Is it ok to ask ex-dissertation supervisor for any future funded phds Following a computer science degree I would love to follow a phd research.
The problem is that all funded phds that are advertised from the specific university do not meet my desired criteria, i.e. research projects that target the  improvement of the quality of life of vulnerable groups of people.

I know that my ex bachelor dissertation supervisor used do research and observe researches of my desired criteria. 

Would it be ok asking him through email whether such oportunities exist? Or would that be just a waste of his time?
The short answer is Yes. Rather, the supervisor might be happy to take you in if she had such a project.

Since you have already worked under the supervisor during your earlier degree and showed some excellent outcomes with research maturity, the supervisor would be really happy to take you in as a research lab member.

If you had not shown any good outcomes during your dissertation, then she might not consider the request.

However, it is not a waste of time to write and ask. Since she knows you she may read your email. Many a time, professors do not read emails even, and that is frustrating.