Grease trap/gravel basin

My three story building has a brick tank at basement level under the rear porch. I was told it was a grease trap for the kitchen and laundry. It also was called a 'gravel basin' but has been disconnected from the downspout because we no longer have gravel on the roof. The downspout now goes to the yard, according to the local code. I would like to replace or eliminate the brick tank because when the sewer backs up due to excessive rainfall, water comes up there and floods the basement (every five years?) What is a solution to the problem? A cast concrete cylinder cut to receive the four inputs and one output? Then cover with a sealed lid as in a sewage ejector setup ?
Who does this kind of work - plumbers or sewer specialists? It's a nice place from which to rod the sewer line to the street about 125 feet away in fronbt of the building.

Also, We had a backwater gate near the exit to the street but it is 50 years old and even after cleaning , no longer swings open easily and debris stops there, especially with the newer low water usage toilets. I removed the flap when it wouldn't open very well. How should I handle the problem? Is there a new version of this type of a device?

thanks.

Gerry

Get a plumber out - he can probably tie the 4 building drains into the sewer using combos and wyes (eliminating the grease/lint/gravel interceptor if not required)...
He can also install a good cleanout fitting at ground level for rodding the line in the future....

You can have him replace the backwater valve in the main line at the same time - they are still made....

basin

If it is either a grease trap or gravel basin, the two are similar but not the same, then the buidings drains should not go through it anyway. If you need a backwater valve, replace the defective one.