A student of Harbin Institute of Technology's Shenzhen graduate school holds up a biofilter covered in greasy oil yesterday. The Buji Water Purification Plant was paralyzed Monday after more than 100,000 tons of untreated sewage discharged that day into the lower reaches of the Buji River over damaged the biofilters at the plant. It will take at least one month for the plant to resume operations, experts say. The environmental protection authority is yet to find the source of the pollution and bring the polluter to justice.Ding Qinglin