The Marysville Wastewater Treatment plant is complete and opened for tours yesterday.  After three years of work, the plant has expanded to increase the capacity by 50 percent.  The $20 million expansion included a new primary clarifying basin, trickling filter, a sanitary sewer overflow tank, a biosolids retention tank, screening system, front office and garage, and electrical and structural improvements to existing equipment.  The project was paid in part by the city with low-interest State Revolving Fund loans; the rest was paid with federal stimulus dollars.  The improvements were required by the Department of Environmental Quality in 2008 when the city’s sewer system regularly met or went over its maximum capacity.