• Waste management in Japan
• Circular economy in Japan
• Waste management in Asia
• Disaster waste management

Features and importance of automatic oxygen supply device (AOSD)

Extremely important research subjects in domestic and industrial wastewater treatment include not only the application of advanced nitrogen and phosphorous removal technologies but also development of energy-saving technologies that could contribute to creation of a low-carbon society. Reducing energy consumption in the wastewater treatment (see related information in the December 2, 2012 issue [in Japanese]) requires optimizing aeration amount as needed. For this reason, conventional bioreactors mainly use an operating system with alternating anaerobic-aerobic process. If, instead, continuous aeration is used for the entire bioreactor, neither nitrification nor denitrification can be performed as desired. Even if the alternating anaerobic-aerobic process is used, setting appropriate intermittent-aeration time has depended largely on past experiences.

Observing the above situation, the Center for Material Cycles and Waste Management Research, jointly with the Chinese Research Academy of Environmental Sciences (CRAES) and other collaborating institutions, has been developing advanced technologies using a system with automatic oxygen supply device (AOSD) in which intermittent-aeration time is automatically controlled based on the amount of oxygen consumed by the microorganisms.

AOSD system uses a dissolved oxygen (DO) sensor to automatically control and provide the amount of oxygen required by the bacteria, protozoa and small metazoa constituting activated sludge to perform oxidation of organic compounds and nitrification and denitrification process in wastewater. It is a new technology featuring a program that controls anaerobic and aerobic operations with a lower limit DO concentrations of 0.2-0.5 mg/L set in consideration of temperature-dependence of nitrifying and denitrifying bacteria. Structure and principles of the AOSD system is illustrated in Fig.1.

In normal operating conditions, aeration blowers run 24 hours a day. With AOSD, however, optimal aeration and non-aeration time is automatically calculated respectively based on parameters such as nitrification/denitrification rates and by using DO and water temperature data that are constantly sent to the Central Processing Unit (CPU). Thus, the use of AOSD can improve water quality and reduce electricity consumption. When influent organic load is high, aeration time is extended, and when it is low, aeration time is shortened. Through this automatic control of the aeration amount, the AOSD system allows advanced treatment of wastewater by keeping near neutral pH which is suitable for biological process. The AOSD also incorporates a program that automatically adjusts aeration time in case the inflow load increases or decreases rapidly. If DO value drops below a set lower limit, the program extends aeration time, and if DO value exceeds a set upper limit, it shortens aeration time.

Characteristics of activated sludge process with AOSD system for domestic wastewater treatment

We compared characteristics of activated sludge process with and without AOSD system in domestic wastewater treatment under the same condition of mixed liquor suspended solids (MLSS) of around 3,000 mg/L and aeration amount of 1.5 L/min, and obtained the following results: (1) with continuous aeration, removal efficiencies were 4% for total nitrogen (T-N), 10% for total phosphorous (T-P), and 65% for chemical oxygen demand (COD); (2) with 30 minute aeration and 60 minute anaerobic mixing, removal efficiencies were 50% for T-N, 10% for T-P, and 65% for COD; (3) with 30 minute aeration and 90 minute anaerobic mixing, removal efficiencies were 60% for T-N, 39% for T-P and 87% for COD; and (4) bioreactors with AOSD demonstrated removal rates of 80% for T-N, 40% for T-P and 90% for COD. Based on the results under the condition of a constant influent concentration and a constant flow rate, the AOSD-controlled system demonstrated potential of much higher removal efficiencies under a fluctuating inflow load condition. These results clearly show that the introduction of the AOSD system can improve the removal efficiencies of T-N, T-P and COD much more than other experimental systems.

We also analyzed energy-saving effect of the AOSD system based on aeration time. Systems without AOSD require aeration of approximately 12 hours a day in order to achieve targeted results. With the AOSD-controlled system, the treatment facilities attained higher performance and energy-saving of 20 to 70%.

Characteristics of AOSD-controlled fluidized bed aeration method for domestic wastewater treatment

At CRAES in China, we conducted performance evaluation of a medium-scale Johkasou (the size for about 30 persons or 10 households) with AOSD-controlled system in fluidized bed aeration method for domestic wastewater treatment (see related information in the March 5, 2007 issue [in Japanese]) of a housing complex. A laboratory was set up for this purpose at the CRAES with support from the Japan International Cooperation Agency (JICA). With influent average concentrations of biological oxygen demand (BOD) of 300 mg/L, suspended solids (SS) of 350 mg/L, CODcr (COD using potassium dichromate) of 600 mg/L, T-N of 80 mg/L, ammonium nitrogen (NH₄-N) of 75 mg/L, and T-P of 10 mg/L, effluent quality was improved significantly. In particular, the introduction of the AOSD system led to highly improved T-N removal rate.

With the blower's operation pattern in the AOSD-controlled fluidized bed aeration system, DO value in the aerobic tank reached 3-4 mg/L during aeration time, and it remained in the range of 0.02-0.5 mg/L during non-aeration time. Electricity consumption of the main blower was 4.7-10 kWh per day, and the mean value was 6.55 kWh. The mixer, which operates during non-aeration time, consumed 2.32-0.87 kWh of electricity. Under the normal operating condition of 24-hour aeration, the main blower (0.55 kW) consumed 13.2 kWh of electricity. It was therefore confirmed that the automatic intermittent aeration control through the AOSD system provides a stable wastewater treatment performance and a power reduction of over 50% on average.

Summary and future outlook of AOSD technology development

The performance evaluation test shows that the program control in the AOSD system achieved higher level of accuracy than the set value. The DO value was controlled within a range of 0.2-4.5 mg/L, and removal efficiency improvement of T-N, NH₄-N, T-P and COD was around 20-50%. Moreover, it demonstrated energy-saving effect by achieving 20-70% reduction of aeration time. The AOSD system which employs an artificial intelligence (AI) based method for calculating respiration rate has proven to be a very important technology for advanced wastewater treatment in terms of protection of water quality in public waters where measures against water pollution and eutrophication (see related information in the December 7, 2009 issue [in Japanese]) are needed, carbon dioxide (CO₂) emission reduction for climate change mitigation, energy-saving and reduction in electricity prices. Japan currently (as of July 2014) suffers from a power supply shortage, and technologies that can reduce power consumption in every economic sector effectively are required. The findings in our study suggested effectiveness of introducing the AOSD system as a breakthrough and innovative technology in the field of wastewater treatment.

As mentioned above, in the comparative performance evaluation of activated sludge process with and without AOSD system for automatically controlling anaerobic and aerobic time, the effectiveness of the system for advancing wastewater treatment and reducing electricity consumption was confirmed. It is becoming increasingly more important to apply this energy-saving technology to business establishments, sewage treatment plant and large-scale Johkasou in Japan as well as in China and other neighboring countries so that the technology can contribute to generating co-benefits of environmental conservation, regeneration and restoration that are linked to creation of the low-carbon society.