Co-conditioning and dewatering of chemical sludge and waste activated sludge

The conditioning and dewatering behaviors of chemical and waste activated sludges from a tannery were studied. Capillary suction time (CST), specific resistance to filtration (SRF), and bound water content were used to evaluate the sludge dewatering behaviors. Zeta potentials were also measured. Experiments were conducted on each sludge conditioned and dewatered separately, and on the sludge mixed at various ratios. Results indicate that the chemical sludge was relatively difficult to be dewatered, even in the presence of polyelectrolyte. When the waste activated sludge was mixed with the chemical sludge at ratios of 1:1 and 2:1, respectively, the dewaterability of chemical sludge improved remarkably while the relatively better dewaterability of the waste activated sludge deteriorated only to a limited extent. As the mixing ratios became 4:1 and 8:1, the dewaterability of the mixed sludge was equal to that of the waste activated sludge. The optimal polyelectrolyte dosage for the mixed sludge was equal to or less than that of the waste activated sludge. It is proposed that the chemical sludges act as skeleton builders that reduce the compressibility of the mixed sludge whose dewaterability is enhanced. Bound water contents of sludge decreased at low polyelectrolyte dosage and were not significantly affected as polyelectrolyte dosage increased. Advantages and disadvantages of co-conditioning and dewatering chemical sludge and waste activated sludge were discussed.     

Electrical field: A historical review of its application and contributions in wastewater sludge dewatering

Electric field-assisted dewatering, also called electro-dewatering, is a technology in which a conventional dewatering mechanism such a pressure dewatering is combined with electrokinetic effects to realize an improved liquid/solids separation, to increase the final dry solids content and to accelerate the dewatering process with low energy consumption compared to thermal drying. Electro-dewatering is not a new idea, but the practical industrial applications have been limited to niche areas in soil mechanics, civil engineering, and the ceramics industry. Recently, it has received great attention, specially, in the fields of fine-particle sludge, gelatinous sludge, sewage sludge, pharmaceutical industries, food waste and bull kelp, which could not be successfully dewatered with conventional mechanical methods. This review focuses on the scientific and practical aspects of the application of an electrical field in laboratory/industrial dewatering, and discusses this in relation to conventional dewatering techniques. A comprehensive bibliography of research in the electro-dewatering of wastewater sludges is included.As the fine-particle suspensions possess a surface charge, usually negative, they are surrounded by a layer with a higher density of positive charges, the electric double layer. When an electric field is applied, the usually negative charged particles move towards the electrode of the opposite charge. The water, commonly with cations, is driven towards the negative electrode. Electro-dewatering thus involves the well-known phenomena of electrophoresis, electro-osmosis, and electromigration. Following a detailed outline of the role of the electric double layer and electrokinetic phenomena, an analysis of the components of applied voltage and their significance is presented from an electrochemical viewpoint. The aim of this elementary analysis is to provide a fundamental understanding of the different process variables and configurations in order to identify potential improvements. Also discussed herein is the investigation of the electrical behaviour of a porous medium, with particular emphasis on porous medium conductivity determination.     

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.     

Enhanced sludge dewatering by dual polyelectrolytes conditioning

Sludge dewatering by dual polyelectrolytes conditioning was investigated in this study. Single polyelectrolyte is utilized in sludge conditioning conventionally, in which charge neutralization and bridging are involved in the reactions. In the current study, both cationic and non-ionic polyelectrolytes were utilized simultaneously in the conditioning. Waste activated sludge was sampled from a synthetic fiber plant, and used in the experiment. Capillary suction time (CST), specific resistance to filtration (SRF), and settling rate were used to evaluate the sludge dewatering behaviors. Zeta potentials were also measured. Sludge dewatering behaviors resulting from single and dual polyelectrolytes conditioning were compared. It was found that sludge conditioned with dual polyelectrolytes performed better in fine particle capture and in the formation of larger flocs that resulted in a better dewaterability and less chance of overdosing. The dosing sequence in dual polyelectrolytes also affected the sludge dewaterability. Reaction mechanisms in dual polyelectrolyte conditioning were proposed based on enhanced flocculation of sludge particles.     

Optimization of sewage sludge conditioning and pressure dewatering by statistical modelling

By using a quadratic model, an assessment was made of the relative importance of different sludge and polyelectrolyte variables with respect to sludge pressure dewatering. It was seen that the polyelectrolyte characteristics and dose dominated the cake dry matter content and that sludge properties were less important, especially the electrophoretic mobility of the sludge, which showed a restricted natural variability over the 10-month sampling period. The developed quadratic model in this study appeared very well suited to quantitatively predict the pressure dewatering properties of sludge, allowing the selection of the dose and polyelectrolyte type that yield the best dewatering result. It was further shown that relatively small deviations from the optimal polyelectrolyte dose caused only small changes in cake dry matter values. The model appeared to be applicable on a long-term basis, as it was able to predict the dewaterability of several sludge samples from the studied wastewater treatment plant after more than 3 years. Finally, the model also allowed an (simplified) economic evaluation, indicating that for high cake disposal costs, the polyelectrolyte should be taken that guarantees the best dewatering results, even if it has to be applied in high doses.     

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。     

Experimental analysis of centrifugal dewatering process of polyelectrolyte flocculated waste activated sludge

The study experimentally investigated the centrifugal separation of moisture from activated sludge subject to cationic polyelectrolyte flocculation. An arm-suspended centrifuge was employed which allowed in-situ detection on all positions of interfaces of centrifuged sludge as functions of time. Experimental results revealed that, sludge flocculation would yield a significant sedimentation effect at the first phase of centrifugation. Therefore, not as suggested in conventional centrifugal-filtration models, the most significant moisture-removal stages included filtrate to flow through a wet cake. Moreover, an optimal rotational speed exists at which the moisture-removal rate reaches a maximum value. New theories/correlations are required to describe centrifugation dewatering of polyelectrolyte flocculated sludge.     

Measurement of additional shear during sludge conditioning and dewatering

Optimum polymer dose is influenced both by the polymer demand of the sludge and the shear applied during conditioning. Sludge exposed to additional shear following conditioning will experience a decrease in cake solids concentration for the same polymer dose. Therefore, it is necessary to measure or quantify the additional shear in order to optimize the conditioning and dewatering. There is currently no direct or indirect method to achieve this. The main objective of this study was to develop a method based on torque rheology to measure the amount of shear that a sludge network experiences during conditioning and dewatering. Anaerobically digested sludge samples were exposed to increasing levels of mixing intensities and times, and rheological characteristics of samples were measured using a torque rheometer. Several rheological parameters were evaluated including the peak torque and totalized torque (area under the rheograms). The results of this study show that at the optimum polymer dose, a linear relationship exists between the applied shear and the area under the rheograms, and this relationship can be used to estimate an unknown amount of shear that the sludge was exposed to. The method is useful as a research tool to study the effect of shear on dewatering but also as an optimization tool in a dewatering automation system based on torque rheology.     

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。     

粉煤灰调理自来水厂排泥水污泥的比阻

自来水厂污泥直排水体会使水环境恶化,为此开展了掺调理剂改善其脱水性能的试验研究。结果表明,自来水厂污泥不宜采用混凝处理,而利用电厂粉煤灰作调理剂,可使污泥比阻大幅度下降,改善了脱水性能;细粉煤灰的最佳投量是20g/100mL,粗粉煤灰的最佳投量是30g/100mL;粉煤灰与污泥混合形成的滤饼掺入一定比例的煤炭后可做成燃料,实现了自来水厂污泥的焚烧处置。     

污泥脱水性能调理技术研究进展

简要介绍了目前污泥脱水性能调理技术现状,介绍了几种新型的污泥脱水性能改善技术——微波技术、微生物絮凝剂技术和壳聚糖絮凝剂理论及在污泥处理方面应用研究现状,提出了这三种技术今后在污泥处理领域应用的研究重点。     

Physico-chemical characteristics of full scale sewage sludges with implications to dewatering

An investigation was carried out for a variety of different sewage sludges in order to establish correlations between sludge composition, structure and dewatering properties. Results indicated that the fraction of extracellular polymeric substances (EPS) in sludges was the most important parameter with respect to sludge structure. With high EPS contents, sludges had a lower shear sensitivity and lower degree of dispersion. This in turn lead to better filterability in terms of low resistance to filtration (SRF). The floc stabilising role of EPS components was not consistent with DLVO-theory, as the zeta-potential increased with increased EPS content due to increased EPS charge content. This indicates that polymer entanglement is a key factor to stable floc structure. This does not rule out the possible change in dispersion due to changed electrostatic repulsion for a given EPS content. While EPS had a good effect on floc stability and filterability, the cake dry matter content decreased with large EPS contents. This could be due to an osmotic pressure related to the polymer charge quantity, or it could be caused by water entrapment in the floc structures. A high degree of sludge dispersion increased the cake dry matter content in filtration. This mechanism is, however, impractical due to high SRF and not important to conditioned sludge. In practice, dewatering also includes sludge expression. Taking this into account, osmosis related to EPS charges is likely to be increasingly important (increasing the negative effect of EPS content on cake dry matter).     

基坑开挖前潜水降水引起的地下连续墙侧移研究

在天津地铁3号线某车站基坑工程开展了现场潜水预降水试验与测试,发现基坑开挖前的潜水预降水即可引起可观的地下连续墙向坑内的位移,进而引起坑外地面及建（构）筑物沉降;并对比了预降水前在地下连续墙墙顶设置水平支撑与否及分段降水时对应的地连墙侧移模式及大小,发现降水前提前在墙顶设置水平支撑能够大幅度减小预降水过程中地连墙墙顶侧移,并使得墙体侧移模式由悬臂型转化为内凸型。对试验依托的实际工程,结合有限元软件ABAQUS建立考虑降水井瞬态降水的三维流固耦合模型,研究了基坑开挖前潜水降水过程地连墙侧移机理。研究表明,基坑开挖前潜水降水导致坑内土体在土体自重和墙体侧压力作用下发生三向固结,从而导致墙体发生侧向位移,同时坑内外墙、土压力发生重分布。基坑提前设置第一道水平支撑再降水、结合信息化施工分层分段降水等均可有效减小基坑开挖前潜水降水引起的变形。     

Electro-dewatering of wastewater sludge: influence of the operating conditions and their interactions effects

Electric field-assisted dewatering, also called electro-dewatering (EDW), is a technology in which a conventional dewatering mechanism such a pressure dewatering is combined with electrokinetic effects to realize an improved liquid/solids separation, to increase the final dry solids content and to accelerate the dewatering process with low energy consumption compared to thermal drying. The application of these additional fields can be applied to either or both dewatering stages (filtration and/or compression), or as a pre-or post-treatment of the dewatering process. In this study, the performance of the EDW on wastewater sludge was investigated. Experiments were carried out on a laboratory filtration/compression cell, provided with electrodes, in order to apply an electrical field. The chosen operating conditions pressure (200-1200 kPa) and voltage (10-50 V) are sufficient to remove a significant proportion of the water that cannot be removed using mechanical dewatering technologies alone. A response surface methodology (RSM) was used to evaluate the effects of the processing parameters of EDW on (i) the final dry solids content, which is a fundamental dewatering parameter and an excellent indicator of the extent of EDW and (ii) the energy consumption calculated for each additional mass of water removed. A two-factor central composite design was used to establish the optimum conditions for the EDW of wastewater sludge. Experiments showed that the use of an electric field combined with mechanical compression requires less than 10 and 25% of the theoretical thermal drying energy for the low and moderate voltages cases, respectively.     

Sequential polymer dosing for effective dewatering of ATAD sludges

Dewatering problems associated with the sludge from autothermal thermophilic aerobic digestion (ATAD) of sludge, result in large chemical conditioning costs for effective dewatering. A variety of chemical conditioners were used to improve dewatering, but none of them were able to dewater the sludge as desired at acceptable conditioning doses. It was found that during the digestion process, chemical precipitation of divalent cations occurred. Some of the ATAD sludge colloids were found to have a positive zeta potential and these were thought to be the precipitated divalent cations. Sequential polymer dosing using either iron or cationic polymer, followed by anionic polymer, was found to improve dewatering. The use of anionic polymer is essential and allows the use of smaller amounts of iron or cationic polymer for effective dewatering. The use of the less expensive anionic polymer along with cationic polymers has the potential to make the use of the ATAD process more economical.     

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.     

Viscosity as a Criterion for Optimum Dosing of Polymers in Waterworks Sludges

This study was carried out to evaluate the use of viscosity measurements as a method of assessing the optimum use of organic polymers as conditioning agents in sludge dewatering. The study focused on sludge from the clarifiers of a water-treatment works. In addition to viscosity measurements, the programme included an appraisal of other techniques for specifying dewater-ability, e.g. the specific resistance to filtration, the capillary suction time and the electrical charge status via streaming-current measurements.     

Impact of sewage sludge conditioning and dewatering on the fate of nonylphenol in sludge-amended soils

The fate of (14)C-labelled p353-nonylphenol (NP) in soils amended with differently treated sludges originating from the same precursor sludge was assessed. The effects of commonly applied conditioning and dewatering techniques were investigated. Nonylphenol was degraded considerably faster in soils amended with liquid sludge, while a significant portion of it remained intact and extractable by organic solvents when sludge had been centrifuged before soil amendment. Mineralization was reduced or even inhibited when freeze-thaw or lime conditioning was applied, respectively. Flocculation by an acrylamide-based cationic polymer led to the formation of a nitro-addition product of nonylphenol in soil, as well to decreased mineralization rates after prolonged incubation times. Possible mechanisms underlying the observations are suggested and discussed.     

Application of microwaves for sewage sludge conditioning

The article focuses on the effect of microwave treatment on the de-waterability of sewage sludge. The specific resistance to filtration, capillary suction time, and dry matter content in centrifuged sludge cake were measured. The quality of sludge liquor obtained after microwave processing was also analysed. It was found out that microwaves improve sludge de-waterability. The quantitative improvement depends on sludge type-better effect of microwave processing were observed in the primary sludge compared to mixed or digested sludges. If microwave treatment is followed by polyelectrolyte conditioning, the effects are considerably better than while these conditioning methods are used separately. Microwave conditioning increased burden of organic matter (BOD(5), COD) in the sludge liquor.     

Waste activated sludge conditioning by direct slurry freezing

It has been widely observed and noted that waste sludges subjected to freezing undergo a conditioning effect that greatly enhances subsequent dewatering. However, wide-spread application of freezing for conditioning purposes has never been practiced because of the high operating costs associated with conventional indirect freezing systems. Furthermore, all reported efforts involved the solid freezing of the experimental sludge. The purpose of this research was to evaluate the technical and economic feasibility of waste activated sludge conditioning for dewatering using a direct slurry freezing system. Liquid butane was used as the refrigerant and it was bubbled through the liquid sludge to form an ice-sludge slurry. Economic investigation included butane recovery techniques.The results of the investigation showed that the dewaterability of waste activated sludge can be greatly improved by slurry freezing with liquid butane. Slurry freezing results in better conditioning and a better quality supernatant than does solid freezing because fewer cellular solids are destroyed by the freezing pressure. The conditioning effect is also comparable to or better than conditioning by widely used processes such as heat treatment. Furthermore, recovery of butane for subsequent reuse reduces the cost of slurry freezing to less than the cost of nearly all conditioning processes in current use.