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The strong movement towards effluent criteria for both nitrogen and phosphorus in Japan has created a need to introduce nutrient removal processes in the existing wastewater treatment plants. However, it is difficult to afford additional space for retrofitting the existing plants to the nitrogen and phosphorus removal process, because most of the treatment plants are located in densely populated areas. Therefore it is necessary to develop a new process, which can decrease the volume of reactor required as well as meet the more stringent standards for nutrients discharge.

A biological nitrogen and phosphorus removal process, five-stage Bardenpho with immobilized nitrifiers, was proposed here as one of the high-end technologies in the case of more stringent discharge standards and space limitation for the treatment plant. Figure 1 shows the schematic diagram of the pilot plant. The reactor was divided into 5 parts defined as anaerobic, first anoxic, first aerobic, second anoxic and final aerobic tank respectively. The second anoxic tank was provided for additional denitrification using nitrate produced in the aerobic tank as the electron acceptor and the endogenous organic carbon as the electron donor. The final aerobic tank was used to strip residual nitrogen gas from solution and to minimize the release of phosphorus in the final clarifier. Mixed liquor from the first aerobic tank was recycled to the first anoxic tank for denitrification. PAC (poly-alum-chloride) was added to the final aerobic tank to support phosphorus removal when it was necessary. Methanol was added to the second anoxic tank in the case of insufficient carbon source. Fluidized immobilization carriers were added to nitrification reactor to keep a high concentration of nitrifying bacteria.

In order to investigate the applicability of the process to the exiting municipal wastewater treatment plant, a pilot scale study was conducted. Relationships between varied operating conditions and nutrient removal performance were examined. The contribution of suspended activated sludge and immobilized carriers to the nitrification, as well as factors affecting denitrification, were investigated. Phosphorus removal capacity was evaluated with phosphorus release and uptake rates in the anaerobic and aerobic condition.

Two types of carriers, bound and entrapped immobilized carriers, were used alternatively in the first aerobic tank with the purpose of decreasing reactor volumes.

The main results of this study can be concluded as follows.

Regardless of a large concentration fluctuation in influent, the process showed a good performance in the removal of nitrogen, phosphorus and organic substances. With the application of the process, it was possible to get annually averaged water quality as follows. BOD5<3 mg/L; T-N<3.0mg/L; T-P<0.3 mg/L

It was observed that the nitrification activity of the immobilized carrier was insensitive to the temperature change compared with the suspended activated sludge. Due to an increased contribution of the carrier on nitrification as temperature decreased, it was possible to maintain the nitrification activity stable under low temperature condition during winter season.

The BOD5 loading affected the denitrification rate extensively and the denitrification rate can be expressed approximately as a linear function of organic loading rate. The unstable denitrification efficiency in the second anoxic tank was attributed to the reduced organic matter through the first anoxic and aerobic tanks including soluble BOD5. Consequently, in order to achieve a high T-N removal efficiency, it was necessary to maintain a stable denitrification performance in the second anoxic tank by means of carbon source addition.

A high BOD5 loading rate corresponded to a high PO4-P release rate and PO4-P concentration in the anaerobic tank. It was also found that the phosphorus uptake rate in the anaerobic tank was proportional to the release rate. In order to achieve a high release rate of phosphorus in anaerobic condition, it was necessary to keep a high BOD5 loading rate in the influent.
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Document Type: Research Article

Publication date: 2001-01-01

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