After repeatedly calling the increasingly squalid conditions on San Francisco’s trash-strewn streets “unacceptable,” Mayor Mark Farrell on Wednesday will unveil a plan to significantly boost spending on street cleaning.

Over the next two years, Farrell intends to add $12.8 million to the Public Works budget to hire dozens of street cleaners, add to the city’s stock of public toilets, and purchase new equipment, including powerful steam cleaners and sweeping machines.

Public outcry over the staggering amounts of litter, used syringes and human waste choking San Francisco’s streets has risen steadily in recent months, with some of the most persistent and visible problems occurring in the South of Market and Tenderloin neighborhoods, as well as the Mission District.

“The conditions on our streets are simply unacceptable, and as mayor, I wanted to make sure it was one of my biggest priorities,” Farrell said. “This is affecting everybody, and now as mayor, I have the ability to positively affect the conditions of our streets across the entire city.”

Street cleanliness is among the central issues Farrell vowed to address during his short time as mayor. On Monday, he announced the city also would set aside $750,000 to hire a 10-person team of health professionals dedicated to cleaning up used needles. City officials set aside $65.4 million for street cleaning in the 2017-18 fiscal year budget.

“After being in City Hall for over seven years, obviously I have my own opinions about what works and what doesn’t, and what has the greatest positive impact on our streets,” Farrell said. “And so, I wanted to fund those initiatives that I believe would have an immediate and lasting impact on our streets and for the residents of San Francisco. They deserve better.”

Just over $3 million a year would be put toward hiring 44 additional street cleaners, four for each of the city’s 11 supervisorial districts. Each supervisor would work closely with Public Works Director Mohammed Nuru to decide which corridors in their districts would be targeted.

Farrell’s plan also calls for building five Pit Stop public toilets and hiring workers to tend to them through contracts with nonprofit groups. The restrooms — currently, 18 are placed in frequently dirty locations — also provide spaces to safely dispose of needles and to get rid of dog waste. Farrell’s proposal also includes money to keep five existing Pit Stops open longer each day, and to increase the areas targeted by the city’s Fix-It team, which responds to a variety of quality-of-life problems, like graffiti or broken streetlights.

Nuru also plans to use the funds to purchase new steam-cleaning trucks to ease the strain on his existing fleet, which he said are pulling double and sometimes triple shifts.

“When you double- or triple-shift machines, you’re going to have a lot of breakdowns,” he said.

Nuru said the funds would go a long way toward addressing chronically dirty streets in some of the city’s most problematic neighborhoods.

“This will bring some consistency to those areas. It’s a game-changer. Nearly $13 million is not a small amount of money. We should see improvements in our commercial corridors and having more bathrooms available — all of those things will make a difference,” he said.

In recent months, Nuru said his department has been forced to shift into “firefighting” mode when it comes to addressing street cleanliness that stems from the city’s homelessness epidemic.

“When you have tents or people living on our streets without the adequate services, whether it’s a bathroom or trash service ... then other parts of the city aren’t getting the normal (amount of) attention,” he said.

Farrell’s proposed budget will be presented to the Board of Supervisors’ Budget and Finance Committee in June, where it could encounter some turbulence. The budget committee is chaired by Supervisor Malia Cohen, who has been a reliable adversary for Farrell since the board installed him as mayor in January, booting her longtime ally, London Breed, who was serving as acting mayor following the death of Ed Lee in December.

Farrell’s announcement Wednesday coincides with his decision to veto a budget supplement passed by the board last week that would have added $1.1 million outside of the normal budget process for citywide street cleaning.

The measure was introduced by Supervisor Jane Kim, who’s also a leading contender in San Francisco’s mayoral race. Kim initially had asked for $2.5 million, but she agreed to whittle down her request during negotiations with the rest of the board.

Farrell had pledged to veto the budget request even before it narrowly passed the board with a 6-5 vote, citing a general aversion to budget supplementals. He also was concerned that Kim’s measure would target cleanup efforts only in her district, which includes SoMa and the Tenderloin. Kim said previously she intended to let Public Works decide how and where to spend the money.

Kim couldn’t be reached for comment Tuesday, but, in a message sent through an aide, she said she intended to go “back to the drawing board to get the resources we need to clean our streets.”

One component of Farrell’s plan, however, targets street cleanliness specifically in Kim’s district. Farrell has proposed spending $550,000 each year over the next two years to fund a team that will clean SoMa streets five days a week.

“SoMa ... has been a major sore spot for residents and businesses and visitors alike, and I believe it deserves special attention,” Farrell said.

Dominic Fracassa is a San Francisco Chronicle staff writer. Email: dfracassa@sfchronicle.com Twitter: @dominicfracassa