Assessment of capillary suction time (CST) test methodologies

Towards globally relevant, small-footprint dewatering solutions: Optimal conditioner dose for highly variable blackwater from non-sewered sanitation

Globally, the sanitation needs of three billion people are met by non-sewered sanitation. Small-footprint treatment technologies are needed that are appropriate for dense urban areas. Blackwater (BW) (or fecal sludge), contains more than 95% liquid, and dewatering it without conditioning requires large footprints. Chemically-enhanced dewatering with conditioners is a promising option to increase dewatering performance and reduce required footprints. However, before implementation of this solution there is a need for increased knowledge on selection and dosing of conditioners. This study evaluated bio-based and synthetic conditioners (chitosan, tannin-, and starch-based, synthetic with and without poly-acrylamide) with 14 types of BW from five countries. The supernatant after settling with jar-tests was analyzed to quantify optimal dose and dewatering performance. The reduction of total chemical oxygen demand (COD) was >55%, achieved by removal of particulate constituents with mainly soluble COD remaining in the supernatant. A reduction in particulate COD could lead to increased efficiency of soluble COD in supernatant treatment. Bio-based conditioners are as effective as synthetic conditioners, and when performance was variable, it was due to differing properties of TSS, TS, EC and pH. Optimal conditioner dose for synthetic conditioners and chitosan could be predicted using concentrations of total solids (TS) (R² > 0.7), whereas optimal dose for starch- and tannin-based conditioners could be predicted with electrical conductivity (EC) (R² > 0.8), and colloid titration (R² > 0.8). In addition, real-time optical TSS and EC sensors could accurately predict chitosan dose for fresh BW treated at source (R² = 0.97, R² = 0.95). This study validates that use of conditioners for dewatering with highly variable BW can be implemented with real-time measurements for optimal dose, in globally relevant implementations.

Highlights of the novel dewaterability estimation test (DET) device

Many industries, which are producing sludge in large quantities, depend on sludge dewatering technology to reduce the corresponding water content. A key design parameter for dewatering equipment is the capillary suction time (CST) test, which has, however, several scientific flaws, despite that the test is practical and easy-to-perform. The standard CST test has a few considerable drawbacks: its lack of reliability and difficulties in obtaining results for heavy sludge types. Furthermore, it is not designed for long experiments (e.g. >30 min), and has only two measurement points (its two electrodes). In comparison, the novel dewaterability estimation test (DET) test is almost as simple as the CST, but considerably more reliable, faster, flexible and informative in terms of the wealth of visual measurement data collected with modern image analysis software. The standard deviations associated with repeated measurements for the same sludge is lower for the DET than for the CST test. In contrast to the CST device, capillary suction in the DET test is linear and not radial, allowing for a straightforward interpretation of findings. The new DET device may replace the CST test in the sludge-producing industries in the future.

The Effects of Aluminium- and Ferric-Based Chemical Phosphorus Removal on Activated Sludge Digestibility and Dewaterability

The uses of Al3+ and Fe3+ salts in chemical phosphorus removal (CPR) in activated sludge plants have increased considerably in recent years and their full impacts on downstream processes such as dewaterability and digestibility are not fully understood. In this research, the effects of CPR on sludge digestibility and dewaterability were investigated in laboratory-scale experiments using sludge samples from a full-scale wastewater treatment plant. The results of the digestibility tests showed a 21% and 36% reduction in the biogas volume generated during anaerobic digestion of surplus activated sludge at 0.1 g/L doses of Al3+ and Fe3+ salts, respectively. This demonstrates that Al3+ dosing for CPR has less of a reduction effect compared with Fe3+ salts on biogas generation during anaerobic digestion of sludge. The dewaterability tests showed that primary sludge dewaterability was improved by up to 25% by Fe3+ and 16% by Al3+, while that of surplus activated sludge was reduced by 64% and 73%, respectively, at a metal salt dose of 50 mg/L. Consequently, a pre-precipitation process during CPR where phosphorus is removed in the primary tank would, therefore, enhance sludge dewaterability.

Study of Sludge Particles Formed during Coagulation of Synthetic and Municipal Wastewater for Increasing the Sludge Dewatering Efficiency

Municipal wastewater sludge was produced by chemical coagulation of synthetic wastewater (sww) based on Synthene Scarlet P3GL disperse dye and real municipal wastewater (nww), coagulated by commercial coagulants PAX (prepolymerised aluminum coagulant) and PIX (a ferric coagulant based on Fe2(SO4)3). An attempt was made to correlate the sludge’s dewatering capacity (in terms of capillary suction time—CST) with operation parameters for wastewater treatment, size distribution and specific surface area of the sludge particles. It was found that the presence of phosphate ions in the system facilitates the removal efficiency of the above-mentioned dye (L) due to the interaction between the dye molecules and H2PO4− ions. Unlike sww, negatively charged organic substances (sorg) in nww are directly adsorbed on the surface of colloidal particles {Fe(OH)3} and {Al(OH)3} (prtc). It was also discovered that an increase in the dose of a coagulant led to an increase of CST for sww sludge and to a decrease of CST for nww sludge. It has been suggested that flocs composed of spherical {Al(OH)3} units possessed more internal space for water than aggregates consisting of rod-shaped {Fe(OH)3} units and, consequently, it is more difficult to remove water from Al-sww sludge than from Fe-sww. The results obtained showed that smaller particles dominate in sww sludge, while larger particles are prevalent in nww sludge. To explain this distinct difference in the size distribution of particles in sludge obtained with the use of Al3+ and Fe3+, simple models of aggregation and agglomeration-flocculation processes (aaf) of treated wastewater have been proposed. Except for PIX in nww, the analyzed particles of the investigated types of sludge were characterized by similar specific surface area (Sps), regardless of the kind of sludge or the applied coagulant. Slightly larger, negatively-charged sorg bridges, anchored directly on the surface of positive prtc are more effective in closing the structure of nww sludge than small L bridges of the dye molecules anchored on the surface of prtc via H2PO4−. All the discovered aspects could lead to improved performance of wastewater treatment plants (WWTP) by increasing the efficiency of sludge dewatering.

Polymer reaction engineering tools to design multifunctional polymer flocculants

A series of multifunctional terpolymers, poly(N-isopropyl acrylamide/2-(methacryloyloxy) ethyl trimethyl ammonium chloride/N-tert-butylacrylamide) [P(NIPAM-MATMAC-BAAM)], were designed to flocculate and dewater oil sands mature fine tailings (MFT). The hydrophobic BAAM comonomer helped in expelling water from the sediments, while the cationic MATMAC comonomer promoted the charge neutralization of negatively charged particles suspended in MFT. The chemical composition distributions of these terpolymers were designed based on the knowledge of the reactivity ratios of all comonomers, instead of by trial and error, as usually done for most polymer flocculants. The binary reactivity ratios of the comonomers were estimated by synthesizing the binary copolymers with various mole fractions of each comonomer in the feed and experimentally measuring the corresponding fraction of comonomer in the copolymers. Polymer reaction engineering tools were used to minimize compositional drift and guarantee the synthesis of terpolymers with narrow chemical composition distributions suitable for MFT dewatering. Focused beam reflectance measurement (FBRM) experiments showed that terpolymers promoted the formation of large MFT flocs (120 μm). The initial settling rate decreased with the increase in flocculant hydrophobicity, likely because the hydrophobic terpolymer segments did not take part in the bridging of the MFT particles. In contrast, the sediment dewaterability increased with the increase in terpolymer hydrophobicity. This study provides guidelines to design a polymer flocculant from first principles and demonstrates the potential of using hydrophobically modified cationic polymers to flocculate MFT effectively.

A novel hydrophobically-modified polyelectrolyte for enhanced dewatering of clay suspension

This work investigates the effect of multifunctional poly (N-isopropyl acrylamide/acrylic acid/N-tert-butylacrylamide) [p(NIPAM-AA-NTBA)] ternary polymer on the sedimentation of kaolin clay - a major fraction of oil sands tailings. A series of linear, uncross-linked p(NIPAM), p(NIPAM/AA), and p(NIPAM/AA/NTBM) were synthesized as random copolymers, where all monomer units were randomly arranged along the polymer backbone and connected by covalent bonds. The ternary copolymer, used as a flocculant, exhibited thermo-sensitivity, anionic nature, and hydrophobic association due to NIPAM, AA, and NTBM, respectively. As the ternary polymer is thermosensitive, it undergoes extended to coil-like conformation, i.e. hydrophilic to hydrophobic transition, above its lower critical solution temperature (LCST). The comonomers NIPAM (above LCST) and NTBM help expel water out of sediments due to their hydrophobicity, while AA promotes charge neutralization of the kaolin clay particles. The effect of number average molecular weight, charge density, and concentration of NTBM on settling behavior of kaolin suspension was examined. Settling test at 50 °C resulted in significantly higher settling rates compared to that at room temperature. Further, the quality of water recovered in each experiment was tested in terms of its turbidity. These results indicate that this novel ternary polymer can be employed to enhance the recovery of water from oil sands tailings containing clays.

Dewatering Oil Sands Tailings with Degradable Polymer Flocculants

We synthesized hydrolytically degradable cationic polymers by micellar radical polymerization of a short-chain polyester macromonomer, polycaprolactone choline iodide ester methacrylate (PCL₂ChMA) with two polyester units, and used them to flocculate oil sands mature fine tailings (MFT). We evaluated the flocculation performance of the homopolymer and copolymers with 30 mol % acrylamide (AM) by measuring initial settling rate (ISR), supernatant turbidity, and capillary suction time (CST) of the sediments. Flocculants made with trimethylaminoethyl methacrylate chloride (TMAEMC), the monomer corresponding to PCL_nChMA with n = 0, have improved performance over poly(PCL₂ChMA) at equivalent loadings due to their higher charge density per gram of polymer. However, MFT sediments flocculated using the PCL₂ChMA-based polymers are easier to dewater (up to an 85% reduction in CST) after accelerated hydrolytic degradation of the polyester side chains. This study demonstrates the potential of designing cationic polymers that effectively flocculate oil sands tailings ponds, and also further dewater the resulting solids through polymer degradation.

Fate of extracellular polymeric substances of anaerobically digested sewage sludge during pre-dewatering conditioning with Acidithiobacillus ferrooxidans culture

This study investigated the fate of extracellular polymeric substances (EPS) of anaerobically digested saline sewage sludge during its preconditioning. Sludge was conditioned with Acidithiobacillus ferrooxidans (AF) culture for 24h in the presence and absence of Fe(2+) as an energy substrate. pH decreased from 7.24 to 3.12 during sludge conditioning process. The capillary suction time (CST) of conditioned sludge significantly decreased to <10s, and specific resistance to filtration (SRF) was reduced by >94% as compared with control within 4h of conditioning with or without Fe(2+), indicating a significant (P<0.001) improvement in sludge dewaterability. A noticeable decrease in extractable EPS was observed in conditioned sludge. The EPS contents showed a significant negative correlation with dewaterability of sludge (P<0.05). The results suggest that bioacidification treatment using A. ferrooxidans effectively improved sludge dewaterability through modification of sludge EPS.

A review on sludge dewatering indices

Dewatering of sludge from sewage treatment plants is proving to be a significant challenge due to the large amounts of residual sludges generated annually. In recent years, research and development have focused on improving the dewatering process in order to reduce subsequent costs of sludge management and transport. To achieve this goal, it is necessary to establish reliable indices that reflect the efficiency of sludge dewatering. However, the evaluation of sludge dewaterability is not an easy task due to the highly complex nature of sewage sludge and variations in solid-liquid separation methods. Most traditional dewatering indices fail to predict the maximum cake solids content achievable during full-scale dewatering. This paper reviews the difficulties in assessing sludge dewatering performance, and the main techniques used to evaluate dewatering performance are compared and discussed in detail. Finally, the paper suggests a new dewatering index, namely the modified centrifugal index, which is demonstrated to be an appropriate indicator for estimating the final cake solids content as well as simulating the prototype dewatering process.

Optimizing chemical conditioning for odour removal of undigested sewage sludge in drying processes

Emission of odours during the thermal drying in sludge handling processes is one of the main sources of odour problems in wastewater treatment plants. The objective of this work was to assess the use of the response surface methodology as a technique to optimize the chemical conditioning process of undigested sewage sludges, in order to improve the dewaterability, and to reduce the odour emissions during the thermal drying of the sludge. Synergistic effects between inorganic conditioners (iron chloride and calcium oxide) were observed in terms of sulphur emissions and odour reduction. The developed quadratic models indicated that optimizing the conditioners dosage is possible to increase a 70% the dewaterability, reducing a 50% and 54% the emission of odour and volatile sulphur compounds respectively. The optimization of the conditioning process was validated experimentally.

WITHDRAWN: Fate of extracellular polymeric substances during flocculation of anaerobically digested sludge using biogenic flocculant

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Impact of different shapes and types of mixers on sludge dewaterability

The most significant operational cost in a treatment plant is related to the dewatering and disposal of sludge. In this research, different shapes of mixers (radial, axial, wheel, three-blade and magnetic) have been tested to assess their influence on sludge dewaterability testing. As well as the shape of mixers, different rapid mixing velocities, rapid mixing times and coagulants also have been used as test parameters. The capillary suction time (CST) test apparatus was used as a rapid measure to assess sludge dewaterability. Findings indicate that the use of magnetic stirrers leads to the lowest sludge dewaterability properties tested using the CST. The magnetic stirrer produced greater vortex and turbulence compared with other types of mixers, so rapid contact between the coagulant and the water occurred. The use of the other mixers produced similar results. However, the application of different coagulants results differently regarding the sludge dewaterability, when using different rapid mixing velocities. Different rapid mixing times did not lead to differences in the CST. During rapid mixing, Moringa oleifera produced the highest CST values followed by alum and ferric. This is likely due to the difference in sludge volume that is produced by each coagulant. Only M. oleifera produced contaminant agglomerates. In comparison, alum and ferric produced contaminant agglomerates and coagulant hydrolysis products.

Innovative enhancement of the design and precision of the capillary suction time testing device

The capillary suction time (CST) test can be used as an alternative to predict the otherwise expensive, hard-to-run specific resistance to filtration test. One of the major drawbacks of the CST test, however, is relatively high variability of test results. The aim of this study was, therefore, to improve the product design by testing the effectiveness of using a funnel sealant to reduce variability. Use of a funnel sealant resulted in increased test precision. There was a reduction of up to 63% in the coefficient of variation and a substantial improvement in the predictability of the specific resistance to filtration test.

Monitoring particle aggregation processes

A wide range of test methods for monitoring particle aggregation processes is reviewed. These include techniques for measuring aggregation rates in fundamental studies and those which are useful in the monitoring and control of practical coagulation/flocculation processes. Most emphasis is on optical methods, including light transmission (turbidity) and light scattering measurements and the fundamentals of these phenomena are briefly introduced. It is shown that in some cases, absolute aggregation rates can be derived. However, even when only relative rates can be obtained, these can still be very useful, for instance in defining optimum flocculation conditions. Some of the methods available for investigating properties of aggregates (flocs), such as size, strength and fractal dimension are also discussed, along with some related properties such as sedimentation rate and filterability of flocculated suspensions.