What action can a tenant take against landlord for not putting tenant's security deposit into separate account

Your question is a good one and should probably be directed to a local Washington, DC attorney. The answer is heavily dependent on local law, but I can give you a basic overview of security deposits for your reference.

Generally, you are correct that a landlord is required to place a security deposit in a separate account. This is known as not "commingling" the funds. Basically, a security deposit remains the tenant's property, and a landlord has a fiduciary duty to keep it separate and maintain an accurate accounting. Also, the landlord has a duty to return unused funds with a final accounting after the tenancy is ended.

I would recommend speaking to a local lawyer because security deposits are heavily dependent on local landlord-tenant ordinances or state law. Each state treats the relationship between landlord and security deposit the same, but the specifics (i.e. interest rate and remedies) are different.

Good luck!

NOTE: This answer is not intended to be legal advice and should not be construed in that way. This answer does not create an attorney-client relationship and no such relationship may be created absent a signed retainer agreement. The author is licensed in Illinois only, and his answer is for educational purposes alone.