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2
Municipal Wastewater, Sewage Sludge, and Agriculture
Historical Perspectives
Wastewater
Large-scale cropland application of municipal wastewater was first practiced about 150 years ago after flush toilets and sewerage systems were introduced into cities in western Europe and North America. The wastewater was discharged without any treatment, and receiving watercourses became heavily polluted. The problem is illustrated by the situation in London in the 1850s when the "stink" from the River Thames obliged the House of Parliament to drench their drapes in chloride of lime (Snow, 1936). Water supplies drawn from the river below the sewage outfall were found by Dr. John Snow to be the source of the cholera outbreaks of the period. The partial solution to the problem was the construction by Sir John Bazalgete of a vast interceptor along the north bank of the River Thames, creating the famed Thames Embankment. This gave relief to central London, but moved the pollution problem downstream. Sir Edwin Chadwick, a lawyer and crusader for public health at the time, was a strong advocate of separate sanitary sewers, and he coined the slogan, "the rain to the river and the sewage to the soil." In this spirit, and to reduce pollution of the Thames downstream, "sewage farms" were established to take the discharges from the interceptor. The agricultural benefits from the farms were incidental to their service in the disposal of the wastewater.
The practice of sewage farms quickly spread. By 1875, there were about 50 such farms providing land treatment in England, and many similar farms served major cities in Europe. By the turn of the century, there were about a dozen sewage farms in the United States. However, the need for a reliable outlet for wastewater was not entirely compatible with the seasonal nature of nutrient and water requirements of crop production. While sewage farms alleviated pollution in the receiving streams, they created a different set of environmental sanitation problems. Hydraulic and pollutant overloading caused clogging of soil pores, waterlogging, odors, and contamination of food crops. The performance improved over the years as operators gained experience with balancing the needs of wastewater disposal and crop growth. Nevertheless, the farms were gradually phased out when the land areas required to accommodate wastes from large cities grew too great to be practical and more effective technologies were developed to remove
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pollutants from wastewater. The processes of primary sedimentation and secondary biological treatment, which were developed in the early part of this century, required much smaller areas for their operations and were capable of producing clarified effluents for direct discharge into a surface water body. These technologies eliminated the need for sewage farms.
In the United States, an estimated 0.69 m3 (182 gal) per capita per day of municipal wastewater is generated (Solley et al., 1993). Municipal wastewater treatment plants (otherwise known as publicly-owned treatment works or "POTWs") currently serve around 75 percent of the U.S. population (EPA, 1995). The remainder are largely served by individual household septic systems. For those served by centralized facilities, the municipal wastewater is collected through a sewer network and centrally treated in a wastewater treatment plant. The collected wastewater contains pollutants originating from households, business and commercial establishments, and industrial production facilities. The general composition of municipal wastewater is well understood. For the purpose of water-quality management, pollutants in municipal wastewater may be classified into the following five categories:
Organic matter (measured as biochemical oxygen demand or BOD),
Disease-causing microorganisms (pathogens),
Nutrients (nitrogen and phosphorus),
Toxic contaminants (both organic and inorganic), and
Dissolved minerals.
Although the exact composition may differ from community to community, all municipal wastewater contains constituents belonging to the above categories. Pollutants belonging to the same category exhibit similar water quality impacts. The objective of wastewater treatment is to remove pollutants so that the effluent meets the water quality requirement for discharge or reuse.
In modern society, thousands of potentially hazardous chemicals are used in household products and commercial and industrial activities. They may be inadvertently or intentionally discharged into the wastewater collection system. Municipal wastewater also contains many types of infectious, disease-causing organisms that originate in the fecal discharge of infected individuals.
In wastewater treatment, the physical and chemical state of pollutants determine the approaches that are employed to remove impurities. For this purpose, pollutants may be further classified (as per Camp and Messerve, 1974) into:
Settleable impurities,
Suspended impurities,
Colloidal impurities, and
Dissolved impurities.
Pollutants sharing the same physical state behave similarly during conventional wastewater treatment.
With more than one hundred years of continuous development, municipal wastewater treatment technology in use today can achieve almost any degree of treatment and removal of
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impurities desired. The conventional municipal wastewater treatment system consists of a series of processes, through which pollutants are removed, step by step, from the water and are concentrated into the solid fraction or sludge (see Chapter 3 for a further description of municipal wastewater and sludge treatment).
Treated effluents are customarily discharged into a surface water body. With advances in wastewater treatment technology, wastewater effluents can achieve consistently high quality and are increasingly reclaimed for reuse. The value of reclaimed water in crop irrigation has long been recognized, particularly where fresh water resources are limited (Webster, 1954; Mertz, 1956; Sepp, 1971). Some wastewater reclamation programs (e.g., in Florida) are also motivated by the need to avoid nutrient overload to sensitive receiving waters. In these cases, beneficial reuse can be more economical and/or technically feasible than employing the advanced wastewater treatment needed to meet the requirements for surface water disposal.
In the United States, irrigating crops with reclaimed wastewater has been generally well accepted, both in the semiarid western states and in Florida. The suitability of water for reuse is influenced by the chemical composition of the source water, mineral pickup due to water use, and the extent of wastewater treatment. These characteristics vary seasonally and from one municipality to another (Pettygrove and Asano, 1985). Dowdy et al. (1976) derived a "typical" chemical composition of treated wastewater effluent from a selected number of cities. This "typical" composition is compared to that of water from the Colorado River—a source for crop production in several western states—for many of the water quality criteria important in irrigation (Table 2.1). Judged against existing guidelines for irrigation water quality criteria (National Academy of Sciences, 1973; Westcot and Ayers, 1985), the chemical composition of treated wastewater effluents, although widely varied, is acceptable for crop irrigation. In addition, treated effluents contain significantly higher amounts of nitrogen and phosphorus—fertilizer elements essential for plant growth.
There are numerous examples of successful agricultural reuse projects in the United States. The wastewater from Bakersfield, California has been used for irrigation since 1912 when raw sewage was used (Pettygrove and Asano, 1985). Currently, reclaimed water from Bakersfield irrigates approximately 2,065 ha (5,100 acres) of corn, alfalfa, cotton, barley, and sugar beets productions with more than 64,000 m3/day (16.9 million gal/day) of primary and secondary effluents from three treatment plants (Pettygrove and Asano, 1985). To avoid wastewater discharge to sensitive receiving waters, the city of Tallahassee, Florida has been using treated effluent for agricultural irrigation on city-owned farmland since 1966. About 68,000 m3/day (18 million gal/day) of secondary effluent are pumped approximately 13.7 km (8.5 miles) and irrigate about 700 ha (1,729 acres) (Roberts and Bidak, 1994).
A seven-year agricultural wastewater reclamation demonstration study was conducted at Castroville, California, and completed in 1987. This study used a wastewater treatment process of secondary (biological) treatment, coagulation, filtration, and disinfection, with the final effluent meeting a quality standard of 2.2 total coliform/100 ml (the standard enforced by California's regulations for wastewater reclamation). The study concluded that the treatment process was acceptable for the spray irrigation of food crops to be eaten raw (Sheikh et al., 1990). The study detected no pathogenic organisms in the treated effluent, and spray irrigation with the treated effluent did not adversely affect soil permeability, did not result in heavy metal accumulation in the soil or plant tissue, and did not adversely affect crop yield, quality, or shelf
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TABLE 2.1 Composition of Secondary Treated Municipal Wastewater Effluents and Irrigation Water
Secondary Effluenta
Parameter
Range
Typical
Colorado Riverb
Irrigation Water Quality Criteriac
Total Solids
U
425
U
NA
Total Dissolved Solids
200–1300
400
668.0
<2000
pH
6.8–7.7
7.0
7.9
6.5–8.4
Biochemical Oxygen Demand
2–50
25
U
NA
Chemical Oxygen Demand
25–100
70
U
NA
Total Nitrogen
10–30
20
U
<30
Ammonia Nitrogen
0.1–25
10
U
NA
Nitrate Nitrogen
1–20
8
0.1–1.2
NA
Total Phosphorus
5–40
10
<0.02
NA
Chloride
50–500
75
55–77
<350
Sodium
50–400
100
71–97
<70
Potassium
10–30
15
4–6
NA
Calcium
25–100
50
66–163
NA
Magnesium
10–50
20
23–28
NA
Boron
0.3–2.5
0.5
0.10–0.54
<3.0
Cadmium (ug/L)
<5–220
<5
<1–69
10
Copper (ug/L)
5–50
20
<10–10
200
Nickel (ug/L)
5–500
10
<1–4
200
Lead (ug/L)
1–200
5
<5
5000
Zinc (ug/L)
10–400
40
<3–12
2000
Chromium (ug/L)
<1–100
1
<1
100
Mercury (ug/L)
<2–10
2
<0.1–0.1
NA
Molybdenum (ug/L)
1–20
5
2–8
10
Arsenic (ug/L)
<5–20
<5
4–16
100
All units in milligrams per liter unless otherwise noted as micrograms per liter (ug/L). U: unavailable. NA: not applicable.
a Adapted from Asano et al., 1984 and Treweek, 1985
b Radtke, et al., 1988
c from Westcot and Ayers, 1985 and National Academy of Sciences, 1973
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life. Over a 10-year period, Yanko (1993) assayed 590 filtered and chlorine-treated secondary effluent samples from wastewater treatment plants in Los Angeles County for enteric viruses. All of the effluent samples had met California's wastewater reclamation standard of 2.2 coliform/100 ml, and only one was found positive for virus (Coxsackie B3).
State regulations of the use of reclaimed wastewater on food crops are aimed at protecting consumers from possible exposure to pathogens (discussed in Chapter 7). The potential health hazard of trace elements (including heavy metals) and toxic organic chemicals has been addressed for a variety of end used of reclaimed wastewater, but they have not been regulated for purposes of agricultural irrigation. As mentioned, concentrations of trace elements in wastewater effluents that have undergone secondary or higher levels of treatment are normally within existing guidelines for irrigation water quality criteria (see Chapter 4 for a discussion of trace elements and organic chemicals).
Sewage Sludge
Before the era of wastewater treatment, municipal wastewater was untreated and sludge did not exist. Sewage sludge is an end product of municipal wastewater treatment and contains many of the pollutants removed from the influent wastewater. Sludge is a concentrated suspension of solids, largely composed of organic matter and nutrient-laden organic solids, and its consistency can range in form from slurry to dry solids, depending on the type of sludge treatment. Agricultural utilization of sewage sludge has been practiced since it was first produced. Given agricultural experience with the use of human excrement, sewage, and animal manure on croplands, the application of municipal wastewater sludge to agricultural lands was a logical development. As an early example, municipal sludge from Alliance, Ohio was used as a fertilizer as early as 1907. During the same period, Baltimore, Maryland used domestic septage in agricultural production (Allen, 1912). The plant nutrient value of sludge has been evaluated by many investigators (Rudolfs and Gehm, 1942; Sommers, 1977; Tabatabai and Frankenberger, 1979), and the nutrient composition is considered to be similar to other organic waste-based soil amendments that are routinely applied on cropland, such as animal manures (as shown in Table 2.2). In addition to major plant nutrients, sludge also contains trace elements that are essential for plant growth. Soils which have been tilled for decades are often deficient in certain trace elements, such as zinc and copper (Martens and Westermann, 1991). Certain calcareous soils are deficient in iron (Martens and Westermann, 1991). Land applications of municipal sludge can help to remedy these trace metal deficiencies (Logan and Chaney, 1983).
Early agricultural sludge use projects were often carried out with little regard for possible adverse impacts to soil or crops (Allen, 1912). A common goal was to maximize the application rate to minimize the cost of sludge disposal. Since the early 1970s, more emphasis has been placed on applying sludge to cropland at an agronomic rate (Hinesly et al., 1972; Kirkham, 1974).
Wastewater treatment authorities attempt to manage the volume of wastewater and type of pollutants discharged into sewers to protect the integrity of the infrastructure, health and well-being of sanitation workers, the performance of wastewater treatment processes, and the impact on receiving waters. Federal and state regulations exert control over the quality of the treated
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TABLE 2.2 Chemical Composition of Sewage Sludge and Animal Manure
Constituent (Unit)
Animal Manurea
Sewage Sludge
Range
Range
Typical
Nitrogen (% dry weight)
1.7–7.8
<0.1–17.6b
3.0
Total phosphorus (% dry weight)
0.3–2.3
<0.1–14.3b
1.5
Total sulfur (% dry weight)
0.26–0.68
0.6–1.5b
1.0
Calcium (% dry weight)
0.3–8.1
0.1–25b
4.0
Magnesium (% dry weight)
0.29–0.63
0.03–2.0b
0.4
Potassium (% dry weight)
0.8–4.8
0.02–2.6b
0.3
Sodium (% dry weight)
0.07–0.85
0.01–3.1b
-
Aluminum (% dry weight)
0.03–0.09
0.1–13.5b
0.5
Iron (% dry weight)
0.02–0.13
<0.1–15.3b
1.7
Zinc (mg/kg dry weight)
56–215
101–27,800b
1200
Copper (mg/kg dry weight)
16–105
6.8–3120c
750
Manganese (mg/kg dry weight)
23–333
18–7,100b
250
Boron (mg/kg dry weight)
20–143
4–757b
25
Molybdenum (mg/kg dry weight)
2–14
2–976b
10
Cobalt (mg/kg dry weight)
1
1–18b
10
Arsenic (mg/kg dry weight)
12–31
0.3–316c
10
Barium (mg/kg dry weight)
26
21–8,980b
-
a Data summarized from Azevado and Stout, 1974
b Data summarized from Dowdy et al., 1976
c Data summarized from Kuchenrither and Carr, 1991
effluent in order to keep contaminants below concentrations that would be harmful to humans and the environment.
Nevertheless, toxic chemicals in low concentrations are introduced into municipal wastewater. Many of these toxic chemicals are removed from the wastewater and concentrated into the sewage sludge by the wastewater treatment process. Sewage sludge also contains human pathogens, although it can be treated to significantly reduce the number of pathogens present. Pathogens and toxic chemical pollutants may be introduced into sludge-amended soil.
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Since about 1970, there has been an intense and concerted effort of scientific research world-wide to better understand the fate of potentially toxic and pathogenic constituents in sludge when sludge is applied to agricultural soils. A search of agricultural research articles (from the computer database, AGRICOLA) revealed more than 2,300 articles published since 1970. The surge of technical information regarding agricultural application of sewage sludge has led to the development of pollutant loading guidelines by the United States and western European countries (McGrath et al., 1994). The World Health Organization is also investigating ways to develop human health-related chemical guideline for using treated municipal wastewater effluents and sewage sludge in agriculture production (Chang, et al., 1993).
Since the late 1970s and early 1980s, source control and industrial wastewater pretreatment programs have been initiated to limit the discharge of industrial pollutants into municipal sewers, and these programs have resulted in a dramatic reduction of toxic pollutants in wastewater and in sludge (see Chapter 3 for a discussion of industrial pretreatment). Municipal wastewater sludge, particularly from industrialized cities, now has significantly lower levels of toxic contaminants—specially heavy metals—than in earlier decades when much of the research on sludge application to cropland was conducted. Tables 2.3, 2.4, and 2.5 show decreasing levels of metals in wastewater or sludge for municipal sewage treatment facilities in Chicago, Baltimore, and Philadelphia from about 1970 through 1985. More recently, a comparison of sludge quality was made between two EPA surveys (Kuchenrither and McMillan, 1990): a study of 40 cities conducted in the late 1970's (EPA, 1982) and the National Sewage Sludge Survey (NSSS) conducted in the late 1980s (EPA, 1990). While the two studies used different sampling and analytic techniques, the comparison (in Table 2.6) shows that most metals have significantly lower concentrations in the NSSS than in the 40-city study, largely as a result of industrial pretreatment programs. The data on organic compounds is difficult to compare as the limits of detection between the two studies varied. EPA's 1991 report to Congress (EPA, 1991) also documents a reduction in sludge metal concentrations from about 1985 to 1990 in a number of POTWs across the country.
For a long time, wastewater treatment authorities in the United States managed land applications of sewage sludge with little governmental attention. Early regulations governing sewage sludge disposal were developed by state public health agencies with the intention of controlling infectious disease. Although federal guidelines for land application of sewage sludge were proposed as early as 1974, comprehensive federal regulations did not exist until 1993. EPA first developed sludge management regulations under the 1972 Federal Water Pollution Control Act to prevent sludge-borne pollutants from entering the nation's navigable waters. In 1977, Congress amended the Act to add a new section, 405(d), that required EPA to develop regulations containing guidelines to (1) identify alternatives for sludge use and disposal; (2) specify what factors must be accounted for in determining the methods and practices applicable to each of these identified uses; and (3) identify concentrations of pollutants that would interfere with each use. In 1987, Congress amended section 405 again and established a timetable for developing sewage sludge use and disposal guidelines. Through this amendment, Congress directed EPA to: (1) identify toxic pollutants that may be present in sewage sludge in concentrations that may affect the public health and the environment and (2) promulgate regulations that specify acceptable management practices and numerical concentration limits for these pollutants in sludge.
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TABLE 2.3 Metal Loadings in Raw Wastewater (in kg/day) Entering the Chicago Area Treatment Facilities in Response to Pretreatment Programs
Cd
Cr
Cu
Pb
Ni
Zn
1971
398
5,197
2,166
2,049
2,443
6,972
1972
343
3,321
1,996
1,793
1,377
4,641
1973
301
2,463
961
1,063
957
4,260
1974
213
1,894
652
735
643
3,403
1975
113
1,522
538
497
386
2,537
1976
132
1,527
685
368
416
2,400
1977
168
1,422
588
536
436
2,587
1984
121
1,185
949
396
702
2,322
Note: Table 2.3 combines data from two different POTWs within the Metropolitan Sewerage District of Greater Chicago. Pretreatment programs began in 1972.
SOURCE: Adapted from Page et al., 1987.
The intent of the 1987 amendment was to ''adequately protect human health and the environment from any reasonably anticipated adverse effect of each pollutant" [Section 405 (d)(2)(D)]. Section 405 also states that any permit issued to a POTW or other treatment works for wastewater discharge should specify technical standards for sludge use or disposal. The Standards for the Use and Disposal of Sewage Sludge, Code of Federal Regulations, Title 40, Part 503 (EPA, 1993) were promulgated in 1993, and are collectively referred to in this report as the "Part 503 Sludge Rule."
Irrigation with Reclaimed Water
Crop Irrigation
The nation encompasses 930.8 million hectares (2,300 million acres) of land, of which 125 million hectares (309 million acres), or 14 percent were used to grow crops in 1993 (USDA, 1992). Figure 2.1 shows the proportion of cropland devoted to different categories of crops. The category "fresh food" includes such produce crops as broccoli, potatoes, or fruit that are bought and consumed fresh, and this is the smallest category in terms of total acreage. Other food crops, such as small grains, vegetables used for commercial processing (canning and processing), peanuts, sugar beets, and sugar cane, are grown on roughly 24 percent of cropland. Crops for domestic animal consumption—feed and hay—occupy the bulk of cropland. Farmers
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TABLE 2.4 Metal Concentrations in Digested Sludge Filter Cake (in mg/kg dry weight) at the Back River POTW, Baltimore, Maryland in Response to Pretreatment Programs
Year
Cd
Cu
Pb
Ni
Zn
1978
51
2,750
680
423
5,000
1979
23
2,540
539
397
3,540
1980
18
2,840
433
381
3,400
1981
19
2,070
493
374
3,410
1982
18
1,110
398
193
2,360
1983
23
1,060
324
214
2,620
1984
26
1,010
372
266
2,750
1985
22
681
346
126
2,030
Note: Source identification began in 1980 and source reduction began in 1981. Based on monthly composites in early years, then biweekly and weekly.
SOURCE: Adapted from Page et al., 1987.
often practice crop rotation, so a variety of crops may be grown on the same piece of land over a period of several years.
Irrigated crop production expanded in the United States from nearly 7.7 million hectares (19 million acres) in 1945 to more than 20.5 million hectares (51 million acres) in 1978. The total amount of irrigated cropland dropped by 1987 to 18.8 million hectares (46 million acres) (Figure 2.2, Council for Agricultural Science and Technology, 1992; USDA, 1992). About 15 percent of harvested crops are grown on the 5 percent of farmland that is irrigated. Irrigation is essential in semiarid and arid regions to produce of many crops including orchard crops, and vegetables (Figure 2.3). Irrigated crops represent 38 percent of the total revenue from crop production in the United States (Bajwa, et al. 1992).
Demand for Irrigation Water
Much of the nation's water withdrawal is used for crop irrigation. In 1990, crop irrigation accounted for 518 million m3/day (137,000 million gal/day) of water or 41 percent of all fresh water withdrawn for all uses from well and surface water (Solley et al., 1993). Irrigation is also a highly consumptive use of water; about 56 percent of the quantity withdrawn is lost to evaporation and plant transpiration, and so is not available as return-flow to surface waters. By comparison, domestic, commercial, industrial and mining uses consume an average of 17 percent of the water withdrawn for these purposes.
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TABLE 2.5 Metal Concentrations in Sludges (in mg/kg dry weight) at Two Philadelphia Wastewater Treatment Plants in Response to Pretreatment Programs
Year
Cd
Cu
Pb
Ni
Zn
Southwest
1974
31
825
1,540
100
3,043
1976
27
1,110
2,710
103
2,650
1977
27
1,400
2,170
185
3,940
1978
16
1,020
1,800
275
4,050
1980
18
986
740
98
2,780
1981
25
971
562
117
2,300
1982
20
940
1,030
113
2,440
1983
12.5
736
421
79
1,700
1984
14.3
1,140
427
111
1,830
1985
15.0
880
373
80
1,730
Northeast
1974
108
1,610
2,270
391
5,391
1976
97
2,240
2,570
372
5,070
1977
71
2,320
2,680
459
3,920
1978
57
1,240
1,620
319
5,910
1980
26
1,210
728
275
3,890
1982
14
985
423
185
2,570
1983
10.9
1,020
351
130
2,110
1984
12.4
1,200
360
130
1,980
1985
17.3
1,270
382
187
2,100
Note: Source identification began in 1976. Liquid sludge analyzed until 1982, and sludge filter cake from 1983 on.
No data available for 1975 and 1979.
SOURCE: Page et al., 1987.
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TABLE 2.6 Comparison of Organics and Trace Elements From the 40-Cities Study Conducted in the Late 1970s and the National Sewage Sludge Survey (NSSS) Conducted in the Late 1980s
Organic Pollutants (µg/kg, unless noted by * = mg/kg)
Percent Detection
Mean Values
40 Cities
NSSS
40 Cities
NSSS
Aldrin
16
3
6.4
1.9
Benzene
93
0
1782
—
Benzo(a)pyrene
21
3
138
—
Bis(2-ethylhexyl)phthalate
100
62
155*
74.7*
Chlorodane
16
0
6.4
—
Dieldrin
16
4
6.4
—
Heptachlor
16
0
6.4
—
Hexachlorobenzene
16
0
155
—
Hexachlorobutadiene
5
0
23
—
Lindane
16
0
6.4
—
Dimethylnitrosamine
5
0
57
—
PCB's
N/A
N/A
—
—
Toxaphene
16
0
6.4
—
Trichloroethene
84
1
8139
—
DDD/DDE/DDT
N/A
N/A
—
—
Trace Elements
(mg/kg, values in O denote composited means by mass in NSSS)
Arsenic
100
60
6.7
9.9
(10)
Beryllium
100
23
1.67
0.4
(1)
Cadmium
100
69
69.0
7.0
(22)
Chromium
100
91
429
119
(268)
Copper
100
100
602
741
(730)
Lead
100
80
969
134
(205)
Mercury
100
63
2.8
5.2
(3)
Molybdenum
75
53
17.7
9.2
(11)
Nickel
100
66
135.1
42.7
(70)
Selenium
100
65
7.3
5.2
(5)
Zinc
100
100
1594
12
(1550)
SOURCE: EPA, 1990.
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TABLE 2.8 The Cost of Fertilizer As a Percentage of All Costs for the Production of Some Food Crops
Crop
Percent of Total Costs
State
Fresh apples
0.6
Pennsylvania
Processing apples
1.3
Pennsylvania
Green tomatoes
2.4
California
Ripe tomatoes
2.6
Pennsylvania
Carrots
3.4
California
Fresh broccoli
4.1
California
Iceberg lettuce
4.9
California
Sweet corn
6.8
California
Tomatoes for processing
10.2
California
Potatoes
11.5
Pennsylvania
SOURCES: University of California Imperial County Cooperative Extension 1992; Harper, 1993.
Use of Sewage Sludge in Agriculture
Potential Role of Sewage Sludge in Crop Production
Based on estimates of the amount of solids produced in typical primary and secondary wastewater treatment processes (Metcalf and Eddy, 1991), the national production of sewage sludge is approximately 7 million metric tons/year. Secondary and higher levels of treatment account for 5.3 million metric tons/year, and the remainder comes from coastal discharges and sewage ponds (EPA, 1993). The quantity of sewage sludge is expected to increase as a greater percentage of the population is served by sewers and as advanced wastewater treatment processes are brought on-line (refer back to Figure 1.1). Currently, 36 percent of sewage sludge is applied to the land for several beneficial purposes, such as agriculture, turfgrass production, or reclamation of surface mining areas. 38 percent is landfilled, 16 percent is incinerated, and the remainder is surface disposed by other methods (EPA, 1993). With the promulgation of the Part 503 Sludge Rule, EPA encouraged agricultural use of sewage sludge.
From a national perspective, sludge has very little impact on agriculture. If all sludge produced in the United States was used agriculturally and applied according to agronomic nitrogen requirements, it would only require an estimated 1.59 percent of the nation's cropland (assuming that the average concentration of available nitrogen in the sludge is 4 percent dry weight and it is applied at 100 kg nitrogen/ha/yr).
Regional and local availability of farmland will, however, affect the potential for increased agricultural use of treated municipal sludge. The ratio of available farmland to sludge produced is an initial consideration in agricultural use of sludge. North Dakota, for example, would require only 0.05 percent of its agricultural land to take up all sludge produced in the state at agronomic rates for nitrogen. The Madison Metro Sewerage District has applied anaerobically digested sludge to private farmland since 1974, and the demand for sludge outstrips
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the supply (Taylor and Northouse, 1992). Rhode Island, on the other hand, would need to utilize 100 percent of its cropland to use up its sludge supply; because that is unlikely, other use or disposal options are required. Table 2.9 shows a comparison of the amounts of cropland required to accommodate in-state sludge applications at agronomic rates. Unlike wastewater effluents, sludge can be transported further distances. For example, contractors are currently shipping some of New York City's treated sludge to northeastern Texas and eastern Colorado for cropland application. Boston, Massachusetts ships a portion of its sludge in the form of heat-dried pellets to Florida for application to cropland and pastures. Some of the sludge from the Los Angeles Basin is being transported by truck for cropland application in Yuma, Arizona.
The cost of transporting sludge for land application must be weighed against the cost and environmental consequence of other sludge disposal options on a case by case basis. If wastewater treatment authorities in urban centers cannot overcome the variety of obstacles to use sludge within reasonable transportation distances, they face the consequences of long distance transportation and its associated costs. These geographical and economical constraints on land application create uncertainty over how much of the nation's sewage sludge will be applied on cropland in the long run.
From the farmer's perspective, other factors limit agriculture use of sewage sludge. Sewage sludge is inherently more difficult to use than chemical fertilizers. In part, this is because the composition of plant nutrients and trace elements vary due to differences among types of sludges (e.g., different water contents or treatment processes) and differences among municipalities and their industrial contributors. The composition of commercial fertilizers are formulated to meet crop requirements. Some have argued that any cost savings derived from substituting sludges for chemical fertilizers may be insignificant (White-Stevens, 1977) and that unless the waste generators offer them payment, the financial incentive for farmers to apply sewage sludge to cropland may be marginal. Others point out that sludge has significant nutrient value, which can range from $100 to $140 per acre (EPA, 1994), and that its effect on soil physical properties can increase crop yield (e.g., Logdson, 1993). Generally, the POTW makes arrangements for hauling and spreading sewage sludge on farmland.
Ecological Linkages Between Urban and Agricultural Systems
The land application of sewage sludge can ecologically link nutrient usage within urban and rural landscapes (Millner, 1994). If nutrients and organic matter are returned to agricultural soil via land application of sludges, the need to supplement the agroecosystem in terms of nutrients will diminish. In this sense, cropland recycling of sewage sludge close to its urban source can conserve energy as does the recycling of crop residues and farm animal manures.
In natural ecosystems, the external inputs to primary food production are solar energy and water (Figure 2.7). Natural ecosystem productivity is sustained through the recycling of nutrients extracted by primary producers (plants) and made available again in the process of organic matter mineralization. In contrast, conventional crop production is enhanced by external inputs of energy, water, nutrients, and chemical herbicides and pesticides (Figure 2.7). The capacity of modern agroecosystems for natural feedback and regulation has been greatly reduced in order to increase crop yields (Risser, 1985; Barrett et al., 1990). When these external inputs
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TABLE 2.9 Amount of Cropland Required to Accommodate In-state Sludge Applications at Agronomic Rates
State
Populationa (millions)
Sludge Producedb (thousands of metric tons)
Cropland Requiredc (thousands of hectares)
Croplandd (thousands of hectares)
Percent of State Cropland
New England
Maine
1.24
24.78
9.91
158
6.3
New Hampshire
1.12
22.38
8.95
43
20.8
Vermont
0.57
11.39
4.56
177
2.6
Massachusetts
6.01
120.10
48.0
79
60.8
Rhode Island
1.00
19.98
7.99
8
100.0
Connecticut
3.27
65.35
26.1
62
42.1
Middle Atlantic
New York
18.70
363.63
149.4
1,539
9.7
New Jersey
7.87
157.27
62.9
166
37.9
Pennsylvania
12.05
240.80
96.3
1,716
5.6
East North Central
Ohio
11.10
221.82
88.7
3,921
2.3
Indiana
5.71
114.11
45.6
4,335
1.1
Illinois
11.70
233.81
93.5
8,162
1.1
Michigan
9.47
189.25
75.7
2,591
2.9
Wisconsin
5.04
100.72
40.3
3,339
1.2
West North Central
Minnesota
4.52
90.33
36.1
7,086
Iowa
2.81
56.15
22.5
8,359
0.51
Missouri
5.23
104.52
41.8
4,987
0.27
North Dakota
0.63
12.59
5.04
10,305
0.84
South Dakota
0.71
14.19
5.68
6,889
0.05
Nebraska
1.60
31.99
12.8
7,285
0.08
Kansas
2.53
50.58
20.2
10,433
0.18
South Atlantic
Delaware
0.70
13.99
5.6
202
2.8
Maryland
4.96
99.12
39.6
578
6.9
Dist. of Columbia
0.58
11.59
4.64
0
0.0
Virginia
6.49
129.69
51.9
1,081
4.8
West Virginia
1.82
36.36
14.5
274
5.3
North Carolina
6.94
138.69
55.5
1,647
3.4
South Carolina
3.64
72.74
29.1
786
3.7
Georgia
6.92
138.29
55.3
1,516
3.6
Florida
13.68
273.38
109.4
931
11.8
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State
Populationa (millions)
Sludge Producedb (thousands of metric tons)
Cropland Requiredc (thousands of hectares)
Croplandd (thousands of hectares)
Percent of State Cropland
East South Central
Kentucky
3.79
75.74
30.3
1,923
1.6
Tennessee
5.10
101.92
40.8
1,731
2.4
Alabama
4.19
83.73
33.5
959
3.5
Mississippi
2.64
52.76
21.1
1,635
1.3
West South Central
Arkansas
2.42
48.36
19.3
2,711
0.71
Louisiana
4.29
85.73
34.3
1,612
2.1
Oklahoma
3.23
64.55
25.8
3,871
0.67
Texas
18.03
360.31
144.1
7,911
1.8
Mountain
Montana
0.84
16.79
6.72
6,200
0.11
Idaho
1.10
21.98
8.79
2,065
0.43
Wyoming
0.47
9.39
3.76
870
0.43
Colorado
3.56
71.14
28.5
3,514
0.81
New Mexico
1.61
32.17
12.9
493
2.6
Arizona
3.93
78.54
31.4
427
7.4
Utah
1.86
37.17
14.9
517
2.9
Nevada
1.39
27.78
11.1
236
4.7
Pacific
Washington
5.25
104.91
42.5
2,701
1.6
Oregon
3.03
60.55
24.2
1,495
1.6
California
31.21
623.69
249.5
3,516
7.1
Alaska
0.60
11.99
4.8
13
36.9
Hawaii
1.17
23.38
9.35
66
14.2
a Estimate of July 1993 population from U.S. Census Bureau.
b Sludge production estimates assume 75% of population is sewered and produces .073 kg of sludge per person per day.
c State cropland required if all sludge produced in-state were to be applied at agronomic rates using the assumption of 4 percent available nitrogen dry weight and an application rate of 100 kilograms per hectare per year.
d 1987 land utilization from U.S. Department of Agriculture, 1992.
exceed the capacity of the agroecosystem to accommodate them, the result is an increase in system outputs of both natural and unnatural byproducts that can cause environmental harm. Society often bears the financial costs required to restore environmental quality and to maintain high crop yields.
Frequently, cropland is removed from crop production and permitted to lie fallow for several years. These fallow fields are often referred to as old-field communities or old-field ecosystems. Various studies on the effects of sludge application to old-field ecosystems have focused on ecological trophic levels, including producers (Maly and Barrett, 1984; Hyder and
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FIGURE 2.7 Diagrams depicting a) natural, and b) human inputs into agroecosystems.
SOURCE: Adapted from Barrett et al., 1990.
Barrett, 1986), primary consumers (Anderson and Barrett, 1982; Brewer et al., 1994), secondary consumers (Brueske and Barrett, 1991), detritivores (Kruse and Barrett, 1985; Levine et al., 1989) and decomposers (Sutton et al., 1991,; Brewer et al., 1994). Thus far, the research indicates that old-field ecosystems are ecologically safe and economically viable sites for sludge disposal (Maly and Barrett, 1984; Carson and Barrett, 1988; Levine et al., 1989; and Brewer
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et al., 1994). These old-fields may again revert to cropland usage due to improved productivity and soil conditioning.
Summary
With continuing advancement in wastewater treatment technology and increasingly stringent wastewater discharge requirements, treated wastewater effluents produced by municipal treatment plants in the United States have achieved consistent high water quality and are increasingly being considered for nonpotable reuse. In the semiarid and arid western states, treated wastewater has been used as a new source of water to help alleviate shortages faced by water-deficient communities. More recently, the need to meet local minimum in-stream water quality limits when treated effluents are discharged into surface water bodies has motivated many municipalities to consider effluent irrigation.
The chemical composition of most treated effluents is within the range defined by accepted irrigation water quality criteria and is comparable to that of water commonly used in crop and landscaping irrigation. At present, treated municipal wastewater probably accounts for much less than one percent of national irrigation water requirements, and it is likely that the level of agricultural use will not significantly increase. Effective barriers to increased use include the limited availability of irrigated agricultural land near municipal centers, and the competition with more cost-effective, higher-value urban uses for reclaimed water.
Much of the wastewater in the United States is produced in regions where agricultural irrigation is not needed or is only occasionally needed. Judging from the acreage of irrigated cropland compared to the availability of reclaimed wastewater and the current pattern of reclaimed water use, only a very small fraction of the food crops in the United States would ever be exposed to reclaimed wastewater.
Treated sewage sludge is an end product of municipal wastewater treatment and contains many of the pollutants that are removed from the influent wastewater during treatment. The nutrients and organic matter in treated sludge resembles those in other organic waste-based soil amendments such as animal manure and organic composts. The use of sludge as a soil conditioner serves to improve soil physical properties in a manner similar to other organic-based soil amendments. While sewage sludge has been land applied since it was first produced, most of the early operations were carried out with little regard for possible adverse impacts to soil, crops, or ground water. In the past two decades, more emphasis has been placed on applying treated sludges to cropland at agronomic rates.
The financial incentive for farmers to use sewage sludge in crop production is debatable. Fertilizers presently account for a relatively small percentage of total crop production costs, and sewage sludge may be more difficult to use than commercial fertilizer. However, the nutrient value of sludge is promoted as a benefit, and the POTW often provides for transport and application of sludge for free or at a nominal cost.
Community-wide source control and industrial wastewater pretreatment programs have resulted in significant reduction of toxic pollutants in wastewater and thus in sewage sludge. Still, land application of treated effluents and treated sludge will increase the level of toxic
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chemicals and pathogens in the soil. The public is concerned about pollutants and pathogens that may contaminate food crops or be transported elsewhere in the environment.
If the total amount of municipal sludge produced in the United States were applied to cropland at agronomic rates, less than 2 percent of the nation's cropland would be necessary to accept it. However, there are some regions where limited cropland acreage may constrain sludge management options. A lack of available disposal options near densely populated urban centers has forced many municipalities to seek distant disposal and land application sites at considerable costs. Given these economic and geographic constraints, it is not likely that all of the sewage sludge will be applied to cropland in the foreseeable future, and thus only a very small percentage of the food crops grown in the United States would ever be exposed to sewage sludge.
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Representative terms from entire chapter:
municipal wastewater
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