Plant-specific effluent concentrations

Plant-Specific Effluent Concentrations, Verification Data... 

In relatively small-scale operations, where frequent shutdowns may be desirable, as at night or during week ends, the mixer-settler plant offers the advantage that concentration gradients are maintained within the plant indefinitely even though operations have stopped. On start up, steady-state conditions are established immediately, product specifications are met with the first effluents, and recycling of the first products withdrawn is unnecessary. The agitated vessel is also eminently suitable for purely batch processes, which are likely to be on a small scale. 

The superior flux and rejection capabilities of the thin film composite membrane has been demonstrated at the municipal wastewater reclamation facility of the Orange County Water District in California. Both asymmetric cellulose acetate and thin film composite membranes were tested on lime clarified secondary effluent. The pilot plants were operated at 85% recovery and the rejections reported in Table 4.5 are the percent rejection of the constituents in the feed-water and not the rejection of the average concentration of the specific constituents in the feed/reject stream. Use of the average concentration would give a higher rejection in both cases. 

The scrubbing liquor is recirculated at the scrubber. A bleed of the scrubbing liquor is required to control the build-up of sulfuric acid, selenium, mercury and other species. This is the so-called weak acid bleed. The volume of this bleed is relatively small. However, it is concentrated since it results from a recirculation loop at the scrubber. Historical data from the Valleyfield operation show that the average concentration of selenium in the weak acid bleed is in the order of 50 mg/1 with peaks up to 180 mg/1 the average mercury concentration is 30 mg/1 (SNC-Lavalin (3)). The concentration of selenium in such a bleed first depends on the specific selenium content of the zinc concentrate being proeessed. It is known that some zinc ore deposits are characterized by high contents of selenium. Unless adequate treatment technologies are available for the removal of selenium fi-om the acid plant effluent, future development of such deposits may encounter environmental obstacles. 

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