The Effluent Trading Pilot project explored the use of a 'trading' mechanism as a means for companies to meet local pretreatment limits for the level of metals in the effluent they discharge to a Publicly-Owned Treatment Works (POTW). Trading allows industrial permittees to achieve the required overall additional reductions in pollutant levels more economically. Controlling metals levels in effluent can be very expensive. Yet when the control measures are instituted, a facility may be able to reduce the levels of metals in its effluent more than is required. For some facilities, instituting the control measures would be so expensive that they would have to shut down some production processes or go out of business. Trading allows facilities within the same POTW service area to work together to control the discharge of metals in a manner that is less expensive for all parties. A company that has instituted control measures that have brought its metals levels down below the local limits can 'sell' these excess reductions. One or more companies with effluent levels in excess of the local limits can buy the reductions as a means of complying with the local limits. A buying company and a selling company negotiate a price for the metals credits, and the permits of the trading partners are adjusted to reflect the amount of credits sold in the trade.