100,000 m³/d WWTP In-Situ Upgrading: MBBR + Denitrification Biofilter + Magnetic Coagulation Meet DB51/2311 Without New Land

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Application of MBBR + Denitrification Biofilter + Magnetic Coagulation Sedimentation for In-Situ Upgrading of a Wastewater Treatment Plant

Published: 2026-5-25  •  Category: Cases  •  Tags: MBBR · Denitrification Biofilter · Magnetic Coagulation · WWTP Upgrading

SECTION 1

1  Project Overview

1.1  Project Background

An industrial park centralized wastewater treatment plant in Chengdu with a capacity of 100,000 m³/d was put into operation in 2013. It adopts the process of aerated grit chamber + AAO + secondary sedimentation tank + D-type filter + UV disinfection, and the effluent quality meets the Class A standard of the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918–2002). The treatment flow ranges from 91,000 to 118,000 m³/d, with industrial wastewater accounting for about 48%. With the release of the Discharge Standard of Water Pollutants for Urban and River Basins in Sichuan Province (DB51/2311–2016) in 2017, the plant is required to comply with the discharge standard for industrial park centralized wastewater treatment plants. The existing effluent basically meets the Class A standard, but the effluent quality fluctuates greatly, with occasional exceedances of TN and TP. At the same time, the high sand content in the influent causes the aerated grit chamber to require frequent emptying for desilting. Therefore, it is necessary to upgrade this plant.

1.2  Influent and Effluent Quality

Based on the statistical data of 2017 and 2018, the comparison between the original design influent quality and the actual influent quality is shown in Table 1. It can be seen that the actual influent COD and BOD₅ are lower than the design values, while the influent SS and TP are far higher than the design values.

Table 1  Comparison Between Design and Actual Influent Quality

Unit: mg/L

The comparison between the actual effluent quality of 2017 and 2018, the original design effluent standard, and the new effluent standard is shown in Table 2. Comparing the new and old standards, the requirements for COD and NH₄-N are tightened, while other indicators remain unchanged. It can be seen from Table 2 that under the old effluent standard, TP and TN cannot stably meet the standard at 100% guarantee rate; under the new effluent standard, NH₄-N cannot stably meet the standard even at 95% guarantee rate. Therefore, achieving stable compliance of TP, TN and NH₄-N with the new discharge standard is the key focus of this upgrading.

Table 2  Comparison of Effluent Quality

Unit: mg/L

1.3  Existing Problems

SECTION 2

2  Upgrading Design Scheme

2.1  Adjustment of Design Water Quality

Based on the measured influent water quality from 2017 and 2018, the design influent quality is projected. The design BOD₅ value is lowered, while the design SS and TP values are increased, with a certain margin maintained relative to the measured values. The design influent and effluent quality for the upgrading are shown in Table 3.

Table 3  Design Influent, Effluent Quality and Removal Rates

Unit: mg/L

2.2  Upgrading Scheme

The main constraints for the upgrading of this plant are: (1) the upgrade must be completed in-situ without adding new land; (2) the influent has high sand content, the aerated grit chamber requires frequent desilting, and a new primary sedimentation tank would also face the problem of sand accumulation; (3) the operating cost after reconstruction must be kept at a relatively low level. Considering these constraints and the upgrading requirements, the process route for each treatment stage is determined.

2.2.1  Pretreatment Enhancement

The effectiveness of pretreatment directly affects the long-term normal operation of the biological tank. The plant has high sand content in the influent, and the aerated grit chamber is overloaded, inevitably leading to some sand entering the biological tank. A rapid solid-liquid separation technology is designed to enhance pretreatment. Its principle is to let the raw water flow through a continuously rotating filter cloth to achieve solid-liquid separation, and use reclaimed water to continuously scour and remove the solid impurities retained on the filter cloth. One advantage of this technology is its high filtration accuracy: the filter cloth pore size is 0.2 mm, which is 1/15 of the conventional fine screen pore size, and can intercept more than 90% of scum, fine sand, hair and other impurities, significantly improving the pretreatment effect. Another characteristic is its high treatment load and small footprint, with a hydraulic retention time of only about 5 seconds.

In the upgrading, three sets of rapid solid-liquid separation devices are installed after the coarse screen, providing a pretreatment capacity of 30,000 m³/d, covering an area of 184 m², operating in parallel with the original fine screen + aerated grit chamber, which is operated at reduced capacity. The industrial enterprises served by the plant are mainly in machinery manufacturing, electronic products and display manufacturing. The characteristic pollutants include heavy metals, fluoride, and refractory COD. The enterprise discharge meets the Wastewater Quality Standard for Discharge into Municipal Sewers (GB/T 31962–2015). To cope with sudden toxic pollutant shocks, a new emergency storage tank is designed with an effective volume of 33,000 m³, fully buried, with landscaping and roads above ground.

2.2.2  Biological Section Modification

The removal of TP, NH₃-N and TN all need to be enhanced in the biological treatment section. Due to land constraints and the inability to increase the total tank volume, three in-situ modifications are made to the original AAO biological tank:

Figure 1  Overall Biological Tank Modification (AAO + MBBR Embedded)

2.2.3  Advanced Treatment Section Modification

The role of the advanced treatment section is to ensure final compliance of TN, TP and SS. The combination of denitrification filter and high-efficiency sedimentation tank is a relatively mature technical route with many application cases, and this project intends to adopt it.

(1) Denitrification Filter Selection

Currently, the most widely used denitrification filters in China are the denitrification deep bed filter and the denitrification biofilter. The Water Supply and Drainage Design Manual Volume 5 (Urban Drainage) and the General Technical Specification for Denitrification Biofilter (GB/T 37528–2019) specify the design parameters of these two filters, as shown in Table 4. It can be seen that the upflow denitrification biofilter using ceramsite filter media has a higher filtration rate and denitrification load compared to the downflow denitrification deep bed filter using quartz sand. Studies have shown that ceramsite filter media, relative to quartz sand, have the advantages of high biofilm carrier capacity, high carbon source utilization, short biofilm formation time, and uniform microbial colony distribution, while the advantage of quartz sand lies in its stronger capacity for TN and SS removal. The upflow filter has a longer backwash cycle and smaller filtration head loss compared to the downflow filter.

Table 4  Main Design Parameters of Denitrification Filter in Standards and Codes

This project has limited land, and the upgraded effluent standards do not impose stricter requirements on TN and SS removal. Considering comprehensively, the upflow denitrification biofilter with ceramsite filter media is chosen as the denitrification unit for advanced treatment. At the same time, a 1 mm pore size internal flow orifice plate fine screen is installed before the filter to reduce particulate impurities entering the filter.

(2) High-Efficiency Sedimentation Tank - Magnetic Coagulation

A magnetic coagulation sedimentation tank is designed. The magnetic coagulation sedimentation tank adds magnetic media + polyaluminum chloride (PAC) + polyacrylamide (PAM) to the filtrate, forming firm flocs through mixing and flocculation, thereby achieving a high flow velocity with a surface hydraulic load of up to 20 m³/(m²·h), far higher than the 6–13 m³/(m²·h) of ordinary high-efficiency sedimentation tanks. In addition to good removal of TP and SS, the magnetic coagulation sedimentation tank also has a certain removal effect on refractory COD, heavy metals, and color. This project has limited land and the influent contains a certain proportion of industrial wastewater, making the magnetic coagulation sedimentation tank more suitable.

(3) Comprehensive Consideration. The denitrification biofilter is placed before the magnetic coagulation sedimentation tank mainly to make full use of the residual BOD₅ in the secondary sedimentation tank effluent as a carbon source for denitrification. A bypass pipe is designed before the filter, allowing the wastewater to flow directly into the magnetic coagulation sedimentation tank without passing through the filter, saving the energy for secondary lifting. The overall process flow after upgrading is shown in Figure 2.

Figure 2  Overall Process Flow After Upgrading (Rapid Solid-Liquid Separation + AAO/MBBR + Denitrification Biofilter + Magnetic Coagulation Sedimentation)

2.3  Main Structures and Design Parameters

SECTION 3

3  Operation Performance Analysis

3.1  Performance of Rapid Solid-Liquid Separation Device

The power consumption of the rapid solid-liquid separation device during operation is 9.12 × 10⁴ kW·h/m³. The separated impurities include domestic waste, fine sand, solid grease, hair and fibers. The average moisture content of the discharged residue is 72.31%, and the dry residue production is 104.6 kg/m³. The dry residue production of the conventional pretreatment facilities is 37.3 kg/m³, and the dry residue production of the rapid solid-liquid separation device is 2.8 times that of conventional pretreatment. The influent and effluent of the device are dried and weighed, and the results show that the interception efficiency of the 0.2 mm pore size filter cloth for sand and hair fibers is 92.99%–94.95%. So far, the device has been in stable operation for three years.

3.2  Performance of Biological Tank

The annual average data of influent and effluent water quality before (2018) and after (2023) the modification are summarized, as shown in Table 5. It can be seen that the effluent quality of the biological tank has been comprehensively improved after the modification. COD, BOD₅, NH₃-N, TN and TP decreased by 14.94%, 12.95%, 62.28%, 25.41% and 39.16%, respectively, with the largest reductions in NH₃-N, TN and TP. At the same time, the stability of each effluent indicator has also significantly improved.

Table 5  Water Quality of Influent and Effluent of Biochemical Tank - Before and After Renovation

The organic matter content of the activated sludge in the biological tank increased after the modification. According to the test data, the average MLVSS/MLSS ratio was 0.58 in 2018 and 0.63 in 2023, an increase of 8.6%. The increase in organic matter content indicates better sludge activity, mainly for two reasons: first, pretreatment has been strengthened, and more inorganic matter such as sand is intercepted by the rapid solid-liquid separation device, reducing the proportion of inorganic components in the activated sludge; second, the addition of the MBBR system in the biological tank increases the total microbial biomass, increasing the organic components in the activated sludge.

In August, the sludge concentrations in the pre-denitrification zone, anaerobic zone, anoxic zone and MBBR zone of the biological tank were measured, with an external sludge recirculation ratio of 100% and an internal recirculation ratio of 200%. Three sets of parallel samples were taken, and the average values are shown in Table 6. It can be seen that the sludge concentrations in the four treatment units of the biological tank differ, with the highest in the MBBR zone, followed by the anoxic zone. The analysis suggests that the biofilm attached to the MBBR carriers continuously detaches and mixes into the activated sludge due to friction, collision and hydraulic scouring, increasing the sludge concentration in this zone by about 3.54% relative to the anoxic zone.

Table 6  Sludge Concentration of Each Unit in the Biochemical Tank

Unit: mg/L

3.3  Performance of Advanced Treatment

Since the TN in the biological tank effluent already meets the standard stably, to save operating costs, no additional carbon source is actually added to the denitrification biofilter. According to the 2023 water quality data, the TN in the biological tank effluent is (7.04 ± 1.01) mg/L. Without considering TN removal in the secondary sedimentation tank, the TN in the denitrification biofilter effluent is (6.43 ± 0.92) mg/L, with a removal rate of 8.7%. The TP in the biological tank effluent is (0.16 ± 0.09) mg/L. Without considering TP removal in the secondary sedimentation tank, the TP in the magnetic coagulation sedimentation tank effluent is (0.08 ± 0.04) mg/L, with a removal rate of 50%.

SECTION 4

4  Reconstruction and Operating Costs

SECTION 5

5  Conclusion