Improved computational model (AQUIFAS) for activated sludge, integrated fixed-film activated sludge, and moving-bed biofilm reactor systems, Part I: Semi-empirical model development

Research was undertaken to develop a model for activated sludge, integrated fixed-film activated sludge (IFAS), and moving-bed biofilm reactor (MBBR) systems. The model can operate with up to 12 cells (reactors) in series, with biofilm media incorporated to one or more cells, except the anaerobic cells. The process configuration can be any combination of anaerobic, anoxic, aerobic, post-anoxic with or without supplemental carbon, and reaeration; it can also include any combination of step feed and recycles, including recycles for mixed liquor, return activated sludge, nitrates, and membrane bioreactors. This paper presents the structure of the model. The model embeds a biofilm model into a multicell activated sludge model. The biofilm flux rates for organics, nutrients, and biomass can be computed by two methods--a semi-empirical model of the biofilm that is relatively simpler, or a diffusional model that is computationally intensive. The values of the kinetic parameters for the model were measured using pilot-scale activated sludge, IFAS, and MBBR systems. For the semiempirical version, a series of Monod equations were developed for chemical oxygen demand, ammonium-nitrogen, and oxidized-nitrogen fluxes to the biofilm. Within the equations, a second Monod expression is used to simulate the effect of changes in biofilm thickness and fraction nitrifiers in the biofilm. The biofilm flux model is then linked to the activated sludge model. The diffusional model and the verification of the models are presented in subsequent papers (Sen and Randall, 2008a, 2008b). The model can be used to quantify the amount of media and surface area required to achieve nitrification, identify the best locations for the media, and optimize the dissolved oxygen levels and nitrate recycle rates. Some of the advanced features include the ability to apply different media types and fill fractions in cells; quantify nitrification, denitrification, and biomass production in the biofilm and mixed liquor suspended solids; and perform dynamic simulations.     

Improved computational model (AQUIFAS) for activated sludge, integrated fixed-film activated sludge, and moving-bed biofilm reactor systems, part II: multilayer biofilm diffusional model

Research was undertaken to develop a diffusional model of the biofilm that can be applied in lieu of a semi-empirical model to upgrade an activated sludge system to an integrated fixed-film activated sludge (IFAS) or moving-bed biofilm reactor (MBBR) system. The model has been developed to operate with up to 12 cells (reactors) in series, with biofilm media incorporated to one or more of the zone cells, except the anaerobic zone cells. The values of the kinetic parameters for the model were measured using pilot-scale activated sludge, IFAS, and MBBR systems. The biofilm is divided into 12 layers and has a stagnant liquid layer. Diffusion and substrate utilization are calculated for each layer. The equations are solved simultaneously using a finite difference technique. The biofilm flux model is then linked to the activated sludge model. Advanced features include the ability to compute the biofilm thickness and the effect of biofilm thickness on performance. The biofilm diffusional model is also used to provide information and create a table of biofilm yields at different substrate concentrations that can be used in the semi-empirical model.     

低剂量臭氧的直接通加对活性污泥的活性、微生物种群及污泥减量化的影响

研究构建了2个容积为1.1 L的好氧活性污泥反应器（即1号和2号反应器）1,号反应器每天直接通加低剂量臭氧（投加量为0.01 g O3/g TSS）,不加臭氧的2号反应器作为对照平行运行,均采用每天换一次人工污水的充/排式操作。运行71 d的结果表明2,个反应器对人工污水COD的处理效果基本相同。反应器运行40 d后1,号反应器的污泥浓度比2号反应器的污泥浓度低1 400～1 700 mg/L并可稳定在8 200 mg/L,污泥减量化效果明显。低剂量臭氧的直接通加明显降低了胞内ATP浓度,并影响了微生物的抗氧化活性,2号反应器的平均超氧化物歧化酶和过氧化氢酶酶活比1号反应器分别高了24.3%和9.5%。PCR-DGGE对两反应器微生物种群的分析结果表明：Uncultured gammaproteobacteria bacteri-um、Nannocystis exedens和Uncultured actinobacterium为1号反应器的主要种群;而2号反应器的主要种群为Uncultured bacte-rium和Uncultured gammaproteobacteria bacterium。     

Simultaneous nitrification-denitrification and clarification in a pseudoliquified activated sludge system.

This paper describes results from a pilot study of a novel wastewater treatment technology, which incorporates nutrient removal and solids separation to a single step. The pseudoliquified activated sludge process pilot system was tested on grit removal effluent at flowrates of 29.4 to 54.7 m3/d, three different solid residence times (SRT) (15, 37, and 57 days), and over a temperature range of 12 to 28 degrees C. Despite wide fluctuations in the influent characteristics, the system performed reliably and consistently with respect to organics and total suspended solids (TSS) removals, achieving biochemical oxygen demand (BOD) and TSS reductions of > 96% and approximately 90%, respectively, with BOD5 and TSS concentrations as low as 3 mg/L. Although the system achieved average effluent ammonia concentrations of 2.7 to 3.2 mg/L, nitrification efficiency appeared to be hampered at low temperatures (< 15 degrees C). The system achieved tertiary effluent quality with denitrification efficiencies of 90 and 91% total nitrogen removal efficiency at a total hydraulic retention time of 4.8 hours and an SRT of 12 to 17 days. With ferric chloride addition, effluent phosphorous concentrations of 0.5 to 0.8 mg/L were achieved. Furthermore, because of operation at high biomass concentrations and relatively long biological SRTs, sludge yields were over 50% below typical values for activated sludge plants. The process was modeled using activated sludge model No. 2, as a two-stage system comprised an aerobic activated sludge system followed by an anoxic system. Model predictions for soluble BOD, ammonia, nitrates, and orthophosphates agreed well with experimental data.     

Effect of operational parameters on the removal of particulate chemical oxygen demand in the activated sludge process.

The removal of particulate material in the aeration basin of the activated sludge process is mainly attributed to bioflocculation and hydrolysis of particulate substrate. The bioflocculation process in the aeration tank of the activated sludge process occurs only under favorable conditions in the system, and several common operational parameters affect its performance. The principal objective of this research was to observe the effect of mixed liquor suspended solids, solids retention time (SRT), and extracellular polymer substances on the removal of particulate substrate by bioflocculation. A first-order particulate removal expression, based on flocculation, accurately described the removal rates for supernatant suspended solids and colloidal chemical oxygen demand. Based on the results presented in this investigation, a mixed liquor concentration of approximately 2200 mg/L, an SRT of at least 3 days, and a contact time of 30 minutes are needed for relatively complete removal of the particulate substrate in a plug-flow reactor.     

Chromium species behaviour in the activated sludge process

The purpose of this research was to compare trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) removal by activated sludge and to investigate whether Cr(VI) reduction and/or Cr(III) oxidation occurs in a wastewater treatment system. Chromium removal by sludge harvested from sequencing batch reactors, determined by a series of batch experiments, generally followed a Freundlich isotherm model. Almost 90% of Cr(III) was adsorbed on the suspended solids while the rest was precipitated at pH 7.0. On the contrary, removal of Cr(VI) was minor and did not exceed 15% in all experiments under the same conditions. Increase of sludge age reduces Cr(III) removal, possibly because of Cr(III) sorption on slime polymers. Moreover, the decrease of suspended solids concentration and the acclimatization of biomass to Cr(VI) reduced the removal efficiency of Cr(III). Batch experiments showed that Cr(III) cannot be oxidized to Cr(VI) by activated sludge. On the contrary, Cr(VI) reduction is possible and is affected mainly by the initial concentration of organic substrate, which acts as electron donor for Cr(VI) reduction. Initial organic substrate concentration equal to or higher than 1000 mgl(-1) chemical oxygen demand permitted the nearly complete reduction of 5 mgl(-1) Cr(VI) in a 24-h batch experiment. Moreover, higher Cr(VI) reduction rates were obtained with higher Cr(VI) initial concentrations, expressed in mg Cr(VI) g(-1) VSS, while decrease of suspended solids concentration enhanced the specific Cr(VI) reduction rate.     

Activated sludge deflocculation in response to chlorine addition: the potassium connection.

Chlorination is often used to control filamentous bulking in activated sludge systems. Pure culture and mixed-liquor experiments showed that soluble potassium (K+) concentrations increased by 2.4 mg/L (80%) and 1.5 to 3.6 mg/L (11 to 30%) in the bulk liquid phase of pure and activated sludge cultures that were exposed to chlorine, relative to unchlorinated controls. Effluent turbidity and total suspended solids from settled mixed liquor increased significantly in both short-term batch and sequencing batch reactor experiments when chlorine mass load increased above 6 milligrams of chlorine per gram mixed liquor volatile suspended solids (mg Cl2/g MLVSS) in a single dose, which correlated with a localized chlorine concentration at the dose point of 10 mg/L as Cl2 or greater. The results support the hypothesis that the glutathione-gated potassium efflux (GGKE) bacterial stress response may contribute to increased effluent turbidity associated with high doses of mixed-liquor chlorination. It is suggested that potassium is a useful parameter to monitor at full-scale facilities when determining chlorine mass doses that should be used to control filaments and minimize increases in effluent turbidity.     

Enhanced excess sludge hydrolysis and acidification in an activated sludge side-stream reactor process with single-stage sludge alkaline treatment: a pilot scale study

A pilot-scale side-stream reactor process with single-stage sludge alkaline treatment was employed to systematically investigate characteristics of excess sludge hydrolysis and acidification with alkaline treatment and evaluate feasibility of recovering a carbon source (C-source) from excess sludge to enhance nutrient removal at ambient temperature. The resulting C-source and volatile fatty acid specific yields reached 349.19 mg chemical oxygen demand (COD)/g volatile suspended solids (VSS) d⁻¹ and 121.3 mg COD/g VSS d⁻¹, respectively, the process had excellent C-source recovery potential. The propionic-to-acetic acid ratio of the recovered C-source was 3.0 times that in the influent, which beneficially enhanced biological phosphorus removal. Large populations and varieties of hydrolytic acid producing bacteria cooperated with alkaline treatment to accelerate sludge hydrolysis and acidification. Physicochemical characteristics indicated that recovered C-source was derived primarily from extracellular polymeric substances hydrolysis rather than from cells disruption during alkaline treatment. This study showed that excess sludge as carbon source was successfully recycled by alkaline treatment in the process.

     

Mechanism of effective nocardioform foam control measures for non-selector activated sludge systems.

Solids retention time (SRT), biological scum trapping and recycle, and the dynamic equilibrium between Nocardioform populations in the foam and the mixed liquor are the controlling factors in activated sludge foaming events caused by Nocardioform bacteria. For the operating modes described in this paper, a cured mixed liquor foaming condition (filament counts of approximately 10(5) intersections/g volatile suspended solids) was only achieved when SRT control, selective wasting, and polymer addition were in effect. Solids retention time control, with the SRT remaining below 1.5 days, and selective wasting will cure a severely foaming mixed liquor, but effects will only be observed after 3 or 4 months after implementation. The combined wastage of Nocardioform bacteria from selective wasting and SRT control can ensure long-term foam control to the operation of a pure-oxygen activated sludge system with foam-trapping features. An SRT of 0.3 days will result in the complete washout of Nocardioform bacteria from the activated sludge system, which can then operate at an SRT of 3 days free of Nocardioform. Polymer addition to mixed liquor is only effective for foam control when a large portion of the system biomass exists as a heavy layer of foam above the mixed liquor.     

Recovery of polyhydroxyalkanoate from activated sludge in an enhanced biological phosphorus removal bench-scale reactor.

A sequencing batch reactor was used to study the possibility of harvesting polyhydroxyalkanoate (PHA) from enhanced biological phosphorus removal (EBPR) processes without compromising treatment quality. Because, in EBPR, the highest PHA concentrations are observed after exposure of the sludge to anaerobic conditions, PHA accumulation was evaluated with collection of waste activated sludge (WAS) at the end of the anaerobic stage, in addition to the traditional removal after the aerobic stage. The system achieved good phosphorus removal, regardless of the point of WAS collection. When sludge was harvested at the end of the anaerobic stage, the PHA content of the sludge ranged from 7 to 16 mg PHA/100 mg mixed liquor volatile suspended solids. Although this level of PHA production is below levels obtained with pure cultures, the demonstrated ability to harvest PHA, while simultaneously satisfying phosphorus removal in an EBPR process, is a key initial step towards of the use of wastewater treatment plants for PHA production.     

Identifying different types of bulking in an activated sludge system through quantitative image analysis

The present study proposes an image analysis methodology for the identification of different types of disturbances in wastewater treatment activated sludge systems. Up to date, most reported image analysis methodologies have been used in activated sludge processes with the aim of filamentous bulking detection, however, other disturbances could be foreseen in wastewater treatment plants. Such disturbances can lead to fluctuations in the biomass contents, affecting the mixed liquor suspended solids (MLSS), and in the sludge settling ability, affecting the sludge volume index (SVI). Therefore, this work focuses on predicting the MLSS and SVI parameters for different types of disturbances affecting an activated sludge system. Four experiments were conducted simulating filamentous bulking, zoogleal or viscous bulking, pinpoint floc formation, and normal operating conditions. Alongside the MLSS and SVI determination, the aggregated and filamentous biomass contents and morphology were studied as well as the biomass Gram and viability status, by means of image analysis.     

Effect of mixed liquor suspended solids concentration on the biodegradation of nitrilotriacetic acid in the activated sludge process

The biodegradation of nitrilotriacetic acid in laboratory simulations of the activated sludge process has been studied at two different concentrations of mixed liquor suspended solids. Responses to changes in the influent nitrilotriacetic acid concentration such as those which might be associated with a “wash day” have been examined.     

实际污水与模拟污水活性污泥系统的特性差异

实验中经常采用人工配置的模拟生活污水,为了研究其与实际生活污水活性污泥系统的特性差异,采用2个序批式间歇反应器(SBR)进行平行实验(厌氧、好氧方式运行),系统地考察了在进水主要组分和运行参数相同的情况下,不同原水对活性污泥系统脱氮、除磷、比好氧速率、污泥絮体形态和出水水质等方面的影响。结果表明,模拟污水系统的硝化活性强于实际污水系统,两者的平均硝化速率分别为7.43 mg NH4+-N/(L.h)和5.55 mg NH4+-N/(L.h)。在前置厌氧段,模拟污水系统的释磷量比实际污水系统高出36.45%。两者在后续好氧阶段都能够充分吸磷。模拟污水系统的平均比好氧速率(SOUR)高达64.54 mg O2/(g MLSS.h),而实际污水系统的则只有32.81 mg O2/(g MLSS.h)。模拟污水系统的污泥絮体疏松,粒径小,形状不规则,沉降性差,沉后出水平均悬浮物浓度(SS)为20 mg/L;而实际污水系统的污泥絮体则密实、粒径大,沉降性好,沉后水十分清澈,SS几乎检测不出。     

蒽醌废水生化处理的活性污泥驯化

通过比较不同的污泥驯化方法研究了活性污泥法处理蒽醌废水的可行性.结果表明,常规的CODCr负荷提升法需要50 d才可以驯化出活性污泥.在进水中添加1 g/L葡萄糖可以快速提高污泥浓度,但是驯化出的污泥活性很低,不能满足废水的处理要求.最好的方法是添加1 g/L葡萄糖和原废水交叉循环驯化法,第28天就可以驯化出高活性的污泥.在平均进水CODCr为1 050 mg/L、HRT为24 h的条件下,CODCr去除率达到95.6%.     

Effects of chlorination on the adhesion strength and deflocculation of activated sludge flocs.

A side effect of the application of chlorine for controlling filamentous bulking is deflocculation of floc-forming bacteria, which may cause unacceptable effluent deterioration depending on dosing. It was assumed that chlorine may adversely affect the adhesion ability of floc bacteria, promoting their erosion in shear flow. The effect of chlorination on the strength of activated sludge flocs was investigated. The adhesion-erosion (AE) model developed by Mikkelsen and Keiding was used to interpret results from deflocculation tests with varying shear and solids concentration. The AE model yields the adhesion enthalpy (deltaHG/R) of cells in sludge flocs and parameters from the model were used to quantify the sludge in terms of floc strength. Two activated sludges with different initial characteristics were studied. The resulting model parameters showed that the AE model was suitable for quantifying the bond energy of particles to the activated sludge exposed to chlorine. For one activated sludge, adhesion of cells was largely unaffected by the applied chlorine doses. A second sludge showed reduced adhesion strength with chlorine, leading to increasing deflocculation. The simple batch test and AE model proved valuable for assessing the effect of chlorination on the flocs in activated sludge. By use of these procedures, it is possible to determine acceptable chlorine dosing to avoid excessive deflocculation and effluent deterioration.     

Specific resistance to filtration of biomass from membrane bioreactor reactor and activated sludge: effects of exocellular polymeric substances and dispersed microorganisms.

This study investigates the effect of dispersed microorganisms and exocellular polymeric substances on biomass dewaterability. Specific resistance to filtration (SRF) was measured for biomass from a membrane bioreactor and a completely mixed activated sludge system. Both laboratory-scale reactors were fed with synthetic wastewater and operated at a high food-to-microorganism ratio (F/M) (1 to 11 kilograms chemical oxygen demand per kilogram mixed liquor volatile suspended solids per day [kgCOD/(kg MLVSS.d)]) and short solids retention times (0.25 to 5 d). The SRF values were affected by strong interactions of three parameters: (1) the mixed liquor suspended solids concentration, (2) the amount of dispersed microorganisms, and (3) the exocellular polymeric substances (EPS) concentration. At F/M smaller than 2 kg COD/(kg MLSS.d) and mixed liquor suspended solids (MLSS) concentration higher than 2000 mg/L, increasing amount of dispersed microorganisms in the biomass yielded higher SRF values. However, at high F/M (> 5 kg COD/kg MLSS.d) and low MLSS concentrations (< 600 mg/L), lower EPS concentrations resulted in slightly smaller SRF values, even though the amount of dispersed microorganisms in the biomass was much higher. Thus, at low MLSS concentrations, EPS concentrations rather than the amount of dispersed microorganisms tend to control SRF.     

Effects of sludge properties on the thickening and dewatering of waste activated sludge.

The thickening and dewatering of waste activated sludge, from a pilot-scale submerged membrane bioreactor and two bench-scale, complete-mix activated sludge reactors (high-shear and low-shear aeration) treating the same municipal primary effluent, were investigated. Solids settling and compaction were measured using the diluted sludge volume index (DSVI) analysis and a batch centrifugation analysis, respectively. Elevated levels of filamentous microorganisms resulted in higher DSVI values and lower centrifuged pellet concentration. Elevated levels of nocardioform bacteria resulted in lower solids float concentrations, and higher colloidal material reduced solids recovery in batch flotation experiments. Sludge filterability, measured as time-to-filter, was shown to be a function of extracelluar polymeric substances and colloidal material, where higher levels of either reduced sludge filterability. Additional research is necessary to confirm these results using full- or demonstration-scale thickening and dewatering units.     

A/O-MBR中MLSS浓度对污泥性能及膜通量的影响

采用国产聚丙烯中空纤维帘式膜组件,进行了A/O-MBR系统处理生活污水的实验研究,主要探讨了系统中污泥浓度(MLSS)与膜通量变化过程的关系。实验结果表明,MLSS由3 805 mg/L升高到6 912 mg/L时,污泥混合液胞外聚合物(EPS)由43 mg/g VSS增加到81 mg/g VSS,EPS中多糖与蛋白质的比例从0.87增加到1.08。同时,污泥相对疏水性(RH)的降低与Zeta电位的升高也在一定程度上共同促进了膜污染速率的上升。实验条件下,当运行时间为60 d左右,MLSS升高至6 200 mg/L时,跨膜压差上升迅速,膜组件清洗周期由初始的22 d缩短为11 d。A/O-MBR中由于MLSS浓度变化而导致的活性污泥混合液特性的变化,是影响膜通量变化的重要原因。     

Oxidative co-treatment using hydrogen peroxide with anaerobic digestion of excess municipal sludge.

The effect of an oxidative co-treatment on anaerobic digestion of a mixture of primary and waste activated sludge was investigated. The oxidant used in this study was hydrogen peroxide (H2O2). A maximum improvement in solid destruction of 15.2% was achieved in the overall process, with a dosage of 2.0 g H2O2/g influent volatile suspended solids (VSS(influent)). All configurations operated at this dosage also showed statistically significant increases in solids removal. A statistically significant enhancement in overall solids destruction was observed for the lower oxidant dosage (0.5 H2O2/g VSS(influent)). Surprisingly, for 1.0 g H2O2/g VSS(influent), only one of the three configurations involving oxidative co-treatment showed significant increases in solids destruction. Special attention was paid to the performance of this process relative to fecal coliforms destruction. Class A biosolids were obtained for all the different hydrogen peroxide dosages used when oxidative co-treatment is combined with a two-stage anaerobic digestion process.     

活性污泥胞外聚合物提取方法研究

考察4种方法(蒸气法、EDTA法、NaOH法和搅拌法)对3种活性污泥(可乐厌氧活性污泥、可乐好氧活性污泥和市政污水污泥)胞外聚合物的提取效率,对比分析了各提取方法对不同样品提取所得胞外聚合物的组成影响,结果表明,EDTA法提取多糖的效率最高,提取市政污水污泥、可乐好氧活性污泥和可乐厌氧活性污泥胞外聚合物糖分别在8.3、16.7和9.7 mg/g VSS左右;NaOH提取蛋白质的效率最高,提取市政污水污泥、可乐好氧活性污泥和可乐厌氧活性污泥胞外聚合物蛋白质分别为:3.15、0.55和5.57 mg/g VSS.对于3种不同种类污泥,EDTA法提取EPS总量效率最高,分别为市政污水污泥10.7 mg/g VSS、可乐好氧活性污泥16.8 mg/g VSS和可乐厌氧活性污泥9.97 mg/g VSS,分别比效率最低的搅拌法高出5倍、5倍和3倍.