9420.pdf

In-house modifications can result in a reduction of the contaminant load discharged from
a plant but may not result in a reduction of the contaminant concentrations of the effluent.
This is due to the fact that contaminant loadings and effluent flows may be reduced by
similar margins. Therefore, effluents may continue to exceed discharge limitations or may
exceed the limits above which surcharges have to be paid.
7.2.9 Wastewater Treatment
Suppliers and manufacturers of wastewater treatment equipment were requested to
provide cost estimates for equipment required to treat a range of flows. The type of
treatment processes selected included fine screening using 600 mm, 500 mm, and
150 mm screens, and dissolved air flotation (DAF). The cost estimates were used to
calculate the range of treatment costs per discharge flow (see Figure 7.3)
A rule-of-thumb markup of 50% of the original cost estimates to account for taxes and
installation was assumed for the calculations. The results are shown for the flow range
of 100 m 3
/day to 6000 m3/day. Cost estimates for DAF do not include any sludge
dewatering equipment, or the cost of waste disposal. Costs also exclude in-plant
modifications to piping, yard work, power supply, and equipment housing which are site
specific which can easily double the total capital cost.
The efficiencies expected for each individual treatment option, shown in Table 7.3, are
based on file data and literature values, as detailed process evaluations could not be
obtained from the manufacturers/suppliers. The, efficiency of a particular wastewater
treatment system is dependent on the nature of the wastewater to be treated. For
example, with respect to screening, the treatment efficiency is dependent on the size
distribution of the particles in the wastewater. If all particles (suspended solids) present
in the wastewater are larger than the openings of the screen to be used, all particles and
the BOD associated with them can be removed. Conversely, suspended solids removal
may not increase much after the installation of finer screens, if the majority of the solids
are too small to be retained by the screen. In addition, screens will not affect dissolved
substances and the BOD which is associated with them. This dissolved BOD may,
however, be removed by chemically assisted DAF.