Strange. If this is a UK airport that caters mainly to domestic flights, then I understand why they're doing it; but if it's an international gateway, passengers won't be arriving with pockets full of pound coins, so will just not use the trolleys.
Instead of messing around with trolley charging, the airports should be looking at the passenger's whole journey needs, and investing in partnerships with other transport providers to get people to their final destinations swiftly and comfortably: bemos, service taxis, minicabs, minibuses, rail, trams, buses, camels, whatever.
In these times of difficulty and low passenger numbers, airports need to think strategically. If you go to Durham railway station and look at the departure screen, you will see a train every hour to Manchester Airport. Nothing to Durham/Tees Valley Airport. Says a lot, really.