Before too long, one or two sewage outflow pipes just won't cut it, sure you can just keep adding more, but you run the risk of really infecting your water table with everyone's poo... Not really the best course of action, so, assuming you have the cash available when you receive your next sewage overflow warning, find a good locations for a sewage treatment facility and plop one of those. Try to keep it away from residental zone - obviously! They don't pollute the ground so can be plopped close to water without serious repercussions on your water supply.
Once your initial sewage treatment facility starts to come under the strain of too much waste, you can just plop an extention to the facility as in the image above. The cost a lot, but they are an environmentally friendly way to control your sewage in your ever-expanding City.
Adding a Sewage Treatment Tank
Oh! Let's not forget that your sewage outflow pipes are nor redundent, so once you are sure your new sewage treatment facility is working fine, you an either close (if you want to wave them for later), or bulldoze your old sewage outflow pipes, both options will save you the hourly running costs of those facilities.
Closing unused sewage outflow pipes