Management of sewage sludge using dewatering units

Transcription

1 Proceedings of International Symposium on Environmental Pollution Control and Waste Management 7-10 January 2002, Tunis (EPCOWM 2002), p Management of sewage sludge using dewatering units Saleh AL-MUZAINI Environmental Sciences Department Kuwait Institute for Scientific Research P. O. Box 24885, Safat 13109, Kuwait Tel: / Fax: / ABSTRACT Sand drying beds are inadequate to handle the projected sludge production at the Jahra treatment plant. The problems of insufficient area for the present drying beds and a lack of additional land around the treatment plant have made necessary assessment of an alternative dewatering unit. In the Jahra treatment plant, there are, at present, 30 drying beds, which are able to dry about 100 m 3 of sludge daily. Sewage sludge is about 0.5 % of the total flow treated at the plant. The daily inflow to the plant is about 45,000 m 3 /d and is increasing. This paper evaluates the dewatering characteristics of the sludge produced in the Jahra treatment plant and presents the results for a testing program of a centrifuge unit for dewatering sewage sludge. The research plan included determination of optimum dosages of coagulants for thickening the sludge, and general descriptions of the centrifuge unit and testing procedures. The results showed that the centrifuge unit could be selected as a sludge dewatering unit able to produce a cake of 20% solids. Heavy metals concentrations in the dry solids are low in concentration and were not likely to cause environmental health problems. KEYWORDS Centrifuge filter, polymer, sewage, treatment plant, Kuwait Introduction The Jahra treatment plant is located 15 km west of Kuwait City and serves two cities, Sulaibiya City on the eastern side and Jahra on western side. The design and construction of the Jahra plant took place in the late 1970s, and the plant was operated in 1981 as a secondary treatment plant. The plant was improved by construction of additional units to reach the performance level of a tertiary plant. It was designed to receive daily 760,000 m 3 of sewage. At present, the plant receives an average daily sewage flow of about 45,000 m 3 (Ministry of Public Works, 1994). Total plant includes primary treatment, secondary treatment and tertiary treatment. About 20% of its treated effluent are used in afforestation projects and the rest is discharged through sea outfalls (Al-Muzaini et al, 1999). The sludge produced at the Jahra plant is directed to prethickeners prior to final dewatering in drying beds. There are two thickeners, 15 m in diameter and with a mean depth of 3.7 m each, accommodating a volume of 730 m 3 of sludge. At present, there are 30 drying beds and they are able to dry about 100 m 3 of sludge daily. The dried sludge is mechanically removed from the drying beds and transported to a dumping area within the plant. As the number of drying beds is not sufficient for drying the total produced sludge, part of it is hauled out of the plant in tankers directly after thickening for disposal at a supervised location. Fig. 1 shows the Jahra treatment plant in relation to Kuwait City. The sludge at the Jahra plant contains more than 95% water, which is responsible for its large volume, and high cost of transportation and final disposal (Ghobrial et al., 1986). Therefore, an effective mechanical filtration system needs to be thoroughly investigated. As a result, a pilot-plant study was conducted to determine the most cost-effective processes to be used in a testing program to select a single dewatering unit for processing the sludge produced at the Jahra sewage treatment plant. As a result of this study, one unit was chosen for pilot-plant testing. This was the centrifuge unit. A study was performed by the Water Environment Research Foundation (WERF) to evaluate the efficiency of the existing dewatering technologies to determine whether centrifuge systems could really be reliable on a long-term basis with minimal maintenance (Gillette and Joslyn, 2001). This study has shown that centrifuge systems could become reliable and cost-effective only with instrument and software improvements. Snyman et al. (2000) showed that sewage sludge is typically dewatered using drying beds, belt filters and centrifuges. However, improvements in mechanical dewatering systems could enhance their efficiency. 867

2 Al-Muzaini Fig. 1. The Jahra sewage treatment plant in relation to Kuwait City. Furthermore, dewatering systems could be maximized by separating out water during the centrifuging. Research has indicated that the addition of polymers increases the velocity of sludge water release and increases the total solids concentration in the sludge cake (Kopp and Dichtl, 2000). Studies have shown that mechanical dewatering units, such as the centrifuge, are used to remove the liquid and to increase the solids concentrations (Al-Muzaini et al., 1999; Al- Layla et al., 1980; Innocenti, 1988; Steel and McGhee, 1979). Research done by Novak (1988) found that using mechanical dewatering processes is an effective method for dewatering solids, and using polymers in mechanical dewatering units was important. Preliminary work indicates that the nature of the sludge, its original solids content, and whether or not a conditioner is added are important in the performance of mechanical dewatering units (Reynold, 1982). The overall performance of the dewatering units, such as centrifuges, could be improved radically if fluctuations in feed sludge characteristics were minimized (Golaszewski and Ronald, 1985). Sludge technologies were reviewed (Hudson 1996; Sawyer et al., 1979), and the quality of the sludge and its dryness were found to be important in selecting the most suitable dewatering process. Therefore, a pilot mechanical dewatering facility was established at the Jahra plant. It provided necessary information about the mechanical dewatering system, and the capacity and necessary operating conditions for a centrifuge sludge dewatering system were determined. Description of Unit Centrifuge Unit Centrifuge units have been used since the 1960s. Improvement in the centrifuge equipment has resulted in wide range of uses for dewatering sludge (Leonard and Parrot, 1978). The most widely used centrifuge for wastewater sludge treatment is the solid bowl or decanter type with an electric motor drive. The centrifuge tested was the decanter type with an electric motor drive. This type of machine consists of a conico-cylindrical rotor that rotates between two bearing blocks supported by a base frame, and includes a decantation bowl, screw conveyor and speed reducer. The unit is driven by horizontal-axle double electric motors. The screw conveyor, which is driven by the speed reducer, turns at a 868

3 Sewage sludge slightly higher speed than the bowl. A flow of the centrifuge process is shown in Fig. 2. The general specifications of the centrifuge used in this experiment are shown in Table 1. Water Supply Centrifuge Unit Polymer Sample Point Water Polymer Polymer Liquid Cake Dosing Feed Discharge Discharge Polymer Polymer Pump Tank Mixing Polymer Stock Unit Pump Tank Sludge Pump Sludge Thickening Sample Point Fig. 2 Flow diagram of the Centrifuging process. Table 1. Technical Specifications for the Centrifuge (LWZ# 20 x 770) Parameter Specification Diameter 380 mm Rotational speed 3000 rpm Centrifuge force 1915 x G Operating temperature C Dryness (solid cake) 30-40% All fittings & piping Stainless steel Description of Operation In general, sludge is fed continuously into a rotating mechanism that separates it into a dense cake containing most of the solids and a diluted central steam containing the remaining fine-density solids. The solid cake is transported along the bowl and exits the discharge ports. The sludge and polymer (no ) at 3.5 mg/l were mixed thoroughly before introduction into the centrifuge. The sludge enters the bowl where it separates into liquid and solids. The solid cake is removed through the discharge point, and the rest is removed through the weir in the end plate. The adjustable parameters in a centrifuge are rotational speed, differential scroll speed, pond depth, centrifuge force, feed rate and polymer dosage. Increased bowl speed increases both cake dryness and solids recovery. 869

4 Al-Muzaini Experimental Program The purpose of the experimental program was to assess the performance of the centrifuge unit as a dewatering system. The performance was evaluated on the basis of polymer dosage, cake dryness, feed loading and solids concentration in the final cake. In this experiment, the unit was operated at its highest level so that it could produce a cake with a high solids content that could be separated easily from the filtrate. Selection of Coagulants Materials and Methods A large number of polyelectrolytes (i.e., polymers) and inorganic chemicals are available for coagulating wastewater sludges. Screening was necessary in order to select the best coagulant for use in wastewater sludge. A jar test was done to determine the optimum dosage of coagulant, and the time required to coagulate the prethickened sludge. The laboratory results indicated that polymer no has the ability to achieve an efficiency of more than 97% at a 3.5- mg/l concentration when coagulating sewage sludge. All conditioning agents were cationic polymers obtained from Allied Colloids Company, Kuwait. Table 2 lists the optimum coagulants, times and dosages (Al-Muzaini et al., 1999). Table 2. Optimum Coagulants, Times and Dosages Coagulant Dosage Range Optimum Coagulant Time Min - Max Concentration (min) (mg/l) (mg/l) Mud Lime Klinker Alum FeCl Klinker Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer Polymer Centrifuge Testing The mechanical dewatering unit was tested and operated at various operating setting to optimize the performance of the unit as well as to assess the effect of the varying parameters on the outputs of the unit. The major variables for the unit tested were as follows: Polymer type, Polymer dose, Rotational speed, Centrifuge force and Cycle time. The unit was tested under various operating conditions to assess the effect of each parameter on the efficiency of the centrifuge and the quality of sludge produced by the unit. Analyses were performed on the centrifuge s influent solids, resultant cake solids and filtrate solids. 870

5 Sewage sludge Sampling and Analyses Grab samples of the cake and filtrates were collected at the end of each test run. Grab samples of the feed sludge to the unit were taken under the different operating conditions. All feed and cake samples were analyzed for total suspended solids and selected heavy metals. All analyses were done in accordance with standard methods (APHA, 1995). Characteristics of Sludge Produced at the Jahra Plant Results and Discussion The feed sludge to the dewatering unit was prethickened sludge which came from clarifier units (Ministry of Public Works 1994). Thickening of sludge is achieved in tanks called thickeners, which are similar in function and construction to clarifiers. To raise the thickened sludges total solids content, the feed sludge was conditioned with polymer no at a 3.5-mg/l concentration. Then, the sludge was dewatered using a centrifuge unit, which resulted in a dry cake of the desired quality. Since, this dewatering unit was operated over various periods of the composition of the feed sludge to the dewatering units varied. Performance of the Centrifuge Unit During the testing program, the operating parameters for the centrifuge unit were varied systematically in a manner to achieve the driest cake. The operating conditions tested were sludge feed, operational speed, centrifuge force and polymer dosage. Table 3 presents data on centrifuge performance. The unit ran satisfactorily during the test period. The unit produced approximately 19-20% solids. The test showed cationic polymer (i.e., polymer no ) to be the most effective and to posses good ability to form a coagulant, leading to the most rapid recipitation. Table 3. Centrifuge Performance Description Value Loading 4.97 kg/min Dewatering sludge Concentrate solids % solids 11,000 mg/1 Polymer dosage 4-3 mg/1 Sludge samples collected from the pilot plant in August 1998 were analyzed for Cd, Cr, Cu, Co, Fe, Mn, Mo, Ni, Pd, Sn, V and Zn as a screening step. All metals were detected and the results are presented in Table 4. Zn, Cu and Ni were selected for their proven toxicity to terrestrial in instances of land disposal (Williams, 1983; Davis, 1984). Cd was selected for its tendency to accumulate in the edible parts of plants (Hansen and Tjell, 1983; and Davis, 1984). Cd, Cu, Pd, and Zn were selected for their toxicity to the marine environment in instances of sea disposal (Forstner and Wittmann, 1981). Fe and Mn were selected for their toxicity potential. From table 5 shows that the concentration of each metal in the dry sludge falls within recommended levels. Trace metal concentrations in sludge is influenced by the level of trace metals in the influent of sewage wastewater, and the type and efficiency of the treatment processes. 871

6 Al-Muzaini Table 4. Concentration of Heavy Metals in Sewage Sludge Samples from the Centrifuge Pilot Plant in Jahra, Kuwait. Metal (mg/kg) Sample No. Cd V Co Zn Cr Cu Fe Mn Mo Ni Pb Sn Note: Sample No. 1- Inlet sample Sample No. 2- Dry sludge from the centrifuge Source: (Al-Muzaini et al., 1999) Conclusions The pilot plant testing was conducted to test a dewatering unit for dewatering sludge in Kuwait. A mechanical dewatering unit was suggested, such as the centrifuge. The main objective of the testing program was to evaluate the characteristics of the sludge produced in the Jahra treatment plant, and study the performance of a centrifuge unit in terms of a technically applicability, feasibility and environmental soundness of this mechanical sludge dewatering technique for use in Kuwait. This was important because of the problems associated with the existing sand drying beds The results of the testing of the sludge dewatering unit can be summarized as follows: The centrifuge unit was operated at a loading rate of 4.97 kg/min and produced a cake with solids of 20% on average. Polymer no was selected and showed good effectiveness for the sludge produced at the Jahra plant. A suitable dosage was estimated to be 3.5 mg/l. Selected heavy metal concentrations in the dry solid produced by the centrifuge unit were within expected levels. Furthermore, the presence of heavy metals in the plants needs further investigation, and should be studied in more detail in the future. After evaluating the data collected during the testing program, the centrifuge was determined to be the mechanical dewatering unit that would meet the Jahra treatment plant s requirements for sludge dewatering management. Acknowledgement The author thanks Dr. Ali Al-Shamlan, Director General of the Kuwait Foundation for Advancement of Sciences (KFAS), for his interest in and support of this study. The study described herein was jointly supported by KFAS and the Kuwait Institute for Scientific Research. The author would like to thank the personnel at the Jahra treatment plant and Ministry of Public Works for their time and support during the study. References Al-Layla, M., S. Ahmed and E. Middlebrooks Handbook of Wastewater Collection and Treatment. London: Garland STPM Press. Al-Muzaini, S.; M. Al-Mutairi; A. Kurian; and R. Al-Nafisi Selection of effective sludge dewatering system. Kuwait Institute for Scientific Research, Report No. KISR, 5466 Kuwait. APHA Standard Methods for the Examination of Water and Wasterwater, 19 th edition, New York: American Public Health Assn. Davis, R.D, Crop uptake of metals Cad, Pd, Cu, Ni, Zn and Cr from sludge treated soil and its implication for soil fertility and for the human diet. In Processing Use of Sewage Sludge. Dordrecht, Holland: D. Reidal Publishing Co., pp

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