up by the crop at certain concentration thresholds, and uptake can only occur if the
metals are in the mobile phase. Additionally, the concentration of heavy metals in the
soil solution is influenced by soil characteristics.
Heavy metals are less available at soil pH over 6.5, due to precipitation phenomena and the presence of higher amounts of organic substances. On the contrary, at
lower pH values, heavy metals become mobile in the soil and can be absorbed by
crops [44]. Not all heavy metals are easily absorbed by plants; lead, chromium, and
mercury, for example, are bound by soil particles and very slowly absorbed by crops,
even when accumulated in the soil. Copper, boron, and zinc are more easily
absorbed by plants, sometime reaching levels of accumulation ten times higher in
a plant than in its originating soil [45]. In this regard, cadmium and nickel represent
the highest risk to human health. The impact of heavy metals on crops is complex,
because antagonistic reactions can occur the influence their absorption [46].
Table 1 shows the heavy metal concentrations in wastewater-irrigated soils
reported by several authors, considering the irrigation length period (short and
long term), type of wastewater, and the effects on soil and plant. The concentration
of some heavy metals exceeds the international threshold values, especially in longterm experimental trials, with consequently accumulation in plants.
Table 2 shows a classification of heavy metals added to the soil with wastewater
when used for crop irrigation, according to risk characteristics and classes relative to
their effect on plant nutrition and human health.
2.3 Microbiological Risks
One of the primary obstacles to the widespread use of reclaimed wastewater for
irrigation of agricultural crops is the possible persistence of pathogenic microorganisms through treatments and their potential contamination of vegetable crops, causing outbreaks of foodborne illness. The risk of biological contamination is primarily
related to the spread of bacteria, viruses, helminths, and protozoa that are harmful to
humans through the soil environment and onto crops which are then ingested.
Bacterial pathogens like Salmonella spp., Shigella spp. enterohaemorrhagic
E. coli serotypes, and Vibrio cholerae are of major concern for public health systems
worldwide. The helminths Ascaris and Tenia spp. and the protozoans intestinal
Giardia and Crysptospridium are also of public health concern. The waterborne
viruses HAV, HEV, rotavirus, and adenovirus are reported to have the greatest risk
of transmission through reused wastewater [1]. The direct detection of such a wide
array of pathogen microorganisms (whose levels are generally low and fluctuating)
by laboratory methods is not an efficient monitoring strategy in terms of monetary
cost and time required for the microbial methods of isolation and confirmation. The
use of microbial indicators of faecal contamination has therefore been considered for
decades by health and environmental authorities worldwide to be the most reliable
method of monitoring water quality and the performance of water treatment systems
[47, 48].
Wastewater Reuse in Agriculture: Effects on Soil-Plant System Properties
85
metals are in the mobile phase. Additionally, the concentration of heavy metals in the
soil solution is influenced by soil characteristics.
Heavy metals are less available at soil pH over 6.5, due to precipitation phenomena and the presence of higher amounts of organic substances. On the contrary, at
lower pH values, heavy metals become mobile in the soil and can be absorbed by
crops [44]. Not all heavy metals are easily absorbed by plants; lead, chromium, and
mercury, for example, are bound by soil particles and very slowly absorbed by crops,
even when accumulated in the soil. Copper, boron, and zinc are more easily
absorbed by plants, sometime reaching levels of accumulation ten times higher in
a plant than in its originating soil [45]. In this regard, cadmium and nickel represent
the highest risk to human health. The impact of heavy metals on crops is complex,
because antagonistic reactions can occur the influence their absorption [46].
Table 1 shows the heavy metal concentrations in wastewater-irrigated soils
reported by several authors, considering the irrigation length period (short and
long term), type of wastewater, and the effects on soil and plant. The concentration
of some heavy metals exceeds the international threshold values, especially in longterm experimental trials, with consequently accumulation in plants.
Table 2 shows a classification of heavy metals added to the soil with wastewater
when used for crop irrigation, according to risk characteristics and classes relative to
their effect on plant nutrition and human health.
2.3 Microbiological Risks
One of the primary obstacles to the widespread use of reclaimed wastewater for
irrigation of agricultural crops is the possible persistence of pathogenic microorganisms through treatments and their potential contamination of vegetable crops, causing outbreaks of foodborne illness. The risk of biological contamination is primarily
related to the spread of bacteria, viruses, helminths, and protozoa that are harmful to
humans through the soil environment and onto crops which are then ingested.
Bacterial pathogens like Salmonella spp., Shigella spp. enterohaemorrhagic
E. coli serotypes, and Vibrio cholerae are of major concern for public health systems
worldwide. The helminths Ascaris and Tenia spp. and the protozoans intestinal
Giardia and Crysptospridium are also of public health concern. The waterborne
viruses HAV, HEV, rotavirus, and adenovirus are reported to have the greatest risk
of transmission through reused wastewater [1]. The direct detection of such a wide
array of pathogen microorganisms (whose levels are generally low and fluctuating)
by laboratory methods is not an efficient monitoring strategy in terms of monetary
cost and time required for the microbial methods of isolation and confirmation. The
use of microbial indicators of faecal contamination has therefore been considered for
decades by health and environmental authorities worldwide to be the most reliable
method of monitoring water quality and the performance of water treatment systems
[47, 48].
Wastewater Reuse in Agriculture: Effects on Soil-Plant System Properties
85