Optimized aeration strategies for nitrogen and phosphorus removal with aerobic granular sludge

Biological wastewater treatment by aerobic granular sludge biofilms offers the possibility to combine carbon (COD), nitrogen (N) and phosphorus (P) removal in a single reactor. Since denitrification can be affected by suboptimal dissolved oxygen concentrations (DO) and limited availability of COD, different aeration strategies and COD loads were tested to improve N- and P-removal in granular sludge systems. Aeration strategies promoting alternating nitrification and denitrification (AND) were studied to improve reactor efficiencies in comparison with more classical simultaneous nitrification–denitrification (SND) strategies. With nutrient loading rates of 1.6 gCOD L⁻¹ d⁻¹, 0.2 gN L⁻¹ d⁻¹, and 0.08 gP L⁻¹ d⁻¹, and SND aeration strategies, N-removal was limited to 62.3 ± 3.4%. Higher COD loads markedly improved N-removal showing that denitrification was limited by COD. AND strategies were more efficient than SND strategies. Alternating high and low DO phases during the aeration phase increased N-removal to 71.2 ± 5.6% with a COD loading rate of 1.6 gCOD L⁻¹ d⁻¹. Periods of low DO were presumably favorable to denitrifying P-removal saving COD necessary for heterotrophic N-removal. Intermittent aeration with anoxic periods without mixing between the aeration pulses was even more favorable to N-removal, resulting in 78.3 ± 2.9% N-removal with the lowest COD loading rate tested. P-removal was under all tested conditions between 88 and 98%, and was negatively correlated with the concentration of nitrite and nitrate in the effluent (r = −0.74, p < 0.01). With low COD loading rates, important emissions of undesired N₂O gas were observed and a total of 7–9% of N left the reactor as N₂O. However, N₂O emissions significantly decreased with higher COD loads under AND conditions.     

Granulation of activated sludge in a pilot-scale sequencing batch reactor for the treatment of low-strength municipal wastewater

Aerobic granulation of activated sludge was achieved in a pilot-scale sequencing batch reactor (SBR) for the treatment of low-strength municipal wastewater (<200 mg L⁻¹ of COD, chemical oxygen demand). The volume exchange ratio and settling time of an SBR were found to be two key factors in the granulation of activated sludge grown on the low-strength municipal wastewater. After operation of 300 days, the mixed liquor suspended solids (MLSS) concentration in the SBR reached 9.5 g L⁻¹ and consisted of approximate 85% granular sludge. The average total COD removal efficiency kept at 90% and NH₄⁺-N was almost completely depleted (∼95%) after the formation of aerobic granules. The granules (with a diameter over 0.212 mm) had a diameter ranging from 0.2 to 0.8 mm and had good settling ability with a settling velocity of 18-40 m h⁻¹. Three bacterial morphologies of rod, coccus and filament coexisted in the granules. Mathematical modeling was performed to get insight into this pilot-scale granule-based reactor. The modified IWA activated sludge model No 3 (ASM3) was able to adequately describe the pilot-scale SBR dynamics during its cyclic operation.     

Development of granular sludge for textile wastewater treatment

Microbial granular sludge that is capable to treat textile wastewater in a single reactor under intermittent anaerobic and aerobic conditions was developed in this study. The granules were cultivated using mixed sewage and textile mill sludge in combination with anaerobic granules collected from an anaerobic sludge blanket reactor as seed. The granules were developed in a single sequential batch reactor (SBR) system under alternating anaerobic and aerobic condition fed with synthetic textile wastewater. The characteristics of the microbial granular sludge were monitored throughout the study period. During this period, the average size of the granules increased from 0.02 ± 0.01 mm to 2.3 ± 1.0 mm and the average settling velocity increased from 9.9 ± 0.7 m h⁻¹ to 80 ± 8 m h⁻¹. This resulted in an increased biomass concentration (from 2.9 ± 0.8 g L⁻¹ to 7.3 ± 0.9 g L⁻¹) and mean cell residence time (from 1.4 days to 8.3 days). The strength of the granules, expressed as the integrity coefficient also improved. The sequential batch reactor system demonstrated good removal of COD and ammonia of 94% and 95%, respectively, at the end of the study. However, only 62% of color removal was observed. The findings of this study show that granular sludge could be developed in a single reactor with an intermittent anaerobic-aerobic reaction phase and is capable in treating the textile wastewater.     

SBR工艺污泥沉降性能的影响因素研究

分别研究了运行方式、运行时间、硝酸盐浓度、曝气量、污泥负荷及曝气时间对SBR工艺中污泥沉降性能的影响.试验结果表明,以厌氧/好氧方式运行时,厌氧≥1 h且好氧≥3h可获得沉降性良好的污泥;以缺氧/好氧方式运行时,缺氧段硝酸盐浓度过高会导致污泥膨胀,缺氧段时间宜控制在1h左右;当有机负荷较高时,曝气量较高或较低均可能导致污泥膨胀,通过降低有机负荷可有效改善污泥沉降性能;在污泥负荷较低、曝气量适当且氮磷充足的条件下,曝气6h时污泥的沉降性较差,将曝气时间延长至10 h,污泥浓度保持稳定,沉降性较好.     

Integration of anammox into the aerobic granular sludge process for main stream wastewater treatment at ambient temperatures

Anaerobic ammonium oxidation, nitrification and removal of COD was studied at ambient temperature (18 °C ± 3) in an anoxic/aerobic granular sludge reactor during 390 days. The reactor was operated in a sequencing fed batch mode and was fed with acetate and ammonium containing medium with a COD/N ratio of 0.5 [g COD/gN]. During influent addition, the medium was mixed with recycled effluent which contained nitrate in order to allow acetate oxidation and nitrate reduction by anammox bacteria. In the remainder of the operational cycle the reactor was aerated and controlled at a dissolved oxygen concentration of 1.5 mg O₂/l in order to establish simultaneous nitritation and Anammox. Fluorescent in-situ hybridization (FISH) revealed that the dominant Anammox bacterial population shifted toward Candidatus “Brocadia fulgida” which is known to be capable of organotrophic nitrate reduction. The reactor achieved stable volumetric removal rates of 900 [g N₂-N/m³/day] and 600 [g COD/m³/day]. During the total experimental period Anammox bacteria remained dominant and the sludge production was 5 fold lower than what was expected by heterotrophic growth suggesting that consumed acetate was not used by heterotrophs. These observations show that Anammox bacteria can effectively compete for COD at ambient temperatures and can remove effectively nitrate with a limited amount of acetate. This study indicates a potential successful route toward application of Anammox in granular sludge reactors on municipal wastewater with a limited amount of COD.     

Design parameters for sludge reduction in an aquatic worm reactor

Reduction and compaction of biological waste sludge from waste water treatment plants (WWTPs) can be achieved with the aquatic worm Lumbriculus variegatus. In our reactor concept for a worm reactor, the worms are immobilised in a carrier material. The size of a worm reactor will therefore mainly be determined by the sludge consumption rate per unit of surface area. This design parameter was determined in sequencing batch experiments using sludge from a municipal WWTP. Long-term experiments using carrier materials with 300 and 350 μm mesh sizes showed surface specific consumption rates of 45 and 58 g TSS/(m² d), respectively. Using a 350 μm mesh will therefore result in a 29% smaller reactor compared to using a 300 μm mesh. Large differences in consumption rates were found between different sludge types, although it was not clear what caused these differences. Worm biomass growth and decay rate were determined in sequencing batch experiments. The decay rate of 0.023 d⁻¹ for worms in a carrier material was considerably higher than the decay rate of 0.018 d⁻¹ for free worms. As a result, the net worm biomass growth rate for free worms of 0.026 d⁻¹ was much higher than the 0.009-0.011 d⁻¹ for immobilised worms. Finally, the specific oxygen uptake rate of the worms was determined at 4.9 mg O₂/(g ww d), which needs to be supplied to the worms by aeration of the water compartment in the worm reactor.     

Effects of oxygen concentration on N-removal in an aerobic granular sludge reactor

In order to optimise nitrogen removal in an aerobic granular sludge system, short- and long-term effects of decreased oxygen concentrations on the reactor performance were studied. Operation at decreased oxygen concentration is required to obtain efficient N-removal and low aeration energy requirement. A short-term oxygen reduction (from 100% to 50%, 40%, 20% or 10% of the saturation concentration) did not influence the acetate uptake rate. A lower aerobic acetate uptake at lower oxygen concentrations was obviously compensated by anoxic acetate uptake. Nitrogen removal was favoured by decreased oxygen concentrations, reaching a value of 34% for the lowest oxygen concentration tested. Long-term effects were evaluated at two oxygen saturation levels (100% and 40%). Nitrogen removal increased from 8% to 45% when the oxygen saturation was reduced to 40%. However, the granules started to disintegrate and biomass washout occurred. It was impossible to obtain stable granular sludge at this decreased oxygen concentration under applied conditions. A solution to obtain stable aerobic granular sludge at low oxygen concentrations is needed in order to make aerobic granular sludge reactors feasible in practice.     

Long-term (1243 days), low-temperature (4-15 °C), anaerobic biotreatment of acidified wastewaters: Bioprocess performance and physiological characteristics

The feasibility of long-term (>3 years), low-temperature (4-15 °C) and anaerobic bioreactor operation, for the treatment of acidified wastewater, was investigated. A hybrid, expanded granular sludge bed-anaerobic filter bioreactor was seeded with a mesophilic inoculum and employed for the mineralization of moderate-strength (3.75-10 kg chemical oxygen demand (COD) m⁻³) volatile fatty acid-based wastewaters at 4-15 °C. Bioprocess performance was assessed in terms of COD removal efficiency (CODRE), methane biogas concentration, and yield, and biomass retention. Batch specific methanogenic activity assays were performed to physiologically characterise reactor biomass.Despite transient disimprovements, CODRE and methane biogas concentrations exceeded 80% and 65%, respectively, at an applied organic loading rate (OLR) of 10 kg COD m⁻³ d⁻¹ between 9.5 and 15 °C (sludge loading rate (SLR), 0.6 kg COD kg[VSS]⁻¹ d⁻¹). Over 50% of the granular sludge bed was lost to disintegration during operation at 9.5 °C, warranting a reduction in the applied OLR to 3.75-5 kg COD m⁻³ d⁻¹ (SLR, c. 0.4-0.5 kg COD kg[VSS]⁻¹ d⁻¹). From that point forward, remarkably stable and efficient performance was observed during operation at 4-10 °C, with respect to CODRE (≥82%), methane biogas concentration (>70%) and methane yields (>4 l_Methane d⁻¹), suggesting the adaptation of our mesophilic inoculum to psychrophilic operating conditions.Physiological activity assays indicated the development of psychroactive syntrophic and methanogenic populations, including the emergence of putatively psychrophilic propionate-oxidising and hydrogenotrophic methanogenic activity. The data suggest that mesophilic inocula can physiologically adapt to sub-optimal operational temperatures: treatment efficiencies and sludge loading rates at 4 °C (day, 1243) were comparable to those achieved at 15 °C (day 0). Furthermore, long-term, low-temperature bioreactor operation may act as a selective enrichment for psychrophilic methanogenic activity from mesophilic inocula. The observed efficient and stable bioprocess performance highlights the potential for long-term, low-temperature bioreactor operation.     

Granulation in high-load denitrifying upflow sludge bed (USB) pulsed reactors

In this work, the effect of the application of a pulse system to anoxic upflow sludge bed (USB) denitrifying reactors for enhancing sludge granulation was studied. In all, three 0.8 L reactors (two operated with flow pulsation, P1 with effluent recycling and P2 without recycling, and one without pulsation and effluent recycling, no pulsation (NP)) were fed with a mixture of NaNO3 and glucose and inoculated with methanogenic granular sludge. The organic loading rate (OLR) and the nitrogen loading rate (NLR) were progressively increased and, at the end of the experiment, extremely high values were obtained (67.5 kgCOD/m3d and 11.25 kgN-NO3-/m3 d). Ammonia and nitrite accumulation in reactor NP were important in the maturation stage, decreasing the denitrification efficiency to 90%, while in reactor P1 only low nitrite values were obtained in the last few days of the experiment. In reactor P2, nitrogen removal was 100% most of the time. Several operational problems (flotation and the subsequent wash out of biomass) appeared in the NP reactor when working at high denitrifying loading rates, while in reactors P1 and P2 there were no notable problems, mainly due to the good characteristics of the sludge developed and the efficient degasification produced by the pulsing flow. The sludge formed in the NP reactor presented a flocculent structure and a total disintegration of the initial methanogenic granules occurred, while a small-sized granular biomass with a high specific density was developed in the pulsed reactors due to the shear stress produced.     

Removal of Pharmaceutical and Personal Care Products (PPCPs) under nitrifying and denitrifying conditions

The contribution of volatilization, sorption and transformation to the removal of 16 Pharmaceutical and Personal Care Products (PPCPs) in two lab-scale conventional activated sludge reactors, working under nitrifying (aerobic) and denitrifying (anoxic) conditions for more than 1.5 years, have been assessed. Pseudo-first order biological degradation rate constants (k_biol) were calculated for the selected compounds in both reactors. Faster degradation kinetics were measured in the nitrifying reactor compared to the denitrifying system for the majority of PPCPs. Compounds could be classified according to their k_biol into very highly (k_biol > 5 L g_SS⁻¹ d⁻¹), highly (1 < k_biol < 5 L g_SS⁻¹ d⁻¹), moderately (0.5 < k_biol < 1 L g_SS⁻¹ d⁻¹) and hardly (k_biol < 0.5 L g_SS⁻¹ d⁻¹) biodegradable.Results indicated that fluoxetine (FLX), natural estrogens (E1 + E2) and musk fragrances (HHCB, AHTN and ADBI) were transformed to a large extent under aerobic (>75%) and anoxic (>65%) conditions, whereas naproxen (NPX), ethinylestradiol (EE2), roxithromycin (ROX) and erythromycin (ERY) were only significantly transformed in the aerobic reactor (>80%). The anti-depressant citalopram (CTL) was moderately biotransformed under both, aerobic and anoxic conditions (>60% and >40%, respectively). Some compounds, as carbamazepine (CBZ), diazepam (DZP), sulfamethoxazole (SMX) and trimethoprim (TMP), manifested high resistance to biological transformation.Solids Retention Time (SRT_aerobic >50 d and <50 d; SRT_anoxic >20 d and <20 d) had a slightly positive effect on the removal of FLX, NPX, CTL, EE2 and natural estrogens (increase in removal efficiencies <10%). Removal of diclofenac (DCF) in the aerobic reactor was positively affected by the development of nitrifying biomass and increased from 0% up to 74%. Similarly, efficient anoxic transformation of ibuprofen (75%) was observed after an adaptation period of 340 d. Temperature (16-26 °C) only had a slight effect on the removal of CTL which increased in 4%.     

Impacts of hydrodynamic shear force on nucleation of flocculent sludge in anaerobic reactor

The Sludge granulation in an anaerobic reactor consists of two steps: nucleation and maturation of nuclei. Nucleation as the starting point is of particular importance. In this paper, the nucleation of flocculent sludge as seed under weak, strong and violent hydrodynamic shear conditions is studied with an original quantitative method, and then the satisfactory linear correlations between the average sludge diameters and the operation time during the nucleation are demonstrated. Nucleation under strong shear conditions with a shear rate of about 8.28 s⁻¹, corresponding to superficial liquid and gas velocities of 2.66 and 0.24 m h⁻¹, develops fastest compared to weak shear conditions with a shear rate of about 0.04 s⁻¹ and violent shear conditions with a shear rate of about 12.42 s⁻¹ with the average augmentation rate of average sludge diameter of 0.57, 0.40 and 0.41 μm day⁻¹ respectively. One of the major mechanisms of the shear force on nucleation is that a high shear force accelerates the extracellular protein secretion of sludge. Although high extracellular protein content benefits nucleation, it is also shown that the extracellular proteins over-produced above around 80.5 mg gVSS⁻¹ leads nuclei to weaken and inhibit nucleation. So the violent shear force would result in disruption and wash-out of nuclei. However, the high extracellular polymers could intensify the shear force by raising the viscosity in the reactor, thus, in practice, it is important to monitor the shear conditions and extracellular protein content of sludge simultaneously in high rate reactors for stable operation.     

用厌氧产氢反应器出水培养好氧颗粒污泥的研究

以厌氧产氢反应器出水为底物，在序批式反应器中研究了好氧颗粒污泥的培养过程。结果表明，以厌氧产氢反应器出水为底物，在60d内能够培养出粒径大、沉降性能优异且对污染物去除能力强的好氧颗粒污泥。在活性污泥的颗粒化过程中，伴随着污泥体积指数的减小。污泥的粒径和沉速增大，反应器内的污泥浓度增加，从而提高了反应器的处理效能。     

Degradation of phthalate esters in an activated sludge wastewater treatment plant

Efficient removal of phthalate esters (PE) in wastewater treatment plants (WWTP) is becoming an increasing priority in many countries. In this study, we examined the fate of dimethyl phthalate (DMP), dibutyl phthalate (DBP), butylbenzyl phthalate (BBP), and di-(2-ethylhexyl) phthalate (DEHP) in a full scale activated sludge WWTP with biological removal of nitrogen and phosphorus. The mean concentrations of DMP, DBP, BBP, and DEHP at the WWTP inlet were 1.9, 20.5, 37.9, and 71.9 μg/L, respectively. Less than 0.1%, 42%, 35%, and 96% of DMP, DBP, BBP, and DEHP was associated with suspended solids, respectively. The overall microbial degradation of DMP, DBP, BBP, and DEHP in the WWTP was estimated to be 93%, 91%, 90%, and 81%, respectively. Seven to nine percent of the incoming PE were recovered in the WWTP effluent. Factors affecting microbial degradation of DEHP in activated sludge were studied using [U-¹⁴C-ring] DEHP as tracer. First order rate coefficients for aerobic DEHP degradation were 1.0×10⁻², 1.4×10⁻², and 1.3×10⁻³ at 20, 32, and 43 °C, respectively. Aerobic degradation rates decreased dramatically under aerobic thermophilic conditions (<0.1×10⁻² h⁻¹ at 60 °C). The degradation rate under anoxic denitrifying conditions was 0.3×10⁻² h⁻¹, whereas the rate under alternating conditions (aerobic-anoxic) was 0.8×10⁻² h⁻¹. Aerobic DEHP degradation in activated sludge samples was stimulated 5-9 times by addition of a phthalate degrading bacterium. The phthalate degrading bacterium was isolated from activated sludge, and maintained a capacity for DEHP degradation while growing on vegetable oil. Collectively, the results of the study identified several controls of microbial PE degradation in activated sludge. These controls may be considered to enhance PE degradation in activated sludge WWTP with biological removal of nitrogen and phosphorus.     

Behavior of polymeric substrates in an aerobic granular sludge system

Particulate and slowly biodegradable substrates form an important fraction of industrial wastewater and sewage. To study the influence of suspended solids and colloidal substrate on the morphology and performance of aerobic granular sludge, suspended and soluble starch was used as a model substrate. Degradation was studied using microscopy, micro-electrode measurements, batch experiments and long term laboratory scale reactor operation. Starch was removed by adsorption at the granule surface, followed by hydrolysis and consumption of the hydrolyzed products. Aerobic granules could be maintained on starch as sole influent carbon source, but their structure was filamentous and irregular. It is hypothesized that this is related to the low starch hydrolysis rates, leading to available substrate during the aeration period (extended feast period) and resulting in increased substrate gradients over the granules. The latter induces a less uniform granule development. Starch adsorbed and was consumed at the granule surface instead of being accumulated inside the granules as occurs for soluble substrates. Therefore the simultaneous denitrification efficiencies remained low. Moreover, many protozoa and metazoans were observed in laboratory reactors as well as in pilot- and full-scale Nereda^® reactors, indicating an important role in the removal of suspended solids too.     

Role of mass transfer in overall substrate removal rate in a sequential aerobic sludge blanket reactor treating a non-inhibitory substrate

A laboratory study was undertaken to explore the role of mass transfer in overall substrate removal rate and the subsequent kinetic behavior in a glucose-fed sequential aerobic sludge blanket (SASB) reactor. At the organic loading rates (OLRs) of 2-8 kg chemical oxygen demand (COD)/m³-d, the SASB reactor removed over 98% of COD from wastewater. With an increase in OLR, the average granule diameter (d_p = 1.1-1.9 mm) and the specific oxygen utilization rate increased; whereas biomass density of granules and solids retention time decreased (13-32 d). The intrinsic and apparent kinetic parameters were evaluated using break-up and intact granules, respectively. The calculated COD removal efficiencies using the kinetic model (incorporating intrinsic kinetics) and empirical model (incorporating apparent kinetics) agreed well with the experimental results, implying that both models can properly describe the overall substrate removal rate in the SASB reactor. By applying the validated kinetic model, the calculated mass transfer parameter values and the simulated substrate concentration profiles in the granule showed that the overall substrate removal rate is intra-granular diffusion controlled. By varying different d_p within a range of 0.1-3.5 mm, the simulated COD removal efficiencies disclosed that the optimal granular size could be no greater than 2.5 mm.     

Characterization of sulfate-reducing granular sludge in the SANI process

Hong Kong practices seawater toilet flushing covering 80% of the population. A sulfur cycle-based biological nitrogen removal process, the Sulfate reduction, Autotrophic denitrification and Nitrification Integrated (SANI^®) process, had been developed to close the loop between the hybrid water supply and saline sewage treatment. To enhance this novel process, granulation of a Sulfate-Reducing Up-flow Sludge Bed (SRUSB) reactor has recently been conducted for organic removal and provision of electron donors (sulfide) for subsequent autotrophic denitrification, with a view to minimizing footprint and maximizing operation resilience. This further study was focused on the biological and physicochemical characteristics of the granular sulfate-reducing sludge. A lab-scale SRUSB reactor seeded with anaerobic digester sludge was operated with synthetic saline sewage for 368 days. At 1 h nominal hydraulic retention time (HRT) and 6.4 kg COD/m³-d organic loading rate, the SRUSB reactor achieved 90% COD and 75% sulfate removal efficiencies. Granular sludge was observed within 30 days, and became stable after 4 months of operation with diameters of 400–500 μm, SVI₅ of 30 ml/g, and extracellular polymeric substances of 23 mg carbohydrate/g VSS. Fluorescence in situ hybridization (FISH) analysis revealed that the granules were enriched with abundant sulfate-reducing bacteria (SRB) as compared with the seeding sludge. Pyrosequencing analysis of the 16S rRNA gene in the sulfate-reducing granules on day 90 indicated that the microbial community consisted of a diverse SRB genera, namely Desulfobulbus (18.1%), Desulfobacter (13.6%), Desulfomicrobium (5.6%), Desulfosarcina (0.73%) and Desulfovibrio (0.6%), accounting for 38.6% of total operational taxonomic units at genera level, with no methanogens detected. The microbial population and physicochemical properties of the granules well explained the excellent performance of the granular SRUSB reactor.     

Biodegradation of the endogenous residue of activated sludge in a membrane bioreactor with continuous or on-off aeration

The goal of this study was to determine the effect of a long sludge retention time on the biodegradation of the endogenous residue in membrane digestion units receiving a daily feed of sludge and operated under either aerobic or intermittently aerated (22 h off-2 h on) conditions. The mixed liquor for these experiments was generated in a 10.4 day sludge retention time membrane bioreactor fed with a synthetic and completely biodegradable influent with acetate as the sole carbon source. It had uniform characteristics and consisted of only two components, heterotrophic biomass X_H and endogenous residue X_E. Membrane digestion unit experiments were conducted for 80 days without any sludge wastage except for some sampling. The dynamic behaviour of generation and consumption of filtered organic digestion products was characterized in the membrane digestion unit systems using three pore filter sizes. Results from this investigation indicated that the colloidal matter with size between 0.04 μm and 0.45 μm was shown to contain a recalcitrant fraction possibly composed of polysaccharides bound to proteins which accumulated in the membrane digestion unit under both conditions. Modelling the membrane digestion unit results by considering a first-order decay of the endogenous residue allowed to determine values of the endogenous residue decay rate of 0.0065 and 0.0072 d⁻¹ under fully aerobic and intermittently aerated conditions, respectively. The effect of temperature on the endogenous decay rate was assessed for the intermittently aerated conditions in batch tests using thickened sludge from tests gave an endogenous decay rate constant of 0.0075 d⁻¹ at 20 °C and an Arrhenius temperature correction factor of 1.033.     

The activated sludge process—IV: Application of the general kinetic model to anoxic-aerobic digestion of waste activated sludge

This paper discusses the application of the general activated sludge model as set out by Dold et al. (Prog. Wat. Technol.12, 47–77, 1980) and extended by Van Haandel et al. (Wat. Res.15, 1135–1152, 1981), to anoxic-aerobic digestion of waste activated sludge. The laboratory scale experimental investigation comprised a 6 day sludge age activated sludge process, the waste sludge from which was fed to a number of digesters operated as follows: single reactor flow-through digesters at 4 or 10 days sludge age (retention times) under aerobic or anoxic-aerobic conditions (with 1.5 and 4 h cycle times) and 3-in-series flow-through aerobic digesters each with 4 days sludge age; all digesters were fed draw-and-fill wise once per day. The general kinetic model simulated accurately all the experimental data without the need to change the values of the kinetic constants. Both theoretical simulations and experimental data indicate that (i) the rate of volatile solids destruction is not affected by the incorporation of anoxic cycles and (ii) the specific denitrification rate constant in a digester is about two-thirds of that in the secondary anoxic reactor of the single sludge activated sludge system; this allows definition of a fourth denitrification rate constant K₄ for the anoxic-aerobic digester with K₄T = 0.046(1.029)^(T-20) mg(NO₃-N) (mgAVSS d)⁻¹, a constant independent of sludge age. An important consequence of (i) and (ii) above is that the denitrification can be integrated readily into the steady state digester model of Marais and Ekama (Wat. SA2, 163–200, 1976) and used for design purposes.     

Low-temperature (7 °C) anaerobic treatment of a trichloroethylene-contaminated wastewater: microbial community development

The feasibility of low-temperature (7 °C) anaerobic digestion for the treatment of a trichloroethylene (TCE) contaminated wastewater was investigated. Two expanded granular sludge bed (EGSB) bioreactors (R1 and R2) were employed for the mineralisation of a synthetic volatile fatty acid based wastewater at an initial organic loading rate (OLR) of 3 kg COD m⁻³ d⁻¹, and an operating temperature of 15 °C. Successive reductions in OLR to 0.75 kg COD m⁻³ d⁻¹, and operational temperature to 7 °C, resulted in stable bioreactor operation by day 417, with COD removal efficiency and biogas CH₄ content ≥74%, for both bioreactors. Subsequently, the influent to R1 was supplemented with increasing concentrations (10, 20, 30 mg l⁻¹) of TCE, while R2 acted as a control. At an influent TCE concentration of 30 mg l⁻¹, although phase average TCE removal rates of 79% were recorded, a sustained decrease in R1 performance was observed, with COD removal of 6%, and % biogas CH₄ of 3% recorded on days 595 and 607, respectively. Specific methanogenic activity (SMA) assays identified a general shift from acetate- to hydrogen-mediated methanogenesis in both R1 and R2 biomass, while toxicity assays confirmed an increased sensitivity of the acetoclastic community in R1 to TCE and dichloroethylene (DCE), which contributed to acetate accumulation. Quantitative Polymerase Chain Reaction (qPCR) analysis of the methanogenic community confirmed the dominance of hydrogenotrophic methanogens in both R1 and R2, representing 71-89% of the total methanogenic population, however acetoclastic Methanosaeta were the dominant organisms, based on 16S rRNA gene clone library analysis of reactor biomass. The greatest change in the bacterial community, as demonstrated by UPGMA analysis of DGGE banding profiles, was observed in R1 biomass between days 417 and 609, although 88% similarity was retained between these sampling points.     

Selective sludge discharge as the determining factor in SBR aerobic granulation: Numerical modelling and experimental verification

Numerical simulation and laboratory experiments were conducted to investigate the determining factor and the underlying mechanism in aerobic sludge granulation in a sequencing batch reactor (SBR). In the numerical simulation, a sectional approach was used to develop a model to describe the biomass dynamics during the granulation process. The growth of different classes of the SBR sludge with different substrate uptake rates and different sludge discharge ratios was simulated. The results indicate that the selective discharge of slow-settling sludge flocs is the key determining factor for granulation. In the laboratory study, experiments were conducted with two identical 2.4-L SBRs, R1 and R2, using different sludge discharge methods - the selective discharge of slow-settling sludge flocs for R1, and mixed, unselective sludge discharge for R2. The SBRs were fed with glucose-based synthetic wastewater at a chemical oxygen demand (COD) loading rate of 1.5 kg/m³-d. The evolution of the microbial community during the experimental process was monitored using the molecular techniques of polymerase chain reaction (PCR), denaturing gradient gel electrophoresis (DGGE) and clone library analysis. Sludge granulation was achieved in less than three weeks in R1, whereas the sludge in R2 remained in the form of flocs. However, some bacterial species had a significant presence in both the R1 granules and the R2 flocs. The results suggest that aerobic granulation may not require the dominance of any particular species. Small and loose sludge flocs were found to have an advantage over larger and dense granules in substrate uptake. Thus, discharge of loose flocs would remove these competitors from the system and makes the substrate more available for uptake and utilisation by biomass in the attached-growth form, resulting in sludge granulation.