437 9 Sludge Management and Treatment Approximately 6.9 million ton of biosolids were generated in the United States in 1998, and about 60% of it was used beneficially in land applications, com- posting, and landfill cover. It is estimated that, by 2010, 8.2 million tons will be generated, and 70% of the biosolids is expected to be used beneficially (USEPA, 1999). Recycling options are described in various documents (Crites and Tchobanoglous, 1998; Crites et al., 2000; USEPA, 1994a, 1995a,c). Sludges are a common by-product from all waste treatment systems, including some of the natural processes described in previous chapters. Sludges are also produced by water treatment operations and by many industrial and commercial activities. The economics and safety of disposal or reuse options are strongly influenced by the water content of the sludge and the degree of stabilization with respect to patho- gens, organic content, metals content, and other contaminants. This chapter describes several natural methods for sludge treatment and reuse. In-plant sludge processing methods, such as thickening, digestion, and mechanical methods for conditioning and dewatering, are not included in this text; instead, Grady et al. (1999), ICE (2002), Metcalf & Eddy (2003), Reynolds and Richards (1996), and USEPA (1979, 1982) are recommended for that purpose. 9.1 SLUDGE QUANTITY AND CHARACTERISTICS The first step in the design of a treatment or disposal process is to determine the amount of sludge that must be managed and its characteristics. Deriving a solids mass balance for the treatment system under consideration can produce a reliable estimate. The solids input and output for every component in the system must be calculated. Typical values for solids concentrations from in-plant operations and processes are reported in Table 9.1. Detailed procedures for conducting mass balance calculations for wastewater treatment systems can be found in Grady et al. (1999), Metcalf & Eddy (2003), Reynolds and Richards (1996), and USEPA (1979). The characteristics of wastewater treatment sludges are strongly depen- dent on the composition of the untreated wastewater and on the unit operations in the treatment process. The values reported in Table 9.2 and Table 9.3 represent typical conditions only and are not a suitable basis for a specific project design. The sludge characteristics must be either measured or carefully estimated from similar experience elsewhere to provide the data for final designs. DK804X_C009.fm Page 437 Thursday, July 21, 2005 8:10 AM © 2006 by Taylor & Francis Group, LLC 438 Natural Wastewater Treatment Systems TABLE 9.1 Typical Solids Content from Treatment Operations Treatment Operation Percent (%) a Typical Dry Solids (kg/10 3 m 3 ) b Primary Settling Primary only 5 150 Primary and waste-activated sludge 1.5 45 Primary and trickling-filter sludge 5 150 Secondary Reactors Activated sludge: Pure oxygen 2.5 130 Extended aeration 1.5 100 Trickling filters 1.5 70 Chemical Plus Primary Sludge High lime (>800 mg/L) 10 800 Low lime (<500 mg/L) 4 300 Iron salts 7.5 600 Thickeners Gravity type: Primary sludge 8 140 Primary and waste-activated sludge 4 70 Primary and trickling filter 5 90 Flotation 4 70 Digestion Anaerobic: Primary sludge 7 210 Primary and waste-activated sludge 3.5 105 Aerobic: Primary and waste-activated sludge 2.5 80 a Percent solids in liquid sludge. b kg/10 3 m 3 = dry solids/1000 m 3 liquid sludge. Source: Metcalf & Eddy, Wastewater Engineering: Treatment, Disposal, and Reuse , 3rd ed., McGraw-Hill, New York, 1991. With permission. DK804X_C009.fm Page 438 Thursday, July 21, 2005 8:10 AM © 2006 by Taylor & Francis Group, LLC Sludge Management and Treatment 439 TABLE 9.2 Typical Composition of Wastewater Sludges Component Untreated Primary Digested Total solids (TS; %) 5 10 Volatile solids (% of TS) 65 40 pH 6 7 Alkalinity (mg/L as CaCO 3 ) 600 3000 Cellulose (% of TS) 10 10 Grease and fats (ether soluble; % of TS) 6–30 5–20 Protein (% of TS) 25 18 Silica (SiO 2 ; % of TS) 15 10 Source: Metcalf & Eddy, Wastewater Engineering: Treatment, Disposal, and Reuse , 3rd ed., McGraw-Hill, New York, 1991. With permission. TABLE 9.3 Nutrients and Metals in Typical Wastewater Sludges Component Median Mean Total nitrogen (%) 3.3 3.9 NH 4 + (as N; %) 0.09 0.65 NO 3 – (as N; %) 0.01 0.05 Phosphorus (%) 2.3 2.5 Potassium (%) 0.3 0.4 Mean Standard Deviation Copper (mg/kg) 741 962 Zinc (mg/kg) 1200 1554 Nickel (mg/kg) 43 95 Lead (mg/kg) 134 198 Cadmium (mg/kg) 7 12 PCB-1248 (mg/kg) 0.08 1586 Source: Data from USEPA (1983, 1990) and Whiting (1975). DK804X_C009.fm Page 439 Thursday, July 21, 2005 8:10 AM © 2006 by Taylor & Francis Group, LLC 440 Natural Wastewater Treatment Systems 9.1.1 S LUDGES FROM N ATURAL T REATMENT S YSTEMS A significant advantage for the natural wastewater treatment systems described in previous chapters is the minimal sludge production in comparison to mechan- ical treatment processes. Any major quantities of sludge are typically the result of preliminary treatments and not the natural process itself. The pond systems described in Chapter 4 are an exception in that, depending on the climate, sludge will accumulate at a gradual but significant rate, and its ultimate removal and disposal must be given consideration during design. In colder climates, studies have established that sludge accumulation proceeds at a faster rate, so removal may be required more than once over the design life of the pond. The results of investigations in Alaska and Utah (Schneiter et al., 1984) on sludge accumulation and composition in both facultative and partial-mix aerated lagoons are reported in Table 9.4 and Table 9.5. A comparison of the values in Table 9.4 and Table 9.5 with those in Table 9.2 and Table 9.3 indicates that the pond sludges are similar to untreated primary sludges. The major difference is that the solids content, both total and volatile, is higher for most pond sludges than for primary sludge, and the fecal coliforms are significantly lower. This is reasonable in light of the very long detention time in ponds as compared with primary clarifiers. The long detention time allows for significant die-off of fecal coliforms and for some consolidation of the sludge solids. All four of the lagoons described in Table 9.4 and Table 9.5 are assumed to be located in cold climates. Pond systems in the southern half of the United States might expect lower accumulation rates than those indicated in Table 9.4. TABLE 9.4 Pond Sludge Accumulation Data Summary Facultative Ponds (Utah) Aerated Ponds (Alaska) Parameter A B C D Flow (m 3 /d) 37,850 694 681 284 Surface (m 2 ) 384,188 14,940 13,117 2520 Bottom (m 2 ) 345,000 11,200 8100 1500 Operated since last cleaning (yr) 13 9 5 8 Mean sludge depth (cm) 8.9 7.6 33.5 27.7 Total solids (g/L) 58.6 76.6 85.8 9.8 Volatile solids (g/L) 40.5 61.5 59.5 4.8 Wastewater, suspended solids (mg/L) 62 69 185 170 Source: Schneiter, R.W. et al., Accumulation, Characterization and Stabilization of Sludges from Cold Regions Lagoons , CRREL Special Report 84-8, U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, NH, 1984. With permission. DK804X_C009.fm Page 440 Thursday, July 21, 2005 8:10 AM © 2006 by Taylor & Francis Group, LLC Sludge Management and Treatment 441 9.1.2 S LUDGES FROM D RINKING -W ATER T REATMENT Sludges occur in water treatment systems as a result of turbidity removal, soft- ening, and filter backwash. The dry weight of sludge produced per day from softening and turbidity removal operations can be calculated using Equation 9.1 (Lang et al., 1985): S = 84.4 Q (2Ca + 2.6Mg + 0.44Al + 1.9Fe + SS + A x ) (9.1) where S = Sludge solids (kg/d). Q = Design water treatment flow (m 3 /s). Ca = Calcium hardness removed (as CaCO 3 ; mg/L). Mg = Magnesium hardness removed (as CaCO 3 ; mg/L). Al = Alum dose (as 17.1% Al 2 O 3 ; mg/L). Fe = Iron salts dose (as Fe; mg/L). SS =Raw-water suspended solids (mg/L). A x = Additional chemicals (e.g., polymers, clay, activated carbon) (mg/L). The major components of most of these sludges are due to the suspended solids (SS) from the raw water and the coagulant and coagulant aids used in treatment. TABLE 9.5 Composition of Pond Sludges Facultative Ponds (Utah) Aerated Ponds (Alaska) Parameter A B C D Total solids (%) 5.9 7.7 8.6 0.89 Total solids (mg/L) 586,000 766,600 85,800 9800 Volatile solids (%) 69.1 80.3 69.3 48.9 Total organic carbon (mg/L) 5513 6009 13,315 2651 pH 6.7 6.9 6.4 6.8 Fecal coliforms ([number/100 mL] × 10 5 ) 0.7 1 0.4 2.5 Total Kjeldahl nitrogen (mg(L) 1028 1037 1674 336 Total Kjeldahl nitrogen (% of TS) 1.75 1.35 1.95 3.43 Ammonia nitrogen (as N; mg/L) 72.6 68.6 93.2 44.1 Ammonia nitrogen (as N; % of TS) 0.12 0.09 0.11 0.45 Source: Schneiter, R.W. et al., Accumulation, Characterization and Stabilization of Sludges from Cold Regions Lagoons , CRREL Special Report 84-8, U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, NH, 1984. With permission. DK804X_C009.fm Page 441 Thursday, July 21, 2005 8:10 AM © 2006 by Taylor & Francis Group, LLC 442 Natural Wastewater Treatment Systems Sludges resulting from coagulation treatment are the most common and are typically found at all municipal water treatment works. Typical characteristics of these sludges are reported in Table 9.6. 9.2 STABILIZATION AND DEWATERING Stabilization of wastewater sludges and dewatering of most all types of sludge are necessary for economic, environmental, and health reasons. Transport of sludge from the treatment plant to the point of disposal or reuse is a major factor in the costs of sludge management. Table 9.7 presents the desirable sludge solids content for the major disposal and reuse options. Sludge stabilization controls offensive odors, lessens the possibility for further decomposition, and signifi- cantly reduces pathogens. Typical pathogen contents in unstabilized and anaero- bically digested sludges are compared in Table 3.10. Research on the use of various fungal strains as a means to stabilize sludges has been conducted with mixed results but may hold promise in some cases (Alam et al., 2004). 9.2.1 M ETHODS FOR P ATHOGEN R EDUCTION The pathogen content of sludge is especially critical when the sludge is to be used in agricultural operations or when public exposure is a concern. Four pro- cesses to significantly reduce pathogens and seven processes to further reduce pathogens are recognized by the U.S. Environmental Protection Agency (EPA), as described by Bastian (1993), Crites et al. (2000), and USEPA (2003a). TABLE 9.6 Characteristics of Water Treatment Sludges Characteristic Range of Values Volume (as percent of water treated) <1.0 Suspended solids concentration 0.1–1000 mg/L Solids content 0.1–3.5% Solids content after long-term settling 10–35% Composition, alum sludge: Hydrated aluminum oxide 15–40% Other inorganic materials 70–35% Organic materials 15–25% Source: Lang, L.E. et al., Procedures for Evaluating and Improving Water Treatment Plant Processes at Fixed Army Facilities , Report of the U.S. Army Construction Engineering Research Laboratory, Champaign, IL, 1985. DK804X_C009.fm Page 442 Thursday, July 21, 2005 8:10 AM © 2006 by Taylor & Francis Group, LLC Sludge Management and Treatment 443 9.3 SLUDGE FREEZING Freezing and then thawing a sludge will convert an undrainable jelly-like mass into a granular material that will drain immediately upon thawing. This natural process may offer a cost-effective method for dewatering. 9.3.1 E FFECTS OF F REEZING Freeze–thawing will have the same effect on any type of sludge but is particularly beneficial with chemical and biochemical sludges containing alum which are extremely slow to drain naturally. Energy costs for artificial freeze–thawing are prohibitive, so the concept must depend on natural freezing to be cost effective. 9.3.2 P ROCESS R EQUIREMENTS The design of a freeze dewatering system must be based on worst-case conditions to ensure successful performance at all times. If sludge freezing is to be a reliable expectation every year, the design must be based on the warmest winter during the period of concern (typically 20 years or longer). The second critical factor is the thickness of the sludge layer that will freeze within a reasonable period if freeze–thaw cycles are a normal occurrence during the winter. A common mistake with past attempts at sludge freezing has been to apply sludge in a single deep layer. In many locations, a large single layer may never freeze completely to the bottom, so only the upper portion goes through alternating freezing and thawing cycles. It is absolutely essential that the entire mass of sludge be frozen completely for the benefits to be realized; also, when the sludge has frozen and thawed, the change is irreversible. TABLE 9.7 Solids Content for Sludge Disposal or Reuse Disposal/Reuse Method Reason To Dewater Required Solids (%) Land application Reduce transport and other handling costs >3 Landfill Regulatory requirements >10 a Incineration Process requirements to reduce fuel required to evaporate water >26 a Greater than 20% in some states. Source: USEPA, Process Design Manual: Land Application of Municipal Sludge , EPA 625/1-83-016, Center for Environmental Research Information, U.S. Envi- ronmental Protection Agency, Cincinnati, OH, 1983. DK804X_C009.fm Page 443 Thursday, July 21, 2005 8:10 AM © 2006 by Taylor & Francis Group, LLC 444 Natural Wastewater Treatment Systems 9.3.2.1 General Equation The freezing or thawing of a sludge layer can be described by Equation 9.2: Y = m ( ∆ T × t ) 1/2 (9.2) where Y = Depth of freezing or thawing (cm; in.). m = Proportionality coefficient (cm (°C·d) –1/2 ) = 2.04 cm (°C·d) –1/2 = 0.60 in. (°F·d) –1/2 . ∆ T =Temperature difference between 0°C (32° F) and the average ambi- ent air temperature during the period of interest (°C; °F). t =Time period of concern (d). ∆ T × t = Freezing or thawing index (°C·d; °F·d). Equation 9.2 has been in general use for many years to predict the depth of ice formation on ponds and streams. The proportionality coefficient m is related to the thermal conductivity, density, and latent heat of fusion for the material being frozen or thawed. A median value of 2.04 was experimentally determined for wastewater sludges in the range of 0 to 7% solids (Reed et al., 1984). The same value is applicable to water treatment and industrial sludges in the same concen- tration range. The freezing or thawing index in Equation 9.2 is an environmental charac- teristic for a particular location. It can be calculated from weather records and can also be found directly in other sources (Whiting, 1975). The factor ∆T in Equation 9.2 is the difference between the average air temperature during the period of concern and 32°F (0°C). Example 9.1 illustrates the basic calculation procedure. Example 9.1. Determination of Freezing Index The average daily air temperatures for a 5-d period are listed below. Calculate the freezing index for that period. Solution 1. The average air temperature during the period is –4°C. 2. The freezing index for the period is ∆T d = [0 – (–4)](5) = 20°C·d. Day Mean Temperature (°C) 10 2–6 3–9 4+3 5–8 DK804X_C009.fm Page 444 Thursday, July 21, 2005 8:10 AM © 2006 by Taylor & Francis Group, LLC Sludge Management and Treatment 445 The rate of freezing decreases with time under steady-state temperatures, because the frozen material acts as an insulating barrier between the cold ambient air and the remaining unfrozen sludge. As a result, it is possible to freeze a greater total depth of sludge in a given time if the sludge is applied in thin layers. 9.3.2.2 Design Sludge Depth In very cold climates with prolonged winters, the thickness of the sludge layer is not critical; however, in more temperate regions, particularly those that expe- rience alternating freeze–thaw periods, the layer thickness can be very important. Calculations by Equation 9.2 tend to converge on a 3-in. (8-cm) layer as a practical value for almost all locations where freezing conditions occur. At 23°F (–5°C), a 3-in. (8-cm) layer should freeze in about 3 days; at 30°F (–1°C) it would take about 2 weeks. A greater depth should be feasible in colder climates. Duluth, Minnesota, for example, successfully freezes sludges from a water treatment plant in 9-in. (23-cm) layers (Schleppenbach, 1983). It is suggested that a 3-in. (8-cm) depth may be used for feasibility assessment and preliminary designs. A larger increment may then be justified by a detailed evaluation during final design. 9.3.3 DESIGN PROCEDURES The process design for sludge freezing must be based on the warmest winter of record to ensure reliable performance at all times. The most accurate approach is to examine the weather records for a particular location and determine how many 3-in. (8-cm) layers could be frozen each winter. The winter with the lowest total depth is then the design year. This approach might assume, for example, that the first layer is applied to the bed on November 1 each year. Equation 9.2 is rearranged and used with the weather data to determine the number of days required to freeze the layer: (9.3) With an 8-cm layer and m = 2.04, the equation becomes: In U.S. customary units (3-in. layer, m = 0.6 in. [°F·d] –1/2 ): t Ym T = () / 2 ∆ t T = 15 38. ∆ t T = 25 0. ∆ DK804X_C009.fm Page 445 Thursday, July 21, 2005 8:10 AM © 2006 by Taylor & Francis Group, LLC 446 Natural Wastewater Treatment Systems 9.3.3.1 Calculation Methods The mean daily air temperatures are used to calculate the ∆T value. The calcu- lations take account of thaw periods, and a new sludge application is not made until the previous layer has frozen completely. One day is then allowed for a new sludge application and cooling, and calculations with Equation 9.3 are repeated to again determine the freezing time. The procedure is repeated through the end of the winter season. A tabular summary is recommended for the data and calculation results. This procedure can be easily programmed for rapid calcula- tions with a spreadsheet or desktop calculator. 9.3.3.2 Effect of Thawing Thawing of previously frozen layers during a warm period is not a major concern, as these solids will retain their transformed characteristics. Mixing of a new deposit of sludge with thawed solids from a previously frozen layer will extend the time required to refreeze the combined layer (solve Equation 9.3 for the combined thickness). If an extended thaw period occurs, removal of the thawed sludge cake is recommended. 9.3.3.3 Preliminary Designs A rapid method, useful for feasibility assessment and preliminary design, relates the potential depth of frozen sludge to the maximum depth of frost penetration into the soil at a particular location. The depth of frost penetration is also depen- dent on the freezing index for a particular location; published values can be found in the literature (e.g., Penner, 1962; Whiting, 1975). Equation 9.4 correlates the total depth of sludge that could be frozen if applied in 3-in. (8-cm) increments with the maximum depth of frost penetration: (9.4a) (9.4b) where ΣY is the total depth of sludge that can be frozen in 3-in. (8-cm) layers during the warmest design year, in inches or centimeters, and F p is the maximum depth of frost penetration, in inches or centimeters. The maximum depths of frost penetration for selected locations in the northern United States and Canada are reported in Table 9.8. 9.3.3.4 Design Limits It can be demonstrated using Equation 9.4 that sludge freezing will not be feasible unless the maximum depth of frost penetration is at least 22 in. (57 cm) for a particular location. In general, that will begin to occur above the 38th parallel of ∑= () −YF p 176 101.(metric units) ∑= () −YF p 176.40(U.S. units) DK804X_C009.fm Page 446 Thursday, July 21, 2005 8:10 AM © 2006 by Taylor & Francis Group, LLC [...]... 4-Chloroaniline Aquatic invertebrates 1.3 33 3-4 1-5 Diazinon Sediment biota 1.1 20 6-4 4-0 Fluoranthene Aquatic community Sediment biota 356.2 2 .9 235.7 10.7 4.2 743 9- 9 6-5 Manganese Aquatic community 7 8 -9 3-3 Methyl ethyl ketone Sediment biota 5.8 10 8 -9 5-2 Phenol Sediment biota 102.4 12 9- 0 0-0 Pyrene Aquatic community 41 .9 Sediment biota 21.1 Soil biota 744 0-2 2-4 Silver 13 .9 4.5 Aquatic community 246.6 Aquatic... and Treatment 4 69 TABLE 9. 13 Hazard Quotient (HQ) Values ≥1 at the 95 th Percentile of the HQ Distribution for Aquatic and Terrestrial Wildlife Via Direct-Contact Pathwaysa CASRN Receptorb Chemical Sediment biota HQ 6 7-6 4-1 Acetone 12 0-1 2-7 Anthracene Sediment biota 744 0-3 9- 3 Barium Aquatic community 744 0-4 1-7 Beryllium Aquatic community 7.8 7 5-1 5-0 Carbon disulfide Sediment biota 1 .9 10 6-4 7-8 4-Chloroaniline... groundwater Adult 2.7 Child 6.4 743 9- 9 6-5 Manganese Drinking water from groundwater Adult 32.3 Child 76.3 14 79 7-6 5-0 Nitrite Drinking water from groundwater Adult 13.6 Child 33.8 14 79 7-5 5-8 Nitrate Drinking water from groundwater Adult 9. 2 Child 23.0 Source: USEPA, Technical Background Document for the Sewage Sludge Exposure and Hazard Screening Assessment, Document No 822-B-0 3-0 01, Office of Water, U.S Environmental... Francis Group, LLC DK804X_C0 09. fm Page 470 Thursday, July 21, 2005 8:10 AM 470 Natural Wastewater Treatment Systems TABLE 9. 14 Human Hazard Quotient (HQ) Values >1 at the 95 th Percentile of the HQ Distribution by Pathway for the Sewage Sludge Lagoon Scenario CASRN Chemical Pathway Receptor HQ 744 0-3 9- 3 Barium Drinking water from groundwater Adult 1.5 Child 3.5 10 6-4 7-8 4-Chloroaniline Drinking water... Equation 9. 4 are plotted in Figure 9. 1, which indicate the potential depth of sludge that could be frozen at all locations in the United States This figure or Equation 9. 3 can be FIGURE 9. 1 Potential depth of sludge that could be frozen when applied in 8-cm layers © 2006 by Taylor & Francis Group, LLC DK804X_C0 09. fm Page 448 Thursday, July 21, 2005 8:10 AM 448 Natural Wastewater Treatment Systems TABLE 9. 9... Requirements are met FIGURE 9. 4 Flowchart to determine the applicability of surface disposal of sludge (From Sigmund, T.W and Sieger, R.B., Water Eng Manage., 140 (9) , 18– 19, 199 3 With permission.) © 2006 by Taylor & Francis Group, LLC DK804X_C0 09. fm Page 468 Thursday, July 21, 2005 8:10 AM 468 Natural Wastewater Treatment Systems TABLE 9. 12 Human Hazard Quotient (HQ) Values >1 at the 95 th Percentile of the... operation Example 9. 5 Determine the area required for a conventional extended-pile composting operation for the wastewater treatment system described in Example 9. 3 (1500 m3 © 2006 by Taylor & Francis Group, LLC DK804X_C0 09. fm Page 464 Thursday, July 21, 2005 8:10 AM 464 Natural Wastewater Treatment Systems sludge production per year at 7% solids) Assume that a site is available next to the treatment plant... 10.2 29. 8 4150 28.3 45.7 6400 Digested primary sludges applied to the bed from 199 0 to 199 2 Accumulated dewatered sludge on the bed March 12, 199 2 Source: Costic & Associates, Engineers Report: Washington Township Utilities Authority Sludge Treatment Facility, Costic & Associates, Long Valley, NJ, 198 3 With permission Another issue of concern in some states is the use of Phragmites on these systems. .. DK804X_C0 09. fm Page 456 Thursday, July 21, 2005 8:10 AM 456 Natural Wastewater Treatment Systems 9. 5 VERMISTABILIZATION Vermistabilization (i.e., sludge stabilization and dewatering using earthworms) has been investigated in numerous locations and has been successfully tested full scale on a pilot basis (Donovan, 198 1; Eastman et al., 2001) A potential cost advantage for the concept in wastewater treatment systems. .. months Example 9. 2 A community near Pittsburgh, Pennsylvania, is considering freezing as the dewatering method for their estimated annual wastewater sludge production of 0.4 million gallons (1500 m3, 7% solids) Maximum frost penetration (from Table 9. 8) is 38 in (97 cm) © 2006 by Taylor & Francis Group, LLC DK804X_C0 09. fm Page 450 Thursday, July 21, 2005 8:10 AM 450 Natural Wastewater Treatment Systems Solution . and Tchobanoglous, 199 8; Crites et al., 2000; USEPA, 199 4a, 199 5a,c). Sludges are a common by-product from all waste treatment systems, including some of the natural processes described in previous chapters text; instead, Grady et al. ( 199 9), ICE (2002), Metcalf & Eddy (2003), Reynolds and Richards ( 199 6), and USEPA ( 197 9, 198 2) are recommended for that purpose. 9. 1 SLUDGE QUANTITY AND CHARACTERISTICS . Richards ( 199 6), and USEPA ( 197 9). The characteristics of wastewater treatment sludges are strongly depen- dent on the composition of the untreated wastewater and on the unit operations in the treatment