In most cases, these metals can be removed by invoking a simple law of chemistry. Raising the pH of the wastewater will cause metal hydroxides to form. And getting those hydroxides to fall out of solution, to precipitate out, is relatively straightforward. Unless the wastewater contains chelated metals. The chelates are molecules in the wastewater that latch on to the metals and won't let them fall out of solution even if pH is raised.

The traditional way of dealing with chelated metals is to add ferrous sulfate and to lower the pH of the wastewater. The iron in the ferrous sulfate replaces the metal held by the chelate, then when the pH of the wastewater is raised, the metal drops out as a metal hydroxide.

Although effective in removing dissolved metals, this process also generates large volumes of wastewater treatment (WWT) sludge, which is a listed hazardous waste (F006), from electroplating operations such as printed wiring board manufacturing.

Substitutes Evaluated
Searching for an alternative to ferrous sulfate that wouldn't generate as much sludge and so would reduce its waste-disposal costs HADCO tried other WWT chemicals such as dimethyl dithio carbamate, or DTC, in two New Hampshire facilities and at its California location.

Although DTC removes as much metal as does ferrous sulfate, it isn't a perfect substitute. The DTC is toxic, and it generates carbon disulfide (another toxic) when mixed with water.

In 1992, the HADCO facility in Salem, New Hampshire, changed its WWT chemistry to a new product developed by Romar (for Removal of Metals And Recycling) Technologies in nearby Danvers, Massachusetts.

"It's a more forgiving process for the operator in that it doesn't require as much attention as the DTC chemistry we've been using," said Lee Wilmot, HADCO's corporate safety, health, and environmental manager, when the company's adoption of the Romar Process was announced in late 1993.

The powder that HADCO now uses, which is called ROMET RP-9000, removes as much metals as did ferrous sulfate or DTC, but generates half the sludge because it creates less nonmetallic solids that contribute to the sludge.

Sludge Generation Falls Dramatically
In 1991, for example, HADCO's Salem facility shipped 67 tons of WWT sludge; two years later sludge shipments dropped by more than 50 percent to 33 tons. At the same time, HADCO maintained continuous compliance with its discharge limits for copper (1.5 mg/l) and lead (0.34 mg/l). In each year, about 16 million gallons of wastewater were treated.

Although it contains about 50 percent water, the WWT sludge that results from this process is rich with minerals mostly copper in HADCO's case but also some tin, lead, and smaller amounts of other metals and so is greatly amenable to recycling. The sludge has a greater concentration of copper than does virgin ore. Even so, HADCO pays about $365 per ton to have its WWT sludge recycled and does not realize a return from the recycling. And because ROMET is more efficient than previous WWT chemicals, HADCO can also treat onsite certain waste acids that had been shipped offsite, further reducing its waste treatment costs by about $8,000 annually.

Wilmot says that from 1990 to 1995, HADCO reduced its sludge generation by 35 percent while its production increased by 70 percent at all facilities that generate wastewater. (HADCO facilities in Hudson and Derry, New Hampshire and in Watsonville, California also use ROMET.)

On a production adjusted basis, according to Wilmot , sludge generation is down about 72 percent. Fifty percent can be attributed to ROMET, and the remaining 22 percent to improved pollution prevention practices.