Natural disinfection of wastewater in marine outfall fields

In this study the natural disinfection effects of marine environment on;was tewater without the process of chlorination before being discharged into th e ocean through submarine outfall pipes were investigated. The effects of f our natural factors, including light intensity, salinity, volumetric mixing ratio of seawater to wastewater and the existence of predators, to the dis infection of wastewater in marine environment were examined. Under the cond ition that with or without the existence of predatory microorganisms in was tewater, experiments are performed based on rotatable central composite des igns with different factor level combinations of the three factors mentione d above. Under each factor level combinations, the numbers of E. coli are m easured at the beginning of each experiment and every half hour later on fo r two hours. Then through statistical analysis, it was found that both ligh t intensity and salinity have significant effects to the die-off rate const ant with or without the existence of predators. The effects of the three en vironment factors can explain a larger portion (about 90%) of the variation s 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 exi stence of predators, which indicates that there may be random effects of pr edatory 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 signific antly larger than that for without the existence of predators. Finally, for the primarily treated sewage from Kaohsiung, Taiwan, by natural disinfecti on it is estimated that it takes about 100 min during the daytime and 196 m in during the nighttime to reach the national guideline concentration of E. coli (1000 cfu/100 ml) of Taiwan in marine environment. (C) 2000 Elsevier Science Ltd. All rights reserved.