When a septic system fails a Title 5 Inspection or the system shows signs of getting tired (ex. constantly backing up into house or ponding on ground surface) you need to contract out with a Civil Engineer or a Registered Sanitarian. The role of the designer is to perform official soil testing on your property in the presence of the local Board of Health agent. The local Board of Health office will need to review the design. In some instances, it may need to be approved by the Massachusetts Department of Environmental Protection, which your designer will inform you of if this is necessary in your situation.
Once you have a septic design
plan you can obtain proposals from Licensed Septic Installers working in your town. I would suggest contacting your Board of Health office for a list of license installers for your town.