The investigation found that Altnagelvin Hospital in Londonderry was due a refund of more than £285,000 from NI Water following the investigation which centred around a billing dispute.Another 19 hospitals across Northern Ireland could be in line for a windfall worth millions after the Consumer Council secured what is its biggest ever repayment.NI Water, however, has said its estimation on the overall amount to be repaid is "substantially lower" than the millions speculated.Council interim chief executive Aodhan O'Donnell said: "This is only the beginning of the story, because the work we have done with Altnagelvin's representatives has implications for all hospitals across Northern Ireland."All the other 19 hospitals, across Northern Ireland are now entitled to a significant refund as well, but they have to apply by 28 February to make sure their refund is calculated and paid back to them as quickly as possible."And any hospital trust can come to us for help and advice and we can work with them to make sure they receive whatever money is due to them."This clarifies a billing issue which dates back some time and it also makes sure the hospitals can now have confidence in their bills."Not only is this the biggest refund the Consumer Council has ever secured, it is great to think this money could make a real difference for patients.Aodhan O'Donnell, Consumer CouncilA cheque for £285,616 was presented to Altnagelvin by NI Water following the billing dispute.It resulted from a law change in what is classified as standard sewage and what is trade effluent, like that produced by laundries or hydrotherapy pools, which usually meant lower charges.NI Water revised its hospital charging policy and this was reflected in bills from 2010.The Consumer Council argued that the changes should have been applied from 2008, when sewerage charges were introduced for businesses and public organisations, and that two-year delay plus technical changes contributed to the refund.A NI Water spokeswoman said a detailed review was conducted of the legislation and the resolution was satisfactory for all parties.She said: "The engagement process is coming to an end on 28 February, however, NI Water remains committed to working with the other hospitals to ensure that any adjustments to their accounts are effected without delay on provision of the required evidence."NI Water welcomed the input of Consumer Council during the investigations required to bring a positive outcome for our customer."However, a number of teams have been involved in this process and NI Water would like to commend the work of its own team in collating the information and ensuring it was checked and validated."On speculation that the publicly owned company could face an overall bill in the millions of pounds, a spokesman added: "At this minute in time NI Water is still engaged with local hospitals and trusts to establish the exact quantum involved in terms of refunds but would be confident that the total would be substantially lower than the "millions of pounds" figure presently being speculated."