Environmental Engineering Reference
In-Depth Information
• Provision of a simple monitoring program to control actual hydraulic and organic
loading rates, treatment efficiencies, and other necessary environmental factors as pH,
T, DO and the sludge dept at selected locations.
• Maintaining in good condition the following basic elements:
The hydraulic conduits and corresponding shutoff devices;
The inlet/outlet structures of the ponds;
Screening devices and pumping stations;
• Cutting the grass along the banks of the ponds and between the different treatment
units;
• Periodical evaluation of WSPS performance and preparation of recommendations for
the future operational plan of the system.
One of the most common reasons for the failure of WSPS to meet the expected treatment
efficiency is the lack of regular desludging and the filling of the volume available for
treatment with sediments. It must be admitted, that in many cases this could not only be a
problem created by negligence during the operational stage, but a problem induced
during the design stage. The design team should prescribe clear directions about the
method of desludging and the period of desludging at all stages, but most importantly,
with respect to AP or PF stages. Also, they should indicate appropriate methods for
sludge disposal. If these provisions have not been included during the design stage, it
would be more difficult to do it after the ponds are operational.
3 REGULATING THE PRACTICE OF IRRIGATION WITH SEWAGE
EFFLUENTS
Wastewater contains physical, chemical and biological constituents that affect its
suitability for reuse in irrigation. Some constituents have beneficial effects in terms of
crop requirements and the improvement of soil characteristics. Others have adverse
effects on soils and crops. Therefore, different regulatory instruments and literature
sources provide guidelines and recommendations with respect to the required range of
variation of concentrations with respect to different water quality constituents, which
could help in the implementation of a safe practice of wastewater reuse in agriculture.
Three categories of constituents are discussed in terms of their impact on crops and soils:
salinity and related parameters, specific ions' impacts, and trace metals and related toxic
effects. In addition, indications for the suitability of water to be used for irrigation
purposes have been made.
3.1
Salinity and soil permeability
Accumulation of salts in the upper soil layers (the root zone) reduces considerably the
ability of plants to extract water from the soil. The effects of soil salinity on plants
depend on the type of soil, irrigation practices, crop type and the growing stage of the
crop. Soils' salinity is measured by the electrical conductivity of the soil. Salinity of
wastewater is expressed by TDS and the correlated parameter EC, or by the concentration
