The City of Regina has rejected a petition from a group that wants to force a referendum on a multimillion-dollar sewage treatment plant upgrade.

On Friday, a senior city official told reporters the petition is invalid because it didn't have enough valid signatures.

Regina Water Watch, the petition organizers, submitted it on June 20, saying it contained more than 24,000 signatures.

By law, if 10 per cent of the population signs and the signatures are valid, a vote must be held.

There were 193,100 people in Regina according to the 2011 census, so the number of valid signatures required would be 19,310.

However, city officials said 4,289 signatures had to be rejected outright and it's believed no more than 90 per cent of the remaining names are valid.

Members of Regina Water Watch say they are upset their petition was rejected. (Dean Gutheil/CBC)

On that basis, the estimated number of valid signatures fell below the threshold and the petition was declared invalid.

Regina Water Watch members said they were outraged at the way the city came to this conclusion.

"I am deeply disappointed and I am horrified by what we just heard," Jim Holmes, a spokesman for the group, said Friday.

Of the rejected signatures, many were not accepted because the attached date didn't have the year, 2013.

Regina's mayor said city council did not direct the clerk on how to assess the petition.

"The review is done independent of council and [the clerk's] decision stands," Fougere said. However, he added he wants to study the clerk's ruling and get public feedback over the weekend.

Fougere said council could still decide to put the wastewater treatment plant issue to a public vote, framing a question of its own.

One of the concerns of the Water Watch group is the city's plan for the new plant to be a P3 project (private-public partnership), which will see private companies not only build but also operate and maintain the facility.

The city says doing it this way will save Regina taxpayers $80 million, but opponents say the city is lowballing the costs.

The city wants construction to start in 2014, with the upgraded plant to go into operation in 2016.