Characteristics of sludge developed under different loading conditions during UASB reactor start-up and granulation

Sludge characteristics available inside the reactor are of vital importance to maximize advantages of UASB reactor. The organic loading rate and sludge loading rate applied during start-up are among the important parameters to govern the sludge characteristics. Effects of these loading rates on the characteristics of the sludge developed are evaluated in six laboratory scale UASB reactors. The sludge characteristics considered are VSS/SS ratio of the sludge, sludge volume index, specific gravity, settling velocity and metal contents of the sludge developed under different loading rates. The experimental results indicate that, for developing good characteristics sludge, during primary start-up from flocculent inoculum sludge, organic loading rate and sludge loading rate should be in the range of 2.0-4.5 kg COD/m3 d and 0.1-0.25 kg COD/kg VSS d, respectively (chemical oxygen demand, COD). Proper sludge granulation and higher COD removal efficiency will be achieved by these loading rates.     

Anoxic treatment of phenolic wastewater in sequencing batch reactor

Studies were conducted on the anoxic phenol removal using granular denitrifying sludge in sequencing batch reactor at different cycle lengths and influent phenol concentrations. Results showed that removal exceeded 80% up to an influent phenol concentration of 1050 mg/l at 6 h cycle length, which corresponded to 6.4 kg COD/m3/d. Beyond this, there was a steep decrease in phenol and COD removal efficiencies. This was accompanied by an increase in nitrite concentration in the effluent. On an average, 1 g nitrate-N was consumed per 3.4 g phenol COD removal. Fraction of COD available for sludge growth was calculated to be 11%.     

Influence of operational parameters on nitrogen removal efficiency and microbial communities in a full-scale activated sludge process

To improve the efficiency of total nitrogen (TN) removal, solid retention time (SRT) and internal recycling ratio controls were selected as operating parameters in a full-scale activated sludge process treating high strength industrial wastewater. Increased biomass concentration via SRT control enhanced TN removal. Also, decreasing the internal recycling ratio restored the nitrification process, which had been inhibited by phenol shock loading. Therefore, physiological alteration of the bacterial populations by application of specific operational strategies may stabilize the activated sludge process. Additionally, two dominant ammonia oxidizing bacteria (AOB) populations, Nitrosomonas europaea and Nitrosomonas nitrosa, were observed in all samples with no change in the community composition of AOB. In a nitrification tank, it was observed that the Nitrobacter populations consistently exceeded those of the Nitrospira within the nitrite oxidizing bacteria (NOB) community. Through using quantitative real-time PCR (qPCR), nirS, the nitrite reducing functional gene, was observed to predominate in the activated sludge of an anoxic tank, whereas there was the least amount of the narG gene, the nitrate reducing functional gene.     

Effects of free cyanide on microbial communities and biological carbon and nitrogen removal performance in the industrial activated sludge process

The changes in process performance and microbial communities under free cyanide (CN⁻) were investigated in a lab-scale activated sludge process treating industrial wastewater. The performance of phenol degradation did not appear to be adversely affected by increases in CN⁻ concentrations. In contrast, CN⁻ was found to have an inhibitory effect on SCN⁻ biodegradation, resulting in the increase of TOC and COD concentrations. Nitratation also appeared to be inhibited at CN⁻ concentrations in excess of 1.0 mg/L, confirming that nitrite-oxidizing bacteria (NOB) is more sensitive to the CN⁻ toxicity than ammonia oxidizing bacteria (AOB). After CN⁻ loads were stopped, SCN⁻ removal, denitrification, and nitrification inhibited by CN⁻ were recovered to performance efficiency of more than 98%. The AOB and NOB communities in the aerobic reactor were analyzed by terminal restriction fragment length (T-RFLP) and quantitative real-time PCR (qPCR). Nitrosomonas europaea lineage was the predominant AOB at all samples during the operation, but an obvious change was observed in the diversity of AOB at the shock loading of 30 and 50 mg/L CN⁻, resulting in Nitrosospira sp. becoming dominant. We also observed coexisting Nitrospira and Nitrobacter genera for NOB. The increase of CN⁻ loading seemed to change the balance between Nitrospira and Nitrobacter, resulting in the high dominance of Nitrobacter over Nitrospira. Meanwhile, through using the qPCR, it was observed that the nitrite-reducing functional genes (i.e., nirS) were dominant in the activated sludge of the anoxic reactor, regardless of CN⁻ loads.     

Effect of temperature and disinfection strategies on ammonia-oxidizing bacteria in a bench-scale drinking water distribution system

The establishment of ammonia-oxidizing bacteria (AOB), a group of autotrophic microorganisms responsible for nitrification in chloraminated distribution systems, was studied in a bench-scale distribution system. The potential significance of temperature and disinfectant residual associated with chloramination in full-scale drinking water distribution systems was assessed. Biofilm development was primarily monitored using AOB abundance and nitrite concentrations. The bench-scale system was initially operated under typical North American summer (22 degrees C) and fall (12 degrees C) temperatures, representing optimal and less optimal growth ranges for these microorganisms. Additional experimentation investigated AOB establishment at a suboptimal winter distribution system temperature of 6 degrees C. The effect of chloramine residual on AOB establishment was studied at higher (0.2-0.6mg/L) and lower (0.05-0.1mg/L) ranges, using a 3:1 (w/w) chlorine:ammonia dosing ratio. Conditions were selected to represent those typically found in a North American distribution system, in areas of low flow and longer retention times, respectively. Finally, the effect of a free chlorine residual on an established nitrifying biofilm was briefly examined. Results clearly indicate that AOB development occurs at all examined temperatures, as well as at selected monochloramine residuals. The maintenance of a disinfectant residual was difficult at times, but was more inhibitory to the nitrifying biofilm than the lower temperature. It can be concluded from the data that nitrification may not be adequately inhibited during the winter months, which may result in more advanced stages of nitrification the following season. Free chlorination can be effective in controlling AOB activity in the short term, but may not prevent reestablishment of a nitrifying biofilm upon return to chloramination.     

Effect of solids retention time on structure and characteristics of sludge flocs in sequencing batch reactors

The effect of solids retention time (SRT) (4-20 d) on sludge floc structure, size distribution and morphology in laboratory-scale sequencing batch reactors receiving a glucose-based synthetic wastewater was studied using image analysis in a long-term experiment over one year. Floc size distribution (>10 microm) could be characterized by a log-normal model for no bulking situations, but a bi-modal distribution of floc size was observed for modest bulking situations. In each operating cycle of the SBRs, the variation in food /microorganisms ratio (0.03-1.0) had no significant influence on floc size distribution and morphology. The results from a long-term study over one year showed that no clear relationship existed between SRT and median floc size based on frequency. However, sludge flocs at the lower SRTs (4-9 d) were much more irregular and more variable in size with time than those at higher SRTs (16 and 20 d). The level of effluent-suspended solids at lower SRTs was higher than that at higher SRTs.     

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.     

Rheological and fractal characteristics of granular sludge in an upflow anaerobic reactor

The rheological and fractal characteristics of the granular sludge in an upflow anaerobic sludge blanket (UASB) reactor were investigated in this study. The influences of sludge concentration and temperature on the rheological properties of the granular sludge were evaluated, and the Bingham model was adopted to describe its rheology. In addition, image analysis was used to determine the sludge fractal dimension. The results indicate that the UASB granular sludge showed a shear-thinning behavior. The relationships between the limiting viscosity and the sludge concentration, as well as the limiting viscosity and temperature could be respectively modeled using an exponential equation and Arrhenius equation well. The Bingham model was able to adequately describe the rheology of the granular sludge. The fractal dimension of the granular sludge, 2.79+/-0.03, was larger than that of some other aggregates, suggesting that the granular sludge were more compact and denser. Furthermore, the relationship between rheological and fractal properties of the granular sludge could be properly described with the model proposed by Shih et al. [1990. Scaling behavior of the elastic properties of colloidal. Phys. Rev. A 42, 4772-4779].     

Characterizing the extracellular and intracellular fluorescent products of activated sludge in a sequencing batch reactor

Three-dimensional excitation-emission-matrix (EEM) fluorescence spectrometry was used to characterize the extracellular and intracellular substances of activated sludge in a sequencing batch reactor (SBR). Parallel factor analysis (PARAFAC) was applied to extract the pure spectra from the overlapped spectra. Three main components, proteins, fulvic- and humic-like substances, were identified from the extracellular substances. Their fluorescence peaks were at an excitation/emission (Ex/Em) of 280/350, 340/400 and 390/450 nm, respectively. The fluorescence of the extracellular proteins had a similar changing pattern with the wastewater chemical oxygen demand, the fulvic-like substance did not vary significantly in a cycle and the humic-like substances accumulated in the substrate uptake phase but decreased later. Proteins and nicotinamide adenine dinucleotide, reduced form (NADH), were identified as the two main intracellular fluorophores, and their fluorescence peaks (Ex/Em) were at 280/340 and 350/450 nm, respectively. The fluorescence intensity scores of the intracellular fluorophores were closely related to the bioreactor performance. Thus, the results of this work provide a foundation for potential utilization of the EEM fluorescence spectroscopy to monitor the activated sludge systems for wastewater treatment.     

Formation of aerobic granules and conversion processes in an aerobic granular sludge reactor at moderate and low temperatures

Temperature changes can influence biological processes considerably. To investigate the effect of temperature changes on the conversion processes and the stability of aerobic granular sludge, an aerobic granular sludge sequencing batch reactor (GSBR) was exposed to short-term and long-term temperature changes. Start-up at 8 degrees C resulted in irregular granules that aggregated as soon as aeration was stopped, which caused severe biomass washout and instable operation. The presence of COD during the aerobic phase is considered to be the major reason for this granule instability. Start-up at 20 degrees C and lowering the temperature to 15 degrees C and 8 degrees C did not have any effect on granule stability and biomass could be easily retained in the system. The temperature dependency of nitrification was lower for aerobic granules than usually found for activated sludge. Due to decreased activity in the outer layers of granules at lower temperatures, the oxygen penetration depth could increase, which resulted in a larger aerobic biomass volume, compensating the decreased activity of individual organisms. Consequently the denitrifying capacity of the granules decreased at reduced temperatures, resulting in an overall poorer nitrogen removal capacity. The overall conclusion that can be drawn from the experiments at low temperatures is that start-up in practice should take place preferentially during warm summer periods, while decreased temperatures during winter periods should not be a problem for granule stability and COD and phosphate removal in a granular sludge system. Nitrogen removal efficiencies should be optimized by changes in reactor operation or cycle time during this season.     

Occurrence of nitrifiers and diversity of ammonia-oxidizing bacteria in developing drinking water biofilms

We studied the population dynamics of nitrifying bacteria during the development of biofilms up to 233 or 280 days on polyvinylchloride pipes connected to two full-scale drinking water distribution networks supplying processed and chloraminated surface water. The numbers of nitrifiers in biofilms were enumerated at intervals of 10–64 days by the most probable number (MPN) method at waterworks and at several study sites in distribution network areas. The numbers of nitrifiers increased towards the distal sites. The highest detected MPN counts of ammonia-oxidizing bacteria (AOB) for study areas 1 and 7 were 500 MPN cm⁻² and 1.0×10⁶ MPN cm⁻², and those of nitrite-oxidizing bacteria (NOB) 96 MPN cm⁻² and 2.2×10³ MPN cm⁻², respectively. The diversity of AOB was determined by PCR amplifying, cloning and sequencing the partial ammonia monooxygenase (amoA) gene of selected biofilm samples presenting different biofilm ages. The PCR primers used, A189 and A682, also amplified a fragment of particulate methane monooxygenase (pmoA) gene of methane-oxidizing bacteria. The majority of biofilm clones (24 out of 30 studied) contained Nitrosomonas amoA-like sequences. There were only two pmoA-like sequences of Type I methanotrophs, and four sequences positioned in amoA/pmoA sequence groups of uncultured bacteria. From both study area very similar or even completely identical Nitrosomonas amoA-like sequences were obtained despite of high difference in AOB numbers. The results show that the conditions in newly formed biofilms in drinking water distribution systems favor the growth of Nitrosomonas-type AOB.     

Combining culturing and 16S rDNA sequencing to reveal seasonal and room variations of household airborne bacteria and correlative environmental factors in nanjing,southeast china

Exposure to bioaerosols poses important health effects on occupants. To elucidate seasonal and room variations of household airborne bacteria, this study investigated 30 residential homes during summer and winter throughout Nanjing, Southeast China, with a humid subtropical climate. Culturing and 16S rDNA sequencing methods were combined in this study. Results showed that the community structure and composition in the same season but different homes show similarity, however, they in the same home but in different seasons show a huge difference, with Sphingomonas (25.3%), Clostridium (14.8%), and Pseudomonas (7.6%) being the dominant bacteria in summer, and Pseudomonas (57.1%) was dominant bacteria in winter. Culturable concentrations of bacteria were also significantly higher in summer (854 ± 425 CFU/m³) than in winter (231 ± 175 CFU/m³), but difference by home or room was relatively minor. More than 80% of culturable bacteria (<4.7 μm) could penetrate into lower respiratory tract. The seasonal variations of bacterial community and concentrations were closely associated with seasonal variations of temperature, humidity, and PM_2.5. Higher concentrations and larger sizes were observed in the bathroom and kitchen, typically with higher humidity than other rooms.     

The contribution of biotic and abiotic processes during azo dye reduction in anaerobic sludge

Azo dye reduction results from a combination of biotic and abiotic processes during the anaerobic treatment of dye containing effluents. Biotic processes are due to enzymatic reactions whereas the chemical reaction is due to sulfide. In this research, the relative impact of the different azo dye reduction mechanisms was determined by investigating the reduction of Acid Orange 7 (AO7) and Reactive Red 2 (RR2) under different conditions. Reduction rates of two azo dyes were compared in batch assays over a range of sulphide concentrations in the presence of living or inactivated anaerobic granular sludge. Biological dye reduction followed zero order kinetics and chemical dye reduction followed second-order rate kinetics as a function of sulfide and dye concentration. Chemical reduction of the dyes was greatly stimulated in the presence of autoclaved sludge: whereas chemical dye reduction was not affected by living or gamma-irradiated-sludge. Presumably redox-mediating enzyme cofactors released by cell lysis contributed to the stimulatory effect. This hypothesis was confirmed in assays evaluating the chemical reduction of AO7 utilizing riboflavin, representative of the heat stable redox-mediating moieties of common occurring flavin enzyme cofactors. Sulfate influenced dye reduction in accordance to biogenic sulfide formation from sulfate reduction. In assays lacking sulfur compounds, dye reduction only readily occurred in the presence of living granular sludge, demonstrating the importance of enzymatic mechanisms. Both chemical and biological mechanisms of dye reduction were greatly stimulated by the addition of the redox-mediating compound, anthraquinone-disulfonate. Based on an analysis of the kinetics and demonstration in lab-scale upward-flow anaerobic sludge bed reactors, the relative importance of chemical dye reduction mechanisms in high rate anaerobic bioreactors was shown to be small due to the high biomass levels in the reactors.     

Evaluation on the impacts of predators on biomass components and oxygen uptake in sequencing batch reactor and continuous systems

An expanded unified model for the biomass fractions, soluble-organic fractions, and oxygen-uptake rates considering extracellular polymeric substances (EPS), intracellular storage products (X_STO), and predators for activated sludge is used to study the impacts of predators on biomass components and oxygen uptake. The new model is applied to evaluate how predation affects the oxygen-uptake rate (OUR) and the different forms of biomass: active bacteria (X_H), X_EPS, and X_STO, under dynamic feast-and-famine and continuous conditions. For the dynamic conditions of a sequencing batch reactor (SBR), eliminating predators from the model increases X_H and X_EPS fractions significantly, and this causes the substantial increases in OUR and MLVSS once the famine period begins. An analysis of how the OUR is distributed among the several respiration processes shows that the predation of X_H is the most significant oxygen utilization rate process in the system under famine conditions of an SBR. Application of the model to simulate the long-term operation of an SBR indicates that predators reach their maximum fraction in the MLVSS (∼4% of MLVSS) at a solids retention time of about 13 days, but they are washed out at a solids retention time less than ∼3 days. Simulation for a continuous system indicates that predators take more time (about 800 h) to reach steady state and reach their maximum fraction (∼5.5%) at an SRT of ∼14 days. Comparison of SBR and continuous systems reveals that the predators have greater impact in the continuous system because the permanent near-famine condition accentuates predation processes.     

Identification of trigger factors selecting for polyphosphate- and glycogen-accumulating organisms in aerobic granular sludge sequencing batch reactors

Nutrient removal performances of sequencing batch reactors using granular sludge for intensified biological wastewater treatment rely on optimal underlying microbial selection. Trigger factors of bacterial selection and nutrient removal were investigated in these novel biofilm systems with specific emphasis on polyphosphate- (PAO) and glycogen-accumulating organisms (GAO) mainly affiliated with Accumulibacter and Competibacter, respectively. In a first dynamic reactor operated with stepwise changes in concentration and ratio of acetate and propionate (Ac/Pr) under anaerobic feeding and aerobic starvation conditions and without wasting sludge periodically, propionate favorably selected for Accumulibacter (35% relative abundance) and stable production of granular biomass. A Plackett-Burman multifactorial experimental design was then used to screen in eight runs of 50 days at stable sludge retention time of 15 days for the main effects of COD concentration, Ac/Pr ratio, COD/P ratio, pH, temperature, and redox conditions during starvation. At 95% confidence level, pH was mainly triggering direct Accumulibacter selection and nutrient removal. The overall PAO/GAO competition in granular sludge was statistically equally impacted by pH, temperature, and redox factors. High Accumulibacter abundances (30–47%), PAO/GAO ratios (2.8–8.4), and phosphorus removal (80–100%) were selected by slightly alkaline (pH > 7.3) and lower mesophilic (<20 °C) conditions, and under full aeration during fixed 2-h starvation. Nitrogen removal by nitrification and denitrification (84–97%) was positively correlated to pH and temperature. In addition to alkalinity, non-limited organic conditions, 3-carbon propionate substrate, sludge age control, and phase length adaptation under alternating aerobic-anoxic conditions during starvation can lead to efficient nutrient-removing granular sludge biofilm systems.     

Microbial community distribution and activity dynamics of granular biomass in a CANON reactor

The application of microelectrodes to measure oxygen and nitrite concentrations inside granules operated at 20 °C in a CANON (Complete Autotrophic Nitrogen-removal Over Nitrite) reactor and the application of the FISH (Fluorescent In Situ Hybridization) technique to cryosectioned slices of these granules showed the presence of two differentiated zones inside of them: an external nitrification zone and an internal anammox zone. The FISH analysis of these layers allowed the identification of Nitrosomonas spp. and Candidatus Kuenenia Stutgartiensis as the main populations carrying out aerobic and anaerobic ammonia oxidation, respectively.Concentration microprofiles measured at different oxygen concentrations in the bulk liquid (from 1.5 to 35.2 mg O₂ L⁻¹) revealed that oxygen was consumed in a surface layer of 100-350 μm width. The obtained consumption rate of the most active layers was of 80 g O₂ (L_granule)⁻¹ d⁻¹. Anammox activity was registered between 400 and 1000 μm depth inside the granules. The nitrogen removal capacity of the studied sequencing batch reactor containing the granular biomass was of 0.5 g N L⁻¹ d⁻¹. This value is similar to the mean nitrogen removal rate obtained from calculations based on in- and outflow concentrations.Information obtained in the present work allowed the establishment of a simple control strategy based on the measurements of NH₄⁺ and NO₂⁻ in the bulk liquid and acting over the dissolved oxygen concentration in the bulk liquid and the hydraulic retention time of the reactor.     

Effects of hydraulic retention time and proteins on sludge toxicity for 4‐chlorophenol wastewater treatment in sequencing batch reactors

Due to the higher uncertainty of environmental risk for pollutants’ treatment by activated sludge, 10 mg/L influent 4‐chlorophenol (4‐CP) treated via a sequencing batch reactor (SBR) was used as acclimated SBR. Another SBR was used as control SBR without 4‐CP. The effects of hydraulic retention time (HRT) and proteins on sludge toxicity for 4‐CP treatment were analysed, and compared to the control SBR. Results showed that the sludge toxicity in acclimated SBR was significantly higher than that of the control SBR. Shortening HRT from 12 to 8 hours was beneficial to degrade 4‐CP and lower sludge toxicity. The identified highly expressed protein of ABC transporter co‐existed in control and acclimated SBRs, while other proteins of TonB‐dependent receptor, heat shock 70 kDa protein and superoxide dismutase in acclimated sludge were overexpressed relative to the control sludge, which played an important function in degrading 4‐CP, resisting 4‐CP toxicity and eliminating sludge toxicity.     

Growth, maintenance and product formation of autotrophs in activated sludge: taking the nitrite-oxidizing bacteria as an example

The autotrophs in activated sludge play an important role in biological wastewater treatment, especially in the nitrification process. Compared with the heterotrophs in activated sludge, information about the growth, maintenance, and product formation of the autotrophs is still sparse. In this work both experimental and modeling approaches are used to investigate the growth, nitrite inhibition, maintenance, and formation of extracellular polymeric substances (EPS) and soluble microbial products (SMP) of the autotrophs, with nitrite-oxidizing bacteria (NOB) in activated sludge as an example. The unified theory for EPS and SMP is integrated into our model to describe the microbial product formation of the NOB. Extensive experiments were carried out using the NOB-enriched in a sequencing batch reactor for the calibration and validation of the developed model. Results show that the NOB spend a considerable amount of energy on maintenance processes. Their apparent growth yield is estimated to be 0.044 mg COD biomass mg⁻¹ N. The model simulations reveal that the concentrations of EPS and SMP in the NOB-enriched culture initially increase, but later decrease gradually, and that the SMP formed in the nitrite oxidation process are biodegradable.     

The role of extracellular polymeric substances in the toxicity response of activated sludge bacteria to chemical toxins

The objective of this study was to evaluate the respiration inhibition induced by octanol, cadmium, N-ethylmaleimide (NEM) and cyanide on activated sludge biomasses with different floc structures but similar physiological characteristics. Mechanical shearing was applied to fresh mixed liquor to produce biomasses with different floc structure properties. Specific oxygen uptake rate assays were conducted on the sheared and unsheared mixed liquors. The results showed that mechanical shearing resulted in release of biopolymers from the floc extracellular polymeric substances (EPS) matrix into the bulk liquid and a simultaneous reduction in floc size. Shearing did not impact biomass viability. The respiration inhibition by octanol and cadmium was more severe in sheared mixed liquor than in the unsheared biomass. Conversely, the respiration inhibition induced by NEM and cyanide was similar for the different mixed liquors tested. We conclude that the EPS matrix functions as a protective barrier for the bacteria inside activated sludge flocs to chemicals that it has the potential to interact with, such as hydrophobic (octanol) and positively charged (cadmium) compounds, but that the toxicity response for soluble, hydrophilic toxins (NEM and cyanide) is not influenced by the presence of the polymer matrix.