The commission shall create three separate funds in respect of the bonds of each series: one fund to be known as the “project fund, series                             “; another fund to be known as the “income fund, series                             “; and another fund to be known as the “debt service fund, series                             “; each fund to be identified by the same series number or letter as the bonds of such series. The money in each fund shall be deposited in such depository and secured in such manner as may be determined by the commission. It is lawful for any bank or trust company incorporated under the laws of this state or of the United States to act as such depository and to furnish such indemnifying bonds or to pledge such securities as may be required by the commission. A separate account shall be kept in each project fund and in each income fund for each project. All expenditures not properly chargeable to the project fund account or to the income fund account of any one project shall be charged by the commission in such proportions as it determines to the project fund accounts or to the income fund accounts, as the case may be, of the projects in respect of which such expenditures were incurred.