G Land Treatment Systems

Land treatment systems include slow rate (SR), overland flow (OF), and soil aquifer treatment (SAT) or rapid infiltration (RI). In addition, the on-site soil absorption systems discussed in Chapter 10 utilize soil treatment mechanisms.

8.1 TYPES OF LAND TREATMENT SYSTEMS

The process of land treatment is the controlled application of wastewater to soil to achieve treatment of constituents in the wastewater. All three processes use the natural physical, chemical, and biological mechanisms within the soil-plant-water matrix. The SR and SAT processes use the soil matrix for treatment after infiltration of the wastewater, the major difference between the processes being the rate at which the wastewater is loaded onto the site. The OF process uses the soil surface and vegetation for treatment, with limited percolation, and the treated effluent is collected as surface runoff at the bottom of the slope. The characteristics of these systems are compared in Table 8.1 and the treatment performance expectations were summarized in Table 1.3 in Chapter 1.

8.1.1 Slow-Rate Systems

The slow rate process is the oldest and most widely used land treatment technology. The process evolved from "sewage farming" in Europe in the sixteenth century to a recognized wastewater treatment system in England in the 1860s (Jewell and Seabrook, 1979). By the 1880s, the United States had a number of slow-rate systems. In a survey of 143 wastewater facilities in 1899, slow rate land treatments systems were the most frequently used form of treatment (Rafter, 1899). Slow rate land treatment was rediscovered at Penn State in the mid-1960s (Sopper and Kardos, 1973). By the 1970s, both the U.S. Environmental Protection Agency (USEPA) and the U.S. Corps of Engineers had invested in land treatment research and development (Pound and Crites, 1973; Reed, 1972). By the late 1970s, a number of long-term effects studies on slow-rate systems had been conducted (Reed and Crites, 1984). A list of selected municipal slow-rate systems is presented in Table 8.2. A large SR system at Dalton, Georgia, occupies 4605 acres of sprinkler irrigated forest, as shown in Figure 8.1 (Crites et al., 2001).

8.1.2 Overland Flow Systems

The overland flow process was developed to take advantage of slowly permeable soils such as clays. Treatment occurs in OF systems as wastewater flows down vegetated, graded-smooth, gentle slopes that range from 2 to 8% in grade. A

TABLE 8.1

Characteristics of Land Treatment Systems

Characteristic

Application method

Preapplication treatment Annual loading (ft/yr) Field area (ac/mgd) Use of vegetation

Disposition of applied wastewater

Slow Rate (SR) Overland Flow (OF)

Soil Aquifer Treatment (RI)

Sprinkler or surface Sprinkler or surface Usually surface

Ponds or secondary Fine screening or primary Ponds or secondary

2-18

60-560

Nutrient uptake and crop revenue

Evapotranspiration and percolation

10-70

16-112

Erosion control and habitat for microorganisms

Surface runoff, evapotranspiration, some percolation

18-360 3-60 Usually not used

Percolation, some evaporation

TABLE 8.2

Selected Municipal Slow-Rate Land Treatment Systems

TABLE 8.2

Selected Municipal Slow-Rate Land Treatment Systems

Flow

System

Location

(mgd)

Area (ac)

Application Method

Bakersfield, California

19.4

5088

Surface irrigation

Clayton County, Georgia

20.0

2370

Solid-set sprinklers

Dalton, Georgia

33.0

4605

Solid-set sprinklers

Lubbock, Texas

16.5

4940

Center-pivot sprinklers

Mitchell, South Dakota

2.45

1284

Center-pivot sprinklers

Muskegon County, Michigan

29.2

5335

Center-pivot sprinklers

Petaluma, California

5.3

555

Hand-move, solid-set sprinklers

Santa Rosa, California

20.0

6362

Solid-set sprinklers

schematic showing both surface application and sprinkler application is presented in Figure 8.2. The treated runoff is collected at the bottom of the slope. The process was pioneered in the United States by the Campbell Soup Company, first at Napoleon, Ohio, in 1954 and subsequently at Paris, Texas (Gilde et al., 1971). Research was conducted on the OF process using municipal wastewater at Ada, Oklahoma (Thomas et al., 1974) and at Utica, Mississippi (Carlson et al., 1974). As a result of this and other research (Martel, 1982; Smith and Schroeder, 1985), over 50 municipal OF systems have been constructed for municipal wastewater treatment. A list of selected municipal overland flow systems is presented in Table 8.3.

FIGURE 8.1 Typical sprinkler irrigation system at the forested slow rate site at Dalton, Georgia.

Evapotranspiration

Evapotranspiration

TABLE 8.3

Municipal and Industrial Overland Flow Systems in the United States

Municipal Systems Industrial Systems

Alma, Arkansas Alum Creek Lake, Ohio Beltsville, Maryland Carbondale, Illinois Cleveland, Michigan Corsicana, Texas Davis, California

Chestertown, Maryland El Paso, Texas Middlebury, Indiana Napoleon, Ohio Paris, Texas Rosenberg, Texas Woodbury, Georgia

Falkner, Michigan Gretna, Virginia Heavener, Oklahoma Kenbridge, Virginia Lamar, Arkansas Minden-Gardnerville, Nevada Mt. Olive, New Jersey Newman, California Norwalk, Iowa Raiford, Florida Starke, Florida Vinton, Louisiana

8.1.3 Soil Aquifer Treatment Systems

Soil aquifer treatment is a land treatment process in which wastewater is treated as it infiltrates the soil and percolates through the soil matrix. Treatment by physical, chemical, and biological means continues as the percolate passes through the vadose zone and into the groundwater. Deep permeable soils are typically used. Applications are intermittent, usually to shallow percolation basins. Continuous flooding or ponding has been practiced, but less complete treatment usually results because of the lack of alternate oxidation/reduction conditions. A typical layout of SAT basins is shown in Figure 8.3 (also see Table 8.4). Vegetation is usually not a part of an SAT systems, because loading rates are too high for nitrogen uptake to be effective. In some situations, however, vegetation can play an integral role in stabilizing surface soils and maintaining high infiltration rates (Reed et al., 1985).

CONTAINMENT I

PREAPPLICATION TREATMENT

EMERGENCY STORAGE

#7 ^

^ #6

#4

#3 ^

^ #2

INFILTRATION BASINS

FIGURE 8.3 Typical layout of soil aquifer treatment basins.

TABLE 8.4

Selected Soil Aquifer Treatment Systems

Hydraulic

Location Loading(ft/yr)

Brookings, South Dakota 40

Calumet, Michigan 115 Darlington, South Carolina 92 Fresno, California 44 Hollister, California 50

Lake George, New York 190

Los Angeles County Sanitary 330

District, California

Orange County, Florida 390

Tucson, Arizona 331

West Yellowstone, Montana 550

8.2 SLOW-RATE LAND TREATMENT

Slow-rate systems can encompass a wide variety of different land treatment facilities ranging from hillside spray irrigation to agricultural irrigation, and from forest irrigation to golf course irrigation. The design objectives can include wastewater treatment, water reuse, nutrient recycling, open space preservation, and crop production.

8.2.1 Design Objectives

Slow-rate systems can be classified as type 1 (slow infiltration) or type 2 (crop irrigation), depending on the design objective. When the principal objective is wastewater treatment, the system is classified as type 1. For type 1 systems, the land area is based on the limiting design factor (LDF), which can be either the soil permeability or the loading rate of a wastewater constituent such as nitrogen. Type 1 systems are designed to use the most wastewater on the least amount of land. The term slow infiltration refers to type 1 systems being similar in concept to rapid infiltration or soil aquifer treatment but having substantially lower hydraulic loading rates. Type 2 systems are designed to apply sufficient water to meet the crop irrigation requirement. The area required for a type 2 system depends on the crop water use, not on the soil permeability or the wastewater treatment needs. Water reuse and crop production are the principal objectives. The area needed for type 2 systems is generally larger than for a type 1 system for the same wastewater flow. For example, for 1 mgd (3785 m3/d) of wastewater flow, a type 1 system would typically require 60 to 150 ac (24 to 60 ha) as compared to the 200 to 500 ac (80 to 200 ha) for a type 2 system.

8.2.1.1 Management Alternatives

Unlike SAT and overland flow, slow-rate systems can be managed in several different ways. The other two land treatment systems require that the land be purchased and the system managed by the wastewater agency. For slow-rate systems, the three major options are (1) purchase and management of the site by the wastewater agency, (2) purchase of the land and leasing it back to a farmer, and (3) contracts between the wastewater agency and farmers for use of private land for the slow rate process. The latter two options allow farmers to manage the slow rate process and harvest the crop. A representative list of small SR systems that use each of the different management alternatives is presented in Table 8.5.

8.2.2 Preapplication Treatment

Preliminary treatment for an SR system can be provided for a variety of reasons including public health protection, nuisance control, distribution system protection, or soil and crop considerations. For type 1 systems, preliminary treatment, except for solids removal, is de-emphasized because the SR process can usually

TABLE 8.5

Management Alternatives Used in Selected Slow-Rate Systems

Purchase and Agency Purchase and

Management by Agency

Flow (mgd)

Lease to Farmer

Flow (mgd)

Farmer Contract

Flow (mgd)

Dinuba, California

1.5

Coleman, Texas

0.4

Camarillo, California

3.8

Fremont, Michigan

0.3

Kerman, California

0.5

Dickinson, North Dakota

1.5

Kennett Square, Pennsylvania

0.05

Lakeport, California

0.5

Mitchell, South Dakota

2.4

Lake of the Pines, California

0.6

Modesto, California

20.0

Quincy, California

0.75

Oakhurst, California

0.25

Perris, California

0.8

Petaluma, California

4.2

West Dover, Vermont

1.6

Winter, Texas

0.5

Sonoma Valley, California

2.7

Wolfeboro, New Hampshire

0.3

Santa Rosa, California

15.0

Sonora, California

1.2

Source: Adapted from Crites, R.W. and Tchobanoglous, G., Small and Decentralized Wastewater Management Systems, McGraw-Hill, New York, 1998.

TABLE 8.6

Pretreatment Guidelines for Slow-Rate Systems

Level of Pretreatment

Acceptable Conditions

Primary treatment

Acceptable for isolated locations with restricted public access Acceptable for controlled agricultural irrigation, except for human food crops to be eaten raw Acceptable for application in public access areas such as parks and golf courses

Biological treatment by lagoons or in-plant processes, plus control of fecal coliform count to less than 1000 MPN per 100 mL

Biological treatment by lagoons or in-plant processes, with additional BOD or SS control as needed for aesthetics, plus disinfection to log mean of 200 MPN per 100 mL (USEPA fecal coliform criteria for bathing waters)

Note: MPN, most probable number; BOD, biological oxygen demand; SS, suspended solids.

Source: USEPA, Process Design Manual for Land Treatment of Municipal Wastewater, EPA 625/1-81-013, U.S. Environmental Protection Agency, Cincinnati, OH, 1981.

achieve final water quality objectives with minimal pretreatment. Public health and nuisance control guidelines for type 1 SR systems have been issued by the EPA (USEPA, 1981) and are given in Table 8.6. Type 2 systems are designed to emphasize reuse potential and require greater flexibility in the handling of waste-water. To achieve this flexibility, preliminary treatment levels are usually higher. In many cases, type 2 systems are designed for regulatory compliance following preliminary treatment so irrigation can be accomplished by other parties such as private farmers.

8.2.2.1 Distribution System Constraints

Preliminary treatment is generally required to prevent problems of capacity reduction, plugging, and localized generation of odors in the distribution system. For this reason, a minimum primary treatment (or its equivalent) is recommended for all SR systems to remove settleable solids and oil and grease. For sprinkler systems, it is further recommended that the size of the largest particle in the applied wastewater be less than one third the diameter of the sprinkler nozzle to avoid plugging.

8.2.2.2 Water Quality Considerations

The total dissolved solids (TDS) in the applied wastewater can affect plant growth, soil characteristics, and groundwater quality. Guidelines for interpretation of water quality for salinity and other specific constituents for SR systems are presented in Table 8.7. The term "restriction on use" does not indicate that the

TABLE 8.7

Guidelines for Interpretation of Water Quality

TABLE 8.7

Guidelines for Interpretation of Water Quality

Slight to

Problem and Related

No

Moderate

Severe

Constituent

Restriction

Restriction

Restriction

Crops Affected

Salinity as TDS (mg/L)

<4S0

450-2000

>2000

Crops in arid areas

affected by high

TDS; impacts vary

Permeability:

SAR = 0-3

TDS >4S0

130-450

<130

All crops

SAR = 3-6

TDS >770

200-770

<200

SAR = 6-12

TDS >1200

320-1200

<320

SAR = 12-20

TDS >1860

800-1860

<800

SAR = 20-40

TDS >3200

1860-3200

<1860

Specific ion toxicity:

Sodium (mg/L)

<70

>70

>70

Tree crops and woody

Chloride (mg/L)

<140

140-3S0

>3S0

ornamentals; fruit

Boron (mg/L)

<0.7

0.7-3.0

>3.0

trees and some field

Residual chlorine (mg/L)

<1.0

1.0-5.0

>5.0

crops; ornamental,

only if overhead sprinklers are used only if overhead sprinklers are used

Note: TDS, total dissolved solids; SAR, sodium adsorption ratio.

Source: Ayers, R.S. and Westcot, D.W., Water Quality for Agriculture, FAO Irrigation and Drainage Paper 29, Revision 1, Food and Agriculture Organization of the United Nations, Rome, 1985.

effluent is unsuitable for use; rather, it means there may be a limitation on the choice of crop or need for special management. Sodium can adversely affect the permeability of soil by causing clay particles to disperse. The potential impact is measured by the sodium adsorption ratio (SAR) which is a ratio of sodium concentration to the combination of calcium and magnesium. The SAR is defined in Equation 8.1.

where SAR Na Ca Mg

Sodium adsorption ratio (unitless). Sodium concentration (mEq/L; mg/L divided by 23). Calcium concentration (mEq/L; mg/L divided by 20). Magnesium concentration (mEq/L; mg/L divided by 12.15).

In type 2 SR systems the leaching requirement must be determined based on the salinity of the applied water and the tolerance of the crop to soil salinity. Leaching requirements range from 10 to 40% with typical values being 15 to 25%. Specific crop requirements for soil-water salinity must be used to determine the required leaching requirement (Reed and Crites, 1984; Reed et al., 1995).

8.2.2.3 Groundwater Protection

Most SR systems with secondary preapplication treatment are protective of the receiving groundwater. The concern over emerging chemical constituents, such as endocrine disruptors and pharmaceutical chemicals, has led to research on the ability of the soil profile to remove these trace organic compounds (Muirhead et al., 2003).

8.2.3 Design Procedure

A flowchart of the design procedure for slow-rate systems is presented in Figure 8.4. The procedure is divided into a preliminary and final design phase. Determinations made during the preliminary design phase include: (1) crop selection, (2) preliminary treatment, (3) distribution system, (4) hydraulic loading rate, (5) field area, (6) storage needs, and (7) total land requirement. When the preliminary design phase is completed, economic comparisons can be made with other waste-water management alternations. The text will focus on preliminary or process design with references to detailed design procedures (Hart, 1975; Pair, 1983; USDA, 1983; USEPA, 1981).

8.2.4 Crop Selection

The selection of the type of crop in a slow-rate system can affect the level of preliminary treatment, the selection of the type of distribution system, and the hydraulic loading rate. The designer should consider economics, growing season, soil and slope characteristics, and wastewater characteristics in selecting the type of crop. Forage crops or tree crops are usually selected for type 1 systems, and higher value crops or landscape vegetation are often used in type 2 systems.

8.2.4.1 Type 1 System Crops

In type 1 SR systems, the crop must be compatible with high hydraulic loading rates, have a high nutrient uptake capacity, a high consumptive use of water, and a high tolerance to moist soil conditions. Other characteristics of value are tolerance to wastewater constituents (such as TDS, chloride, boron) and limited requirements for crop management. The nitrogen uptake rate is a major design variable for design of a type 1 system. Typical nitrogen uptake rates for forage, field, and tree crops are presented in Table 8.8. The largest nitrogen removal can be achieved with perennial grasses and legumes. Legumes, such as alfalfa, can

Haccp Voorbeeld Visverwerkingsbedrijf
FIGURE 8.4 Flowchart of the design procedure for slow rate land treatment.

fix nitrogen from the air; however, they will preferentially take nitrate from the soil solution if it is provided. The use of legumes (clovers, alfalfa, vetch) in type 1 systems should be limited to well-draining soils because legumes generally do not tolerate high soil moisture conditions. The most common tree crops for type 1 systems are mixed hardwoods and pines (Nutter et al., 1986). Tree crops provide revenue potential as firewood, pulp, or biomass fuel. Tree species with high growth response such as eucalyptus and hybrid poplars will maximize nitrogen uptake.

TABLE 8.8

Typical Nitrogen Uptake Values for Selected Crops

Nitrogen Uptake

Nitrogen Uptake

TABLE 8.8

Typical Nitrogen Uptake Values for Selected Crops

Nitrogen Uptake

Crop

(lb/acyr)

Crop

(lb/ac-

Forage crops

Eastern forest

Alfalfaa

200-600

Mixed hardwoods

200

Brome grass

115-200

Red pine

100

Coastal Bermuda grass

350-600

White spruce

200

Kentucky bluegrass

175-240

Pioneer succession

200

Quackgrass

210-250

Aspen sprouts

100

Orchard grass

220-310

Southern forest

Reed canary grass

300-400

Mixed hardwoods

250

Ryegrass

160-250

Loblolly pine

200-2

Sweet clovera

155

Lake states forest

Tall fescue

130-290

Mixed hardwoods

100

Field crops

Hybrid poplar

140

Barley

110

Western forests

Corn

155-180

Hybrid poplar

270

Cotton

65-100

Douglas fir

200

Grain sorghum

120

Potatoes

200

Soybeansa

220

Wheat

140

a Legume crops can fix nitrogen from the air but will take up most of their nitrogen from applied wastewater nitrogen.

Source: USEPA, Process Design Manual for Land Treatment of Municipal Wastewater, EPA 625/1-81-013, U.S. Environmental Protection Agency, Cincinnati, OH, 1981.

8.2.4.2 Type 2 System Crops

Crop irrigation or water reuse systems can use a broad variety of crops and landscape vegetation including trees, grass, field, and food crops. Field crops often include corn, cotton, sorghum, barley, oats, and wheat.

8.2.5 Hydraulic Loading Rates

Hydraulic loading rates for SR systems are expressed in units of in./wk (mm/wk) or ft/yr (m/yr). The basis of determination varies from type 1 to type 2.

8.2.5.1 Hydraulic Loading for Type 1 Slow-Rate Systems

The hydraulic loading rate for a type 1 system is determined by using the water balance equation:

where

Lw = Wastewater hydraulic loading rate (in./mo; mm/mo).

ET = Evapotranspiration rate (in./mo; mm/mo).

The water balance is generally used on a monthly basis. The design values for precipitation and evapotranspiration are generally chosen for the wettest year in 10, to be conservative. For slow-rate systems, the surface runoff (tailwater) is usually captured and reapplied. An exception is the forested type 1 system, where surface and subsurface seepage is allowed by the regulatory agency. Seepage (the surfacing of groundwater) may occur on or off the site without causing water quality problems.

The design percolation rate is based on the permeability of the limiting layer in the soil profile. For type 1 systems, the permeability is often measured in the field using cylinder infiltrometers, sprinkler infiltrometers, or the basin flooding technique. The range of soil permeability is usually contained in the detailed soil survey from Natural Resources Conservation Service (NRCS). Although the given range is often wide (0.2 to 0.6 in./hr; 5 to 15 mm/hr), the lower value is often used in preliminary planning. The design percolation rate is calculated from the soil permeability taking into account the variability of the soil conditions and the overall cycle of wetting (application) and drying (resting) of the site:

where

K = Permeability of limiting soil layer (in./hr; mm/hr).

0.04 to 0.10 = Adjustment factor to account for the resting period between applications and the variability of the soil conditions.

Using either NRCS permeability data or field test results, it is recommended that the daily design percolation rate should range from 4 to 10% of the total rate. Selection of the adjustment factor depends on the site and the degree of conser-vativeness desired. For most SR systems, the wetting period is 5 to 15% of a given month. If the soil is only wet for 5% of the time, then only that percent of the time (in a given month) should be used as percolation time. The 4% factor should be used when the soil type variation is large, when the wet/dry ratio is small (5% or less), and the soil permeability is less than 0.2 in./hr (5 mm/hr). The high percentages, up to 10%, can be used where soil permeabilities are higher, the soil permeability is more uniform, and the wet/dry ratio is higher than 7%.

8.2.5.2 Hydraulic Loading for Type 2 Slow-Rate Systems

For crop irrigation systems, the hydraulic loading rate is based on the crop irrigation requirements. The loading rate can be calculated using Equation 8.4:

where

Lw = Wastewater hydraulic loading rate (in./yr; mm/yr). ET = Crop evapotranspiration rate (in./yr; mm/yr). Pr = Precipitation rate (in./yr; mm/yr). LR = Leaching requirement (fraction). E = Irrigation efficiency (percent).

The leaching requirement depends on the crop, the total dissolved solids (TDS) of the wastewater, and the amount of precipitation. The leaching requirement is typically 0.10 to 0.15 for low TDS wastewater and a tolerant crop such as grass. For higher TDS wastewater (750 mg/L or more), the leaching requirement can range from 0.20 to 0.30. The irrigation efficiency is the fraction of the applied wastewater that corresponds to the crop evapotranspiration. The higher the efficiency, the less water that percolates through the root zone. Sprinkler systems usually have efficiencies of 70 to 80%, while surface irrigation systems usually have efficiencies of 65 to 75%.

8.2.6 Design Considerations

Design considerations for both types of SR systems are described in the following text. Considerations for nitrogen loading, organic loading, land requirements, storage requirements, distribution systems, application cycles, surface runoff control, and underdrainage are presented.

8.2.6.1 Nitrogen Loading Rate

The limiting design factor (LDF) for many SR systems is the nitrogen loading rate. The total nitrogen loading (nitrate nitrogen, ammonia nitrogen, and organic nitrogen) is important because the soil microorganisms will convert organic nitrogen to the plant-available inorganic forms. Limitations on the total nitrogen loading rate are based on meeting a maximum nitrate nitrogen concentration of 10 mg/L in the receiving groundwater at the boundary of the project (usually 20 to 100 ft or 6 to 30 m downgradient of the wetted field area). To make certain that the groundwater nitrate nitrogen concentration limit is met, the usual practice is to set the percolate nitrate nitrogen concentration at 10 mg/L prior to commingling of the percolate with the receiving groundwater.

The nitrogen loading rate must be balanced against crop uptake of nitrogen, denitrification, and the leakage of nitrogen with the percolate. The nitrogen balance is given in Equation 8.5:

TABLE 8.9

Denitrification Loss Factor for Slow-Rate Systems

Type of Wastewater

High-strength wastewater

Moderate-strength industrial wastewater Primary effluent Secondary effluent Tertiary effluent

Carbon/Nitrogen Ratio

Warm Climate f Factor

0.25

0.15

Cold Climate f Factor

0.25

Source: Adapted from Crites, R.W. and Tchobanoglous, G., Small and Decentralized Wastewater Management Systems, McGraw-Hill, New York, 1998.

where

Ln = Nitrogen loading rate (lb/ac-yr; kg/ha-yr). U = Crop uptake of nitrogen (lb/ac-yr; kg/ha-yr).

f = Fraction of applied nitrogen lost to nitrification/denitrification, volatilization, and soil storage (see Table 8.9). A = Conversion factor (0.23; 10.0). Cp = Concentration of nitrogen in percolate (mg/L). P = Percolate flow (in./yr; m/yr).

By combining the nitrogen balance and water balance equation, the hydraulic loading rate that will meet the nitrogen limits can be calculated using Equation 8.6:

where Lwn is the hydraulic loading rate controlled by nitrogen (in./yr; m/yr), Cw is the concentration of nitrogen in the applied wastewater (mg/L), and the other terms are as defined previously.

Crop uptake of nitrogen can be estimated from Table 8.8. The fraction of applied nitrogen that is lost to denitrification, volatilization, and soil storage depends on the wastewater characteristics and the temperature. The fraction will be highest for warm climates and high-strength wastewaters with carbon-to-nitrogen ratios of 20 or more (see Table 8.9).

TABLE 8.1G

BOD Loading Rates at Industrial Slow-Rate Systems

TABLE 8.1G

BOD Loading Rates at Industrial Slow-Rate Systems

Location

Industry

BOD Loading Rate, Cycle Average (lb/ac-d)

Almaden Winery; McFarland, California

Winery stillage

420

Anheuser-Busch; Houston, Texas

Brewery

360

Bronco Wine; Ceres, California

Winery

128

Citrus Hill; Frostproof, Florida

Citrus

448

Contadina; Hanford, California

Tomato processing

84-92

Frito-Lay; Bakersfield, California

Potato processing

84

Harter Packing; Yuba City, California

Tomato and peach processing

150-351

Hilmar Cheese; Hilmar, California

Cheese processing

420

Ore-Ida Foods; Plover, Wisconsin

Potato processing

190

SK Foods; Lemoore, California

Tomato processing

210

TRI Valley Growers; Modesto, California

Tomato processing

200

Source: Data from Crites and Tchobanoglous (1998) and Smith and Murray (2003).

Source: Data from Crites and Tchobanoglous (1998) and Smith and Murray (2003).

8.2.6.2 Organic Loading Rate

Organic loading rates do not limit municipal SR systems but may be important for industrial SR systems. Loading rates for biological oxygen demand (BOD) often exceed 100 lb/ac-d (110 kg/ha-d) and occasionally exceed 300 lb/ac-d (330 kg/ha-d) for SR systems applying screened food processing and other high-strength wastewater. A list of industrial SR systems with organic loading rates in the above range is presented in Table 8.10. Odor problems have been avoided in these systems by providing adequate drying times between wastewater applications. Organic loading rates beyond 450 lb/ac-d (500 kg/ha-d) of BOD should generally be avoided unless special management practices are used (Reed et al., 1995). Procedures for managing organic loadings from high-strength industrial wastewater are presented in Section 8.6.

8.2.6.3 Land Requirements

The land requirements for a slow-rate system include the field area for application, plus land for roads, buffer zones, storage ponds, and preapplication treatment. The area can be calculated using Equation 8.7:

where

A = Field area (ac; ha). Q = Annual flow (Mgal/yr; m3/yr). 0.027 = Conversion constant.

Lw = Design hydraulic loading rate (in./yr; mm/yr).

The design hydraulic loading rate can be based on soil permeability, crop irrigation requirements, or nitrogen loading rate. Modification to the land requirement based on storage is discussed in the section on storage.

Example 8.1. Land Area for a Slow-Rate System

Calculate the land requirements for a type 1 slow-rate system for a community of 1000 persons. The climate is moderately warm, and the design wastewater flow rate is 65,000 gal/d. A partially mixed aerated lagoon produces an effluent with 50 mg/L BOD and 30 mg/L total nitrogen. A site has been located that has relatively uniform soil with a limiting soil permeability (K) of 0.2 in./hr. The selected mix of forage grasses will take up 300 lb/ac-yr of nitrogen. The water balance of evapotranspiration and precipitation shows a net evapotranspiration of 18 in./yr.

Solution

1. Calculate the design percolation rate using Equation 8.3. Use a 7% factor to account for relatively uniform soil and moderate permeability: P (annual) = K(0.07)(24 hr/d)(365 d/yr)

2. Calculate the wastewater loading rate Lw using Equation 8.2. Lw = (ET - Pr) + P

Lw = (Net ET) + P Lw = 18 in. + 123 in. Lw = 141 in./yr

3. Calculate the field area based on soil permeability limits using Equation 8.7:

65,000 gal 365 d 1Mgal = 23.7 Mgal

d yr 106 gal yr

4. Calculate the hydraulic loading rate controlled by nitrogen, using a percolate nitrate nitrogen limit of 10 mg/L (Equation 8.6). Use a denitrification percentage of 25%:

5. Calculate the field area based on nitrogen limits using Equation 8.7:

A = Q = 23.7Mgal/yr = 9.6 ac 0.027 x L^n 0.027 x 91.2 in./yr

6. Calculate the organic loading rate, assuming that 9.6 ac based on the nitrogen limits will be the required field area:

0.065 Mgal 50 mg

Therefore, the BOD loading is not limiting because it is much less than 450 lb/ac-d.

7. Determine the field area required. Because the area for nitrogen limits (9.6 ac) is larger than the area required for soil permeability (6.2 ac), the required field area is 9.6 ac.

Comment

Nitrogen is the limiting design factor for this example. 8.2.6.4 Storage Requirements

Wastewater is usually stored during periods when it is too wet or too cold to apply to the fields. Except for forested sites, where year-round application is possible, most systems also store wastewater during crop harvesting, planting, or cultivation. Storage for cold weather is generally required by most state regulatory agencies unless it can be shown that groundwater quality standards will not be violated by winter applications and that surface runoff will not occur as a result of wastewater application. The conservative estimate of the storage period is to equate it to the nongrowing season for the crop selected. A more exact site-specific method is to use the water balance as shown in Example 8.2.

Example 8.2. Storage Requirements for a Slow-Rate System Estimate the storage requirements for the SR system from Example 8.1 using the water balance approach. The monthly precipitation and evapotranspiration data are presented in the following table. The temperatures are too cold in January for wastewater application. The maximum percolation rate is 10.3 in./month.

Crop

Month Evapotranspirationa 10-Year Rainfall1,

Crop

Month Evapotranspirationa 10-Year Rainfall1,

(1)

(2)

(3)

January

1.1

7.2

February

2.0

7.0

March

2.7

4.5

April

3.9

3.0

May

5.6

0.4

June

7.0

0.1

July

8.6

0.1

August

7.4

0.2

September

5.9

0.6

October

3.7

1.2

November

2.0

4.0

December

1.2

4.8

Total

51.1

33.1

a Forage crop évapotranspiration.

b Average distribution of rainfall for the wettest year in 10.

a Forage crop évapotranspiration.

b Average distribution of rainfall for the wettest year in 10.

Solution

1. Determine the available wastewater each month:

Monthly flow Area

(65,000 gal

( 365 d ^

i \ yr

1 yr J

43,560 ft2

7.48 gal

V ft

2. Complete the water balance table as shown on the next page:

Crop 10-Year Design Wastewater Available Change in Cumulative

Month Evapotranspiration3 Rainfall' Percolation0 Loadingd Wastewater"5 Storage' Storage

January

1.1

7.2

6.1

0.0

7.6

+7.6

8.5

February

2.0

7.0

10.3

5.3

7.6

+2.3

10.8'

March

2.7

4.5

10.3

8.5

7.6

-0.9

9.9

April

3.9

3.0

8.1

9.0

7.6

-1.4

8.5

May

5.6

0.4

3.7

8.9

7.6

-1.3

7.2

June

7.0

0.1

2.0

8.9

7.6

-1.3

5.9

July

8.6

0.1

0.4

8.9

7.6

-1.3

4.6

August

7.4

0.2

1.7

8.9

7.6

-1.3

3.3

September

5.9

0.6

3.6

8.9

7.6

-1.3

2.0

October

3.7

1.2

6.4

8.9

7.6

-1.3

0.7

November

2.0

4.0

10.3

8.3

7.6

-0.7

0.0

December

1.2

4.8

10.3

6.7

7.6

+0.9

0.9

Total

51.1

33.1

73.2

91.2

91.2

a Forage crop evapotranspiration.

b Average distribution of rainfall for the wettest year in 10. c Maximum percolation rate is 10.3 in./mo.

d Loading rate is limited by percolation rate from November through March (January has zero loading due to cold weather); loading rate for April through October is limited by the annual nitrogen loading. e Based on 65,000 gal/d and a field area of 9.6 ac. f Available wastewater minus the wastewater loading. g February is the maximum month.

3. The design percolation rate (column 4 data) is 10.3 in./mo when that much rainfall or wastewater is applied. From April to October, the wastewater loading is limited by the nitrogen loading, and the design percolation rate is the difference between the wastewater loading (column 5) and the net evapotranspiration (evapotranspiration - precipitation) (column 2 minus column 3).

4. The wastewater loading is limited by the nitrogen balance from April through October; by the precipitation and percolation rates for November, December, February, and March; and by the cold weather in January.

5. Determine the change in storage by subtracting the wastewater loading (column 5) from the available wastewater (column 6). Enter the amount in column 7.

6. The cumulative storage calculations (column 8) begin with the first positive month for storage in the fall/winter (December). The maximum month for storage is February, with a value of 10.8 in. This depth is converted to million gallons as follows:

Storage volume = (10.8 in.)(9.6 ac)

( ft ^

( 43,560 ft2 ^

112inJ

Mgal 106 gal ft3

7. Convert the required storage volume into equivalent days of flow:

Days of storage =

Comment

The estimated storage volume from the above procedure can be adjusted during final design to account for the net gain or loss in volume from precipitation, evaporation, and seepage. In the wettest year in 10, the storage volume should be reduced to zero at one point in time during the year. To estimate the area needed for the storage pond, divide the required volume in ac-ft by a typical depth, such as 10 ft. The net precipitation falling on the surface area can then be added to the storage volume. Typical seepage rates that are allowed by state regulations range from 0.062 to 0.25 in./d. These state standards for pond seepage are becoming more stringent, and compaction or lining requirements are becoming more common; therefore, a conservative approach would be to assume zero seepage.

TABLE 8.11

Comparison of Suitability Factors for Distribution Systems

Distribution System

Sprinkler systems: Portable hand move

Wheel-line (side-roll) Solid set

Center pivot or linear Traveling gun

Surface systems: Graded borders, narrow

(border strip) 15 ft wide Graded borders, wide, up to 100 ft Straight furrows

Drip systems: Drip tube or microjets

Suitable Crops

Pasture, grain, alfalfa, orchards, vineyards, vegetable and field crops All crops less than 3 ft high No restriction All crops except trees Pasture, grain, alfalfa, field crops, vegetables

Pasture, grain, alfalfa, vineyards Pasture, grain, alfalfa, orchards Vegetables, row crops, orchards, vineyards

Orchards, landscape, vineyards, vegetables

Minimum Infiltration Rate (in./hr)

0.10

0.02

Maximum

10-15 No restriction 15 15

No restriction

8.2.6.5 Distribution Techniques

The three principal techniques used for effluent distribution are sprinkler, surface, and drip application. Sprinkler distribution is often used in the newer SR systems (see Figure 8.1), in most industrial wastewater (high solids content), and in all forested systems. Surface application includes border strip, ridge-and-furrow, and contour flooding. Drip irrigation should only be attempted with high-quality filtered effluent. A comparison of suitability factors for distribution systems is presented in Table 8.11. Selection of the distribution technique depends on the soil, crop type, topography, and economics. Of the sprinkler systems, the portable hand move and solid set are most common for small systems because of the relatively high flow rates required for the other systems. Continuous-move systems usually require 300 to 500 gal/min (1135 to 1890 L/min) to operate.

8.2.6.6 Application Cycles

Sprinkler systems operate between once every 3 days and once every 10 days or more. Surface application systems operate once every 2 to 3 weeks. For all systems, the total field area is divided into subsections or sets which are irrigated sequentially over the application cycle. For type 1 systems, the application schedule depends on the climate, crop, and soil permeability. For type 2 (crop irrigation) systems, the schedule depends on the crop, climate, and soil moisture depletion.

8.2.6.7 Surface Runoff Control

The surface runoff of applied wastewater from SR systems is known as tailwater and must be contained on-site. Collection of tailwater and its return to the distribution system or storage pond are integral parts of the design of surface application systems. Sprinkler systems on steep slopes or on slowly permeable soil may also use tailwater collection and recycle. A typical tailwater return system consists of a perimeter collection channel, a sump or pond, a pump, and a return forcemain to the storage or distribution system. Tailwater volumes range from 15% of applied flows for slowly permeable soils to 25 to 35% for moderately permeable soils (Hart, 1975). Storm-induced runoff does not need to be retained on-site; however, stormwater runoff should be considered in site selection and site design. Erosion caused by stormwater runoff can be minimized by terracing steep slopes, contour plowing, no-till farming, and grass border strips. If effluent application is stopped before the storm, the stormwater can be allowed to drain off the site.

8.2.6.8 Underdrainage

In some instances, subsurface drainage is necessary for SR systems to lower the water table and prevent water-logging of the surface soils. The existence of a water table within 5 ft indicates the possibility of poor subsurface drainage and should lead to an examination of the underdrains. For small SR sites (less than 10 ac or 4 ha) the need for underdrains may make the site uneconomical to develop. Underdrains usually consist of 4 to 6 in. (100 to 150 mm) of perforated plastic pipe buried 6 to 8 ft (1.8 to 2.4 m) deep. In sandy soils, drain spacings are 300 to 400 ft (91 to 122 m) apart in a parallel pattern. In clayey soils, the spacings are much closer, typically 50 to 100 ft (15 to 30 m) apart. Procedures for designing underdrains are described in Van Schilfgaarde (1974), USDA (1972), and USDoI (1978).

8.2.7 Construction Considerations

In most instances, the slow rate site can be developed according to local agricultural practices (Crites, 1997). Local extension services, NRCS representatives, or agricultural engineering experts should be consulted. One of the key concerns is to pay attention to the soil infiltration rates. Earthworking operations should be conducted to minimize soil compaction, and soil moisture should generally be substantially below optimum during these operations. High-flotation tires are recommended for all vehicles, particularly for soils with high percentages of fines. Deep ripping may be necessary to break up hardpan layers, which may be present below normal cultivation depths.

8.2.8 Operation and Maintenance

Proper operation of an SR system requires management of the applied wastewater, crop, and soil profile. Applied wastewater must be rotated around the site through the application cycle to allow time for drying maintenance, cultivation, and crop harvest. The soil profile must also be managed to maintain infiltration rates, avoid soil compaction, and maintain soil chemical balance. Compaction and surface sealing can reduce the soil infiltration or runoff. The causes can include (WEF, 2001):

1. Compaction of the surface soil by harvesting or cultivating equipment.

2. Compaction from grazing animals when the soil is too wet (wait 2 to 3 d after irrigation to allow grazing by animals).

3. A clay or silt crust can develop on the surface as the result of precipitation or wastewater application.

4. Surface clogging as a result of suspended solids application.

The compaction, solids accumulation, and crusting of surface soils may be broken up by cultivating, plowing, or disking when the soil surface is dry. At sites where clay pans (hard, slowly permeable soil layers) have formed, it may be necessary to plow to a depth of 2 to 6 ft (0.6 to 1.8 m) to mix the impermeable soil layers with more permeable surface soils. A check of the soil chemical balance is required periodically to determine if the soil pH and percent exchangeable sodium are in the acceptable range. Soil pH can be adjusted by adding lime (to increase pH) or gypsum (to decrease pH). Exchangeable sodium can be reduced by adding sulfur or gypsum followed by leaching to remove the displaced sodium.

8.3 OVERLAND FLOW SYSTEMS

Overland flow is a fixed-film biological treatment system in which the grass and vegetative litter serve as the matrix for biological growth. Process design objectives, system performance design criteria and procedures, and land and storage requirements are described in this section.

8.3.1 Design Objectives

Overland flow (OF) can be used as a pretreatment step to a water reuse system or can be used to achieve secondary treatment, advanced secondary treatment, or nitrogen removal, depending on discharge requirements. Because OF produces a surface water effluent, a discharge permit is required (unless the water is reused).

In most cases, the discharge permit will limit the discharge concentrations of BOD and total suspended solids (TSS), and that is the basis of the design approach in this chapter.

8.3.2 Site Selection

Overland flow is best suited to sites with slowly permeable soil and sloping terrain. Sites with moderately permeable topsoil and impermeable or slowly permeable subsoils can also be used. In addition, moderately permeable soils can be compacted to restrict deep percolation and ensure a sheet flow down the graded slope. Overland flow may be used at sites with existing grades of 0 to 12%. Slopes can be constructed from level terrain (usually the minimum of a 2% slope is constructed). Steep terrain can be terraced to a finished slope of 8 to 10%. At the wastewater application rates in current use, the site grade is not critical to performance when it is within the range of 2 to 8% (Smith and Schroeder, 1982). Site grades of less than 2% will require special attention to avoid low spots that will lead to ponding. Grades above 8% have an increased risk of short-circuiting, channeling, and erosion.

8.3.3 Treatment Performance

Overland flow systems are effective in removing BOD, TSS, nitrogen, and trace organics. They are less effective in removing phosphorus, heavy metals, and pathogens. Performance data and expectations are described in this section.

8.3.3.1 BOD Loading and Removal

In municipal systems, the BOD loading rate typically ranges from 5 to 20 lb/ac-d. Biological oxidation accounts for the 90 to 95% removal of BOD normally found in OF systems. Based on experience with food processing wastewater, the BOD loading rate can be increased to 100 lb/ac-d (110 kg/ha-d) for most wastewater without affecting BOD removal. The industrial wastewater system at Paris, Texas, continues to remove 92% of applied BOD (Tedaldi and Loehr, 1991). BOD removals from four overland flow systems are presented in Table 8.12 along with the application rate and slope length. A typical BOD concentration in the treated runoff water is about 10 mg/L.

8.3.3.2 Suspended Solids Removal

Overland flow is effective in removing biological and most suspended solids, with effluent TSS levels commonly being 10 to 15 mg/L. Algae are not removed effectively in most OF systems because many algal types are buoyant and resist removal by filtration or sedimentation (Peters et al., 1981). If effluent TSS limits are 30 mg/L or less, the use of facultative or stabilization ponds that generate high algae concentrations is not recommended prior to overland flow. If OF is otherwise best suited to a site with an existing pond system, design and operational

TABLE 8.12

BOD Removal for Overland Flow Systems

Application Rate

Location

Wastewater Type

(gal/ftmin)

Ada, Oklahoma

Raw wastewater

0.10

Primary effluent

0.13

Secondary effluent

0.27

Easley, South Carolina

Raw wastewater

0.29

Pond effluent

0.31

Hanover, New Hampshire

Primary effluent

0.17

Secondary effluent

0.10

Melbourne, Australia

Primary effluent

Influent BOD

Effluent BOD

120 150 8

120 70 8

180 200 23

150 28 15

100 72 9

100 45 5

TABLE 8.13

Nitrogen Removal for Overland Flow Systems

Parameter

Type of wastewater

Application rate (gal/ft-min)

BOD/N ratio

Total nitrogen (lb/ac-yr): Applied Removed Crop uptake

Nitrification/denitrification

Removal, mass balance (%)

Denitrification (% of total removal)

Total nitrogen (mg/L): Applied Runoff

Nitrogen removal, concentration basis (%)

Ada, Oklahoma

Screened raw wastewater

0.10

1070 980 100 880

Hanover, New Hampshire

0.17

850 790 190 600

36.6

Utica, Mississippi

Primary effluent Pond effluent

590 445-535 220 225-325

75-90

50-60

63-79

Source: USEPA, Process Design Manual for Land Treatment of Municipal Wastewater, EPA 625/181-013, U.S. Environmental Protection Agency, Cincinnati, OH, 1981.

procedures are available to overcome the algae removal issue. The application rate should not exceed 0.12 gal/min-ft (0.10 m3/m-hr) for such systems, and a nondischarge mode of operation can be used during algae blooms. In the non-discharge mode, short application periods (15 to 30 min) are followed by 1- to 2-hr rest periods. The OF systems at Heavener, Oklahoma, and Sumrall, Michigan, operate in this manner during algae blooms (WEF, 2001).

8.3.3.3 Nitrogen Removal

The removal of nitrogen by OF systems depends on nitrification/denitrification and crop uptake of nitrogen. The removal of nitrogen in several OF systems is presented in Table 8.13, which shows that denitrification can account for 60 to 90% of the nitrogen removed with denitrification rates of 800 lb/ac-yr or more. Up to 90% removal of ammonia was reported at 0.13 gal/min-ft (0.10 m3/hr-m) at the OF system at the City of Davis, California, where oxidation lagoon effluent was applied (Kruzic and Schroeder, 1990). Further research at the Davis site proved that the wet/dry ratio was also very important (Johnston and Smith, 1988). The effect of the wet/dry ratio in ammonia removal is illustrated in Figure 8.5.

FIGURE 8.5 Effect of wet/dry ratio on the removal of ammonia by overland flow. (From Johnston, J. and Smith, R., Operating Schedule Effects on Nitrogen Removal in Overland Flow Treatment Systems, paper presented at the 61st Annual Conference of the Water Pollution Control Federation, Dallas, TX, 1988.)

Wet-Dry Ratio

FIGURE 8.5 Effect of wet/dry ratio on the removal of ammonia by overland flow. (From Johnston, J. and Smith, R., Operating Schedule Effects on Nitrogen Removal in Overland Flow Treatment Systems, paper presented at the 61st Annual Conference of the Water Pollution Control Federation, Dallas, TX, 1988.)

To obtain effective nitrification, the wet/dry ratio must be 0.5 or less. At Sacramento County, California, secondary effluent was nitrified at an application rate of 0.70 gal/min-ft (0.54 m3/hr-m). Ammonia concentrations were reduced from 14 to 0.5 mg/L (Nolte Associates, 1997). At Garland, Texas, nitrification studies were conducted with secondary effluent to determine whether a 2-mg/L summer limit for ammonia and a 5-mg/L winter limit could be attained. Application rates ranged from 0.43 to 0.74 gal/min-ft (0.33 to 0.57 m3/hr-m). Winter values for effluent ammonia ranged from 0.03 to 2.7 mg/L and met the effluent requirements. The recommended application rate for Garland was 0.56 gal/min-ft (0.43 m3/hr-m) for an operating period of 10 hr/d and a slope length of 200 ft (61 m) with sprinkler application (Zirschky et al., 1989).

8.3.3.4 Phosphorus and Heavy Metal Removal

Phosphorus removal in OF is limited to about 40 to 50% because of the lack of soil-wastewater contact. If needed, phosphorus removal can be enhanced by the addition of chemicals such as alum or ferric chloride. Heavy metals are removed using the same general mechanisms as with phosphorus: absorption and chemical precipitation. Heavy metal removal will vary with the constituent metal from about 50 to about 80% (WEF, 2001).

8.3.3.5 Trace Organics

Trace organics are removed in OF systems by a combination of volatilization, absorption, photodecomposition, and biological degradation. If removal of trace organics is a major concern, Reed et al. (1995) and Jenkins et al. (1980) should be reviewed.

0.20

0.04

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