Cake Solids Content Research Articles

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.

The role of compactibility in liquid–solid separation of wastewater sludges

The dewaterability of sludges is generally evaluated by the measurement of specific resistance to filtration (SRF) and capillary suction time (CST). It is generally, but not truly, agreed that the lower figures of these parameters indicate the easiness of sludge dewatering. The biological sludges which have the lower particle size and EPS contents may be characterised as hard-to-filter sludges based on SRF and CST measures. However, cake solids concentration of such a kind of sludge can be unexpectedly higher when the centrifugation is used as the dewatering mechanism. This study introduces compactibility, which is cake solids concentration of sludges after centrifugation as a new measure of dewaterability of hard-to-filter sludges. The results of this study claim that although the filterability indexes (CST and SRF) are moderately high for hard-to-filter sludges, their compactibility might be high indicating particle packing characteristics.

Improvement of the thickening and dewatering characteristics of activated sludge by electroflotation (EF)

The performances of electroflotation (EF) on the thickening of activated sludge were investigated using laboratory scale batch flotation reactors. Four activated sludges including bulking sludges were tested. After 30 minutes of EF operation, 57-84% of sludge volume reduction could be achieved by EF, while only about 1.5-14% could be obtained by gravity thickening for the same period. After thickening the effluent water quality in terms of TCOD, SS, and turbidity was improved by EF operation for all sludge samples. In addition, the EF thickened sludge showed much better dewaterability both in SRF and cake solid content. It is induced that the air bubbles entrapped in the thickened sludge play a key role in the observed dewaterbility improvement.

The role of carbohydrate and protein parts of extracellular polymeric substances on the dewaterability of biological sludges

There are many factors affecting the biological sludge dewaterability such as particle size distribution, floc structure, extracellular polymeric substances (EPS), etc. In this research, the role of the protein and carbohydrate parts of EPS (EPS(carbohydrate), and EPS(protein)) on the dewaterability of biological sludges was investigated. The sludge EPS composition was altered by feeding the sludges of same origin, in different reactors, with synthetic media having carbon to nitrogen (C/N) ratios of 8, 19 and 30 (in terms of COD/NH3-N), respectively. EPS in sludge samples were extracted by a cation exchange resin (CER). The characteristics of EPS were investigated by analytical methods and by using FT-IR spectroscopy. The dewaterability of the sludges was determined in terms of filterability and compactibility. Filterability, as filterability constant (X), and compactibility, as cake solids concentration, of sludges were determined by using the capillary suction time (CST) test and the centrifugation, respectively. The floc structure of sludge samples was also observed microscopically. Filterability and compactibility of the sludge samples were improved considerably with the increasing carbohydrate part and the decreasing protein part of the sludge EPS. EPS(protein) was inversely related to the cake solids concentration, which might be explained by the water holding capacity of EPS(protein). Filterability and compactibility of sludges improved by the increase of the size and strength of the flocs.

Alum Sludge Dewatering Using Moringa oleifera as a Conditioner

An active component of Moringa oleifera (MO) has been used in comparison with synthetic polymers and alum for the conditioning of chemical sludge from a drinking water treatment plant. The comparison was based on dewatering characteristics of the conditioned sludge determined by capillary suction time (CST), specific resistance to filtration (SRF), sand column drainage and shear strength tests. The results indicated that MO showed comparable conditioning effect as alum. Polyelectrolytes were more effective than MO and alum. Sludge conditioned with MO and alum, as in dual chemical conditioning, showed better results than MO alone. According to CST, SRF and sand drainage results, optimum doses for MO, alum and polyelectrolytes were 125, 63, and 1.8 kg/t, respectively. Comparison of the two polyelectrolytes showed that the cationic polyelectrolyte was more effective. For sand drainage tests both polyelectrolytes improved the drainage rate by 2 orders of magnitude. MO and alum improved the drainage rate by about 4.2 times. On the other hand, the improvements in cake solids concentration were similar for all the chemical conditioners. Flocs from MO and alum were relatively stronger compared to those of the polyelectrolytes. From the results of the study it could be concluded that MO alone or in combination could be effectively used and replace alum for dewatering of chemical sludge.

Effect of Sludge Water Distribution on the Liquid–Solid Separation of a Biological Sludge

Water distribution in sludge is worthy of investigation due to its importance in the liquid–solid separation of sludge. Biological sludge contains different physical states of water, including free water, interstitial water, vicinal water, and water of hydration, associated with sludge solids. Conventional dewatering processes can remove only the free water and a part of the interstitial water. Release of interstitial water held inside the cell structure requires the disruption of sludge cells. This study investigates the effect of water distribution on the liquid–solid separation of a biological sludge. Sludge cells were disrupted by alkali treatment, NaCl treatment, heat treatment, and sonication. Water distribution in sludge was determined by differential scanning calorimetry (DSC) and centrifugation. The results of this study indicated that the sludge cell disruption changed the water distribution in sludge and improved its compactibility (as a measure for liquid–solid separation). It released a considerable amount of interstitial water trapped inside the cells. Cell disruption created extra surfaces for water binding and increased the unfreezable water content (mostly vicinal water, water of hydration, and a fraction of interstitial water). Compactibility (liquid–solid separation property) of the sludge in terms of cake solids content was improved to about 2–7.5 folds depending on the disruption method used.

Effects of humus soil on the settling and dewatering characteristics of activated sludge

Effects of humus soil on the settling and dewatering characteristics of the activated sludge were investigated. Addition of the humus soil improved the sedimentation characteristics of the activated sludge remarkably. The initial lag phase was diminished and the zone settling velocity increased as the humus soil dosage increased. For estimation of the chemical effect of the humus soil on sludge settling, cation mixtures equivalent to the cations leaching from the humus soil were added and only the chemical effect was examined. The physical effect was then isolated by subtracting the chemical effect. The chemical effect on sludge settling was about30∼50% of the equivalent humus soil powder dosage and the physical effect was more important in improvement of the sludge settling properties. Dewatering characteristics of the activated sludge were dependent on both the concentration and distribution of the cations leaching from the humus soil. Divalent cations, calcium and magnesium from the humus soil decreased the bound water content of the sludge, which in turn increased floc density and cake solids content. However, excess sodium ion increased the SRF (specific resistance to filtration) slightly. Addition of humus soil to thickened activated sludge improved the dewatering properties, which could be well explained by the effect of the cations leaching. The SRF showed a good trend with the monovalent to divalent cation (M/D) ratio, which could be utilized as a useful tool for evaluation of the sludge dewatering characteristics.

Ultra-rapid freezing of water treatment residuals

An alum water treatment residual (“sludge”) was conditioned by freezing at ultra-rapid rates. Ultra-rapid rates were obtained by freezing on liquid nitrogen. The purpose of the experiments was to determine the conditions necessary for such rapid freezing to improve dewaterability. Contrary to results reported in the literature, ultra-rapid freezing worked extremely well. Although the conditioned flocs (“zots”) were not as dense or granular as obtained at slower freezing rates, dewaterability was still greatly improved. Dewaterability, as measured by cake solids content (CSC), capillary suction time (CST), filterability coefficient ( χ) and aggregate volume index (AVI), was best for samples with an initially high solids content and for samples that had been cured or stored at sub-freezing temperatures, for long times. Zot size analyses showed that zots decreased in size following ultra-rapid freezing. The results are explained using a proposed conceptual model of the residuals freezing process.

Investigation into alternative sludge conditioning prior to dewatering

The principal objective of this work is to provide a more effective alternative for sludge conditioning prior to mechanical dewatering. The common use of polyelectrolytes often results in the final cake solids concentration not sufficiently high for the sludge to be beneficially reused or safely disposed. The pyrolysed domestic refuse (char) was used to increase the structural strength and permeability of biological sludges in order to increase final cake solids concentration. The laboratory experiments have proven that the mixture of char and a small quantity of polyelectrolyte (0.5 to 1kg per ton of dry solids), used as a conditioner prior to centrifugation and filtration tests, produced cake solids concentration superior to that obtained by using polyelectrolyte on its own. Laboratory dewatering by filtration with admixture of char and polymer, resulted in cake solids concentration of 30 and above 40 per cent.

Water treatment plant residuals management

Water treatment plants produce enormous amounts of sludge. Due to increased environmental concern, recently, there is considerable amount of pressure on the water authorities for the safe disposal of the sludge. Currently, the most acceptable form of disposal is sanitary landfilling, which requires sludge to be dewatered to 20-40% of solids. One of the most commonly used dewatering techniques is sand drying beds. This process is popular because of its reliability, ease of use and low cost. However, one of the basic concerns with this process is the requirement of a large area of land. This could be overcome by chemical conditioning of the sludge, which can accelerate the dewatering rate substantially. At this stage, however, there are no guidelines for the design of the sand drying beds for treating chemically conditioned sludges. This study is conducted to investigate the design and performance of sand drying beds for treating chemically conditioned water treatment plant (WTP) sludge. Experiments are conducted using laboratory scale sand drainage columns. The results indicate that the drainage rates, after adding polymer, increased by 9 times for clarifier sludge and 20-25 times for filter backwash sludge. Thus, the land area requirement for sand drainage bed can be drastically reduced through polymer addition. Optimal dose of polymer is found to be between 3 and 6 kg/t of dry solids, with dilute sludges requiring high dosages. The cake solids concentration increased from 3 to 12% as the polymer dose increased from 0 to 16 kg/t of dry solids. The variation of drainage rates and cake solids with solids/hydraulic loading rates are found to vary depending on the type of sludges and polymers. The filtrate quality is found to be independent of polymer dose and solids loading rates.

Investigation into alternative sludge conditioning prior to dewatering

The principal objective of this work is to provide a more effective alternative for sludge conditioning prior to mechanical dewatering. The common use of polyelectrolytes often results in the final cake solids concentration not sufficiently high for the sludge to be beneficially reused or safely disposed. The pyrolysed domestic refuse (char) was used to increase the structural strength and permeability of biological sludges in order to increase final cake solids concentration. The laboratory experiments have proven that the mixture of char and a small quantity of polyelectrolyte (0.5 to I kg per ton of dry solids), used as a conditioner prior to centrifugation and filtration tests, produced cake solids concentration superior to that obtained by using polyelectrolyte on its own. Laboratory dewatering by filtration with admixture of char and polymer, resulted in cake solids concentration of 30 and above 40 per cent.

Water treatment plant residuals management

Water treatment plants produce enormous amounts of sludge. Due to increased environmental concern, recently, there is considerable amount of pressure on the water authorities for the safe disposal of the sludge. Currently, the most acceptable form of disposal is sanitary landfilling, which requires sludge to be dewatered to 20.40% of solids. One of the most commonly used dewatering techniques is sand drying beds. This process is popular because of its reliability, ease of use and low cost. However, one of the basic concerns with this process is the requirement of a large area of land. This could be overcome by chemical conditioning of the sludge, which can accelerate the dewatering rate substantially. At this stage, however, there are no guidelines for the design of the sand drying beds for treating chemically conditioned sludges.This study is conducted to investigate the design and performance of sand drying beds for treating chemically conditioned water treatment plant (WTP) sludge. Experiments are conducted using laboratory scale sand drainage columns. The results indicate that the drainage rates, after adding polymer, increased by 9 times for clarifier sludge and 20-25 times for filter backwash sludge. Thus, the land area requirement for sand drainage bed can be drastically reduced through polymer addition. Optimal dose of polymer is found to be between 3 and 6 kg/t of dry solids, with dilute sludges requiring high dosages. The cake solids concentration increased from 3 to 12% as the polymer dose increased from 0 to 16 kgA of dry solids. The variation of drainage rates and cake solids with solids/hydraulic loading rates are found to vary depending on the type of sludges and polymers. The filtrate quality is found to be independent of polymer dose and solids loading rates.

Laboratory and full-scale studies of liquid stream viscosity and streaming current for characterization and monitoring of dewaterability

Many laboratory tests are available to evaluate the dewaterability of a given sludge with a specific type of conditioner. However, most of these are empirical and shed little light on dewaterability mechanisms. The most common laboratory measurements are also not amenable to continuous or flow-through use for actual process application. Viscosity and streaming current are two methods of characterizing the effectiveness of sludge conditioning which may be advantageous in these aspects. Both can be used as measurements characterizing the liquid stream—centrate or filtrate—from the dewatering process. Thus, this study assessed the utility of determining the liquid stream viscosity or streaming current using both lab and full scale experiments. Results were compared to other laboratory measurements of dewaterability, such as the capillary suction time (CST), the conditioned sludge streaming current (SC), and centrate total solids. They were also compared to CST, cake solids content, filtrate turbidity, and percent solids recovery in full-scale dewaterability evaluations. A significant correlation between the centrate viscosity and CST was established, with both related to dewaterability as shown by other parameters. There was also a clear relationship between the CST, the filtrate SC, and these with other dewaterability measures, such as the solids recovery, in the full-scale testing. A minimum CST, SC values near-zero for both centrate and conditioning sludge, and minimum centrate or filtrate viscosity all correlate with maximum solids recovery. These results are of significance both in explaining conditioning and dewatering mechanisms and for possible application in feedback control of polymer dose.

Measurement of chemical sludge floc density and implications related to sludge dewatering

ABSTRACT: An isopyenic centrifugation procedure was developed for estimating the characteristic floc density present in several chemical sludges. Issues addressed included selection of a low osmotic pressure gradient medium; selection of appropriate centrifugal acceleration and time values for testing; viscosity effects; and need for temperature control. Increases in sludge floc density were predictive of an increase in cake solids concentration following mechanical dewatering. Most sludge water removal during dewatering was hypothesized to be external to the floc structure.

Dewatering Characteristics of Oily Sludge

Proper sludge conditioning facilitates better dewatering of sludges. Sludge conditioning techniques required for different sludges bigbly depend on the properties of the sludges. Dewatering characteristics of oily sludges were studied for different degrees of oil content in sludge with lime as sludge conditioner. The optimum dosage of lime was found to range from 6% to 10% to yield favourable characteristics of the sludge for better filterability. Also the established correlations between the combinations of dewatering characteristics, such as specific resistance, capillary suction time, cake solid content and filter yield provide means of quick indication for the nature of the sludges.

Use of dilatometric and drying techniques for assessing sludge dewatering characteristics

ABSTRACT: Various methods have been used to characterize the water distribution present in waste sludge samples. This study investigated constant temperature (35°C) drying and dilatometric techniques for quantifying the bound water content of sludges. Preference was stated for the use of dilatometric methods. Bound water content decreased following polymer or freeze‐thaw conditioning of waste chemical and biological sludges. Also, mechanical dewatering produced a significant decrease in bound water content and corresponding increase in dewatered cake solids concentration. Interstitial water was found to have a key role in defining dewatered cake solids concentrations.