Rheological and fractal characteristics of unconditioned and conditioned water treatment residuals

Methods, mechanisms, models and tail gas emissions of convective drying in sludge: A review

In tandem with the population and economic growth worldwide, the scale of wastewater treatment has been increasing each year. Thus, a large amount of sludge is being produced. If the problem of sludge treatment and disposal cannot be effectively solved, it will cause serious environmental pollution. The premise of sludge drying is that sludge is "harmless" and can be "recycled." Currently, the studies on convective drying focus on the direction of thin-layer drying, fluidized bed drying, spray drying and pneumatic drying. This paper systematically reviews the convective drying technology of sludge. First, the effects of air velocity temperature, relative humidity and particle size on the drying effect are precisely described, as well as the four different drying stages in the drying process, including preheating, constant rate drying, first falling rate drying, and second falling rate drying stages. Second, the research progress of different convective drying treatment technologies and the application of eight mathematical models of thin-layer drying in this field are elaborated. The effects of sludge shrinkage formation mechanisms and sludge viscous resistance generation during the drying process are also discussed in detail. The formation mechanism of sludge shrinkage and the effect of sludge viscosity resistance during drying are also elaborated. Finally, the main dry tail gases and restraining methods are elaborated during the drying process. This paper will provide a structured reference for the related research of sludge convective drying in the future.

Micro-interfacial mechanisms on sludge dewaterability enhancement using cerium chloride for preparation of carbon-based functional material

In this work, sludge conditioning efficiency of cerium chloride (CeCl₃) in combination with organic polymers was evaluated, the floc microstructure and extracellular polymeric substances (EPS) properties under flocculation conditioning were analyzed. The interaction mechanisms between EPS and Ce(III) were systematically investigated through two-dimension correlation spectroscopy, X-ray photoelectron spectroscopy, and confocal laser scanning microscopy. In addition, the adsorption and catalytic abilities of Ce-sludge based carbon (SBC) in tetracycline (TC) removal were evaluated. The results showed that CeCl₃ conditioning performed well in improving sludge dewaterability, and CeCl₃ and cationic polyacrylamide showed a synergistic effect in sludge conditioning. Contents of EPS decreased as the dosage of CeCl₃ increased because of charge neutralization and complexation reactions. 2D-UV-FTIR heterospectral correlation spectroscopy analysis suggested the reaction activity of EPS to Ce(III) followed the order of humic acid > protein > polysaccharide. The decrease of α-helix content improved the hydrophobicity of proteins in EPS, which was responsible for sludge dewaterability improvement in CeCl₃ conditioning. Besides, the SBC was prepared with CeCl₃ conditioned sludge for treating water containing TC. Ce-SBC had a dual function of adsorption and Fenton-like activity. This work provides a sludge recycling process that coupled chemical conditioning to pyrolysis carbonization to prepare functional carbon-based materials.

Variations in macro and micro physicochemical properties of activated sludge under a moderate oxidation-in situ coagulation conditioning: Relationship between molecular structure and dewaterability

A moderate Mn(Ⅶ) oxidation-in situ Fe(Ⅲ) coagulation (Mn(Ⅶ)-Fe(Ⅱ)) conditioning strategy was proposed to improve sludge filterability and release the water bound in extracellular polymeric substances (EPS). During Mn(Ⅶ)-Fe(Ⅱ) conditioning, flocs disintegration and cell disruption, macro and micro physicochemical properties of activated sludge, especially of EPS, were investigated. Relationships between protein molecular structures in EPS fractions of three layers and sludge dewaterability were also determined. Besides, factor analysis was used to explain the variances of "functional" protein secondary structures, which may have an important effect on sludge dewaterability. Results showed that sludge filterability (CST₀/CST) increased by 2.40 times and partial bound water was released at 2 min oxidation of 120 mg/g KMnO₄ with subsequent FeCl₂ termination at equivalent molar ratio of 1:3. During this strategy, EPS wrapping on cells surface was disintegrated, and a limited influence occurred on cells. It was also found that CST and bound water content were strongly correlated with aggregated strands, random coil and β-turn percentages in slime (R² >-0.82, p < 0.05). Moreover, the dewaterability also presented strong negative correlations with aggregated strands, β-sheet and β-turn in TB-EPS (R² >-0.78, p < 0.05). This suggested that unfolding and despiralization of slime protein and moderate weakening rigidity of tightly bound EPS (TB-EPS) protein were beneficial for improvement of sludge dewaterability. In addition, the strong correlation between percentages of "functional" protein secondary structures and factor 1 score (R² > 0.85, p < 0.05) demonstrated that EPS migration, rather than only EPS disintegration, may have a key impact on the formation of some protein secondary structures in slime and TB-EPS. The observed protein secondary structures, which may affect sludge dewaterability, were probably derived from inner EPS.

Coagulation/flocculation in dewatering of sludge: A review

Sludge disposal is an integral part of wastewater treatment systems, and its cost usually accounts for more than half of the total operation cost. Sludge disposal technology is facing challenges and opportunities simultaneously and can still be improved. Sludge dewatering is an essential process in sludge disposal, and it is important for the effective reduction of the final processing cost. Coagulation/flocculation is a relatively mature, cost-effective, user-friendly sludge dewatering technology. In this work, coagulation/flocculation and their combinations with other pretreatments, including dewatering mechanisms, are reviewed. Various coagulants/flocculants used in sludge dewatering, including inorganic coagulants, organic synthetic and natural polymeric flocculants, and bioflocculants, are introduced in detail because coagulants/flocculants are the key in coagulation/flocculation. The different factors that influence the dewatering performance of these coagulants/flocculants are also presented briefly. Moreover, aiming at the complicated composition of sludge and its treatment difficulty, the prospects and technical developments of coagulation/flocculation in sludge dewatering are discussed.

Experimental rheological procedure adapted to pasty dewatered sludge up to 45 % dry matter

Wastewater sludge are characterized by complex rheological properties, strongly dependent on solids concentration and temperature. These properties are required for process hydrodynamic modelling but their correct measurement is often challenging at high solids concentrations. This is especially true to model the hydrodynamic of dewatered sludge during drying process where solids content (TS) increases with residence time. Indeed, until now, the literature mostly focused on the rheological characterization of sludge at low and moderate TS (between 4 and 8%). Limited attention was paid to pasty and highly concentrated sludge mainly because of the difficulties to carry out the measurements. Results reproducibility appeared to be poor and thus may not be always fully representative of the effective material properties. This work demonstrates that reproducible results can be obtained by controlling cracks and fractures which always take place in classical rotational rheometry. In that purpose, a well-controlled experimental procedure has been developed, allowing the exact determination of the surface effectively sheared. This surface is calculated by scattering a classical stress sweep with measurements at a reference strain value. The implementation of this procedure allows the correct determination of solid-like characteristics from 20 to 45% TS but also shows that pasty and highly concentrated sludge highlight normal forces caused by dilatancy. Moreover the surface correction appears to be independent of TS in the studied range.

Characterization of anaerobic granular sludge using a rheological approach

High-rate anaerobic granular sludge reactors have been developed and are widely used for wastewater treatment. An accurate estimate of sludge rheological properties is required for the design and efficient operation of the digestion process. The present work determined the rheological behavior of anaerobic granular sludge obtained from a full-scale bioreactor at different solid concentrations, operation temperatures and particle sizes, and highlighted common features in flow and dynamic measurements. The granular sludge showed a shear-thinning behavior with a yield stress under flow measurements and a viscoelastic property in dynamic measurements. The structure of granules was nearly temperature-independent in the range of operational temperature (20-70 °C), but the total solid concentration and particle size had significant effects on not only the rheological properties, but also the operation of the bioreactors. In addition, anaerobic granular sludge could cross over from the strong-link regime to the weak-link regime as the solid concentration increased. Furthermore, we adopted a Wagner-type constitutive model to describe the time-dependent and non-linear viscoelastic behaviors of anaerobic granular sludge, and then evaluated its validity and limitation.

Evaluation of Dewatering Performance and Fractal Characteristics of Alum Sludge

The dewatering performance and fractal characteristics of alum sludge from a drinking-water treatment plant were investigated in this study. Variations in residual turbidity of supernatant, dry solid content (DS), specific resistance to filtration (SRF), floc size, fractal dimension, and zeta potential were analyzed. Sludge dewatering efficiency was evaluated by measuring both DS and SRF. Results showed that the optimum sludge dewatering efficiency was achieved at 16 mg∙L(-1) flocculant dosage and pH 7. Under these conditions, the maximum DS was 54.6%, and the minimum SRF was 0.61 × 10(10) m∙kg(-1). Floc-size measurements demonstrated that high flocculant dosage significantly improved floc size. Correlation analysis further revealed a strong correlation between fractal dimension and floc size after flocculation. A strong correlation also existed between floc size and zeta potential, and flocculants with a higher cationic degree had a larger correlation coefficient between floc size and zeta potential. In the flocculation process, the main flocculation mechanisms involved adsorption bridging under an acidic condition, and a combination between charge neutralization and adsorption-bridging interaction under neutral and alkaline conditions.

Essence of disposing the excess sludge and optimizing the operation of wastewater treatment: rheological behavior and microbial ecosystem

Proper disposal of excess sludge and steady maintenance of the high bioactivity of activated sludge in bioreactors are essential for the successful operation of wastewater treatment plants (WWTPs). Since sludge is a non-Newtonian fluid, the rheological behavior of sludge can therefore have a significant impact on various processes in a WWTP, such as fluid transportation, mixing, oxygen diffusion, mass transfer, anaerobic digestion, chemical conditioning and mechanical dewatering. These are key factors affecting the operation efficiency and the energy consumption of the entire process. In the past decade-due to the production of large quantities of excess sludge associated with the extensive construction of WWTPs and the emergence of some newly-developed techniques for wastewater purification characterized by high biomass concentrations-investigations into the rheology of sludge are increasingly important and this topic has aroused considerable interests. We reviewed a number of investigations into the rheology of sludge, with the purpose of providing systematic and detailed analyses on the related aspects of the rheological behavior of sludge. It is clear that, even though considerable research has focused on the rheology of sludge over a long time period, there is still a need for further thorough investigation into this field. Due to the complex process of bio-treatment in all WWTPs, biological factors have a major influence on the properties of sludge. These influences are however still poorly understood, particularly with respect to the mechanisms involved and magnitude of such impacts. When taking note of the conspicuous biological characteristics of sludge, it becomes important that biological factors, such as the species composition and relative abundance of various microorganisms, as well as the microbial community characteristics that affect relevant operating processes, should be considered.

Phosphorus recovery from wastewater by struvite crystallization: property of aggregates

Struvite crystallization is a promising method to remove and recover phosphorus from wastewater to ease both the scarcity of phosphorus rock resources and water eutrophication worldwide. To date, although various kinds of reactor systems have been developed, supporting methods are required to control the struvite fines flushing out of the reactors. As an intrinsic property, aggregation is normally disregarded in the struvite crystallization process, although it is the key factor in final particle size and therefore guarantees phosphorus recovery efficiency. The present study developed a method to analyze the characteristics of struvite aggregates using fractal geometry, and the influence of operational parameters on struvite aggregation was evaluated. Due to its typical orthorhombic molecular structure, struvite particles are prone to crystallize into needle or rod shapes, and aggregate at the corners or edges of crystals. The determined fractal dimension (Dpf) of struvite aggregates was 1.52-1.31, with the corresponding range of equivalent diameter (d0.5) at 295.9-85.4 μm. Aggregates formed in relatively low phosphorus concentrations (3.0-5.0 mmol/L) and mildly alkaline conditions (pH 9.0-9.5) displayed relatively compact structures, large aggregate sizes and high aggregation strength. Increasing pH values led to continuous decrease of aggregate sizes, while the variation of Dpf was insignificant. As to the aggregate evolution, fast growth in a short time followed by a long steady stage was observed.

Effect of conditioning by PAM polymers with different charges on the structural and characteristic evolutions of water treatment residuals

Three types of polyacrylamide (PAM) flocculants with different charges (cationic PAM WD4960, nonionic PAM M351, and anionic PAM WDA110) were used for water treatment residuals (WTRs) conditioning, and the physicochemical, morphological and structural characteristics of raw and conditioned WTRs were investigated. Rheological methods were employed to analyze the internal structural transition between the raw and conditioned WTRs under a typical dosage of WD4960. Results showed that when the raw WTRs were conditioned with the polymers, the optimum dosage of WD4960 was 4.82 g/kg total suspended solid (TSS) while that of both M351 and WDA110 was 7.24 g/kg TSS. The residual PAM content in the supernatant of the WTR matrix conditioned at the optimum WD4960 dosage was 5.59 mg/L, which is the least among the supernatants obtained with the three types of PAM. Furthermore, the visible fulvic acid (FA) in the supernatant disappeared and the intensity of the ultraviolet FA decreased. The average diameter of irregularly shaped aggregates in the WTR suspensions increased from 35.73 μm to several hundred micrometers with increasing PAM dosage. The size of WTR aggregates conditioned at the optimum WD4960 dosage was much larger than that of aggregates obtained at the optimum M351 or WDA110 dosages. Two-dimensional fractal dimension (D2) values presented an increasing trend with increasing PAM dosage. D2 values of the conditioned WTR aggregates were 1.87, 1.76, and 1.83 at optimum WD4960, M351, and WDA110 dosages, respectively. The cationic PAM (CPAM) WD4960 thus appears to be a more ideal conditioner for WTRs. Consistent relationships were observed among the capillary suction time (CST), average particle size, and D2 values of the conditioned WTR aggregates at the optimum WD4960 dosage. Mass fractal dimensions (D(f)) indicated that both the raw and WD4960-conditioned WTRs behave like weak-link flocs/aggregates. D(f) values (log G'-log TSS) of the WTR aggregates before and after conditioning with the optimum WD4960 dosage were 2.79 and 2.81, respectively. Although the transition from an alum/ferric hydrogel to a cross-linked gel occurred during the WD4960-conditioning process, only a small amount of the alum/ferric hydrogel was destroyed, and the conditioned WTR aggregates showed a gel-like matrix in which residual alum-ferric hydrogels were embedded in cross-linked WD4960 molecules.

Differences in rheological and fractal properties of conditioned and raw sewage sludge

Rheological tests for raw and conditioned activated sludge (AS) or anaerobic digested sludge (ADS) show that power-law relationships can be used to describe the evolution of several theological parameters, i.e., limiting viscosity (eta(infinity)), yield stress (tau(y)), cohesion energy of the sludge network (Ec), and storage modulus (G'), with total suspended solid (TSS) content in raw and conditioned sludge. A gel-like structure that behaves similar to weak-link flocs/aggregates was observed in AS and ADS. As derived from the double-logarithmic plots of G'-TSS content, the mass fractal dimensions of the raw and conditioned AS or ADS flocs/aggregates were 2.70 and 2.53 or 2.85 and 2.79, respectively. The rheological tests also indicate that both polymer conditioning and increased TSS content led to improved elastic behavior, cohesion energy, and yield stress of the sludge network, as well as expanded the corresponding linear viscoelastic range. The porosity of AS or ADS flocs/aggregates will be improved by polymer conditioning.

Effect of solid contents on the controlled shear stress rheological properties of different types of sludge

Controlled shear stress (CSS) test was used to study the effect of solid contents on the corresponding rheological parameters for sludge. Three types of sludge with or without conditioning, including activated sludge (AS), anaerobic digested sludge (ADS), and water treatment residuals (WTRs), were collected for the CSS test. Results showed that the yield stress and the cohesion energy of the sludge networks were improved with increased total suspending solid (TSS) contents in most cases. For the conditioned AS/ADS and the raw WTRs, exponential law was observed in the relationships between cohesion energy of material networks or yield stress and the TSS contents, whereas for the conditioned WTRs, only exponential law dependence was found between the parameters of shear modulus or critical strain and the TSS contents.