Design Application Rates

Application rates for biosolids can be limited by nitrogen or by pollutants (heavy metals listed in Table 10.1). For most biosolids that are land applied

TABLE 10.2 Typical Slope Limitation for Land Application

Slope (%) Comment

0-3 Ideal, no concern for runoff or erosion of liquid or dewatered biosolids.

3-6 Acceptable, slight risk of erosion; surface application of liquid or dewatered biosolids is acceptable.

6-12 Injection of liquid biosolids required for general cases, except in closed drainage basin and/or when extensive runoff control is provided; surface application of dewatered biosolids is generally acceptable.

12-15 No application of liquid biosolids should be made without extensive runoff control; surface application of dewatered biosolids is acceptable, but immediate incorporation into the soil is recommeded.

>15 Suitable only for sites with good permeability where the length of slope is short and where areas with a steep slope constitute only a minor part of the total application area.

Source: Metcalf & Eddy, 2003.

in the United States, the nitrogen content is the limiting factor. Thus, the design application rate for agricultural use is determined by the crops to be grown and the nitrogen uptake values for those crops. Table 10.3 lists the nitrogen uptake rates for selected crops. The table also includes phosphorus and potassium uptake rates for forage and field crops.

Application Rates Based on Nitrogen Regular nutrient analyses of biosolids provide the database for the nitrogen-based application rate for a particular source of biosolids. Biosolids application rates are typically calculated to supply the available nitrogen provided by commercial fertilizers. Plant-available nitrogen in biosolids can be estimated using the equation

where npa nh4

= plant available nitrogen in the application year, g/dry kg (lb/dry ton)

= ammonia nitrogen in biosolids, % = volatilization factor for ammonia = 0.5 for surface-applied liquid biosolids = 0.75 for surface-applied dewatered biosolids = 1.0 for injected liquid or dewatered biosolids = nitrate nitrogen in biosolids, % = organic nitrogen in biosolids, %

TABLE 10.3 Nutrient Uptake Rates for Selected Crops




Crop kg/ha • yr lb/ac-yr kg/ha • yr lb/ac-yr kg/ha • yr lb/ac-yr

Forage crops Alfafa" Bromegrass Coastal bermudagrass Kentucky bluegrass Quackgrass Reed canarygrass Ryegrass Sweet clover" Tall fescue Orchardgrass Field crops Barley Corn Cotton

Grain sorghum Potatoes Soybeans" Wheat Trees Eastern forests Mixed hardwoods Red pine White spruce Pioneer

225-540 130-225 400-675

succession Southern forests Mixed hardwoods Southern pine 220-320 Lake state forests Mixed hardwoods Hybrid poplar Western forest Hybrid poplar 300-Douglas fir 150-

200-480 115-200 355-600

200-270 180-240

235-280 335-450

200-280 175 150-325 250-350

125 175-200 75-110 135 230 250 160

110 280 280

400 -250

210-250 300-400

180-250 155 135-290 225-310

110 155-180 65-100 120 205 225 145

100 250 250


270-355 135-225

22-35 40-55 35-45

30-45 40-45

60-85 20 30 20-50

20-30 35-50 30-40

25-40 35-40

55-75 18 25 18-45

175-225 245 225

275 315

270-325 100 300 225-315

20 110 40 70

245-325 30-55 20-45

155-200 220 200

245 280

240-290 90 270 200-280

18 100 35 60

220-290 25-50 18-40

a Legumes will also take nitrogen from the atmosphere.

fm = mineralization factor for organic nitrogen

= 0.5 for warm climates and digested biosolids = 0.4 for cool climates and digested biosolids = 0.3 for cold climates and composted biosolids F = conversion factor, 1000 g/kg (2000 lb/ton) of dry solids

The biosolids application rate based on available nitrogen is then calculated from the equation



AN = biosolids application rate based on nitrogen, dry metric tons/ha-yr (tons/ac-yr)

NPA = plant available nitrogen in biosolids, g/kg (2000 lb/ton)

Application Rates Based on Pollutants The pollutants (heavy metal) of concern are listed in Table 10.1. The biosolids application rates based on pollutant loadings can be calculated using the equation

cpf where

AP = maximum amount of biosolids that can be applied, metric tons/ha-yr (tons/ac-yr) LP = maximum amount of pollutant that can be applied, metric tons/ha-yr (tons/ac-yr) CP = pollutant concentration in biosolids, mg/kg F = conversion factor, 0.001 kg/metric ton (2000 x 10-6 lb/ton)

Land Requirements By comparing the values calculated using equations (10.2) and (10.3), the minimum biosolids application rate is determined, and then the field area required can be calculated using the equation


A = application area required, ha (ac)

B = biosolids production, metric tons/yr (tons/yr) of dry solids L = nitrogen or pollutant application rate metric tons/ha-yr (tons/ac-yr) of dry solids

Design Example 10.1 A wastewater treatment plant produces 4 dry metric tons per day (4.4 tons/d) of dewatered digested biosolids. Following are the average concentrations of nitrogen and pollutants in the biosolids:

ammonia nitrogen :


nitrate nitrogen :


organic nitrogen :


arsenic (As) :


cadmium (Cd ) :


copper (Cu ) :

2200 mg/kg

lead ( Pb) :


mercury (Hg ) :


nickel ( Ni ) :

400 mg/kg

selenium (Se) :


zinc ( Zn ) :


Determine the application area required based on the nitrogen uptake rate for ryegrass and the pollutant loading rates when the biosolids are applied on the surface of the land.

1. Plant-available nitrogen in biosolids: From equation (10.1),

Npa = (0.005)(0.75) + 0.0002 + (0.03)(0.4)(1000g/kg) = 16g/kg (32 lb/ton)

2. Biosolids application rate based on nitrogen: From Table 10.3, the average nitrogen uptake rate for ryegrass is 230 kg/ha-yr (205 lb/ac-yr). From equation (10.2),

= 230kg/ha yr = 14.4 dry metric tons/ha • yr (6.4 dry tons/ac-yr) 16g/kg

3. Land area required based on nitrogen application:

yearly biosolids production = (4 metric tons/d)(365 d/yr)

= 1460 metric tons/yr (1610 tons/yr)

From equation (10.4),

A = l460metricto'"/y- = ioi ha (250 ac ) 14.4 metric tons/ha • yr

Note: The initial nitrogen content of the soil is assumed to be zero. After the initial application, the mineralization rate of organic nitrogen will decrease in subsequent years; therefore, the plant-available nitrogen will also decrease, thus requiring a larger area for biosolids application.

4. Suitability of biosolids based on pollutant concentrations: Compare the concentrations of metals in the biosolids to the ceiling concentration and the exceptional quality pollutant concentration values in Table 10.1. All metal concentrations are below the ceiling concentration limits. Therefore, the biosolids are suitable for land application.

The concentrations of cadmium, copper, and lead are above the values for exceptional quality biosolids. Therefore, annual application rates for those three metals need to be calculated.

5. Biosolids application rates based on pollutant loading rates:

Cadmium: annual pollutant loading rate = 1.9 kg/ha-yr (Table 10.1) From equation (10.3), the maximum application rate

(55mg/kg)(0.001 kg/metric ton)/(mg/kg) _ 34.5 metric tons/ha • yr (15.3 tons/ac-yr)


S (2200mg/kg)(0.001 kg/metric ton)/(mg/kg) _ 34.1 metric tons/ha • yr (15.2 tons/ac-yr)


(700mg/kg)(0.001 kg/metric ton)/(mg/kg) = 21.4 metric tons/ha • yr (9.6 tons/ac-yr)

Based on the application rates above, the limiting rate is 21.4 metric tons/ha-yr of bisolids based on lead.

6. Land area required based on pollutant loading: From equation (10.4),

. 1460 metrictour/yr .

7. Comparing the area required for biosolids application based on nitrogen loading (101 ha) and lead loading (68 ha), the nitrogen loading is more limiting. Therefore, 101 ha (250 ac) of land is initially required for the application of biosolids.

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