Upgrading and Renovation Project of Wastewater Treatment Plant with A2/O-MBBR Process
With the continuous increase in public environmental awareness, wastewater treatment plants need to actively carry out upgrading and renovation activities, adopt advanced technologies to treat wastewater, achieve wastewater reuse, and contribute their share to sustainable social development. A major challenge encountered during the upgrading and renovation of wastewater treatment plants is nitrogen and phosphorus removal. By utilizing MBBR technology, this problem is effectively resolved. This paper focuses on the township wastewater treatment plant in Xichou County, which employs a combined process of pretreatment + A2/O secondary biological treatment process + cloth media filtration + sodium hypochlorite disinfection. The biological treatment section utilizes integrated wastewater treatment equipment (including a pre-anoxic tank, anaerobic tank, anoxic tank, aerobic tank, inclined tube sedimentation tank, cloth media filter, and disinfection tank).
1 Project Overview
The sewage pipe network construction supporting the township wastewater treatment plant in Xichou County, Wenshan Zhuang and Miao Autonomous Prefecture, Yunnan Province, includes projects in six townships: Dongma, Lianhuatang, Banggu, Fadou, Bolin, and Xinmajie. The total length of supporting sewage pipe networks in these townships is about 39.182 km, with pipe diameters ranging from DN200 mm to DN500 mm, using High-Density Polyethylene Double Wall Corrugated Pipes (HDPE). Integrated pumping stations are constructed in Lianhuatang and Xinmajie townships. In Xinmajie Township, there is a Q=25 m³/h, DN150 mm pressure water supply PE pipe of 50 m, and in Lianhuatang Township, a Q=25 m³/h, DN200 mm pressure water supply PE pipe of 15 m. The total construction area of the wastewater treatment plant is 3,482 m², including a comprehensive building, integrated wastewater treatment equipment, transformer and distribution room, monitoring room, regulation tank, sludge tank, reuse water tank, sludge dewatering room and sludge storage shed, screen channel, lift pump station, and emergency tank.
2 Water Quality Analysis and Selection of Main Process
2.1 Influent and Effluent Water Quality
Comprehensive analysis of the influent water quality of the Xichou County township wastewater treatment plant shows that its concentration is stable with a slight downward trend. As the current process is a high-efficiency wastewater treatment process, the volume of the treatment tanks is not large, and its tolerance to shock loads is not strong. Therefore, the guarantee rate standard for influent water quality indicators cannot be set too high; this time it is set at 90%. Furthermore, the plant receives 500 m³ of landfill leachate daily. When designing the final influent water quality, it is necessary to rely on the overall trend of water quality to efficiently complete the relevant design work. The water quality indicators are shown in Table 1.
The BOD₅/CODcr ratio in the wastewater is 0.35, indicating easily biodegradable wastewater; the BOD₅/TN ratio is 3. To meet the effluent TN standard, additional treatment measures are required, such as adding an external carbon source; the BOD₅/TP ratio is 26.3, which is suitable for biological phosphorus removal.
Currently, the residual amounts of NH₃-N and TN are relatively high, and the removal efficiency is poor. This indicates that nitrification of NH₃-N cannot be fully carried out in the old aerobic tank. Since an anoxic tank was not originally set up, the denitrification process did not occur. Nitrogen removal was only achieved by discharging excess sludge, and the nitrification-denitrification method was not employed.
2.3 Main Process
After thorough analysis of the specific situation of the Xichou County township wastewater treatment plant, the upgrading and renovation had to be completed within the plant site. The space within the plant area is very limited. When determining the wastewater treatment process, it was necessary to comprehensively consider the site conditions and make reasonable use of the existing biochemical tank treatment process. After extensive research, adopting the A2/O-MBBR process (referred to as the MBBR process) effectively addressed land use and operational issues. This approach facilitated the three-dimensional expansion of the biochemical tank capacity and enabled the active construction of anoxic and anaerobic tanks. The MBBR process combines activated sludge with biofilm. Its advantages are manifested in its relatively small footprint, long biological chain, ability to achieve ideal effluent water quality standards, and stable operation. The biofilm method for nitrogen removal also shows good results during low-temperature seasons. The MBBR process flow is shown in Figure 1.
2.4 Advantages of the MBBR Process
Comparing the MBBR process, fixed-media biofilm methods, and activated sludge processes, the MBBR process stands out with the most prominent advantages, specifically: ① The suspended carriers are mainly made of modified materials such as PP and PE, offering good durability. As the suspended carriers are easy to start up and operate, problems like clumping and clogging rarely occur. Therefore, when applied to the aeration system and effluent devices of the wastewater treatment system, their depreciation rate and replacement frequency are very low. ② The MBBR process possesses strong nitrogen removal capability. Aerobic, anoxic, and anaerobic environments can coexist on the suspended carriers, allowing both nitrification and denitrification reactions to be completed within a single reactor. Nitrifying bacteria can grow rapidly on the biofilm formed on the suspended carriers, achieving optimal nitrification. ③ The MBBR process has good tolerance to shock loads, enhancing effluent stability and resistance to toxic substances. ④ By adopting the MBBR process, reasonable upgrading and renovation of the original treatment equipment can be utilized, with almost no change to the land use, thus saving space. ⑤ Traditional wastewater treatment requires adding carrier support frames in the aeration tank, whereas the MBBR process eliminates this step, thereby reducing the difficulty of maintaining aeration devices and managing the carriers.
3 Biochemical Tank Renovation Plan
3.1 Construction of New Anaerobic and Anoxic Tanks
After demolishing the buildings on the west side of the plant's biochemical tank area, new anoxic and anaerobic tanks were constructed on the cleared land. The anoxic zone was modified from the initial section of the existing biochemical tank. Active construction of the anoxic and anaerobic tanks was carried out. Their plan dimensions and effective volume must meet relevant usage requirements, and the hydraulic retention time was scientifically planned to enable them to play an important role. During the construction of the anoxic tank, the minimum temperature was controlled to >12°C, and management of indicators such as mixed liquor suspended solids concentration, denitrification nitrate concentration, and denitrification rate was well implemented. Insufficient carbon source may occur in winter; an appropriate amount of carbon source can be added to enhance denitrification efficiency. The newly built anoxic tank is equipped with a total of 16 units of 5 kW vertical turbine mixers; the existing biochemical tank anoxic zone is equipped with a total of 8 sets of 5 kW vertical propellers; the anaerobic tank is equipped with a total of 6 sets of 6.5 kW submersible mixers.
Comparing the difficulty coefficients of phosphorus removal and nitrogen removal tasks, nitrogen removal is evidently more challenging. Usually, satisfactory phosphorus removal effects can be obtained by chemical phosphorus removal methods. To optimize nitrogen removal effects, when temperatures are low and influent total nitrogen is high, sludge can be recycled to the anaerobic section to ensure a longer retention time in the anoxic section.
3.2 Renovation of Existing Biochemical Tanks
After renovation, the existing biochemical tank is divided into four parts: A dividing wall is added between the first and fourth parts. The areas before and after the dividing wall in these two parts are the anoxic zone and the carrier zone (MBBR zone), and the MBBR zone and degassing zone, respectively. The second and third parts are both MBBR zones. Adding a dividing wall in the fourth part can control the dissolved oxygen concentration of the internal recycle mixed liquor within a reasonable range. Furthermore, equipment such as screens and perforated pipe aerators are installed in the MBRR zone to improve the operational efficiency of the biochemical tank. After the renovation of the biochemical tank aerobic zone is completed, the total effective tank volume of the degassing zone and MBBR zone reaches 38,000 m³. The degassing zone is equipped with a total of 12 units of 18.5 kW axial flow pumps, with 4 as standby; pure HDPE suspended carriers are used.
3.3 Renovation of Blower Room and Aeration System
There are 4 blowers in the blower room: 3 are old blowers with an inlet flow rate of 480 m³/min, and one is a new blower. Water cooling is the main cooling method for the old blowers, with a power of 830 kW each; air cooling is the main method for the new blower, with a power of 670 kW. Comparing the operational status of the old and new blowers, the new blower operates more efficiently and effectively. The old blowers not only have low operational efficiency but also require expensive maintenance and repair costs.
When designing the aeration volume for the aerobic zone, it should be based on the highest oxygen demand in the aerobic zone, with a final selected value of 720 m³/min. The configuration of perforated aeration pipes should be based on the air volume of the 4 blowers. The work of replacing the old blowers should be carried out effectively. Repurchasing 3 new blowers to replace the old ones is beneficial for reducing the aeration volume. When replacing the aeration pipes, only the old aeration pipes inside the aerobic tank are replaced.
3.4 Sludge Treatment System
The main sludge treatment equipment used in the Xichou County township wastewater treatment plant is a sludge thickening and dewatering filter press. Comprehensively analyzing sludge dewatering and thickening processes, integrating sludge thickening and dewatering operations can minimize capital investment costs and reduce the dosage of high-polymer flocculants. To avoid environmental damage from sludge treatment, mechanical sludge thickening and dewatering technology was chosen to efficiently control environmental and atmospheric pollution.
3.5 Deodorization System
There are many methods for treating odors, commonly used ones include biological, chemical, and physical methods. Different odor treatment methods have significant differences in their deodorization mechanisms, application conditions, and technical types. After comprehensively analyzing the specific circumstances of this project and considering the advantages and disadvantages of different deodorization technologies, the ion deodorization process was ultimately selected to carry out the relevant operations.
3.6 Key Points of Process Renovation
3.6.1 Carrier Selection
When selecting suspended carriers, it must be ensured that the manufacturing material has sufficient corrosion resistance and the total effective specific surface area meets the effluent standards, thereby guaranteeing the biomass. Simultaneously, the service life, wear resistance, and strength of the suspended carriers must meet standards, with a service life maintained at over 15 years.
3.6.2 Carrier Accumulation
As water flows, the carriers change position, causing a large number of carriers to accumulate in front of the interception screens. After some time, the interception screens may become clogged. Increasing aeration is used to flush away the accumulated carriers. A head loss occurs at each interception screen. A large number of carriers accumulate under the pressure of the water level difference across the screen. As the water level difference increases, the amount of carrier accumulation also increases. A carrier recycle device is installed in the carrier zone. Driven by an airlift device, carriers at the end of the carrier zone are returned to the front end, preventing carrier accumulation.
3.7 Analysis of Post-Renovation Operational Effectiveness
The total investment for this project is 219.91 million yuan. The average unit operating cost is 0.4 yuan/m³, and the average unit total cost is 0.5 yuan/m³. After the upgraded renovation project was completed and put into operation, its water flow effect is very satisfactory, the operational status is good, and the effluent water quality standards can meet relevant requirements.
4 Conclusion
During the construction of this upgrading and renovation project, existing structures were effectively utilized. By rationally employing MBBR technology, the layout renovation work achieved good results without increasing the footprint, significantly enhancing the nitrogen and phosphorus removal capacity of the wastewater treatment system and optimizing pollutant removal efficiency. MBBR technology is highly advanced, possessing not only the advantages of conventional wastewater treatment technologies but also efficiently utilizing the high treatment capacity of its special carriers, significantly improving pollutant purification efficiency.
Based on analysis and demonstration, to ensure the rationality of the plan, it is recommended to adopt the MBBR process scheme. By performing in-situ renovation of the original biological system, adding carriers to the aerobic zone to increase its load capacity ensures nitrogen treatment meets standards. Subsequent use of high-density sedimentation tanks + cloth media filters to control SS and TP can guarantee stable effluent meeting the Grade 1A standard. The MBBR process, as well as various combined processes that incorporate MBBR into activated sludge systems, operate stably, are easy to operate and adjust, have strong tolerance to changes in influent quality and quantity, offer good nitrogen and phosphorus removal effects, and represent an economical, efficient, and stable wastewater treatment method. As national and local requirements for effluent quality from wastewater treatment plants increase, this process is a very suitable solution for projects facing challenges such as early construction with processes unable to meet new requirements, limited land availability, high land costs, and funding difficulties. It is bound to be more widely applied in the upgrading and renovation of municipal or industrial wastewater treatment plants.
Furthermore, during this renovation project, targeted denitrification pathway control measures were taken based on actual conditions when renovating the biochemical tanks, including strengthening the management of indicators like denitrification nitrate concentration and denitrification rate. The process renovation focused on improving carrier selection and accumulation management. By integrating renovations to the blower room and aeration system, sludge treatment system, and deodorization system, the comprehensive treatment capacity of the wastewater treatment plant was enhanced.