A two-step chemical precipitation process involving hydroxide
precipitation and sulfide precipitation combined with "field separation "
technology such as magnetic separation, dissolved air flotation, vortex
separation, or expanded plastics flotation, effectively removes chelated
and non-chelated heavy metal precipitates and other fine particles from
water. In the first-step, the non-chelated heavy metals are precipitated
as hydroxides and removed from the water by a conventional liquid/solids
separator such as an inclined plate clarifier to remove a large
percentage of the dissolved heavy metals. The cleaned water is then
treated in a second precipitation step to remove the residual heavy
metals to meet discharge limits. In the second precipitation step, any
metal precipitant more effective than hydroxide for metal precipitation
can be used. The invention improves metal removal, lowers cost because
fewer chemicals are used, produces less sludge, and reduces the discharge
of toxic metals and metal precipitants to the environment. Magnetic
separation is preferred for the separation of particles precipitated in
the second stage. Similar methods can be employed for separation of other
particulates from water. Particulates can also be removed by causing them
to adhere to particles of expanded plastic, forming a floc lighter than
water, so that the floc can be removed by flotation.