Professor Lemmer is a biologist with specialization in limnology and microbiology. Her doctoral thesis (1985) dealt with the microbiology of scum-forming actinomycetes in wastewater treatment. Since 1981 she has worked as a research scientist in the field of microbial ecology. The main focus area of this research is sludge separation problems in wastewater treatment and the degradation of xenobiotics in wastewater and the environment. The aim of this research is to elucidate microbial population structures and their taxonomy, physiology and activity in these systems. The findings of her research activities are translated directly into everyday practice.
In addition to initiating research projects, she supervises research groups and students on doctoral and master's degree programs. She is the author of numerous publications in peer-reviewed journals and of articles in books and trade magazines aimed at wastewater practitioners and professionals. Her lectures and courses on microscopy play a significant role in the in-service training of wastewater practitioners in the field of wastewater microbiology. She also teaches environmental microbiology at TUM.