Natural disinfection of wastewater in marine outfall fields

In this study the natural disinfection effects of marine environment on wastewater without the process of chlorination before being discharged into the ocean through submarine outfall pipes were investigated. The effects of four natural factors, including light intensity, salinity, volumetric mixing ratio of seawater to wastewater and the existence of predators, to the disinfection of wastewater in marine environment were examined. Under the condition that with or without the existence of predatory microorganisms in wastewater, experiments are performed based on rotatable central composite designs with different factor level combinations of the three factors mentioned above. Under each factor level combinations, the numbers of E.coli are measured at the beginning of each experiment and every half hour later on for two hours. Then through statistical analysis, it was found that both light intensity and salinity have significant effects to the die-off rate constant with or without the existence of predators. The effects of the three environment factors can explain a larger portion (about 90%) of the variations exhibiting in the estimated die-off rate constants in the case of without the existence of predators than that (about 50%) for the case with the existence of predators, which indicates that there may be random effects of predatory microorganisms in wastewater causing more variations in the die-off rate constants. Furthermore, through paired t-test, it also indicates that the die-off rate constants for with the existence of predators is significantly larger than that for without the existence of predators. Finally, for the primarily treated sewage from Kaohsiung, Taiwan, by natural disinfection it is estimated that it takes about 100 min during the daytime and 196 min during the nighttime to reach the national guideline concentration of E. coli (1000 cfu/100 ml) of Taiwan in marine environment.