Population dynamics of anaerobic microbial consortia in thermophilic methanogenic sludge treating paper-containing solid waste.

To maintain a stable thermophilic (55 degrees C) anaerobic digestion treating toilet paper-containing garbage, it is necessary to operate the digester at long hydraulic retention time (HRT) and low organic loading rate (OLR). Critical conditions of the digestion were investigated by operating the digester at HRT 23 days and OLR 3.4 gCOD(Cr)/L/d (R1) or HRT 14 days and OLR 5.6 gCOD/Cr)/L/d (R2) separately. Characteristics of methanogenesis of the two digesters were examined by measuring gas generation volume and volatile fatty acids (VFA) concentration, and the populations of four anaerobic acidogens and three methanogens were analyzed using quantitative PCR method. In digester R1, methanogenic activity was unstable but it could be recovered by stopping feeding as though VFA accumulation occurred. The population of acidogens and two methanogens were maintained at 10(11) - 10(13) copies/L, however, the population of Methanoculleus could not be recovered after methanogenesis recovering. In digester R2, the period of methanogenesis was significantly shorter than that in digester R1. Both the acidogens and the methanogens could not be maintained at a stable concentration. It is suggested that the critical HRT to sustain the population of acidogens in this process should be longer than 14 days and for all kinds of methanogens, HRT should be longer than 23 days.     

Microbial degradation of estrogens using activated sludge and night soil-composting microorganisms.

In order to investigate the potential for microbial degradation of estrogens, and the products formed, activated sludge collected from Korea (ASK) and night soil-composting microorganisms (NSCM) were used to degrade estrogens. Results showed that both ASK and NSCM degraded almost 100% of the natural estrogens estrone (E1), 17beta-estradiol (E2), and estriol (E3) from initial concentrations of 20-25 mg/L, while synthetic estrogen, ethynylestradiol (EE2), was not degraded. Analysis of degradation products of E2 by using HPLC-ECD and a consecutive first-order reaction calculation confirmed that E2 was sequentially degraded to E1, which was further degraded to other unknown compounds by ASK and NSCM. We then used the yeast two-hybrid assay to show that the unknown degradation products did not appear to possess estrogenic activity when E1, E2 or E3 were degraded to below the detection limit after 14 days of incubation, indicating that ASK and NSCM not only degrade natural estrogens, but also remove their estrogenic activities.     

The effect of flow equalisation and prefermentation on the sludge production and sludge characteristics in a BNR plant.

A full-scale study investigated the influence of diurnal flow equalisation and prefermentation on the characteristics of sludge. The diurnal variations in the sludge concentration and the level of sludge blanket in the secondary clarifiers were evened out significantly with the use of an equalization basin. Stable conditions in the aeration basin and in the secondary clarifiers contributed to the improvements in the performance of the BNR plant. A decrease in the waste activated sludge production and an improvement in the settleability were also observed. The low WAS yield was attributed to the low yield COD compounds produced by the prefermentation, longer sludge age and constant conditions obtained by the flow equalisation. Some evidence was found that good settling properties would be related to the amount of suspended solids fed to the biological process as well as to the good performance of the biological process.     

Microbial characteristics of biogas

The microbial diversity of biogas was analyzed in order to examine the aerosolization behavior of microorganisms. Six biogas samples were analyzed: five from mesophilic and thermophilic anaerobic digestors treating different wastes, and one from landfill. Epifluorescent microscopic counts revealed 10(6) prokarya m(-3). To assess the difference occuring in aerosolization, 498 biogas-borne 16S ribosomal DNA were analyzed and compared to published anaerobic digestor microbial diversity. Results show a large microbial diversity and strong discrepancy with digestor microbial diversity. Three different aerosolisation behaviour patterns can be identified: (i) that of non-aerosolized microorganisms, Deltaproteobacteria, Spirochaetes, Thermotogae, Chloroflexi phyla and sulfate-reducing groups, (ii) that of passively aerosolized microorganisms, including Actinobacteria, Firmicutes and Bacteroidetes phyla and (iii) that of preferentially aerosolized microorganisms, including Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, as well as strictly aerobic and occasionally pathogenic species, presented high levels of aerosolization.     

Process performance of dry, batch anaerobic digestion of sewage sludge: methanogenic seed mixtures

A dry, batch anaerobic digestion (DBAD) process was tested on two sewage sludge types with different methanogenic seed fractions under laboratory conditions. The aim was to indicate optimal sludge:seed mixing ratios and analyse process performance based on degradation rate and reactor-specific methane production. The attained results were compared with the performance of a liquid-state, laboratory-scale stirred reactor (SR). A mixing ratio of at least 1:1.25 (sludge:seed) yielded processes free from significant inhibitions. Further seeding increments resulted in slightly better performances, but much lower sludge fractions treated in the reactors. Compared with the SR process, the DBAD reactors produced comparable degradation rates albeit in a significantly longer process and with somewhat lower reactor-specific methane production rates. These findings indicate that the DBAD method may provide a viable alternative to liquid-state processes if sludge drying is already applied and reactor volume requirements are of importance.     

Process performance and change in sludge characteristics during anaerobic digestion of sewage sludge with ozonation.

A new process configuration combining anaerobic digestion with ozonation, and operated at long SRT, was studied with the objective of on-site reduction in sludge quantity and improving biogas recovery. The process performance with respect to solid reduction efficiency and other important process parameters like accumulation of inorganic solids, changes in sludge viscosity and dewatering characteristics were evaluated from the data of long term pilot scale continuous experiments conducted using a mixture of primary and secondary municipal sewage sludge. Due to sludge ozonation and long SRT, high VSS degradation efficiency of approximately 80% was achieved at a reactor solid concentration of 6.5%. A high fraction of inorganic solid (>50%) consisting mainly of acid insoluble and iron compounds was found to accumulate in the reactor. The high inorganic content accumulated in the digested sludge did not, however, contribute to the observed increase in sludge viscosity at high solid concentration. The sludge viscosity was largely found to depend on the organic solid concentration rather than the total solid content. Moreover, higher inorganic content in the digested sludge resulted in better sludge dewaterability. For a quick assessment of the economic feasibility of the new process, an economic index based on the unit cost of digested sludge disposal to unit electric cost is proposed.     

Physical characteristics of a waste activated sludge: conditioning responses and correlations with a synthetic surrogate.

The efficient and economic management of waste activated sludge (WAS) requires a proper understanding of the sludge's material properties. Though there has been much study of WAS, an adequate linkage between its physical and chemical properties has been elusive. In particular, the conditioning and dewatering of WAS are expensive operations, and the addition of polymer to WAS leads to even more complex material behavior that is difficult to optimize. This paper reports on an extensive characterization of WAS, both with and without polymer conditioning. We combined the classical "jar test" approach with less conventional rheometric and electrokinetic measurements. In addition to the use of sampled WAS, a synthetic surrogate sludge was formulated, attempting to duplicate properties of WAS and allow more extensive characterization of a reproducible surrogate to WAS. Results with both the synthetic surrogate and WAS indicated that the traditional, electrokinetic, and rheological properties were related. However, the dose optima by rheometry were somewhat higher in both cases.     

Microbial community structure of activated sludge during aerobic granulation in an annular gap bioreactor.

A novel annular gap reactor was designed to create a controlled shear environment in which aerobic granular sludge could be developed. The bacterial and eukaryal community structures during two aerobic granular sludge experiments were tracked using denaturing gradient gel electrophoresis (DGGE). The first granule cultivation experiment, using an organic loading rate of 1.6 kg/m3d COD, resulted in biomass that was dominated by filamentous bacteria and Zoogloea ramigera colonies. A second experiment with a higher organic loading rate of 6 kg/m3d COD developed a granule-like morphology but was ultimately dominated by filamentous fungi. Species identification via DGGE band purification and DNA sequencing closely matched the observed sludge morphology and behavior.     

Strength characteristics of aerobic granular sludge

Aerobic granular sludge has a number of advantages over conventional activated sludge flocs, such as cohesive and strong matrix, fast settling characteristic, high biomass retention and ability to withstand high organic loadings, all aspects leading towards a compact reactor system. Still there are very few studies on the strength of aerobic granules. A procedure that has been used previously for anaerobic granular sludge strength analysis was adapted and used in this study. A new coefficient was introduced, called a stability coefficient (S), to quantify the strength of the aerobic granules. Indicators were also developed based on the strength analysis results, in order to categorize aerobic granules into three levels of strength, i.e. very strong (very stable), strong (stable) and not strong (not stable). The results indicated that aerobic granules grown on acetate were stronger (high density: >150 g T SSL(-1) and low S value: 5%) than granules developed on sewage as influent. A lower value of S indicates a higher stability of the granules.     

The effects of hydraulic retention time and feed COD concentration on the rate of hydrolysis of primary sewage sludge under methanogenic conditions.

A series of completely mixed methanogenic anaerobic digesters have been operated to determine the rate of hydrolysis of primary sewage sludge. The hydraulic retention time was reduced from 60 d to when the system failed (approximately 5 d), while the feed COD concentration was 40, 25, 13 and 2 gCOD/L. A steady state model based on first order kinetics was developed to simulate the hydrolysis rate at each retention time and feed concentration. With the mean value for the hydrolysis rate constant (0.992 +/- 0.492 d(-1)), this model was able to accurately predict the effluent COD for all steady state operating conditions. However, the effluent COD concentration was relatively insensitive to the exact value for this constant. The model provides a framework for analysis of anaerobic digestion experimental data, to enable meaningful comparisons.     

污泥固化土强度特性研究

为明确污泥固化处理后的强度变化特征,并从水分转化角度揭示其变化机理,对不同水泥添加量和不同养护龄期条件下污泥固化土的强度特性进行了室内试验研究。结果表明:(1)污泥固化土的无侧限抗压强度与水泥添加量呈线性正相关,固化系数k随着养护龄期的增加而增大。0~14 d,k由0.807 9增大到1.251 8,增长幅度较大,为54.94%;14~28 d,k由1.251 8增大到1.488 7,增长幅度明显减小,为18.92%。(2)无侧限抗压强度与养护龄期之间呈现良好的正相关性,二者拟合的相关系数在0.969 8~0.999 7之间。(3)水泥主要与污泥中结合势能较低的自由水发生水化反应,随着水泥添加量的增加,自由水百分量降低明显,下降幅度达到47.7%,结合水百分量降低缓慢,下降幅度仅9%;且水化反应在14 d内基本完成,14 d内水化反应消耗的自由水含量约为28 d内的80%。研究成果可为污泥固化土这种特殊性质土体的基本力学性质研究提供基础参数,为实际固化施工工艺的选择提供了参考。     

The impact of metal transport processes on bioavailability of free and complex metal ions in methanogenic granular sludge

Bioavailability of metals in anaerobic granular sludge has been extensively studied, because it can have a major effect on metal limitation and metal toxicity to microorganisms present in the sludge. Bioavailability of metals can be manipulated by bonding to complexing molecules such as ethylenediaminetetraacetate (EDTA) or diethylenetriaminepentaacetate (DTPA). It has been shown that although the stimulating effect of the complexed metal species (e.g. [CoEDTA](2-)) is very fast, it is not sustainable when applied to metal-limited continuously operated reactors. The present paper describes transport phenomena taking place inside single methanogenic granules when the granules are exposed to various metal species. This was done using magnetic resonance imaging (MRI). The MRI results were subsequently related to technological observations such as changes in methanogenic activity upon cobalt injection into cobalt-limited up-flow anaerobic sludge blanket (UASB) reactors. It was shown that transport of complexed metal species is fast (minutes to tens of minutes) and complexed metal can therefore quickly reach the entire volume of the granule. Free metal species tend to interact with the granular matrix resulting in slower transport (tens of minutes to hours) but higher final metal concentrations.     

Impact of sludge retention time on sludge characteristics and microbial community in MBR

In this study, the impact of sludge retention time (SRT) on sludge characteristics and microbial community and the effect on membrane fouling in membrane bioreactor (MBR) was investigated. The results show that MBR with longer SRT has less fouling propensity, in agreement with other studies, despite the fact that the MBR with longer SRT contained higher MLSS and smaller particle size. However, much more soluble microbial products (SMPs) were released in MBR with shorter SRT. More slime on the membrane surface was observed in MBR with shorter SRT while sludge cakes formed on the membrane surface in MBR with longer SRT. The results show that SMP contributes to the severe fouling observed in MBR with shorter SRT, which is in agreement with other studies showing that SMPs were the major foulants in MBR. Under different SRTs of operation, the bacterial community structures of the sludge obtained by use of polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) were almost identical, but those on the membrane surface differed substantially. It suggests that, although SRT has impact on sludge characteristics, it doesn't affect the microbial community in the suspension.     

Microbial Fe(II)-oxidation by nitrate in activated sludge

The oxidation of Fe(II) to Fe(III) by addition of nitrate and nitrite to activated sludge was studied to determine whether the process was biological or chemical (chemodenitrification). It was shown that the process was mainly biological, although the microorganisms involved have not yet been described. Investigations in a full scale treatment plant suggested that the process most likely took place m the anoxic (denitrification) tank. Details of the kinetics and stoichiometry have not yet been determined, but the process may he of significance for keeping Fe(III) oxidized, which is important for P-removal and for floc structure. Furthermore, in some treatment plants, the oxidation may also be of significance for nitrate removal (denitrification).     

Factors affecting measurement of specific methanogenic activity.

Specific methane production (SMP) tests have been used to determine the potential loading rate capacity of anaerobic reactors, to characterize biomass prior to its use as an inoculum for new anaerobic reactors, to detect changes in biomass activity during operation, or to assess the occurrence of toxic conditions. SMP tests also provide a basis for estimating specific methanogenic activity in mixed anaerobic cultures. SMP protocols used to date have varied widely in both procedure and objective. Tests conducted by the present authors indicated that biomass concentration, substrate type and concentration, and mixing intensity are factors that can affect the results of SMP tests.     

Microbial community analysis during continuous fermentation of thermally hydrolysed waste activated sludge

Acidogenic fermentation of thermally hydrolysed waste activated sludge was carried out at laboratory scale in two reactors operated under different hydraulic retention times (HRT). Process performance was assessed in terms of volatile fatty acid (VFA) composition and yield. The diversity of the microbial population was investigated by constructing a 16S rRNA gene library and subsequent phylogenetic analysis of clones. Fluorescence in situ hybridization (FISH) was used to assess the relative abundance of different bacterial groups. Bacteroidetes and Firmicutes were the dominant taxonomic groups representing 93% of the total sequences obtained in the reactor with 4 d HRT. A similar VFA yield (0.4-0.5 g VFA(COD) g SCOD(-1)) was obtained for the HRTs tested (1-4 d), indicating that extended retention times were not useful. Within Firmicutes, Clostridia was the major group detected in the clone sequences. These had close affiliation to Sporanaerobacter acetigenes, suggesting organisms of this group were important for hydrolysis of the protein fraction of the substrate. However, FISH analysis failed to detect the major portion of the bacteria, and this is most likely due to the lack of appropriate probes. This work emphasizes the diversity of fermentative communities, and indicates that more work is needed to identify and detect the important members.     

Functional analysis of microbial community in phenol-degrading aerobic granules cultivated in SBR.

Phenol-degrading aerobic granules were cultivated in a sequencing batch reactor with an influent phenol concentration of 500 mg l(-1). Eight strains were isolated from aerobic granules to characterize the functional redundancy of the microbial community in the granules. The specific oxygen utilization kinetics show the eight strains possessed different phenol-degrading activities, with half-saturation constants (Ks) ranging from 0.4 to 70.5 mg phenol l(-1). Two isolates belonging to dominant populations expressed differing functions. The first strain was linked to the function of phenol degradation as this strain has the highest phenol-degrading ability among all isolates, while the second strain was linked to the maintenance of the granule structure because of its strong self-flocculation activity. This study could be used to exploit the granule-based system for treating high-strength wastewaters.     

Equalization characteristics of an upflow sludge blanket-aerated biofilter (USB-AF) system.

Equalization characteristics of the upflow sludge blanket-aerated bio-filter (USB-AF) were investigated with the fluctuated raw domestic sewage. Recycle of nitrified effluent from AF to USB triggered the equalization characteristics of the sludge blanket on both soluble and particulate organic matter. Increment of EPS in sludge blanket by nitrate recycle was detected and removal of turbidity and particulates increased at higher recycle ratios by bio-flocculation. Increased TCOD removal in the USB was due to both denitrification of recycled nitrate and entrapment of the particulate organic matter in sludge blanket. Capture of both soluble and particulate organic matter increased sludge blanket layer in the USB, which improved the reactor performances and reduced the organic load on the subsequent AF. Overall TCOD and SS removal efficiencies were about 98% and 96%, respectively in the USB-AF system. Turbidity in the USB effluent was about 44, 20 and 5.5 NTU, at recycle ratios of 0, 100 and 200%, respectively. Particle counts in the range 2-4 microm in the USB effluent were higher than those in influent without nitrate recycle, while particle counts in the range of 0.5-15 microm in the USB effluent decreased 70% at recycle ratio of 200%. The major constituent of EPS extracted from anaerobic sludge was protein and total EPS increased from 109.1 to 165.7 mg/g-VSS with nitrate recycle of 100%. Removal efficiency and concentration of T-N in the UBS-AF effluent was over 70% and below 16 mg/L, respectively.     

Mineralisation of hazardous organic compounds in a sludge reed bed and sludge storage.

Linear alkylbenzene sulphonates (LAS) and nonylphenolethoxylates (NPE) may be detrimental to the environment if spread in inappropriately large concentrations. Mineralisation of LAS and NPE in mesophilic digested sludge was observed during a 9-month monitoring programme where three separate treatment methods were investigated. Sludge was treated in a sludge reed bed under aerobic conditions, by storage in a container under anaerobic conditions, and by storage in a sludge pile turned over mechanically at intervals to improve the oxygen influx. Treatment in a sludge reed bed was shown to be effective. Mineralisation of 98% of LAS and 93% of NPE was observed. Only limited mineralisation occurred on the surface of the sludge stored in a container. A reduction of LAS and NPE of 90% and 43%, respectively, was observed in the sludge which was stored in a pile and frequently turned.     

Metabolic interactions in methanogenic and sulfate-reducing bioreactors.

In environments where the amount of electron acceptors is insufficient for complete breakdown of organic matter, methane is formed as the major reduced end product. In such methanogenic environments organic acids are degraded by syntrophic consortia of acetogenic bacteria and methanogenic archaea. Hydrogen consumption by methanogens is essential for acetogenic bacteria to convert organic acids to acetate and hydrogen. Several syntrophic cocultures growing on propionate and butyrate have been described. These syntrophic fatty acid-degrading consortia are affected by the presence of sulfate. When sulfate is present sulfate-reducing bacteria compete with methanogenic archaea for hydrogen and acetate, and with acetogenic bacteria for propionate and butyrate. Sulfate-reducing bacteria easily outcompete methanogens for hydrogen, but the presence of acetate as carbon source may influence the outcome of the competition. By contrast, acetoclastic methanogens can compete reasonably well with acetate-degrading sulfate reducers. Sulfate-reducing bacteria grow much faster on propionate and butyrate than syntrophic consortia.