16593 16 tủ tài liệu bách khoa

CHAPTER 16 WATER TREATMENT PLANT RESIDUALS MANAGEMENT David A Cornwell, Ph.D., P.E President Environmental Engineering & Technology, Inc Water treatment plants typically produce some type of waste stream The quality and characteristics of these waste streams are related to the main treatment process Furthermore, waste streams can impact the finished water quality of the treatment process itself This is especially true when the waste is stored internal to the process or recycled Despite the strong linkage between the treatment process and its waste streams, however, water treatment plant waste management has historically been treated as a stand-alone management issue Whatever the treatment process produced was dealt with in a technically appropriate manner With increasing costs associated with managing waste streams, it has become prudent to consider the waste stream quality and characteristics as part of the overall evaluation and design of the main water treatment process.Water treatment processes produce unique waste streams, each of which has different associated waste handling costs The waste streams must be viewed as part of the overall process to be optimized when determining the most economical method for meeting a specific set of finished water quality goals As an example, it is now recognized that storing solids in a sedimentation basin is not desirable from a water quality perspective Including solids storage considerations in the overall design of the water treatment process will then influence the decision regarding the type of sedimentation basin to install Similarly, some filter media combinations produce more spent filter backwash waste than others In determining the main treatment process components, waste streams should be considered in the overall decision tree, not viewed as an issue that is handled separately It is interesting to note the goals of waste treatment as described in the last edition of this book (1990)—“What must be removed? Where will it be disposed? What treatment is necessary to prepare it for disposal?” Recently, however, a new set of issues other than just disposal of the waste has become important Instead of disposal, the first approach to end use is now beneficial utilization and solids treatment Systems are often geared to preparing a material that can be used Minimization of the liquid volumes of waste produced is also increasingly important Water quality issues associated with storing residuals in the process train or associated with recy16.1 16.2 CHAPTER SIXTEEN cling the water back to the treatment plant have become important in planning a waste management system The original term used to describe all water treatment plant wastes was sludge In fact, sludge is really only the solid or liquid-solid component of some types of waste streams The term residuals is now used to describe all water treatment plant process wastes, either liquid, solid, or gaseous Hydrolyzing metal salts or synthetic organic polymers are added in the water treatment process to coagulate suspended and dissolved contaminants and yield relatively clean water suitable for filtration Most of these coagulants and the impurities they remove settle to the bottom of the settling basin where they become part of the waste stream These residuals are referred to as alum, iron, or polymeric sludge (even though they may be made up largely of water), being named after the primary coagulant used These residuals account for approximately 70 percent of the water plant solids generated Similar solids, called lime sludge, are produced in treatment plants where water softening is practiced, and these lime or lime/soda ash plants account for an additional 25 percent of the industry’s solids production It is therefore apparent that most of the waste generation where solids are produced involves water treatment plants using coagulation or softening processes The above wastes are solid/liquid wastes in that the liquid waste (water) contains suspended solids (and, as indicated above, are referred to as sludges) Other solid/liquid wastes produced in the water industry include wastes from iron or manganese removal plants, spent GAC, spent precoat filter media, wastes from slow sand filter plants, and spent filter backwash water (SFBW) The water industry also produces liquid-phase residuals, referred to as such because the liquid phase (water) contains primarily dissolved solids which are within the liquid phase itself These wastes are often called brines or concentrates and include spent brine from ion exchange regeneration, reject water from membrane systems (although microfilter and ultrafilter membranes will produce a concentrate containing suspended solids, they are included in this category), reject water from electrodialysis plants, and spent regenerant from specific adsorption media such as activated alumina Gas-phase residuals are produced as off-gases from air stripping systems, and off-gas from ozone contactors The major types of treatment plant residuals are shown in Table 16.1 This chapter primarily addresses characterization, handling, and ultimate utilization of sludges Some introduction to the handling of liquid-phase residuals is included More information on handling other residuals can be found in AWWA (1996) and Cornwell, Bishop, Gould, and Vandermeyden (1987) A good presentation on softening pellets is in Cornwell and Koppers (1990) QUANTITY OF SOLID/LIQUID RESIDUALS GENERATED Most conventional coagulation plants produce two major residuals—residuals from the sedimentation basin (commonly referred to as sludge) and residuals from backwashing a filter (referred to as spent filter backwash water) The quantity of these solid/liquid residuals generated from water treatment plants depends upon the raw water quality, dosage of chemicals used, performance of the treatment process, method of sludge removal, efficiency of sedimentation, and backwash frequency One of the most difficult tasks facing the utility or engineer in planning and designing a residuals treatment process is determining the amount of material (volume and solids) to be handled The solids quantity is usually determined as an 16.3 WATER TREATMENT PLANT RESIDUALS MANAGEMENT TABLE 16.1 Major Water Treatment Plant Wastes Solid/liquid residuals Alum sludges Iron sludges Polymeric sludges Softening sludges Spent filter backwash water Spent GAC or discharge from carbon systems Slow sand filter wastes Wastes from iron and manganese removal plants Spent precoat filter media Softening pellets Liquid-phase residuals Ion-exchange regenerant brine Waste regenerant from activated alumina Membrane concentrate GAC transport water Gas-phase residuals Air stripping off-gases Ozone off-gases annual average for a given design year and is a function of flow demand projections Sometimes overlooked, but very important, is information on seasonal or monthly variations It is not unusual for order of magnitude differences in sludge production to exist for different months of the year The amount of alum (or iron) sludge generated can be calculated fairly closely by considering the reactions of alum or iron in the coagulation process Using an empirical relation to account for the sludge contribution from turbidity will improve the estimate, and the contribution from other sources can be added as required When alum is added to water as aluminum sulfate, the reaction with respect to sludge production is typically represented by the simplified equation that includes three waters of hydration in the product (Cornwell et al., 1987; Cornwell and Koppers, 1990; AWWA, 1996) Al2(SO4)3 ⋅ 14H2O + 6HCO3− = 2Al(OH)3 ⋅ 3H2O + 6CO2 + 11H2O + 3SO4−2 (16.1) If inadequate alkalinity is present, lime or sodium hydroxide is normally added to maintain the proper pH The three waters of hydration satisfy the covalent bonding number of six for aluminum Not including the waters of hydration in the reaction will tend to underestimate the amount of solids that are produced This chemically bound water increases the sludge quantity, increases the sludge volume, and also makes it more difficult to dewater because the chemically bound water cannot be removed by normal mechanical methods Commercial alum has a molecular weight of 594 and contains two moles of aluminum, each with a molecular weight of 27 Therefore, alum is about 9.1 percent aluminum (54/594) The resulting aluminum hydroxide species (Al(OH)3 ⋅ 3H2O) has a molecular weight of 132, and therefore, mg/L of aluminum will produce 4.89 mg of solids (132/27), or mg/L of alum added to water will produce approximately 0.44 mg/L of inorganic aluminum solids (0.091 × 4.89) Suspended solids present in the raw water produce an equivalent weight of sludge solids because they are nonreactive It can be assumed that other additives, such as polymer and powdered activated carbon, produce sludge on a one- 16.4 CHAPTER SIXTEEN to-one basis The amount of sludge produced in an alum coagulation plant for the removal of suspended solids is then: S = 8.34 Q (0.44Al + SS + A) where (16.2) S = sludge produced (lb/day) Q = plant flow (mgd) Al = dry alum dose (mg/L) (as 17.1 percent Al2O3) SS = raw water suspended solids (mg/L) A = additional chemicals added, such as polymer, clay, or powdered activated carbon (mg/L) Note: To convert from lb/day to kg/day, multiply by 0.45 If iron is used as the coagulant, then the equivalent product of equation 16.1 is Fe(OH)3 ⋅ H2O with a molecular weight of 161 The solids production equation becomes: S = 8.34 Q (2.9 Fe + SS + A) 3+ (16.3) 2+ where the iron dose is expressed as mg/L of Fe added or produced via Fe oxidation (note that significant Fe2+ in the raw water will also produce sludge at a factor of 2.9 if it is oxidized) For iron coagulants, the solids production is best expressed as a function of iron because iron coagulant is purchased in may different forms It should not be interpreted from equations 16.2 and 16.3 that iron produces several times the amount of sludge that alum produces.The units for the coagulant are significantly different for the two equations In reality, mole of coagulating equivalent of iron produces about 20 to 25 percent more dry-weight sludge than mole of aluminum, based on the ratio of molecular weights of the product When iron is purchased as ferric chloride (FeCl3), the coagulant dose is usually reported as equivalent dry weight of chemical without waters of hydration (although this should be confirmed with the manufacturer) and, thus, the coagulant has a molecular weight of 162.3.This results in the production of 1.0 mg of solids produced for each milligram of ferric chloride (FeCl3) added Ferric sulfate is reported by different manufacturers with different waters of hydration and the individual products need to be referenced A treatment plant in the mid-Atlantic area of the United States had an average raw water turbidity of 4.5 ntu for the period 1991 through 1994 and used an average ferric chloride dose of 11.5 mg/L (as FeCl3) After conducting a correlation study between SS and turbidity, they found the b value, to convert from turbidity (TU) to suspended solids (SS), to be 1.4 at an average flow of 198 mgd What was the annual sludge production? EXAMPLE 16.1 The solution can be found using equation 16.3 In this equation, the coagulant dose is expressed as Fe Therefore, convert FeCl3 dose to Fe dose by: SOLUTION MW Fe 56 ᎏᎏ = ᎏ (11.5 mg/L) = mg/L as Fe MW FeCl3 161 and the solids production S = 8.34 Q (2.9 Fe + bTU) S = 8.34 (198) [2.9(4) + 1.4 (4.5)] = 29,558 lb/day or 149 lb/MG (million gallons) WATER TREATMENT PLANT RESIDUALS MANAGEMENT 16.5 Polyaluminum chloride (PACl) is the third major coagulant used Care especially needs to be used when converting PACl dose to solids production, because each manufacturer may use different strengths and utilities report these doses differently Some utilities report PACl dose as a neat solution, some as Al2O3, and some as PACl product.A “typical” manufactured PACl liquid contains about 30 to 35 percent PACl and around 10 percent Al2O3 One manufactured product contains 33.3 percent PACl and 10.3 percent Al2O3 or, in this case, the PACl itself contains 30.9 percent Al2O3 This is equivalent to 16.4 percent aluminum, and therefore mg of PACl (as PACl) will produce 0.8 mg of solid product (0.164 × 4.89) The above equations can then be used to track yearly or even daily variation changes in sludge dry weight produced One difficulty in applying the relationships is that most plants not routinely analyze raw water suspended solids concentrations The logical correlation is to equate a turbidity unit to a suspended solids unit Unfortunately, the relationship is generally not to 1: SS (mg/L) = b ⋅ TU (16.4) The value of b for low-color, predominately turbidity removal plants can vary from 0.7 to 2.2 (Cornwell et al., 1987) It may vary seasonally for the same raw water supply A utility can therefore either continually measure suspended solids, or it may be possible to develop a correlation between turbidity and suspended solids Figure 16.1 shows one such correlation for a low-color raw water source (Cornwell, 1981) Ideally, this correlation should be done weekly until information is learned as to seasonal variations in the suspended solids/turbidity relationship Afterward, a monthly correlation may be sufficient Another complication exists for raw water sources that contain a significant amount of total organic carbon (TOC) Total organic carbon can be a large contributor to the sludge production Values of b for low-turbidity, high-TOC raw waters can be as high as 20, but unless turbidity and TOC vary together, a correlation between suspended solids and turbidity will not exist Figure 16.2 shows the relationship between calculated and measured solids production done by the City of Philadelphia (EE&T, 1996) The City used iron as the coagulant during this time period A correlation was developed that showed the ratio of suspended solids to turbidity was 1.4 The calculated quantities using equation 16.3 were within percent of the measured quantities Through careful calibration and measurements, a complete solids mass balance can be prepared, as was done by the City of Philadelphia and shown in Figure 16.3 Through similar theoretical considerations, a general equation has been developed (Cornwell et al., 1987; AWWA, 1981) for plants that use a lime softening process for carbonate hardness removal with or without the use of alum, iron, or polymer The equation is: S = 8.34 Q [2.0 Ca + 2.6 Mg + 0.44 Al + 2.9 Fe + SS + A] where (16.5) S = sludge production (lb/day) Ca = calcium hardness removed as CaCO3 (mg/L) Mg = magnesium hardness removed as Mg(OH)2 (mg/L) Fe = iron dose as Fe (mg/L) Al = dry alum dose (mg/L) (as 17.1 percent Al2O3) Q = plant flow (mgd) SS = raw water suspended solids (mg/L) A = other additives (mg/L) The preceding equations or prediction procedures allow estimation of the dry weight of sludge produced For sludge productions from noncarbonate hardness 16.6 CHAPTER SIXTEEN FIGURE 16.1 Suspended solids versus turbidity (Source: Cornwell, 1981.) removal or when sodium hydroxide is used, see AWWA (1981) These equations not predict the volume of sludge that will be produced Volumes and suspended solids concentrations of sludges leaving the sedimentation basins or clarifiers are a function of raw water quality, treatment, and the sludge removal method When basins are cleaned only periodically by manual procedures accumulating sludges tend to compact and thicken at the bottom There is often a stratification of solids with the heavier particles settling to the bottom and the hydroxide, or lighter, particles at the top However, the actual volume produced will depend largely on the amount of water used to flush the solids out of the basin during cleaning With increasing finished water quality standards, there will be a trend to remove the solids as quickly as possible, generally with continuous collection equipment In this case, the solids concentrations will be lower because compaction height and time have been less Solids concentrations using continuous collection equipment for sludges produced with alum or iron coagulants and for low- to moderate-turbidity raw waters will be about 0.1 to 1.0 percent leaving the sedimentation basin Some of the upflow clarifier devices will produce sludge at a concentration below 0.1 percent, whereas some of the sludge blanket clarifiers can produce sludge at over percent solids concentration The higher the ratio of coagulant-to-raw-water-solids, the lower the solids concentration and the higher WATER TREATMENT PLANT RESIDUALS MANAGEMENT 16.7 FIGURE 16.2 Baxter WTP theoretical versus measured residuals quantities (Source: EE&T, 1996.) the sludge volume Coagulant sludges from highly turbid raw waters may be in the to percent solids concentration range and occasionally higher Sludge volumes from sedimentation basins tend to be 0.1 to percent of the raw water flow, with one survey (Cornwell and Susan, 1979) finding an average of 0.6 percent Softening sludges will concentrate higher, usually as a function of the CaCO3:Mg(OH)2 ratio and the type of clarifier Conventional sedimentation basins may only produce solids concentrations of to percent, whereas sludge blanket clarifiers can produce solids concentrations of up to 30 percent Sludge volumes will correspondingly vary considerably, from 0.5 to percent of the water plant flow Spent filter backwash water is characterized by its large water volume, high instantaneous flow rate, and low solids concentration Filters can be backwashed at anywhere from 15 to 30 gpm/ft2, depending upon the media size and water temperature, and the backwash time may be 15 to 20 Backwash water volumes are in the range of to 10 percent of plant production Accurate plant records often exist on the amount of backwash water used The percentage of plant production used for backwashing can be computed from the ratios of the unit run volumes For example, a filter producing water at gpm/ft2 with a 24-h run time has a unit production of 5760 gal/(run ft2 ) If it is backwashed at 20 gpm/ft2 for 20 min, the unit volume of backwash water is 400 gal/(run ft2 ), for a ratio of about percent backwash water compared with production water Spent filter backwash water will typically contain 10 to 20 percent of the total solids production and have suspended solids concentrations of 30 to 300 mg/L depending upon the applied turbidity to the filters and the ratio of backwash water to production PHYSICAL AND CHEMICAL CHARACTERISTICS OF SOLID/LIQUID RESIDUALS Physical characterization of water plant wastes is primarily directed at solid/liquid waste streams of various percent suspended solids concentrations Solid/liquid wastes are terms used to describe free-flowing liquids that are predominantly water all the way up to mixtures that are predominantly solids and behave like a soil texture Therefore, whenever referring to the physical properties of sludge, it is important to know the suspended solids concentration of the solid/liquid mixture to assess FIGURE 16.3 Baxter WTP baseline residuals distribution (Source: EE&T, 1996.) 16.8 WATER TREATMENT PLANT RESIDUALS MANAGEMENT 16.9 the physical state Cornwell and Wang (Cornwell et al., 1992) used the Atterberg limit test to classify a sludge’s physical state The Atterberg test was originally developed to describe quantitatively the effect of varying the water content on the consistency of fine-ground soils The test consists of measuring five limits; however, the liquid limit and the plastic limit are the most applicable to sludges Figure 16.4 shows the relative location of the liquid and plastic limits The plastic limit identifies the solids concentration at which a sludge transitions from a semisolid to a plastic stage (the plastic state ranges from soft butter to stiff putty) The liquid limit is the solids concentration below which the sludge exhibits viscous behavior; the consistency could be described as ranging from soft butter to a pea soup–type slurry Coagulant sludges that were tested had liquid limits in the 15 to 20 percent solids concentrations range Solids concentrations below but near this range would result in a material that still had free water associated with it but may not flow Generally, a coagulant sludge is still free flowing up to an to 10 percent solids concentration The plastic limit for coagulant sludges was found to be anywhere from 40 to 60 percent solids concentration Knocke and Wakeland (1983) divided the physical properties of sludge into macroproperties and microproperties Macroproperties include parameters such as specific resistance, settling rates, and solids concentrations The indices described above could be considered macroproperties Microproperties included particle size distribution and density Vandermeyden et al (1997) studied the micro- and macroproperties of about 80 water plant sludges Coagulant sludges had a median particle FIGURE 16.4 Relative location of liquid and plastic limits 16.10 CHAPTER SIXTEEN diameter of 0.005 mm with a range of approximately 0.001 to 0.03 mm, as shown in Figure 16.5 Lime sludge had a similar range, but the median diameter was 0.012 mm They also measured the specific gravity of the solid material in the sludge mixtures, as shown in Figure 16.6 The coagulant residuals had an average specific gravity of 2.32 and the lime residuals averaged 2.50 Koppers (Cornwell and Koppers, 1990) reported the dry density of coagulant sludges to be about 2.5 Knocke et al (1993) found densities for coagulant sludges to range from 2.45 to 2.86, lime sludges to be 2.47, and polymer sludges to be 1.60 Vandermeyden et al (1997) also evaluated drainage properties of the 80 residuals using the capillary suction time (CST), specific resistance (SR), and time to filter (TTF) tests The CST test is a fast and relatively simple test that is performed to FIGURE 16.5 Average particle diameter for coagulant and lime residuals (Source: Vandermeyden et al., 1997.) FIGURE 16.6 Specific gravity distribution for coagulant and lime residuals (Source: Vandermeyden et al., 1997.) WATER TREATMENT PLANT RESIDUALS MANAGEMENT Clarification length (in) Bowl radius (in) Radius to pool (in) rpm Pilot unit Full-scale 31 8.4 5.1 3250 91.6 14.5 9.5 3150 16.37 To calculate the sigma values, Eq 16.22 needs to be used.The volume of sludge or water in the pool, V, can be found as: V = π L (r22 − r12) where L is the clarification length Therefore, for the pilot unit: V = π (31)(8.42 − 5.12) = 4339 in3 and ω is found as ΂ ΃ ΂2π ᎏᎏ revolution ΃ ω = (rpm) ᎏ 60 s rad ΂ ΃ = 3250 ᎏ (2π) = 340 rad/s 60 Using Eq 16.22: (4,339) (340)2 Σ1 = ᎏᎏᎏ (32.2) (12) ln (8.4/5.1) Σ1 = 2,601,462 in3 = 1,505 ft3 Similarly, Σ2 − 13,320 ft3, such that the sigma ratio, or scale-up factor, is 13,320/ 1,505 = 8.85 and, from Eq 16.21, the design flow for the full-scale centrifuge would be 40(8.85) = 354 gpm Filter Presses The filter press is another process option available for dewatering sludge, and it generally results in the production of the highest final cake concentration of any of the mechanical dewatering devices Early filter presses were frequently used in Europe for dewatering thin slurries, such as china clays and wastewater sludges Their practical use for water treatment residues began around 1965 Experiments commenced in England in 1956, but were disappointing until the advent of the use of polymers as conditioners The first known use in the United States was at the Atlanta Waterworks and at the Little Falls Treatment Plant of the Passaic Valley Water Commission At the commencement of a filter cycle, sludge is forced into contact with the cloth, which retains the solid matter while passing the liquid filtrate Very quickly the cloth becomes coated with a cake of sludge solids, and all future filtering occurs through this cake, which increases in depth as succeeding layers build up The type of cloth does not affect the rate of filtration after the first few minutes, and it can be ignored from a theoretical point of view Filter presses (Figure 16.22) are very heavy, cumbersome pieces of equipment demanding costly foundations and relatively large buildings Apart from minor 16.38 CHAPTER SIXTEEN refinements, for several decades filter press design changed little until the advent of the diaphragm filter The original design of the plate and frame filter (Figure 16.23) consisted of a series of frames into which sludge is passed under high pressure, up to 225 psi (1570 kPa), to dewater the sludge against the outer cloth-covered plate The depth of the cake was consequently fixed, being governed by the distance between filter plates Different sizes of plates were manufactured to give cakes of, for example, 3⁄4, 1, or 11⁄2 in (19, 25, or 38 mm) depth Such filters have been used to dewater sludge in an acceptable manner for many years A considerable change in design resulted with the introduction of diaphragm filters (Figure 16.24) The advantage of this system is that the thickness of the cake is infinitely variable within the limits of the machine dimensions Sludge is filtered through a cloth for a fixed period of time, perhaps 20 min, at which stage the sludge supply is cut off and water or compressed air is applied behind an expandable diaphragm that further squeezes water out of the sludge The cake is dislodged by shaking or by rotating the cloth, depending on the manufacturer’s design, and falls into a hopper for disposal Hanging cakes, where the cake refuses to leave the cloth, an unhappy feature of the older plate and frame press, is consequently eliminated Diaphragm presses also have the advantage that conditioning of the sludge, although desirable in some circumstances, is not always necessary As a result, although output is somewhat reduced, a considerable amount of capital and operations costs are eliminated A diaphragm press is currently installed at the Moores Bridges Water Treatment Plant in Norfolk, Virginia, and was designed for a peak output of 138,000 lb (62,600 kg) of solids per week FIGURE 16.22 Conventional filter press (Source: Courtesy of US Filter–Zimpro Products, Rothschild, WI.) WATER TREATMENT PLANT RESIDUALS MANAGEMENT FIGURE 16.23 16.39 Plate and frame filter press schematic (Source: Innocenti, 1988.) RECYCLE It is common for many utilities to recycle various streams back to the beginning of the treatment process Recycling is most often done due to water conservation measures, to improve plant operation, or due to the lack of being able to discharge the liquids to a watercourse or sewer The liquid volume associated with sedimentation underflow and spent filter backwash water can be to 10 percent of the plant production In water-scarce areas, the water is too valuable a resource to discharge, and returning it to the treatment process is a necessary water conservation measure Lime softening plants find that recycling solids can provide nucleation sites for calcium carbonate precipitation, thereby making more efficient use of lime by not having to overfeed chemical to start the precipitation process Discharge into watercourses is regulated by states under the National Pollutant Discharge Elimination System (NPDES) In some areas, an NPDES permit cannot be obtained or a sewer is not available making recycling the only option Recycling can be back to a terminal reservoir, to the beginning of the treatment train, or in some cases to an intermediate point in the treatment process Possible streams that could be recycled include the following: Filter-to-waste Spent filter backwash water a With the solids from filtration b Without the solids from filtration (after settling) Clarifier or sedimentation basin sludge Sludge thickener overflow (supernatant) Sludge lagoon overflows 16.40 CHAPTER SIXTEEN FIGURE 16.24 Diaphragm filter press (Source: Courtesy of US Filter–JWI Products, Holland, MI.) Dewatering operation liquid wastes a Pressate from belt or filter press b Centrate from centrifuge c Leachate from sand drying bed Recycling of these wastes may upset the treatment process and affect the quality of the finished water The impacts could be caused by the solids in some of the recycle streams, or by constituents in the recycle streams The principal constituents that could be in recycle streams and be of water quality concern include: ● ● ● ● ● ● Microbiological contaminants, including Giardia and Cryptosporidium parasites Total organic carbon Disinfection by-products Turbidity and suspended solids Metals, such as manganese, aluminum, and iron Taste- and odor-causing compounds Giardia and Cryptosporidium cysts and oocysts can be present in spent filter backwash water and sedimentation basin sludges at relatively high concentrations (Cornwell and Lee, 1993) For example, one plant studied had Giardia and Cryptosporidium concentrations of more than 150 cysts/L in the spent filter backwash water, as compared with 0.2 to cysts/L in the raw water WATER TREATMENT PLANT RESIDUALS MANAGEMENT 16.41 Laboratory- and full-scale confirmation in that research showed that sedimentation was effective in reducing particles (and cyst levels) in the spent filter backwash prior to recycle However, very low overflow rates [less than 0.05 gpm/ft2 (0.12 m/h)] were required to achieve 70 to 80 percent particle removal in the cyst size range A nonionic polymer was effective in increasing particle removals to more than 90 percent at overflow rates of 0.2 to 0.3 gpm/ft2 (0.5 to 0.75 m/h) Nearly all coagulant sludges contain high concentrations of manganese Quantities of dissolved manganese in the water surrounding sludge can be in the range of to mg/L, and upon storage of the solids, the release of manganese to the surrounding water can reach 20 to 30 mg/L As sludge accumulates in manually cleaned sedimentation basins, the manganese levels in the clarified water may gradually increase Therefore, some manganese will be released to sludge thickener overflows and recycled to the plant or will be released in manually cleaned sedimentation basins to the clarified water Normally, the manganese concentrations are low unless large spikes of waste streams are recycled However, if accumulated sludge is allowed to occupy a significant portion of the thickener or manually cleaned basin, or if a hydraulic upset occurs, a situation could develop in which large concentrations of manganese are recycled or released from the manually cleaned sedimentation basin In the research by Cornwell and Lee (1993), it was generally found that if the solids were removed from the waste streams prior to recycle, total trihalomethane formation potential (TTHMFP) in the recycle streams was no higher than in the raw waters The same was found for total organic carbon (TOC) However, without solids removal,TTHMFP and TOC levels can be quite high in the waste streams.The recycle streams may contain preformed trihalomethane (THM), and therefore the THM concentration leaving the plant with recycle is sometimes found to be higher than that without recycle This increase could impact a utility’s distribution system THM average Waste streams can be treated prior to recycle and the type of treatment depends upon the types of contaminants to be removed and the degree of removal required Generally, sedimentation is sufficient to remove 80 percent of the solids leaving a relatively clear supernatant to be recycled Polymer or coagulant assistance can increase the removal efficiency of particles The supernatant stream could also be disinfected prior to recycle using ozone, chlorine dioxide, or perhaps potassium permanganate If particulate removal above that achievable by sedimentation is required, filters could be added or membrane technology employed If recycle of streams is necessitated by conservation or discharge issues, it appears that an appropriate technology can be selected to allow for recycle without impacting finished water; however, more research and site-specific investigations are needed to determine appropriate treatment for each situation MEMBRANE AND ION EXCHANGE RESIDUALS The water industry produces liquid wastes primarily from ion exchange or membrane processes If the wastes contain a high total dissolved solids (TDS) content, they are often referred to as brine wastes This would apply to ion exchange processes or to electrodialysis (ED) or membrane processes used to remove salt from seawater or brackish groundwater Membrane processes that produce a low-TDS waste are referred to as concentrate Ion exchange has been used for a number of years as a softening process Most large plants that have used ion exchange have 16.42 CHAPTER SIXTEEN been located near coastal areas so that brine wastes were discharged to the ocean Many of these large plants have been abandoned due to corrosion and high maintenance costs However, ion exchange is still practiced by small treatment systems Ion exchange columns are generally backwashed, or regenerated, at a rate of to gpm/ft2 for about 10 The frequency of regeneration depends upon the hardness level and type of resin being used The amount of concentrate or brine produced by membrane processes will vary by application and membrane type Table 16.4 shows a range of concentrate production for various types of membranes Conventional methods of concentrate disposal involve discharge to surface bodies of water, spray irrigation combined with a dilution stream, deep well injection, drain fields, boreholes, or into the sewer Concentrate disposal methods must be evaluated in light of geographic, environmental, and regulatory impacts Table 16.5 summarizes concerns and requirements associated with conventional concentrate disposal methods (AWWA, 1996) ULTIMATE DISPOSAL AND UTILIZATION OF SOLIDS The treatment and disposal of water treatment plant residuals is rapidly becoming an integral part of operating water treatment plant facilities as local, state, and federal regulations require more stringent standards for traditionally practiced residuals management programs The discharge of untreated residuals to most surface waters is severely restricted under the National Pollutant Discharge Elimination System (NPDES) of the Clean Water Act Discharge of residuals to sanitary sewers is becoming more restrictive through wastewater pretreatment standards, the availability of wastewater treatment plant capacity, the capability of digesters to handle the mostly inorganic residuals, and the wastewater plants’ effluent standards Sanitary landfill disposal of dewatered liquid/solid residuals has become very costly, and the residuals consume valuable space in a disposal system that is already subject to shortages in the future As a result, beneficial use programs for residuals are increasingly being considered by utilities, not only as a cost-effective alternative but also as a publicly acceptable management practice The desired residuals management goal for any water utility is to operate an economically efficient and environmentally attractive residuals management plan by developing one or more long-term agreements, which will allow for the proper utilization of residuals in a beneficial application The beneficial use markets that have exhibited the greatest potential for success with coagulant sludges are the following: TABLE 16.4 Membrane Concentrate Generation Membrane process Percent recovery of feed water Percent disposal as concentrate UF NF Brackish water RO Seawater RO ED 80 to 90 80 to 95 50 to 85 20 to 40 80 to 90 10 to 20 to 20 15 to 30 60 to 80 10 to 20 Source: AWWA, 1996 WATER TREATMENT PLANT RESIDUALS MANAGEMENT 16.43 TABLE 16.5 Concerns and Requirements Associated with Conventional Disposal Methods Disposal method Regulatory concerns Other requirements Disposal to surface water Receiving stream limitations Radionuclides Odors (hydrogen sulfide) Low dissolved oxygen levels Sulfide toxicity Low pH Mixing zone Possible pretreatment Multiple-port diffusers Modeling of receiving stream Deep well injection Confining layer Upcoming to USDWs Injection well integrity Corrosivity Well liner Monitoring well Periodic integrity test Water quality of concentrate must be compatible with the water quality in the injection zone Spray irrigation Groundwater protection Monitoring wells Possible pretreatment Backup disposal method Need for irrigation water Availability of blend waters Drainfield or borehole Groundwater protection Monitoring wells Proper soil conditions and/or rock permeability Sanitary sewer collection systems Effect on local wastewater treatment plant performance (toxicity to biomass or inhibited settleability in clarifiers) None Source: AWWA, 1996 Commercial products The use of sludges in making commercial products has been successful in such areas as turf farming and topsoil blending In both cases, the sludge is a substitute for natural soil material and offers economical benefits to the commercial user Brick manufacturing is a potential market for coagulant sludges and experiences by the Santa Clara Valley Water District (Migneault, 1988) and the City of Durham, North Carolina (Rolan, 1976), indicate that this can be a workable program Co-use with biosolids Incorporating sludge in biosolids management programs, such as land application and composting, can be beneficial Blended products tend to have lower metal concentrations making the product more marketable Also, for utilities that operate both a water and wastewater facility, permitting, record keeping, and monitoring requirements are reduced when the sludge and biosolids are managed under one program Land application Land application of sludge to agricultural or forested land is a feasible beneficial use alternative It is not widely practiced because the need has only been here quite recently and residuals must frequently compete with biosolids for appropriate sites Application of coagulant sludges to turf farms is a management option that benefits a farmer’s harvest and supplements the farmed areas with a new “soil” base Turf 16.44 CHAPTER SIXTEEN grass has a relatively low nutrient demand but requires significant moisture levels, particularly for initial growth phases Dewatered sludge applied to a turf farm at the beginning of the seeding process can provide excellent water retention capabilities The preferred method of application is with a manure-type spreader The application rate is a function of the type of grass, supplemental fertilization, and residuals quality A 1-in application depth of residuals at a 20 percent solids concentration is equal to a solids loading rate of 17 dry tons per acre or a mass loading rate of 1.7 percent Greenhouse and leaching experiments undertaken by the University of Buffalo (Van Benschoten, 1991) showed the success of using residuals in grass growing Sludge mixed with native soils at 25, 50, 75, and 100 percent loadings clearly enhanced turf grass growth in the greenhouse studies.As shown in Figure 16.25, measured turf grass growth in sludge/soil mixtures was up to three to four times the growth observed in native soils This improvement in yield allows a turf farmer to bring his fields to harvest sooner and, therefore, provides a marketable turf product sooner than experienced under current harvesting conditions Increases in labile phosphorus (as determined by the Bray-1 phosphorus tests) followed the recorded increases in grass mass, indicating a beneficial increase in soil phosphorus available to plants Commercial producers of topsoil utilize a variety of raw soil products to develop a marketable product for nurseries, homeowners, professional landscapers, and so forth In this process, the raw soils are screened and blended with some organic material before being sold as a product Sludge can be blended during the topsoil production process to increase the nutrient value and water retention capabilities To a topsoil producer, the sludge is a raw material that can be obtained at little or no cost and increases the profit margin on the final product The amount of sludge added to the topsoil may be 10 percent or less and is a function of consistency, quality, and availability The acceptable quality of the sludge is determined by individual topsoil producers An example of metal limits established by one commercial topsoil producer is shown in Table 16.6 (Vandermeyden and Cornwell, 1993) Blending sludge with topsoil can be a mutually beneficial operation for the utility and the producers Key elements for long-term success are reasonably consistent solids quality, reliable delivery of dewatered residuals, and strong contractual arrangement between all parties FIGURE 16.25 University of Buffalo greenhouse study: Grass mass versus percent residuals loading Note: Each bar shows the sum of three harvests and represents a unique test (Source: Van Benschoten, J E., J N Jensen, and A R Griffin 1991 Land Application of Water Treatment Plant Sludge Prepared for the Erie County, NY, Water Authority.) WATER TREATMENT PLANT RESIDUALS MANAGEMENT 16.45 TABLE 16.6 Topsoil Blending: Example of Metal Content Limits (ppm) Parameter Preapproved Requires review Not accepted Cadmium Chromium Copper Lead Mercury Nickel Zinc 1200 Source: Vandermeyden and Cornwell, 1993 Sludge can be mixed with biosolids and, subsequently, be a coproduct in the overall end-use management strategy of the biosolids For a utility that operates both water and wastewater facilities, this type of solids management program has certain benefits including: (1) it avoids separate permitting and monitoring of the residuals, (2) it provides a beneficial and often cost-effective end-use avenue for the residuals, and (3) it reduces most of the metal concentrations in the biosolids product because of the diluting effect of the residuals Even in situations in which the water and wastewater facilities are owned and operated by separate entities, the residuals can enhance the quality of the biosolids product and provide a source of revenue Residuals can be incorporated with biosolids in a variety of methods, including: ● ● ● ● Discharge of liquid residuals to the sanitary sewer Discharge of liquid residuals at the wastewater plant influent Discharge of liquid residuals at the wastewater plant solids handling stage Blending of dewatered residuals with dewatered biosolids Water plant residuals containing high concentrations of inorganic compounds or suspended solids can significantly impact primary settler overflow quality, digester space, and digester efficiency Residuals should also not degrade the end-use biosolids product quality, such as lowering nutrient values or increasing/introducing higher metal concentrations This is particularly true for land application and composting processes The land application of residuals can be an environmentally safe and costeffective method for residuals management Due to the variable characteristics of different raw water sources, allowable residuals application rates must be considered according to the composition of each individual residual Heavy metals and the potential for phosphorus binding are the primary concerns associated with land application Alum sludge, when added to soil, has the capability to absorb inorganic phosphorus, which prohibits plants from extracting P from the soil for plant growth However, with good soil management and crop selection, P depletion can be prevented with proper loading rates and P fertilization The aluminum concentration of alum sludge can range from to 15 percent of the total dry solids mass, which is 50 to 100 percent higher than the concentration of the aluminum in most soils (Elliott and Dempsey, 1991) Elliott and Dempsey (1991) determined that for a 10-ton-peracre sludge loading rate with an Al concentration of 30 percent, the soil Al level was reported to only increase by 0.3 percent Aluminum phytotoxicity is dependent on Al solubility and is not a problem when the pH range in the soils is maintained between pH to 6.5 (Elliott and Dempsey, 1991) Cornwell et al (1992) has found 16.46 CHAPTER SIXTEEN that aluminum does not leach from the sludges but remains complexed within the solids matrix The addition of alum sludge to soils could also change the soils, physical structure, or bulk density A soil with a high-bulk density is more compact, which is unfavorable to plant growth because root penetration is restricted A low-bulk density (less compacted soil) has more pore space for air and water, which is beneficial for plant growth (Tisdale and Nelson, 1975) Rengasamy et al (1980) found that soil mixed with alum residuals increased soil aggregation and moisture retention and, as a result, increased the dry yield of maize Bugbee and Frink (1985) demonstrated that improvements in aeration and moisture retention, promoted by the addition of alum residuals, were made to offset the phosphorus deficiency in lettuce The impact of land application on various crops has been investigated by Virginia Polytechnic Institute and State University Novak (1993) studied the impact of alum and PACl residuals on corn when applied at 1.3 to 2.5 percent by dry weight (13 to 25 tons per acre) Crop yields from the treated plots were not statistically different from the untreated plots Mutter et al (1994) studied the impact of PACl residuals on wheat when applied at 2, 4, and percent No negative effects on wheat grain or biomass yield were observed at these loading rates Although soil aluminum levels were significantly increased at these loading rates, after two crop rotations the soil Al concentrations were found to be similar to background Al levels The wheat leaf tissues were slightly increased in Al concentration as compared with the control This increase in Al concentration, however, was not found to be significant Lime sludges have been land-applied for over 40 years In many farming regions, the application of nitrogen fertilizers causes a reduction in soil pH Farmers normally apply sufficient quantities of lime to obtain the desired soil pH Lime sludges are high in CaCO3 and provide the same or better neutralizing value as commercially available limestone (AWWA, 1981) The Ohio Department of Health (AWWA, 1981) reported that liming materials typically available to farmers have a total neutralizing power (TNP) of 60 to 90 The department evaluated sludges from seven utilities and found the TNP of lime sludges to range between 92 to 100, or better than that of commercially available materials In Illinois, a calcium carbonate equivalent performed on several sludges indicated that the softening sludges were superior to agricultural limestones available locally The sludge can be applied to farm lands by either spraying liquid sludge (