Sewer rates in Lansdale will not increase this year.   A study presented by a consultant during a joint meeting of two borough council committees Wednesday night concludes that the $6 a month hike in the unit charge implemented last summer is currently enough to keep the sewer fund in the black.   Borough officials also say the study is a useful benchmark and provides them with a model that will be an invaluable tool down the road as decisions are made about rates and other issues affecting the sewer department.