Investigation of the diversity of homoacetogenic bacteria in mesophilic and thermophilic anaerobic sludges using the formyltetrahydrofolate synthetase gene.

Homoacetogenic bacteria are strict anaerobes capable of autotrophic growth on H(2)/CO(2) or CO, and of heterotrophic growth on a wide range of sugars, alcohols, methoxylated aromatic compounds and one carbon compounds, yielding acetate as their sole metabolic end-product. Batch activity tests on anaerobic granular sludge, using H(2)/CO(2) as a substrate and 2-bromoethanesulfonate (BES) as a specific methanogenic inhibitor revealed that H(2)/CO(2) conversion and concomitant acetate production commenced only after a lag period of 60-100 h. This finding suggests that the homoacetogenic population of digester sludge could be maintained by heterotrophic growth on sugars or other organic compounds, rather than by autotrophic growth on H(2)/CO(2). In the present study, two upflow anaerobic sludge bed (UASB) reactors were operated at 37 degrees C and 55 degrees C for two distinct trial periods, each characterised by the application of influents designed to enrich for homoacetogenic bacteria. Specific primers designed for the amplification of the functional gene encoding formyltetrahydrofolate synthetase (FTHFS), a key enzyme in the acetyl-CoA pathway of acetogenesis, were used as a specific probe for acetogenic bacteria. The diversity of acetogens in the granular sludge cultivated in each reactor was revealed by application of FTHFS targeted PCR. Results show that biomass acetogenic composition was dependent upon the operational temperature of the reactor and the substrate supplied as influent.     

Microbial characteristics of a methanogenic phenol-degrading sludge.

Microbial properties of a methanogenic granular phenol-degrading sludge were characterized using the 16S rRNA/DNA-based techniques, including polymerase chain reaction (PCR) amplification, cloning, DNA sequencing, and fluorescence in situ hybridization (FISH). The sludge was sampled from an upflow anaerobic sludge blanket reactor, which removed 98% of phenol (up to 1260 mg/l) in wastewater at 26 degrees C with 12 hours of hydraulic retention. Based on DNA analysis, the Eubacteria in the sludge was composed of 13 operational taxonomy units (OTUs). Two OTUs, one resembling Clostridium and the other remotely resembling Desulfotomaculum, were likely responsible for the conversion of phenol to benzoate, which was further degraded by five Syntrophus-resembling OTUs to acetate and H2/CO2; methanogens lastly converted acetate and H2/CO2 into methane. The role of six remaining OTUs remains unclear. Overall, the sludge was composed of 26 +/- 6% Eubacteria and 74 +/- 9% methanogens, of which 54 +/- 6% were acetotrophic Methanosaetaceae, 14 +/- 3% and 3 +/- 2% were hydrogenotrophic Methanomicrobiales and Methanobacteriaceae, respectively.     

Behavior of cellulose-degrading bacteria in thermophilic anaerobic digestion process.

Previously, we found that the newly isolated Clostridium sp. strain JC3 became the dominant cellulose-degrading bacterium in thermophilic methanogenic sludge. In the present study, the behavior of strain JC3 in the thermophilic anaerobic digestion process was investigated quantitatively by molecular biological techniques. A cellulose-degrading experiment was conducted at 55 degrees C with a 9.5 L of anaerobic baffled reactor having three compartments (Nos. 1, 2, 3). Over 80% of the COD input was converted into methane when 2.5 kgCOD m(-3) d(-1) was loaded for an HRT of 27 days. A FISH probe specific for strain JC3 was applied to sludge samples harvested from the baffled reactor. Consequently, the ratio of JC3 cells to DAPI-stained cells increased from below 0.5% (undetectable) to 9.4% (compartment 1), 13.1% (compartment 2) and 21.6% (compartment 3) at day 84 (2.5 kgCOD m(-3)d(-1)). The strain JC3 cell numbers determined by FISH correlated closely with the cellulose-degrading methanogenic activities of retained sludge. A specific primer set targeting the cellulase gene (cellobiohydrolaseA: cbhA) of strain JC3 was designed and applied to digested sludge for treating solid waste such as coffee grounds, wastepaper, garbage, cellulose and so on. The strain JC3 cell numbers determined by quantitative PCR correlated closely with the cellulose-sludge loading of the thermophilic digester. Strain JC3 is thus important in the anaerobic hydrolysis of cellulose in thermophilic anaerobic digestion processes.     

The performance and microbial diversity of temperature-phased hyperthermophilic and thermophilic anaerobic digestion system fed with organic waste.

The objective of this study was to evaluate the performances and microbial diversities for development of the effective hyperthermophilic digester system that consists of a hyperthermophilic reactor and hyperthermophilic or thermophilic reactor in series. Lab-scale reactors were operated continuously fed with artificial kitchen garbage. The effect of temperature on the acidification step was firstly investigated. Results indicated that 20.8% of COD solubilization was achieved at 70 degrees C, with 12.6% at 80 degrees C. The average protein solubilization reached 31% at 80 degrees C. Methane conversion efficiency following the acidification was around 85% on average at 55 degrees C, but decreased with increasing temperature and methane gas was not produced over 73 degrees C. As well, bacteria affiliated with the methanogens dominated the population below 65 degrees C, while those affiliated with acidogens were predominant over 73 degrees C. These results indicated that the hyperthermophilic process has considerable benefits to treat wastewater or waste containing high concentration of protein.     

Treatment of dye works wastewater using anaerobic-oxic biological filter reactor packed with carbon fibre and aerated with micro-bubbles.

A new anaerobic-oxic biological filter reactor, which was packed with carbon fibre and aerated with micro-bubbles, was proposed. The reactor performance was examined using dye works wastewater compared with the activated sludge reactor. Effluent SS from the experimental reactor was significantly lower than that from the activated sludge reactor, and transparency was higher. Temperatures of the activated sludge reactor were over 35 degrees C and DOC removal ratios were 40-80% depending on the influent wastewater. On the other hand, the DOC removal efficiency of the experimental reactor was over 70%, when the reactor temperature was over 22 degrees C. In the anaerobic zone, sulphate reduction occurred predominantly and acetate was produced. In the oxic reactor, sulphur oxidation and organic removal occurred. When the amount of sulphate reduction in the anaerobic zone increased, DOC and colour in effluent decreased. The sulphate reducing activity of biofilm at 30 degrees C was three times higher than those at 20 degrees C. The sulphate reducing activity of biofilm in the oxic zone was higher than those in the anaerobic zone, meaning that the sulphate reduction-oxidation cycles were established in the biofilm of the oxic zone. Microbial community of sulphate reducing bacteria was examined by in situ hybridisation with 16S rRNA targeted oligonucleotide probes. Desulfobulbus spp. was most common sulphate reducing bacteria in the anaerobic zone. In the oxic zone, Desulfobulbus spp. and Desulfococcus spp. were observed.     

Methane production and solids destruction in an anaerobic solid waste reactor due to post-reactor caustic and heat treatment.

This study was undertaken to determine the feasibility of caustic and heat treatment of sludge from a dry anaerobic reactor (DAR) with respect to increased methane production and solids destruction. The DAR was operated semi-continuously at 55 degrees C on sized-reduced municipal solid waste at a solids retention time of 15 days. A respirometer was employed to monitor the extent and rate of methane production from anaerobic biodegradation of DAR sludge that was treated with caustic and heat. Results indicate that caustic and heat treatment at 50 degrees C and 175 degrees C increased methane production by 22% and 52%, respectively. Also, volatile solids destruction increased from 46% to 58% and 83%, respectively. Based on these results, economic analysis for a full-scale 10(5) kg/d facility suggests that annual project revenue for 50 degrees C and 175 degrees C treatment is estimated at $21,000 and $445,000, respectively.     

Autotrophic nitrogen removal from anaerobic supernatant of Florence's WWTP digesters.

In municipal WWTP with anaerobic sludge digestion, 10-20% of total nitrogen load comes from the return supernatant produced by the final sludge dewatering. In recent years a completely autotrophic nitrogen removal process based on Anammox biomass has been tested in a few European countries, in order to treat anaerobic supernatant and to increase the COD/N ratio in municipal wastewater. This work reports the experimental results of the SHARON-ANAMMOX process application to anaerobic supernatant taken from the urban Florentine area wastewater treatment plant (S. Colombano WWTP). A nitritation labscale chemostat (7.4 L) has been started-up seeded with the S. Colombano WWTP nitrifying activated sludge. During the experimental period, nitrite oxidising bacteria wash-out was steadily achieved with a retention time ranging from 1 to 1.5 d at 35 degrees C. The Anammox inoculum sludge was taken from a pilot plant at EAWAG (Zurich). Anammox biomass has been enriched at 33 degrees C with anaerobic supernatant diluted with sodium nitrite solution until reaching a maximum specific nitrogen removal rate of 0.065 kgN kg(-1) VSS d(-1), which was 11 times higher than the one found in inoculum sludge (0.005 kgN kg(-1) VSS d(-1). In a lab-scale SBR reactor (4 L), coupled with nitritation bioreactor, specific nitrogen removal rate (doubling time equal to 26 d at 35 degrees C and at nitrite-limiting condition) reached the value of 0.22 kgN kg(-1) VSS d(-1), which was approximately 44 times larger than the rate measured in the inoculum Anammox sludge.     

Anammox process for synthetic and practical wastewater treatment using a novel kind of biomass carriers

The anammox process, as an alternative to conventional nitrogen removal technologies, has abstracted much attention in recent years. In this study, one column-type reactor using a novel support material--net type acrylic fiber (Biofix)--was used for anammox treatment. The Biofix reactor was operated at 25 degrees C (peak summer temperature, 31.5 degrees C). Over 330 days of operation for synthetic wastewater treatment, the nitrogen loading rates of the reactor were increased to 3.6 kg-N/m3/d and T-N removal efficiencies reached to 81.3%. For the practical anaerobic sludge digester liquor treatment, the average TN removal efficiency of 72% was obtained. A protein substance was shown to be the most abundant extracellular polymeric substances (EPS) in the granular sludge with almost two times more in the attached sludge of the Biofix reactor. Considering the EPS levels and observation by scanning electron microscopy, the anammox granules in the Biofix reactor were showing dense state. Results of DNA analyses indicated that the KSU-1 strain might prefer relatively low nutrient levels, while the KU2 strain might be better suited for the high media concentration. Other kinds of bacteria were also identified with the potentials for consuming the dissolved oxygen in the influent and facilitating anammox bacteria surviving under aerobic conditions.     

Syntrophic acetate oxidation in two-phase (acid-methane) anaerobic digesters

The microbial processes involved in two-phase anaerobic digestion were investigated by operating a laboratory-scale acid-phase (AP) reactor and analyzing two full-scale, two-phase anaerobic digesters operated under mesophilic (35 °C) conditions. The digesters received a blend of primary sludge and waste activated sludge (WAS). Methane levels of 20% in the laboratory-scale reactor indicated the presence of methanogenic activity in the AP. A phylogenetic analysis of an archaeal 16S rRNA gene clone library of one of the full-scale AP digesters showed that 82% and 5% of the clones were affiliated with the orders Methanobacteriales and Methanosarcinales, respectively. These results indicate that substantial levels of aceticlastic methanogens (order Methanosarcinales) were not maintained at the low solids retention times and acidic conditions (pH 5.2-5.5) of the AP, and that methanogenesis was carried out by hydrogen-utilizing methanogens of the order Methanobacteriales. Approximately 43, 31, and 9% of the archaeal clones from the methanogenic phase (MP) digester were affiliated with the orders Methanosarcinales, Methanomicrobiales, and Methanobacteriales, respectively. A phylogenetic analysis of a bacterial 16S rRNA gene clone library suggested the presence of acetate-oxidizing bacteria (close relatives of Thermacetogenium phaeum, 'Syntrophaceticus schinkii,' and Clostridium ultunense). The high abundance of hydrogen consuming methanogens and the presence of known acetate-oxidizing bacteria suggest that acetate utilization by acetate oxidizing bacteria in syntrophic interaction with hydrogen-utilizing methanogens was an important pathway in the second-stage of the two-phase digestion, which was operated at high ammonium-N concentrations (1.0 and 1.4 g/L). A modified version of the IWA Anaerobic Digestion Model No. 1 (ADM1) with extensions for syntrophic acetate oxidation and weak-acid inhibition adequately described the dynamic profiles of volatile acid production/degradation and methane generation observed in the laboratory-scale AP reactor. The model was validated with historical data from the full-scale digesters.     

Anaerobic digestion elutriated phased treatment of piggery waste.

The performance of a novel high-rate anaerobic process, the anaerobic digestion elutriated phased treatment (ADEPT) process, for treating a slurry-type piggery waste (55 g COD/L and 37 g TS/L) was investigated. The ADEPT process consists of an acid elutriation slurry reactor for hydrolysis and acidification, followed by an upflow anaerobic sludge bed reactor for methanification. This process provides stable and high system performance with short HRT (7.4 d) and better effluent quality (2 g SCOD/L and 0.68 g VSS/L) due to the alkaline pH condition for hydrolysis/acidification phase, high refractory solids removal and ammonia toxicity reduction. The optimum pH and HRT for hydrolysis/acidogenesis of the piggery waste were 9 and 5 days at both 35 degrees C and 55 degrees C conditions. The hydrolysis and acidification rate in the mesophilic reactor were 0.05 d(-1) and 0.11 d(-1), meaning that hydrolysis was a limiting step. SCOD production by the hydrolysis was about 0.26 g SCOD/g VS(fed) (3.6 g SCOD/g VS reduction). Methane production and content in the system were 0.3 L CH4/g VS(fed) (0.67 L CH4/g VS destroyed) and 80%, respectively, corresponding to 0.23 L CH4/g COD removal (@STP).     

Difficulties in maintaining long-term partial nitritation of ammonium-rich sludge digester liquids in a moving-bed biofilm reactor (MBBR).

Nitrogen can be eliminated effectively from sludge digester effluents by anaerobic ammonium oxidation (anammox), but 55-60% of the ammonium must first be oxidized to nitrite. Although a continuous flow stirred tank reactor (CSTR) with suspended biomass could be used, its hydraulic dilution rate is limited to 0.8-1 d(-1) (30 degrees C). Higher specific nitrite production rates can be achieved by sludge retention, as shown here for a moving-bed biofilm reactor (MBBR) with Kaldnes carriers on laboratory and pilot scales. The maximum nitrite production rate amounted to 2.7 gNO2-Nm(-2)d(-1) (3 gO2m(-3)d(-1), 30.5 degrees C), thus doubling the dilution rate compared to CSTR operation with suspended biomass for a supernatant with 700 gNH4-Nm(-3). Whenever the available alkalinity was fully consumed, an optimal amount of nitrite was produced. However, a significant amount of nitrate was produced after 11 months of operation, making the effluent unsuitable for anaerobic ammonium oxidation. Because the sludge retention time (SRT) is relatively long in biofilm systems, slow growth of nitrite oxidizers occurs. None of the selection criteria applied - a high ammonium loading rate, high free ammonia or low oxygen concentration - led to selective suppression of nitrite oxidation. A CSTR or SBR with suspended biomass is consequently recommended for full-scale operation.     

Co-degradation of phenol and m-cresols by upflow anaerobic sludge blanket reactor.

The study was performed to assess the efficacy of an upflow anaerobic sludge blanket reactor for the degradation of mixtures of phenol and m-cresol. The experiments were performed in an upflow anaerobic sludge blanket reactor. The reactor was seeded with digested sewage sludge and was initially operated at 24 HRT. A phenol concentration of 200 mg/L was fed to the reactor to acclimatize the microorganisms to phenols. Subsequently the dosages of phenols were increased to 400 mg/L, 500 mg/L, and 600 mg/L. Cresols were introduced in the reactor when phenol removal efficiency of 77% was achieved at phenol concentration of 600 mg/L. Different phenol to m-cresol ratios were tried and the performance of the reactor was evaluated for each case. The result demonstrates that it is important to consider phenol/ m-cresol ratio to avoid toxic effects and both can be co-degraded successfully under anaerobic conditions provided proper acclimatization time is given.     

Changes of microbial characteristics of retained sludge during low-temperature operation of an EGSB reactor for low-strength wastewater treatment.

In this study, a lab scale EGSB reactor was operated for 400 days to investigate the influence of temperature-decrease on the microbial characteristic of retained sludge. The EGSB reactor was started-up at 15 degrees C seeding with 20 degrees C-grown granular sludge. The influent COD of synthetic wastewater was set at 0.6-0.8 gCOD/L. The process-temperature was stepwise reduced from 15 degrees C to 5 degrees C during 400 days operation. Decrease of temperature of the reactor from 15 degrees C to 10 degrees C caused the decline of COD removal efficiency. However, continuous operation of the EGSB reactor led the efficient treatment of wastewater (70% of COD removal, 50% of methane recovery) at 10 degrees C. We confirmed that the both acetate-fed and hydrogen-fed methanogenic activities of retained sludge clearly increased under 15 to 20 degrees C. Changes of microbial profiles of methanogenic bacteria were analyzed by 16S rDNA-targeted DGGE analysis and cloning. It shows that genus Methanospirillum as hydrogen-utilizing methanogen proliferated due to low temperature operation of the reactor. On the other hand, genus Methanosaeta presented in abundance as acetoclastic-methanogen throughout the experiment.     

UASB reactor for domestic wastewater treatment at low temperatures: a comparison between a classical UASB and hybrid UASB-filter reactor.

The performance of an upflow anaerobic sludge blanket (UASB) reactor and a hybrid UASB-filter reactor was investigated and compared for the treatment of domestic wastewater at different operational temperatures (28, 20, 14 and 10 degrees C) and loading rates. For each temperature studied a constant CODt removal was observed as long as the upflow velocity was lower than 0.35 m/h. At these upflow velocities similar removals were observed for both reactor types at 28 and 20 degrees C, 82 and 72% respectively. However, at 14 and 10 degrees C the UASB reactor showed a better COD removal (70% and 48%, respectively) than the hybrid reactor (60% and 38%). COD removal resulted from biological degradation and solids accumulation in the reactors. At 28 degrees C, a constant 200 g sludge mass was observed in both reactors and COD removal was attributed to biological degradation only. At lower temperatures, solids accumulation was observed in addition to biological degradation with an increase in reactor sludge as the temperature decreased. The decrease in biological degradation at lower temperatures was offset by solids accumulation and explains the similar overall COD removal efficiency observed at 28 degrees C, 20 degrees C and 14 degrees C. The decrease in temperature was also followed by an increase in the effluent TSS concentration in both reactors. At 14 and 10 degrees C a lower effluent TSS concentration and better performance was observed in the UASB reactor.     

Relative kinetics of anaerobic digestion under thermophilic and mesophilic conditions

With several advantages over the conventional mesophilic anaerobic digestion, such as better sludge quality and higher biogas production, thermophilic anaerobic digestion is regarded as a promising alternative for sludge digestion. Primary and activated sludges are complex materials, and historically, analysis of kinetics has been largely on whole sludge, without analysis of individual components. This paper analyses relative digestion kinetics of pure substrates designed to target main stages of sludge digestion under thermophilic and mesophilic conditions. Hydrolysis rate of cellulose was significantly influenced by temperature with hydrolysis coefficients of--at 55 degrees C (0.7 +/- 0.1 day(-1)), 60 degrees C (0.8 +/- 0.2 day(-1)), 65 degrees C (1.1 +/- 0.2 day(-1)) and 70 degrees C (1.2 +/- 0.2 day(-1)) over 38 degrees C (0.4 +/- 0.1 day(-1)). This strongly follows the Arrhenius relationship, with an activation energy (E(A)) of 31 +/- 4 kJ mol(-1), corresponding to an increase of 1.5x for each 10 degrees C of temperature increase. Glucose uptake was rapid with a wide variety of fermentation products detected under mesophilic conditions, while uptake was slower under thermophilic conditions with acetate and propionate being dominant products. Propionate acetogenesis and acetate-utilizing methanogenesis kinetics were not influenced by temperatures. Hydrolysis is widely regarded as a rate-limiting step in sludge digestion, thus improvements in hydrolysis rates as measured during this study have the potential for significant improvements in overall apparent sludge digestion rates.     

Grey water treatment in upflow anaerobic sludge blanket (UASB) reactor at different temperatures

The treatment of grey water in two upflow anaerobic sludge blanket (UASB) reactors, operated at different hydraulic retention times (HRTs) and temperatures, was investigated. The first reactor (UASB-A) was operated at ambient temperature (14-25 degrees C) and HRT of 20, 12 and 8 h, while the second reactor (UASB-30) was operated at controlled temperature of 30 degrees C and HRT of 16, 10 and 6 h. The two reactors were fed with grey water from 'Flintenbreite' settlement in Luebeck, Germany. When the grey water was treated in the UASB reactor at 30 degrees C, total chemical oxygen demand (CODt) removal of 52-64% was achieved at HRT between 6 and 16 h, while at lower temperature lower removal (31-41%) was obtained at HRT between 8 and 20 h. Total nitrogen and phosphorous removal in the UASB reactors were limited (22-36 and 10-24%, respectively) at all operational conditions. The results showed that at increasing temperature or decreasing HRT of the reactors, maximum specific methanogenic activity of the sludge in the reactors improved. As the UASB reactor showed a significantly higher COD removal (31-64%) than the septic tank (11-14%) even at low temperature, it is recommended to use UASB reactor instead of septic tank (the most common system) for grey water pre-treatment. Based on the achieved results and due to high peak flow factor, a HRT between 8 and 12 h can be considered the suitable HRT for the UASB reactor treating grey water at temperature 20-30 degrees C, while a HRT of 12-24 h can be applied at temperature lower than 20 degrees C.     

Detection of fatty acid beta-oxidizing syntrophic bacteria by fluorescence in situ hybridization.

A new 16S rRNA-targeted oligonucleotide probe, specific for the cluster of fatty acid beta-oxidizing syntrophic bacteria of the family Syntrophomonadaceae was designed for fluorescence in situ hybridization. This probe was evaluated with target as well as non-target cultures. Moreover this probe was assessed with butyrate and oleate degrading enrichment cultures and methanogenic sludges from full-scale plants. The results showed that the probe revealed the presence of fatty acid beta-oxidizing syntrophic bacteria in some of the samples analyzed. However, cell quantification was possible only in enrichment cultures and in a flocculent sludge from a reactor that treats lipid-rich wastewaters, but not in methanogenic granular sludges from upflow anaerobic sludge blanket reactors.     

Upgrading of Florence wastewater treatment plant: co-digestion and nitrogen autotrophic removal.

In recent years a completely autotrophic nitrogen removal process based on Anammox biomass has been tested in a few European countries in order to treat anaerobic supernatant and to increase the COD/N ratio in municipal wastewater. This work reports experimental results on a possible technical solution to upgrade the S. Colombano treatment plant which treats wastewater from the Florentine urban area. The idea is to use 50% of the volume of the anaerobic digester in order to treat external sewage sludge (as septic tank sludge) together with waste activated sludge and to treat the resulting effluent on a SHARON-ANAMMOX process in order to remove nitrogen from the anaerobic supernatant. Anaerobic co-digestion, tested in a 200 L pilot plant, enables low cost treatment of septic tank sludge and increases biogas production; however, it also increases the nitrogen load re-circulated to the WWTP, where nitrogen removal efficiency is already low (<50%), due to the low COD/N ratio, which limits predenitrification efficiency. Experimental results from a SHARON process tested in a lab-scale pilot plant show that nitrite oxidising bacteria are washed-out and steady nitrite production can be achieved at retention times in the range 1 - 1.5 days, at 35 degrees C. In a lab-scale SBR reactor, coupled with a nitration bioreactor, maximum specific nitrogen removal rate under nitrite-limiting conditions (with doubling time equal to about 26 days at 35 degrees C) was equal to 0.22 kgN/kgSSV/d, about 44 times the rate measured in inoculum Anammox sludge. Finally, a cost analysis of the proposed upgrade is reported.     

Full scale fluidized bed anaerobic reactor for domestic wastewater treatment: performance, sludge production and biofilm.

This paper describes the performance, sludge production and biofilm characteristics of a full scale fluidized bed anaerobic reactor (32 m3) for domestic wastewater treatment. The reactor was operated with 10.5 m x h(-1) upflow velocity, 3.2 h hydraulic retention time, and recirculation ratio of 0.85 and it presented removal efficiencies of 71+/-8% of COD and 77+/-14% of TSS. During the apparent steady-state period, specific sludge production and sludge age in the reactor were (0.116+/-0.033) kgVSS. kgCOD(-1) and (12+/-5)d, respectively. Biofilm formed in the reactor presented two different patterns: one of them at the beginning of the colonization and the other of mature biofilm. These different colonization patterns are due to bed stratification in the reactor, caused by the difference in local-energy dissipation rates along the reactor's height, and density, shape, etc. of the bioparticles. The biofilm population is formed mainly of syntrophic consortia among sulfate reducing bacteria, methanogenic archaea such as Methanobacterium and Methanosaeta-like cells.     

Evaluation of sludge properties in a pilot-scale UASB reactor for sewage treatment in a temperate region

In this study, continuous operation of a pilot-scale upflow anaerobic sludge blanket (UASB) reactor for sewage treatment was conducted for 630 days to investigate the physical and microbial characteristics of the retained sludge. The UASB reactor with a working volume of 20.2 m(3) was operated at ambient temperature (16-29 °C) and seeded with digested sludge. After 180 days of operation, when the sewage temperature had dropped to 20 °C or lower, the removal efficiency of both total suspended solids (TSS) and total biochemical oxygen demand (BOD) deteriorated due to washout of retained sludge. At low temperature, the cellulose concentration of the UASB sludge increased owing to the rate limitation of the hydrolytic reaction of suspended solids in the sewage. However, after an improvement in sludge retention (settleability and concentration) in the UASB reactor, the process performance stabilized and gave sufficient results (68% of TSS removal, 75% of total BOD removal) at an hydraulic retention time (HRT) of 9.7 h. The methanogenic activity of the retained sludge significantly increased after day 246 due to the accumulation of Methanosaeta and Methanobacterium following the improvement in sludge retention in the UASB reactor. Acid-forming bacteria from phylum Bacteroidetes were detected at high frequency; thus, these bacteria may have an important role in suspended solids degradation.