Welsh Water invested a record £1m a day last year, according to latest figures.
The investment was made to improve drinking water safety, environmental quality, customer service and energy efficiency.
The annual results also show a rise in pollution incidents, with serious incidents trebling, which the company put down to increased monitoring.
The not-for-profit company owned by Glas Cymru also paid a 'dividend' of £22 per customer.
Welsh Water's annual results show that a £361m capital programme was successfully completed, bringing investment over the past five years to more than £1.5bn - equivalent to more than £1,000 for every household.
When Glas Cymru acquired Welsh Water in 2001, Welsh Water had the highest water bill in the industry and the second highest sewerage bill.
Glas Cymru says that this year the water bill is below the industry average and the sewerage bill is closer to the industry average - an achievement it says has been made possible through cost reduction.
But the number of Category 1 and 2 incidents - the most serious - rose from three to nine, while Category 3 incidents went up from 230 to 311.
And the number of written complaints also rose, from 12,486 in 2008-2009 to 13,313 in 2009-2010.
Glas Cymru chairman Lord Burns said: "The prudent financial policies that we have pursued since 2001 mean that we have been able to deliver our largest ever capital investment programme.
"This investment has been highly beneficial to the economy in Wales during the recent recession and will allow us to protect drinking water safety for many years to come, as well as to start to respond to the long term challenges of climate change."
A spokeswoman said that the increase in pollutiuon figures came from increased monitoring.
"2008 saw excellent performance by Welsh Water, with very few serious pollution incidents recorded," she said.
"The figures reported for categories one and two during last year are very similar to those recorded in 2007.
"The increase in category three incidents is a result of increased monitoring due to the introduction of telemetry technology on the Combined Sewer Overflows and sewers.
"All problems detected are now reported to the Environment Agency Wales.
"We are concerned about the number of pollution incidents from our assets and are committed to reducing them as part of our £1.2bn investment plan over the next five years."
Welsh Water is facing significant challenges over the next five years.
Following the result of Ofwat's Price Review in November 2009, the business plans to reduce its day-to-day running costs by 20% while continuing to make improvements.
Customers are also expected to benefit from a reduction of £30 a year in the average household bill by 2015.
Glas Cymru is unique amongst UK utility companies in that it has no shareholders. It reinvests all its financial surpluses for the benefit of Welsh Water's customers.