Cleaners Ruining Wastewater Quality

Question:

We do acid and alkaline zinc as well as copper/nickel/chrome plating. We have a conventional wastewater pretreatment system that uses caustic as the main neutralization chemical. Our wastewater quality to the sewer is typically excellent, except when we treat our spent cleaners. We have tried reducing the rate at which we bleed the cleaners into the treatment system. We have also tried adding calcium chloride, ferrous sulfate and aluminum sulfate. While these work, they require very large dosages resulting in greatly increased filter cake generation. Any other suggestions? N.A.

Answer:

The reason why your cleaners are wreaking havoc on your wastewater pretreatment system is that they are designed to keep soils, including your heavy metals, in suspension because they contain chelating agents. You have several options.

First, you can work with your chemical suppliers to investigate alternative cleaners with chemistries that would not interfere with your pretreatment system, at least to the degree that the present cleaners do. The challenge, though, is to find a cleaner that both achieves the cleaning quality that you need and, at the same time, not interfere with wastewater pretreatment. Some of the alternative cleaners require additional equipment such as oil skimmers and filters.

Speaking of oil skimmers and filters, you should also investigate extending the life of your baths through the removal of dirt and oils that are removed from the parts. We have clients that have extended their cleaner life 2-4 times with employing a simple oil/water separator followed by a cartridge filter; another client uses a membrane system. The amount and type of dirt and oils will determine the most effective removal system and materials. There are a number of suppliers of such systems listed in Product Finishing’s Directory and Technology Guide (www.pfonline.com/suppliers.html); try these sections: cleaning and phosphating equipment; filtration equipment, solutions; and oil skimmers, separators.

If extending the life of your baths is successful but you still run into wastewater pretreatment interferences, you should investigate the off-site treatment and disposal of the spent cleaner. We know one facility that finds it cheaper to have their spent cleaners pumped out of their cleaner tanks and transported to an off-site treatment facility.

A fourth option is to set up a batch pretreatment tank for your waste cleaners. Maybe your current holding tank could serve this purpose. The likely additional equipment is mixing by either an electrical or air operated mixer or a recirculating pump with several eductors on its discharge. Here you can use the chemicals you mentioned above, using less chemicals overall since you are not treating your entire wastewater flow, and generating less sludge. There are three other pretreatment chemistries that we suggest you try. Contact your chemical supplier to evaluate the effectiveness of cationic polymers with and without ferric chloride; cationic polymers can be very effective and do not generate the volumes of filter press cake. Another chemistry is the use of magnesium sulfate (Epsom salts) at a dose of 40-60 lbs for each 200 gal of spent cleaner. A third option is the use of your waste hydrochloric acid; use this acid to reduce pH of spent cleaner to less than four. If there is significant oil, it may float and will need to be skimmed off before metering into the wastewater pretreatment system. The beauty of a batch pretreatment tank is that you can easily experiment with a variety of chemistries to see what works best for your facility. It can be cheaper to operate since you are adding chemicals to a much smaller volume of wastewater.