Unsatisfactory levels of Legionella was found seven times in one Belfast City Council-owned facility last year.
The spa area of Avoniel Leisure Centre had to close on three occasions, with follow-up water tests after these closures accounting for the additional failed tests.
Legionella is a type of bacteria that an cause Legionnaires' disease, a serious lung infection that can prove fatal.
The first failed test happened on January 24, 2017 when a count of 20,000 cfu/litre (a microbiology term) was recorded - which is 20 times the limit for closing the pool.
Day-to-day running of Belfast's leisure centres was outsourced to social enterprise Greenwich Leisure Ltd (GLL) in 2015.
GLL said: "Routine water samples of the spa pool are carried out on a monthly basis to ensure the highest compliance with health and safety legislation.
"The spa pool and infrastructure at Avoniel is much older than at other leisure centres and requires a lot more maintenance resulting in more closures, as pipework needs updated and maintained.
"Following some of the closures in 2017, a full overhaul of the system was carried out in partnership with Belfast City Council.
"As part of GLL and Belfast City Council's continued commitment to investing and maintaining some of the older leisure facilities in Belfast, a refurbishment of the pool and pool tank at Avoniel was also completed this week."
Figures show all incidents of unsatisfactory levels of Legionella were found since GLL took over the management of Belfast's leisure centres in January 2015.
There were two other incidents where unsatisfactory levels of Legionella were found in Belfast spa pools, both at Shankill Leisure Centre, on June 25, 2015 and October 17, 2016.
Neither of these incidents required the pool to be closed as the reading fell below 1,000 cfu/litre, the level at which the Health and Safety Executive advises a closure should take place.
Belfast City Council said: "Health and safety at leisure centres is a top priority for both Belfast City Council and GLL, which operates the centres on our behalf.
"Environmental health officers take water samples routinely from all pools on a regular basis which are then sent to an outside laboratory for testing.
"When the results are returned any necessary action is taken immediately.
"As part of the council's strategic partnership with GLL and the independent Active Belfast Board we meet regularly to review service standards and performance through a range of mechanisms, and are confident that GLL operates within all health and safety guidelines."
In April GLL defended itself against claims from union Unite - which represents some workers in the leisure centres - that changes to staff practices had resulted in thousands of pounds worth of damage at leisure facilities.
Addressing staff numbers, GLL said it had increased the number of pool plant operators from 15 to 55 since taking over the operation, which has increased the number of pool plants operators on shift across its 14 facilities.
"An independent review of leisure centre operations was carried out in 2017 and no evidence was found to suggest that GLL operates outside of health and safety legislation or guidance," it added.
The other pool closures happened on July 26, when a reading of 2,800 cfu/litre of Legionella was recorded during tests, and on November 17, when 2,600 cfu/litre of Legionella was recorded.