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The Osaka Prefectural Government seeks to introduce an advanced wastewater treatment system at the wastewater treatment plant, to preserve water quality in Osaka Bay and to create a new water circulation process. However, due to space limitation at the existing wastewater treatment plants in urban areas, it is difficult to introduce the conventional type of advanced wastewater treatment system. As such, there is a need for a new type of space and energy saving wastewater treatment system. Since October 2000, an experimental study has been in progress at the actual practical level at the Konoike Sewage Treatment Plant of the Neyagawa Regional Sewage System in Osaka prefecture.

This process consists of pre-coagulation and sedimentation process, and nitrification and denitrification process, using attached-growth media of biofilm, without using activated sludge. To complete denitrification in the 2nd Anoxic tank, organic matter is required as hydrogen donor in the 2nd Anoxic tank. The organic matter is obtained as organic acid by fermentation of pre-coagulated sludge. This system can reduce organic load to biological treatment process and the HRT of the biological treatment is shortened to 8 hours (at design maximum daily flow, 3,500m3/day).

The effluent from a grit chamber at the Konoike Sewage Treatment Plant located in Osaka Prefecture, Japan, was introduced to the pre-coagulation process. FeCl3 and an anionic high molecular coagulant were added, in amounts of 0.1mg-Fe/mg-SS and 0.5mg/L, respectively. The retention time at the primary sedimentation tank was 1.4 hours.

The pre-coagulation and sedimentation process is followed by the biological reaction tanks, which consist of four tanks. The names of the respective tanks and their retention times are as follows: 1st anoxic tank (1.8 hours), aerobic tank (3.6 hours), 2nd anoxic tank (1.6 hours) and Post-aeration Tank (1.0 hours). The total retention time is 8 hours. Media were added to the aerobic tank at the additive ratio of 10%, to promote nitrification and to remove organic materials and were aerated by diffuser. To prevent media from becoming one-sided, they are returned to the inlet area of an aerobic tank from the end point of the tank, by means of an air lift pump. Media were also added to 1st and 2nd Anoxic Tanks, at the additive ratio of 17%, to promote denitrification. A draft tube was used as an agitator. Media are made of polyvinyl alcohol (PVA) and have an outside diameter of 4mm. The influent goes through the 1st Anoxic Tank and the Aerobic Tank, where recycled nitrification/denitrification takes place, and then it enters the rear stage of the 2nd Anoxic Tank, for denitrification of the remaining NOx-N, followed by the Rost-aeration Tank, where the removal of organic materials and adjustment of DO take place, and finally to a final sedimentation tank, where solid-liquid separation proceeds. Denitrification occurs in the anoxic tank with addition of organic matter as hydrogen donor. The organic matter (hydrogen donor) for denitrification in the anoxic tank is intended to be produced from the pre-coagulated sludge (Somiya et al., 1994; Somiya et al., 1998). As a hydrogen donor required in the 2nd Anoxic Tank, it is planned to use organic acid, to be produced separately by using pre-coagulation sedimentation sludge, in the future, but in the present case, methanol was added to the No. 2 Anoxic Tank. This System can reduce organic load to biological process and the HRT of the biological treatment is shortened to 8 hours (at design maximum daily flow 3,500m3/day process).

Study on the operation condition of the Organic Acid Fermentation Tank was conducted, to promote efficient production of organic acid, using pre-coagulation sludge generated by the System. After being temporarily reserved in a reservation tank, the pre-coagulation sludge from the wastewater treatment system was sent to a fermentation tank, where organic acid was produced under the respective conditions. Fermentation sludge was taken out periodically, and the concentrated sludge generated in the primary sedimentation tank was sent to the fermentation tank, and SRT (solid retention time) of the fermentation tank was adjusted.

As a result of an experimental study on the advanced sewage treatment system conducted using an existing line used for conventional activated sludge process, the following things were clarified:

It was possible to control T-P in the effluent of the Pre-Coagulation and Sedimentation Tank to below 1.0 mg/L, by setting the amount of coagulants to be added to the Pre-Coagulation and Sedimentation Tank as follows:

0.1 mg-Fe/mg-SS of ferric chloride (FeCl3) and 0.5 mg/L of anionic high-molecular coagulant.

According to the result of wastewater treatment conducted during the period between December 2001 and February 2002 (the average effluent volume was 3,400 m3/day), the average T-P was 0.3 mg/L, achieving the target value. SS was 8.1 mg/L, and T-N was 6.8 mg/L. Their target values were almost achieved. BOD slightly exceeded the target value. The average ATU-BOD was 5.6 mg/L. It was assumed that the removal rate of BOD would become higher thanks to complete nitrification in the Oxic Tank.

According to the result of the organic acid fermentation pilot plant experiment, the organic acid generation rate was 7 ∼ 13 percent under the following conditions:

pH in the Organic Acid Fermentation Tank was 5.0; the temperature of water Acid Fermentation was 35°C; the solid retention time of the Organic Acid Fermentation Tank varied from 0.6 days to 4 days.

When the water temperature was lowered to 15°C, the organic acid generation rate was reduced to half of the generation rate obtained when the water temperature was 35°C.

When the pH level in the Organic Acid Fermentation Tank was between 6.0 and 7.0, the consumption of generated organic acids was promoted by methanol fermentation. Meanwhile, when pH was 5.0, methanol fermentation was little. Therefore, the optimum pH level was determined to be 5.0.

It was found, from a study on soluble BOD and organic acid COD, that the conversion factor of organic acid COD to soluble BOD was 1.

To generate the amount of organic acids required when the System is operated at design maximum daily flow, the conditions of the Organic Acid Fermentation Tank must be as follows: pH = 5.0, water temperature = 35°C, and SRT = 2 days.

About 25 ∼ 40 percent of T-N, and about 11 ∼ 24 percent of T-P, in raw sludge were liquated as a result of organic acid fermentation.

About 50 percent of soluble phosphorus in the organic acid supernatant was coagulated by the remains of coagulants that were added to the Pre-Coagulation and Sedimentation Tank.

When organic acid supernatant was added to No. 2 Anoxic Tank in lieu of methanol, the target values were almost achieved for T-N and T-P.

Document Type: Research Article


Publication date: 2002-01-01

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