Environmental Engineering Reference
In-Depth Information
Pb
mg/l
0.41
0.49
0.70
0.65
1.14
0.68
0.65
0.01
0.01
0.5
0.2
(0.142) (0.192) (0.741) (0.452) (0.382) (0.523)
(0.141)
Zn
mg/l
0.12
0.16
0.18
0.17
0.23
0.17
0.15
--
3
20
1
(0.071) (0.042)
(0.051) (0.132) (0.164) (0.074)
(0.062)
* WHO guidelines;
** South African guidelines;
Values in parenthesis reflect standard deviations of the mean values
• Sewage sludge land disposal, if applied adequately, leads to the improvement of soil
quality in terms of organic content and nutrients.
• In cases of sludge land disposal and beneficial sludge reuse in agriculture, the
enforcement of municipal by-laws limiting the discharge of wastes with high metals
concentrations into the sewer system is essential in order to limit the metals
concentrations in sewage sludge. The wastewater treatment process does not provide
for reduction of metals, thus the only way to reduce these concentrations is the source
control.
• Proper monitoring and control of pollutant concentrations in soils is essential, as soils
saturated with pollutants might act as a time bomb and release them into groundwater
after saturation limits are reached. Thus, the operational period of the dedicated site
would depend on such monitoring practice and must stop before these limiting
concentrations are reached.
In relation to the specific case of SEM disposal on Crowborough pastures, the following
conclusions could be made:
• The soils' quality has been improved in relation to organic carbon and nutrients content;
• The levels of metals concentrations in the soil are within prescribed limits, but the soil
has been acidified in the most often irrigated areas.
• Long term irrigation, high hydraulic and pollution loads and improper irrigation practice
have result edin elevated concentrations of metals in groundwater, with Pb, Cr and Ni
being of greatest concern. This makes the aquifer water quality unsuitable for
beneficial use and could be treated as a serious hazard for the environment.
The further use of the pasture at Crowborough farm as a land disposal site for SEM could
be recommended only if proper measures are implemented, such as the:
• Improvement of pH of the soil and additional investigation in respect to soil
characteristics;
• Reduction of the present hydraulic load. This could be achieved by:
Upgrading the BNR plant to prevent the overloading of the trickling filters line
(already under consideration by the municipal authorities);
Reduction of the incoming effluent by diverting part of the sewage to one or more
new local treatment plants;
Pumping part of SEM to new disposal sites.
