Technical and economic feasibility assessment of metals reduction in pulp and paper mill wastewaters

Pulp and paper mills may have to remove metals from wastewater streams to comply with limitations derived from the Great Lakes Initiative (GLI) surface water quality criteria of March, 1995. The metals in order of greatest to least concern are mercury, cadmium, copper, chromium, nickel, and zinc, and arsenic. Mercury poses a particularly severe compliance problem because dilution in the mixing zone cannot be considered in calculating its discharge limit, and the mercury criterion must be satisfied in the plant discharge. This study evaluated metal removal processes with potential to achieve the proposed metals limitations. This identified the electrocoagulation and insoluble sulfide processes with the greatest potential to reduce metals to GLI criteria. Cost analyses of these processes suggested total costs (capital plus operating and maintenance costs) in the range of $0.40 to $0.80 per 1000 gallons of water treated. Applying the median production-normalize bleached kraft effluent flow value of 20,054 gal/ton from NCASI's 1996 Wastewater and Solid Waste Survey results in production-normalize costs of $8 to $16 per ton of pulp. Several process variations with potential for reducing costs below these levels were identified during the cost analysis. Overall, it is less costly to treat a low-volume, more concentrated in-plant stream than to treat total mill effluent. In-plant treatment should be considered if the in-plant stream contains a high percentage of the total plant metals loading and the stream can be effectively treated.