Testing waste water for pollutants. Technician holding a phial of waste water that has been treated to show its hydrogen sulphide (H2S) concentration (blue). It is being compared with a range of indicator samples which increase in H2S concentration from left to right. A similar indicator range for phenol concentrations (pink) can be seen behind the blue one. Rigorous tests like these are conducted on waste water to determine if it is safe to flow into unpolluted water sources.