ST. PETERSBURG — Historic rains last year overwhelmed the city's ancient wastewater system, sending 31.5 million gallons of untreated and partially treated sewage gushing into the waters of Tampa Bay and Boca Ciega Bay in August.

Mayor Rick Kriseman called it a once-in-a-century calamity.

Then it happened again.

Last month, 9.8 million of gallons of sewage — estimated to be 30 to 50 percent untreated sewage — spilled into the bay. Untreated sewage also gushed from manholes in Clam Bayou, leaked into the locker room at Al Lang Stadium and befouled the basement of the Vinoy Renaissance resort.

St. Petersburg officials say this latest dump in June — the result of Tropical Storm Colin — was unavoidable. In the 10 months between floods, they say they have not dithered to solve the city's sewage issues.

"Unfortunately, 10 months is just too short of a time to really do anything of significant impact," said Claude Tankersley, public works administrator.

But was this latest sewage dump really unavoidable? Did the city heed the alarm bells of 2015?

St. Petersburg had options. But the option officials chose was to do nothing.

• • •

The city used to have four wastewater facilities. Last year it closed its oldest sewer plant, the Albert Whitted Water Reclamation Facility, located on the waterfront by the small airport with the same name.

It was closed because of the costs of maintaining the aging facility and a state rule change that would have raised those costs. But in doing so, it removed more than 12 million gallons of capacity from the city's sewer system.

At the time, city officials said the city's largest wastewater treatment facility, the Southwest plant next to Eckerd College, could handle the downtown sewage once processed by Albert Whitted.

Those promises were shattered last August when heavy rains overwhelmed the plant, dousing Eckerd College and Clam Bayou with more than 30 million gallons of sewage — about half of it untreated sewage.

Eckerd's orientation turned into a foul affair as activities scheduled on the school's waterfront access at Frenchman's Creek had to be rescheduled because of high levels of bacteria. Local civic associations howled in protest.

St. Petersburg officials vowed to never let that happen again.

City officials came up with a plan: move two high-powered pumps from the shuttered Albert Whitted plant to Southwest. That fix would add more capacity by increasing the plant's ability to get rid of cleaned wastewater by pumping it underground into deep injection wells.

They also said a 15 million-gallon tank at Southwest could be used for emergency storage, if needed.

Wastewater officials told City Council members they were confident that the plan would avoid major spills.

"They had the tank, they had the pumps and it was high-fives!" said Walter Donnelly of the Alliance for Bayway Communities Civic Association, who has followed the sewage crisis closely.

• • •

But like generals planning for the last war, St. Petersburg was ill-equipped for the next sewage crisis: There was no plan for dealing with short, intense bursts of rain, like from a tropical storm.

Last year, nearly 15 inches of rain steadily fall through August in the city. But last month, the city found itself dealing with nearly 10 inches of rain dumped on it in a matter of days by Tropical Storm Colin.

The downpour was too much for the front-end or intake part of the Southwest plant that initially treats untreated sewage. Tankers­ley made the call to divert the flow away from Southwest to Albert Whitted.

In hindsight, the city may have had the capacity to store the 9.8 million gallons of sewage that ended up in the bay last month. But city officials didn't know that until the rain stopped.

As the storm flooded the sewer system, Tankersley said he didn't want to risk another spill near the college and neighborhoods.

During that crisis, officials also learned they couldn't use that new, almost-completed 15 million-gallon tank at Southwest, which officials had touted as critical emergency storage. The contractor had plugged up the pipes and no one from the city had checked until a mad scramble at the height of the June storm.

In the end, the plan — the pumps and the tank — failed. More wastewater was dumped into Tampa Bay.

City officials initially refused to release information about how much was being pumped into the bay, saying it wasn't a public record.

Eventually, they confirmed that it was 9.8 million gallons.

• • •

In the aftermath of Tropical Storm Colin, Tankersley sent millions of gallons of sewage to the old Albert Whitted plant. But it could only store waste, not fully treat it. The shuttered plant's storage capacity was just 5 million gallons. When it ran out of space, the city started dumping into the bay.

So on Thursday, the City Council unanimously endorsed a plan to create more space at Albert Whitted to hold the sewage overflow created by the next storm.

The plan is to spend $400,000 to repair and convert structures to accommodate 3 million gallons of added storage at the old plant. The work should take about five months.

It's a quick and cheap solution — but it's no silver bullet. After all, if the extra storage had been in place for the tropical storm, the city would still have ended up dumping millions of gallons into Tampa Bay. There would have still been a visible plume of sewage fouling the bay's waters.

Still, why didn't the city expand storage at Albert Whitted after last August's imbroglio?

The City Council didn't ask those questions Thursday. Tankersley started the meeting by saying he's done answering questions about June's incident.

"Sometimes we can get mired in the past, spend so much time rehashing past issues that we're not paying attention to the present," he said. "At this point, I don't plan on discussing it anymore."

Kriseman also believes second-guessing about June's dumps serves little purpose.

"After the fact, it's easy to say now that was the wrong decision because you had additional information available to you that you didn't have at the time you made that decision," he said.

• • •

Being able to use the 15 million-gallon tank at Southwest — a key part of the plan to avoid future spills— would have helped.

Once the tropical storm hit, the city scrambled to see if the tank could be used for emergency storage. But city workers had not checked the tank beforehand. So no one knew that the contractor had plugged the tank's pipes, rendering it useless.

When asked why that tank wasn't ready to go, the mayor's chief of staff, Kevin King replied, "Looking at hindsight doesn't really serve anyone well."

Another potential fix that was never explored: The city could have installed temporary piping to Southwest's biggest tank — the one that was plugged and unavailable. Such piping would have allowed that tank to store untreated sewage during last month's storm, perhaps avoiding the dump in Tampa Bay altogether.

But the city promised Eckerd College when it built the tank that it wouldn't fill it with anything but nearly clean "reject" water.

Filling it with sewage "would have been breaking a promise to Eckerd," Leavitt said.

• • •

City officials dispute the viability of these potential options. They say there was no way to avoid June's sewage dump.

They say the decision to dump into Tampa Bay rather than risk spills at the Southwest plant were made in the midst of a crisis, when they could not have known how much rain would fall.

Some believe that Kriseman's decision to divert sewage from Southwest — which ran only slightly above capacity during the height of the crisis — was more about politics than good wastewater management.

After all, the fiercest criticism last year came from the college and the surrounding neighborhoods, which were affected by the 2015 spills.

Council member Karl Nurse said it was likely a political decision to divert the sewage away from the Southwest plant and, ultimately, into Tampa Bay.

"I think it's pretty clear that they know where people were most wound up about this," Nurse said.

Kriseman said he was offended by Nurse's comments.

"It played no factor," he said.

• • •

So what's next?

The city is conducting an 18-month study to determine where it needs to fix the leaky pipes that fill with rainwater during a storm, increasing the volume of waste that flows into the sewer plants.

St. Petersburg will also spend $35 million over the next two years at its Southwest plant, increasing capacity to 50 million gallons per day — 2½ times its current capacity.

And Kriseman, after months of opposing spending the city's $6.5 million BP settlement money on sewers, finally relented.

The council approved spending $3 million in BP money to fix pipes last month. Tankersley hopes that work will start by the beginning of October.

That delay angered Nurse, who has pushed for more action and fewer studies.

"My frustration is I don't want to wait 18 months (until) you know everything before you do anything," Nurse said at Thursday's meeting.

The city must plan carefully and spend wisely, Kriseman said, as acting haphazardly will waste money.

Reaching for a Sierra Club award on a shelf in his office last week, Kriseman said the last two sewage dumps drive home the need to spend more time and money — to study climate change.

The sewer system is a symptom of a larger problem, he said. The changing climate is putting more pressure on the city's infrastructure.

"We have to look at this as giving us an opportunity," the mayor said.

Contact Charlie Frago at cfrago@tampabay.com or (727) 893-8459. Follow @CharlieFrago.