Advanced Nitrogen Removal in WWTPs: Pure Biofilm MBBR Process Design & Performance Analysis | Case Study

Engineering Design and Performance of a Pure Biofilm MBBR Process for Advanced Nitrogen Removal

With the comprehensive advancement of China's ecological civilization construction, the discharge standards for wastewater treatment plants (WWTPs) have become increasingly stringent. The Grade A standard of the "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants" (GB 18918-2002) requires TN ≤ 15 mg/L, while local standards in regions like Beijing and Shandong explicitly set the limit at TN ≤ 10 mg/L. These elevated standards extend beyond just water quality limits, placing stricter demands on effluent stability. Consequently, there is a pressing need to enhance the nitrogen removal capacity of treatment processes. One approach is to increase the carbon source dosage in the existing process to improve denitrification, but this leads to high operational costs and increased carbon emissions. Alternatively, adding advanced nitrogen removal facilities, often employing biofilm methods to efficiently enrich denitrifying bacteria, can enhance TN removal, reduce the need for external carbon sources, and lower carbon emissions. The Moving Bed Biofilm Reactor (MBBR), with its advantages of strong functional bacteria enrichment, small footprint, and simple operation and maintenance, has been widely applied in the construction, expansion, and upgrading of WWTPs. It can stably achieve discharge standards better than quasi-Class IV surface water quality and holds significant potential and advantages for advanced nitrogen removal in WWTPs. This article takes a WWTP in Shandong as a case study to analyze the design rationale and operational performance of applying a pure biofilm MBBR process for advanced nitrogen removal, aiming to provide a technical reference for efficient wastewater denitrification.

1. Project Overview

1.1 Project Introduction

A WWTP in Shandong was constructed in two phases. The first phase, utilizing the BIOLAK process, was officially commissioned in November 2003 with a treatment capacity of 40,000 m³/d. The layout of the BIOLAK process and the available area for upgrade are shown in Figure 1. Initially, the effluent quality met the Grade B standard of GB 18918-2002. By 2020, through enhanced carbon source dosing and the addition of advanced treatment, the effluent quality was improved to the Grade A standard. By 2023, after three years of operation, the overall effluent quality could generally meet the Grade A standard, but it faced two major challenges regarding nitrogen removal:

High Carbon Source Dosage: To achieve the target of TN ≤ 15 mg/L, a substantial amount of external carbon source was required. Calculations based on process sections showed a C/N ratio as high as 5.9, whereas the AAO process in the plant's second phase required only a C/N of 4.5–5.0 to ensure stable TN compliance. The large carbon source addition also adversely affected the aerobic nitrification process, increasing the oxygen demand in the aerobic zone.

Poor Stability of Nitrogen Removal: Since nitrification and denitrification occurred in the same tank under differing required conditions, operational parameters needed frequent adjustment based on influent changes. Controlling NH₃-N and TN was contradictory, making it difficult to maintain a stable balance between nitrification and denitrification. The system's shock load resistance was average, leading to poor effluent stability.

Therefore, an upgrade of the original BIOLAK process was necessary, with the core objectives of resolving the conflict between nitrification and denitrification, reducing nitrogen removal operational costs, and improving effluent stability.

1.2 Upgrade Challenges

As the BIOLAK process was unsuitable for in-tank modification to enhance performance, the plan was to strengthen treatment by constructing a new advanced nitrogen removal unit. The original BIOLAK process focused primarily on nitrification with denitrification as secondary, while the new process would focus on denitrification. Given the actual renovation needs, the project faced two major challenges: limited available land for the new process and high operational efficiency requirements.

Limited Available Land for New Process: The new construction had to be completed within the existing plant site, which had essentially no reserved land. Construction was only possible on a greenbelt adjacent to the BIOLAK tanks, with an available area of 400 m². This meant the new project's footprint per unit water treated had to be ≤ 0.01 m²/(m³·d).

High Operational Efficiency Requirements: This was not a simple upgrade but a further optimization of the biochemical functional zone. The new unit was expected to handle a nitrogen removal load of 20 mg/L. This process not only had to be completed on limited land but also needed to reduce the carbon source dosage compared to the original BIOLAK denitrification while ensuring stable denitrification performance. Thus, high demands were placed on both nitrogen removal efficiency and carbon source utilization efficiency.

2. Process Comparison and Selection

After treatment by the BIOLAK process, the effluent TN consists mainly of nitrate nitrogen. Currently, mature advanced nitrogen removal processes primarily utilize biofilm methods, characterized by microorganisms efficiently enriching on carrier surfaces in an attached state, offering significantly higher functional bacteria enrichment efficiency than conventional activated sludge processes. Biofilm processes can be further divided into fixed-bed and moving-bed types based on carrier fluidization, as shown in Figure 2. Denitrifying filters, typical fixed-bed biofilm processes, use fixed granular filter media as microbial growth carriers. By adding an external carbon source, they leverage the denitrification of the biofilm and the filtration of the media to achieve simultaneous removal of NO₃^--N, SS, and other pollutants. Advantages include stable treated water quality, no need for secondary clarifiers, and a compact layout, making them widely used in WWTP upgrades as an advanced treatment unit to strengthen TN removal from secondary effluent. However, operational focus must be on the impact of C/N on advanced denitrification efficiency. The Pingtang WWTP Phase I upgrade project, also with a capacity of 40,000 m³/d, used a denitrifying filter + high-efficiency dissolved air flotation (DAF) as the advanced treatment process to raise effluent TN to quasi-Class IV surface water standards, achieving a footprint of about 0.045 m²/(m³·d), saving land and enabling efficient treatment, but with a C/N as high as 18.34. To meet new local standards for effluent TN, the Chengdu No. 9 Water Reclamation Plant adopted a high-density sedimentation tank and denitrifying deep-bed filter as the upgrade process, with a C/N of 5.7, achieving advanced treatment under high standards. The Dingqiao WWTP in Haining could not meet the Grade A discharge standards required for the Qiantang River Basin. Gao Feiya et al. used a denitrifying deep-bed filter for advanced TN treatment, simultaneously removing SS and TP, bringing effluent quality close to quasi-Class IV standards, but with a high C/N of 15.68, leading to high nitrogen removal costs. Additionally, filter processes require regular backwashing, typically using air-water scouring, which can impact operational stability.

instability in denitrifying filters, research on applying sulfur-based autotrophic denitrification (SAD) to denitrifying filters has gained attention. SAD utilizes elemental sulfur or sulfur compounds as electron donors under anaerobic or anoxic conditions to reduce NO₃^--N to N₂. It offers advantages such as good denitrification efficiency, no need for an organic carbon source, low operational cost, and low sludge production. Song Qingyuan et al. studied the nitrogen removal effect of a SAD filter on secondary effluent. After optimizing pilot conditions, nitrate removal remained stable above 95%, but the media consumption rate reached 20% annually, accompanied by increased effluent sulfate concentration and decreased pH. To avoid secondary pollution risks from SAD, Li Tianxin et al. prepared media by pelletizing a mixture of sulfur and limestone powder. Adding a certain proportion of limestone to the filter bed neutralized the generated acidity and produced CaSO₄ precipitate, lowering effluent sulfate concentration and effectively addressing the issues of acid production and high sulfate levels. However, the limestone occupied space meant for electron donor media within the system, weakening advanced denitrification capacity, increasing effluent hardness, and raising operational costs. Current research on SAD technology is primarily at lab and pilot scale, with insufficient engineering experience for reference. Further applied research is needed before industrial-scale promotion.

MBBR is a typical representative of fluidized-bed biofilm processes and a new wastewater treatment technology that has received significant attention in recent years. It uses suspended carriers with a density close to water to specifically enrich microorganisms, forming a biofilm to achieve advanced nitrogen removal. Fluidized-bed biofilm processes also avoid issues of media clogging and backwashing. Currently, pure biofilm MBBR for advanced WWTP denitrification has over 20 years of successful operational experience abroad and is seeing increasingly wider application in China. Zheng Zhijia et al. used a two-stage pure biofilm MBBR process for advanced denitrification. At C/N=4.0, the system's effluent nitrate nitrogen stabilized at (1.87 ± 1.07) mg/L, with an average TN removal rate of 93.3%. A development zone WWTP in a certain city constructed a new MBBR bio-tank as tertiary advanced treatment for enhanced denitrification. The TN removal load in the anoxic section of the pure biofilm MBBR was 1.1 g/(m²·d), improving system denitrification reliability. Gao Yanbo et al., aiming to increase the original plant's capacity, constructed a new two-stage AO pure biofilm MBBR bio-tank, achieving stable effluent TN below 5 mg/L with high denitrification efficiency. Thus, the pure biofilm MBBR process shows great potential for advanced nitrogen removal in WWTPs, combining advantages like high carbon source utilization efficiency, high treatment load, and small footprint. However, it also places higher demands on equipment, requiring reliable equipment to support stable process operation. A comparison of common advanced nitrogen removal processes is shown in Table 1.

Based on a comprehensive comparison, although the SAD process requires no carbon source addition, its current application is not yet mature and carries secondary pollution risks, so it was not considered for this upgrade. Although denitrifying filters are widely used, they are mostly employed in WWTP upgrades where the design influent/effluent TN is often 15/12 mg/L, handling a relatively small TN removal load. Since this project required meeting long-term, high TN removal demands, operation would significantly shorten the filter's backwashing cycle, increasing operational difficulty and instability. The pure biofilm MBBR process combines advantages like high carbon utilization efficiency, no need for backwashing, mature application, and no secondary pollution. Considering the process challenges and renovation requirements, the project ultimately selected the construction of a new pure biofilm MBBR bio-tank (hereinafter referred to as the MBBR tank) as the advanced nitrogen removal solution for the first phase, designed with a C/N=4.5, and a planned investment payback period of 7.37 years.

3. New Construction Plan

3.1 Process Flow

The wastewater treatment process flow after renovation is shown in Figure 3. The plant's influent passes through fine screens, vortex grit chambers, and primary sedimentation tanks before entering the BIOLAK bio-tank for the removal of organic matter, ammonia nitrogen, etc. It is then lifted by pumps into the MBBR tank for advanced TN removal. The MBBR tank is designed for an influent TN of 35 mg/L and an effluent TN ≤ 15 mg/L. The MBBR effluent is lifted by secondary pumps to the plant's existing advanced treatment for solid-liquid separation and sludge wasting. The final effluent is disinfected before discharge to the receiving river. Surplus sludge is thickened, dewatered, and transported off-site for disposal.

3.2 New MBBR Tank

The MBBR tank employs an AO process, constructed using Lipp tanks for modular assembly, completed in 30 days. The total system hydraulic retention time (HRT) is 1.43 hours. SPR-III type specialized aerobic and anoxic suspended carriers are added inside the tanks, with a 60% fill ratio in the aerobic zone and 55% in the anoxic zone. The carriers are oblate cylindrical, 25 mm in diameter and 10 mm in height, with an effective specific surface area ≥ 800 m²/m³. The anoxic zone is equipped with 4 MBBR-dedicated variable-frequency mixers (SPR chemical power type), N=5.5 kW each, providing uniform and sufficient fluidization for the carriers. After biofilm maturation, 2 mixers are routinely operated, with the other 2 as hot standby. The aerobic zone uses screw blowers for aeration. A single blower has an air capacity of 14.50 m³/min, pressure 90 kPa, N=22 kW. One set of aerobic zone dedicated perforated pipe diffusers (SPR type) is installed. Due to the low required aeration volume, the existing Phase I blowers can usually be utilized, with the new blower and Phase I blowers serving as mutual backups. New material interception screens (SPR type), 12 mm thick, with a designed service life of 30 years, are installed in both the aerobic and anoxic zones.

3.3 New Supporting Facilities

• Influent System: Effluent from the BIOLAK bio-tank is lifted into the MBBR tank. 4 inlet pumps are installed (2 duty, 2 standby), each with Q=840 m³/h, H=65 kPa, N=30 kW.
• Carbon Source Dosing System: The effluent from the Phase I BIOLAK bio-tank contains only COD that is difficult to utilize. To ensure advanced denitrification in the anoxic zone of the MBBR tank, sodium acetate is used as the external carbon source. 4 metering pumps are installed (2 duty, 2 standby), each with Q=300 L/h, H=200 kPa, N=0.37 kW.