Wastewater treatment for metals

Wastewater treatment systems for metals are pretty well defined for precipitation systems. The incoming solution is pH adjusted to the optimum range for precipitating the metal as a hydroxide. In difficult situations a sulfide is added to increase recovery. The treated water is run through a clarifer to settle the solids. The effluent water is pH adjusted, if needed, to meet city limits and the hydroxide sludge is filter pressed to a cake for recycling.

Wastewater precipitation systems operate on the principal that give enough time and a low enough flow, a solid will settle out of a liquid. A pretty simple concept but much harder in practice than in theory. The way this is accomplished in practice is with a lamellae plate clarifier. By using a series of plates about 1" apart set at a 45-60 degree angle and forcing the water to run through the channels, a short settling distance is created. Using a flow rate of about 1/4 gpm/surface sq. ft. of plate, the solids drop to the plates before the water can carry them out. As they accumulate on the plates, they slide to the bottom and off into a sump where they are eventually removed.

The standard heavy metal wastewater treatment methodology for metals removal has been hydroxide precipitation with or without the addition of a sulfide. The sulfide results in a lower solubility than hydroxide precipitation alone. In some cases, neither seems to be able to consistently obtain satisfactory results. We have found in most cases, this was cause by either and undersized system (flow too high for the settling area) or incompatible constituents resulting in a carryover of the flock. The two solutions are obvious, size the system correctly in the first place or add a polishing filter to the end of the system. Our polishing filters can take a 20 NTU (NTU is a measure of optical clarity) effluent stream down to 0.2 NTU and operate completely automatic.

Another major problem with this technology is with chelates. Chelates are organic compounds that hold metals in solutions at high pH. Hydroxide precipitation depends on the insoluble metal hydroxide forming and the chelating agent prevents this. Some sulfides and strong reducing agents can break weak chelates but EDTA is more difficult.

Another method of breaking chelates is to do a substitution. A non hazardous metal is added that the chelate prefers over the target metal. For EDTA at high pH, Calcium, Magnesium, and Iron are all preferred over the hazardous divalent metals such as Copper, Nickel, and Zinc. Adding any one of these would increase the precipitation of the divalent metals.