Process Design Land Application

The basic design approach is based on the underlying assumption that, if sludge is applied at rates that are equal to the requirements of the design vegetation, over the time period of concern there should not be any greater impact on the groundwater than from normal agricultural operations. The design loading, based

TABLE 9.16

Ceiling Concentrations


Ceiling Concentration

(mg/kg, dry weight basis)






Mercury Molybdenum Nickel Selenium



Source: Data from Bastian (1993) and Crites et al. (2000).

TABLE 9.17

Cumulative Pollutant Loading Rates

Cumulative Pollutant Loading Rate Pollutant (kg/ha)

Arsenic 41

Cadmium 39

Copper 1500

Lead 300

Mercury 17

Molybdenum" —

Nickel 420

Selenium 100

Zinc 2800

" Molybdenum was dropped in 1994 and a new value has not been set. Check the USEPA website for current values.

Source: Bastian, R.K., Summary of 40CFR Part 503, Standards for the Use or Disposal of Sewage Sludge, U.S. Environmental Protection Agency, Washington, DC., 1993.

initially on nutrient requirements, is modified as required to satisfy limits on metals and toxic organics. As a result of this design approach, extensive monitoring should not be required, and the use of sludge by private farmers is made possible. As the loading increases, as it may in forests and on dedicated sites, the potential for nitrate contamination of the groundwater increases, and it is then usually necessary to design a municipally owned and operated site to ensure proper management and monitoring. Metals

The following is extracted from the 40 CFR Part 503.13 pollutant limits (Bastian, 1993). Bulk sewage sludge or sewage sludge sold or given away in a bag or other container must not be applied to the land if the concentration of any pollutant in the sewage sludge exceeds the ceiling concentration shown in Table 9.16 (chromium was removed from Table 9.16 in 1994). If bulk sewage sludge is applied to agricultural land, forest, a public contact site, or a reclamation site, either the cumulative loading rate for each pollutant must not exceed the cumulative loading rate for the pollutants shown in Table 9.17 or the concentration of any pollutant in the sewage sludge must not exceed the ceiling concentration shown in Table 9.16. If bulk sludge meets the "high-quality" pollutant concentrations shown in Table 9.18, the cumulative pollutant loading rates (Table 9.17) do not apply because these materials can be applied at agronomic rates for 100 years without concerns about limiting cumulative loading rates.

If bulk sewage sludge is applied to a lawn or a home garden, the concentration of each pollutant in the sewage sludge must not exceed the concentrations shown in Table 9.18. If sewage sludge products are sold or given away in a bag or other container for application to the land, either the concentration of each pollutant in the sewage sludge must meet the pollutant concentrations in Table 9.18 or the product label will provide product use directions to limit the annual pollutant loading rates shown in Table 9.19. Equation 9.6 shows the relationship between the annual pollutant loading rate (APLR) and the annual whole sludge application rate (AWSAR):



APLR = Annual pollutant loading rate (kg/ha per 365-d period). C = Pollutant concentration (mg/kg of total solids on a dry weight basis).

AWSAR = Annual whole sludge application rate (mt/ha per 365-d period on a dry weight basis). 0.001 = A conversion factor.

TABLE 9.18

Pollutant Concentrations (High Quality)











Monthly Average Concentration (mg/kg, dry weight basis)

41 39 1500 300 17

420 100 2800

a Molybdenum was dropped in 1994 and a new value has not been set. Check the USEPA website for current values.

Source: Bastian, R.K., Summary of 40CFR Part 503, Standards for the Use or Disposal of Sewage Sludge, U.S. Environmental Protection Agency, Washington, DC., 1993.

Equation 9.6 can be modified to calculate the lifetime loading of the heavy metals:


where LWSAR is the lifetime whole sludge application rate (mt/ha), CPLR is the cumulative pollutant loading rate (kg/ha), and the other terms are as defined previously.

To determine the AWSAR or LWSAR for a sewage sludge, analyze a sample of the sewage sludge to determine the concentration of each pollutant listed in Table 9.19. Insert the proper APLRs from Table 9.19 or the proper CPLRs from Table 9.17 and the milligrams of pollutants per kilogram of dry solids into Equation 9.6 or 9.7 to determine the annual or cumulative whole sludge application rate. The AWSAR or LWSAR for the sewage sludge is the lowest value calculated for the various metals. For example, a measured concentration of copper of 2000 mg/L and an APLR of 75 kg/ha per 365-d period would yield an AWSAR of 2000/(75 x 0.001) = 26,667 mt/ha per 365 d. Calculate values for the other metals and select the lowest AWSAR for design.

TABLE 9.19

Annual Pollutant Loading Rate











Annual Pollutant Loading Rate (kg/ha per 365-d Period)

75 15 0.85

21 5 140

a Molybdenum was dropped in 1994 and a new value has not been set. Check the USEPA website for current values.

Source: Bastian, R.K., Summary of 40CFR Part 503, Standards for the Use or Disposal of Sewage Sludge, U.S. Environmental Protection Agency, Washington, DC., 1993.

Some states may have more stringent metal limits than those presented here; therefore, it is essential to consult local regulations prior to design of a specific system. Phosphorus

Some states require that the nutrient-limited sludge loading be based on the phosphorus needs of the design vegetation to ensure even more positive protection. This also provides a safety factor against nitrate contamination, as most sludges contain far less phosphorus than nitrogen, but most crops require far more nitrogen than phosphorus, as shown in Table 8.8. If optimum crop production is a project goal, this approach will require supplemental nitrogen fertilization. Equation 9.8 can be used to determine the phosphorus-limiting sludge loading; it is based on the common assumption (USEPA, 1978) that only 50% of the total phosphorus in the sludge is available:


RP = Phosphorus-limited annual sludge application rate, assuming 50% availability in the sludge (mt/ha; ton/ac).

UP = Annual crop uptake of phosphorus (kg/ha; lb/ac); see Chapter 3 of this text for further discussion and USEPA (1983) for more exact data for midwestern crops.

CP = Total phosphorus in sludge, as a decimal fraction (equation has already been adjusted for 50% availability, but this could be adjusted if data are available to indicate a higher or lower percent available total phosphorus) Nitrogen

Calculation of the nitrogen-limited sludge loading rate is the most complicated of the calculations involved because of the various forms of nitrogen available in the sludge, the various application techniques, and the pathways nitrogen can take following land application. Most of the nitrogen in municipal sludges is in organic form, tied up as protein in the solid matter. The balance of the nitrogen is in ammonia form (NH3). When liquid sludges are applied to the soil surface and allowed to dry before incorporation, about 50% of the ammonia content is lost to the atmosphere through volatilization (Sommers et al., 1981). As a result, only 50% of the ammonia is assumed to be available for plant uptake if the sludge is surface applied. If the liquid sludge is injected or immediately incorporated, 100% of the ammonia is considered to be available.

The availability of the organic nitrogen is dependent on the "mineralization" of the organic content of the sludge. Only a portion of the organic nitrogen is available in the year the sludge is applied, and a decreasing amount continues to be available for many years thereafter. The rate will be higher for sludges with higher initial organic nitrogen content. The rate drops rapidly with time, so for almost all sludges after the third year it is down to about 3% per year of the remaining organic nitrogen.

For the first few years of a sludge application, the nitrogen contribution from mineralization can still be significant. It is essential to include this factor when the design is based on annual applications and nitrogen is the potential limiting parameter. The nitrogen available (to plants) during the application year is given by Equation 9.9, and the available nitrogen from that same sludge in subsequent years is given by Equation 9.10. When annual applications are planned, it is necessary to repeat the calculations using Equation 9.10 and then add the results to those of Equation 9.9 to determine the total available nitrogen in a given year. These results will converge on a relatively constant value after 5 to 6 yr if sludge characteristics and application rates remain about the same.

Available nitrogen in the application year is given by:

TABLE 9.20

Typically Assumed Mineralization Rates for Organic Nitrogen in Wastewater Sludges

Time after Sludge Application

Mineralization Rate (%)

Raw Sludge Anaerobic Digested Composted

TABLE 9.20

Typically Assumed Mineralization Rates for Organic Nitrogen in Wastewater Sludges

Raw Sludge Anaerobic Digested Composted









































Source: Data from Sommers et al. (1981) and USEPA (1983).

Source: Data from Sommers et al. (1981) and USEPA (1983).


Na = Plant-available nitrogen in the sludge during the application year (kg/mt dry solids; lb/ton dry solids).

NO3 = Percent nitrate in the sludge (% as a decimal).

kv = Volatilization factor = 0.5 for surface-applied liquid sludge, 1.0 for incorporated liquid sludge and dewatered digested sludge applied in any manner.

NH4 = Fraction of ammonia nitrogen in sludge (as a decimal).

fn = Mineralization factor for organic nitrogen in first year n = 1 (see Table 9.20 for values).

No = Fraction of organic nitrogen in sludge (as a decimal).

Nitrogen available in subsequent years is

where Npn is the plant-available nitrogen available in year n from mineralization of sludge applied in a previous year (kg/mt or lb/ton dry solids), (No)n is the decimal fraction of organic nitrogen remaining in the sludge in year n, and the other terms are as defined previously.

The nitrogen-limiting annual sludge loading is then calculated using Equation 9.11:

Rn = Annual sludge loading in year of concern (mt/ha; ton/ac). UN = Annual crop uptake of nitrogen (kg/ha; lb/ac) (see Table 8.8). N = Plant-available nitrogen from current year's sludge, from Equation 9.9

(kg/mt or lb/ton dry solids). Npn = Plant-available nitrogen from mineralization of all previous applications (kg/mt or lb/ton dry solids).

In addition to the available nitrogen calculated above, it is also necessary that nitrogen from any other source be included when calculating agronomic rates. Calculation of Land Area

Equation 9.6, Equation 9.8, and Equation 9.11 should be solved to determine the parameter limiting the sludge loading. Some regulatory authorities require limits on constituents other than nitrogen, phosphorus, or metals. The limiting parameter for design will then be the constituent that results in the lowest calculated sludge loading. The application area can then be determined using Equation 9.12. The area calculated using this equation is only the actual application area; it does not include any allowances for roads, buffer zones, and seasonal storage:

Rl where

A = Application area required (ha; ac).

Qs = Total sludge production for the time period of concern (mt or ton dry solids).

Rl = Limiting sludge loading rate as defined by previous equations (mt/ha-yr; ton/ac-yr) for annual systems or for the time period of concern.

It is not likely that the design procedure described above will result in the ideal balance of nitrogen, phosphorus, and potassium for optimum crop production in an agricultural operation. The amounts of these nutrients in the sludge to be applied should be compared with the fertilizer recommendations for the desired crop yield, and supplemental fertilizer applied if necessary. USEPA (1983) gives typical nutrient requirements for crops in the midwestern states; agricultural agents and extension services can provide similar data for most other locations.

Annual applications are a common practice on agricultural operations. Forested systems typically apply sludge on a 3- to 5-yr interval due to the more difficult site access and distribution. The total sludge loading is designed using the equations presented above; however, because of mineralization of the larger single application, there may be a brief period of nitrate loss during the year of sludge application.

Reclamation and revegetation of disturbed land generally require a large quantity of organic matter and nutrients at the start of the effort to be effective. As a result, the sludge application is typically designed as a one-time operation, and the lifetime metal limits given in Table 9.17 are controlling on the assumption that the site might someday be used for agriculture. A single large application of sludge may result in a temporary nitrate impact on the site groundwater. That impact should be brief and preferable to the long-term environmental impact from the unreclaimed area. When cumulative metal loading limits control the sludge loading, the same total application area will be necessary for either agricultural or reclamation projects.

Forest systems may require the largest total land area of the three concepts because of access and application difficulties. Application of liquid sludge has been limited to tank trucks with sprinklers or spray guns. The maximum range of these devices is about 120 ft (37 m). To ensure uniform coverage, the site will require a road grid on about 250-ft (76-m) centers or limit applications to 120 ft (37 m) on each side of the existing road and firebreak network.

Experience has shown that tree seedlings do poorly in fresh anaerobically digested sludge (Cole et al., 1983). It may be necessary to wait for 6 months before planting to allow for aging of the sludge. Weeds and other undergrowth will crowd out new seedlings, so herbicides and cultivation may be necessary for at least 3 years (Sopper and Kerr, 1979). Sludge spraying on young deciduous trees should be limited to their dormant period to avoid heavy sludge deposits on the leaves.

Example 9.6

Find the area required for sludge application in an agricultural operation. Assume the following characteristics and conditions: anaerobically digested sludge production, 3 mt/d dry solids; sludge solids content, 7%; total nitrogen, 3%; ammonia nitrogen, 2%; nitrate, 0; arsenic, 50 ppm; cadmium, 18 ppm; copper, 400 ppm; lead, 430 ppm; mercury, 20 ppm; nickel, 80 ppm; selenium, 50 ppm; zinc, 900 ppm (ppm = mg/kg). A marketable crop is not intended, but the site will be planted with a grass mixture. It is expected that the orchard grass will eventually dominate. The local regulatory authorities accept the USEPA metal limitations and allow a design based on nitrogen fertilization requirements. A parcel of land is available within 6 km of the treatment plant.


1. A preliminary cost analysis indicates that transport of the liquid sludge to the nearby site will be cost effective, so further dewatering will not be required, and the application technique will be surface application.

2. Metal limits (from Table 9.17) are As, 41 kg/ha; Cd, 39 kg/ha; Cu, 1500 kg/ha; Pb, 300 kg/ha; Hg, 17 kg/ha; Ni, 420 kg/ha; Se, 100 kg/ha; and Zn, 2800 kg/ha. The annual nitrogen uptake of the grass will be 224 kg/ha-yr (from Table 8.5). The mineralization rates for anaerobi-cally digested sludge will be 20, 10, 5, and 3%, etc. (from Table 9.20).

3. The lifetime metal loadings are calculated using Equation 9.7:

C x 0.001 For arsenic,

LWSAR = 41kg/ha = 820 mt dry sludge per ha 50 x 0.001


Cd LWSAR = 2167 mt dry sludge per ha.

Cu LWSAR = 3750 mt dry sludge per ha.

Pb LWSAR = 698 mt dry sludge per ha.

Hg LWSAR = 850 mt dry sludge per ha.

Ni LWSAR = 5250 mt dry sludge per ha.

Se LWSAR = 2000 mt dry sludge per ha.

Zn LWSAR = 3111 mt dry sludge per ha. Lead results in the lowest sludge loading and is therefore the limiting metal parameter. As a result, 698 mt/ha of sludge can be applied during the useful life of the site if sludge conditions remain the same. If all of the metal concentrations had been below the pollutant concentration limits shown in Table 9.18, heavy metal constraints would not affect the sizing of the facility.

4. Use Equation 9.9 and Equation 9.10 to calculate the available nitrogen in the sludge. Because the liquid sludge will be surface applied, volatilization losses will occur, and kv will equal 0.5. Assume that organic nitrogen equals total nitrogen less ammonium nitrogen:

N = (Kn )[(NO3 ) + kv (NH4 ) + fn (No )] = (1000)[(0) + (0.5)(0.02) + (0.20)(0.01)] = (1000)(0.012) = 12 kg / mt dry solids

The residual nitrogen in this sludge in the second year is:

= 0.008 (as a decimal fraction)

The second year mineralization is:

Residual nitrogen in the third year is

Similarly, mineralization in the third year is 0.0004; in the fourth year, 0.0002; in the fifth year, 0.0002; etc. The total available nitrogen in the second year is the second-year contribution plus the residual from the first year:

(Na )2 = (N a )1 + KNfl (Na )2 = 12 + (1000)(0.0008) = 12.8 kg / mt dry sludge


= 12 + (1000)(0.0008 + 0.0004) = 13.3 kg / mt dry sludge

(Na)4 = 13.4 kg/mt dry sludge; (Na)5 = 13.6 kg/mt, etc. Assuming that the sludge characteristics stay the same, the available nitrogen will remain at about 13.6 kg/mt dry sludge from the fifth year on. 5. Use Equation 9.11 to calculate the annual nitrogen-limited sludge loading. Use 13.6 kg/mt as the steady-state value from step 4:

Rn = ——-= 224 = 16.5 mt / ha / yr of dry sludge

Higher loadings may be applied during the first 2 years if desired, as the full cumulative effects of mineralization will not be realized until the third year.

6. Use Equation 9.12 to find the required application area. Because food chain crops are not involved, the annual loading is based on the nitrogen limits:

A = Q = (3 mt / d)(365 d / yr) / (16.5 mt / ha • yr) = 66 ha Rl

7. Determine the useful life of the site for sludge application. This will eliminate restrictions on potential future land uses, including production of human food crops. The lead-limited sludge loading calculated in step 3 will control.

A system design for a reclamation site would typically use a single sludge application. The total annual sludge production is 1095 mt/yr (3 mt/d x 365 d/yr). At a single loading of 698 mt/ha, 1.6 ha of land would require reclamation each year. Reclamation project designs must ensure that sufficient land will be available for each year of the intended operational life.

9.8.3 Design of Surface Disposal Systems

The design of surface disposal systems typically includes all of the factors discussed for land application systems, as metals and nutrients may still control the sludge loading and useful life of the site. In addition, sludges intended for surface disposal systems may contain a larger fraction of biodegradable material than typical municipal sludges and have significant concentrations of toxic or hazardous substances. These materials, more common in petroleum and many industrial sludges, are quite often organic compounds. Their presence, if degrad-able, may control the frequency as well as the size of the design unit loading on the system. If the pollutants are nondegradable, the application site should more properly be considered as a disposal or containment operation; information on such systems may be found elsewhere (Sittig, 1979; USACE, 1984). The primary mechanism for degradation of organic chemicals in soil is due to the activity of the soil microorganisms. Volatilization may be significant for some compounds (Brown, 1983; Jenkins and Palazzo, 1981) and plant uptake may be a factor if vegetation is a system component, but biological reactions are the major treatment mechanism. Design Approach

The design approach for these organic materials is based on their half-life in the soil system. This is analogous in some respects to the mineralization rate approach for nitrogen management. If, for example, a substance in the sludge has a 1-yr half-life and the sludge is applied on an annual basis, half of the mass of the substance will still be left in the soil at the end of the first year. At the end of the second year, three quarters of the annual mass applied will still be in the soil, and so forth, until at the seventh year the mass remaining in the soil will be very close to the amount of the annual application. It is suggested that, for compounds with a half-life of up to 1 year, the amount allowed to accumulate in the soil should not exceed twice the annual application of the substance (Brown, 1983; Burnside, 1974). This can be achieved by adopting an application schedule that is equal to one half-life for the substance of concern.

Soil texture and structure, moisture content, temperature, oxygen level, nutrient status, pH, and the type and number of microorganisms present influence the biological reactions in the soil. The optimum conditions for all of these factors are essentially the same as those required for successful operation of an agricultural land application system. An aerobic soil with a pH of 6 to 7, a temperature of at least 50°F (10°C), and soil moisture at field capacity would represent near-optimum conditions for most situations. An additional special concern with toxic organics is their impact on soil microbes. A unit loading that is too high may actually sterilize the soil. Mixing of the soil and the sludge reduces this risk and promotes aeration and contact between the microbes and the waste. As a result of this need for mixing, surface vegetation is not typically a treatment component in systems designed for short-half-life sludges. Data Requirements

Characterization of the sludge constituents is a critical first step in design, especially if potentially toxic or hazardous organic compounds are present. Essential data include inorganic chemicals, electrical conductivity, pH, titratable acids and bases, moisture (water) content, total organic matter, volatile organic compounds, extractable organic compounds, residual solids, and a biological assessment to determine acute and genetic toxicity. The inorganic chemicals might include the same metals, nutrients, and halides and other salts that would be included in an analysis for land application designs. Half-Life Determination

The degradation and half-life of complex organic compounds are typically determined in the laboratory by a series of soil respirometer tests. Representative samples of soil and sludge are mixed in a proportional range and placed in sealed flasks, which in turn are placed in an incubation chamber. Humidified, carbondioxide-free air is passed through each flask. The carbon dioxide evolved from microbial activity in the flask is picked up by the air and then collected in columns containing 0.1-^ sodium hydroxide. The sodium hydroxide solutions are changed about three times a week and then titrated with hydrochloric acid. Detailed procedures can be found in Brown (1983) and Stotzky (1965). The typical incubation period is up to 6 months. The control tests are run at 68°F (20°C), but, if field temperatures are expected to vary by more than 18°F (10°C), the half-life at these other temperatures should also be determined. In some cases it is desirable to verify laboratory results with pilot studies in the field. Soil samples are taken on a routine basis after application and mixing of the sludge and soil. The analysis should include total organics as well as compounds of specific concern. In addition to measurements of carbon dioxide evolution by the respirometer tests, it is recommended that the organic fractions of the original sample and that of the final soil-sludge mixture be determined. The degradation rates are then determined using Equation 9.13 and Equation 9.14. For total carbon degradation,


Dt = Fraction of total carbon degraded over time t. [C02]w = Cumulative C02 evolved by soil-waste mixture. [C02]S = Cumulative C02 evolved by unamended soil. C = Carbon applied with the sludge.

For organic carbon degradation,

where D,

0 = Fraction of organic carbon degraded over time t. Cr0 = Amount of residual carbon in the organic fraction of the final sludge-soil mixture. CS = Amount of organic carbon extracted from the unamended soil. Ca0 = Amount of carbon in the organic fraction of the applied sludge.

The degradation rates of individual organic subfractions are also determined by Equation 9.14. The half-life for the total organics or for a specific waste is determined using Equation 9.15:

Dt where t1/2 = Half-life of the organics of concern (d).

t = Time period used to produce the data for Equation 9.13 or Equation 9.14 (d).

Dt = Fraction of carbon degraded over time t.

If vegetation is to be a routine treatment component in the operational system, greenhouse and pilot field studies are necessary to evaluate toxicity and develop optimum loading rates. Greenhouse studies are easier and less costly to run, but field studies are more reliable. Systems designed only for soil treatment need not be tested unless vegetation is planned as a post closure activity.

Because a range of sludge-soil mixtures is tested in the respirometers, it is also possible to determine the concentration at which acceptable microbial activity occurs. It is then possible to determine the annual loading from this value and the previously determined half-life:


Cyr = Annual application rate for the organic of concern (kg/ha/yr; lb/ac/yr). Cc = Critical concentration at which acceptable microbial toxicity occurs

(kg/ha; lb/ac). t1/2 = Half-life of the organic of concern (yr).

The loading rate is then calculated using a variation of Equation 9.6:

Cw where

R0C = Loading rate limited by organics (kg/ha/yr; lb/ac-yr).

Cyr = Annual application rate for organic of concern (Equation 9.16)

(kg/ha-yr; lb/ac-yr). Cw = Fraction of the organic of concern in the sludge (as a decimal).

If the half-life of the organic of concern is less than 1 year, the R0,C calculated from Equation 9.16 may be applied on a more frequent schedule. In this case, the number of applications becomes:

where N is the number of applications per year, and t1/2 is the half-life (yr).

The land area required is then determined using Equation 9.12. As with land application systems, the calculations are performed for nutrients, metals, and other potentially limiting factors. The limiting parameter for design is then the constituent requiring the largest land area as calculated by Equation 9.12. Loading Nomenclature

Depending on industrial conventions and practices, the loading rates and application rates used in the design calculations may be expressed in a variety of units; for example, in the petroleum industry, it is common to express the loading in terms of barrels per hectare. In most cases, the sludge is mixed with the surface soil. This surface zone, termed the incorporation zone, is typically 6 in. (15 cm) thick. As a result, the loading is also often expressed as kilograms per meter of incorporation zone or as a percentage of a contaminant (on a mass basis) in the incorporation zone. The calculations below illustrate the various possibilities.

One barrel (bbl) of oil contains 42 gal (159 L), which is about 315 lb (143 kg) of oil. One cubic foot of "typical" soil contains about 80 lb of soil (1270 kg of soil). One acre of treatment area with a 6-in. (15-cm) incorporation zone contains (0.5)(43,560) = 21,780 ft3/ac (1520 m3/ha). At 41 bbl oil per acre (100 bbl/ha), the mass loading will be (41)(316)/21,780 ft3/ac = 0.595 lb/ft3 (9.53 kg/m3) of incorporation zone. At 203 bbl/ac (500 bbl/ha), the mass loading (on a percentage basis) will be (203)(316)/(21,780)(79) = 3.75% oil in the incorporation zone.

Example 9.7

Find the land area required for treatment of a petroleum sludge produced at a rate of 5 mt/d, containing 15% critical organics. The following data were obtained with respirometer tests:

• CO2 produced (90 d) = 1500 mg (waste + soil) = 100 mg (soil only).

• A field test indicated that the critical application (Cc) for maintenance of the soil microbes was 71,500 kg/ha-yr (3.75%).


1. Use Equation 9.13 to determine evolved CO2 on a total carbon basis:


2. Determine the half-life for the organic compounds using Equation 9.15: t _ (0.5)(t) _ (0.5)(90)

3. Determine the application rate for the critical compounds using Equation 9.16:

4. Determine the organic-controlled loading rate using Equation 9.17:


5. Determine the required land area using Equation 9.12:

6. To complete the design calculations, the area required for nutrients, metals, and any other limiting substances should be determined. The largest of these calculated areas will then be the design treatment area. Site Details for Surface Disposal Systems

The site selection procedure and design will depend on whether the site is to be permanently dedicated for a treatment/disposal operation or if it is to be restored and made available for unrestricted use following the operational life. A system of the former type may be operated as a treatment system, but ultimately one or more of the waste constituents will exceed the specified cumulative limits, so the site must be planned as a disposal operation. Criteria for these disposal operations can be found in Sittig (1979) and USACE (1984). The general site characteristics are similar for both land application and surface disposal systems. A major difference is often the control of runoff. Off-site runoff is not generally permitted for either type of operation; however, in the case of agricultural sludge operations, runoff is contained but then may be allowed to infiltrate on the application site. Runoff is a more serious concern for surface disposal operations, as the sludge may contain mobile toxic or hazardous constituents.

The site is typically selected, constructed on a gentle slope (1 to 3%), and subdivided into diked plots. The purpose is to induce controlled runoff and ensure minimum infiltration and percolation. A complete hydrographic analysis is required to determine the criteria for design of collection channels, retention basins, and structures to prevent off-site runoff from entering the site. Such designs should be based on the peak discharge from a 25-year storm, and the retention basins for a 25-year, 24-hour-return-period storm. The discharge pathway from the retention basin will depend on the composition of the water. In many cases, it may be land applied using one or more of the techniques described in Chapter 8. Special treatments may be required for critical materials; sprinklers or aeration in the retention basin are often used to reduce the concentration of volatile organics.

If clay or other liners are a site requirement, then underdrains will be necessary. Under drains may also be required to control groundwater levels in an unlined site and to ensure maintenance of aerobic conditions in the incorporation zone. Any water collected with these drains must also be retained and possibly treated.

The site design must also consider the application method to be used, and appropriate access for vehicles must be provided. Sprinklers and portable spray guns have been used with liquid sludges. In this case, the civil engineering aspects of site design are quite similar to those for the overland-flow concept described in Chapter 8. Dry sludges can be spread and mixed with the same type of equipment that would be used for land application operations.

On-site temporary storage may also be a requirement, particularly in colder climates. Optimal soil temperatures for microbial activity are 68°F (20°C) or higher. If lower temperatures are expected, the interval between applications can be extended (as determined by field or respirometer tests), or the sludge can be stored during the cold periods.

The soil temperatures for bare soil surfaces are commonly greater than the ambient air temperature by 5 to 9°F (3 to 5°C) during daylight hours. Surface soils at many land treatment sites may exceed ambient temperatures by 9 to 18°F (5 to 10°C) because of microbial activity and increased radiation absorption when dark, oily wastes are incorporated (Loehr and Ryan, 1983). In the general case, it can be assumed that active degradation is possible when the ambient air temperatures are 50°F (10°C) or higher and no frost remains in the soil profile. On this basis, the operational season for a surface disposal system may be slightly longer than for a land application system in the same location.


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