Switching from Lime to Magnesium Hydroxide

Q. We use a conventional metal precipitation wastewater pretreatment system consisting of sulfuric acid to lower pH to 4.0 to break chelating chemicals, lime slurry to raise pH to 9.5, addition of flocculant and then clarification. We are considering switching to magnesium hydroxide since jar tests indicate a 30% reduction in treatment residue at a pH of 8.5 and still meet our metal finishing pretreatment limits. Our flow consists of 1/3 tumbling and deburring and 2/3 nickel/chrome plating with a combined flow of 20–30 gpm. W.M.

 

A. A change from lime to magnesium hydroxide is certainly worthwhile to investigate. Its advantages include lower soluble metals in wastewater effluent, better floc formation, lower solids generation and, generally, higher solids content in the filter press cake. However, magnesium hydroxide has a very slow reaction time (30–60 minutes). Therefore, periodic lab tests are needed to determine the likely required dose, and the slurry has to be fed upstream of the pH adjustment tank in order to give it the needed reaction time, otherwise, much of the magnesium hydroxide will be wasted. If there is insufficient detention time in your system, an additional pH adjustment tank of sufficient volume would need to be installed in front of your existing pH adjustment tank since a small amount of lime slurry or sodium hydroxide will likely be needed to “top off” the magnesium hydroxide in order to achieve consistent pH control for metal precipitation. Because magnesium hydroxide is a slurry, be sure to provide mixing of the slurry and a water flush of the feed line(s).

Based upon your information, your wastewater filter press cake is a RCRA listed hazardous waste (EPA F006), which requires very expensive handling, transportation and disposal. Have you investigated separate wastewater treatment systems for your two waste streams? The tumbling/deburring wastewater is the likely source of chelators requiring the acid pretreatment stage and, if treated separately, it is almost a sure bet that the solids generated by the tumbling/deburring wastewaters would be a non-hazardous waste and disposed into a sanitary landfill at significant cost savings.

While tumbling/deburring wastewaters are usually high in solids, they are typically low in heavy metals (except for iron, of course) unless large amount of stainless steel parts are processed. You may want to investigate this waste stream and perform treatability studies on it. You may find that it may only need screening for media recovery, flow equalization, free oil removal, pH adjustment and gross solids removal by settling before it combines with the treated wastewater from the nickel/chrome plating operation for discharge.

Other advantages for separate treatment system include elimination of acid pretreatment of chelators, significant reduction of RCRA hazardous waste, reduced alkali demand for pH control due to elimination of acid pretreatment, and increased recycling potential of the metal-bearing filter press cake from nickel/chrome plating because of the removal of the “junk” tumbling solids.