The town’s Water Conservation Oversight Committee  is rewriting a local bylaw to give it more teeth, eliminating the provision for voluntary conservation in an effort to ensure that residents adhere to usage limits set by the state Department of Environmental Protection  and the federal Environmental Protection Agency.  Under permits issued by the agencies, Billerica may draw up to 65  gallons of water per person, per day,  from the Concord River.  As work on the bylaw continues, the town remains under a mandatory outside water ban, allowing odd-numbered houses to water on odd-numbered calendar days and even-numbered houses to water on even-numbered calendar days.  To help residents further conserve water, the town Department of Public Works is offering water conservation tips on its website, www.billericadpw.org.