انت هنا الان : شبكة جامعة بابل > موقع الكلية > نظام التعليم الالكتروني > مشاهدة المحاضرة
الكلية كلية الهندسة
القسم الهندسة البيئية
المرحلة 4
أستاذ المادة سولافة عامر محمد علي باش
4/11/2011 4:29:07 PM
Sludge Treatment and Disposal
CONTENTS
1. INTRODUCTION
2. SLUDGE THICKENING
3. SLUDGE CONDITIONING
4. SLUDGE DEWATERING
5. SLUDGE DIGESTION
6. SLUDGE DISPOSAL
7. BIBLIOGRAPHY
1. INTRODUCTION
Sludge is the end product of wastewater treatment, whether biological or physical/chemical treatment. Primary sludge is from 3 to 6 percent solids.
Treatment objectives are reduction of the sludge and volume, rendering it suitable for ultimate disposal.
Secondary objectives are to utilize the generated gas if anaerobic digestion is selected as part of the sludge management strategy. In addition, an attempt should be made to sell/utilize the sludge as a soil conditioner rather than paying to dispose of it.
2. SLUDGE THICKENING
Thickening is provided to reduce the volume of sludge. Two basic types of thickeners work by gravity or flotation and use either continuous or batch processes. Gravity thickeners are essentially settling tanks with or without mechanical thickening devices.
2.1 GRAVITY THICKENERS: A gravity thickener will be designed on the basis of hydraulic surface loading and solids loading. The design principles are to be the same as those for sedimentation tanks. Bulky sludges with a high Sludge Volume Index (SVI) require lower loading rates. The use of chemical additives (lime or polyelectrolytes) also allows higher loading rates. The minimum detention time and the sludge volume divided by sludge removed per day (which represents the time sludge is held in the sludge blanket) is usually less than two days. Table 1 gives mass loadings to be used for designing gravity thickeners.
2.2 FLOTATION THICKENING: Flotation thickening causes sludge solids to rise to the surface where they are collected. This is accomplished by using a dissolved air flotation process. The process is best suited for activated sludge treatment where solids contents of 4 percent or higher are obtained. Table 2 provides design values for flotation thickening.
Table 1: Mass loadings for designing thickeners
Table 2: Air flotation parameters
3. SLUDGE CONDITIONING
3.1 CHEMICAL CONDITIONING. Chemical additives may be used to improve sludge dewaterability by acting as coagulants. Chemicals commonly used for this are ferric chloride (FeCl3), lime (CaO), and organic polymers. The application of chemical conditioning is very dependent on sludge characteristics and operating parameters; therefore, a treatability study will be used to determine specific design factors such as chemical dosages. Nevertheless, Table 3 provides a range of dosages which are typical for various sludge types.
Table 2: Dosage of chemicals for various types of sludges
(conditioners in percentage of dry sludge solids)
3.2 PHYSICAL CONDITIONING. Physical conditioning is primarily by heat. Heat conditioning involves heating at 350 to 390 degrees Fahrenheit for 30 minutes at 180 to 210 pounds per square inch gauge. Dewaterability is improved dramatically and pathogens are destroyed as well. The main disadvantage is the return of high biochemical oxygen demand loading to the wastewater stream.
4. SLUDGE DEWATERING
Dewatering reduces the moisture content of the sludge so that it can more easily be disposed of by landfill, incineration, heat drying, composting or other means. The objective is a moisture content of 60 to 80 percent, depending on the disposal method. EPA Manual 625/1-82-014 provides information on the capabilities of the various dewatering devices and a methodology for selecting the cost-effective device. Because all dewatering devices are dependent upon proper sludge conditioning, a carefully designed chemical feed system should be included as part of the dewatering facility.
4.1 BELT PRESS FILTRATION. Belt filter presses employ single or double moving belts to continuously dewater sludges through one or more stages of dewatering. All belt press filtration processes include three basic operational stages: chemical conditioning of the feed sludge; gravity drainage to a non-fluid consistency; shear and compression dewatering of the drained sludge. When dewatering a 50:50 mixture of anaerobically digested primary and waste activated sludge, a belt filter press will typically produce a cake solids concentration in the 18-23 percent range.
4.1.1 ADVANTAGES AND DISADVANTAGES. Table 4 lists some of the advantages and disadvantages of the belt filter press compared to other dewatering processes.
Table 4: Advantages and disadvantages of belt filter presses
4.2 SLUDGE DRYING BEDS. Sludge drying beds rely on drainage and evaporation to effect moisture reduction. These beds are open; and as such, are very susceptible to climatic conditions such as precipitation, sunshine, air temperature, relative humidity, and wind velocity. For example, sludge drying in 6 weeks in the summer would take at least 12 weeks to dry in the winter. Sludge bed drying efficiency can be improved significantly by covering the bed with glass or plastic and by providing artificial heat. Heat could be supplied using waste biogas as a fuel or waste heat from the base power plant. Figure 2 illustrates a typical bed.
Figure 1: Plan and section of a typical sludge drying bed
6. SLUDGE DIGESTION.
6.1 AEROBIC SLUDGE DIGESTION. The major function of sludge digestion (and its principal advantage) is the stabilization of the sludge in terms of volatile content and biological activity. Aerobic digestion accomplishes this through biological oxidation of cell matter which is done without the production of volatile solids or high biochemical oxygen demand liquor associated with anaerobic digestion.
6.1.1 MODES OF OPERATION. Aerobic digesters can be either continuous or intermittent batch operations. With batch operation, waste sludge feed will be discontinued at a specified time before digested sludge withdrawal. In continuous operation, supernatant is constantly withdrawn. This mode of operation is used when phosphorus is a problem and low phosphorus levels are required in the effluent because batch operation produces high phosphorus concentrations in the supernatant.
6.1.2 DESIGN FACTORS. A summary of design factors is given in Table 13. The tank is open which can be a problem in cold climates with mechanical aeration; no heating is required although some increase in volatile solids reduction can be obtained with increased temperature. Tank design is similar to aeration basin design with the addition of a sludge thickening apparatus. A major disadvantage of aerobic digestion is the high energy requirement.
6.2 ANAEROBIC SLUDGE DIGESTION.
6.2.1 PROCESS DESCRIPTION. Anaerobic sludge digestion is the destruction of biological solids using bacteria which functions in the absence of oxygen. This process produces methane gas which can be used as an energy source and can make anaerobic digestion more economically attractive than aerobic digestion. The larger the treatment plant, the greater the economic incentive to use anaerobic digestion. However; anaerobic digestion is considerably more difficult to operate than aerobic digestion. The methane produced could be of great benefit in cold regions as a supplemental source of heat. Therefore, the decision to use anaerobic digestion must carefully evaluate the operational capability of the installation.
6.2.2 OBJECTIVES. The objectives of anaerobic digestion are the stabilization of organic solids, sludge volume reduction, odor reduction, destruction of pathogenic organisms, useful gas production, and the improvement of sludge dewaterability. Volatile solids typically are reduced by 60 to 75 percent, with final volatile matter contents of 40 to 50 percent.
BIOSOLIDS DISPOSAL
The treated biosolids must be disposed of in
a safe and environmentally sound manner. Sewage biosolids must be stabilized before land application. Stabilization is a process that involves reducing bacteria and other pathogenic organisms. It is accomplished by a process of sewage digestion or other acceptable methods such as lime stabilization. The digestion process reduces the number and types of pathogens (viruses, bacteria, fungi and parasites), reduces the volume of material, and stabilizes organic matter, reducing the potential for odors. Biosolids, which is 8 - 9.5% solids, is typically removed once a year and injected 8 inches into farmland. There are occasions where biosolids are spread over farmland, usually hayfields. Extra precautions must be taken for surface application such as isolation distances and the slope of the farmland. Biosolids acts as a fertilizer supplement and before land application of biosolids can be done, biosolids must be thoroughly tested, correct application rates must be calculated, and farmland soil must be tested and permitted.
BIBLIOGRAPHY
1. J. Paul Guyer, P.E., R.A., Fellow ASCE, Fellow AEI (2011) “Introduction to Sludge Handling, Treatment and Disposal”, Continuing Education and Development, Inc.
2. http://home.comcast.net/~hollywastewater/Process.htm
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
|