Organic Waste Recycling

Land treatment of sludge 469

During wet weather, which could cause soil compaction and run-off
problems, the sludge is taken to a temporary storage site with a sludge storage
capacity of 63,000 m^3.

9.4 TOXIC SUBSTANCES VS CROP GROWTH

Toxic substances in sludge can be categorized as heavy metals and toxic organic
compounds. The U.S.EPA had collected sludge samples from more than 200
representative wastewater treatment facilities and analyzed for more than 400
pollutants. They found that many toxic compounds traditionally subject to
regulations were either not present or were present at levels that would not pose
a health or environmental risk (Walsh 1995). Hence, only the heavy metal limits
(see Table 9.11) and faecal coliform standards, reported in section 9.3 are being
emphasized.
Some of the factors affecting plant uptake of heavy metals are as follows
(Bitton et al. 1980):

1. Levels of toxic elements in the wastewater or sludge and their
   characteristics;


2. Species of plants grown, their age, condition, and rooting depth.
   Some plants are known to be metal accumulators.


3. Background concentration of toxic elements in the soil and their
   distribution;


4. Ability of soil chemical constituents to convert toxic elements to
   non-available chemical compounds. This ability is in turn affected
   by the nature of toxic element and the type of soil, for example:
   a. pH of the soil solution
   b. Organic and clay content and type of the soil
   c. Phosphate level in the soil
   d. Cation exchange capacity (CEC) of the soil
   e. Adsorption and precipitation.

The above factors determine the deleterious effect of heavy metals and other
substances on plants and soils. In soils with high pH, most heavy metals occur
in precipitated, unavailable forms. Similarly, the phosphorus uptake by plants is
also retarded at high pH. Thus, calcareous solids are generally less amenable to
manifestations of heavy metal toxicity than are the neutral or acidic soils.
More serious consideration is being given at present to the fate of cadmium,
mercury, selenium, molybdenum, arsenic, copper, and zinc in the food chain.
The other heavy metals do not seem to accumulate in edible portions of crops
and are essentially phyto-toxins (some substances in trace amounts may, in fact,
be advantageous for crop growth). Zinc is more readily absorbed than most