Catalysis of the reaction: 1,1-dichloro-2,2-bis(4-chlorophenyl)ethane = HCl + 1-chloro-2,2-bis(4-chlorophenyl)ethene. 1,1-dichloro-2,2-bis(4-chlorophenyl)ethane is also known as DDD; 1-chloro-2,2-bis(4-chlorophenyl)ethene is also known as DDMU.