aeration
bacterium
basin
biological reactor
bioreactor
brown
clarifier
cleaning
contamination
dirty
environment
excrement
factory
filter
foam
industrial
industry
pipe
plant
pollution
pool
processing
purification
purify
recycling
sanitary
sanitation
secondary
sediment
sedimentation
seethe
seething
sewage
sewage treatment
sewer
sludge
stir
tank
toxic
urban
waste
waste water
wastewater
water