Two Phase Digestion

As discussed in Section 5.1.2, anaerobic digestion involves two major phases hydrolysis and acid formation together, and methane production. In the three preceding high-rate digestion processes, both phases take place in a single reactor. In two-phase digestion, the two major phases are divided into separate tanks coupled in series. Pilot studies (Ghosh et al., 1975 Lee et al., 1989) and data from two full-scale facilities (Ghosh et al., 1995) have shown that better sludge digestion can be...

Two Stage Digestion

Two-stage digestion is carried out in a high-rate digestion tank coupled in series with a second tank (sometimes called a secondary digester), as shown in Figure 5.7. The second tank is neither heated nor mixed. Its main function is to allow gravity concentration of digested solids and decanting of supernatant liquor. Decanting reduces the volume of digested sludge, requiring further processing and disposal. A secondary digester fitted with a floating cover can also provide storage for digested...

Incineration

Incineration is complete combustion, which is the rapid exothermic oxidization of combustible elements in sludge. Dewatered sludge will ignite at temperatures of 420 to 500 C (788 to 932 F) in the presence of oxygen. Temperatures of 760 to 820 C (1400 to 1508 F) are required for complete combustion of organic solids. In the incineration of sludge, the organic solids are converted to the oxidized end products, primarily carbon dioxide, water vapor, and ash. Particulates and other gases will also...

Dewatered Sludge Percent Solids

Figure 6.4 Theoretical quicklime dosages for class A and class B dry lime stabilization. lower line shows the dose required for class B pH requirements, and the upper line shows the dose for class A temperature requirements. The figure shows that the quicklime requirement for class B stabilization theoretically increases with increased solids, whereas the quicklime requirement for class A stabilization decreases with increased solids. This is because of the predominance of the heating...

Advanced Alkaline Stabilization Technologies

Technologies using materials other than lime for alkaline stabilization have been used by a number of municipalities. These technologies are modifications of traditional dry lime stabilization, and most of them use materials such as cement kiln dust, lime kiln dust, portland cement, or fly ash. The most common modifications are the supplemental drying and addition of other chemicals or bulking agents. The principal advantages of advanced alkaline stabilization processes include (1) the product...

Dry Lime Stabilization

In dry lime stabilization (also known as postlime stabilization), dry lime (hydrated lime or quicklime) is mixed with dewatered sludge cake to raise the pH of the mixture. The process requires adequate mixing to avoid pockets of putrescible material and to produce a homogeneous mixture. An effective mixer is a pug mill in which two screw conveyors or paddle mixers rotate in opposite directions. Other types of mixers include plow blender, paddle mixer, and screw conveyor. Figure 6.3 shows the...

Pathogen and Vector Attraction Reduction

Pathogens are disease-causing organisms such as certain bacteria, fungi, viruses, protozoans and their cysts, and intestinal parasites and their ova. Vector attraction is the characteristic of sewage sludge that attracts rodents, flies, mosquitoes, or other organisms capable of transporting infectious agents. Subpart D of Part 503 covers alternatives for reducing pathogens in sewage sludge and domestic septage, as well as options for reducing the characteristic of sewage sludge that attracts...

Turovskiy And Westbrook

S. (2001), Reduction of Energy Consumption in Wastewater Sludge Treatment, Florida Water Resources Journal, March, pp. 34-36. -, and Westbrook, J. D. (2002), Recent Advances in Wastewater Sludge Composting, Water Engineering and Management, October, pp. 29-32. -, Goldfarb, L. L., Pavlovskiy, L. L., and Minz, M. S. (1972), Method of Sludge -, Goldfarb, L. L., Zamoschin, L. V., and Zhukov, Z. A. (1978), Method of Effluent Sludge Treatment, U.S. patent 4,125,465.

Technological Improvements

The principal author of this book, together with the Medical Hygiene Institute in Russia and several wastewater treatment plants, conducted research on aerobic sludge digestion (Turovskiy, 2001). The object of this research was to obtain sanitarily harmless biosolids with an effective sludge digestion technology that was also cost-effective. Thickening Experiments with activated sludge thickening were conducted with samples from treatment plant thickeners using simulators. The thickening of...

Physical and Biological Properties

Raw primary sludge particle size distribution is greater than 7 mm (5 to 20 ), 1 to 7 mm (9 to 33 ), and smaller than 1 mm (50 to 88 ), of which about 45 is less than 0.2 mm. In activated sludge, the approximate distribution is 90 below 0.2 mm, 8 between 0.2 and 1 mm, 1.6 between 1 and 3 mm, and 0.4 over 3 mm. The organic part of the sludge decays more rapidly, with an increase in the quantity of finely dispersed and colloidal particles and bound water resulting in a decrease in the separation...

Methods of Thermal Drying

Preheater Exhaust Fan

Dryers are commonly classified on the basis of the predominant method of transferring heat to the wet solids being dried. These methods are convection (direct drying), conduction (indirect drying), radiation (infrared drying), or a combination of these. In convection (direct drying), heat transfer is accomplished by direct contact between the wet sludge and hot gases. The heat of the inlet gas provides the latent heat required for evaporating the liquid from the sludge. The vaporized liquid is...

Autothermal Thermophilic Aerobic Digestion

Wastewater Sludge

Autothermal thermophilic aerobic digestion (ATAD) is a sludge digestion process that is capable of achieving a high degree of stabilization and pathogen reduction. The process is characterized by high reaction rates achieved at a thermophilic temperature of 40 to 70 C. The temperatures are attained by using the heat released by the exothermic microbial oxidation process. Approximately 15,000 kJ of heat is generated per kilogram of volatile solids destroyed. In a completely mixed and aerated...

Chemical Conditioning

Thickener Cross Section

Chemical conditioning is the most common conditioning process for sludge thickening and dewatering. Conditioning by adding chemicals can be viewed as coagulation or flocculation by neutralization of colloidal surface charge by oppositely charged organic polymers or inorganic chemicals. Particle size is the most important characteristic of the dewaterability of sludge. By adding chemicals, the particle size increases and the bound water decreases. Different sludges have different dewatering...

Liquid Lime Stabilization

In liquid lime stabilization (also known as prelime stabilization), lime slurry is mixed with liquid sludge to produce class B biosolids. Figure 6.2 is a schematic of a liquid lime stabilization system. This is usually a batch treatment system in which the contact tank for mixing lime slurry with liquid sludge is Figure 6.2 Typical liquid lime stabilization process schematic. Figure 6.2 Typical liquid lime stabilization process schematic. sized, based on the quantity of sludge and the number of...

Process Design Considerations

Factors that govern the process design of conventional aerobic digesters include feed sludge characteristics, temperature, volatile solids reduction, oxygen requirements, and mixing. Other system design considerations and operational considerations are discussed later in this chapter. Feed Sludge Characteristics Because the aerobic digestion process is similar to the activated sludge process, the same concerns, such as variations in influent characteristics and materials that are toxic to...

Aerated Static Pile Process

The identifying feature of the aerated static pile system is a grid of aeration piping for forced aeration. A blower or fan aerates the pile. The aerated static pile process consists of mixing of dewatered sludge with a bulking agent (usually, wood chips), construction of the composting pile over the grid of aeration piping, composting, screening of the compost, and curing and storage. Figure 7.5 shows an aerated static pile process of composting. The aeration grid is usually made of 100- to...

Windrow Process

In the windrow process, dewatered sludge mixed with a bulking agent is formed in long parallel rows or windrows. The width of a typical windrow is 2 to 4.5 m (6 to 14 ft) at the base and the height is 1 to 2 m (3 to 6.5 ft). Depending on the characteristics of the equipment used for mixing and turning of the windrows, the cross section of the pile may be triangular or trapezoidal. Windrow composting is commonly performed at open outdoor sites. However, in areas of significant precipitation, it...

Zoological Characteristics of Compost

Compost has the ability to enrich soil with beneficial invertebrates that stimulate the growth of soil organisms. The zoological population influences the physical, chemical, and microbial factors in soil. Some zoological populations in the mixture of sludge, bulking agent, and amendment survive the composting process. They penetrate the compost pile when temperature decreases to 30 to 35 C during the cooling stage. When the compost is stored, the zoological populations increase. The number of...

Supernatant

Supernatant from an anaerobic system generally contains high concentrations of dissolved and suspended solids, organic materials, nitrogen, phosphorus, and other materials. When returned to the treatment plant influent, these materials may impose an extra load on the liquid treatment processes. Table 5.6 presents reported characteristics of supernatant from two-stage digestion systems at different wastewater treatment plants. These characteristics represent a summary of the wide range of data...

B Cross Section of a rectangular tank Figure 311 Dissolved air flotation system

Settling Tank Figures

The floatation tank can be circular or rectangular and made of steel or concrete. Smaller tanks are usually steel and come completely assembled. For large installations requiring multiple tanks or large tanks, concrete tanks are more economical. Rectangular tanks have several advantages over circular units. In rectangular tanks, skimmers skim the entire surface and the flights can be closely spaced, allowing more efficient skimming. In a rectangular tank, bottom sludge flights are usually...

Units Of Measure

Wastewater Sludge Processing, By Izrail S. Turovskiy and P. K. Mathai Copyright 2006 John Wiley & Sons, Inc. A.2 Abbreviations for U.S. Customary Units A.2 Abbreviations for U.S. Customary Units A.3 Conversion from SI Units to U.S. Customary Units pounds per thousand cubic foot per day Some of the units listed are metric units commonly used in environmental engineering. Some of the units listed are metric units commonly used in environmental engineering. A.4 Conversion from U.S. Customary...

Composting Process

Composting of wastewater sludge is an aerobic biothermal process that decomposes the organic constituents. It can be described by the formula C6H12O6 + 6O2 6CO2 + 6H2O + 674 kcal It can also be performed as an anaerobic process described by the formula The aerobic process provides a higher caloric content. It goes much faster than the anaerobic process, and decomposition of the organic constituents produces a stable humuslike material. Microbiology Composting represents the combined activity of...

Dry Lime Mixing

Mixing of dewatered sludge cake and lime (or other alkaline materials) is the most critical component of dry lime stabilization. The goal is to provide intimate contact between the sludge cake and lime and to produce a homogenized product. The mixing step is typically accomplished using a mechanical mixer such as a pug mill, plow blender, paddle mixer, or screw conveyor. Dewatered sludge and lime are introduced at the head end of the mixer. Sludge cake with a low moisture content and in large...

Pressure Filter Press

Pressure filter press dewatering is a batch process in which dewatering is achieved by forcing the water from the sludge under high pressure. It produces a cake that is drier than that produced by any other dewatering alternative. Another advantage is that the high solids capture results in good filtrate quality. Disadvantages include high capital cost, relatively high operation and maintenance costs, high chemical costs, and a large area requirement for the equipment in small wastewater...

Belt Filter Press

Disk Belt Filter Press

A belt filter press (BFP) is a continuous-feed sludge dewatering machine with two porous moving belts that has a gravity drainage zone and mechanically applied pressure zones. Belt filter presses have been used in Europe first for dewatering paper pulp and later modified to dewater wastewater sludge. It was introduced in North America in the mid-1970s, mainly because of its ability to dewater secondary sludge economically, and its reduced energy requirements compared to centrifuges and vacuum...

System Design Considerations

Factors that must be considered in designing conventional aerobic digesters include method of operation, tank volume and detention time, tank design, and aeration and mixing equipment. Typical design criteria for aerobic digestion are presented in Table 4.1. Method of Operation The two primary operational modes for conventional aerobic digesters are batch or continuous operation, referring to the manner in which supernatant is withdrawn from the process. Figure 4.3 shows cross sections of...

Composting Mixture

Sludge Composting Device

The new technology requires quicklime to be mixed with the dewatered sludge just prior to adding a bulking agent (sawdust, wood chips, bark, etc.) and recycled compost. Refer to Figure 7.13 for the schematic of the proposed 1.Hopper with conveyor for dewatered sludge 2.Silos for quicklime with processing unit and pneumatic pumping of lime 3.Hopper for bulking agents and recycled compost 6.Discharge conveyor for mixture to be composted 11. Cover over composting piles 1.Hopper with conveyor for...

Examples Of Composting In Europe

Several examples of different types of composting systems that have been in use in Europe are described below. All these systems are open-air processes, mostly on asphalt pavements. In Finland, two wastewater treatment plants (city of Lappeenranta and city of Loensiy) compost sludge mixed with ground bark as the bulking agent at a ratio of 1 1 by volume in 3-m-high windrow piles. Every three weeks, piles are shoveled over by scoop loader. After two weeks of processing the temperature rises to...

Composting Process Control

Experiments show that the type and population of microorganisms varies during the composting process. It is therefore critical to control the composting environment so that the microorganisms can flourish. The composting environment parameters include the compost pile temperature, moisture content of the compost, oxygen and carbon dioxide levels in the compost pile, and the availability of nutrients, including carbon, nitrogen, phosphorus, and potassium for the microorganisms. These parameters...

Examples Of Composting In The United States

Aerated Static Pile Composting

In the United States, studies of windrow and aerated static pile composting were conducted in the mid-1970s in Beltsville, Maryland, and in Carson, California. As of 2003, there were more than 200 wastewater sludge composting facilities in the United States. Most of the composting facilities are aerated static pile systems, and some are in-vessel systems. Windrow composting is rarely used because of the potential odor generation and the large area requirement. The city of Eustis, Florida, uses...

Drying Beds

Concrete Sludge Drying Beds

Drying beds are the most widely used method of municipal wastewater sludge dewatering in the United States. They have been used for more than 100 years. Although the use of drying beds might be expected in small plants and in warmer, sunny regions, they are also used in several large facilities and in northern climates. In the United States, a majority of wastewater treatment plants with less than 5-mgd capacity use drying beds for biosolids dewatering. In Russia and other Eastern European...

Process Description 721 Factors Influencing Composting

Composting represents the combined activity of a succession of mixed populations of bacteria, actinomycetes, and fungi at different stages of the process. The principal factors that affect the biology of composting are moisture, temperature, pH, nutrient concentration, and oxygen supply. Moisture Decomposition of organic matter depends on moisture. Less than 40 moisture may limit the rate of decomposition. The optimum moisture content is 50 to 60 . Moisture content is also important for the...

Theoretical Aspects Of Composting

The Composting Process

For the best results in a composting process, it is important to have appropriate mixing of sludge cake with bulking agents and recycled materials. For the process to operate in good condition, it needs to have the optimum mass balance, moisture, temperature, pH, nutrients, and air. Figure 7.11 shows a mass balance diagram that can be used for all three composting systems windrow, aerated static pile, and in-vessel. In the diagram A' weight of ready compost in one day A weight of recycled...

Advantages and Disadvantages of Composting

The main advantages of sludge composting are the following Compost has an abundance of nutrients and is suitable for a wide variety of end uses, such as landscaping, topsoil blending, and growth media. Compost has less nitrogen than biosolids from other stabilization processes, due to the loss of ammonia during composting. However, nitrogen in compost is released more slowly and is available to plants over a long period of time, which is more consistent with plant uptake needs. Well-composted...