MIL-HDBK-1005/9A
solubility will depend on ionic strength, temperature, and
degree of chemical complexation. Bench scale or pilot plant
testing must be conducted to determine actual metal removals at
various adjusted wastewater pH values. Both soluble and total
metal concentrations in the treated effluent should be measured
during the testing program.
d) Sludge Production. Determine volume, mass,
settleability, and dewaterability of sludges produced during
treatability study. Using TCLP, determine the toxicity for
alternative chemical precipitants (selection of lime versus
caustic soda can affect the results of TCLP).
e) Coagulants. Coagulants such as iron salts, alum,
and polyelectrolytes may be required to enhance flocculation and
settleability of the metal precipitates.
f) Process Design. Use batch treatment systems
whenever possible. Provide reaction tankage for staged
treatment with each stage capacity sufficient to treat total
volume of wastewater expected during the treatment period. Use
batch tank as reactor and preliminary settler. Expected batch
treatment cycle is approximately 4 hours. If greater tankage
volume is required to accommodate duration of wastewater flow,
consider off-line storage of excess wastewater rather than
continuous flow treatment.
(1) Hydroxide precipitation. Provide mixed tank
with feed and multiple supernatant drawoff lines or drawoff by
telescoping valve (if batch reactor will also be used as
settling basin). If sulfate concentration in waste is high,
calcium sulfate scale will be a problem when lime is used.
Consider caustic soda to avoid scaling.
(2) Sulfide precipitation. Provide mixed tank
with sulfide (gas or Na2S slurry) feed and multiple supernatant
drawoff lines or drawoff by telescoping valve (if batch reactor
will also be used as settling basin). Maintain reactor pH
between 8 to 9.5 using a caustic to minimize formation of toxic
hydrogen sulfide gas. Provide excess sulfide in reactor to
drive precipitation reaction. Provide post-treatment aeration
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