454 Organic Waste Recycling: Technology and Management
9.3 System design and sludge application rates
The design of land treatment of wastewater and sludge is usually based on the
limiting design parameter (LDP). LDP is the design value in which other
constraints such as nutrient requirements or public health problems will be
overcome. For example, the annual sludge application rate on an agricultural
land may be determined by N or P requirement, and the useful life of the site
may be limited by cadmium (Cd) loading. Sludge should be applied to
agricultural land at a rate equal to the N uptake rate of the crop unless less
application rates are required because of cadmium limitations. The basis for N
loading criteria is to minimize nitrate leaching to groundwater. The annual limit
for Cd loading is to minimize crop uptake and the potential long-term, sub-
clinical adverse effects on human health. The lifetime limit of a land application
site is established on the basis of maximum cumulative loadings of heavy metals
such as lead (Pb), zinc (Zn), copper (Cu), and nickel (Ni) and Cd. These limits
are designed to allow for growth and use of food-chain crops at future dates.
Since the liquid content of sludge is typically low, the hydraulic capacity of
the site soils is seldom the LDP for design. An exception might be toxic and
hazardous waste sites where very permeable soils would permit too rapid
percolate movement.
A preliminary estimate of the land area required for screening purposes can
be determined for municipal sludge with Table 9.8. Estimates of soil treatment
area for industrial sludge should be based on the critical LDP such as metal
concentration, etc. Slope limitation for the land application of the sludge is
given in Table 9.9.
For the agricultural utilization of sludge, the design approach is based on the
utilization of sludge as a supplement or replacement for commercial fertilizers.
As a result the annual application is based on either the N or P needs of the crop
in a particular soil. In addition, both the annual and cumulative sludge loadings
must be consistent with regulatory limits on pathogens and metals. Table 9.10
summarizes the U.S. EPA limits on cumulative heavy metal loadings for
agricultural land as a function of soil cation exchange capacity (CEC). CEC has
been employed as an indicator of soil properties because it has been shown
experimentally that most metals in sludge-amended soils are not present as an
exchangeable cation, or these metals are not exchangeable with a neutral salt.
Therefore soil with a higher CEC value would minimize the plant availability of
sludge-borne metals more than soil with a lower CEC value (U.S. EPA 1983).