Geoscience Reference
In-Depth Information
Table 10.1 Maximum
allowable concentrations for
different metals in soils
Maximum allowable concentrations
(mg/Kg dry weight basis)
Metals
Arsenic
75
Cadmium
85
Copper
4300
Lead
840
Mercury
57
Molybdenum
75
Nickel
420
Selenium
100
Zinc
7500
concentrations of each of these metals allowed by the USEPA, which are consid-
ered safe for human contact (OEPA 2002 ). Chromium was later removed from the
list due to a lack of evidence of human toxicity (McBride 2003 ). On the other hand,
pathogen levels are not risk based. Instead, through treatment and use limitations,
pathogens are limited through requirements to reduce their incidence and resultant
potential exposure (NRC 2002 ). The Part 503 establishes criteria for classification
of biosolids as Class A or B dependent on the level of treatment.
10.2.1 Class A Biosolids
Biosolids that have gone through extensive treatment to reduce pathogens below
detectable levels and enhance vector attraction reduction (VAR) are termed Class
A biosolids, also known as exceptional quality (EQ) biosolids. Biosolids must meet
both prescribed densities of either Salmonella sp. or fecal coliform and treatment-
process control requirements to be classified as Class A. EQ biosolids can be bagged
and are deemed safe to sell to the public given that they meet these requirements.
They can also be applied to land if there are no pathogen-related restrictions attached
to the application site. The WWTPs distributing EQ biosolids need only a facility
permit for land application, not a land permit. The WWTPs are not required to keep
records of land sites to which EQ biosolids are applied. EQ biosolids can be applied
to land under the same restrictions of other soil amendment products or fertilizers
(NRC 2002 ).
10.2.2 Class B Biosolids
Class B biosolids are treated, but may still contain measurable levels of live bacte-
ria and active viruses after treatment. That means Class B biosolids contain more
pathogens than Class A biosolids. Class B biosolids must meet the organism con-
trol requirements of Part 503 in at least one of seven alternatives: fecal coliform
limitation, aerobic digestion, anaerobic digestion, lime stabilization, air drying,
composting, or equivalent process to significantly reduce pathogens. Unlike Class
 
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