Organic Waste Recycling

462 Organic Waste Recycling: Technology and Management

f = mineralization rate (Table 9.14), as a decimal. Subscripts refer
to the year of concern
ON = percent organic nitrogen remaining in the sludge in a particular
year, as a decimal. Subscripts refer to the year of concern.

A system with continuous annual applications will have to solve Equation 9.3
for each of the subsequent year, i.e.: NPR2, NPR3, NPR4etc. The calculations will
converge on a relatively constant value after 5 to 6 years if the sludge
composition remains the same.

3. The annual sludge loading (SNy) based on nitrogen is determined
   with:

Un

SNt = (9.4)

(NP + NPR1......)

Where:

SNt = annual sludge loading in year (t) of concern, dry ton/ha

Un = crop uptake of N kg/(ha-year)

NP, NPR1 = from Equations 9.2 and 9.3

In addition to the above three-step procedure, attention should be given to
possible loss of sludge or nutrients due to surface run-off. This sludge loss can
be significant in areas having high rainfall intensity and soil erosion. According
to Battele Memorial Institute (1987), the extent of sludge loss is dependent upon
rainfall intensity, soil erosivity, soil slope, type of crops being grown, and
erosion control practice implemented at the site. Under conditions where high
rainfall and steep slope prevail or when sludge application is conducting during
flooding and raining, the magnitude of sludge loss due to surface runoff should
be taken into consideration.

Example 9.2

Determine land area for application of anaerobically digested municipal sludge
with the following conditions:

a) Sludge production: 22 dry tons/day

b) Sludge characteristics: Pb = 500 ppm, Cd = 50 ppm, Ni = 100 ppm,

Cu = 500 ppm, Zn = 2,000 ppm, Total N = 2.5%, NH 4 = 1.0%, NO 3

= 0.