Elsevier

Water Research

Volume 35, Issue 3, February 2001, Pages 786-794
Water Research

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

https://doi.org/10.1016/S0043-1354(00)00326-2Get rights and content

Abstract

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.

Introduction

The activated sludge process is the most important treatment method for a wide range of wastewaters. However, sludge from wastewater treatment plants usually exhibits resistance to mechanical dewatering. The chemicals are therefore added to the wastewater sludge as conditioner to enhance dewaterability. Nevertheless, sludge dewatering has been pointed out as the most expensive and least understood process (Bruus et al., 1992). The characteristics of the conditioning and dewatering of sludge have received extensive study recently. The sludges include sediments from clarifiers, screenings, grit, scum, and septage. Depending on sources of generation, they are classified as primary sludge, biological sludge and chemical sludge. Both chemical and biological sludges could be generated at the same site, since pretreatment of industrial wastewaters has been widely practiced for pollution control. The wastewater originating from the industries contains several conventional parameters as well as a number of micropollutants. It is therefore common to have a physicochemical pretreatment unit process in conjunction with a biological unit process, so that the micropollutants can be removed and effective treatment can be achieved. Consider tannery wastewater as an example; chemical precipitation is required to remove the chromium and chemical oxygen demand (COD) from wastewater before the wastewater flows into a biological treatment unit (Tunay et al., 1994; Ates et al., 1997).

The wastewater composition, chemicals used, and treatment units mainly determine the amount and properties of chemical sludge. There are carbonate precipitates, phosphate precipitates, hydroxide precipitates, inert solids, and polymer solids (Cheremisinoff, 1994). The chemical sludge from a tannery, for example, consists mostly of oil and grease, magnesium, calcium and chromium hydroxides precipitates, inert solids, and debris from pig hide (Martin and Parkin, 1986; Langlais and Shivas, 1989; Chuan and Liu, 1996).

Biological sludge is relatively hard to be dewatered compared with the primary sludge. It is due to the small size of colloidal and fine particles that flow through the primary clarifier and are eventually removed in the secondary clarifier (Cheremisinoff, 1994). Activated sludge is a heterogeneous mixture of particles, microorganisms, colloids, organic polymers and cations (Jorand et al., 1995). The biological cells contain 99% water which can be divided into free water, surface water, interstitial water and bound water (Vesilind and Martel, 1990).

In recent years, polyelectrolytes have become a primary choice as conditioner for sludge dewatering operations. Conventionally, a cationic polyelectrolyte is used in the waste activated sludge conditioning in which two main mechanisms are involved: charge neutralization and interparticle bridging (Bohm and Kulicke, 1997). Good control of polyelectrolyte dose is critical in the sludge conditioning, since an overdose will increase the cost and reduce sludge dewaterability. The optimal polyelectrolyte dosage is usually associated with the colloidal surface of minimum surface charge and a tendency to aggregate to form large floc (Christensen et al., 1993). Since some of the chemical sludges are positively charged, an anionic polyelectrolyte may be used as the conditioner to facilitate the polymer adsorption onto the sludge surfaces. For the wastewater treatment plants that generate both chemical and waste-activated sludges on-site, questions arise whether it is wise to co-condition both the sludge, or rather treat them separately. Only very limited information is available. It has been indicated that wastewater containing fiber from a pulp and paper mill enhances the dewaterability of the sludge from a POTW (Eckenfelder Jr. and Musterman, 1994). Besides, it is recommended, without detailed elaboration, to dewater chemical and waste activated sludges together, so that better dewatering efficiency can be achieved (WPCF, 1983; Cheremisinoff, 1994). In fact, very little is known about the conditioning and dewatering behaviors of the mixture of chemical and waste activated sludges. Yet, in a survey among industrial wastewater treatment plants in Taiwan, it is found that co-conditioning and dewatering of the chemical and biological sludges is a common practice.

In the current study, basic properties of the chemical and the waste activated sludges from a tannery were first characterized. The conditioning and dewatering of each sludge were investigated. The co-conditioning and dewatering behaviors of mixed chemical and waste activated sludges were then examined and compared. The major objectives are to study if it is advantageous in co-conditioning and dewatering the chemical and waste activated sludges from a tannery, and to elucidate the mechanisms in conditioning and dewatering of the mixed sludge.

Section snippets

Materials and methods

The sludge samples were obtained from the Chen-Sam tannery located in Pintung, Taiwan. The Chen-Sam tannery manufactures leather using pig hide as the raw material. Its production processes are soaking–liming–deliming, bating–tanning–neutralization–dyeing, and fatliquoring. Its wastewater treatment plant consists of a physicochemical process with coagulation–flocculation and sedimentation units, and an activated sludge process. The process flow chart is shown in Fig. 1. The amount of sludge

Zeta potential

Fig. 2 demonstrates that the zeta potential of the chemical sludge was originally equal to 0.5 mV. The coagulant used probably caused the nearly neutral value. Upon the addition of the cationic polyelectrolyte, it shifted slightly to positive values ranging from 6 to 8 mV. The surfaces of the waste activated sludge were originally negatively charged with the zeta potential of −26 mV. The zeta potential was significantly affected by polyelectrolyte, as it shifted towards the less negative values as

Summary

The chemical sludge from a tannery was found to be very difficult to be dewatered by polyelectrolyte conditioning. The waste activated sludge from the same plant showed a much better dewaterability. When the chemical sludge was mixed with waste-activated sludge at volumetric ratios of 1 : 4 and 1 : 8, the dewaterability of the mixed sludge was equal to or better than that of the waste activated sludge. The optimal polyelectrolyte dosage for the mixed sludge was the same as that of the waste

Acknowledgements

The authors thank the National Science Council of Taiwan for financial support of this work, under contract NSC-87-2218-E-011-006, and the assistance from the Chen-Sam Tannery Co.

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