated the existence of the polymers and their dynamic changes under alternating anaerobic-anoxic conditions, consistent with the chemical analytical results. In particular, Raman analysis for the first time unraveled the co-existence of S0/Sn2- species stored either intracellularly or extracellularly; and the dynamic conversions between S0/Sn2- and other sulfur species suggest that there might be a universal pool of bioavailable sulfur. The results reveal the mechanisms underlying the structural dynamics and changes in storage states of the relevant polymers that are functionally relevant to the carbon/phosphorus/sulfur-cycles during different metabolic phases. These mechanisms would otherwise not be obtained only using a traditional chemical analysis-based approach.

Elucidating the effects of starvation and reactivation on anaerobic sulfidogenic granular sludge: Reactor performance and granular sludge transformation.

In biological wastewater treatment, the bacteria starvation always challenges the stability of system operation. Yet, the effects of starvation and possibility of reactivation are less understood for anaerobic sulfidogenic system. Sulfidogenic systems use sulfate as electron acceptor for organic chemical oxygen demand (COD) degradation, so it will encounter two kind of starvations: (i) complete stoppage of wastewater flow (named complete food starvation) and (ii) remaining organic COD but with very low level sulfate in the influent (named sulfate starvation). In this study, the relative long-term starvation (over 30-day) and subsequent reactivation were studied in two lab-scale continuous gas recirculation sulfate-reducing upflow sludge bed (CGR-SRUSB) reactors. It was observed that the complete food starvation and sulfate starvation decreased the COD removal rate (in the similar range of 60-65%) and the specific sulfidogenic activity (about 45% and 61% respectively), as well as increasing the sludge flotation potential (SFP) from <15% to 58% and 35% respectively. Moreover, the following restoration experiments proved that the perturbed systems could be reactivated within 10-15 days for both cases. The results of investigating the mechanisms showed the performance deterioration were highly attributed to the starvation-induced granular sludge transitions, with respect to the changing of sludge physico-chemical properties (permeability, porosity, hydrophobicity and viscocity) and microbial stuctures (sulfate-reducting bacteria and extracellular polymeric substances). The outcomes of this study can provide useful information for dealing with the prolonged starvation problems in sulfidogenesis-based systems in industrial and municipal wastewater treatment.

Integrated fixed-film activated sludge membrane bioreactors versus membrane bioreactors for nutrient removal: A comprehensive comparison.

This research elucidates the pollutants (nutrients and carbon) removal performance and nitrous oxide (N2O) emissions of two pilot plants. Specifically, a University of Cape Town (UCT) Membrane Bioreactor (MBR) plant and an Integrated Fixed Film Activated Sludge (IFAS)-UCT-MBR plant were investigated. The plants were fed with real wastewater augmented with acetate and glycerol in order to control the influent carbon nitrogen ratio (C/N). The short-term effect of the inlet C/N ratio variation (C/N = 5 mgCOD/mgN and C/N = 10 mgCOD/mgN) on the behaviour of both plants was investigated. The results showed that the IFAS-UCT-MBR configuration provided the best performance in terms of pollutants removal at the two investigated C/N ratios. Furthermore, the lowest N2O emission (with respect to the influent nitrogen) was observed in the IFAS-UCT-MBR configuration, thus suggesting a potential beneficial effect of the biofilm in the emission reduction. However, the membrane of the IFAS-UCT-MBR showed a greater fouling tendency compared to the UCT-MBR configuration. This result, likely related to the biofilm detached from carriers, could seriously affect the indirect GreenHouse Gas emissions due to the increase of the energy requirement for permeate extraction with the increase of membrane fouling.

Denitrifying sulfur conversion-associated EBPR: Effects of temperature and carbon source on anaerobic metabolism and performance.

The recently developed Denitrifying Sulfur conversion-associated Enhanced Biological Phosphorus Removal (DS-EBPR) process has demonstrated simultaneous removal of organics, nitrogen and phosphorus with minimal sludge production in the treatment of saline/brackish wastewater. Its performance, however, is sensitive to operating and environmental conditions. In this study, the effects of temperature (20, 25, 30 and 35 °C) and the ratio of influent acetate to propionate (100-0, 75-25, 50-50, 25-75 and 0-100%) on anaerobic metabolism were investigated, and their optimal values/controls for performance optimization were identified. A mature DS-EBPR sludge enriched with approximately 30% sulfate-reducing bacteria (SRB) and 33% sulfide-oxidizing bacteria (SOB) was used in this study. The anaerobic stoichiometry of this process was insensitive to temperature or changes in the carbon source. However, an increase in temperature from 20 to 35 °C accelerated the kinetic reactions of the functional bacteria (i.e. SRB and SOB) and raised the energy requirement for their anaerobic maintenance, while a moderate temperature (25-30 °C) resulted in better P removal (≥93%, 18.6 mg P/L removal from total 20 mg P/L in the influent) with a maximum sulfur conversion of approximately 16 mg S/L. These results indicate that the functional bacteria are likely to be mesophilic. When a mixed carbon source (75-25 and 50-50% acetate to propionate ratios) was supplied, DS-EBPR achieved a stable P removal (≥89%, 17.8 mg P/L for 400 mg COD/L in the influent) with sulfur conversions at around 23 mg S/L, suggesting the functional bacteria could effectively adapt to changes in acetate or propionate as the carbon source. The optimal temperatures or carbon source conditions maximized the functional bacteria competition against glycogen-accumulating organisms by favoring their activity and synergy. Therefore, the DS-EBPR process can be optimized by setting the temperature in the appropriate range (25-30 °C) and/or manipulating influent carbon sources.

Quantitative characterization and analysis of granule transformations: Role of intermittent gas sparging in a super high-rate anaerobic system.

Knowledge of leveraging biomass characteristics is essential for achieving a microbial community with a desired structure to optimize anaerobic bioreactor performance. This study investigates the successive granule transformations in a high-rate anaerobic system with intermittent gas sparging and sequential increases in organic loading rates (OLRs), by establishing the correlations between the granule microstructures and reactor operating parameters. Over the course of a 196-day lab-scale trial, the granules were visualized in various stages using scanning electron microscopy, and digital image processing was applied for further quantifying their surface properties. Correlation analyses revealed that irregularities of the granule microstructures (surface properties, specific surface area and pore volume) emerged at stage 4 when the OLR was 13.31 kg COD/m3·day and in stage 5 in the absence of gas sparging. The loading ratio (substrate surface loading to upward velocity) was identified to be the main parameter controlling the granule transformations, and the surface structures were classified into three categories for further interpretation. Confocal laser scanning microscopy analyses showed that the granule core started to hollow out from stage 4. It is also found that a rough granule surface helped accelerate the growth of the granular diameter under gas sparging. Overall, this study not only establish quantitative correlations between the granules microstructures and reactor operating parameters, but also shed light on the use of intermittent gas sparging to control the surface properties of anaerobic granules in high-rate anaerobic bioreactors.

Development of biochemical sulfide potential (BSP) test for sulfidogenic biotechnology application.

The determination of organics biodegradability and corresponding biodegradation kinetics provides valuable information on the optimal design and operation of anaerobic biotechnology especially for sulfidogenesis. This study proposes a deterministic method, i.e. a biochemical sulfide potential (BSP) test, and compares it to the conventional biochemical methane potential (BMP) test in terms of their ability to characterize sulfate-laden organic waste biodegradability. It demonstrated 1.48 times higher degradation of volatile suspended solids (VSS) and 2.60 times more chemical oxygen demand (COD) conversion in its major metabolites than the BMP test. Moreover, it required only four days to complete, compared to the 35 days required by the BMP test. Through the two-substrate first-order hydrolysis model, it was revealed that the shortened time was attributed to the enhanced degradation rates from both readily (eight times) and slowly (nearly 10 times) biodegradable organic substrates in the BSP test compared with the BMP test for the same sulfate-laden organic waste. The findings highlight the inappropriateness of the BMP test to sulfidogenic applications due to the underestimated predictions of organic waste biodegradability and excessive time requirements. Furthermore, the ability of the BSP test to identify the average elemental composition (CxHyOzNaPbSc) of substrate biodegradable particulate organics (BPO) is explored and verified using a casein-based validation test. Using BPO elemental composition as the input variable, a BSP biochemical kinetic model is thereby developed to predict BSR performance and possible dynamic process control. Overall, this study demonstrates the applicability and advantages of the BSP test in sulfidogenic applications for characterization of organics biodegradability and identification of BPO average elemental composition, furthermore develops a process model utilizing the derived BPO average elemental composition to provide optimized reactor retention time and substrates feed mixture for optimum performance.

Characterization of a new continuous gas-mixing sulfidogenic anaerobic bioreactor: Hydrodynamics and sludge granulation.

Continuous gas recirculation (CGR) was demonstrated in this study to be an effective method to mitigate the persistent problem of sludge flotation in high-rate sulfate-reducing upflow sludge bed (SRUSB) reactors that do not produce much gas. The effects of hydraulic- and CGR-mixing on the mixing regime of the SRUSB reactors were investigated over a period of 45 d at the average shear rates of 0.9, 1.5, 2.7, 4.2 and 7.2 s-1 (Phase I). CGR-mixing at 4.2 s-1 resulted in the smallest reactor short-circuiting flow of 1.3 ±â€¯0.1% and the smallest dead zone volume of 0.2 ±â€¯0.01% at a lower power consumption (0.0007 W) than hydraulic mixing. In Phase II, the SRUSB reactor with CGR-mixing at 4.2 s-1 was re-inoculated and operated for 150 days. Within the first 65 days, the sludge transformed into micro-granules (300-350 µm) with a high sulfate-reducing bacteria (SRB) activity (0.62 ±â€¯0.05 g COD/(g MLVSS·day)), a low sludge flotation potential (<20%) and a high settleability (SVI5/SVI30 < 1.3). These results are attributed to the following sludge properties: (i) a low ratio of loosely-bound to tightly-bound extracellular polymeric substances (0.06-0.1), (ii) weakly adhesive surface properties as demonstrated by a strongly negative zeta potential (-23 ±â€¯2 mV), a low hydrophobicity (37 ±â€¯3%) and a low viscosity (0.7 ±â€¯0.1 mPa s), and (iii) small size granules resulting in strong mass transfer (sulfate and COD penetration into the granule core) and a homogeneous structure (SRB detected throughout the granule).

Optimizing mixing mode and intensity to prevent sludge flotation in sulfidogenic anaerobic sludge bed reactors.

Sludge flotation is a notorious problem in anaerobic wastewater treatment that can occur under various operational conditions and even cause the anaerobic process to completely fail. Despite having been documented for over three decades, its causes and remedies remain elusive, particularly for low-gas-production anaerobic processes such as sulfidogenic and anammox processes. This paper systematically studies sludge flotation in an anaerobic sulfidogenic process for saline domestic sewage treatment. Three lab-scale sulfidogenic reactors were operated in parallel with different modes of mixing (hydraulic, mechanical and pneumatic) at various mixing intensity levels at shear rates ranging from 0.7 to 6.6 s-1 to investigate reactor performance and sludge properties and their relationships with sludge flotation potential. The results indicate that a sulfidogenic reactor with low flotation potential have sludge with low hydrophobicity, low viscosity, and low (more negative) surface charge, while the sludge particle surfaces have high compactness and low roughness. These sludge properties enabled a sludge flotation potential of less than 20% to be maintained. Furthermore, our results show that i) mixing and extracellular polymeric substances (EPS), ii) EPS and sludge properties, and iii) sludge properties and sludge flotation potential are all strongly correlated (all the Spearman's rank correlation coefficients (Rs) are either over 0.64 (if positively correlated) or under -0.64 (if negatively correlated), at the 95% confidence level). Accordingly, sludge flotation can be resolved by controlling reactor mixing. Our findings provide a method to optimize the design and operation of anaerobic sulfidogenic reactors that can be extended to similar low-gas-production anaerobic bioreactors.

Greenhouse gases from membrane bioreactors: Mathematical modelling, sensitivity and uncertainty analysis.

In this study a new mathematical model to quantify greenhouse gas emissions (namely, carbon dioxide and nitrous oxide) from membrane bioreactors (MBRs) is presented. The model has been adopted to predict the key processes of a pilot plant with pre-denitrification MBR scheme, filled with domestic and saline wastewater. The model was calibrated by adopting an advanced protocol based on an extensive dataset. In terms of nitrous oxide, the results show that an important role is played by the half saturation coefficients related to nitrogen removal processes and the model factors affecting the oxygen transfer rate in the aerobic and MBR tanks. Uncertainty analysis showed that for the gaseous model outputs 88-93% of the measured data lays inside the confidence bands showing an accurate model prediction.

Bacterial community structure and removal performances in IFAS-MBRs: A pilot plant case study.

The paper reports the results of an experimental campaign carried out on a University of Cape Town (UCT) integrated fixed-film activated sludge (IFAS) membrane bioreactor (MBR) pilot plant. The pilot plant was analysed in terms of chemical oxygen demand (COD) and nutrients removal, kinetic/stoichiometric parameters, membrane fouling and sludge dewaterability. Moreover, the cultivable bacterial community structure was also analysed. The pilot plant showed excellent COD removal efficiency throughout experiments, with average value higher than 98%, despite the slight variations of the influent wastewater. The achieved nitrification efficiency was close to 98% for most of the experiments, suggesting that the biofilm in the aerobic compartment might have sustained the complete nitrification of the influent ammonia, even for concentrations higher than 100 mg L-1. The irreversible resistance due to superficial cake deposition was the mechanism that mostly affected the membrane fouling. Moreover, it was noticed an increase of the resistance due pore blocking likely due to the increase of the EPSBound fraction that could derive by biofilm detachment. The bacterial strains isolated from aerobic tank are wastewater bacteria known for exhibiting efficient heterotrophic nitrification-aerobic denitrification and producing biofilm.