Top-down view of a water sample. Bacteria (green dots) drift and
multiply; settled cells attach to the pipe floor as a biofilm (the
mottled patch), a protected colony that disinfectant struggles to
reach. Raise the dose to kill free-floating cells — the biofilm
keeps a residual population alive underneath even after the water
looks clear.
dN/dt = r . N . (1 - N/K) - k_d . N
r = r0 . Q10^((T-20)/10) (growth speeds up with warmth)
K = K0 . (BOD / BOD0) (nutrient load sets carrying capacity)
k_d = k . C^n (Chick-Watson disinfection kinetics)
- Organic pollution (BOD) — more nutrients raise the carrying capacity K.
- Water temperature — growth rate roughly doubles every 10°C warmer (Q10 = 2).
- Disinfectant dose — kill rate follows Chick-Watson kinetics: higher dose gives a faster log-reduction.
- Chlorine vs UV — chlorine keeps working after dosing (residual) but barely penetrates biofilm; UV kills instantly on contact but leaves no residual, so regrowth resumes.
The coliform index estimates CFU per 100 mL. This sim marks the
sample "Unsafe" above roughly 50 CFU/100mL, "Caution" between 1 and 50,
and "Safe" below 1 with a biofilm under 5%.