Fig. 11. Virus removal simulation (assuming first-order removal with respect to concentration) with increasing time of operation. Conditions: 1 x 10⁷ plaque forming unit (pfu)/mL of virus added in septic tank effluent (STE) dosed at 5 cm/d. IZ, infiltrative zone (0–4 cm); VZ, vadose zone (4–60 cm).