The Research Of Sludge Granulation In A Zero-valent Iron Anaerobic Reactor

The upflow anaerobic sludge blanket (UASB) reactor has become the most popular high-efficiency reactor for anaerobic treatment of wastewater all over the world. The formation of anaerobic granular sludge is considered as a prerequisite for the successful operation of the UASB reactor. However, one major problem of the UASB reactor is that it requires extremely long start-up period to sludge granulation. The reasons for the slow granulation are mainly ascribed to slow growth of methanogens and the excessive susceptibility of methanogens to the variation of operational conditions such as pH, nutrient balance or the presence of toxic compounds. Especially, the high organic load may cause acidification which leads to the sudden disintegration of the established granules.The subject comprehensively considered the principle of the formation of the particles and the process of the research, a further intelligent of the ZVI in wastewater treatment is to couple with anaerobic processes. When the ZVI is used under the anaerobic condition, it is expected to be helpful for creating an enhanced anaerobic environment that may improve the performance of anaerobic wastewater treatment owing to its reductive property. Meantime, the anaerobic condition can protect the ZVI from rusting. According to the experimental study and theoretical analysis, following conclusion can be draw:(1) The removel efficiency of COD was high during the whole start-up period, the removel rate kept above 70%, the highest rate could reach 85%, and the reactor was opering stable, the change of efficiency caused by the influent was small, while the removel of efficiency in common UASB was 15%-20% lower than the ZVI-UASB, and the fluctuation was obvious in the UASB reactor.(2) The anaerobic environment in the ZUI-UASB reactor was enhanced. Under the condition of zero-valant iron, the pH in the reactor maintained about 6.5-7.0 during the whole process of the granulation. The stability is significant to the anaerobic microorganisms which were sensitive to the variation of pH, especially for the methanogens. While the pH in the common UASB reactor was fluctuation obviously with the change of the organic loading rate, the pH was low that is not conducive to the rapid growth of methanogens. The ORP kept below-300mv which was about 70mv lower than the common UASB, the extreme anaerobic statement in the reactor plays an active role for the good performance of an anaerobic reator. (3) After the hydraulic circulation was operated around the ZVI bed, the sludge granulation was apparently enhanced, while the normal UASB was still not good. The reason was ascribed that the effects of the ZVI on the granulation was intensified. On one hand, the ZVI reaction benefited the growth of methanogens by acid buffering and decrease of ORP; On the other hand, Fe2+ leaching from the ZVI advanced the granule aggregation. Due to the fact that the cirlulation was only conduced in the ZVI unit which was located in the upper half of reactor, the sludge was not obviously washed out. As results, not only the time needed for startup was shortened, but also the performance of the reactor for wastewater treatment was improved.