The impact of floc size on respiration inhibition by soluble toxicants--a comparative investigation

Activated sludge facilities are susceptible to upset by shock loads of toxic compounds. We hypothesized that floc size plays an important role in determining the sensitivity of mixed liquor to shock by cadmium and 2,4-dinitrophenol (DNP). To test this hypothesis, heterotrophic respiration inhibition experiments were conducted using mixed liquor from a pilot-scale membrane bioreactor (MBR) and full-scale activated sludge (FSAS) facility with gravity settling secondary clarifiers that were operated under similar process conditions. MBR mixed liquor flocs were both 41% smaller and 2 and 1.25 times more sensitive to equivalent soluble cadmium and DNP concentrations, respectively, compared to FSAS mixed liquor flocs. Similarly, FSAS mixed liquor that had been sheared (resulting in a smaller average floc diameter) was 1.5 times more sensitive to soluble cadmium than non-sheared FSAS mixed liquor. These results suggest that activated sludge process conditions that create smaller floc particles, such as the use of membranes for liquid-solid separation, are more susceptible to upset events caused by shock loads of cadmium and DNP. The particle size distribution (PSD) and average floc diameter of a mixed liquor suspension should be measured and reported when stating the inhibition concentration of a specific toxicant.     

A comparative study of fouling-related properties of sludge from conventional and membrane enhanced biological phosphorus removal processes

The physical and biochemical properties of activated sludge mixed liquor, including floc size distribution, zeta potential, relative hydrophobicity, and bound and unbound (soluble) extracellular polymeric substances (EPS), were examined in this study to evaluate their relationship to membrane fouling. Mixed liquors from a membrane enhanced biological phosphorus removal (MEBPR) process and a conventional enhanced biological phosphorus removal (CEBPR) process were compared. It was found that the floc size distribution and the amount of soluble EPS in the mixed liquor were the most important properties that significantly influenced the fouling propensity of sludge. Contrary to the literature, the content of EPS bound in activated sludge flocs was not found to be directly associated with membrane fouling, and sludge surface properties such as zeta potential and relative hydrophobicity were not closely related to the observed differences in the fouling tendencies of the two types of sludge.     

旁路微氧污泥减量效果及其影响因素

通过对旁路微氧污泥减量技术中好氧污泥在微氧池中的减量效果及其影响因素的研究,发现污泥减量效果与微氧池的污泥浓度(MLSS)、好氧污泥与厌氧污泥的比例(α)、微氧池的氧化还原电位(ORP)、微氧池的污泥停留时间有关。当微氧池的α=2∶8、MLSS为10 000 mg/L时,减量效果最佳;通过不同MLSS和不同α值两组试验,得出在最佳值时的减量率分别为19.15%和19.61%。低ORP值条件下微氧池污泥颗粒细碎,中位粒径为20.24μm,而好氧污泥的中位粒径为32.18μm。同时,混合液中溶解性大分子有机物含量明显增加。该工艺使污泥有更充分的时间进行内源呼吸和EPS的离解释放,从而实现了污泥减量。     

Influence of loosely bound extracellular polymeric substances (EPS) on the flocculation, sedimentation and dewaterability of activated sludge

Laboratory experiments on the activated sludge (AS) process were carried out to investigate the influence of microbial extracellular polymeric substances (EPS), including loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS), on biomass flocculation, sludge settlement and dewaterability. The heat EPS extraction method was modified to include a mild step and a harsh step for extracting the LB-EPS and TB-EPS, respectively, from the sludge suspension. Six lab-scale AS reactors were used to grow AS with different carbon sources of glucose and sodium acetate, and different sludge retention times (SRTs) of 5, 10 and 20 days. The variation in the bioreactor condition produced sludge with different abundances of EPS and different flocculation and separation characteristics. The sludge that was fed on glucose had more EPS than the sludge that was fed on acetate. For any of the feeding substrates, the sludge had a nearly consistent TB-EPS value regardless of the SRT, and an LB-EPS content that decreased with the SRT. The acetate-fed sludge performed better than the glucose-fed sludge in terms of bioflocculation, sludge sedimentation and compression, and sludge dewaterability. The sludge flocculation and separation improved considerably as the SRT lengthened. The results demonstrate that the LB-EPS had a negative effect on bioflocculation and sludge-water separation. The parameters for the performance of sludge-water separation were much more closely correlated with the amount of LB-EPS than with the amount of TB-EPS. It is argued that although EPS is essential to sludge floc formation, excessive EPS in the form of LB-EPS could weaken cell attachment and the floc structure, resulting in poor bioflocculation, greater cell erosion and retarded sludge-water separation.     

Effect of temperature shocks on membrane fouling in membrane bioreactors

Temperature is known to influence the biological performance of conventional activated sludge systems. In membrane bioreactors (MBRs), temperature not only affects the bioconversion process but is also shown to have an effect on the membrane performance. Four phenomena are generally reported to explain the higher resistance for membrane filtration found at lower temperatures: (1) increased mixed liquor viscosity, reducing the shear stress generated by coarse bubbles, (2) intensified deflocculation, reducing biomass floc size and releasing EPS into the mixed liquor, (3) lower backtransport velocity and (4) reduced biodegradation of COD. Although the higher resistance at low temperatures has been reported in several papers, the relation with supernatant composition has not been investigated before. In this paper, the composition of the soluble fraction of the mixed liquor is related to membrane performance after exposing the sludge to temperature shocks. Flux step experiments were performed in an experimental system at 7, 15, and 25° Celsius with sludge that was continuously recirculated from a pilot-scale MBR. After correcting the permeate viscosity for temperature, higher membrane fouling rates were obtained for the lower temperature in combination with low fouling reversibility. The soluble fraction of the MBR mixed liquor was analysed for polysaccharides, proteins and submicron particle size distribution. At low temperature, a high polysaccharide concentration was found in the experimental system as compared to the MBR pilot. Upon decreasing the temperature of the mixed liquor, a shift was found in particle size towards smaller particles. These results show that the release of polysaccharides and/or submicron particles from sludge flocs could explain the increased membrane fouling at low temperatures.     

Relationship between floc composition and flocculation and settling properties studied at a full scale activated sludge plant

The variation in activated sludge floc composition, flocculation and settling properties was studied at a full scale plant over a 2-year period. A comprehensive set of process parameters was analysed and related to the floc properties to increase the understanding of the factors affecting floc formation. The composition of the activated sludge showed a seasonal change with higher concentrations of extractable extracellular polymeric substances (EPS) during the winter months. The protein content of the total sludge and EPS increased significantly during the winter. This coincided with higher effluent suspended solids concentrations and increased shear sensitivity of the sludge flocs. Only poor correlations between EPS contents and stirred sludge volume index (SSVI) could be observed. High iron concentrations in the sludge due to dosage of iron salt to precipitate phosphorus were found to have a negative impact on the settling and compaction properties of the sludge, whereas it had a positive impact on floc stability. Higher organic loading due to by-passed primary settlers leads to improved settling and compaction properties.     

胞外聚合物的生成特性及其对污水生物处理的影响

微生物产生的胞外聚合物(EPS)是活性污泥生物絮体的主要组成部分,而且不同基质条件会对其产生不同的影响。在活性污泥中,EPS的存在对生物絮凝、沉降及脱水性能等具有重要的影响,在控制和改善污泥处理过程中起着举足轻重的作用;阐述了EPS对膜污染的贡献,及其对金属离子的络合作用。     

Active biomass in activated sludge: Comparison of respirometry with catalase activity measured using an on-line monitor

On-line measurements of the activity of activated sludge biomass may allow more efficient operation of treatment plants. A novel monitor for activity which can be used on-line follows oxygen evolution from hydrogen peroxide by catalase activity in activated sludge. Results from this monitor were compared with assays of oxygen uptake rate (OUR) by respirometry. Samples from the five activated sludge plants were assayed and showed, as expected, that the mixed liquor suspended solids (MLSS) and volatile suspended solids (VSS) levels were a poor measure of the activity of the sample determined by OUR. However, the activity of the biomass determined by the catalase activity biomass monitor correlated well with the respiration rate. This monitor should provide a simple and robust alternative to OUR measurements in activated sludge plants.     

Long-term formation of microbial products in a sequencing batch reactor

Activated sludge in a sequencing batch reactor (SBR) is subjected to alternating feast-and-famine conditions, which may result in the enhanced production of microbial products: extracellular polymeric substances (EPS), soluble microbial products (SMP), and internal storage products (X_STO). In this work, the long-term formation of these three microbial products by activated sludge in an SBR is investigated using an expanded unified model with a parallel experimental study. We also use the model to compare the impacts in an SBR to those in a continuous-flow activated sludge system. The model captures all experimental trends for all components with solids retention time (SRT) for global steady state and within a cycle. At an SRT of 20 days, the active microorganisms constitute about 28% of the mixed liquor volatile suspended solids (MLVSS); the remaining biomass is comprised of residual inert biomass (X_I) of 40%, EPS of 31%, and X_STO of ∼1%. The active biomass becomes a smaller fraction with the increasing SRT, while the inert biomass becomes increasingly dominant. For soluble components, effluent chemical oxygen demand (COD) is dominated by SMP, which varies to some degree in a cycle, peaking as external substrate becomes depleted. Within the SBR cycle, external substrate (S) declines strongly in the first part of the cycle, and SMP shows a small peak at the time of S depletion. X_STO is the only biomass component that varies significantly during the cycle. It peaks at the time that the input substrate (S) is depleted. Simulation for a continuous-flow activated sludge system and comparison with an SBR reveals that the constant “famine” conditions of the continuous system lead to lower EPS and X_STO, but higher MLVSS and X_I.     

Effect of PAC addition on sludge properties in an MBR treating high strength wastewater

This paper examines the sludge characteristics in a submerged membrane bioreactor (MBR) operated on a high strength wastewater from an alcohol distillery. Two membrane bioreactors, each with a 30 μm mesh filter, were investigated with and without addition of powdered activated carbon (PAC). Experiments were conducted with varying organic loading rates ranging from 3.4 to 6.9 kgCOD m⁻³ day⁻¹ and the specific oxygen uptake rate (SOUR), sludge volume index (SVI), mixed liquor suspended solids (MLSS), particle size and extracellular polymeric substances (EPS) were monitored over a 180 day period. Respirometric experiments did not show enhancement in microbial activity with PAC supplementation. Addition of PAC decreased the SVI thereby perceptibly improving sludge dewaterability. The sludge particle size, which increased with time, appeared to be independent of PAC addition but was influenced by the aeration intensity. PAC also did not affect the sludge EPS concentration; however, the EPS composition, in terms of protein/carbohydrate (polysaccharide) ratio was altered resulting in a high P/C ratio. FTIR analysis of the sludge samples indicated that the functional groups associated with the sludge polysaccharides appear to be involved in its interaction with PAC.     

The influence of sludge age on heavy metal removal in the activated sludge process

In a laboratory simulation of the activated sludge process ten heavy metals were added continuously to the system which was allowed to equilibrate at six sludge ages between 3 and 18d. Cobalt, manganese and molybdenum removals were poor and were unaffected by changes in the sludge age. The highest removal efficiencies for the other metals occurred at the 15d sludge age. Chromium (trivalent) and cadmium had the highest removal efficiencies, typically greater than 50%. The behaviour of the majority of the metals which were removed to a significant extent was related to one of the parameters influenced by sludge age, i.e. mixed liquor suspended solids, effluent suspended solids or effluent chemical oxygen demand. The metals which were poorly removed showed little affinity for the activated sludge, while most metals exhibited maximum specific uptake by the mixed liquor at a sludge age of 9–12d. However, the affinity of silver for the mixed liquor continued to increase as the sludge age increased to 18d. An affinity series, based on an arbitrary measure of the specific accumulation of metals by the mixed liquor, indicated that chromium, cadmium and silver were most readily adsorbed by the activated sludge.     

A review of physical relationships between form and function in wastewater biotreatment and a new theory relating activated sludge and biofilm systems behaviour

In this review paper, a physical systems analysis based on thermodynamics has been applied to the observed behaviour of activated sludge as a physical system. The origin of the microecology of activated sludge and its relationship to biofilms and the metabolic consequences of the extracellular polysaccharide (EPS) formation are explained for the first time. Feast‐famine conditions are shown to be the leading candidate for the evolution of the ability of biofilm forming bacteria and activated sludge ‘floc‐formers’ to form EPS. The basis for competition for RBCOD between low F:M filamentous heterotrophic bacteria and ‘floc‐forming’ heterotrophic is presented and its relationship to activated sludge bulking described.     

Characterization of extracellular polymeric substances produced by mixed microorganisms in activated sludge with gel-permeating chromatography, excitation-emission matrix fluorescence spectroscopy measurement and kinetic modeling

In this work the extracellular polymeric substances (EPS) produced by mixed microbial community in activated sludge are characterized using gel-permeating chromatography (GPC), 3-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy measurement and mathematical modeling. Chromatograms of extracted EPS exhibit seven peaks, among which proteins have four peaks and polysaccharides have three peaks. Evolution of the chromatogram area indicates that the quantity of produced EPS increases significantly in the substrate utilization process. With the parallel factor analysis (PARAFAC) approach, two components of the polymer matrix are identified by the EEM analysis, one as EPS proteins at Ex/Em 280/340 nm and one matrix associated as fulvic-acid-like substances at 320/400 nm. The proteins and fulvic-acid-like substances in the EPS increase in the substrate utilization phase, but decrease in the endogenous phase. To have a better insight into EPS production, the kinetic modeling of EPS is performed with regard to their molecular weight distribution and chemical natures identified by GPC and EEM. In this way, the dynamics of these important microbial products are better understood.     

Promotion and inhibition of oxygen transfer under fine bubble aeration by activated sludge

The promotion and inhibition of inactivated and activated sludge on oxygen mass transfer (OMT) were studied using lab‐scale experiments. The results showed that the α‐values and oxygen transfer efficiency (OTE) decreased with increasing mixed liquor suspended solids (MLSS) concentration (1–10 g/L). Although OMT promotion rate by microbial respiration in activated sludge system increased from 39.8–97.5% for the α‐values and OTE, the two parameters were found to fall sharply when MLSS concentration was over 5 g/L. This indicated that the sludge concentration is a major influence factor on OMT in activated sludge system. Such results provide valuable knowledge for the operating optimization of the aeration system in wastewater treatment process.     

厌氧消化污泥脱水性能变化的试验与分析

研究了剩余污泥在中温厌氧消化条件下脱水性能的变化及其作用机制。剩余污泥厌氧消化过程中,消化污泥的比阻(SRF)相比于剩余污泥有一定的减小,消化污泥的过滤速度有一定的改善,但改善不明显。聚丙烯酰胺(PAM)、FeCl3和聚合氯化铝(PAC)3种絮凝剂调理试验显示,消化污泥的最佳投药量相对于剩余污泥均有所增加,说明消化污泥脱水性能变差。分析了2种污泥中胞外聚合物(EPS)含量及污泥颗粒特性的变化,表明消化过程导致EPS的降解并向液体中释放。随着EPS含量的减少,由EPS架桥形成的较大絮体解体成为较小的污泥颗粒,污泥中小颗粒的比例增加,污泥的脱水性能变差。     

The immunochemical detection of stress proteins in activated sludge exposed to toxic chemicals

The heat shock protein, GroEL, was found to be induced in activated sludge cultures exposed to perturbations of chemicals (cadmium, pentachlorophenol, and acetone) or heat stress. In laboratory activated sludge reactors, GroEL was rapidly induced (within minutes) in the presence of 5 mg/l or greater total cadmium. At 5 mg/l cadmium, however, moderate to insignificant changes in activated sludge process performance indicators [effluent suspended solids concentration, chemical oxygen demand (COD) removal, and specific oxygen uptake rate] were observed. As total cadmium concentrations increased above 5 mg/l, there was a significant and consistent increase in effluent volatile suspended solids concentrations from activated sludge sequencing batch reactors relative to unstressed controls. These results indicate that stress proteins may serve as sensitive and rapid indicators of mixed liquor toxicity which can adversely impact treatment process performance, but that GroEL may not be a good candidate protein for this purpose.     

基于絮体结构指标的丝状菌膨胀污泥SVI预测

为了建立更加准确的膨胀活性污泥沉降性表征方法,筛选出5个反映污泥微观絮体结构的特征指标:絮体大小(SZ)、伸长性(ST)、密实性(CP)、规则性(RG)和丝状菌(FL),分析众多特征参数降维的可能性,建立丝状菌膨胀污泥絮体结构特征指标体系,以偏最小二乘回归法(PLS)探索膨胀污泥絮体结构特征指标与污泥容积指数(SVI)的关系,建立丝状菌膨胀污泥的SVI预测函数。结果表明,污泥絮体特征参数之间具有较好的相关性,可采用主成分分析法(PCA)进行降维。污泥絮体结构特征指标与丝状菌膨胀活性污泥的SVI之间有较强的线性相关性(R2=0.779),据此建立了SVI预测模型,丝状菌膨胀污泥的SVI实测值与预测值呈现较强的线性相关性(R2=0.801),模型能较好地预测丝状菌污泥膨胀。     

Effect of pump shear on the performance of a crossflow membrane bioreactor

The influences of pumping shear on the performance of a crossflow membrane bioreactor (MBR) were investigated. To compare the intensity of pumping shear, two types of pumps (a centrifugal pump and a rotary one) were used in turn to recirculate mixed liquor. Rotary pump system imposed much stronger shear to microbial floc than centrifugal one and resulted in severe floc breakage. Colloidal particles and organics were liberated from microbial floc by shear and caused rapid loss of membrane permeability by the formation of dense cake layers on the surface of membrane. Recirculation of mixed liquor with a rotary pump gradually increased the soluble COD in the bioreactor and deteriorated microbial activity. After 7 days' operation, specific oxygen uptake rate (SOUR) of microorganisms in rotary pump system reduced to 78%, of initial condition. With a centrifugal pump, however, buildup of soluble COD was not observed and change in microbial activity was negligible. Sludge yield in MBR process was lower than that (0.4-0.5 g MLVSS gCOD(-1)) reported in a conventional activated sludge process: 0.3 g MLVSS gCOD(-1) for the centrifugal pump system and 0.2 g MLVSS gCOD(-1) for the rotary pump system.     

Extracellular polymeric substances (EPS) properties and their effects on membrane fouling in a submerged membrane bioreactor

A pilot-scale submerged membrane bioreactor (MBR) for real municipal wastewater treatment was operated for over one year in order to investigate extracellular polymeric substances (EPS) properties and their role in membrane fouling. The components and properties of bound EPS were examined by the evaluation of mean oxidation state (MOS) of organic carbons, Fourier transform infrared (FT-IR) spectroscopy, three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy, and gel filtration chromatography (GFC), etc. Test results showed that MOS of organic carbons in the bound EPS was ranging from −0.14 to −0.51, and major components could be assessed as proteins and carbohydrates. FT-IR analysis confirmed the presence of proteins and carbohydrates in the bound EPS. The organic substances with fluorescence characteristics in the bound EPS were identified as proteins, visible humic acid-like substances and fulvic acid-like substances by EEM technology. GFC analysis demonstrated that EPS had part of higher MW molecules and a broader MW distribution than the influent wastewater. It was also found that a high shear stress imposed on mixed liquor could result in the release of EPS, which would in turn influence membrane fouling in MBRs. Bound EPS solution was observed to have a stronger potential of fouling than mixed liquor. During long-term operation of the MBR, bound EPS demonstrated positive correlations with membrane fouling while temperature was verified as a negative factor affecting EPS concentration. Compared to tightly bound EPS (TB-EPS), loosely bound EPS (LB-EPS) showed more significant correlations with membrane fouling. This critical investigation would contribute towards a better understanding of the behavior, composition and fouling potential of EPS in MBR operation.     

污泥龄对活性污泥絮体中磷形态与分布的影响

以污泥龄分别为4和20 d的两组SBR反应器为研究对象,利用SMT法测定活性污泥、细菌细胞和EPS中磷的形态与分布,探讨了污泥龄对活性污泥中磷形态和分布的影响。研究结果表明,高SRT污泥的TP含量约为低SRT污泥的1.56倍,这主要是由于前者较后者有更高的OP和AP含量。高SRT污泥中细菌细胞和EPS的TP含量均高于低SRT污泥中的,且高SRT污泥中EPS的OP含量远大于低SRT污泥中的。高SRT污泥的细菌细胞和EPS分别较低SRT污泥的细菌细胞和EPS有更大的厌氧释磷量和好氧吸磷量,前者厌氧释磷和好氧吸磷主要源自其OP含量的变化,后者则主要源自其IP含量的变化;在厌氧/好氧反应过程中,污泥中EPS的IP含量改变主要源自其NAIP含量的变化。