There could be several things that can cause black lines down the page when your are printing on your Brother MFC-1815, however, if it only seems to happen when you are using your scanner / document feeder, firstly check to see if there is dirt or damage on the scanner glass.
Even a small smudge or drop of dirt on the scanner glass can cause the copies to come out with a black line down the page due to the movement of the scanner as it scans a document. If you still get the issue after the glass has been cleaned, check the advice on the following page for more tips on how to fix black lines down the page: https://www.tonergiant.co.uk/blog/2017/05/black-lines-down-the-page-4-causes-and-how-to-fix/