Shigeki ISHIHARA
Development of Phosphorus Removal System in Biofilm Reactor
Takashi Yamaguchi
Phosphate recovery from sewage is desirable, because of phosphate resource depression. Conventional technology has some problems, e.g. cost of aeration and disposal of excess sludge. Since UASB+DHS system is less excess sludge and less motive aeration energy, and it attracts attention as future generation sewage treatment. But this system doesn't remove phosphate at all. As a result, UASB+DHS discharge contains low concentration phosphorus. Low concentration phosphorus is difficult to recover, and causes eutrophication problem.
So, we developed phosphate recovery system intended for municipal discharge (UASB+DHS discharge). The equipment is airtight DHS whose gas phase goes over anaerobic and aerobic, and PAOs (Polyphosphate Accumulating Organisms) retains in the sponge. However, the result was not stable. And we didn¡Çt know what conditions were causes of deteriorate.
To stable phosphorus remove, we had to find the optimate conditions for EBPR (Enhanced Biological Phosphorus Removal). Plugflow reactor was set up as simulation of DHS. This time we used biofilm reactor by vinyl tube. This reactor can change conditions in easily. In aerobic time, the phosphorous water is supplied and same amount of air is supplied, too. In anaerobic time is supplied organic water (acetate: propionate = 1: 1). We researched adopted condition of EBPR.
The first experiment was discussed of possibility of phosphorous remove in biofilm reactor. That result was concentrated 75 mgP/L from 5 mgP/L. It could say this biofilm reactor as can phosphorus remove as DHS reactor. The next experiment was discussed of optimum 1 cycle time (1cycle time is anaerobic time and aerobic time). If the cycle time is too short PAOs can¡Çt have sufficient time to phosphorus release and intake. By contrast, the cycle time is too long competitive organisms are grow. The cycle time were set 4,6,8,12,16,18,24 hours. And all of aerobic time and anaerobic time ratio was 1:2. HRT is 10 min (360 ml/hour). 12 hour cycle and 16 hour cycle showed steady phosphorus release and intake. Every cycle¡Çs COD was taken. It indicates many of competitive organisms existed. Other cycle¡Çs phosphorus intake and release were not much.
From the result of biofilm of reactor, 16 hour cycle was the best condition of phosphorus intake and release in EBPR. But PAOs ratio is expected not to a lot in biofilm. Because the biofilm¡Çs total phosphorus concentration was not different from ordinal excess sludge¡Çs. At the same time, the reactor had very high concentration of sledges. It indicates if PAOs ratio is too less, but enable to remove phosphorus and recovery high concentration of phosphorus water.
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