The grant contribution, from funds made available pursuant to sections 644.500 to 644.561, made toward the cost of such water pollution control projects shall not exceed fifty-five percent of the estimated reasonable cost thereof as determined by the clean water commission. The state may charge interest at or below market rates, on loans made pursuant to this chapter, except that the state may charge interest on loan payments, delinquent under the terms of the loan, not exceeding twenty percent per annum. The state may charge an administrative fee on assistance made pursuant to this chapter.