Four full-scale systems wastewater treatment plants (WWTPs) were used as study sites. All of these WWTPs were designed and operated for biological nitrogen removal (BNR) by using nitrification-denitrification processes. In general, the WWTPs in Thailand operated at higher values of temperature, HRT and SRT. Influents and effluents from these sites are compared and discussed in terms of BNR, dominant nitrifying microorganisms and WWTP design. Nitrogen removal was observed in all the sites and correlated to the influent total N (TN) to BOD ratio. Polymerase chain reaction coupled with denaturing gradient gel electrophoresis was used to identify dominant bacteria involved in nitrogen transformations: ammonia-oxidizing bacteria (AOB), nitrite-oxidizing bacteria (NOB), and nitrate reducing bacteria (NRB). AOB Nitrosomonas sp. was found only in Thailand where aerobic HRT was ≥ 4 hours and SRT was ≥15 days. Furthermore, AOB Nitrosospira sp. were found only in Japan at aerobic HRT ≤ 4 hours and SRT≤ 13 temperature (21-27°C). NOB Nitrospira sp. was found at aerobic HRT ≥ 4 hours and SRT ≥ 6 days. Interestingly, Nitrotoga sp. was found in the aerobic tank one in Thailand and one in Japan and co-occurred with NRB Burkholderia denitrificans. The higher wastewater temperature and lower influent nitrogen concentration in Thailand appear to promote a different AOB and NOB community structure than in Japan. The most important factor affecting TN removal was the influent TN to BODratio.
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