When developing cost effective technologies for wastewater treatment systems, one should evaluate water and chemical conservation methods first. Next, modifications to the existing treatment system should be considered, since they require the least capital expense. Finally, new wastewater treatment systems should be evaluated.
The first questions are what the facilities' constraints are in terms of capital and real estate. Quite often, capital is scarce and management will want to exhaust all options with respect to modifying the existing system, even at the expense of high operating costs.
The second question should be, "Which technologies are in common use?" If a process is in widespread use, it has passed the test of time with others and is cost-effective. Next comes a determination of whether or not this process is technically feasible at the plant. This step may require months of bench scale and pilot testing to develop a comfort level. If this testing is positive, the search is done, and it is time to move to the design phase. If the testing does not prove a common technology, then facilities need to move into the newer technologies, such as membranes.
In the early 1980s, when the original metal finishing wastewater regulations were promulgated, the following steps were typically followed:
Step 1: Shop for wastewater equipment.
Step 2: Get sticker shock.
Step 3: Consider water and chemical conservation methods to reduce the capital and operating cost of the wastewater equipment.
Step 4: Shop for wastewater equipment with reduced chemical loadings and flow rates.
Why not eliminate Steps 1 and 2 and proceed directly to Step 3? There is no reason for equipment shopping or sticker shock when you can reduce the cost of wastewater treatment through water and chemical conservation. Reducing water and chemical consumption by 25-50 % is commonplace.
The list of potential cost savings is endless, but consider the following:
Consider Modifications or Replacement of the Existing System
After conservation methods, the most cost-effective approach is to modify your current treatment system. Many metal finishers, however, will simply replace their wastewater plant built in the '80s that has outlived its useful life due to corrosion and an outdated relay-logic control panel. The following technologies should be considered. The cost-effectiveness of these technologies varies with your situation and often other constraints, such as available space, will dictate the solution.
Ion exchange systems may be used as stand-alone treatment or as polishers to existing hydroxide-based systems. Ion exchange systems will meet low levels and may render a wastewater suitable for reuse. The water must be filtered to avoid physically plugging the ion exchange vessel. The technology consists of passing the water through a pressurized vessel filled with ion exchange media. Dissolved metal ions replace hydrogen ions on the surface of the media. The systems are regenerated with an acid solution and concentrated metal-bearing brine is produced. Most ion exchange systems are regenerated on-site and the brine is typically hauled to a hazardous wastewater treatment facility. Evaporation of the brine may be used to reduce the volume hauled.
The following two examples demonstrate the process of selecting treatment technologies.
Problem 1: A metal finisher has a 20 year old, 200-gpm hydroxide treatment system that uses the additive DTC to meet NPDES limits. Approximately 1,000 mg/liter of calcium chloride is also added to control oil. The system is in excellent physical condition, and the plant recently updated its system with a new state-of-the art control panel. The current DTC cost is $32,000 per year and would have doubled to more than $60,000 if it had had to comply the new MP&M regulations. The plant's environmental manager is concerned about increasing DTC usage because the effluent has occasionally failed aquatic toxicity limits. Physical space is at a premium and the plant manager wishes to know if there is a simple process change that is more cost-effective and eliminates DTC usage.
Solution: The least intrusive change would be to increase chemical additives, but DTC was the only compound that was effective during jar testing. A ferrous iron co-precipitation system will not require a building addition, but rather a simple changeout of the pH control tank to a specially designed reactor. The new control system is PLC controlled and could be modified to accommodate this change.
Jar testing demonstrated compliance with a ferrous dosage of 600 mg/liter and no DTC or calcium chloride. Annual cost savings in chemicals was projected at approximately $90,000 per year with a savings of approximately $10,000 per year in sludge generation. The installed capital cost of the reactor is $55,000 and can be installed during a short production shutdown. The plant has budgeted the conversion during a holiday shutdown rather than wait for the upcoming regulations.
Problem 2: A manufacturer of electronic components has a 25-year-old, sodium hydroxide precipitation system treating 50 gpm of waste containing lead, copper, and nickel. Five years ago, a sand filter was added to improve its compliance record with the existing metal finishing standards. The existing treatment system is in poor condition, with extensive corrosion of the process equipment as well as the control panel. Plant management desires a highly reliable, cost-effective system that requires minimal maintenance and management and is open to the idea of a new system. The plant has room for a small treatment system and space for a building addition.
Solution: A survey of the manufacturing equipment showed that one of the printed circuit board etchers produced as much metals in its waste stream as the other three etchers combined. It was decided to perform the wastewater evaluation assuming that this machine was replaced with a more efficient unit. The revised waste stream was projected to have a total metals content of approximately 20 mg/liter versus the previous 50 mg/liter at a reduced flow rate of 30 gpm.
Bench scale testing showed that ferrous iron co-precipitation as well as hydroxide precipitation with DTC addition could meet requirements. Ion exchange was tested and found to meet the limits. A cost comparison was made between a new precipitation system and ion exchange with both on-site and off-site regeneration. The installed capital costs and operating costs were estimated as follows:
|Ferrous iron co-precipitation||
|Hydroxide precipitation with DTC||
|Ion exchange with on-site regeneration and brine evaporator||
|Ion exchange with off-site regeneration||
The evaluation team selected the ion exchange with off-site regeneration system since it greatly minimizes wastewater treatment labor, provides a high level of treatment, and requires minimal capital investment since the ion exchange contactors are leased.
Planning for an efficient and cost-effective system should never be put off. It requires time to evaluate and implement both conservation efforts and new wastewater treatment technologies.