Process Transformation and Operation Performance of EMBR-RO-ECOP Coupled System

Abstract

Aiming at the characteristics of printing and dyeing wastewater such as complex water quality, high organic content and poor biodegradability, a coupled process of “hydrolysis acidification – aerobic biochemical treatment – enhanced membrane bio-reactor (EMBR) – reverse osmosis (RO) – pulse electrocatalytic oxidation (ECOP)” was adopted to transform the wastewater treatment system of a printing and dyeing plant.

Practical operation results show that the combined process can effectively reduce COD and chroma of printing and dyeing wastewater, but has insignificant removal effect on total nitrogen. When the influent COD of the EMBR system is below 120 mg/L and the RO system operates at a concentration ratio of 3.3 times, the RO concentrate enters the ECOP system for advanced treatment. The effluent pollutant concentration is lower than the limit value in Table 2 of Discharge Standard of Water Pollutants for Textile Dyeing and Finishing Industry (GB 4287-2012). The overall water reuse rate of the process increases from 70% to 86%, saving 58,000 tons of water annually and reducing annual expenditure by more than 360,000 CNY.

1. Introduction

With the acceleration of industrialization, the printing and dyeing textile industry has entered a stage of rapid development, discharging a large volume of organic wastewater. Printing and dyeing wastewater is characterized by large discharge volume, high organic pollutant content, toxic heavy metals and carcinogenic substances, deep chroma, large pH fluctuation and dramatic water quality changes.

At present, the secondary treatment processes of printing and dyeing wastewater are mainly physicochemical and biological methods. Although biological treatment has good organic matter removal efficiency, the B/C ratio of effluent decreases after biological treatment, resulting in poor biodegradability. Meanwhile, the growing demand for reclaimed water in printing and dyeing plants requires reverse osmosis (RO) systems to concentrate and reuse biochemically treated wastewater.

RO concentrate features high concentration ratio, high salinity, high concentration of refractory organic matter, high chroma and low B/C ratio, which cannot be effectively treated by conventional biochemical and physicochemical methods. Catalytic ozonation can degrade refractory pollutants efficiently, but has the disadvantages of high investment and high operating cost.

Enhanced Membrane Bio-Reactor (EMBR) is a new wastewater treatment technology combining membrane bioreactor and biological technology. By adding biological fillers into the membrane tank to attach microorganisms, it further improves the removal of organic matter such as COD, and has the advantages of high biochemical efficiency, strong impact load resistance, stable effluent quality, small footprint and easy automatic control.

Electrocatalytic oxidation with pulse (ECOP) uses metal oxide electrodes with catalytic performance to generate hydroxyl radicals with strong oxidation capacity to oxidize pollutants in water, completely decomposing them into CO₂ and H₂O. It has the advantages of simple operation and convenient maintenance, and its treatment cost is only half of powdered activated carbon adsorption process.

A printing and dyeing plant in Jiaxing has a wastewater treatment capacity of 1000 m³/d, originally adopting the process of hydrolysis acidification – aerobic – secondary sedimentation – final sedimentation – MBR – RO. In the early stage, due to low reclaimed water demand, the RO system recovery rate was only 40%–50%, and the COD of RO concentrate was lower than the discharge limit and could be discharged directly. With the increase of high-quality reclaimed water demand, the RO recovery rate needs to be increased to more than 70%, resulting in RO concentrate COD of about 330 mg/L, far exceeding the discharge standard.

The original plan of powdered activated carbon adsorption had the problems of high consumption cost, low automation, complicated operation and additional solid waste disposal cost. Based on the on-site operation and water quality conditions, the EMBR-RO-ECOP process was adopted for advanced treatment, which can effectively improve the water reuse rate and ensure stable discharge of RO concentrate.

2. Project Overview

2.1 Wastewater Source and Quality

The wastewater of the plant mainly comes from loose wool dyeing and garment washing sections. The main components of the wastewater are reactive dyes, acid dyes and disperse dyes. Pollutants include natural organic substances (wax, colloid, hemicellulose, grease, etc. from natural fibers) and synthetic organic substances (dyes, auxiliaries, sizing agents, etc.). The raw water quality and discharge limits are shown in Table 1.

Table 1 Raw Water Quality and Discharge Limits of Printing and Dyeing Wastewater

Note: The discharge standard follows the indirect discharge limit in Table 2 of GB 4287-2012.

2.2 Original Process and Existing Problems

The original treatment process was hydrolysis acidification → aerobic tank → secondary sedimentation tank → final sedimentation tank → MBR → RO. With a RO recovery rate of 40%–50%, the COD of RO concentrate was below 200 mg/L and could be discharged directly. The overall water reuse rate was about 70%.

The main problems after increasing reuse demand were as follows:

1. When the RO recovery rate was increased to above 70%, the COD concentration of RO concentrate rose to about 330 mg/L, exceeding the discharge standard and requiring advanced treatment.
2. The proposed powdered activated carbon adsorption process had high operating cost, large manual demand and secondary solid waste pollution, which was not suitable for long-term stable operation.
3. The original MBR system had limited COD removal capacity, and the high organic load of RO influent would further shorten the service life of RO membranes and increase the frequency of chemical cleaning.
4. Process Design and Main Facilities

3.1 Transformed Process Flow

After transformation, the overall process is as follows: raw wastewater → regulating tank → primary sedimentation tank → hydrolysis acidification tank → aerobic tank → secondary sedimentation tank → final sedimentation tank → EMBR system. The EMBR effluent is divided into two parts: 500 m³/d is directly reused as low-quality reclaimed water; the rest enters the RO system for advanced treatment, with a RO recovery rate of 72%, and the RO permeate is reused as high-quality reclaimed water. The RO concentrate enters the ECOP system for advanced treatment, and the effluent meets the discharge standard before being discharged.

The overall water reuse rate of the system reaches 86%, saving 3840 tons of tap water per day. The EMBR system further reduces the COD of RO influent, and the ECOP process efficiently removes refractory organic matter from RO concentrate, with the advantages of simple operation and low maintenance cost.

3.2 Main Structures and Equipment Parameters

(1) Primary Sedimentation Tank

Reused existing structure, semi-underground reinforced concrete structure, size 4 m × 5.5 m × 5.5 m, effective water depth 5 m, effective volume 605 m³, hydraulic retention time 2.6 h.

(2) Hydrolysis Acidification Tank

Reused existing structure, semi-underground reinforced concrete structure, size 44 m × 5.5 m × 5.5 m, effective water depth 5 m, effective volume 1210 m³, hydraulic retention time 28.8 h, dissolved oxygen concentration 0.5 mg/L.

(3) Aerobic Tank

Reused existing structure, semi-underground reinforced concrete structure, size 44 m × 5 m × 5.5 m, effective water depth 5 m, effective volume 1100 m³, hydraulic retention time 26.2 h, dissolved oxygen concentration 2–3 mg/L, sludge concentration 2500–3500 mg/L. Equipped with 2 Roots blowers (1 in operation, 1 standby).

(4) Secondary and Final Sedimentation Tanks

Reused existing structures, semi-underground reinforced concrete structure, size 36 m × 2.5 m × 5.5 m, effective water depth 5 m, effective volume 450 m³, surface load 0.5 m³/(m²·h). Each equipped with 1 sludge scraper, and the secondary sedimentation tank is equipped with 2 sludge return pumps (1 in operation, 1 standby).

(5) EMBR Membrane Tanks

2 tanks, reused existing structures, semi-underground reinforced concrete structure. Single tank size 2.8 m × 2.5 m × 5.5 m, effective water depth 4.5 m, effective volume 31.5 m³ per tank.

• Membrane modules: 96 pieces of curtain-type hollow fiber membrane modules, 48 pieces per membrane rack, single membrane area 30 m², total water production capacity 42 m³/h. Equipped with 3 self-priming pumps (2 in operation, 1 standby), 2 backwash pumps (1 in operation, 1 standby), 1 backwash security filter and 2 Roots blowers (1 in operation, 1 standby).
• Biological fillers: hydrophilic polyurethane gel fillers, particle size (20±1) mm, specific surface area 4000 m²/m³, relative density 1.02, dosing ratio 18%.

(6) RO System

Reused existing equipment, adopting 54 pieces of DOW 8060 anti-fouling brackish water membrane elements, 9 pieces of 6-core membrane housings, two-stage design, system design recovery rate 72%. Equipped with 1 security filter, 1 booster pump, 1 high-pressure pump and 1 set of membrane cleaning device.

(7) ECOP Unit

Newly constructed, adopting integrated ECOP device. Electrolytic cell is made of PP + A3 steel, size 3 m × 1.5 m × 1.8 m, effective volume 4.5 m³, hydraulic retention time 42 min.

• Electrode plates: titanium-based modified lead oxide electrodes, mesh structure.
• Operating parameters: working current 500 A, working voltage 50 V, working current density 10 mA/cm², electrolysis time 42 min.
• Supporting facilities: 1 set of 70V/600A rectifier, 1 produced water delivery pump.

4. Operation Performance and Pollutant Removal Efficiency

4.1 COD Removal Performance of EMBR System

Biological fillers were added to the membrane tank and microorganisms were inoculated on October 22. The COD change trend before and after filler addition was monitored.

Within 10 days after microbial inoculation, the average influent COD of the system was 120 mg/L, and the average effluent COD was 110 mg/L, with almost no change in COD removal rate, indicating that the microorganisms in the membrane tank had not grown to the required quantity.

10 days after inoculation, the average influent COD was 110 mg/L, and the average effluent COD was 96 mg/L. The COD removal rate increased significantly: the average COD removal amount increased from 9.07 mg/L to 13.55 mg/L, and the average COD removal rate increased from about 7.5% to 12.4%. At this stage, microorganisms in the EMBR system began to multiply in large numbers, and COD in wastewater was degraded by aerobic microorganisms.

4.2 Total Nitrogen Removal Performance of EMBR System

Monitoring of influent and effluent TN of the EMBR system from August 25 to December 22 showed that the TN values of influent and effluent were basically the same before and after adding biological fillers, indicating that the EMBR system had almost no removal effect on TN.

The main reason is that TN removal mainly occurs in the denitrification stage of the biochemical system, and the TN of EMBR effluent is mainly affected by the previous biochemical section. The EMBR system is mainly used for further removal of organic matter and solid-liquid separation, and lacks an anoxic denitrification environment.

4.3 COD Removal Performance of ECOP System

The adjustable parameters of the ECOP system include current, voltage and influent flow. During commissioning, the current was set at 550 A and the voltage at 52 V. The RO system recovery rate was gradually increased from 40% to 72% by August 27.

Before pH adjustment, the COD removal amount of the ECOP system was only about 90 mg/L, while the COD of RO concentrate remained at 400–450 mg/L, greatly exceeding the design treatment capacity. This was mainly because the water temperature of the biochemical system exceeded 40 ℃, which inhibited microbial activity and led to excessive COD in the effluent of the biochemical system.

Two improvements were carried out: first, strengthening the daily monitoring of the biochemical system; second, optimizing the operating conditions of the electrocatalytic oxidation system through laboratory experiments. Laboratory experiments found that the pH of RO concentrate was 8–8.5 (weakly alkaline), and pH was an important influencing factor for COD removal. When the pH of RO concentrate was adjusted to about 6.7, the COD removal effect of the electrocatalytic oxidation system was significantly improved.

After adjusting the pH to 6.7 with hydrochloric acid on site, when the influent COD of ECOP was less than 330 mg/L, the effluent COD was less than 200 mg/L. The system operated stably in the following month, and the effluent COD index met the discharge requirements.

4.4 TN and Chroma Removal Performance of ECOP System

Monitoring from October 22 to December 22 showed that the ECOP system had no obvious removal effect on TN in the wastewater. The main reason is that TN in RO concentrate mainly exists in the form of nitrate nitrogen. The electrocatalytic oxidation system can oxidize ammonia nitrogen in wastewater, but has limited removal effect on other forms of nitrogen. To ensure the effluent TN meets the standard, it is necessary to focus on controlling the operating conditions of the front-end biochemical system.

The chroma of ECOP influent, 30 min electrolysis effluent, 60 min electrolysis effluent and 90 min electrolysis effluent was 173 times, 56 times, 46 times and 32 times respectively. The ECOP system has a significant removal effect on chroma: most of the color in wastewater is basically removed after 30 min of electrolysis, and the effluent chroma is less than 80 times, which is lower than the indirect discharge limit of GB 4287-2012 Table 2.

5. Economic Benefit Analysis

The operating cost of the system mainly includes electricity consumption and chemical agent cost.

• Electricity cost: The average electricity consumption per ton of water is (1.76±0.1) kW·h. Calculated at 0.7 CNY/(kW·h), the electricity cost per ton of water is 1.23 CNY.
• Chemical cost: Including daily maintenance agents and chemical cleaning agents, the chemical cost per ton of water is 0.46 CNY.

The total operating cost is 1.69 CNY per ton of water, which is only about 50% of the powdered activated carbon adsorption scheme, with obvious economic advantages.

After the transformation, the overall water reuse rate increases from 70% to 86%, with an annual water saving of 58,000 tons and an annual cost reduction of more than 360,000 CNY.

6. Engineering Application Insights

6.1 Applicable Scenarios

The EMBR-RO-ECOP coupled process is particularly suitable for the following scenarios:

• Upgrading and transformation of printing and dyeing textile wastewater treatment systems to improve reclaimed water reuse rate and reduce fresh water consumption
• Advanced treatment of RO concentrate from textile, chemical and other industries to achieve stable compliance of refractory organic wastewater
• Deepening and upgrading of existing MBR wastewater treatment systems to improve effluent quality and reduce organic load of subsequent membrane systems
• Wastewater reuse projects with limited site area, making full use of the high volumetric efficiency of EMBR and electrocatalytic oxidation processes

6.2 Key Design Considerations

• EMBR filler configuration: The filler dosing ratio should be controlled at 15%–20% to ensure sufficient biofilm attachment area while avoiding excessive filler accumulation affecting membrane scouring effect and water flow state.
• ECOP pH optimization: For weakly alkaline RO concentrate, it is recommended to adjust the pH to 6.5–7.0 before entering the electrocatalytic oxidation unit, which can significantly improve the generation efficiency of hydroxyl radicals and reduce the operating power consumption per unit COD removal.
• Front-end biochemical temperature control: The operating temperature of the aerobic biochemical system should be controlled below 38 ℃ to avoid excessive temperature inhibiting microbial activity, which leads to the increase of organic load in subsequent EMBR and RO systems.
• RO recovery rate matching: The RO recovery rate should be reasonably set according to the raw water quality and effluent requirements. Excessively high recovery rate will lead to a sharp increase in the concentration of pollutants and salinity in the concentrate, increasing the treatment difficulty and cost of the subsequent advanced treatment unit.

6.3 Operation and Maintenance Best Practices

• Establish a regular maintenance system for EMBR membranes, implement standard backwash and chemical cleaning procedures, control membrane fouling rate, and extend membrane service life.
• Regularly check the electrode plate status of the ECOP system, clean the electrode surface scale in time, and replace severely worn electrodes on schedule to maintain stable oxidation efficiency.
• Set up pH linkage control at the inlet of ECOP to automatically adjust the acid dosing amount according to the inlet pH value, ensuring the stable operation of the electrocatalytic oxidation reaction in the optimal pH range.
• Strengthen the operation management of the front-end biochemical system, control the effluent COD and water temperature stability, and reduce the load fluctuation of the subsequent advanced treatment system.

7. SYNERAQUA Technical Perspective

At SYNERAQUA, we recognize that the EMBR-RO-ECOP coupled process is a highly cost-effective technical solution for printing and dyeing wastewater reclamation and RO concentrate treatment. It solves the common pain points of high treatment cost and large solid waste generation in traditional RO concentrate treatment schemes, and achieves the dual goals of improving water reuse rate and ensuring stable effluent compliance.

Our modular printing and dyeing wastewater reuse solution integrates enhanced biochemical treatment, membrane separation and advanced oxidation units, with factory prefabrication and standardized configuration, which can greatly shorten the on-site transformation period. Equipped with our intelligent operation platform, the system realizes real-time dynamic adjustment of ECOP current density and pH according to RO concentrate load, further reducing comprehensive energy consumption by 10%–15% compared with conventional fixed-parameter operation. We also provide full-cycle technical services including process scheme design, equipment supply, commissioning and operation guidance, helping printing and dyeing enterprises achieve stable wastewater reuse while optimizing total operating cost and reducing solid waste output.

8. Conclusion
9. The EMBR system can enhance the COD removal effect. After transformation, the average COD removal amount increases from 9.07 mg/L to 13.55 mg/L, and the average COD removal rate increases from 7.5% to 12.4%.
10. The ECOP system can effectively reduce COD in RO concentrate. After adjusting pH with acid, the COD removal amount of RO concentrate increases from about 90 mg/L to about 130 mg/L, and the effluent COD is stable below 200 mg/L, meeting the discharge standard.
11. The ECOP system has no obvious removal effect on TN in wastewater mainly in the form of nitrate nitrogen. The operating parameters of the front-end A/O system should be adjusted to ensure the stability of effluent TN.
12. When the influent COD of the EMBR system is less than 120 mg/L, the RO system operates at a recovery rate of 70%, and the effluent COD of the ECOP system is stable below 200 mg/L. The overall EMBR-RO-ECOP system operates stably, and the effluent quality meets the requirements of Discharge Standard of Water Pollutants for Textile Dyeing and Finishing Industry(GB 4287-2012).
13. The operating cost of the 1000 m³/d printing and dyeing wastewater advanced treatment process is 1.69 CNY per ton of water. The annual water saving is 58,000 tons, and the annual cost reduction is more than 360,000 CNY, with good economic and environmental benefits.

FAQ

Q1: Why is electrocatalytic oxidation chosen instead of powdered activated carbon adsorption for RO concentrate treatment?
Powdered activated carbon adsorption has three main disadvantages: first, activated carbon is a consumable, with continuous dosing cost and high long-term operation cost; second, the automation degree is low, requiring manual dosing and sludge treatment, with large labor input; third, the saturated activated carbon and filter sludge belong to hazardous waste, which requires additional disposal cost and has secondary environmental risks. Electrocatalytic oxidation degrades organic matter through electrochemical action without adding a large amount of chemical agents, producing less sludge, with low operation cost and high automation, which is more suitable for long-term stable operation.

Q2: Why does the EMBR system have almost no removal effect on total nitrogen?
Total nitrogen removal relies on the combined action of nitrification in aerobic environment and denitrification in anoxic environment. The EMBR system is an aerobic membrane tank, which mainly undertakes the functions of further organic matter degradation and solid-liquid separation, and lacks an anoxic denitrification environment. Therefore, it cannot remove nitrate nitrogen, and the total nitrogen of effluent is basically consistent with that of influent. The total nitrogen control of the whole system still needs to be realized by optimizing the operation of the front-end A/O biochemical system.

Q3: Why does pH adjustment significantly improve the COD removal efficiency of electrocatalytic oxidation?
The core of electrocatalytic oxidation for organic matter removal is the generation of hydroxyl radicals (·OH) with strong oxidation capacity. Under weakly acidic conditions, the side reactions of oxygen evolution on the electrode surface are inhibited, and more current is used for the generation of oxidizing active species, which improves the current efficiency. At the same time, the oxidation potential of hydroxyl radicals is higher under acidic conditions, and the degradation rate of organic matter is faster. Therefore, adjusting the pH of weakly alkaline RO concentrate to near neutral or weakly acidic can significantly improve the treatment efficiency and reduce the unit treatment cost.

Q4: What are the key points to improve the reuse rate of printing and dyeing wastewater reuse systems?
The key points include three aspects: first, stabilizing the effluent quality of the front-end biochemical system, reducing the organic load and chroma of the membrane system influent, which is the basis for increasing RO recovery rate; second, optimizing the pretreatment process before RO, using EMBR and other processes to further reduce COD and suspended matter, extending the membrane cleaning cycle and service life; third, selecting a cost-effective RO concentrate advanced treatment process to ensure that the concentrate meets the discharge standard while the recovery rate increases, avoiding the increase of reuse rate leading to excessive total treatment cost.