MIL-HDBK-1005/9A
storage tank, metering pump, and in-line mixing element. The
addition of alkali can be controlled by simple instrumentation
for pH control.
The precipitated sulfides are removed during
subsequent oil/water separation equipment such as DAF or IAF
(refer to paragraph 3.10.6).
(3) Aeration. Air injection into oily wastewater
can prevent sulfides. In this technique, wastewater is aerated
for an extended period of time, such as up to 24 hours to
prevent ORP conditions. Aeration can cause release of existing
sulfides due to mixing and should be avoided in enclosed areas.
The sulfide formation during storage can also be
prevented by the addition of a supplemental oxygen source, such
as nitrate salts. The bacteria will consume nitrates instead of
sulfates and sulfides will not be formed. The addition of
nitrates has a disadvantage in that it enhances biological
activity in wastewater and may significantly increase suspended
solids loading.
3.9.4
Dissolved Metals Removal
a) General. Oily wastewater may contain significant
amounts of dissolved metals. The removal of the metals may be
necessary due to discharge regulations particularly for direct
discharging plants or for improved plant performance. Metals
such as iron, zinc, lead, copper, and nickel can be reduced to
low levels by chemical precipitation.
b) Treatment. The removal of dissolved metals can be
accomplished by raising wastewater pH above 8 by adding lime or
sodium hydroxide. At this pH, metals form highly insoluble
precipitates. The minimum solubilities of different metals
occur at different pH values as shown in Figure 14. Therefore,
a laboratory investigation is essential to determine the optimum
pH level. For plants using sulfide control by chemical
precipitation, additional treatment for metals removal may not
be necessary. As the alkaline solution is added for sulfide
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