Modeling cake buildup under TMP-step filtration in a membrane bioreactor: cake compressibility is significant

Fouling is inevitable in membrane bioreactors (MBRs) due to the complex nature of activated sludge, which contains a broad variety of potential foulants. Filter cakes that build up from sludge particles are traditionally highly compressible due to both the deformation of the individual sludge particles and the rearrangement of these particles in the cake. However, this phenomenon has been little examined in studies of fouling mechanisms in MBR systems. This study examines the properties of the cake layer, modeling the cake buildup and specific cake resistance (α), including compressibility, in terms of pressure-dependent α.The changes in fouling resistance during transmembrane pressure (TMP)-step filtration in an MBR setup were simulated using an empirical pressure dependence of the specific cake resistance and a simple mass balance model. The total change in fouling resistance in each TMP step could be divided into an initial rapid change in specific cake resistance due to filter cake compression followed by simple cake buildup. By including cake compression in this simple model, the model fitted the data with high precision. We demonstrated that compressibility should be considered when describing cake fouling in MBRs.     

Lignite aided dewatering of digested sewage sludge

Mechanical dewatering is commonly used to increase the solids content of municipal sludge prior to its disposal. However, if the rate of filtration is slow, mechanical dewatering can be expensive. In this study, the use of lignite to improve the sludge dewatering is investigated. The effectiveness of lignite conditioning of polyelectrolyte-flocculated sludge is examined using mechanical compression tests. Results show that lignite conditioning in conjunction with polyelectrolyte flocculation gives much better dewatering than the polyelectrolyte flocculation alone. Using Darcy's filtration theory, the specific cake resistance and permeability of the compressed cakes are obtained. Both of these parameters are significantly improved after lignite conditioning. Mercury porosimetry tests on compressed cakes show that the porosity of the lignite-conditioned sludge cake is much higher than that of the polyelectrolyte-flocculated sludge and it increases with increasing doses of lignite. The mercury porosimetry results show that the lignite pore volume of pores greater than 0.5 μm are reduced with increasing sludge ratio indicating that sludge is trapped within these pores, whereas smaller pores are unaffected. The yield stress curves for sludge, lignite and sludge-lignite mixtures show that the sludge filter cake is very compressible, but the lignite-conditioned cake has a range of compressibility which although more than lignite indicate that the cake is relatively incompressible at low pressures. Thus, lignite conditioning acts to maintain the permeability of the filter cake during compression dewatering by resisting cake compression. This leads to a trade-off between the rate of dewatering and the solids content of the compressed cake. With lignite conditioning, the dewatering rate can be increased by a factor of five for the same degree of water removal.     

Compression and swelling of activated sludge cakes during dewatering

A drainage/filtration apparatus was developed for automatically determining sedimentation velocity and dewatering rate. Pressure-step testing was used to study filter cake compressibility, resistance, and swelling. Activated sludge was analysed, and the data indicate that the sludge is highly compressible even at low pressures (10 kPa). Furthermore, compressed sludge cakes swell if the pressure is released. Hence, the average specific cake resistance decreases if the pressure is released, though the resistance is higher after the compression cycle than before. Sludge must be dewatered under low pressure, because higher pressure only compresses the cake and does not improve the dewatering rate.     

Biomass growth and activity in a membrane bioreactor with complete sludge retention

This work reports the main results of a bench scale membrane bioreactor operated for more than 100 days without sludge withdrawal and fed on real municipal wastewater. The experiments were oriented towards three main objectives. Firstly, the performance of the system was evaluated under two different volumetric loading rates (0.8 and 1.7 g CODL(react.)(-1)d(-1)). Secondly, biomass growth and accumulation of solids were assessed and a relationship between sludge concentration and volumetric loading rates was proposed. Thirdly, biomass activity was evaluated through respirometric tests, and endogenous and maximum respiration rates of heterotrophic and nitrifying bacteria were determined. The experimental campaign showed that these systems are easy to manage and very rapid to start-up. The SS concentrations under equilibrium conditions for both experimental periods were slightly lower than 10 times the volumetric loading rates, and the organic loading rates reached the same equilibrium value of 0.12 g CODgTSS(-1)d(-1). Furthermore, under equilibrium conditions the system showed very limited sludge production (0.12 g VSSgCOD(rem)(-1)) and low biomass activity, although it readily responded to load variations. Further work is being carried out to evaluate the performance over the long term.     

Novel cake characteristics of waste-activated sludge

Breaking down the time limit constraints for conventional compression-permeation (C-P) cell test, this work has, for the first time, experimentally evaluated the cake characteristics of viable waste-activated sludge subject to polyelectrolyte flocculation and to freeze/thaw treatment under a pressure range of 25-200 kPa. There exists a threshold pressure exceeding which the cake structure would significantly deteriorate. Also, the present biological sludge is a "super-compactible" sludge, whose compactibility is greater than most data ever reported in open literature. The information presented herein has implications to filter design/operation and can be used as a reference data set for examining the existing filtration theories.     

Sludge quality aspects of full-scale reed bed drainage

Sludge-drying reed beds can be a cost-effective and sustainable solution to surplus activated sludge dewatering and mineralization, especially for small wastewater treatment plants. However, the simplicity as well as low energy and monitoring requirements of this technology are often counterbalanced by frequent operational problems consisting of slow and insufficient dewatering, poor vegetation growth, odor, and overall poor mineralization of the sludge residues. The main reason is that the general rules for facility design and operation are based on empirical experience rather than on the actual and current sludge parameters. In this study a new method for the assessment of activated sludge drainage properties has been applied to determine the reasons behind operational problems faced by the operators of reed bed facility accepting surplus activated sludge from two wastewater treatment plants in Esbjerg, Denmark. The importance of sludge quality monitoring as well as the damaging effect of shear forces, oxygen depletion, and long-distance sludge transportation were demonstrated. Finally, more general guidelines for reed bed facility design and operation are given, based on experimental data from seven full-scale plants.     

Sludge dewatering with cyclodextrins

Cyclodextrins (CDs) increase the cake solids and drainage rate of belt-pressed biological or primary sludge when added to the sludge slurry along with conventional conditioning chemicals. These benefits are obtained at very low CD dosage. A 2.8 percentage point increase in cake solids was obtained in a full-scale trial with mixed primary and biological sludge from a paper mill. CDs also decrease the specific resistance to filtration and increase the capture rate of solids during belt pressing. Mechanistic studies showed that CDs increase the surface tension of c-PAM polymers in water and reduce the turbidity, indicating that they are able to aggregate the charged polymers, and, by inference, sludge particles treated with the polymer. A mechanism is proposed where CDs reduce excessively charged regions of the polymer-treated surface of the sludge, thereby facilitating its flocculation.     

考虑淤泥处置场荷载特点的软土地基处理方法

淤泥处置场占地面积较大,对深厚的软土地基,若采用常规的复合地基处理方式无疑将大规模增加工程投资。本文考虑淤泥处置场的荷栽特点,将淤泥的处置过程当作分级加载的过程,分析地基在处置场运行期间地基承载力的提高,通过控制作业程序,使淤泥堆填的速度和荷载与地基承载力随时间的增长相适应,同时设置加筋砂石垫层,为软土地基提供排水层,减小库区的不均匀沉降,提高地基承载力,。最后结合某淤泥处置场工程,分析了该地基处理思路的可行性,对淤泥处置场的方案提出了优化建议。本文的计算方法和地基处理方式可为软土地区污泥处置和垃圾填埋等工程提供有益的参考,     

Operational strategies for an activated sludge process in conjunction with ozone oxidation for zero excess sludge production during winter season

A pilot-scale activated sludge system coupled with sludge ozonation process was operated for 112 days of a winter season without excess sludge wasting. The concept of this process is that the excess sludge produced is first disintegrated by ozone oxidation and then recirculated to a bioreactor in order to mineralize the particulate and soluble organic compounds. The basis of operation was to determine either the optimal amount of sludge in kg SS ozonated each day (SO) or the optimal ozonation frequency under the variable influent chemical oxygen demand (COD) loading and temperature conditions, since the ozone supply consumes costly energy. The optimal SO was obtained using the theoretically estimated sludge production rate (SP) and experimentally obtained ozonation frequency (n). While the SP was mainly subject to the COD loadings, sludge concentration was affected by the temperature changes in winter season. The optimal n was observed between 2.5 and 2.7 at around 15 degrees C, but it was doubled at 10 degrees C. Mixed liquor suspended solids (MLSS) concentration was leveled off at around 5000 mg/L in bioreactor at 15 degrees C, but the volatile fraction of MLSS was fixed around 0.7 indicating that there was no significant inorganic accumulation. Suspended solids (SS) and soluble COD in effluents kept always a satisfactory level of 10 and 15 mg/L with sufficient biodegradation. It was recommended to apply a dynamic SO under variable influent COD loadings and temperature conditions to the activated sludge system without excess sludge production for saving energy as well as system stabilization.     

Characterisation of adsorbents prepared by pyrolysis of sludge and sludge/disposal filter cake mix

Copper and zinc removal from water (pH = 5.0) using adsorbents produced from slow and fast pyrolysis of industrial sludge and industrial sludge mixed with a disposal filter cake (FC), post treated with HCl, is investigated in comparison with a commercial adsorbent F400. The results show that a pseudo-second order kinetics model is followed. The Langmuir-Freundlich isotherm model is found to fit the data best. The capacity for heavy metal removal of studied adsorbents is generally better than that of commercial F400. The dominant heavy metal removal mechanism is cation exchange. Higher heavy metal removal capacity is associated with fast pyrolysis adsorbents and sludge/FC derived adsorbents, due to enhanced cation exchange. Improvement of Zn²⁺ removal via 1 N HCl post-treatment is only effective when exchangeable cations of the adsorbent are substituted with H⁺ ions, which boost the cation exchange capacity. Increase of temperature also enhances metal removal capacity. Fast pyrolysis sludge-based adsorbents can be reused after several adsorption-desorption cycles.     

Pressurised electro-osmotic dewatering of activated and anaerobically digested sludges: electrical variables analysis

Pressurised electro-osmotic dewatering (PEOD) of two sewage sludges (activated and anaerobically digested) was studied under constant electric current (C.C.) and constant voltage (C.V.) with a laboratory chamber simulating closely an industrial filter. The influence of sludge characteristics, process parameters, and electrode/filter cloth position was investigated. The next parameters were tested: 40 and 80 A/m², 20, 30, and 50 V—for digested sludge dewatering; and 20, 40 and 80 A/m², 20, 30, and 50 V—for activated sludge dewatering. Effects of filter cloth electric resistance and initial cake thickness were also investigated.The application of PEOD provides a gain of 12 points of dry solids content for the digested sludge (47.0% w/w) and for the activated sludge (31.7% w/w). In PEOD processed at C.C. or at C.V., the dewatering flow rate was similar for the same electric field intensity. In C.C. mode, both the electric resistance of cake and voltage increase, causing a temperature rise by ohmic effect. In C.V. mode, a current intensity peak was observed in the earlier dewatering period. Applying at first a constant current and later on a constant voltage, permitted to have better control of ohmic heating effect. The dewatering rate was not significantly affected by the presence of filter cloth on electrodes, but the use of a thin filter cloth reduced remarkably the energy consumption compared to a thicker one: 69% of reduction energy input at 45% w/w of dry solids content. The reduction of the initial cake thickness is advantageous to increase the final dry solids content.     

A review of secondary sludge reduction technologies for the pulp and paper industry

The broader application of the activated sludge process in pulp and paper mills, together with increased production, have amplified sludge management problems. With sludge management costs as high as 60% of the total wastewater treatment plant operating costs, and increasingly stringent environmental regulations, it is economically advantageous for pulp and paper mills to reduce their biosolids production. In order to provide a state-of-the-art review of secondary sludge reduction technologies, we have considered the scenarios of lower sludge production through process modifications, and waste-activated sludge reduction through post-treatment. A critical evaluation of all candidate reduction technologies indicates that sludge reduction through treatment process changes appears more appealing than post-treatment alternatives. The former approach offers a clear advantage over the latter in that the treatment process changes reduce sludge production in the first place, thus decreasing sludge management cost. Although it is technically feasible to eliminate the need for sludge disposal, it is unlikely to be economically feasible at this time.     

壳聚糖与硅藻土调理市政污泥

以污泥比阻为评价指标,综合考虑脱水率、泥饼含水率及过滤时间因素,研究使用壳聚糖与硅藻土对污泥单独调理的最佳条件及联合调理改善污泥脱水性能的效果,并与聚丙烯酰胺调理污泥的效果进行了对比。结果表明,先投加0.5g/g硅藻土调理污泥,再投加5 mg/g壳聚糖以30r/min搅拌反应150s,污泥比阻下降了95.43%,脱水率上升至91.02%,泥饼含水率降至83.13%,过滤时间降至29.5s。壳聚糖与硅藻土联合调理的效果明显优于壳聚糖或硅藻土单独调理的效果,且其联合调理改善污泥脱水综合性能的效果优于聚丙烯酰胺调理污泥的效果。     

Biological hydrolysis and acidification of sludge under anaerobic conditions: the effect of sludge type and origin on the production and composition of volatile fatty acids

New wastewater treatment processes resulting in considerably reduced sludge production and more effective treatment are needed. This is due to the more stringent legislations controlling discharges of wastewater treatment plants (WWTPs) and existing problems such as high sludge production. In this study, the feasibility of implementing biological hydrolysis and acidification process on different types of municipal sludge was investigated by batch and semi-continuous experiments. The municipal sludge originated from six major treatment plants located in Denmark were used. The results showed that fermentation of primary sludge produced the highest amount of volatile fatty acids (VFAs) and generated significantly higher COD- and VFA-yields compared to the other sludge types regardless of which WWTP the sludge originated from. Fermentation of activated and primary sludge resulted in 1.9-5.6% and 8.1-12.6% COD-yields, soluble COD (SCOD)/total COD (TCOD), in batch experiments, respectively. The COD-yields for primary, activated and mixed sludge were 19.1%, 6.5% and 21.37%, respectively, in semi-continuous experiments operated at solids retention time (SRT) of 5d and temperature of 37 degrees C. The benefit of fermentation for full-scale application was roughly estimated based on the experiments performed in semi-continuous reactors. The results revealed that even though the VFA production of primary sludge was higher compared to activated sludge, substantial amounts of VFA could be produced by fermentation of activated sludge due to the substantially higher production of activated sludge in WWTPs.     

阳离子表面活性剂改善污泥脱水性能的机理研究

研究了两种含有不同长度疏水链的阳离子表面活性剂CSG—1和CSG-2对活性污泥脱水性能的影响，探讨了两种活性剂在活性污泥体系中的作用机理。结果表明，投加约为干污泥质量10％的CSG-1可使滤饼的含水率降至78％左右，此时污泥的沉降效果较好，污泥毛细吸水时间（CST）也较短（约为62s）；与CSG-1相比，CSG-2虽可使滤饼含水率降至约74％，但其对污泥过滤性能和沉降效果的改善均较差。对CSG—1和CSG-2改善污泥脱水性能的机理研究表明，阳离子表面活性剂通过静电引力和范德华力而被吸附于污泥表面，改变了活性污泥絮体的特性，并引起胞外聚合物（EPS）的分布发生变化，从而改善了污泥的脱水性能，同时也导致了絮体颗粒粒径发生了显著变化。     

厦门城市污泥深度脱水处理及资源化利用研究

结合厦门污水处理厂污泥深度脱水工艺的应用情况,考察了预浓缩—FeC l3和CaO调质—高压隔膜厢式压滤机对污泥的深度脱水效果,并研究了泥饼的处理、处置和资源化利用情况。结果表明,污泥经深度脱水后,泥饼含水率60%,自然放置7 d后,含水率可进一步降至45%左右,满足填埋的要求,另外还可用于制砖和作园林绿化的土壤基质,也可作焚烧处置。污泥深度脱水过程中产生的滤液可循环利用,回流至进水处时,对出水水质影响很小,且可为脱氮过程提供碳源和碱度;回流至重力浓缩池时,可改善污泥沉降性能,提高脱水效率。与传统污泥脱水工艺相比,该工艺具有经济优势。     

Comparative study of laboratory-scale thermophilic and mesophilic activated sludge processes

Laboratory-scale mesophilic (20-35 degrees C) and thermophilic (55 degrees C) activated sludge processes (ASPs) treating diluted molasses wastewater were compared in effluent quality, removal of different COD fractions, sludge yield, floc size, and sludge settleability. The effect of polyaluminium chloride (PAC) with high cationic charge on sludge settleability and effluent quality was also studied. In the ASPs, the hydraulic retention time was 12h in both processes, corresponding to a volumetric loading rate of 3.2+/-1.0 kg COD(filt) m(-3)d(-1). The mesophilic ASP gave 79+/-18% and 90+/-2% and the thermophilic ASP gave 50+/-6% and 67+/-11% total COD (COD(tot)) and GF/A-filtered COD (COD(filt)) removals, respectively. The higher COD values in the thermophilic effluent were due to dispersed particles, such as free bacteria, measured as colloidal COD (COD(col)=difference between soluble COD (COD(sol)) and COD(filt)). Both ASPs gave high (90%) COD(sol) removals. Aeration (24-h) of the thermophilic ASP effluent at 35 degrees C reduced the density of the free bacteria by half, which was also measured as 90% COD(col) removal. Post-aeration of thermophilic effluent at 55 degrees C had no influence either on the density of free bacteria or on the COD(col) values. The use of PAC did not increase COD(tot) or COD(filt) removals in either process, but reduced the mesophilic and increased the thermophilic sludge volume index values.     

腐殖土改善活性污泥沉降与脱水性能的研究

考察了腐殖土对活性污泥沉降和脱水性能的改善效果.结果表明,投加腐殖土可显著改善污泥的沉降和脱水性能,随着腐殖土投量的增加,活性污泥的初沉速度、压缩比、泥饼含固率均明显提高,污泥容积指数(SVI)、污泥比阻(SRF)及毛细吸水时间(CST)均明显降低;当活性污泥浓度为2 300 mg/L、腐殖土的投加量为5.0 g/L时,污泥的初沉速度由原来的1.72 m/h增至3.01m/h,压缩比由原来的2.86增至7.14,SVI由原来的152 mL/g降至61 mL/g;当污泥浓度为7 300mg/L、腐殖土的投加量为5.0 g/L时,污泥比阻由原来的1.33×1012m/kg降至5.7×1011m/kg,CST由原来的20.3 s降至15.7 s,泥饼含固率由原来的13.4%增至33.0%.     

Compression dewatering of municipal activated sludge: effects of salt and pH

Even after mechanical dewatering, activated sludge contains a large amount of water. Due to its composition and biological nature this material is usually highly compressible and known to be difficult to dewater. In the present work, two treatments (salt addition and pH modification) are proposed to highlight some aspects which could explain the poor dewaterability of activated sludge. Dewatering tests are carried out in a pressure-driven device in order to well examine both, filtration and compression stages. Physico-chemical parameters, such as surface charge, hydrophobicity, extracellular polymeric substances (EPS) content and filtrate turbidity are measured on the tested sludge, for a better analysis of dewatering results.The dewatering ability of the sludge is widely linked to the cohesion of the flocculated matrix and the presence of fine particles. Both treatments alter the flocculated matrix and release fine particles. The release of fine particles tends to clog both, the filter cake and the filter medium. Consequently, the filtration rate decreases due to higher resistances to the flow. On another hand, the polymeric matrix breakdown enables to release some water trapped within the floc to the bulk liquid phase and thus facilitates its removal, which tends to decrease the moisture content of the filter-cake. It also impacts the compression dewatering step. The more destroyed structures lead to less elastic cakes and thus a slower primary consolidation stage. At the opposite, the mobility of the broken aggregates within the filter-cake does not seem to be improved by size reduction (the kinetics of the secondary consolidation stage are not significantly modified).     

Evaluation of alternative sludge settleability indices

Proposed modifications of the standard sludge volume index (SVI) to eliminate its unpredictable variation with suspended solids (SS) concentration include stirring, conduct of the test at constant SS concentration, and dilution. We have tested these modifications by making correlations between the respective indices and the content of filamentous organisms in activated sludge expressed in terms of total extended filament length. The results indicate that diluted SVI is the best index of sludge settleability among those tested. Correlation of diluted SVI with specific functional relationships between settling velocity and SS concentration would enable the application of this parameter to prediction of thickening capacity in secondary clarifiers.