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:
- Reducing rinse water flow rates with automatic valves controlled by conductivity sensors. Conductivity sensors are much more reliable than in the past.
- Use RO water for all metal finishing uses. Many plants use unreasonably high amounts of water to compensate for poor water quality. RO equipment has dropped considerably in price as the technology has become widespread in industrial and commercial applications.
- Automatic chemical feeders, particularly for use with liquid alkaline cleaners, can produce cost savings. Typically, without automatic feeders, operators renew the bath at the beginning of the shift, and the bath is made very stout to last the entire shift. With automatic feeders, constant feed of chemicals results in less chemical usage.
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.
- Ferrous iron co-precipitation is often more effective and typically less costly to operate than a hydroxide based system. Ferrous iron co-precipitation can be used to either replace the current hydroxide-based system or as a polishing step to the hydroxide-based system. These systems are well proven to meet very low metals limits, often less than 20 ppb of the individual metals. In addition, hexavalent chromium is reduced and co-precipitated along with other metals. This technology forms a dense iron sludge that physically entraps containments as it is formed. The system is effective in the presence of chelators and oils since they are caught up in the matrix. Modifying an existing system consists of replacing the existing pH adjustment tank with a special reactor and modifying the system's control sequence per the supplier's recommendations. A new reactor and clarifier are required to use this technology as a polisher to an existing hydroxide system.
- Chemical additives may be added to existing precipitation systems to reduce metals below their hydroxide solubility limits. These chemicals have become widespread. These compounds are typically sold as liquids and metered in at the pH adjustment tank. Dithiocarbamate (DTC) forms an insoluble metal sulfide precipitate and is particularly useful in removing metals from chelating agents such as ethylenediamine tetracetic acid (EDTA). DTC forms a double bond with metals that is much stronger than the bond between chelants and metals. DTC's attraction is so strong that it physically removes the metals from the chelant's weak grasp. The main drawback of DTC, however, is the toxicity of its breakdown products that should be evaluated fully prior to implementation. Chemical suppliers also have a number of other products including chelating polymers that scavenge the soluble metals in a hydroxide-based system.
- Membrane systems continue to improve in performance and have lower capital costs. Ultrafiltration (UF) systems are fine filters that may reduce metals concentrations by removing total suspended solids to low levels. Dissolved metals are typically not removed beyond the metal hydroxide solubility limits by UF. UF is widespread in metal finishing and oily wastewater treatment, and membrane life is much less of a worry than it was 10 years ago. Nanofiltration and RO are much less widespread and should be considered in the development stage for wastewater treatment. Nanofiltration will physically reject divalent metal ions as well as cations such as sulfates. RO will produce effluent with metals at the analytical detection limits, but it is the riskiest and costliest technology available. The shortcoming on all membrane systems has been in membrane life, and only pilot plant testing can prove the technology for your application. The cost of this testing, however, may be very worthwhile since wastewater treated by membranes may be suitable for recycle.
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.