History and Objectives: Considering the high cost of mechanical treatment of wastewater and the necessity to investigate more economical methods, specially the ones involving natural treatment processes, a study on efficiency of integrated stabilization pond (ISP) was conducted in Shoosh Sewage Treatment Plant during 1999.

Materials and Methods: This ISP experimental study was conducted during three seasons with two types of raw sewage feeding. The system consists of an anaerobic pond in combination with facultative pond in series with a high rate and a maturation samples from inflow and effluent were taken on weekly basis and COD, BOD5, TSS, total nitrogen and total phosphorous were checked, the resulting reduction in mentioned parameters formed the basis of a statistical evaluation.

Results: Pollutants reduction was higher in spring compared with summer and fall. BOD5 with 89% reduction showed the highest and total phosphorous with 60% showed the lowest reduction levels during the said period. The level of sewage pollutant reduction during three stages of the study shows that the anaerobic pond in combination with facultative pond registed the highest percentage of pollutant removal.

Conclusion: ISP systems are effective in treatment of sewage. Up flow of sewage in fermentation pit has a positive effect on the system efficiency and shows the fermentation pits considerable role in facultative ponds. Since there is little available experience about ISP in Iran, more investigation on the subject is recommended.