How MBBR Transformed A Major Chinese Wastewater Plant: A 10-Year Case Study

Analysis of 10-Year Operation Practice of MBBR Process in Licunhe Wastewater Treatment Plant

Capacity expansion and upgrading have always been hot topics in the field of wastewater treatment. For the upgrading of wastewater treatment plants, new processes are required not only to have high treatment efficiency but also to possess good shock resistance and the feasibility of in-situ retrofitting. Therefore, the Moving Bed Biofilm Reactor (MBBR) process is a good choice. This process adds suspended carriers in-situ into the biological tank to enrich biofilm, thereby achieving enhanced pollutant removal. It has advantages such as high treatment load, strong shock resistance, and small footprint. Since the successful use of the MBBR process for Class A upgrading in Wuxi Lucun Wastewater Treatment Plant in 2008, the wastewater treatment capacity using the MBBR process in China has exceeded 35 million m³/d, and it has been widely applied in fields such as upgrading, capacity expansion, upgrading + capacity expansion, high-standard effluent, micro-polluted water treatment, and point source pollution treatment.

As a wastewater treatment plant that underwent upgrading relatively early, Licunhe Wastewater Treatment Plant first adopted the MBBR process for upgrading in 2010, and then used the MBBR process for capacity expansion in 2015. Based on more than 10 years of MBBR process operation in Licunhe Wastewater Treatment Plant, this paper introduces its operational performance, related equipment issues, etc., which can provide a reference for new construction, renovation, and expansion of other wastewater treatment plants.

1  Overview of Licunhe Wastewater Treatment Plant

Licunhe Wastewater Treatment Plant is located on the south bank of the lower reaches of Licunhe River, Shibei District, Qingdao City. By the end of 2020, the total treatment capacity reached 250,000 m³/d. The design influent and effluent quality are shown in Table 1.

 Since its construction, the plant has completed three upgrading/capacity expansion projects, all implemented in-situ within the plant without expanding the site. The facilities in the plant are divided into three phases. The total treatment capacity of Phase I and II was 170,000 m³/d, and after expansion in 2015, the treatment capacity reached 205,000 m³/d. Phase III was demolished, renovated, and newly built on the original site in 2015, with a treatment capacity of 45,000 m³/d.

The technical route for upgrading and the technical route for upgrading the biological tank of Licunhe Wastewater Treatment Plant: In 2010, the plant underwent its first upgrading, requiring effluent ammonia nitrogen, TP, SS, COD, and BOD₅ to be upgraded from the Class II standard of the "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant" (GB 18918-2002) to the Class A standard. Due to land constraints, only in-situ upgrading technology could be selected. At that time, there was little experience to draw on. Ultimately, the MBBR process was selected for the biological section to enhance nitrogen removal, and an advanced treatment system was added to meet the requirements for SS and TP removal. The core goal of this upgrading was to control ammonia nitrogen, so suspended carriers were added in part of the aerobic zone. The effective specific surface area of the suspended carriers was >450 m²/m³, and the filling ratios for Phase I and II were 40% and 48%, respectively. Using MBBR for upgrading was simple, easy, convenient, reliable, and the effect was stable. In 2015, the plant underwent its second renovation. The treatment capacity was increased from 170,000 m³/d to 250,000 m³/d. At the same time, all core indicators including TN were required to meet the Class A standard, and through optimized operation control, further meet the quasi-Class IV surface water standard. Considering the smooth connection with the existing process and the smoothness of the upgrading, as well as the comprehensive investment and operating costs, the MBBR technical route was ultimately continued. The core goal of this upgrading was to control ammonia nitrogen and TN while increasing the treatment capacity. On one hand, to address TN compliance, the biological section was upgraded from the traditional A²/O process to the Bardenpho process. By adding a post-anoxic zone, the limitation of the recirculation ratio on TN removal was overcome, ensuring stable effluent TN compliance. On the other hand, to address the ammonia nitrogen issue, Phase I and II supplemented the aerobic MBBR zone with suspended carriers having an effective specific surface area >800 m²/m³. Phase III was a new project, adding suspended carriers with an effective specific surface area >800 m²/m³ at a filling ratio of 48%. In 2020, the effluent of the plant was required to meet the quasi-Class IV surface water standard, and the effluent was used as ecological replenishment water for Licunhe River. Since the biological section had already fully considered meeting the quasi-Class IV surface water standard during the second renovation, this upgrading did not modify the biological tank. Only adding "air flotation + ozone advanced oxidation" in the advanced treatment section ensured stable effluent quality compliance.

Licunhe Wastewater Treatment Plant adopted the MBBR process for continuous upgrading. Although the technical routes were different, the core was the same: adding suspended carriers to the aerobic zone, using the advantage of suspended carriers to efficiently enrich autotrophic bacteria to enhance the nitrification process; at the same time, increasing the total effective surface area of the suspended carriers in the aerobic zone by increasing the filling ratio or using suspended carriers with a larger effective specific surface area, increasing the volumetric load, saving aerobic zone volume, and then reallocating the saved aerobic volume to the pre-anoxic zone or post-anoxic zone through volume redistribution, enhancing TN removal.

According to the upgrading milestones of the plant, the operation from 2010 to 2020 was divided into four stages. The operational performance of each stage is shown in Table 2. It can be seen from Table 2 that the effluent in each stage could meet the discharge standards.

2  Operational Performance of the MBBR Process

The core functions of the MBBR process are nitrification and denitrification, and denitrification is premised on nitrification. For municipal wastewater treatment plants, in actual control, meeting effluent ammonia nitrogen standards is key, because nitrification is easily affected by low temperature, shock loads, organic matter, and other factors, causing fluctuations in effluent quality.

2.1 Stability of Nitrification Operation

The compliance rates of ammonia nitrogen in the four operation stages are shown in Figure 1.

It can be seen from Figure 1 that in Stage I, based on the Class II effluent standard of the plant, the actual compliance rate of the Class A standard (ammonia nitrogen ≤5 mg/L) was 93.72%, and stable Class A standard operation could not be achieved. After the MBBR process renovation, the compliance rate of the Class A standard (ammonia nitrogen ≤5 mg/L) in Stage II increased to 99.18%. 0.82% of the ammonia nitrogen data were >5 mg/L, but still less than 8 mg/L. Analysis indicates this was mainly due to low-temperature operation. After further upgrading, the compliance rate of the Class A standard (ammonia nitrogen ≤5 mg/L) in Stage III reached 100%, and the compliance rate of the Class IV standard (ammonia nitrogen ≤1.5 mg/L) of the "Environmental Quality Standards for Surface Water" (GB 3838-2002) reached 93.99%. By Stage IV, the effluent ammonia nitrogen was (0.44±0.28) mg/L, and the compliance rate of the Class IV surface water standard (ammonia nitrogen ≤1.5 mg/L) had reached 100%. In addition, it can be seen from Figure 1 that from Stage I to Stage IV, the inflection point of the curve gradually moved backward and the maximum ammonia nitrogen value gradually decreased, indicating that the effluent ammonia nitrogen of the system was more stable and the shock load resistance was stronger.

December, January, and February of each year are the low-temperature operation period of the plant. The distribution of effluent ammonia nitrogen in the four stages during this period is shown in Figure 2. It can be seen from Figure 2 that from Stage I to Stage IV, the concentration distribution of effluent ammonia nitrogen gradually converged toward higher standards, and the low-temperature resistance became stronger and stronger. During the activated sludge control stage (Stage I), the low-temperature resistance was weak, and the effluent ammonia nitrogen concentration fluctuated greatly. With the addition of suspended carriers, the fluctuation of effluent ammonia nitrogen in Stage II weakened, and the low-temperature resistance of the system was enhanced. With the second renovation, the total effective surface area of the suspended carriers increased, the effluent ammonia nitrogen was stably below 5 mg/L, and the Class A compliance rate of the system reached 100%. As the biofilm on the suspended carriers gradually matured, by Stage IV, the treatment capacity and low-temperature resistance of the system were further strengthened, and the effluent ammonia nitrogen stably met the Class IV surface water standard. A wastewater treatment plant in Inner Mongolia, under winter low-temperature conditions (6–8 ℃), using the MBBR process, had an average effluent ammonia nitrogen of 1.5 mg/L, with an ammonia nitrogen removal rate of 94.2%, showing good low-temperature resistance. A wastewater treatment plant in Tianjin used the MBBR process for upgrading. Under low-temperature conditions of 2–6 ℃, the nitrification rate of the biofilm on the suspended carriers was 10 times that of the activated sludge, ensuring stable ammonia nitrogen compliance at low temperatures. From the overall low-temperature operation of the four stages at Licunhe Wastewater Treatment Plant, the addition of suspended carriers strengthened the low-temperature resistance of the system, ensuring stable effluent compliance throughout the year.

In addition to facing the problem of low temperature in winter, the system also frequently encounters water quality and quantity shocks (see Figure 3), which brings great challenges to stable effluent compliance. The influent flow of Licunhe Wastewater Treatment Plant shows a typical pattern of "high in summer, low in winter". Summer is mainly affected by the rainy season flood season and the combined sewer system, with total influent periodically exceeding the design capacity. It can be seen from Figure 3 (a) that from May 2013 to December 2015, the actual influent flow of the plant continuously exceeded the design value, with the exceedance rate close to 100%, lasting for a long time. During this stage, the effluent ammonia nitrogen was (1.3±0.3) mg/L, with an average removal rate of 96.4%, and the nitrification effect of the system was stable. It can be seen from Figure 3 (b) that from January 2017 to December 2020, the influent water quality had a serious exceedance phenomenon, mainly reflected in organic pollutants. The exceedance rate of BOD₅ reached 29.17%, and the resulting exceedance rate of influent TN reached 31.25%. The actual influent ammonia nitrogen/TN <0.5, resulting in a large ammonification load of organic nitrogen and a large nitrification load of ammonia nitrogen, bringing great challenges to the stable compliance of the plant. Through optimized operation control, measures such as increasing the system dissolved oxygen were adopted to ensure that the effluent ammonia nitrogen was (0.8±0.6) mg/L, achieving stable compliance discharge.

During the operation of the activated sludge process, the direct impact of influent organic shock is that the nitrification capacity of the system weakens, and the recovery time is long. For the MBBR process, since the microorganisms responsible for nitrification are mainly enriched in the biofilm on the suspended carriers, the adverse effects on nitrification caused by activated sludge control are effectively reduced, preventing the loss of nitrifying bacteria and ensuring stable ammonia nitrogen compliance. Huang Qing et al. found that before and after organic shock, the nitrification rate of the suspended carriers was not affected, while the nitrification rate of the activated sludge decreased by 44% after the shock. It can be seen that the MBBR process has strong resistance to water quality and quantity shocks.

2.2 Comparison of TN Removal Effect Before and After Renovation

The TN treatment effect of the plant from 2010 to 2020 is shown in Figure 4. It can be seen from Figure 4 that before 2015, the effluent TN was (19.62±4.63) mg/L, with a TN removal rate of 67.6%. Facing the TN compliance requirement in Stage III and the problem of high influent TN, the Bardenpho process was used to re-divide the original A²/O volume, achieving the goal of high TN removal. In Stage III, the effluent TN had dropped to (7.75±2.60) mg/L, fully achieving quasi-Class IV surface water effluent. Therefore, when upgrading for quasi-Class IV surface water standards in 2020, the biological tank was not modified. Due to the outstanding advantages of the Bardenpho process in TN control, with increasingly stringent effluent standards for wastewater treatment plants, the Bardenpho process is becoming more and more widely used. For example, a wastewater treatment plant in Zhejiang using the Bardenpho process had an average effluent TN of 8.43 mg/L, stably meeting the "Discharge Standard of Major Water Pollutants for Municipal Wastewater Treatment Plant" (DB33/2169-2018). A wastewater treatment plant in Tianjin using the Bardenpho process had an average effluent TN of 6.82 mg/L, stably meeting the Class A standard of the "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant" (DB12/599-2015).

3 MBBR Specialized Equipment and Maintenance

3.1 Suspended Carriers

Suspended carriers are the core equipment for enriching biofilm and achieving pollutant removal. Licunhe Wastewater Treatment Plant uses SPR series flat cylindrical suspended carriers, made of high-density polyethylene (HDPE), with a diameter of (25±0.5) mm and a height of (10±1) mm. As of the writing date, the earliest batch of suspended carriers added has been in operation for more than 10 years, ensuring stable biochemical performance. No replacement or supplementation has been made during operation. In engineering practice, the earliest overseas project using pure HDPE suspended carriers has been in stable operation for 31 years, and Wuxi Lucun Wastewater Treatment Plant in China has also been in operation for 14 years with good results. The long service life of suspended carriers not only helps the stable performance of the MBBR process but also avoids subsequent operating costs. A one-time investment can maintain long-term stable operation.

The SPR series suspended carriers at Licunhe Wastewater Treatment Plant include SPR-Ⅰ, SPR-Ⅱ, and SPR-Ⅲ, as shown in Figure 5. The core parameter, effective specific surface area, gradually increases. About one week after adding the suspended carriers, a biofilm attaches, appearing light yellow. As the system operates, the biofilm gradually matures, mainly brown. The color of the suspended carriers is mainly related to influent water quality, substrate concentration, operation time, addition zone, etc. Some reports also show yellow or black, but this does not affect the core function. The biofilm on the suspended carriers has a strong ability to enrich nitrifying bacteria, reaching more than 23%, which is more than 15 times that of activated sludge, confirming the fundamental reason for the enhanced nitrification by suspended carriers at the microbial level. The effective specific surface area of the three types of suspended carriers gradually increases, meaning that under the same filling ratio, the treatment capacity per unit volume becomes stronger and stronger. This is also the basis for further capacity expansion and upgrading of the MBBR process in the future. One of the industry development directions of the MBBR process is to develop suspended carriers with larger effective specific surface areas, but it is also necessary to pay attention to the influence of other parameters such as void ratio and wall thickness to prevent one-sided gains.

3.2 Screens

Screens are key to ensuring the specific enrichment of suspended carriers and the safe operation of the MBBR process. Licunhe Wastewater Treatment Plant first used stainless steel screens with a thickness of 3 mm. After 5 years of operation, suspended carrier leakage occurred. The problem was repaired in a timely manner through underwater construction, and then 5 mm thick stainless steel screens were used. After 3 years of operation, suspended carrier leakage occurred again. Both leakages had a significantly shorter life than expected. The analysis of the reason was mainly due to the operating environment. The high content of inorganic sand and gravel in the influent, combined with the effects of cavitation, electrochemical corrosion, acid and alkali corrosion, accelerated the wear of the screens. To fundamentally solve the problem, the manufacturer upgraded the stainless steel screens with high wear to non-metallic wear-resistant material. The wear index of the non-metallic wear-resistant material is about 1/12 to 1/6 of that of different types of stainless steel. The smaller the wear index, the better the wear resistance; it also avoids the problem of increased wear caused by electrochemical corrosion of stainless steel. Compared with stainless steel screens, non-metallic wear-resistant screens have a longer life, reaching more than 30 years, and can better ensure the safe operation of the MBBR process.

3.3 Mixers

Mixers are one of the fluidization power sources for suspended carriers. During the first renovation of Licunhe Wastewater Treatment Plant, push-flow mixers were installed in the aerobic MBBR zone to assist the fluidization of suspended carriers, achieving a good engineering fluidization effect. However, in actual operation, the collision between the suspended carriers and the mixers, as well as the inconsistent spatial filling ratio of the suspended carriers near the blades, easily led to damage to the mixer motors and wear of the blades, resulting in high maintenance and replacement costs. Therefore, for the MBBR process, the aerobic zone should first be de-mixerized, and the fluidization of suspended carriers should be achieved only through aeration. For the anoxic zone, since the mixer is the only power source for the anoxic zone, it is necessary to develop mixers specifically for MBBR. For example, by optimizing the rotation speed, shape, surface curvature, and linear speed of the outer edge of the blades of the mixer, and if necessary, using simulation methods for design and installation optimization. In short, it is necessary to achieve low-consumption fluidization of suspended carriers and a long service life of the mixers.

3.4 Fluidization Tank Configuration

The tank configuration of the MBBR process is a complete presentation of the influent and effluent system, aeration system, and fluidization system. The two MBBR process fluidization tank configurations at Licunhe Wastewater Treatment Plant are shown in Figure 6. In 2010, the large-scale MBBR cases in China were only Wuxi Lucun Wastewater Treatment Plant and Jining Zhongshan Public Utilities Wastewater Treatment Plant. The first renovation of Licunhe Wastewater Treatment Plant followed the model of Wuxi Lucun Wastewater Treatment Plant, using a circulating flow tank configuration. The circulating flow tank configuration is similar to an "oxidation ditch", requiring push-flow mixers in the suspended carrier zone. The suspended carriers form a horizontal circulating flow under the combined action of aeration and mixer push. This tank configuration was also the first successfully applied tank configuration for the MBBR process. The micro-mixing tank configuration does not require push-flow mixers; the fluidization of suspended carriers is achieved only through aeration. Through single-side influent and single-side effluent, the flow velocity of water in the tank is reduced, and with reasonable aeration arrangement, the suspended carriers can form a vertical circulating flow in the tank: the upper part flows from the effluent end to the influent end, and the lower part flows from the influent end to the effluent end. In terms of operation, both tank configurations can achieve good fluidization of suspended carriers, ensuring the effect of the MBBR process. The key to the application of the micro-mixing tank configuration is the arrangement of aeration pipes and the uniformity of influent distribution. It is necessary to effectively avoid non-fluidization of suspended carriers and short-circuiting. If necessary, simulation software should be used to simulate hydraulic fluidization. In addition, in design, it can be considered to control the perforated aeration and fine-bubble aeration with separate blowers to reduce mutual influence and improve the simplicity of control.

3.5 Operation and Maintenance

The core of MBBR process control is the fluidization of suspended carriers. Fluidization is the basis for mass transfer and oxygen transfer of the biofilm on the suspended carriers and is the key for the biofilm to exert its pollutant removal capacity. For the maintenance of tanks containing suspended carriers, special pumps are often used to transfer the carriers to adjacent tanks or other functional zones. After the maintenance is completed, the suspended carriers are pumped back. When emptying the tank, it is necessary to pay close attention to the liquid level difference before and after the screen and empty it in an orderly manner. When starting and stopping under special circumstances, attention should be paid to starting the aeration first when starting the system, so that the suspended carriers are fluidized before influent and recirculation are turned on; when stopping the system, the recirculation and influent should be turned off first, and finally the aeration should be turned off. The core is to ensure carrier fluidization and prevent a liquid level difference at the screen. Currently, with the development and innovative practice of underwater equipment for the MBBR process, its service life has been greatly improved, and the process can achieve long-term safe operation without maintenance. The MBBR process does not have strict requirements for pretreatment. The fine screen used for pretreatment at Licunhe Wastewater Treatment Plant has a bar spacing of 5 mm, and no screen clogging caused by improper pretreatment has occurred.

3.6 Feasibility of Pure-Media MBBR

Licunhe Wastewater Treatment Plant adopted the MBBR process renovation in the form of in-situ embedding, which is a hybrid sludge-biofilm system. As an activated sludge enhancement process, although the biofilm on the suspended carriers has the ability to enrich high-abundance, high-activity microorganisms, in the hybrid system, its main role is to compensate for the insufficient treatment capacity of activated sludge. As early as 2015, the plant began to verify the feasibility of the pure-media MBBR process. The pure-media MBBR process was used to treat the effluent of the secondary sedimentation tank for deep denitrification. The results showed that the denitrification load could reach 0.45 kg/(m³·d) under low-temperature conditions, and the effluent TN could be reduced to 5 mg/L, with good operation performance. At present, there are engineering examples of pure-media MBBR processes applied to municipal wastewater treatment, achieving high discharge standards in a short hydraulic retention time. Moreover, the pure-media MBBR process does not enrich activated sludge, not only eliminating the need for secondary sedimentation tanks, shortening the process flow, but also avoiding problems such as sludge bulking. Operation control is simple, providing new ideas and process options for wastewater treatment. This is also the future development direction of the MBBR process.

4 Conclusions and Recommendations

Licunhe Wastewater Treatment Plant in Qingdao used the MBBR process for two in-situ renovations of the biological section, achieving the goals of in-situ upgrading and capacity expansion. The discharge standard was upgraded from the Class II standard of GB 18918-2002 to the quasi-Class IV surface water standard, and the treatment capacity was increased from 170,000 m³/d to 250,000 m³/d. The MBBR process has been in operation for 10 years with stable treatment performance. The effluent ammonia nitrogen and TN reached (0.44±0.28) mg/L and (7.75±2.60) mg/L, respectively. The efficient enrichment capacity of nitrifying bacteria in the biofilm on the suspended carriers is the basis for the excellent nitrification performance of MBBR, ensuring that the MBBR process has good resistance to low temperatures and water quality and quantity shocks. The core of MBBR process operation and maintenance control is to control the fluidization of suspended carriers, which requires reliable design and equipment support, placing higher demands on process manufacturers.

The development directions of the MBBR process are: ① Develop suspended carriers with larger effective specific surface areas to increase the treatment capacity per unit volume, further reduce footprint or achieve continuous upgrading and capacity expansion; ② Use more wear-resistant non-metallic wear-resistant material screens to improve the safety of process operation; ③ Develop customized mixers to improve the service life of mixers and achieve low-consumption fluidization; ④ Develop an automatic fluidization monitoring system to replace manual labor and simplify operation and maintenance; ⑤ Enrich the process application forms and adopt pure biofilm methods to meet the diverse needs of wastewater treatment.