Land treatment of sludge 459- Nutrients and excessive salts do not leak into surface or subsurface
water supplies. - Crops are not contaminated with pathogens that will become health
risks to the farm operators and consumers.
Application rate based on heavy metal concentrations
It is apparent from Table 9.10 that sludge with significant metal concentrations
might have an annual application limited by Cd. According to this table, the
maximum lifetime application of cadmium is 5 kg/ha for soils with a cation
exchange capacity (CEC) less than 5 meq/100 g, 10 kg/ha for soils with a CEC
between 5 and 15 meq/100 g, and 20 kg/ha for soils with CEC exceeding 15
meq/100 g. A further restriction is that sludge containing Cd in excess of 25
mg/kg should not be applied to sites where tobacco, leafy vegetables or root
crops are grown. The state of California, U.S.A., additionally recommends that
the annual application of Cd from sludge should not exceed 0.5 kg/ha on land
use for production of crops grown for human consumption (Crites and Richard
1987).
Application rate based on nitrogen
Since much of the sludge nitrogen is in the organic form, as shown in Tables 9.3
and 9.4, it is not all readily available to the plants. Other forms of nitrogen in
sludge are ammonia nitrogen and very little nitrate. When applied on land, part
of organic nitrogen will be mineralized or biologically converted into ammonia
and/or nitrate to become available to the plants. Table 9.14 contains suggested
mineralization rates for organic nitrogen present in a few types of sludge. The
organic nitrogen not mineralized in the first year (or the first growing season)
will be mineralized in the subsequent years according to these mineralization
rates. Because of the high temperatures prevailing in the tropics,
Sripanomtanakorn and Polprasert (2002) found the percent N mineralization of
anaerobically digested sludge in the first growing season to be 27-42%, higher
than the 20% value shown in Table 9.14.
Under a high pH condition some ammonia nitrogen will be lost through
volatilization (Equation 6.1 and Figure 6.9). Ammonia nitrogen is readily
absorbed by soil but nitrate nitrogen is not. Nitrates that are not uptaken by the
plants and not biologically denitrified (or converted into gaseous nitrogen) may
be leached into groundwater.
The following example shows a method to calculate sludge application rate
in the first year based on nitrogen uptake rate of a crop and considering mass
balance of nitrogen in soil.