An investigation has been launched into potential harm caused to patients at a kidney dialysis unit run by the East and North Hertfordshire NHS Trust.
Unpurified water is believed to have been used for dialysis treatment at Bedford Renal Unit, with 23 patients undergoing blood transfusions because of low haemoglobin levels "likely" caused by the water quality, the NHS trust said.
The unit was closed for six weeks and the equipment that purified the water replaced.
It is understood that one patient was taken to hospital, after suffering a stroke, with a possible link with the water quality being investigated.
During the unit's closure, patients undergoing dialysis had to travel to other centres, with sites in Oxford and Northampton offering assistance.
The NHS trust said that all patients continued to receive dialysis, with no cancellations and, where possible, patients continued to be treated by staff who usually care for them.
Adam Sewell-Jones, chief executive of the East and North Hertfordshire NHS Trust, said: "I am grateful to staff, many of whom travelled with their patients to work in other units, and the other trusts for their willingness to support.
"I also apologise to our patients for the inconvenience experienced during this period."
The NHS trust said it communicated regularly with affected patients throughout the unit's closure, and those who had to travel to receive their dialysis were not impacted financially.
Dialysis removes waste products and excess fluid from the blood when the kidneys stop working properly.
If the water used in dialysis is not purified, it can expose patients to contaminants or bacteria that could be harmful to people without fully functioning kidneys.
A spokesperson for the East and North Hertfordshire NHS Trust said: "The Bedford Renal Unit reopened to patients following necessary improvements to the water treatment plant.
"We are currently conducting a formal investigation to determine the cause of the issue with the water and expect this investigation to conclude on July 31.
"We are unable to comment further whilst the investigation takes place, but will provide a full statement once this has concluded."
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