Environmental Engineering Reference
In-Depth Information
sludge, if not killed, may leach into ground water (Powelson et al.
1991
). There are
four major types of human pathogenic (disease-causing) organisms (bacteria, viruses,
protozoa, and helminthes), and all may be present in domestic sewage. The actual spe-
cies and quantity of pathogens present in domestic sewage from a particular munici-
pality depends on the health status of the local community, and may vary signiicantly
at different times. The concentration of pathogens in treated sewage sludge (biosolids)
also depends on the reductions achieved by wastewater and sewage-sludge treatment
processes. During the typical wastewater treatment process, the microorganisms pres-
ent in sewage are reduced in number and become concentrated in the sewage sludge.
However, some pathogens are still present in the efluent, which can contaminate rec-
reational waters and drinking water supplies (Rose et al.
1996
) (Table
3
).
Lewis and Gattie (
2002
) reported that, of all the pathogenic organisms present
in sewage sludge, enteric viruses are of the utmost risk to humans, owing to their
resistance to high pH and heat treatment, high infectivity, and survivability (Gibbs
et al.
1994
; Lewis and Gattie
2002
). The bacteria (e.g., fecal coliform,
Listeria
monocytogenes
, and enterococci) found in sludge are capable of surviving anaero-
bic digestion (Sidhu
2000
; Gerba et al.
2002
; Estrada et al.
2004
). After waste
treatment processes and land application of sewage sludge, regrowth may also
occur (and does occur with
Salmonella
sp). Several plant disease-causing patho-
gens have also been reported to exist in sewage sludge (Santos and Bettiol
2003
;
Al-Zubeiry
2005
). Contamination of ground and surface water by chemicals and
pathogens, and odor from volatile organics are some of the potential problems
associated with the use of sludges on cropland (USEPA
1994
).
Bioaerosols are particulate matter of microbial, plant or animal origin that mea-
sure less than 20 mm in diameter (Goyer et al.
2001
). They consist of pathogenic or
nonpathogenic live or dead bacteria, viruses, molds, pollens, etc. Bioaerosols are of
considerable concern because they are associated with a wide range of health prob-
lems such as contagious infections, allergies, and cancer (Bray and Ryan
1991
;
Douwes et al.
2003
). These are also of concern from the use of sewage sludge
amendment, since bioaerosols can transmit many enteric microorganisms (Pahren
and Jakubowski
1980
). Land application of sewage sludge can result in the transport
of pathogens through aerosols downwind of sludge storage sites, contamination of
ground water, drinking-water wells, stock ponds, or food chain contamination from
eating food grown in sludge-treated land.
4
Conclusions
The land application of sewage sludges to agricultural soils, and associated prac-
tices, is the most cost-effective management technique for disposing of sewage
sludges and offers potential improvements over conventional disposal methods,
such as landilling or incineration. One advantage of such land application of slud-
ges is that it recycles plants nutrients that are present in the sludge. Nutrients present
in sewage sludge are also useful to the soil microbial biomass. Using sewage sludge
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