A FIELD REPORT BY RON MARLETT Hill Bros. Chemical – San Jose Branch 

 

Recently IPEC completed installation of a wastewater treatment system on a military base. The system is used to treat various metal contaminated streams originating from the base’s large scale plating operation. 

 

Per contract with the government, IPEC was required to operate the system to prove its capability of treating the various spent plating concentrates found at the base. As the chemical supplier I was involved in treating two solutions….. 

 

Solution #1: 

 

The first was a Sulfuric Acid / Sodium Dichromate solution. The initial pH of the 300 gallons to be treated in IPEC’s batch system was well below one and the Hexavalent chrome concentration was very high. Our first step was adding Mag Hydroxide to increase the pH to 3. Magnesium Hydroxide is typically slower than caustic, when increasing pH, however in this case it actually worked faster because it decreased the amount of exotherm and thus allowed for continuous operation. Our second step was adding Sodium Meta-bisulfite to reduce chrome from its Hexavalent form to a less toxic trivalent form which can be precipitated by hydroxides. Our third step was adding Mag Hydroxide to increase the pH to 7.5 and precipitate the chrome. After mixing at 7.5 for 45 minutes to allow for a thorough reaction. DTC was added to the solution to precipitate the remaining chrome and any other trace metals. After mixing another 20 minutes, we discharged the solution from the batch treatment tank to the filter press. The effluent, (tap water clarity in appearance), was discharged to a 35,000 gallon day tank awaiting testing and final disposal to the sewer. The filter cake went to the sludge dryer and was sent for disposal. 

 

Solution #2: 

 

This was an Electroless Nickel plating bath concentrate. 25 gallons of this highly complex concentrate, (pH 4), was added to the batch treatment system. It was then diluted with 350 gallons of water due to the very high metal concentration. Our first step was to increase the pH of the solution to approximately 9.5. This was achieved by using Mag Hydroxide to increase the pH to 8 and then “topping” off with Sodium Hydroxide to reach 9.5. After mixing the solution while at 9.5 and noticing near complete precipitation, we “polished” with DTC, mixed, and discharged to the filter press. As with the chrome solution, the effluent was crystal clear and was sent to the storage tank awaiting testing and discharge to the sewer. The dry filter cake was disposed of. 

 

Treating solutions from start to finish using IPEC’s equipment and Hill Brothers’ chemistry is an experience I found very rewarding. Reading books, reviewing technical information, & watching others operate systems is helpful, but hands-on-experience is the best educational tool around!