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CHARACTERIZATION, UTILIZATION, AND DISPOSAL OF MUNICIPAL SLUDGE: THE STATE OF-THE-ART

Muhammad H. Al-Malack*, Nabil S. Abuzaid, Alaadin A. Bukhari and Mohammed H. Essa King Fahd University of Petroleum & Minerals Dhahran, Saudi Arabia

. . . .

*Address for Correspondence: KFUPM Box 1150 King Fahd University of Petroleum & Minerals Dhahran 31261 Saudi Arabia e-mail: [email protected]

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ABSTRACT The rapid increase in sludge production, following massive investment in new wastewater treatment plant installations and upgrading of existing facilities, creates the need for adoption of economically and environmentally acceptable sludge management schemes, which take into account the worldwide growing interest in reuse and recovery possibilities. This paper presents a literature review concerning sludge generation, production, and treatment. Alternatives of sludge utilization and disposal are also presented. In addition, the paper provides information on the regulations on municipal sludge reuse and disposal, set by related agencies, mainly the United States Environmental Protection Agency (U.S.EPA). The regulations were formulated to protect the environment, the health of workers handling municipal sludge, and the health of the public. Keywords: Municipal Sludge Generation, Production, Characteristics, Treatment, Utilization, Disposal, Regulations.

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CHARACTERIZATION, UTILIZATION, AND DISPOSAL OF MUNICIPAL SLUDGE: THE STATE OF-THE-ART

SLUDGE GENERATION AND PRODUCTION Solids generation in biological wastewater treatment plants is difficult to estimate, where all suspended solids in the influent do not appear in the sludge. Some are biologically metabolized to soluble or gaseous end-products in the biological treatment process or sludge digestion process. Some soluble wastewater components will be transformed into biological solids that can be reduced during digestion. Solids generation in a biological treatment process is a function of the type and the operation of the process. The sources of sludge in a wastewater treatment plant vary according to the type of plant and its method of operation [1]. The principal sources of sludge at municipal wastewater treatment plants are the primary sedimentation basin and the secondary clarifiers. Additional sludge may also come from chemical precipitation, nitrification­denitrification facilities, screening, grinder, and filtration devices. Table 1 shows the principal sources of solids and sludge and the types generated [1].

Table 1. Sources of Solids and Sludge from a Conventional Wastewater Treatment Plant [1]. Unit Operation or Process Screening Types of Solids or Sludge Coarse solids Remarks Coarse solids are removed by mechanical and hand-cleaned bar screens. In small plants, screenings are often comminuted for removal in subsequent treatment units. Scum removal facilities are often omitted in grit removal facilities. In some plants, scum removal facilities are not provided in preaeration tanks. If the preaeration tanks are not preceded by grit removal facilities, grit deposition may occur in preaeration tanks. Quantities of sludge and scum depend upon the nature of the collection system and whether industrial wastes are discharged to the system. Suspended solids are produced by the biological conversion of BOD. Some form of thickening may be required to concentrate the waste sludge stream from biological treatment. Provision for scum removal from secondary settling tanks is a requirement of the U.S. Environmental Protection Agency. The characteristics of the end products depend on the characteristics of the sludge being treated and the operations and processes used. Regulations for the disposal of residuals are becoming increasingly stringent.

Grit removal Preaeration

Grit and scum Grit and scum

Primary sedimentation

Primary sludge and scum

Biological treatment

Suspended solids

Secondary sedimentation

Secondary sludge and scum

Sludge-processing facilities

Sludge, compost, and ashes

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Steel [2] reported that the per capita production of suspended solids could be assumed as 91 g/capita/day. If there is a moderate amount of industrial wastes, this may rise to 100 g/capita/day, and if it is a combined sewage with considerable industrial waste, it may be 113 g/capita/day. If ground garbage is added to the sewage, then there will be an additional 32 to 50 g/capita/day. Schmidtke [3] reported that, without sludge digestion, the per capita sludge production rates in Ontario are approximately 80 and 115 g/d for primary and secondary treatment, respectively. Adding iron or aluminum salts for phosphorus removal will increase sludge production by about 40% for primary treatment or 25% for secondary treatment. Koch et al. [4] evaluated procedures for estimating sludge production. They concluded that more detailed data needed to be collected on wastewater characteristics, the treatment process, and the operating parameters to reliably predict sludge production. Lishman et al. [5] investigated the effect of temperature on wastewater treatment under aerobic and anoxic conditions. They concluded that the observed yields were 35 to 52 percent higher for the anoxic reactors than they were for the aerobic ones. Jardin and Popel [6] investigated the effect of an enhanced biological phosphorus removal process on waste activated-sludge production and the type of phosphorus storage in two continuousflow activated-sludge systems in pilot-scale experiments. They reported that the additional uptake of phosphorus resulted in an increase in the inorganic sludge mass, but the organic sludge mass did not change. Davis and Cornwell [7] reported that for primary treatment, the quantities of sludge produced may be 0.25 to 0.35 percent by volume of wastewater treated. When treatment is upgraded to activated sludge, the quantities increase to 1.5 to 2.0 percent of the volume of wastewater treated. Use of chemicals for phosphorus removal can add another 1.0 percent. Vesilind et al. [8] reported figures of dry sludge production in Europe ranging from 30 to 124 g/capita/day. In Metropolitan Seattle, Machno [9] reported that the amount of solids generated per household is 125 g/day (about 35 g/capita/day). In the United Kingdom, raw primary sludge produces 52 g/capita/day, co-settled activated produces 74 g/capita/day and co-settled activated

Table 2. Sludge Production and Methods of Disposal in 1990 [10]. Population (total) (million) Austria Belgium Denmark France West Germany Greece Ireland Italy Luxembourg Netherlands Portugal Spain Switzerland UK Japan USA

a b

Country

Population served (%) 48 33 100 64 90 44 30 92 90 47 47 80 84 42 -

Sludge produced (TDS ×1000/yr) 320 75 130 700 2500 15 24 800 15 282 (871) 200 280 215 1075 2440 800 ­1600

b a

Disposal method (%) Agriculture 13 31 37 50 25 3 28 34 81 44 80 10 60 51 24 16 Landfill 56 56 33 50 63 97 18 55 18 53 13 15 30 16 41 43 Incineration 31 9 28 0 12 0 0 11 0 3 0 10 20 5 22 21 Other 0 4 2 0 0 0 54 0 1 0 7 30 0 28 13 21

7.8 9.9 5.1 56 62 10 3. 57 0.4 15 10.3 39 6.4 57 123 249

The production of sludge in the Netherlands is expected to go from 282 000 TDS/yr in 1990 to 871 000 TDS/yr in 2000. A range of 800 million to 1.6 billion wet tons in the United States. April 2002

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tertiary sludge produces 76 g/capita/day [10]. Sakai et al. [11] reported that no excess wastewater sludge was produced in a full-scale wastewater treatment plant if the return sludge was ozonated. Soluble solids concentration of 2 to 15 mg/l in the effluent were higher than those in the treatment processes without ozonation. The annual sludge production in 1990 for a number of countries is shown in Table 2 [10]. It should be noted that while the population of each country is given, the population served by wastewater facilities is typically about one half of the total. Table 3 shows data on the quantities of sludge produced from various processes and operations [1]. It is worth mentioning that the quantity of sludge produced varies widely. Corresponding data on the sludge solids concentration to be expected from various processes are given in Table 4 [1]. SLUDGE CHARACTERISTICS The characteristics of sludge to be measured are strongly related to its ultimate fate. For example, if the sludge is to be thickened by gravity, its settling and compaction characteristics are important. On the other hand, if the sludge is to be digested anaerobically, the concentrations of volatile solids and heavy metals are of importance. The first characteristic, solids concentration, is perhaps the most important variable in defining the volume of sludge to be handled, and determining whether the sludge behaves as a liquid or a solid. The specific gravity of inorganic solids is about 2­2.5 and that of the organic fraction is 1.2­1.3. Table 5 shows solids concentrations in various municipal sludges [12].

Table 3. Typical Data for the Physical Characteristics and Quantities of Sludge Produced from Various Wastewater Treatment Operations and Processes [1]. Specific gravity of sludge solids 1.4 1.25 1.45 1.30 1.30 1.20 1.20 Specific gravity of sludge 1.02 1.005 1.025 1.015 1.01 1.005 1.005 Dry solids, lb/10 3 gal Range 0.9 ­ 1.4 0.6 ­ 0.8 0.5 ­ 0.8 0.7­1.0 0.7­1.0 0.1 ­ 0.2 0.1 ­ 0.2 Typical 1.25 0.7 0.6 0.8 a 0.8 a 0.15 0.15

Treatment operation or Process

Primary sedimentation Activated sludge (waste sludge) Trickling Filtration (waste sludge) Extended aeration (waste sludge) Aerated lagoons (waste sludge) Filtration Algae removal Chemical addition to primary sedimentation tanks for phosphorus removal Low lime (350 ­500 mg/l) High lime (800 ­1600 mg/l) Suspended-growth nitrification Suspended-growth denitrification Roughing filter

a b c d

1.9 2.2 -c 1.20 1.28

1.04 1.05 1.005 1.02

2.0 ­ 3.3 5.0 ­ 11.0 0.1 ­ 0.25 -

2.5 b 6.6 b 0.15 -d

Assuming no primary treatment Sludge in addition to that normally removed by primary sedimentation Negligible Included in sludge production from biological secondary treatment processes

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Table 4. Expected Sludge Concentrations from Various Treatment Operations and Processes [1]. Sludge solids concentration, % dry solids Range Primary settling tank Primary sludge Primary sludge to a cyclone Primary sludge and trickling-filter humus Primary sludge with iron addition for phosphorus removal Primary sludge with low lime addition for phosphorus removal Primary sludge with high lime addition for phosphorus removal Scum Secondary settling tank Waste activated sludge With primary settling Without primary settling High purity oxygen activated sludge With primary settling Without primary settling Trickling-filter humus sludge Rotating biological contractor waste sludge Gravity thickener Primary sludge only Primary and waste activated sludge Primary sludge and trickling-filter humus Dissolved-air flotation thickener Waste activated sludge only With chemical addition Without chemical addition Centrifuge thickener Waste activated sludge only Gravity belt thickener Waste activated sludge only with chemical addition Anaerobic digester Primary sludge only Primary and waste activated sludge Primary sludge and trickling-filter humus Aerobic digester Primary sludge only Primary and waste activated sludge Waste activated sludge only Typical

Operation or process application

4.0 ­ 10.0 0.5 ­ 3.0 3.0 ­ 8.0 4.0 ­ 10.0 2.0 ­ 8.0 4.0 ­ 16.0 3.0 ­ 10.0

5.0 1.5 4.0 5.0 4.0 10.0 5.0

0.5 ­ 1.5 0.8 ­ 2.5 1.3 ­ 3.0 1.4 ­ 4.0 1.0 ­ 3.0 1.0 ­ 3.0

0.8 1.3 2.0 2.5 1.5 1.5

5.0 ­ 10.0 2.0 ­ 8.0 4.0 ­ 9.0

8.0 4.0 5.0

4.0 ­ 6.0 3.0 ­ 5.0

5.0 4.0

4.0 ­ 8.0

5.0

3.0 ­ 6.0

5.0

5.0 ­ 10.0 2.5 ­ 7.0 3.0 ­ 8.0

7.0 3.5 4.0

2.5 ­ 7.0 1.5 ­ 4.0 0.8 ­ 2.5

3.5 2.5 1.3

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The rheological characteristics of sludge are very important because they are one of only few truly basic parameters describing the physical nature of sludge. Sludge varies from a Newtonian fluid, where shear is proportional to the velocity gradient, to a plastic fluid, where a threshold shear must be reached before the sludge starts to move. Most wastewater sludges are pseudoplastic. McCartney and Tingley [13] developed a rapid infrared moisture content measurement technique. The method was as accurate as moisture contents determined by other standard methods. The physical characteristics of sludge from different treatment processes are given in Table 6 [10]. The chemical characteristics of sludge are of great importance for several reasons. Characteristics of sludge that affect its suitability for beneficial use include organic content (usually measured as volatile solids), nutrients, pathogens, metals, and toxic organics. Henry et al. [14] developed a new technique for field determination of nitrogen mineralization from sludge, which provides a simple and inexpensive test that yields accurate results. Table 7 shows the mean concentrations of several elements in different sludge types, while typical nutrient values of sludge as compared to commercial fertilizers are reported in Table 8 [1, 10]. Moreover, Table 9 gives a range of typical chemical compositions of different sludges [10].

Table 5. Solids Concentration in Various Sludge Types [12]. Sludge type Primary sludge Waste activated sludge Fixed film waste sludge Primary and waste activated sludge Primary and fixed film sludge Aerobically digested sludge (thickened) Anaerobically digested sludge (thickened) Solids concentration % 5 ­ 8% 0.5 ­ 2.0% 3 ­ 10% 2.5 ­ 4% 3 ­ 5% 1 ­ 2% 6 ­ 12%

Table 6. Physical Characteristics of Sludge [10]. Parameter Dry solids Volatile solids Sludge specific gravity Solids specific gravity Shear strength (kN/m2) Energy content (MJ/kg VS) Particle size (90%) Primary sludge 2 ­ 6% 60 ­ 80% 1.02 1.4 <5 12 ­ 22 < 200 µm Secondary sludge 0.5 ­ 2% 50 ­ 70% 1.05 1.25 <2 12 ­ 20 < 100 µm Dewatered sludge 15 ­ 35% 30 ­ 60% 1.1 1.2 ­ 1.4 < 20 25 ­ 30 < 100 µm

Table 7. Concentrations of Several Elements in Different Sludge Types [10]. Parameter K Na Ca Mg Ba Fe Al Mean Concentration (% of dry solids) Anaerobic 0.52 0.7 5.8 0.58 0.08 1.6 1.7 Aerobic 0.46 1.1 3.3 0.52 0.02 1.1 0.7 All 0.4 0.57 4.9 0.54 0.06 1.3 1.2

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Trace elements in sludge are those inorganic chemical elements that, in very small quantities, can be essential or detrimental to plants and animals. The term `heavy metals' is used to denote some of the trace elements present in sludge. Several investigators described analytical methods for measuring trace metals and organic compounds [15­20].

Table 8. Comparison of Nutrient Levels in Commercial Fertilizers and Wastewater Sludge [1]. Nutrients, % Nitrogen Fertilizers for typical agricultural use Typical values for stabilized sludge 5 3.3 Phosphorus 10 2.3 Potassium 10 0.3

Table 9. Typical Chemical Composition of Sludges [10]. Parameter pH Alkalinity (mg/l CaCO3) Nitrogen (N% of TS) Phosphorus (P2O5% of TS) Fats, grease (% of TS) Protein (% of TS) Organic acids (mg/l as Hac) Primary sludge 5­ 8 500 ­ 1500 1.5 ­ 4 0.8 ­ 2.8 6 ­ 30 20 ­ 30 6800 ­ 10 000 Anaerobically digested sludge 6.5 ­ 7.5 2500 ­ 3500 1.6 ­ 6 1.5 ­ 4 5 ­ 20 15 ­ 20 2700 ­ 6800 0.5 ­ 7.6 1.1 ­ 5.5 Aerobically digested sludge

Table 10. Concentrations of Heavy Metals in Sludge [1]. Dry sludge, mg/kg Metal Range Arsenic Cadmium Chromium Cobalt Copper Iron Lead Manganese Mercury Molybdenum Nickel Selenium Tin Zinc 1.1 ­ 230 1 ­ 3410 10 ­ 99 000 11.3 ­ 2490 84 ­ 17 000 1000 ­ 154 000 13 ­ 26 000 32 ­ 9870 0.6 ­ 56 0.1 ­ 214 2 ­ 5300 1.7 ­ 17.2 2.6 ­ 329 101 ­ 49 000 Median 10 10 500 30 800 17000 500 260 6 4 80 5 14 1700

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With respect to microbiological characteristics of municipal sludges, wastewaters generally contain four major types of pathogens: bacteria, protozoa, viruses, and helminths. The concentrations of these pathogenic organisms in wastewater depend on the health condition of the community. In times of epidemics, these concentrations would be high. Since pathogens come from relatively large volumes of wastewater, when ending up in the sludge, they, in general, become concentrated and very infectious. Gaspard et al. [21] performed a parasitological analysis of helminths on 89 sludge samples, three sediments, and seven composts. The average concentration of helminths was 130 eggs per 100 grams of dry matter. Sludge from all types of treatment (mesophilic anaerobic and aerobic digestion, composting, and liming) contained 10 or more viable eggs per 100 grams of dry matter. Antibiotic resistance of Escherichia coli in sludge and wastewater was affected by location but not the digestion process [22]. The E.coli strains in digested municipal sludge from El Paso, Texas had more antibiotic resistance than those from any other site. Table 11 shows some disease-producing protozoa and helminths in sludges, while Table 12 shows the levels of indicator pathogenic organisms in municipal sludges [10, 23]. Table 13 shows survival times of various pathogens in soil and on plant surfaces [23].

Table 11. Pathogenic Organisms in Wastewater and Sludge [23]. Organism Protozoa Cryptosporidium Entamoeba histolytica Giardia lamblia Balantidium coli Toxoplasma gondii Gastroenteritis Acute enteritis Giardiasis Diarrhea and dysentery Toxoplasmosis Disease/symptoms

Helminths Ascaris lumbricoides Ascaris suum Trichuris trichiura Toxocara canis Taenia saginata Taenia solium Necatur americanus Hymenolepis nana Digestive and nutritional disturbances; abdominal pain, vomiting May produce symptoms such as coughing, chest pain, and fever Abdominal pain, diarrhea, anemia, weight loss Fever, abdominal discomfort, muscle aches, neurological symptoms Nervousness, insomnia, anorexia, abdominal pain, digestive disturbances Nervousness, insomnia, anorexia, abdominal pain, digestive disturbances Hookworm disease Taeniasis

Table 12. Levels of Indicator and Pathogenic Organisms in Different Sludges (no. per gram of dry weight) [10]. Sludge (untreated) Primary Secondary Mixed Total coliform 10 6 ­ 10 8 107 ­ 10 8 10 ­ 10

7 9

Faecal coliform 10 6 ­ 107 107 ­ 10 9 10 ­ 10

5 6

Faecal Streptococci 10 6 10 6 10

6

Salmonella species 4 × 10 2 9 × 10 2 5 × 10

2

Pseudomonas aeruginosa 3 × 10 3 1 × 10 4 10 ­ 10

3 5

Enteric viruses 0.002 ­ 0.004 0.015 ­ 0.026 --

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Table 13. Survival Times Of Various Pathogens In Soil And On Plant Surfaces [23]. SOIL Pathogen Absolute Maximum 1 year 6 months 10 days 7 years Common Maximum 2 months 3 months 2 days 2 years PLANTS Absolute Maximum 6 months 2 months 5 days 5 months Common Maximum 1 month 1 month 2 days 1 month

Bacteria Viruses Protozoan cysts Helminth ova

SLUDGE THICKENING, DEWATERING, CONDITIONING, AND STABILIZATION Thickening is the pre-processing of sludge prior to dewatering. Thickeners are used in wastewater treatment plants where solids must be concentrated to increase the efficiency of further treatment. Thickening is economically attractive because considerable volume reduction is achieved with even modest increases in sludge solids concentrations. Such reduction can provide significant savings in the cost of dewatering, digestion, or other downstream activities. The proper location of the thickener in a wastewater treatment plant is important. If the sludge is to be digested, thickening a blend of raw, primary, and waste activated sludge would be very efficient [24]. On the other hand, if raw primary and secondary sludges are to be dewatered and incinerated, the sludge should be thickened separately and blended immediately before dewatering. The three commonly used methods for sludge thickening are gravity, flotation, and centrifugation methods. Thickening by gravity is the most common concentration process in use at wastewater treatment plants due to its simple and inexpensive operation. Gravity thickening is a sedimentation process similar to that which occurs in all settling tanks. The degree to which waste sludge can be thickened depends on many factors. The most important factor is the type of sludge being thickened and its volatile-solids concentration. Bulky biological sludge, particularly from an activated sludge process, will not concentrate to the same degree as raw primary sludge. The degree of biological treatment and the ratio of primary to secondary sludge will affect the ultimate solids concentration obtained by gravity thickening [24]. The design parameters for gravity thickeners include the floor loading and the supernatant overflow rates. Another design factor is the sludge volume ratio (SVR), which is defined as the volume of the sludge blanket divided by the daily volume of sludge pumped from the thickener. Values for SVR are normally maintained between 0.5 and 2 days, with the lower values being used during warmer weather. Flotation thickening units are useful in sewage treatment plants for handling waste-activated sludge. They have the advantage over gravity thickening tanks of offering higher solids concentrations and lower initial equipment cost due to higher hydraulic and solids loadings which can be applied. Flotation thickeners in wastewater treatment are often used to thicken activated sludge before digestion. According to Butler et al. [25] dissolved air flotation can be used to thicken a mixture of primary and waste activated sludges. The advantages of co-thickening these sludges are a significant reduction in the organic loading to the secondary treatment process and reduction of the amount of grit transferred to the sludge digesters. Centrifugation was shown to be an effective method to thicken a portion of the wastewater sludge before digestion [26]. The thickened portion was then diluted with wastewater sludge to achieve the desired six percent solids content in the digester. Although centrifuges have been used widely for dewatering, they have had limited use for thickening because of their relatively high cost. They have been used for thickening waste activated sludge where space limitations or sludge characteristics make other methods unsuitable. Sludge dewatering is a physical unit process used to remove as much water as possible from sludge to produce a highly concentrated cake. Dewatering differs from thickening, as the sludge should behave as a solid after it has been dewatered. Metcalf and Eddy [1] reported some reasons for performing dewatering.

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A sludge suspension contains bulk water, which is not bound to the sludge particles, and bound water. Kopp and Dichtl [27] reported that in a sewage sludge suspension, four different types of water can be distinguished according to their physical bonding to the sludge particles. These are free water, interstitial water, surface water, and intracellular water. The free water content represents the largest part in sewage sludge, which can be separated mechanically. The interstitial water is kept between the interstices of the sludge particles and microorganisms in the sludge floc, which is bound physically by active capillary forces. The surface water covers the entire surface of sludge particles in several layers of water molecules and is bound by adsorptive and adhesive forces. The surface water is physically bound to the particles and cannot move freely. The intracellular water contains the water in cells as well as water of hydration. Smith and Vesilind [28] reported that intracellular water can only be determined together with the surface water and is often called bound water content. Bound water, which contributes the smallest proportion of water content, has the strongest physical­chemical bonding to the particles and can only be removed thermally. Dewatering devices primarily remove free water; some interstitial water can be removed as well, but it is likely that the major fraction of bound water is vicinal water that cannot be removed mechanically. Robinson and Knocke [29] reported that freeze­thaw conditioning physically disrupts the floc and cell structure and produces the greatest degree of bound water release for dewatering. Lajoie et al. [30] reported that the abundance of zoogleal clusters and centrifuged solids content were negatively correlated, as determined by linear regression. Wu and Huang [31] concluded that recycling-sludge operation can decrease the average bound water content by nearly 13­24 percent and increase the effective floc density, thereby forming a compact structure with a high fractal dimension and low floc porosity. Chin et al. [32] investigated the use of capillary suction time (CST) as a measure of sludge dewaterability and reported that CST was a good index for sludge filterability. Besides the use of chemical coagulants and anaerobic digestion to improve the dewaterability of sludge, Cantet et al. [33] reported that the addition of talqueous powders improved sludge dewaterability. A number of sludge dewatering techniques are currently in use. The selection of a sludge-dewatering system depends on: (1) the characteristics of the sludge to be dewatered; (2) the available space; and (3) the moisture content requirements of the sludge cake for ultimate disposal. When land is available and the sludge quantity is small, natural dewatering systems are most attractive. These include drying beds and drying lagoons. Mechanical dewatering systems are generally selected where land is not available. Common mechanical sludge-dewatering systems include vacuum filter, centrifuge, filter press, and belt filter press. Ockier et al. [34] reported that mechanical dewatering of the sludge directly extracted from municipal wastewater treatment plants is often applied. Dorical et al. [35] reported that sludge dewatering efficiency varied substantially at the mills. On the other hand, Harvey and Boulanger [36] concluded that 65 percent of the reduction of chemical cost could be achieved by focusing on areas such as blend ratio and consistence, equipment modification, and optimization of the chemical addition points. Drying Beds Sludge drying beds are the oldest method of sludge dewatering and are still used extensively in small-to-medium sized plants to dewater sludge. They are relatively inexpensive and provide dry sludge cake. In recent years, many advances have been made to conventional drying bed technology, and new systems are used on medium- and large-sized plants. These variations of the drying beds are: (1) conventional sand; (2) paved; (3) wire-wedge; and (4) vacuum assisted. Conventional Sand Beds Typical sand beds consist of a layer of coarse sand 15­25 cm in depth and supported on a gravel bed (0.3 ­2.5 cm) that incorporates selected tiles or perforated pipe under-drain. Sludge is placed on the bed in 20 ­30 cm layers and allowed to dry. Sludge cake removal is manual by shoveling into wheelbarrows, trucks, scraper, or front-end loader. The underdrained liquid is returned to the plant. The drying period is 10 ­15 days, and the moisture content of the cake is 60 ­70 percent. Sludge loading rate is 100 ­300 kg dry solids per m2 per year for uncovered beds. Several investigators such as Hossam and Saad [37], Marklund [38], Marklund [39], and Nishimura et al. [40] studied the use of conventional sand beds.

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Paved Drying Beds Paved drying beds may be of a drainage type, decanting type, or a combination. The drainage-type paved drying beds are rectangular and are similar to the conventional drying beds with vehicular track for cake removal. The paving is made of concrete, asphalt, or soil cement. The drainage pipe is placed below the unpaved area of the drying bed. In the decanting type, the sludge feed pipe is vertical and is at the center of the bed. The settled sludge is agitated periodically by a tractor-mounted horizontal auger or other device to regularly mix and aerate the sludge to promote evaporation and percolation. Solids concentration may range between 40 to 50 percent for 30 ­ 40 days of drying period in an arid climate for a 30 cm sludge layer [1]. Lienard et al. [41] studied dewatering of activated sludge in experimental reed-planted and unplanted sludge concrete drying beds. They concluded that reeds were found to contribute to maintaining a high and regular liquid conductivity in the sludge, which allows easier and higher dosing of planted beds. Wedge-Wire Drying Beds In wedge-wire drying beds, artificial media made of stainless steel wire wedge and high-density polyurethane formed into panels have been successfully utilized. Drainage and evaporation are the two mechanisms utilized to form a sludge cake. The U.S. EPA reported the following advantages for the system: (1) no clogging of the media; (2) constant and rapid drainage; (3) higher throughput rate than sand beds; (4) easier removal of sludge cake; (5) ability of drying difficult-to-dewater sludge; and (6) ease of maintenance [42]. Vacuum-Assisted Drying Beds Dewatering of sludge can be accelerated by applying vacuum to the drying bed. The operation of a vacuum-assisted sludge-drying bed involves the application of preconditioned sludge to a depth of 30 ­75 cm, while solids levels of 14­23 percent are reached in the sludge cake. The solids loading per application ranges from 5 to 20 kg/m2. Drying Lagoons Drying lagoons may be used as a substitute for drying beds for the dewatering of digested sludge. However, lagoons are not suitable for dewatering of untreated sludges, limed sludges, or sludges with a high-strength supernatant because of their odor and nuisance potential. Metcalf and Eddy [1] reported that climate, precipitation, and low temperatures affect the performance of drying lagoons, like that of drying beds. Lagoons are most applicable in areas with high evaporation rates. Dewatering by subsurface drainage and percolation is limited by increasingly stringent environmental and groundwater regulations. It may be necessary to line the lagoon, if a groundwater aquifer used for a potable water supply underlies the lagoon site. Evaporation is the main dewatering mechanism utilized in drying lagoons. Sludge is removed mechanically, usually at a solids content of 25 to 30 percent. The cycle time for lagoons varies from several months to several years. Solids loading ranges from 36 to 39 kg/m3.yr of lagoon capacity. Schweizer et al. [43] and Tarven et al. [44] conducted studies to improve the performance of sludge lagoons by using Roller Compacted Concrete (RCC) and covers. Vacuum Filter A vacuum filter consists of a rotating cylindrical drum covered with a filtering material or fabric, partially submerged in a vat of conditioned sludge. A vacuum is applied inside the drum to extract water, leaving the solids, or filter cake, on the filter medium. As the drum completes its rotational cycle, a blade scrapes the filter cake from the filter and the cycle begins again. There is a wide variety of filter fabrics, ranging from Dacron to stainless steel coils, each with its own advantages. Vacuum filters produce a sludge cake with solids content ranging between 15 and 30 percent. Gingerich et al. [45] investigated the use of applied direct pressure and constant voltage direct current to dewater anaerobically and aerobically digested municipal sludge. They reported that final cake solids were increased to 50 percent with an applied voltage of 60 V. Increasing pressure, voltage, or time may enhance final cake solids, and additional water may be removed from a conventionally dewatered cake by further application of dc voltage.

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Centrifuge Centrifuges are analogous to sedimentation tanks except that the separation of suspended particles is accelerated by a centrifugal force that is higher than the gravity force. In a typical unit, sludge is pumped into a horizontal, cylindrical bowl, rotating at 800 to 2000 rpm. Polymers used for sludge conditioning are also injected into the centrifuge. The solids are spun to the outside of the bowl where they are scraped out by a screw conveyor. The liquid, or concentrate, is returned to the treatment plant, ahead of the primary sedimentation tank. Davis and Cornwell [7] reported that centrifuges are sensitive to changes in the concentration or composition of sludge, as well as to the amount of polymer applied. The sludge cake from the centrifuge contains 20 to 35 percent solids. Filter Press Filter presses are also called plate and frame presses or recessed pressure filters. Filter presses consist of round or rectangular recessed plates that, when pressed together, form hollow chambers. A filter cloth is mounted on the face of each individual plate. The sludge is pumped under high pressure (350­1575 kN/m2) into the chamber. The water passes through the cloth while the solids are retained and form a cake on the surface of the cloth. The sludge filling continues until the press is effectively full of cake. Filter presses were reported to attain up to 40 percent solids. The filter is then mechanically opened, and the dewatered cake drops from the chamber onto a conveyor belt for removal. Table 14 shows typical dewatering performance of filter presses [46]. Krieger [47] discussed new developments in the field of reject and sludge dewatering. He noted that the Kufferath Company has developed a new screw press to be used specifically for dewatering of sludges. Rehmat et al. [48] described a laboratory scale sludge press. The effects of applied pressure and press time on filtrate flow rate and sludge cake solids were discussed, as well as the effects of various combinations of primary and secondary sludges.

Table 14. Typical Dewatering Performance of Filter Presses [46]. Chemical Dosage Type of Sludge Feed Solids (percent) 4 (percent dry solids) FeCl3 Primary and secondary Anaerobically digested, 4 Primary and secondary Thermally conditioned, Primary and secondary 14 0 0 12 60 6 16 5 40 5 CaO 15 5 40 Filter Yield (kg/m2.h) Cake Solids (percent)

Belt Filter Press The belt filter press operates by bending a sludge cake contained between two filter belts around a roll, which introduces shear and compressive forces in the cake. This allows water to find its way to the surface and out of the cake, thereby reducing the cake moisture content. The press consists of two converging belts mounted on rollers. The lower belt is made of fine wire mesh and is very porous. As conditioned sludge moves onto the belt, some of the entrained water drains through the belt in the gravity drain zone. The sludge passes through the press zone where the pressure provided by the converging belts and rollers causes dewatering. Table 15 shows typical data for various types of sludges dewatered on belt filter press [46]. Bullard et al. [49] investigated the factors that influence belt filter press performance. They reported that characterizing the inherent dewatering potential of an activated sludge would help in refining the treatment processes leading to enhancement of dewatering potential. Such an action can result in reduced water content of dewatered cake solids, increased belt filter press production capacity, and reduced conditioning chemicals usage. It was reported that the use of belt filter presses for dewatering of municipal sludges provides cost-effective solutions for sludge dewatering [50].

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Chemical conditioning is performed using either inorganic chemicals or organic polyelectrolytes. Inorganic chemicals used include ferric chloride, lime, and a combination of ferrous sulfate and lime. For conditioning of raw or digested sludges, prior to dewatering in presses or vacuum filters, a combination of lime and lime salts has traditionally been used. Ferrous sulfate, ferric sulfate, or ferric chloride, may also be used. Lime is used for pH adjustment and to provide some degree of odor control and disinfection. Calcium carbonate and other calcium compounds formed by reaction with constituents of sludge no doubt assist in providing a granular structure for dewatering. Iron salts, which should be added before lime, are thought to form positively-charged complexes which react with negatively-charged sludge solids and also form hydroxides which act as flocculating agents. The introduction of polyelectrolytes in sludge conditioning has undoubtedly been the most significant development in the past years. Essentially, both anionic and cationic polyelectrolytes may be effective in sludge conditioning. Many polyelectrolytes are of high molecular weight and of high charge density. The required dose of polyelectrolyte is critical to the performance of the dewatering equipment and to the cost of operation. Several investigators used organic and inorganic chemicals, and heat as a mean of sludge conditioning [51­58]. Sludge stabilization processes are used to convert raw wastewater sludge to inoffensive forms by decreasing the organic content in the sludge or by otherwise rendering them inert. This is especially essential when sludge is to be disposed of on land. This process renders the sludge or sludge end-products pathogen-free. Much recent legislative emphasis is placed on successfully stabilizing sludge or compost products. The four major stabilization processes are anaerobic digestion, aerobic digestion, chemical (lime, chlorine, and oxygen) stabilization, and heat treatment. Reimers

Table 15. Typical Data for Various Types of Sludge Dewatered on Belt Filter Presses [46]. Type of Sludge Raw P WAS P+WAS P+TF 3 ­ 10 0.5 ­ 4 3­ 6 3­ 6 360 ­ 680 45 ­ 230 180 ­ 590 180 ­ 590 1­ 5 1 ­ 10 1 ­ 10 2­ 8 28 ­ 44 20 ­ 35 20 ­ 35 20 ­ 40 Feed Solids (percent) Solids Loading Rate (kg/m.belt width.h) Polymer Dose (g/kg) Cake Solids (percent)

Anaerobically Digested P WAS P+WAS 3 ­ 10 3­ 4 3­ 9 360 ­ 590 40 ­ 135 180 ­ 680 1­ 5 2 ­ 10 2­ 8 25 ­ 36 12 ­ 22 18 ­ 44

Aerobically Digested P+WAS P+TF 1­ 3 4­ 8 90 ­ 230 135 ­ 230 2­ 8 2­ 8 12 ­ 20 12 ­ 30

Oxygen Activated WAS 90 ­ 180 90 ­ 180 4 ­ 10 15 ­ 23

Thermally Conditioned P+WAS 290 ­ 910 290 ­ 910 0 25 ­ 50

P = primary sludge; WAS = waste activated sludge; TF = trickling filter sludge.

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et al. [59] reviewed current and future advances of disinfection technologies such as heat drying, biological processes, and alkaline treatment. Switzenbaum et al. [60] evaluated different stability criteria for different disinfection processes. The authors concluded that no single method can assess stability for all types of biosolids and recommended tests for all stabilization methods evaluated. Vesilind and Hsu [61] noted that specific criteria based on the use of the biosolids are needed. Leboucher et al. [62] suggested that the measurement of the potential of biosolids to generate hydrogen sulfide be used as a stability criterion. Anaerobic digestion is one of the most commonly employed processes for sludge stabilization. The process, which biologically reduces the amount of volatile suspended solids that must be handled by subsequent dewatering and ultimate disposal operations, renders the organic material nonputrescible, and destroys a large number of pathogenic organisms. A major gaseous end-product of anaerobic digestion is methane, which is often used as a source of fuel in wastewater treatment plants [1]. Digested sludge is an excellent soil conditioner and has found some utility for this purpose. A major advantage associated with anaerobic digestion is its low energy requirement. Power consumption is much less than that required for aerobic stabilization or heat treatment. The methane produced is normally more than sufficient to generate the heat required to maintain optimum digester temperatures and thus provides a surplus energy source that can be used elsewhere. The attractions of anaerobic digestion of sludge are methane production, 30 to 50 percent reduction in sludge volume, odor-free sludge end product, and pathogen-free sludge. Several investigators reviewed, described, and presented results related at anaerobic digestion [63­ 68]. Aerobic digestion is somewhat similar to the activated sludge process. Sludge is fed to a tank where it is mixed aerobically. The main objective of the process is to reduce the solids content for ultimate disposal. The volatile solids are reduced as in anaerobic digestion and, thus, a stabilized highly fertilizable humus is produced. Advantages of this process are low capital cost, easy operation, low odor, non-explosive gas, and more purified supernatant than in anaerobic digestion. The disadvantages are high operation costs for power and oxygen, reduced performance in cold weather, and difficult to dewater sludge [1]. Aerobic digestion has the advantage of a complete food chain and a mixed and varied ecology including some anaerobic and facultative organisms. The sludge produced from the aerobic process during the first days of aeration often shows a marked increase in the sludge volume index, while drainability is severely hampered. After about ten days of aeration, however, the situation improves. Chemical stabilization includes chlorine and lime stabilization. Lime has long been used for sludge stabilization, where high concentrations cause an increase in pH resulting in destruction of most of the biological life. Quicklime has also been used for stabilization of raw and digested sewage sludge. It has been determined that a pH of 11­11.5, at 15 °C and 4-hr detention time destroys all E. coli and Salmonella typhosa. Some spore-forming bacteria may survive, but few pathogens form spores. Higher organisms such as hookworm and amoebic cysts may survive at least 24 hr at high pH. Lime also tends to eliminate odors and improve sludge dewaterability. It was found that the addition of lime to raw primary sludge produced a significant drop in specific resistance to filtration, a measure of how well a sludge dewaters. Lime addition to raw sludge does not render it permanently stable because the pH eventually drops and surviving organisms, or organisms that recontaminate the sludge, can create nuisance conditions. Oxygen can also be used to stabilize sludge in either of two ways: biologically or chemically at elevated pressure and temperature. Biological stabilization with oxygen resembles aerobic digestion except that pure oxygen is used instead of air. Chemically, oxygen can be used to stabilize sludge at a sufficiently high temperature, pressure, and reaction time. Sludges produced thus are sterile and normally are easily dewatered. The heat treatment process involves heating sludge for short periods of time under pressure. It is essentially a conditioning process, which prepares sludge for dewatering without the use of chemicals. In addition, the sludge is sterilized and generally rendered inoffensive. Composting is also utilized to stabilize municipal sludge for reuse purposes. Compost stability/maturity has become a critical issue for land application of compost because immature compost can be detrimental to plant growth and the soil environment. Wu et al. [69] compared several methods of evaluating the stability/maturity of sludge compost. They concluded that pH, electrical conductivity (EC), CO2 evolution rate, seed germination rate, and Dissolved Organic Carbon (DOC) could be used to monitor the stabilization and maturation processes. Vermicomposting is the use of

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earthworms as a waste treatment technique, and is gaining popularity. Ndegwa et al. [70] investigated the effect of stocking density and feeding rate on vermicomposting of sludge. They reported that vermicomposting produces a product that is homogeneous, has desirable aesthetics, contains lower levels of contaminants, tends to hold more nutrients over a longer period, and has no impact on the environment. Certain parameters need to be established for the design of an efficient and economic vermicomposting system. Aggelides and Londra [71] investigated the effect of compost produced from town wastes and sewage sludge on the physical properties of a loamy and clay soil. The organic fertilizer was produced by composting 62 percent town wastes, 21 percent sewage sludge, and 17 percent sawdust by volume. The product was applied to loamy and clay soils in areas characterized by a semi-arid climate. They reported that the chemical properties of the soils were affected directly by the amendment compost. Moreover, the physical properties of the amended soils were improved in terms of the saturated and unsaturated hydraulic conductivity, water retention capacity, bulk density, total porosity, pore size distribution, soil resistance to penetration, aggregation, and aggregate stability. SLUDGE REUSE AND DISPOSAL Sewage sludge production is a continuous process and requires a flexible and secure range of outlets for its disposal to be economically and environmentally acceptable. The majority of wastewater sludge is disposed of on land, with approximately three-quarters being used as a soil conditioner and the remainder buried in landfills. The predominant treatment and disposal options available include municipal landfill, incineration (ash is landfilled), sludge farming or composting, fertilizer, and others such as lagoons and ocean disposal. Four options are known for utilizing sewage sludge on land as a resource, other than in agriculture. These are its use as a forest fertilizer, as a soil conditioner for the restoration of disturbed soils, as a soil forming material for reclaiming derelict land, and for producing soil for use on green areas in the urban environment. With the exception of ocean dumping, all sludge treatment technologies are land based, which present formidable problems, including environmental acceptability, air and water pollution problems, disposal of hazardous wastes on land, and groundwater and soil contamination potentials. Several investigators have reported different means by which sludge can be disposed or utilized [16, 72­79]. Environmental concerns regarding land disposal are surface water and groundwater pollution, contamination of the soil and crops with toxic substances, and transmission of human and animal diseases. Problems of sludge on land may be environmental, agronomic, and operational. Environmental problems are largely associated with emissions to the atmosphere of odor, ammonia, and pathogens. The agronomic and operational aspects are very much interrelated and primary concerns are soil conditions and crop response in relation to accuracy and timing of applications. The effect of wastewater sludge in co-disposal landfills of municipal solid waste and wastewater sludge on the degradation rate and leachate quality was investigated in field and laboratory lysimeter studies by Rohrs et al. [80]. In treatment with sludge addition, chemical oxygen demand (COD) and nickel concentrations in the leachate were reduced, while ammonium and phosphorus concentrations increased. Coastal cities have discharged digested sludge into the ocean for decades. In recent years, this practice has been questioned by regulatory agencies, particularly in the United States. The principal environmental concerns are degradation of recreational waters, buildup of solids on the sea bottom, and toxicity to marine life. The contaminants involved are the same as those related to disposal on land: heavy metals, pathogens, and organic pollutants. The Helsinki agreement called for the banning of ocean dumping of sludges by 1987. By 1995 most developed countries had put this ban into practice [10]. Another principal method of disposal is incineration. Important considerations in evaluating incineration methods include the composition of the sludge feed and the amount of auxiliary fuel required. Air pollution constraints and resultant equipment and treatment requirements, as well as ash disposal are also important [24]. Advantages, disadvantages, and economical and environmental viability of various engineering designs for disposal of solid wastes, such as wastewater sludge and municipal incinerator fly ash, were assessed and compared [81]. The effect of incineration of municipal sludge on air quality has been studied by several researchers [82­ 84].

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Capping of municipal sludge is a cost-effective remediation method for soft contaminated sludge and the U.S. EPA agreed to permit capping as a remediation method. Aydilek et al. [85] investigated the consolidation characteristics of wastewater sludge. They concluded that the finite-strain model predicted the behavior observed in both the field and laboratory more accurately than the consolidation theory. To eliminate the possibility of transmitting contaminants to humans, the preferred vegetation is a non-food-chain crop such as cotton. Grass is considered acceptable, provided cattle are restricted from grazing for a specified period after sludge application. The variety and numbers of bacteria, viruses, and parasitic organisms pathogenic to humans and animals found in wastewater relate to the state of health of the contributing community. Although treatment processes reduce their numbers, often considerably, the effluent and sludge would still contain some of the species. REGULATIONS FOR THE REUSE AND DISPOSAL OF SLUDGE The volume of municipal sludges is expected to increase significantly as a result of a number of recent environmental developments world-wide including: 1. The Helsinki Agreement called for banning ocean dumping of sludges by 1987. By 1995 most developed countries had put this ban into practice, e.g. New York and most of the United States by 1992; Australia and New Zealand by 1993 [10]. 2. EU Environmental Directive on Urban Wastewater, requiring better than secondary treatment and in sensitive areas nutrient removal as well [10]. 3. In the United States, `Part 503', the USEPA Standards for the use and disposal of sewage sludge [86]. 4. In New Zealand, the public health guidelines for the safe use of sewage effluent and sewage sludge on land [87]. Application of sludge to land, where feasible, is the most desirable alternative because it uses sludge in a natural cycle. Ideally, sludges produced from wastewater treatment processes contain all foreign matter introduced into water through domestic use that has not been removed or transformed into neutral substances by the wastewater treatment process. Except for chemical additions, the residues accumulated in wastewater treatment processes largely consist of organics and minerals ultimately derived from soil. Home use of synthetic agents may introduce metals above ambient levels and toxic compounds into sludge. Pathogens or parasites occur in sludge as a result of sewage being a carriage vehicle for human excrement. The U.S. Environmental Protection Agency's Part 503 Sludge Regulations impose significant monitoring, reporting, and recordkeeping requirements on publicly owned treatment works (POTWs). These requirements will help ensure environmental quality with long-term application of sludge or biosolids on land. Sludge is considered clean if the concentration of certain pollutants is not exceeded. Additionally, certain pathogens and vector reduction requirements must also be met for land application. Some of the common pathogens of concern that appear in municipal wastewater and sludge include ascarids (Ascaris lumbricoides and Toxocara), whipworms (Trichuris sp.), tapeworms (Hymenolepis sp. and Taenia sp.), amoeba (Entamoeba coli), and giardia (Giardia lamblia). The requirements for pathogens vary, depending on the classification of the sludge, either Class A or Class B. Class A sludge has either fecal coliform densities under 1000 MPN per gram of dry solids or Salmonella sp. densities under 3 MPN per 4 grams of dry solids. For Class B sludge, the fecal coliform density requirement is relaxed to under 2 × 106 MPN per gram of dry solids. Class A pathogens requirements shall be met when bulk sewage is applied to a lawn or home garden or sold or given away in a bag or other container for application to the land. The Class B pathogen requirements and site restrictions shall be met when bulk sewage sludge is applied to agricultural land, forest, a public contact site, or a reclamation site. Recently, and besides the U.S. EPA standards for use or disposal of sewage sludge, the National Institute for Occupational Safety and Health released a report which recommended that workers who handle Class B sludge should follow standard personal hygiene practices and use appropriate personal protective equipment where needed to prevent potential health problems [88]. The pathogens in sewage sludge pose a disease risk only if there are routes by which the pathogens are brought into contact with humans or animals. A principal route for transport of pathogens is vector transmission. Part 503 of the U.S. EPA regulations contain 12 options, summarized in Table 16, for demonstrating reduced vector attraction of sewage

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Table 16. Summary of Requirements for Vector Attraction Reduction Under Part 503 [23]. Requirement Option 1 What is Required At least 38% reduction in volatile solids during sewage sludge treatment Most Appropriate for Sewage sludge processed by: - Aerobic biological treatment - Anaerobic biological treatment - Chemical oxidation Only for anaerobically digested sewage sludge that cannot meet the requirement of Option 1 Only for aerobically digested sewage sludge with 2% or less solids that cannot meet the requirements of Option 1 (e.g. sewage sludges treated in extended aeration plants) Sewage sludges from aerobic processes (should not be used for composted sludges) Composted sewage sludge (Options 3 and 4 are likely to be easier to meet for sludges from other aerobic processes Alkali-treated sewage sludge (alkalies include lime, fly ash, kiln dust, and wood ash) Sewage sludges treated by an aerobic or anaerobic process (i.e. sewage sludges that do not contain unstabilized solids generated in primary wastewater treatment) Sewage sludges that contain unstabilized solids generated in primary wastewater treatment (e.g. any heat-dried sewage sludge) Sewage sludge applied to the land or placed on a surface disposal site. Domestic septage applied to agricultural land, a forest, or a reclamation site, or placed on a surface disposal site. Sewage sludge applied to the land or placed on a surface disposal site. Domestic septage applied to agricultural land, a forest, or a reclamation site, or placed on a surface disposal site Sewage sludge or domestic septage placed on a surface disposal site Domestic septage applied to agricultural land, a forest, or a reclamation site or placed on a surface disposal site

Option 2

Less than 17% additional volatile solid loss during bench-scale anaerobic batch digestion of the sewage sludge for 40 additional days at 30°C to 37°C Less than 15% additional volatile solids reduction during benchscale aerobic batch digestion for 30 additional days at 20°C

Option 3

Option 4

Specific oxygen uptake rate at 20°C is 1.5 mg oxygen/hr/g total sewage sludge solids Aerobic treatment of the sewage sludge for at least 14 days at over 40°C with an average temperature of over 45°C

Option 5

Option 6

Addition of sufficient alkali to raise the pH to at least 12 at 25°C and maintain a pH12 for 2 hours and a pH11.5 for 22 more hours Percent solids 75% prior to mixing with other materials

Option 7

Option 8

Percent solids 90% prior to mixing with other materials

Option 9

Sewage sludge is injected into soil so that no significant amount of sewage sludge is present on the land surface 1 hour after injection, except Class A sewage sludge which must be injected within 8 hours after the pathogen reduction process

Option 10

Sewage sludge is incorporated into the soil within 6 hours after application to land or placement on a surface disposal site, except Class A sewage sludge which must be applied to or placed on the land surface within 8 hours after the pathogen reduction process

Option 11

Sewage sludge placed on a surface disposal site must be covered with soil or other material at the end of each operating day PH of domestic septage must be raised to 12 at 25°C by alkali addition and maintained at 12 for 30 minutes without adding more alkali

Option 12

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sludge [23]. These requirements are designed to either reduce the attractiveness of sewage sludge to vectors (options 1 through 8 and 12) or prevent the vectors from coming in contact with the sewage sludge (options 9 through 11). Metals, which are toxic and bio-accumulative as well as nutrients, are constituents of interest in sludge. The composition of sludge varies widely depending on industrial activity and treatment applied. Some typical values for heavy metals are given in Table 10 [1]. If the sludge contains high concentrations of toxic organics, it will be a hazardous waste subject to more restrictive disposal measures. With respect to heavy metals in sewage sludge, Section 503.13 of the U.S. EPA standards for the use or disposal of sewage sludge states that: 1. Bulk sewage sludge or sewage sludge sold or given away in a bag or other container shall not be applied to the land if the concentration of any pollutant in the sewage sludge exceeds the ceiling concentration for the pollutant (shown herein in Table 17) [23]. 2. If bulk sewage sludge is applied to agricultural land, forest, a public contact site, or a reclamation site, either: (i) The cumulative loading rate for each pollutant shall not exceed the cumulative pollutant loading rate for the pollutant (Table 17), or (ii) The concentration of each pollutant in the sewage sludge shall not exceed the concentration for the pollutant (Table 17). 3. If bulk sewage sludge is applied to a lawn or a home garden, the concentration of each pollutant in the sewage sludge shall not exceed the concentration for the pollutant (Table 17). 4. If sewage sludge is sold or given away in a bag or other container for application to the land, either (i) The concentration of each pollutant in the sewage sludge shall not exceed the concentration for the pollutant (Table 17), or (ii) The product of the concentration of each pollutant in the sewage sludge and the annual whole sludge application rate for the sewage sludge shall not cause the annual pollutant loading rate for the pollutant in Table 17 to be exceeded. In comparison, Table 18 shows the Canadian standards and guidelines on heavy metals in sewage sludges for land application [12].

Table 17. Concentration Limits on Sludge Quality and Application [23]. Ceiling Concentration (mg/kg) 75 85 3000 4300 840 57 75 420 100 7500 Cumulative Pollutant Loading Rate (kg/ha) 41 39 3000 1500 300 17 420 100 2800 Monthly Average Concentration (mg/kg) 41 39 1200 1500 300 17 420 36 2800 Annual Pollutant Loading Rate (kg/yr/ha) 2.0 1.9 150 75 15 0.85 21 5.0 140

Pollutant Arsenic Cadmium Chromium Copper Lead Mercury Molybdenum Nickel Selenium Zinc

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Table 18. Canadian Regulations on Heavy Metals in Sewage Sludge for Land Application [12]. Pollutant Arsenic Cadmium Chromium Cobalt Copper Lead Mercury Molybdenum Nickel Selenium Zinc Maximum Acceptable Concentration (mg/kg) 75 20 -150 -500 5 20 180 14 1850 Cumulative Acceptable Loading (kg/ha) 15 4 -30 -100 1 4 36 2.8 370

Table 19. Pollutant Concentration- Active Sewage Sludge Unit without a Liner and Leachate Collection [23]. Pollutant Arsenic Chromium Nickel Concentration (mg/kg) 73 600 420

The U.S. EPA has also regulated disposal of sewage sludge. Subpart C of part 503-Standards for the Use or Disposal of Sewage Sludge states that, for active sewage sludge unit without liner and leachate collection system, the concentration of pollutants shall not exceed those concentrations listed in Table 19 [23]. In Europe, the problem of sewage sludge treatment and disposal has regularly taxed the minds of legislators. Whether at EC level or in Member States, legislation has governed these processes for many years. Over the last decade, annual sludge production has risen as more sewage is treated to a higher standard. As an example, sludge production is currently running at over 960 000 tonnes of dry solids (tds) per year in the U.K. About 18% of sludge was disposed of at sea until 1998 when this option was banned by the EC. Recycling sludge to agricultural land has absorbed some of the excess (about 60% is recycled to land now), but a certain proportion is inevitably landfilled and incinerated. There is still a need for alternative disposal routes. A recent development in this field has been the implementation of the Safe Sludge Matrix agreed between the water companies and the British Retail Consortium. This provides agreed procedures and quality standards to the use of sewage sludge in agriculture. The Government's proposed amendments to the long standing Statutory Controls for the Agricultural Use of Sewage Sludge incorporate the main tenets of this agreement. These controls effectively give the Safe Sludge Matrix a statutory basis and introduce the concepts of treated and enhanced treated sludge [89]. The Safe Sludge Matrix sets minimum acceptable levels of treatment and use for any sewage sludge based product used in agriculture. This goes beyond the currently regulated cropping and grazing restrictions. From the end of 2001, water companies will cease the application of raw sludge to farmland. Where grassland is used for grazing, treated sludge must be injected. Land used for growing vegetables and in horticulture will receive only Advanced Treated

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sludge. This is an excellent start, but many commentators in the water industry are concerned that the work put in trying to safeguard recycling of sewage sludge to agricultural land will be undermined if the European Council of Ministers accepts the European Commission's revisions to the sludge regulations next year. The likely implementation date is 2004/5. There is concern that some aspects may not be possible to meet in any case. Under the previous EU Directive it is legal to apply sludge with levels of lead up to 1 200 mg/kg on agricultural land. The new regulations propose to reduce this level initially to 750 mg/kg in the short term, probably by 2004. In the medium and long term, that is 2015 and 2025, it is proposed that these levels will be further reduced to 500 mg/kg and 200 mg/kg, respectively. Permissible levels for zinc, copper, cadmium, mercury, and nickel would also be reduced. If any of these levels are exceeded, then the application of that sludge to land will not be allowed. In addition to metals and pathogens, such as E. Coli and Salmonella, sludge treatment (or source controls) must also reduce the levels of certain organic compounds. The organic compounds listed in the draft document include nonylphenols, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenols (PCBs), halogenated organics, and dioxins. Under the new regulations, only sludge that has undergone advanced treatment, over and above the usual digestion process that is used to treat the majority of sewage sludge, can be applied to certain categories of agricultural land. For instance, advanced treated sludge only should be used on land growing salad or soft fruit crops. Advanced treated sludge can also be used on most land with no or lesser restrictions on cropping methods. Levels of heavy metals in soils, under the new proposals, will also be limited. Water companies must test the soil before applying sewage sludge. If tests show that the limit values for heavy metals have already been reached then sludge must not be applied. HEALTH PROBLEMS ASSOCIATED WITH SLUDGE Wastewater sludge typically contains organics (protein, carbohydrates, fats, oils, greases, chemicals, etc.), pathogens (bacteria, viruses, and parasites), heavy and toxic metals, and toxins (pesticides, and household and industrial chemicals). All of these present a risk to humans and the environment, and because of this, sludge presents not only a sizable disposal problem, but obvious risks and hazards. Untreated sludge contains a number of harmful pollutants such as viruses, bacteria, parasites, and fungi. Many of the viruses that cause disease in man enter the sewers with feces or other discharges and have been identified in sludge. Wastewater treatment, particularly chemical coagulation or biological processes followed by sedimentation, concentrates viruses in sludge. Raw primary and waste activated sludges contain 10 000 to 100 000 PFU (plaque forming units) per 100 ml. Man may be exposed to pathogens in wastewater sludge in a variety of ways and at greatly varying concentrations. There is no firm scientific evidence to document a single confirmed case where human disease is directly linked to exposure to pathogens from wastewater sludge. Viable pathogens, however, have been isolated from intermediate points in the sludge management system, such as from surface runoff from sludge treated fields. Gaspard et al. [90] performed a parasitological analysis of helminths on 89 biosolids samples. The average concentration of helminths was 130 eggs per 100 g of dry matter. Studies of fecal coliform and Salmonella sp. regrowth in sandy soils amended with biosolids demonstrated that, in most cases, concentrations of these organisms decreased to below detection after a long hot, dry summer in the field [91]. CONCLUSIONS It is clear that sludge is an unavoidable part of wastewater treatment. Measures have to be implemented in order to deal with sludge complications. Nowadays, the area of sludge disposal is a growing concern, due to the substantial increase in sludge production. This increase is due to the increasing awareness concerning environmental pollution and the consequent increasingly stringent effluent discharge standards. Hence, the use of secondary and, in some locations, tertiary treatment of wastewater was found to be a necessary practice in order to comply with those standards. As the quantity of sludge increases, so does the number of regulations attempting to control this material, as well as other potentially toxic or hazardous wastes. This paper has reviewed the state of the art development in sludge related research

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and has covered sludge production, characteristics, treatment, utilization, related health problems, and regulations governing reuse and disposal of sludge. Sludge could pose health hazards if not handled properly, due to its harmful characteristics. On the other hand, sludge can be usefully utilized as a fertilizer and soil conditioner, if properly treated. ACKNOWLEDGEMENT The authors would like to express their thanks to King Abdulaziz City for Science and Technology (KACST) for providing financial support to this work, which is part of project number AR-18-28. The Research Institute of King Fahd University of Petroleum & Minerals (KFUPM) is also to be thanked for its support. REFERENCES

[1] [2] [3] Metcalf and Eddy. Wastewater Engineering: Treatment, Disposal, and Reuse, 3rd edn. New York: McGraw-Hill, 1991. E.W. Steel, Water Supply and Sewerage, 4th edn. Tokyo, Japan: McGraw-Hill, 1960. N.W. Schmidtke, "Sludge Generation, Handling and Disposal at Phosphorus Control Facilities in Ontario, in Characterization, Treatment and Use of Sewage Sludge", in Proceedings of Second European Symposium, Vienna: Commission of the European Communities, 1981, pp 190­225. C.M. Koch, J.S. Lee, J.R. Bratby, and D.B. Barber, "A Critical Evaluation of Procedure for Estimating Biosolids Production", in Proceedings of the Conference on Water Residuals Biosolids Management: Approaching the Year 2000, Philadelphia: Water Environment Federation, 1997. L.A. Lishman, R.L. Legge, and G.J. Farquhar, "Temperature Effects on Wastewater Treatment under Aerobic and Anoxic Conditions", Water Research, 34(8) (2000), pp. 2263­2276. N. Jardin and J. Popel, "Waste Activated Sludge Production of the Enhanced Biological Phosphorus Removal Process", Water Environ. Res., 69 (1997), p. 375. M.L. Davis and D.A. Cornwell, Environmental Engineering, 3rd edn. Boston, Mass: McGraw-Hill, 1998. P.A. Vesilind, G.C. Hartman, and E.T. Skene, Sludge Management and Disposal for the Practicing Engineer. Chelsea, MI, U.S.A.: Lewis Publishers, 1986. P.S. Machno, "Biosolids Quantity and Quality: A Comparison of Primary and Secondary Treatment", in Proceedings of Sludge 2000, Cambridge, 1992

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Paper Received 18 March 2001; Revised 21 November 2001; Accepted 12 February 2002.

April 2002

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