In DC, you pay the interest earned. We keep the deposit in a separate interest bearing savings account and return to the tenant what interest it earned.
I just checked that account, our tenants have been in 7 years. The interest seems to be accruing currently at the rate of $0.05/month. They have earned $7.59 on their deposit in the last 7 years. I think we looked into whether we could at least put it in a money market account and that was not allowed by the rules.