Section 2Z. There shall be established and set up on the books of the commonwealth a separate fund to be known as the Commonwealth Sewer Rate Relief Fund. The fund shall consist of all amounts credited to the fund and any income derived from the investment of amounts credited to the fund. All amounts credited to the fund shall be held in trust and used solely for the purposes of this section. Amounts credited to the fund shall be available to mitigate sewer rate increases due to debt service obligations created by issuing eligible indebtedness. For the purposes of this section, eligible indebtedness shall mean debt issued on or after January 1, 1990, which has a final date of maturity more than 5 years after the date of issuance and which is incurred, wholly or in substantial part, to finance or refinance the cost of planning, design or construction of a water pollution abatement project, or part of such a project, required to be constructed to meet the Federal Water Pollution Control Act, 33 U.S.C. sections 1251 et seq., and sections 26 to 53, inclusive, of chapter 21, or any wastewater collection or transportation project related thereto. Eligible indebtedness shall not include any indebtedness for which the issuer has received assistance provided from state grants. Notwithstanding this section, eligible indebtedness shall include indebtedness incurred to finance the Metrowest Water Supply Tunnel and the Chicopee Valley Aqueduct Redundancy Project. Eligible indebtedness shall include indebtedness incurred under loan agreements under chapter 275 of the acts of 1989 which exceeded $50,000,000 by June 30, 1995, and the debt service attributable to those agreements for any year, for purposes of this section, shall be the net obligation borne by the issuer after application of any credits, subsidies or assistance, however characterized, provided under the aforementioned laws. No city, town, district, commission, agency, authority, board or other instrumentality of the commonwealth or any of its political subdivisions which is responsible for the ownership or operation of wastewater treatment projects and is authorized to finance all or any part of the cost of such projects through the issuance of eligible indebtedness, in this section called an issuer, shall receive relief authorized by this section in excess of 20 per cent of its annual debt service obligations due to eligible indebtedness. The division of local services of the department of revenue, in consultation with the department of environmental protection, shall develop guidelines to certify an issuer eligible indebtedness and shall create a process to distribute funds equitably to eligible issuers, in order to mitigate extraordinary increases in sewer costs. Funds disbursed in any fiscal year shall be disbursed on or before March 31 of the fiscal year. The board, office or commission responsible for setting sewer charges in each city, town, district or commission that either receives aid itself or is a member of a regional entity that receives aid under this section shall certify to the division of local services that it has reduced sewer charges to reflect its share of any such aid. No expenditure shall cause the fund to be in deficit at the end of the fiscal year.