Since Thai people have traditionally used Thai-style toilets, wastewater in Bangkok, Thailand, is characterized by its water quality referred to as greywater without feces contamination. The water tends to be easily decomposed by microorganisms in sewers under hot and stagnant conditions. The contamination with night soil affected microbial fauna and flora of activated sludge formed in sewage works, as being suggested by simulated laboratory experiments. Nitrification was also promoted by the “dilution effect” with rain, alongside the “shading effect” of blacken particles of tropical fungal origins suspended in the discharged water courses. These findings suggest that the better solution of wastewater management in the monsoon tropical climates is not western-style sewage treatment but region-specific alternative options such as stabilization ponds with fish culture and composting toilets for agricultural manure use.