Thanks for the tip! This was indeed the problem!
Yes, I could reproduce the same behavior on different machines too. In each case, I used openmpi installed with apt-get install libopenmpi-dev . Based on your suggestion, I removed openmpi, and installed mpich also with apt-get, then recompiled both PETSc-SLEPc and FreeFem. This solved the memory leakage. Furthermore, now that I use mpich instead of openmpi, the programs do not start sleeping processes (these I mentioned in this post) . Moreover, I think my programs also run faster!
Thanks for the help, I really appreciate it!