Hmmm... interesting additional info, SirK. I'd consider offering the tenants the service to pay you to take care of the trash. If you have to do it, give them an invoice...
'Course, that doesn't help the old neighborly relations very well, does it? A better approach might be to ask them what they think can be done to make it easier to manage the trash. There may be another reasonable solution that you all can live with.