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Pollution Prevention and Control Technologies for Plating Operations
Section 6 - Wastewater Treatment
6.3 COSTS OF CONVENTIONAL WASTEWATER TREATMENT
6.3.1 Capital Costs
6.3.2 Operating Costs
Capital costs for conventional end-of-pipe treatment equipment are presented in Exhibits 6-16, 6-17 and 6-18. For each unit operation, both basic equipment costs and installed equipment costs are presented. Installation costs (38% over basic equipment costs) include: engineering (10%), shipping (5%), piping (8%) and electrical/instrumentation (15%). All equipment costs are for packaged, skid mounted systems. The unit operations are based on the following retention times:
- cyanide oxidation first stage: 60 minutes
- cyanide oxidation second stage: 120 minutes
- chromium reduction: 30 minutes
- neutralization: 30 minutes
- precipitation: 30 minutes
- sludge thickening: 24 hours
- polishing filter: 10 minutes
The major operating cost items for conventional treatment (labor, treatment reagents and sludge disposal) are shown in Exhibit 6-19. This chart is based on the results from the NCMS/NAMF Users Survey, which are discussed in Section 6.4. An individual shop can use the results of the survey and other data presented in this text to accurately estimate their chemical treatment reagent costs, based on the individual wastestream (i.e., Cr, CN, acid/alkaline) flow rates and characteristics.
Treatment reagent prices have risen dramatically over the past 20 years due to inflation and increased manufacturing costs (Exhibit 6-20). During this time period, most reagent costs have increased by a factor of two to five times. The actual reagent unit costs paid by respondents is significantly impacted by the annual quantity of reagent that is purchased. This is due to the packaging costs for small quantities vs the lack of packaging for bulk quantities. Exhibit 6-21 presents average treatment reagent prices paid by respondents, based on usage rates.
The quantity of reagents used by respondents was somewhat unexpected. As discussed in Section 6.4, on the average, respondents use much greater amounts of reagents than needed to satisfy chemical equations governing the treatment process reactions (i.e., stoichiometric requirements). This effect is due to several factors (based on conversations with survey respondents): (1) wastewaters contain buffering, chelating and other types of substances that interfere with the treatment processes and cause higher than expected reagent requirements; (2) wastestreams can be highly variable both in terms of flow and pollutant concentration, which causes dramatic swings in chemical feed and results in some wasted reagent; (3) shops must meet effluent regulations 100% of the time, rather than be in compliance Òon the average,Ó which causes them to add excess reagents; (4) local effluent requirements for cyanide or a particular metal species are often below the Federal limitations and can only be met by overdosing the wastewater with reagents; (5) treatment system designs may not be ideal; (6) instruments (e.g., ORP and pH) and chemical feed systems may not function perfectly all the time; and (7) operator ignorance or error.
The following are typical ranges of treatment reagent usage rates for survey respondents for each of the steps in the conventional treatment process3. Examples of calculations using these factors are presented in Exhibit 6-22.
Chromium Reduction:
- H2SO4: 1 to 10 lbs/1,000 gal, and 2 lbs/lb Cr+6
- NaHSO3: 3 to 15 lbs/lb Cr+6, or
- SO2: 2 to 10 lbs/lb Cr+6
- NaOH (50%): 3 to 6 lbs/1,000 gal
Cyanide Oxidation:
- NaOCl (15%): 8 to 40 gal/lb CN
Precipitation:
- NaOH (50%): 2 to 6 lbs/1,000 gal, and
- 5 lbs/lb Cr, and
- 4 lbs/lb other metals
Flocculation:
- Polymer: 0.1 to 0.3 lb/1,000 gal