Section 22. (1) For the purpose of providing the funds necessary to carry out the powers and duties of the insolvency fund, the board of directors shall assess the generators at such time and for such amounts as the board finds necessary. Assessments shall be due not less than sixty days after prior written notice to the generators and shall accrue interest at twelve per cent per annum on and after the due date. Failure without just cause to pay any lawful assessment pursuant to this section and in accordance with the plan shall constitute a violation of this chapter.

(2) There shall be two classes of assessments, as follows:

(a) Class A assessments shall be made for the purpose of meeting administrative costs and other expenses. Class A assessments may be made whether or not related to a particular impaired or insolvent licensee.

(b) Class B assessments shall be made to the extent necessary to carry out the powers and duties of the insolvency fund under section nineteen with regard to an impaired or insolvent licensee.

(3) The amount of any Class A assessment shall be determined by the board and may be made on a pro rata or non-pro rata basis. If pro rata, the board may provide that it be credited against future Class B assessments. A non-pro rata assessment shall not exceed one hundred and fifty dollars per generator in any one calendar year.

(4) Class B assessments against generators shall be made pro rata in the proportion that the gallons of liquid hazardous waste or pounds of solid hazardous waste generated in the commonwealth by each assessed generator during the calendar year preceding the assessment bears to the total of such wastes generated by all generators in the commonwealth for the calendar year preceding the assessment. For purposes of this assessment, the department of environmental protection shall, at the request of the insolvency fund, determine the amount of such wastes in total and for each generator. One gallon of liquid hazardous waste shall be considered the equivalent of eight pounds of solid hazardous waste for purposes of calculating the assessment.

(5) Assessments for funds to meet the requirements of the insolvency fund with respect to an impaired or insolvent licensee shall not be made until necessary to implement the purposes of sections fifteen to thirty, inclusive. Classification and computation of assessments under this subsection shall be made expeditiously and with reasonable accuracy, recognizing that exact determinations may not always be possible.

(6) The insolvency fund may abate or defer, in whole or in part, the assessment of a generator if, in the opinion of the board, payment of the assessment would endanger the ability of the generator to continue in operation. In the event an assessment against a generator is abated, or deferred in whole or in part, the amount by which such assessment is abated or deferred may be assessed against the other generators in a manner consistent with the basis for assessments set forth in this section.

(7) The total of all assessments upon a generator shall not in any one calendar year exceed ten per cent of such generator's average annual gross revenues received in the commonwealth during the three calendar years preceding the assessment. If the maximum assessment, together with the other assets of the insolvency fund, does not provide in any one year an amount sufficient to carry out the responsibilities of the insolvency fund, the necessary additional funds shall be assessed as soon thereafter as permitted by sections fifteen to thirty, inclusive.

(8) The board may, by an equitable method as established in the plan of operation, refund to generators, in proportion to the contribution of each generator to the insolvency fund, the amount by which the assets of the insolvency fund exceed the amount the board finds is necessary to carry out during the coming year the obligations of the insolvency fund, including any assets accruing from assignment, subrogation, net realized gains and income from investments. A reasonable amount may be retained to provide funds for the continuing expenses of the insolvency fund and for future losses.