r-strategists are your fast-growing organisms thriving in high F/M and in changing environments due to influent and other rapidly changing environmental conditions. In wastewater, we previously thought of r-strategists as the young sludge heterotrophic organisms. K-strategists are typically slower growing organisms that have higher substrate affinity - a complex way to say they can continue to grow at lower concentrations of food.
However, even among slower growing niche organisms you can have genera classified as r or K strategists. Such as nitrifiers (AOB & NOB), NOB genera Nitrobacter are r-strategists versus Nitrospira, a K strategist. If you have high nitrite, you favor Nitrobacter, but in most wastewater with low levels of NO2, you favor Nitrospira. Some of the growth rate and substrate affinity differences are slight in these niche organisms, but small changes in operations can shift which organism grows in the system.
For a particularly good discussion on r/K Strategists in wastewater check out the following article:
Qidong Yin , et al. "The r/K selection theory and its application in biological wastewater treatment processes," Science of the Total Environment, Issue 824 February 2022.