Research Progress on Process Operation and Application of MBBR Systems at Low Temperatures
Overview
The Moving Bed Biofilm Reactor (MBBR) process is one of the widely used biofilm wastewater treatment technologies at present. Compared to conventional activated sludge processes, MBBR offers advantages such as effective effluent quality, strong resistance to shock loads, and no requirement for sludge return or backwashing. During the low-temperature period in winter, especially in northern regions and southwestern plateaus, air temperatures can easily drop below 5°C, and water temperatures can fall below 15°C. Low temperatures may lead to non-compliance of effluent indicators such as Chemical Oxygen Demand (COD), ammonia nitrogen, and total nitrogen in MBBR systems. Biofilm nitrogen removal includes aerobic nitrification and anoxic denitrification, and temperature is one of the key factors affecting these processes. As temperatures decrease, the nitrification rate of bacteria in activated sludge systems gradually declines, with a significant reduction in nitrification capacity when temperatures fall below 8°C. This article systematically elaborates on the operation of MBBR processes under low-temperature conditions from aspects such as microbial communities, carrier enhancement technologies, and process combinations and manipulation, providing references for further research and application.
1. Research on Microbial Communities in Low-Temperature MBBR Systems
Currently, the core process in wastewater treatment plants is biological treatment. Low temperatures in winter (≤15°C) inhibit the activity of nitrifying bacteria in bioreactors, affecting the nitrification process and limiting the system's nitrogen removal capacity. Nitrifying bacteria are autotrophic with long generation cycles and are sensitive to temperature changes, with an optimal growth temperature range of 20–35°C.
1.1 Microbial Activity
Biofilms in MBBR reactors grow attached to carrier surfaces, supporting the growth of microorganisms with long generation cycles, thereby increasing the content of nitrifying bacteria in the system. Compared to activated sludge processes, MBBR exhibits stronger nitrification performance at low temperatures, making it widely used in low-temperature wastewater treatment. Low temperature is one of the important environmental factors affecting the nitrification performance of this reactor. Temperature reduction leads to decreased cell membrane fluidity and enzyme catalysis, reduced material transport and metabolic rates, thereby affecting the stability of nucleic acid secondary structures and inhibiting DNA replication, mRNA transcription, and translation. When temperatures fall below the cytoplasmic freezing point, ice crystals form within cells, causing severe structural damage. Studies by Qiu Tian et al. showed that the ammonia oxidation and nitrite oxidation activities of MBBR biofilm at 10°C were 55% and 56% of those at 20°C, respectively. Zheng Zhijia et al. tested the nitrification rates of activated sludge in a wastewater treatment plant in summer (20°C) and winter (8°C), finding that the ammonia nitrogen nitrification rate at 8°C was 48.5% of that at 20°C. The impact of low temperature on the nitrification capacity of biochemical tanks includes two aspects: firstly, low temperature affects the activity of nitrifying bacteria communities, and secondly, prolonged low temperatures reduce the population of nitrifying bacteria in activated sludge.
1.2 Microbial Community Competition
As nitrifying bacteria are autotrophic, other microbial communities significantly impact the nitrification process and compete strongly with nitrifying bacteria. Houweling et al. conducted MBBR process experiments, showing that at 4°C, MBBR has certain nitrification potential, but excessive growth of heterotrophic microorganisms within the system reduced the nitrification rate to some extent. Shao Shuhai et al. indicated that the nitrogen removal effect of single-stage MBBR is not ideal due to competition between nitrifying and heterotrophic bacteria. Han Wenjie et al. studied microbial community changes and biological distribution patterns in a wastewater treatment plant using MBBR hybrid processes during low-temperature seasons, finding that the number of microbial species in suspended carrier biofilms was lower than that in activated sludge from the same system, with uneven species distribution. The addition of suspended carriers enhanced microbial diversity in the system, while influent and operational modes had a certain selectivity on the microbial community composition. Wu Han et al. simulated domestic wastewater treatment using three sequential batch MBBR reactors with different filler types. By gradually reducing temperatures (25, 20, 15, 10, 6, and 5°C) to cultivate and acclimate biofilms for low-temperature wastewater, they found that different microorganisms dominated in the three reactors. High-throughput sequencing results showed that at 5°C, microorganisms degrading organic matter were predominant in all three reactors; one reactor successfully acclimated and enriched psychrophilic nitrifying bacteria, while the other two had higher abundances of nitrogen-fixing bacteria unfavorable for nitrogen removal.
1.3 Acclimation of Psychrophilic Microorganisms
The acclimation and enrichment enhancement technology for low-temperature dominant microbial communities is an effective method to improve the operational efficiency and stability of MBBR under low-temperature conditions. Through progressive induction and optimized cultivation, dominant populations are screened and applied, utilizing the strong tolerance of the microbial communities to reduce the impact of low temperatures, offering long-term stability potential. Wang Dan et al. found that under winter low-temperature conditions, adding activated sludge containing cold-tolerant microbial communities to achieve an activated sludge-biofilm symbiotic hybrid bioreactor offered advantages such as rapid startup, quick biofilm formation, and stable treatment effects. Delatolla et al. discovered that decarbonizing the system at 1°C increased nitrifying active biomass, thickened the biofilm, effectively increased the number of viable cells during low-temperature operation, and enhanced the system's nitrification performance. Additionally, NO, N₂H₄, NH₂OH, etc., are key intermediates that stimulate the anaerobic ammonium oxidation (anammox) process and alleviate the inhibition of anammox bacteria by NO₂. Zekker et al., in a study treating high-concentration wastewater (ammonia nitrogen concentration 740 mg/L) with an MBBR system, found that adding NO significantly accelerated the anammox process, and the abundance of ammonia-oxidizing bacteria increased proportionally during system operation.
2. Research on Carrier Enhancement Technologies for MBBR at Low Temperatures
The selection of suspended MBBR fillers is one of the core technologies of this process for wastewater treatment and a key factor affecting process efficiency and engineering costs. Commonly used filler types include honeycomb fillers, semi-soft fillers, and composite fillers, among others. Practical applications may encounter issues such as filler clogging, agglomeration, and aging. Under low-temperature conditions, biofilm formation on MBBR fillers is slower, potentially prolonging equipment startup periods, hindering normal process operation, resulting in poor shock load resistance, and failing to achieve expected treatment effects. Industrially used MBBR suspended carriers vary in size and shape and are made from high molecular polymers such as high-density polyethylene (HDPE), polyethylene (PE), or polypropylene (PP) through methods like melt extrusion or granulation. With the large-scale engineering application of this process, the variety of commercial carriers has gradually increased. Carrier design and processing can be tailored to water quality and microbial growth characteristics, enabling targeted optimization and improvement to enhance MBBR biofilm systems under low-temperature conditions. In practical applications, carrier modifications primarily focus on improving specific surface area, hydrophilicity, bio-affinity, magnetic properties, etc., to enhance carrier mass transfer, biofilm formation, and wastewater treatment performance.
2.1 Magnetic Loading
Current research has explored using magnetic fields to optimize MBBR's wastewater treatment capacity at low temperatures. Magnetic fields of certain strengths can enhance pollutant removal in biological treatment processes. Under weak magnetic fields, organic pollutants are enriched on the surface of magnetic biological carriers through magnetic aggregation and adsorption, aided by magnetic forces, Lorentz forces, and magneto-colloidal effects. Within an appropriate intensity range, magnetic fields can improve microbial oxygen utilization, enhance microbial growth metabolism and enzyme activity, and increase cell membrane permeability. Jing Shuangyi et al. studied the comparative effects of adding magnetic carriers [polyethylene, neodymium iron boron magnetic powder (Nd₂Fe₁₄B), and polyquaternium-10 (PQAS-10), etc.] versus commercial carriers in MBBR reactors. Results showed that under low-temperature conditions, magnetic carriers significantly improved biofilm nitrification activity, promoted extracellular polymeric substance (EPS) secretion, and maintained and improved biofilm morphology and structure. Magnetic carriers enriched more nitrifying bacteria genera, with relative abundances of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria increased by 1.82 times and 1.05 times, respectively, compared to commercial carriers, and acclimated and enriched two unique nitrifying bacteria genera.
2.2 Carrier Modification
Besides magnetic loading, affinity modification of traditional carrier materials like polyethylene is also an important way to enhance filler biofilm formation performance. Sun Bo et al. used novel nano suspended fillers to treat low-temperature domestic wastewater. At 10–12°C, the biofilm formation period for nano fillers was less than 18 days, shorter than other fillers, with system COD removal rate stable at around 75%, demonstrating good promotion value. Ren Yanqiang et al. used honeycomb suspended fillers made from highly hydrophilic polymer alloy materials to treat effluent from the primary sedimentation tank of a wastewater treatment plant under low-temperature conditions. Results indicated that these suspended fillers effectively improved the attachment capacity of surface-active microorganisms, aiding in enhancing the treatment effects of the MBBR process. Han Xiaoyun et al. used soft polyurethane foam with a developed pore structure as an immobilized carrier to fix efficient cold-tolerant microbial communities separated from activated sludge. After adding this filler to the reactor, pollutant treatment effects significantly improved, with COD removal rate of 82% and biochemical oxygen demand (BOD) removal rate of 92% under low-temperature conditions. Chen et al. used an MBBR process with polyvinyl alcohol (PVA) gel filler inoculated with HN-AD bacteria to treat livestock and poultry breeding wastewater instead of activated sludge. Under different carbon-to-nitrogen ratios (C/N), the performance of different carriers varied significantly. The porous structure of PVA gel provided protection for bacteria, resulting in more stable performance. Microbial analysis showed that the MBBR process with PVA gel carriers favored the growth of autotrophic bacteria and HN-AD bacteria (Paracoccus and Acinetobacter).
3. Process Combination and Regulation of MBBR at Low Temperatures
This system has unique requirements for biofilm formation on filler surfaces, highlighting the importance of process combination and Regulation. Stable nitrification in MBBR can be achieved through Regulation of process parameters and ratios; compensating for the effects of low temperature through stricter constraints is a relatively direct and effective method.
3.1 Aeration
The MBBR process is currently mainly applied in aerobic environments. The aeration rate and method in the reactor directly affect the dissolved oxygen (DO) content in the system and the characteristics of biofilm formation, thereby influencing the degradation level of pollutants. Chen Long et al., during industrial wastewater treatment, effectively addressed difficulties in biofilm formation using measures such as Batch Aeration, achieving COD removal rate of 95.5% and ammonia nitrogen removal rate of 91%. Persson et al. used MBBR to treat mixed wastewater of kitchen waste and black water after anaerobic pretreatment at 10°C, achieving complete nitrification through intermittent aeration. Bian et al. found that controlling a constant ratio between DO and total ammonia nitrogen concentration optimized effluent effects at low temperatures; when the control ratio did not exceed 0.17, the nitrification process remained stable at 6°C.
3.2 Carbon-to-Nitrogen Ratio (C/N)
There is obvious competition between nitrifying and heterotrophic bacteria; therefore, C/N regulation becomes an important parameter affecting the balance between organic matter and nitrogen degradation in the system. Chen et al. showed that in MBBR systems, when C/N was between 4–15, COD removal rate was above 90%. When C/N decreased to 1, COD removal rate dropped significantly. The system's ammonia nitrogen removal efficiency first increased and then decreased with reducing C/N. Chen et al. explored the impact of C/N on the performance of an A/O-MBBR reactor treating mariculture wastewater. Results indicated that reducing C/N is beneficial to improving COD and ammonia nitrogen removal efficiency.
3.3 Hydraulic Retention Time
Hydraulic Retention Time (HRT) determines the active sludge load within the reaction system. Too high or too low HRT can affect the treatment efficiency and construction/operational costs of MBBR systems. Selecting a reasonable HRT is crucial for stable system operation. Van et al. applied MBBR for agricultural non-point source pollution control at low temperatures. Research showed that at 5°C, as HRT decreased, pollutant removal efficiency significantly declined, with 8 hours being the minimum retention time to ensure nitrate denitrification to nitrogen gas. Wang Chuanxin et al. treated domestic wastewater with an anoxic/aerobic biofilm system, focusing on the characteristics of simultaneous nitrification and denitrification in MBBR at low temperatures. Results showed that the system adapted well to seasonal temperature drops by extending HRT, stabilizing effluent COD and ammonia nitrogen concentrations to meet standards. Shitu used a novel sponge filler as an MBBR biofilm carrier to study its water treatment effect at different HRTs. Results indicated that water treatment effects was best at HRT 6 h. Zhao Wenbin et al. showed that the optimal HRT for pollutant removal in wastewater by MBBR systems under low-temperature conditions was 24 h. Han Lei et al. studied the pollutant removal rate when HRT was reduced from 15.4 h to 11.0 h in a DE oxidation ditch + MBBR combined process. Results showed that as HRT shortened, pollutant removal efficiency gradually decreased, but effluent quality could still meet water quality target requirements, reflecting the strong shock load resistance of the MBBR system.
3.4 Process Combination
Deng Rui et al. studied a two-stage A/O-MBBR process for treating municipal wastewater. Under conditions of low water temperature and low influent concentration, this combined process demonstrated strong shock load resistance and temperature adaptability, stable operation, and convenient operation, showing good application prospects in wastewater treatment. Luostarinen et al. studied the treatment effects of MBBR process on dairy wastewater after anaerobic pretreatment at low temperatures. Results showed that the process could remove 40%–70% of COD, 50%–60% of nitrogen, and the combination of Upflow Anaerobic Sludge Blanket (UASB) and MBBR could remove 92% of COD, 99% of BOD, and 65%–70% of nitrogen. Ru Chun et al. used a modified Bardenpho-MBBR + magnetic loading precipitation process to renovate a wastewater treatment plant. By adjusting carbon source dosing points and implementing multi-point influent and multi-point reflux in the system, efficient utilization of externally added carbon sources was achieved, ensuring nitrification and denitrification effects at 8.7°C, with stable effluent quality better than discharge standards.
Conclusion
Under low-temperature conditions, microbial activity in MBBR systems decreases, and there is obvious competition between heterotrophic microorganisms treating organic matter and autotrophic microorganisms treating ammonia nitrogen. Therefore, based on the composition of raw water pollutants and effluent Indicators requirements, suitable C/N should be fully considered. Measures such as improving and acclimating low-temperature dominant strains, targeted enrichment, and increasing the abundance of dominant populations on carriers should be implemented for key indicators to ensure effluent quality.
Carrier enhancement is an important means to improve the low-temperature tolerance of MBBR systems and enhance process degradation efficiency. Specific measures mainly include magnetic loading and structural treatment of carriers. Magnetic loading can enhance the attachment of nitrifying bacteria at low temperatures, strengthen the EPS secretion process, and improve bacterial activity; optimizing carrier structure and surface properties can accelerate pollutant mass transfer efficiency, improve their ability to solidify and protect microbial communities, and maintain more stable system performance.
The MBBR process itself possesses certain low-temperature resistance characteristics. However, with continuously improving effluent quality standards for wastewater treatment plants, Working conditions adjustment and process combination of MBBR under low-temperature conditions have become important research content for process breakthrough. For different types of wastewater, optimal Working conditions conditions should be determined based on actual situations. Meanwhile, reasonable process combinations can effectively enhance the shock load resistance, temperature adaptability, and system stability of MBBR systems towards pollutants.