Long-term effect of weak nitrogen limitation on polyhydroxyalkanoates production of propionate-fed activated sludge operated at long sludge retention time

A sequencing batch reactor (SBR) submitted to aerobic dynamic feeding condition was fed with weakly nitrogen-limited substrate (COD/N ratio of 60?mg/mg, propionate as the sole carbon source), and operated at 15?days sludge retention time (SRT). Then, the nitrogen-limited sludges at different cultivating periods (45th, 120th, 195th and 240th days) were harvested to conduct aerobic batch experiments of PHAs production under various nitrogen concentrations. Experimental results indicated that PHAs accumulating behavior of the propionate-fed sludge declined significantly after long-term cultivation, implying that weak nitrogen limitation and long SRT operation cannot achieve a stable PHAs producing performance of the sludge. Supplementing enough nitrogen was consequently a requisite to guarantee the stability of the propionate-fed PHAs accumulating culture. Besides, the sludge with higher PHAs content in SBR did not ensure a better PHAs producing capability in the batch PHAs production phase. Batch experiments also indicated that the propionate-fed sludge with 0.7?N?mmol/L exhibited a better PHAs producing capability than with other nitrogen concentrations.     

Influence of sludge retention time on tolerance of copper toxicity for polyphosphate accumulating organisms linked to polyhydroxyalkanoates metabolism and phosphate removal

This study explored the influence of sludge retention time (SRT) on tolerance of copper invasion for polyphosphate accumulating organisms (PAOs) in an enhanced biological phosphorus removal (EBPR). The experimental data showed the anaerobic polyhydroxyalkanoates (PHA) storage for the sludge at 10d SRT was less influenced by copper invasion than those at 5d and 15d SRTs. The reaction of PAOs aerobically taking up phosphate for the sludge at 5d or 15d SRT almost ceased at 2 mg Cu L⁻¹, whereas PAOs in the sludge at 10d SRT retained half of the ability to take up phosphate. Both the PHAs degradation and synthesis rates decreased with increasing copper concentration, regardless of the SRTs. However, the copper inhibition of the former was greater than that of the later.     

Low temperature thermo-chemical pretreatment of dairy waste activated sludge for anaerobic digestion process

An investigation into the influence of low temperature thermo-chemical pretreatment on sludge reduction in a semi-continuous anaerobic reactor was performed. Firstly, effect of sludge pretreatment was evaluated by COD solubilization, suspended solids reduction and biogas production. At optimized condition (60 °C with pH 12), COD solubilization, suspended solids, reduction and biogas production was 23%, 22% and 51% higher than the control, respectively. Secondly, semi-continuous process performance was studied in a lab-scale semi-continuous anaerobic reactor (5 L), with 4 L working volume. With three operated SRTs, the SRT of 15 days was found to be most appropriate for economic operation of the reactor. Combining pretreatment with anaerobic digestion led to 80.5%, 117% and 90.4% of TS, SS and VS reduction respectively, with an improvement of 103% in biogas production. Thus, low temperature thermo-chemical can play an important role in reducing sludge production.     

Dry-thermophilic anaerobic digestion of simulated organic fraction of Municipal Solid Waste: Process modeling

Solid retention time (SRT) is a very important operational variable in continuous and semicontinuous waste treatment processes since the organic matter removal efficiency - expressed in terms of percentage of Dissolved Organic Carbon (% DOC) or Volatile Solids (% VS) removed - and the biogas or methane production are closely related with the SRT imposed. Optimum SRT is depending on the waste characteristics and the microorganisms involved in the process and, hence, it should be determined specifically in each case.In this work a series of experiments were carried out to determine the effect of SRT, from 40 to 8 days, on the performance of the dry (30% Total Solids) thermophilic (55 °C) anaerobic digestion of organic fraction of Municipal Solid Wastes (OFMSW) operating at semicontinuous regime of feeding.The experimental results show than 15 days is the optimum SRT (the best between all proved) for this process. Besides, data of organic matter concentration and methane production versus SRT have been used to obtain the kinetic parameters of the kinetic model of Romero García (1991): the maximum specific growth rate of the microorganisms (μ_max = 0.580 days⁻¹) and the fraction of substrate non-biodegradable (α = 0.268).     

Fractionation of proteins and carbohydrates of extracellular polymeric substances in a membrane bioreactor system

The major operational problem associated with membrane bioreactors (MBR) is membrane fouling, for which extracellular polymeric substances (EPS) are primarily responsible. In this work both the soluble and bound EPS (i.e. SMP and EPS) produced in an MBR system operating under sludge retention times (SRT) of 10, 15, 20 and 33 days were fractionized by means of membranes having variable molecular weight cutoffs (300 kDa, 100 kDa, 10 kDa & 1 kDa). The results show that increasing the SRT leads to a reduction of SMP and EPS and that these reductions are more pronounced for the SRTs in the range 10–20 days. This reduction is more significant for carbohydrates than for proteins. The decrease of SMP and EPS with increasing SRT from 10 to 20 days led to a significant decrease of the level of fouling. The further increase of SRT to 33 days did not significantly impact on the level of fouling as the SMP and EPS concentrations did not change much.     

An integrated UASB-sludge digester system for raw domestic wastewater treatment in temperate climates

To improve the performance of an upflow anaerobic sludge blanket (UASB) reactor treating raw domestic wastewater under temperate climates conditions, the addition of a sludge digester to the process was investigated. With the decrease in temperature, the COD removal decreased from 78% at 28 °C to 42% at 10 °C for the UASB reactor operating alone at a hydraulic retention time of 6 h. The decrease was attributed to low hydrolytic activity at lower temperatures that reduced suspended matter degradation and resulted in solids accumulation in the top of the sludge blanket. Solids removed from the upper part of the UASB sludge were treated in an anaerobic digester. Based on sludge degradation kinetics at 30 °C, a digester of 0.66 l per liter of UASB reactor was design operating at a 3.20 days retention time. Methane produced by the sludge digester is sufficient to maintain the temperature at 30 °C.     

Improvement strategy on enhanced biological phosphorus removal for municipal wastewater treatment plants: full-scale operating parameters, sludge activities, and microbial features

The poor quality of effluent discharged by municipal wastewater treatment plants (WWTPs) is threatening the safety of water ecology. This study, which integrated a field survey, batch tests, and microbial community identification, was designed to improve the effectiveness of the enhanced biological phosphorus removal (EBPR) process for WWTPs. Over two-thirds of the investigated WWTPs could not achieve total P in effluent lower than 0.5 mg/L, mainly due to the high ratio of chemical oxygen demand to P (28.6-196.2) in the influent. The rates of anaerobic P release and aerobic P uptake for the activated sludge varied from 0.22 to 7.9 mg/g VSS/h and 0.43 to 8.11 mg/g VSS/h, respectively. The fraction of Accumulibacter (PAOs: polyphosphate accumulating organisms) was 4.8 ± 2.0% of the total biomass, while Competibacter (GAOs: glycogen-accumulating organisms) accounted for 4.8 ± 6.4%. The anaerobic P-release rate was found to be an effective indicator of EBPR. Four classifications of the principal components were identified to improve the EBPR effluent quality and sludge activity.     

An evaluation of aerobic and anaerobic sludges as start-up material for microbial fuel cell systems

The operation of microbial fuel cells (MFCs) seeded with the same quantities of aerobic or anaerobic sludge has been compared. The two sludges consisted of mixed cultures obtained from the aerobic reactor and anaerobic digester, respectively, of a municipal Wastewater Treatment Plant (WWTP). Both the sludges were diluted with their sedimentation supernatant to avoid modifying their metabolism. The results show that the type of sludge has a major impact on the performance of the system. Seeding an MFC with anaerobic acclimated sludge leads to a more rapid start-up of electricity production and the absence of a lag period. In the MFC seeded with anaerobic sludge, the steady-state operation conditions were achieved in less than 10 days, while in the aerobic sludge-seeded MFC more than 20 days were necessary to achieve this regime. The anaerobic sludge also led to better performance of the MFC. Thus, maximum power densities above 300mWm(-2) were obtained for such systems (i.e. two times higher than that achieved with the aerobic sludge-seeded MFC in the same setup). This better performance is a direct consequence of the greater availability of Chemical Oxygen Demand (COD) in anaerobic sludge. However, the performance is not a consequence of the coulombic efficiency in the use of the COD to produce electricity because the aerobic sludge-seeded MFC doubles this figure with respect to the anaerobic sludge-seeded system.     

A review and update of the microbiology of enhanced biological phosphorus removal in wastewater treatment plants

Enhanced biological phosphorus removal (EBPR) from wastewater can be more-or-less practically achieved but the microbiological and biochemical components are not completely understood. EBPR involves cycling microbial biomass and influent wastewater through anaerobic and aerobic zones to achieve a selection of microorganisms with high capacity to accumulate polyphosphate intracellularly in the aerobic period. Biochemical or metabolic modelling of the process has been used to explain the types of carbon and phosphorus transformations in sludge biomass. There are essentially two broad-groupings of microorganisms involved in EBPR. They are polyphosphate accumulating organisms (PAOs) and their supposed carbon-competitors called glycogen accumulating organisms (GAOs). The morphological appearance of microorganisms in EBPR sludges has attracted attention. For example, GAOs as tetrad-arranged cocci and clusters of coccobacillus-shaped PAOs have been much commented upon and the use of simple cellular staining methods has contributed to EBPR knowledge. Acinetobacter and other bacteria were regularly isolated in pure culture from EBPR sludges and were initially thought to be PAOs. However, when contemporary molecular microbial ecology methods in concert with detailed process performance data and simple intracellular polymer staining methods were used, a betaproteobacteria called ‘Candidatus Accumulibacter phosphatis’ was confirmed as a PAO and organisms from a novel gammaproteobacteria lineage were GAOs. To preclude making the mistakes of previous researchers, it is recommended that the sludge ‘biography’ be well understood – i.e. details of phenotype (process performance and biochemistry) and microbial community structure should be linked. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dynamic fouling behaviors of submerged nonwoven bioreactor for filtration of activated sludge with different SRT

The flux variations and resistances accumulated during filtration of activated sludge with sludge retention time (SRT) of 15, 30, and 60 days were analyzed to investigate the dynamic fouling behavior in a submerged nonwoven bioreactor. Different SRT values varied sludge condition and particle size distribution in the supernatants, which caused dissimilar fouling characteristics. Short-term fouling of the nonwoven bioreactor during filtration of activated sludge with SRT of 15 days was fully reversible, and the resistance percentages of solutes, colloids, and suspended solids were 6%, 27%, and 67%, respectively. On the other hand, significant increases of colloid resistance, such as with the filtration of activated sludge with SRT of 30 and 60 days, were related to the occurrence of irreversible fouling. The phenomenon of pore blocking by particles or colloids with size analogous to the pore of nonwoven fabric was a decisive factor leading to irreversible fouling in the large-pore materials.     

Effect of increasing the surface area of primary sludge on anaerobic digestion at low temperature

Two sets of reactors were operated at 15 °C and at sludge retention times (SRTs) of 65 days and 30 days, respectively. Each set was operated at six different mixing velocities. Shear forces provided by mixing affected particle size distribution and resulted in different sludge surface areas. The aim was to investigate the effect of increasing primary sludge surface area on anaerobic digestion at low temperature. The maximum surface areas at the applied mixing velocities were 5926 cm²/cm³of sludge and 4672 cm²/cm³ of sludge at SRTs of 65 days and 30 days, respectively. The corresponding calculated methanogenesis were 49% and 15% at SRTs of 65 days and 30 days, respectively. Maximum total solids (TS) reductions were 26% and 11% at 65 days and 30 days SRTs, respectively. Lipase activity increased with increasing SRT and sludge surface area. Results clearly showed that increasing sludge surface area improved sludge digestion at 15 °C.     

Quinone profiling of bacterial communities in natural and synthetic sewage activated sludge for enhanced phosphate removal

Respiratory quinones were used as biomarkers to study bacterial community structures in activated sludge reactors used for enhanced biological phosphate removal (EBPR). We compared the quinone profiles of EBPR sludges and standard sludges, of natural sewage and synthetic sewage, and of plant scale and laboratory scale systems. Ubiquinone (Q) and menaquinone (MK) components were detected in all sludges tested at molar MK/Q ratios of 0.455 to 0.981. The differences in MK/Q ratios were much larger when we compared different wastewater sludges (i.e., raw sewage and synthetic sewage) than when we compared sludges from the EBPR and standard processes or plant scale and laboratory scale systems. In all sludges tested a Q with eight isoprene units (Q-8) was the most abundant quinone. In the MK fraction, either tetrahydrogenated MK-8 or MK-7 was the predominant type, and there was also a significant proportion of MK-6 to MK-8 in most cases. A numerical cluster analysis of the profiles showed that the sludges tested fell into two major clusters; one included all raw sewage sludges, and the other consisted of all synthetic sewage sludges, independent of the operational mode and scale of the reactors and the phosphate accumulation. These data suggested that Q-8-containing species belonging to the class Proteobacteria (i.e., species belonging to the beta subclass) were the major constituents of the bacterial populations in the EBPR sludge, as well as in standard activated sludge. Members of the class Actinobacteria (gram-positive bacteria with high DNA G+C contents) were the second most abundant group in both types of sludge. The bacterial community structures in activated sludge processes may be affected more by the nature of the influent wastewater than by the introduction of an anaerobic stage into the process or by the scale of the reactors.     

Scanning electron microscopy of biofilm development in anaerobic fixed-bed reactors: Influence of the inoculum

Summary Scanning electron microscopy was applied to evaluate the influence of inoculum on efficiency of initial biofilm formation and reactor performance. Five anaerobic fixed-bed reactors were inoculated with anaerobic sludges from different sources and operated in parallel under identical conditions with defined wastewater and acetate, propionate and butyrate as constituents In all sludges Methanothrix sp. was the predominant acetotroph. The reactors inoculated with anaerobic sludge adapted to the wastewater achieved the highest space loading with 21.0 g COD/l·d after 58 days. The inoculation with granular sludge from an upflow anaerobic sludge blanket (UASB) reactor resulted in significantly less reactor efficiency. Time course of biofilm formation and biofilm thickness (ranging from 20–200 m) depended on the type of inoculum.     

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

Assessing contemporary anaerobic biotechnologies requires proofs on reliable performance in terms of renewable bioenergy recovery such as methane (CH₄) production rate, CH₄ yield while removing volatile solid (VS) effectively. This study, therefore, aims to evaluate temperature-phased anaerobic sequencing batch reactor (TPASBR) system that is a promising approach for the sustainable treatment of organic fraction of municipal solid wastes (OFMSW). TPASBR system is compared with a conventional system, mesophilic two-stage anaerobic sequencing batch reactor system, which differs in operating temperature of 1st-stage. Results demonstrate that TPASBR system can obtain 44% VS removal from co-substrate of sewage sludge and food waste while producing 1.2 m³CH₄/m³_system/d (0.2 m³CH₄/kgVS_added) at organic loading rate of 6.1 gVS/L/d through the synergy of sequencing-batch operation, co-digestion, and temperature-phasing. Consequently, the rapid and balanced anaerobic metabolism at thermophilic stage makes TPASBR system to afford high organic loading rate showing superior performance on OFMSW stabilization.     

Radiolabelled proteomics to determine differential functioning of Accumulibacter during the anaerobic and aerobic phases of a bioreactor operating for enhanced biological phosphorus removal

Proteins synthesized by the mixed microbial community of two sequencing batch reactors run for enhanced biological phosphorus removal (EBPR) during aerobic and anaerobic reactor phases were compared, using mass spectrometry‐based proteomics and radiolabelling. Both sludges were dominated by polyphosphate‐accumulating organisms belonging to Candidatis Accumulibacter and the majority of proteins identified matched closest to these bacteria. Enzymes from the Embden–Meyerhof–Parnas pathway were identified, suggesting this is the major glycolytic pathway for these Accumulibacter populations. Enhanced aerobic synthesis of glyoxylate cycle enzymes suggests this cycle is important during the aerobic phase of EBPR. In one sludge, several TCA cycle enzymes showed enhanced aerobic synthesis, suggesting this cycle is unimportant anaerobically. The second sludge showed enhanced synthesis of TCA cycle enzymes under anaerobic conditions, suggesting full or partial TCA cycle operation anaerobically. A phylogenetic analysis of Accumulibacter polyphosphate kinase genes from each sludge demonstrated different Accumulibacter populations dominated the two sludges. Thus, TCA cycle activity differences may be due to Accumulibacter strain differences. The major fatty acids present in Accumulibacter‐dominated sludge include palmitic, hexadecenoic and cis‐vaccenic acid and fatty acid content increased by approximately 20% during the anaerobic phase. We hypothesize that this is associated with increased anaerobic phospholipid membrane biosynthesis, to accommodate intracellular polyhydroxyalkanoate granules.     

Effect of anaerobic reaction time on denitrifying phosphorus removal and N2O production

Nitrous oxide (N₂O) is a highly potent greenhouse gas; however, the characteristics of N₂O production during denitrification using poly-β-hydroxyalkanoates (PHA) as a carbon source are not well understood. In this study, effects of anaerobic reaction time (AnRT) on PHA formation, denitrifying phosphorus removal and N₂O production were investigated using a laboratory-scale anaerobic/anoxic/oxic sequencing batch reactor (An/A/O SBR). The results showed that operation of the An/A/O SBR for 0.78 SRT (47 cycles) after the AnRT was shortened from 90 min to 60 min resulted in anaerobically synthesized PHA improving by 1.8 times. This improvement was accompanied by increased phosphorus removal efficiency and denitrification. Accordingly, the N₂O-N production was reduced by 6.7 times. Parallel batch experiments were also conducted with AnRTs of 60, 90 and 120 min. All results indicated that in addition to the amount of anaerobically synthesized PHA, the kinetics of PHA degradation also regulated denitrifying phosphorus removal and N₂O production.     

Effect of low solids retention time and focused pulsed pre-treatment on anaerobic digestion of waste activated sludge

The interacting effects of Focused Pulsed (FP) treatment and solids retention time (SRT) were evaluated in laboratory-scale digesters operated at SRTs of 2-20 days. Anaerobic digestion and methanogenesis of waste activated sludge (WAS) were stable for SRT ? 5 days, but the effluent soluble organic compounds increased significantly for SRT = 2 days due to a combination of faster hydrolysis kinetics and washout of methanogens. FP treatment increased the CH₄ production rate and TCOD removal efficiency by up to 33% and 18%, respectively, at a SRT of 20 days. These effects were the result of an increase in the hydrolysis rate, since the concentrations of soluble components remained low for SRT ? 5 days. Alternately, FP pre-treatment of WAS allowed the same conversion of TCOD to CH₄ with a smaller SRT and digester size: e.g., 40% size savings with a CH₄ conversion of 0.23 g CH₄-COD/g COD_in.     

Acetate Degradation at 70°C in Upflow Anaerobic Sludge Blanket Reactors and Temperature Response of Granules Grown at 70°C

Anaerobic acetate degradation at 70°C and at 55°C (as a reference) was studied by running laboratory upflow anaerobic sludge blanket (UASB) reactors inoculated with mesophilic granular sludge. In UASB reactors fed with acetate-containing media (3 g of chemical oxygen demand [COD] per liter, corresponding to 47 mM acetate) approximately 50 days was needed at 70°C and less than 15 days was needed at 55°C to achieve an effluent COD of 500 to 700 mg/liter. In the UASB reactors at both 70 and 55°C up to 90% of the COD was removed. Batch assays showed that sludges from two 70°C UASB reactors, one run at a low effluent acetate concentration and the other run at a high effluent acetate concentration, exhibited slightly different responses to temperatures in the range from 37 to 70°C. Both 70°C sludges, as well as the 55°C sludge, produced methane at temperatures of 37 to 73°C. The 55°C sludge exhibited shorter lag phases than the 70°C sludges and higher specific methane production rates between 37 and 65°C.     

Effect of sludge age on methanogenic and glycogen accumulating organisms in an aerobic granular sludge process fed with methanol and acetate

The influence of sludge age on granular sludge formation and microbial population dynamics in a methanol- and acetate-fed aerobic granular sludge system operated at 35°C was investigated. During anaerobic feeding of the reactor, methanol was initially converted to methane by methylotrophic methanogens. These methanogens were able to withstand the relatively long aeration periods. Lowering the anaerobic solid retention time (SRT) from 17 to 8 days enabled selective removal of the methanogens and prevented unwanted methane formation. In absence of methanogens, methanol was converted aerobically, while granule formation remained stable. At high SRT values (51 days), γ-Proteobacteria were responsible for acetate removal through anaerobic uptake and subsequent aerobic growth on storage polymers formed [so called metabolism of glycogen-accumulating organisms (GAO)]. When lowering the SRT (24 days), Defluviicoccus-related organisms (cluster II) belonging to the α-Proteobacteria outcompeted acetate consuming γ-Proteobacteria at 35°C. DNA from the Defluviicoccus-related organisms in cluster II was not extracted by the standard DNA extraction method but with liquid nitrogen, which showed to be more effective. Remarkably, the two GAO types of organisms grew separately in two clearly different types of granules. This work further highlights the potential of aerobic granular sludge systems to effectively influence the microbial communities through sludge age control in order to optimize the wastewater treatment processes.     

Effect of nitrite from nitritation on biological phosphorus removal in a sequencing batch reactor treating domestic wastewater

Although nitrite effect on enhanced biological phosphorus removal (EBPR) has been previously studied, very limited research has been undertaken about the effect of nitrite accumulation caused by nitritation on EBPR. This paper focused on nitrite effect from nitritation on EBPR in a sequencing batch reactor treating domestic wastewater. Results showed that nitrite of below 10 mg/L did not inhibit P-uptake and release; whereas EBPR deterioration was observed when nitrite accumulation reached 20 mg/L. Due to P-uptake prior to nitritation, nitrite of 20 mg/L has no effect on aerobic P-uptake. The main reason leading to EBPR deterioration was the competition of carbon source. Batch tests were conducted to investigate nitrite effect on anaerobic P-release. Under sufficient carbon source, nitrite of 30 mg/L had no impact on poly-β-hydroxyalkanoate (PHA) storage; contrarily, under insufficient carbon source, denitrifiers competing for carbon source with phosphorus accumulating organisms resulted in decrease of PHA synthesis and P-release.