Section 31. A public entity may apply to the division for a planning grant by the commonwealth for the purpose of assisting the public entity in developing a comprehensive water pollution abatement plan for such public entity. Upon approval by said division, the division may grant for such purpose, a sum not to exceed fifteen thousand dollars. In no case shall a grant exceed the reasonable and necessary cost of such planning, after subtracting therefrom any federal grant made to such public entity for the same purpose. The division may direct that such planning be undertaken in such manner as to maximize federal reimbursement, or to minimize the cost to the commonwealth, including, in appropriate cases, metropolitan-wide comprehensive planning and programming for co-ordinating activities affecting the development of the area. Planning grants authorized under this section need not be repaid to the commonwealth, and shall be in addition to financial assistance made available under other sections of this chapter.