ONE PIECE of good news this week was the $150,000 grant the MacArthur Foundation gave to the Philadelphia prison system to study ways to shrink the population in city jails.

The timing couldn't be better. Council is set to deliberate a bill to purchase a tract of land near the Delaware River to build a new city jail with the stated aim of replacing the city's 90-year-old House of Correction, which houses 1,500 inmates in crowded conditions.

Some are protesting the site for the prison - along an increasingly valuable and active stretch of riverfront. Others worry that once the new prison is built, it won't replace the old House of Corrections, but supplement it -allowing the city to incarcerate more people.

The Nutter administration says it doesn't want to incarcerate more people. The city spends $243 million a year to staff and run its jails and detention facilities. More prisoners means higher costs, about $95 to house each inmate for one day.

The reverse is true, too. A lower prison population will yield savings for the city.

In 2008, the city prison population was 9,399. As of last month, it was 7,909. The goal is to get it down to 6,000 or 6,500.

That won't be done by letting convicted criminals out of jail. That group represents a relatively small slice of the prison population. Only about 25 percent of those in jail are serving time because of a criminal conviction.

The reality is that the vast majority of inmates are in jail awaiting trial or former inmates sent to jail for probation or parole violations.

You may be innocent until proven guilty, but you will go to jail to await trial if you cannot post bail. About 15 percent will spend more than 120 days awaiting trial.

If the judicial system could give all those charged a speedy trial, that 15 percent number could be lowered.

That example highlights an important fact. While the prison system must house everyone sent to it, it has no say over who goes. Those decisions are made by the police, prosecutors, judges, and probation and parole officers.

Any plan to lower the numbers has to have buy-in by those parties. And it will take a coordinated effort to work on the various parts of the criminal justice machine that churns out defendants and parole violators headed for jail.

One way would be to release on their own recognizance people awaiting trial for minor crimes rather than set bail. In many cases, these people do raise the needed bail money - but only after spending several days or weeks in prison.

Another would be to find a way to handle technical violators of probation or parole without giving them a "Go to Jail" card.

It's not up to us to prescribe ways to lower the prison population. That's up to the experts, the people with deep knowledge of how the system works.

But, we hope this grant - which was sought by the Nutter administration and could lead to a larger, second round of funding-will spur them to move on this issue now and have an action plan ready for the next mayor.