Alternate Chemistry - Generating Less F006 Sludge
A major Midwest specialty hardware plating company that was discharging the plant's wastewater to the local POTW was experiencing metal limit excursions for some of the regulated metal limits. In addition, the sludge volume was reaching the capacity of both the plate and frame press as well as the gas-fired rotary dryer. The chemistry consisted of sodium hydroxide (50 percent caustic) for pH adjustment/metal precipitation as well as large quantities of antifoam and an anionic polymer for flocculation in the clarifier.
Over a six weeks period, numerous jar studies were run to establish alternative blended chemistries that would work consistently with the ever-changing waste stream while consistently meeting all regulated metal discharge limits. A reduction in sludge volumes also was sought.
Figure 1 is a flow diagram of the wastewater streams and treatment plant. The streams are first separated to treat for chromatic and cyanide. Acids and cleaners are collected in separate tanks and batch treated, and cyanide post district and reduce chrome are mixed into a Multi-stage Equilizator (EQ) tank. A blended precipitant produced by Water Specialists is controlled by an ORP controller in the first stage, and the coagulant is baseline fed to the second stage of the EQ tank by flow. The flocculent is then fed to the wastewater as it is introduced into the Upflow Clarifier.
A startup date was set and in eight hours the chemistry had changed. The following effects were observed:
- The clarifier cleared up, and operators could see more than ten feet down into the clarifier.
- The metal numbers were non-detect by AA testing to less than 50 percent of hydroxide program to non-detect.
- Operators were able to treat the water at a lower pH than the caustic program so the amount of acid needed to adjust pH for water discharge was greatly reduced.
- The polymers demand was reduced by 25 percent.
- The amount of sludge being generated dropped dramatically (approximately 75 percent)
Once over six months of data had been collected, operators generated an overall cost analysis on the return on investment for wastewater treatment to illustrate the overall savings that were realized while using the higher priced specialty blend chemistries. This analysis is shown below .
Daily Cost Comparison:
| Hydroxide Precipitation Costs | Water Specialists Treatment Program | |
| Caustic to raise pH | $97.50 (to 9.10) | $16.25 (to 8.9) |
| Precipitant | $59.58 | |
| Flocculant | $25.95 | $15.93 |
| Sludge Drying Cost | $259.00 | $57.56 |
| Sludge Disposal Cost | $96.21 | $19.24 |
| Acid to pH adjust | $10.00 | $1.10 |
| Total Annual Cost | $153,397.90 | $60,255.15 (39%+ Savings) |
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