Waste Management Inc. (NYSE: WM) added one of the nation's largest private recycling firms to its portfolio for an undisclosed amount. The Houston-based company bought Greenstar LLC, also based in Houston, from Dublin-based NTR Plc, for an undisclosed amount, according to a statement. Greenstar manages about 1.5 million tons of recycled material for more than 12,000 customers through 12 material recovery facilities, including seven single-stream plants, and a brokerage business for recovered material. Operations…