Organic Waste Recycling

460 Organic Waste Recycling: Technology and Management

Table 9.14 Mineralization rates for organic N in wastewater sludge (U.S. EPA 1983)

Year of sludge Mineralization rate %
application Raw sludge Anaerobically
digested sludge

Composted sludge

First
Second
Third
Fourth
Fifth
Sixth
Seventh
Eighth
Ninth
Tenth

40

20

10

5 3 3 3 3 3 3

20

10

5 3 3 3 3 3 3 3

10

5 3 3 3 3 3 3 3 3

Example 9.1

If corn is to be grown in a sandy loam soil, estimate the application rate of a
digested sludge which has the following nitrogen analysis: organic N = 20,000
mg/L, ammonia N = 1,500 mg/L, nitrite N = 5 mg/L, nitrate N = 50 mg/L. This
sludge contains 21,555 mg/L total N or 2.15% N by weight.
The annual nitrogen uptake rate for corn is about 224 kg/ha. Assume the
nitrogen mineralized during the first year is 20% of the organic nitrogen and that
the volatilized fraction is 50% of the ammonia nitrogen. Also assume that no N
flows into the groundwater, no denitrification occur, and initial available
nitrogen content of the soil is negligible.

The mass balance is:
N applied = N volatilized + N leached to groundwater + N used by plants +
N lost through denitrification. (9.1)

The amount of N applied is calculated as kg/ha. N is volatilized in the form
of ammonia nitrogen. Since 20% of the organic is mineralized (to ammonia), the
total available ammonia N is 1,500 + 0.2 (20,000) = 5,500 mg/L, and the
volatilized ammonia is 0.5 (5,500) = 2,750 mg/L (or about 0.275% N is lost
through volatilization). If the annual application rate of sludge is X kg/ha dry
weight, the total N volatilized is 2.75 x 10-3 (X) kg/ha.
The amount of N loss to groundwater and due to denitrification is zero. The
amount N used by the corn is 224 kg/(ha-year).
The total N applied is the product of dry solids application rate, X kg/ha
times the fraction of total nitrogen or 21.555 x 10-3 (X) kg/ha.