What is the flight that returned to base after dumping fuel due to toilet issues?
I'm going to be a bit lazy and only point you in the right direction (sorry):
The Aviation Herald website is a simple, yet excellent source of aviation &quot;occurences&quot;. There is a search tool at the top of the page, here is a link to a sample search I did using word lavatory:
Avherald.com search with word lavatory
...and below is a sample report picked from the results of the aforementioned search.

Incident: Jet2.com B738 near Cork on Aug 4th 2019, lavatory issues
By Simon Hradecky, created Monday, Aug 5th 2019 21:46Z, last updated Monday, Aug 5th 2019 21:46Z
A Jet2.com Boeing 737-800, registration G-JZBC performing flight LS-167 from Glasgow,SC (UK) to Lanzarote,CI (Spain), was enroute at FL360 about 90nm south of Cork (Ireland) when the aircraft turned around due to problems with the lavatories and diverted to Manchester,EN (UK) for a safe landing on runway 23R about one hour later. A passenger reported toilets were leaking water flooding the aircraft. The airline reported the aircraft diverted to Manchester to permit maintenance to service the toilets. A replacement Boeing 737-800 registration G-GDFV reached Lanzarote with a delay of 3.5 hours.

Searching the avherald.com database with relevant words will give you a plethora of cases for your white paper.