An increasing number of central banks implement monetary policy via two standing facilities: a lending facility and a deposit facility. In this paper we show that it is socially optimal to implement a non-zero interest rate spread. We prove this result in a dynamic general equilibrium model where market participants have heterogenous liquidity needs and where the central bank requires government bonds as collateral. We also discuss how an change in the outstanding stock of collateral - as widely seen in the recent crisis where central banks increased the number of eligible assets - affects the economy.