Magnesium Hydroxide Controls Sulfide Corrosion in Rural Pennsylvania Sewer System

The East Nittany Valley Joint Municipal Authority operates a wastewater collection system that serves 11 villages in north central Pennsylvania. Because of the undulating terrain, the sewer system incorporates multiple grinder pump pressure sewers, eight pump stations and force mains, as well as gravity sewers. Approximately 205,000 gal/d of wastewater is conveyed through the system to a regional facility in Lock Haven for treatment. Constructed and started up in 2002, the system experienced severe corrosion and odor complaints in certain areas within a year after start-up. The wastewater discharging from the Interchange force main was found to be highly septic, with sulfide concentrations as high as 5 mg/L and headspace H2S concentrations of up to 150 ppm. Concrete corrosion was evident in many parts of the system, and some manholes had been rehabilitated after only 3 years of service.Various chemical control strategies were evaluated, including iron salts, hydrogen peroxide, air/oxygen injection, nitrates, and magnesium hydroxide. A screening analysis showed nitrate and magnesium hydroxide to be the two most likely candidates for this application. Preliminary cost analyses indicated that magnesium hydroxide (Mg(OH)2) offered significant savings over nitrate for this application. Further, it appeared that much of the sulfide could be controlled with a single dosing point of magnesium hydroxide vs. multiple injection locations for nitrate solutions.A full-scale demonstration of magnesium hydroxide was conducted in the fall of 2006. At a dosage of about 22 gal/d, H2S levels in the manhole at the discharge of the Interchange force main were reduced from an average of approximately 100 ppm to 2 ppm, a reduction of 98 percent. Similar results were found at the discharge of the Lizardville force main that is two pump stations downstream of the Interchange force main. Here, H2S was reduced from an average of 49 ppm to 2.4 ppm, a 95 percent reduction. This was achieved by a single dosage point nearly 4 miles upstream.