The effect of high speed mixing and polymer dosing rates on the geometric and rheological characteristics of conditioned anaerobic digested sludge (ADS).

Wastewater sludges are non-Newtonian fluids because their shear rates change with shear stress and no linear relationship is observed between their shear stress and shear rate. Therefore, it is necessary to condition sludges with polyelectrolytes prior to dewatering to increase in the dewaterability. Since 1978, researchers have observed that the yield strength of conditioned sludge increased with the addition of polymer up to the optimum dose. Then sludge rheogram was used as a control parameter to optimize the addition of polymers, and the peak height in the shear stress vs. shear rate curve was an indication of sludge conditionablity, where the optimum polymer conditioning corresponding to the highest peak was obtained. However, few studies have addressed the effects of distinct conditioning factors on the appearance of peaks in such rheograms. In this study, the impact of factors such as high speed mixing and polymer dosing rates on the geometric and rheological characteristics of conditioned anaerobic digested sludge (ADS) with the polymer zetag7557 were investigated through the jar test method. The results showed that both the high speed mixing time and polymer dosing time had important effects on the emergence of the initial peak in test curves obtained using the Haake RV20 and Flokky rheometers for evaluation of conditioned ADS. A high speed mixing time within 60 s or a polymer dosing time of 5 s was sufficient for observation of the initial peak in flow curves, and both peak height and area decreased as the high speed mixing times were prolonged in most cases in this study. As same as the high speed mixing time, the extension of zetag7557 dosing time can also lead to the gradual decrease in the initial peak size of test curves, and form small aggregates with a decrease in two-dimensional fractal dimensions (D(2)). Although the initial peak in the test curves was observed when high speed mixing intensities increased up to 300 rpm, there were several differences in the peak height and area observed on the Haake RV20 test rheograms and Floccky test curves. In addition, a high speed mixing intensity of 300 rpm was found to lead to the formation of smaller and less compact aggregates than other mixing intensities. All of the rheological and geometric parameters were somewhat correlated with fractal dimension-D(2P) (based on regression analysis of the logarithmics of area and perimeter). However, D(2L)(based on regression analysis of the logarithmics of area and maximum diameter) did not show good correlation with any other parameters. The median diameters of the aggregates were well correlated with one-dimensional fractal dimensions (D(1)).

Assessment of rheological methods for a correlation to sludge filterability.

To achieve good dewaterability, the conditioning process should create sludge flocs providing a porous cake during compression but also a high resistance to erosional stresses. This suggests that sludge's rheological characteristics, some of which (like the yield stress or the storage modulus) represent its network strength, should be related to the material's dewatering properties. Previous efforts to verify such a correlation--which would allow prediction of full scale dewaterability--have not provided strong correlations. In this work, commonly accepted rheological and dewatering tests were applied to conditioned sludges. Correlations were seen between different rheological parameters, particularly yield stress and specific energy, but not between rheological and filterability properties. Even using high pressure filtration tests, which should reflect the sludge's responses to normal and shear stresses, results were related to the measured rheological properties only in an indirect manner: a threshold shear strength is required beyond which further strength confers no improvement in filterability. Thus, common rheological tests are unlikely to provide useful information regarding full scale filterability.

Improving the assessment of risk from pathogens in biosolids: fecal coliform regrowth, survival, enumeration, and assessment.

Reactivation or regrowth of fecal coliform bacteria in biosolids has recently become a concern due to knowledge that Class B materials may fail to meet this criterion after storage or even after land application. In this paper, data show the two types of fecal coliform increases that have been characterized: immediate reappearance of large concentrations directly after dewatering; and the rapid, but less immediate, increases that follow dewatering with some biosolids after dewatering. The latter phenomenon is shown to extend over a time period of days prior to gradual decrease in fecal coliform numbers. Modeling shows that anaerobic or fermentative growth cannot simulate the observed growth, but that a straightforward biokinetic model can duplicate the observed conditions if a doubling time of one hour is assumed, which is supported by literature. Thus regrowth cannot be ruled out as the underlying phenomenon.

The importance of structural and gel fractions in determining shear sensitivity of sludge.

Shear sensitivity measurements with variation of ingredients in a synthetic sludge can be related to biosolids' structural properties and particulate amount. The shear sensitivity is primarily controlled by the gel fraction, with alginate and the associated cations increasing the gel-like behavior. However the effect of particulate fraction on shear sensitivity should also be taken into account. Shear sensitivity is thus a complex parameter including two types of response: the resistance to dispersed mass concentration into centrate, and the maintenance of desired strength while the solids concentration increases in the shear field.

Toward efficient sludge processing using novel rheological parameters: dynamic rheological testing.

Materials known as complex fluids exhibit a transitional behaviour between their solid and liquid states. Sludges produced at different treatment stages of wastewater treatment are examples of these materials, since they exhibit solid behaviour within shorter time frames and liquid behaviour over longer durations. These sludges can also change from solid-like to liquid-like, or vice versa, when subjected to even a slight deformation; this stems from their complex makeup, which includes both particulate materials and polymeric gels, including both biopolymers and the synthetic polymers used in conditioning them. This paper focuses on two means of quantifying dynamic rheological test results: (1) the linear viscoelastic behaviour (LVE) of sludge samples, as measured by the strain sweep analysis storage (G') and loss moduli (G"); and (2) the flow curve, which presents the relationship between the shear viscosity and shear rate and determines the yield stress values of samples using the strain sweep test mode. With these methods, sludges can be more accurately characterised, allowing better prediction of their behaviour in treatment and transport processes.

Evaluation of conditioning responses of thermophilic-mesophilic anaerobically and mesophilic aerobically digested biosolids using rheological properties.

One of the most crucial processes in biosolids management is stabilisation, which is typically accomplished using either anaerobic or aerobic digestion processes. Although there are many advantages to both of these methods--such as reduction of volatile organic matter, pathogen content, and offensive odours--one common disadvantage is that digested biosolids exhibit poor dewaterability characteristics. The deterioration in dewaterability of digested biosolids leads to increases in polymer requirements, connoting higher conditioning costs. Many studies have examined this phenomenon based on conventional filterability tests such as capillary suction time and specific resistance to filtration. However, these test methods are limited in their ability to predict full-scale dewatering behaviour (particularly in centrifugation), and do not regard handling properties as being important for sludge transport. For this purpose, new rheometric methods may assist in predicting sludge behaviour, potentially allowing optimisation of stabilisation and conditioning processes relative to the desired sludge properties. This paper investigates the possible utility of these methods. Classical methods for characterising biosolids, such as filtration tests, are compared with yield stress values and other rheological properties as determined using the steady-shear rate sweep test. The rheological behaviours are compared for different sludge types, including raw and polymer conditioned thermophilic anaerobically, mesophilic anaerobically, and mesophilic aerobically digested biosolids. The paper demonstrates the determination of specific energy values associated with breakdown of the flocculated matrix. Robust modelling approaches are employed to optimise the conditioning of the digested samples, and to shed light on commonly used rheological models such as Bingham, Herschel-Bulkley, and Ostwald equations.

Enhanced sludge conditioning by enzyme pre-treatment: comparison of laboratory and pilot scale dewatering results.

The effect of enzyme pre-treatment on dewaterability of anaerobically digested sludge was investigated at both laboratory and pilot scale. Our results revealed a significant increase in cake solid content (27% cake solids compared to 18% without enzyme pre-treatment), using an enzyme dose of only 20 mg/L. In order to assess practical application, enzyme pre-treatment was applied at the Wilmington, Delaware (U.S.) wastewater treatment plant, using a pilot-scale centrifuge. However, the efficiency reached in laboratory scale could not be obtained in pilot scale, where the final cake solids content did not exceed 20%. Centrifuge and belt filter press (simulated by Crown Press) dewatering were compared in terms of the process efficiencies in the absence and presence of enzyme pre-treatment. Possible factors that might cause the differences were tested by experimental and statistical comparisons. Results indicated that the higher shear applied in centrifugation is responsible for the lack of improved cake solids. The network strength of sludge determined by rheological measurements revealed that enzymatic treatment weakens the gel structure of the sludge floc through the hydrolysis of extracellular polymeric substances; this allows improved dewatering by filtration processes, but leads to floc deterioration when subjected to high shear during centrifugation.

Use of drainability and filterability simulations for evaluation of oxidative treatment and polymer conditioning of sludge.

In the traditional view of sludge structure, a two-phase system is visualized. The solid phase is often illustrated as consisting of uniform, spherical, negatively charged particles. This simplistic picture limits the understanding of both conditioning and dewatering. Specifically, it limits us to only two strategies for destabiiization: charge neutralization (using cationic coagulant species with either uniform or patch effects), and bridging (using polymeric flocculants). In fact, the situation is more complex in many respects. In this paper, we compared chemical conditioning of anaerobically digested biosolids using (1) conventional polymeric flocculation, and (2) treatment by chemical oxidants. Since these strategies may alter dewatering behavior by very different mechanisms, we report comparisons using a lab-scale simulator of belt press dewatering, including separate steps for the drainage and filtration processes. This allows greater insight into the effects of these conditioning methods and how they might be properly employed.

Reactivation and/or growth of fecal coliform bacteria during centrifugal dewatering of anaerobically digested biosolids.

Fecal coliform bacteria are used as indicator organisms for the presence of pathogens. In sludges, it has often been assumed that the counts of fecal coliforms after digestion (where the sludges may also be called biosolids) are representative of the counts when the sludge is disposed or recycled, such as by land application. The possibility has been raised, however, that dewatering processes can lead to increased counts of fecal coliforms and, by inference, human pathogens. This paper presents data from previous studies of this possibility; the results were inconsistent but showed observable increases in fecal coliforms at one treatment plant. Additional studies were then performed at another treatment facility, which showed statistically significant increases in fecal coliform counts after dewatering and two days of aging. The increases exceeded two orders of magnitude and included two centrifuge types and two biosolids types. Artifacts of media type and enumeration method have been excluded, and shearing of the material by commercial blender did not produce the same effects.

Physical characteristics of a waste activated sludge: conditioning responses and correlations with a synthetic surrogate.

The efficient and economic management of waste activated sludge (WAS) requires a proper understanding of the sludge's material properties. Though there has been much study of WAS, an adequate linkage between its physical and chemical properties has been elusive. In particular, the conditioning and dewatering of WAS are expensive operations, and the addition of polymer to WAS leads to even more complex material behavior that is difficult to optimize. This paper reports on an extensive characterization of WAS, both with and without polymer conditioning. We combined the classical "jar test" approach with less conventional rheometric and electrokinetic measurements. In addition to the use of sampled WAS, a synthetic surrogate sludge was formulated, attempting to duplicate properties of WAS and allow more extensive characterization of a reproducible surrogate to WAS. Results with both the synthetic surrogate and WAS indicated that the traditional, electrokinetic, and rheological properties were related. However, the dose optima by rheometry were somewhat higher in both cases.

Direct generation of electricity from sludges and other liquid wastes.

Recently it has been shown that electrical energy can be harvested from marine sediments, simply by connection of an electrode (anode) in anaerobic marine sediments to an electrode in the aerobic zone above the sediments. We have now shown that similar applications are available in sludge treatment. Using a reactor with graphite foil electrodes in an aerated aerobic and anaerobic sludge zone, electrical current was generated, and enhanced when an additional organic substrate (acetate) was added. Electron microscopy, x-ray diffraction, and PCR examination of the anode surface showed no surface colonization and no increase in Geobacter relative to a control, indicating that microbial use of the anode as an electron acceptor wa indirect through the use of redox mediators. Given the demonstration of electricity generation from sludge, the potential for similar applications, using other organic waste sources, is evaluated.

Analysis of sulfur and nitrogen odorants using solid-phase microextraction and GC-MS.

An analytical method involving solid-phase micro-extraction (SPME) and gas chromatography-mass spectrometry (GC-MS) was applied to analyze biosolids odors. A selective ion monitoring (SIM)-based MS method was developed, using SPME injections of odorant standards under the full-scan mode to select the quantification and confirmation ions for each odorant. The odorants analyzed in this study include: dimethylsulfide, dimethyldisulfide, methyl mercaptan, hydrogen sulfide, carbon disulfide, trimethylamine and dimethylamine. We have used this method to quantify parts-per-billion levels of odorant vapors produced during anaerobic incubation of digested wastewater sludge. Important considerations for expedient and accurate calibration under static and dynamic flow conditions are discussed. The SPME-GC-MS method may give a positive intercept in the calibration curve, especially under static sampling conditions, which sets a practical detection limit for odor analysis.

Contaminants in sludge: implications for management policies and land application.

Policies on sludge (or biosolids) management vary widely, particularly when decisions must be made on what to do with the final product. This paper examines the two principal rationales with which such decisions are made, and through which scientific knowledge is included in the process. These rationales are risk analysis (risk assessment and management), and the criterion of sustainability. Both are found to be potentially arbitrary due to the difficulty in defining the individual constituents necessary to relate environmental phenomena to environmental policy. To place the difficulties in a practical context, this paper presents research results from three recent projects concerned with contaminants in sludge (phosphorus, flocculant polymers, and polymer-surfactant aggregates), and uses the findings to exemplify the dilemma encountered in policy making. A path forward is proposed.

Aerobic and anaerobic biodegradability of a flocculant polymer.

Flocculant polymers are used to improve the efficiency of separation processes used in wastewater treatment. The subsequent fate and effects of these additives are uncertain, however, with some previous reports indicating them to be biodegradable while others indicate complete recalcitrance. The biodegradability of a common flocculant polymer was therefore evaluated, using both aerobic and anaerobic batch assays. Knowledge of the polymer's chemical composition also allowed degradation stoichiometries to be calculated for complete biodegradation and also for incomplete degradation to several hypothesized end products. Results showed conclusively that the polymer was subject to partial degradation by both aerobic and anaerobic cultures. Measured oxygen consumption under aerobic conditions, and gas production under anaerobic conditions, both indicate that the partial destruction of pendant cationic moieties occurs, but that the polymer's CH2 backbone remains essentially intact. These results allow seemingly contradictory previous reports to be explained. The findings are relevant to the environmental fate of these polymers as well as certain treatment process effects.

Relationships between biosolids treatment process and soil phosphorus availability.

Laws mandating phosphorus (P)-based nutrient management plans have been passed in several U.S. Mid-Atlantic states. Biosolids (sewage sludge) are frequently applied to agricultural land and in this study we evaluated how biosolids treatment processes and biosolids P tests were related to P behavior in biosolids-amended soils. Eight biosolids generated by different treatment processes, with respect to digestion and iron (Fe), aluminum (Al), and lime addition, and a poultry litter (PL), were incubated with an Elkton silt loam (fine-silty, mixed, active, mesic Typic Endoaquult) and a Suffolk sandy loam (fine-loamy, siliceous, semiactive, thermic Typic Hapludult) for 51 d. The amended soils were analyzed at 1 and 51 d for water-soluble phosphorus (WSP), iron-oxide strip--extractable phosphorus (FeO-P), Mehlich-1 P and pH. The biosolids and PL were analyzed for P, Fe, and Al by USEPA 3050 acid-peroxide digestion and acid ammonium oxalate, Mehlich-1, and Mehlich-3 extractions. Biosolids and PL amendments increased extractable P in the Suffolk sandy loam to a greater extent than in the Elkton silt loam throughout the 51 d of the incubation. The trend of extractable WSP, FeO-P, and Mehlich-1 P generally followed the pattern: [soils amended with biosolids produced without the use of Fe or Al] > [PL and biosolids produced using Fe or Al and lime] > [biosolids produced using only Fe and Al salts]. Mehlich-3 P and the molar ratio of P to [Al + Fe] by either the USEPA 3050 digestion or oxalate extraction of the biosolids were good predictors of changes in soil-extractable P following biosolids but not PL amendment. Therefore, the testing of biosolids for P availability, rather than total P, is a more appropriate tool for predicting extractable P from the biosolids-amended soils used in this study.

Chemical and physical pretreatment of ATAD biosolids for dewatering.

This paper reports on two modifications to the typical practice of polymer conditioning intended to enhance the dewaterability of autothermal thermophilic aerobic digested (ATAD) biosolids. One modification was a chemical pretreatment, using ferric chloride prior to polymer conditioning, and the other was a physical pretreatment using an electrical arc technology before polymer addition. These modifications were evaluated separately and in combination, in laboratory conditioning experiments using ATAD biosolids. Results showed that neither ferric chloride nor the arc treatment is sufficient to produce dewaterable biosolids without the use of polymer. However, use of a ferric chloride dose as low as 100 g per kg of dry solids reduced the polymer requirement by about 50%. The arc treatment provided further enhancement of dewaterability, but the observed benefits might not justify its use without further optimization. For this purpose, additional methods were utilized to assist in understanding the pretreatment mechanisms.

Conditioning, thickening, and dewatering: research update/research needs.

The production of sludges that are amenable to beneficial reuse (and thereby merit the term "biosolids") has become a high priority concern in wastewater treatment. Regulations in many countries have become stringent in this regard, and the production of suitable biosolids now influences the selection, design, and operation of upstream processes. Among these are the conditioning, thickening, and dewatering of sludges. This paper briefly reviews current technology trends in these process areas, placing them in the context of overall needs for improved biosolids management. It is foreseen that conditioning, thickening and dewatering processes will need to operate at higher performance and reliability levels to maintain disposal and reuse flexibility. The chemistry of conditioning must be more clearly determined since secondary interactions of the conditioning additives may be important in ways that are not currently recognized.