San Diego is in the midst of spending roughly $3 billion on a massive new water treatment system, but city officials can’t or won’t tell customers how that will affect their water bills.

New water recycling plants will eventually purify enough sewage to provide a third of the city’s drinking water.

In November, the City Council approved the first, $1.4 billion phase of the project.

Water bills will rise, that much is certain. Yet, officials in Mayor Kevin Faulconer’s administration are refusing to estimate by how much.

In December, Voice of San Diego asked the city to estimate how much customers’ bills will increase because of the Pure Water project.

The city, after weeks of delay, finally declined last week to offer any estimate because “there is no simple calculation” they could perform.

That’s an odd response, because Pure Water itself is one of the most complex and ambitious public works projects in California. For it to work, the city must flawlessly remove bacteria and other pollution from raw sewage – any failure could not only endanger public health but also set back other similar projects for a generation.

During a December interview, several city officials – among them city CFO Rolando Charvel and Johnnie Perkins, a top city official charged with fixing a series of unrelated messes in the water department – provided various reasons as to why it would be unreasonable to provide an estimate now, four years before the project starts delivering water to customers.

Charvel said because different customers used different amounts of water, any estimates would not apply universally.

Perkins argued that the project is so important that it’s not appropriate to look at the costs per customer.

“Ratepayers are fully aware that rates were going up to pay for Pure Water,” Perkins said.

Pure Water is ultimately an attempt to kill two birds with one stone. Not only will it provide a drought-proof source of water, but it helps with another long-standing city problem. For years, the city has promised to build Pure Water in order to avoid spending $2 billion to upgrade the Point Loma treatment plant, which dumps treated sewage into the ocean. If Pure Water were only about treating sewage or only about getting a new water supply, it probably wouldn’t make financial sense.

That still doesn’t explain why the city won’t calculate its costs, though.

Two previous water department leaders – Halla Razak, who left the city for another job, and Vic Bianes, who retired amid an ongoing investigation by VOSD and NBC 7 Responds – have also declined in interviews over the past several years to say how much the project might cost ratepayers, including countless customers who already struggle to pay their water bills.

To the city’s credit, the calculation may be complicated and some numbers remain up in the air. The city already received a low-interest, $614 million federal loan to help build the project but it’s seeking further subsidies. Those could lower the project’s costs. Higher construction costs, though, could drive up costs. An unexpected dispute with San Diego Gas & Electric has already increased Pure Water’s costs by tens of millions of dollars.

There is also the tricky issue of dividing up how much costs different customers bear. City of San Diego water customers will pay for the bulk of the project, but suburban cities that use the city’s sewer system will also chip in.

But the city’s refusal to produce an estimate contrasts with other public water agencies that have offered some estimates before they embark on a major project.

In 2012, the San Diego County Water Authority estimated that a new $1 billion desalination plant in Carlsbad would increase customers’ bills by about $5 a month.

Last year, the Metropolitan Water District of Southern California said a $17 billion pair of tunnels to help bring Northern California water south would cost Southern California water customers about $5 a month too.

There has been quibbling with those numbers, but at least they exist to quibble with.

Meanwhile, private water companies, like California American Water, which serves Coronado and Imperial Beach, plays by stricter rules because they are regulated by the California Public Utilities Commission.

Before Cal American make a major decision, it has to say how much the project will cost and how that will affect customers’ bills. In 2016, when it filed its most recent request to increase rates, it estimated down to the cent how much a typical customer’s bill would go up.

Public agencies – like the city water department – by contrast, have a lot of freedom to do what they want, the theory being they are eventually accountable to the public.