Abstract
Practical performance of ten individual type gappei-shori johkasous, on-site treatment facilities for household wastewater, were surveyed during a one year period. Effluent BOD and COD were below 20mg·l-1 with a cumulative frequency of 70% and 80%, respectively. Mixed liquor in the contact aeration tank was recycled to the first chamber of the anaerobic filter tank in the five facilities of which their airlift type pumps could be used to transfer sloughed biofilm. Only one notch type system could easily regulate the amount of the recycled water as 3-4Q where Q was the influent water amount per day, while the amount of recycle changed frequently in the other four facilities. The effluent BOD, N-BOD, T-N, NO3-N and DKN concentrations were reduced during the recycle operation. Their average values in the recycle mode were 12mg·l-1, 4.5mg·l-1, 10mg·l-1, 4.9mg·l-1 and 4.3mg·l-1, respectively. It was clear that the biological denitrification took place during the recycle operation resulting in the decrease not only in T-N and NO3-N but also in N-BOD which also caused the decrease of BOD. A number of operations in different recycle ratio suggested that effluent T-N could be around 10mg·l-1 with the recycle ratio below 10Q, and it would be over 20mg·l-1 with the recycle ratio near to 20Q.
