Parameter Optimization, Degradation Mechanism and Techno-Economic Evaluation

Abstract

In this study, electrochemical oxidation process was applied to treat high-ammonia-nitrogen wastewater from a pharmaceutical factory, and the effects of initial pH, electrolyte concentration, current density and electrode gap on treatment performance were systematically investigated. Results show that weak alkaline conditions, increased electrolyte concentration, higher current density and reduced electrode gap all contribute to improved degradation performance. Considering both treatment efficiency and energy consumption, the optimal operating conditions were determined as follows: initial pH of 8.0, electrolyte (NaCl) mass concentration of 6.0 g/L, current density of 20 mA/cm², electrode gap of 1 cm, and electrolysis time of 3.0 h. Under these conditions, the removal efficiencies of ammonia nitrogen and chemical oxygen demand (COD) reached 84.76% and 45.93% respectively, with an operating energy consumption of 69.12 kW·h/m³ and corresponding electricity cost of 55.30 yuan/m³. This process can simultaneously reduce ammonia nitrogen and remove a large amount of organic matter from pharmaceutical wastewater, delivering favorable environmental and social benefits.

1. Introduction

China is one of the largest producers of antibiotics and exporters of active pharmaceutical ingredients (APIs) in the world. According to the annual report of drug administration statistics, by the end of 2020, there were 7,690 valid drug manufacturing licenses nationwide, including 4,460 API and preparation manufacturers. In the pharmaceutical production process, a large volume of wastewater is generated from chemical synthesis and fermentation processes. This type of wastewater is characterized by complex composition, high COD concentration and high ammonia nitrogen content. Discharge of high-ammonia-nitrogen wastewater into water bodies easily causes eutrophication, damages aquatic organisms, severely deteriorates aquatic ecosystems and threatens human health. Therefore, it is essential to adopt effective treatment measures for high-ammonia-nitrogen pharmaceutical wastewater.

Currently, mainstream treatment technologies for high-ammonia-nitrogen wastewater include biological process, air stripping, chemical precipitation, adsorption, membrane absorption and breakpoint chlorination. However, each technology has inherent limitations. For biological treatment, high-toxicity antibiotic pollutants in pharmaceutical wastewater inhibit microbial activity and increase treatment difficulty. Air stripping causes secondary air pollution. Chemical precipitation requires high chemical dosage and generates large amounts of sludge.

As an emerging and efficient wastewater treatment technology, electrochemical oxidation has the advantages of wide applicability, easy automation, environmental friendliness and operational safety. It has been successfully applied in drinking water disinfection, municipal wastewater treatment and various industrial wastewater treatment, attracting increasing attention in the environmental field.

The degradation mechanism of electrochemical oxidation includes direct oxidation and indirect oxidation. Direct oxidation degrades pollutants through electron transfer directly on the electrode surface, while indirect oxidation relies on strong oxidizing species such as hydroxyl radicals (·OH) and active chlorine (HClO) generated during electrolysis. For ammonia nitrogen removal, the dominant pathway is decomposition by HClO generated through indirect oxidation. Compared with traditional breakpoint chlorination, electrochemical oxidation can continuously generate HClO oxidant in chloride-containing electrolyte, and direct oxidation reactions also occur between anode and cathode, resulting in higher ammonia nitrogen removal efficiency.

In this study, electrochemical oxidation was adopted to treat high-ammonia-nitrogen pharmaceutical wastewater. The effects of four key operating parameters (initial pH, electrolyte concentration, current density and electrode gap) on treatment performance were systematically investigated to determine the optimal operating conditions. Energy consumption analysis was also carried out to provide reference for the research and industrial application of electrochemical oxidation technology in high-ammonia-nitrogen pharmaceutical wastewater treatment.

2. Experimental Section

2.1 Experimental Wastewater

Wastewater samples were collected from a pharmaceutical company in Hebei Province, China. The main water quality indices are shown in Table 1.

Table 1 Wastewater Quality Indices

2.2 Experimental Equipment and Reagents

Main instruments: PHS-3C pH meter (Leici, Hangzhou), ATX239 electronic analytical balance (Shimadzu, Japan), 84-1A magnetic stirrer, 5B-3C(V8) COD/ammonia nitrogen rapid analyzer, UV-1780 UV-Vis spectrophotometer (Shimadzu, Japan), KGY-100A/30V DC regulated power supply.

Reagents: Dilute hydrochloric acid (0.5 mol/L), sodium hydroxide solution (0.5 mol/L), sodium chloride (analytical grade), sodium hypochlorite (30% available chlorine), sodium nitrite (analytical grade), all purchased from commercial chemical reagent suppliers.

2.3 Experimental Setup and Procedure

The electrocatalytic oxidation experimental device consists of an organic glass electrolytic cell (effective volume of 500 mL), a DC regulated power supply and a magnetic stirrer. The electrode assembly includes Ti/RuO₂-IrO₂ dimensionally stable anodes and titanium plate cathodes, with electrode dimensions of 6 cm × 10 cm, placed vertically and parallel in the wastewater.

For each test, 500 mL wastewater sample was poured into the electrolytic reactor. The initial pH was adjusted with dilute HCl and NaOH solution, and NaCl was added as supporting electrolyte. The solution was continuously stirred during electrolysis. Samples of 1 mL were collected at electrolysis time of 0 (initial), 1.0, 1.5, 2.0, 2.5 and 3.0 h, diluted and then analyzed.

Four key operating parameters were investigated: initial pH (6.0, 8.0, 10.0), electrolyte mass concentration (4.0, 6.0, 8.0 g/L), current density (10, 20, 30 mA/cm²) and electrode gap (1, 2, 3 cm).

2.4 Analytical Methods

• pH was measured according to Water Quality – Determination of pH Value – Glass Electrode Method(GB/T 6920-1986).
• CODCr was measured according to Water Quality – Determination of the Chemical Oxygen Demand – Fast Digestion Spectrophotometric Method(HJ/T 399-2007).
• Ammonia nitrogen was measured according to Water Quality – Determination of Ammonia Nitrogen – Nessler’s Reagent Spectrophotometry(HJ 535-2009).
• Total nitrogen was measured according to Water Quality – Determination of Total Nitrogen – Alkaline Potassium Persulfate Digestion UV Spectrophotometric Method(HJ 636-2012).
• Total dissolved solids were measured according to Water Quality – Determination of Total Salt – Gravimetric Method(HJ/T 51-1999).

Specific energy consumption (Ec, kW·h/m³) was calculated by Equation (1):

Where: U = system voltage (V), I = current (A), t = electrolysis time (h), V = wastewater volume (m³).

3. Results and Discussion

3.1 Comparison of Different Treatment Processes

Before electrochemical oxidation tests, the treatment performance of three different processes was compared: nitrite method, breakpoint chlorination and electrochemical oxidation.

Results show that the nitrite method achieved 71.50% ammonia nitrogen removal after 3.0 h reaction at 150℃, but showed poor COD removal, as the reaction mainly occurs between nitrite and ammonium ions to generate nitrogen, with only a small fraction of organic matter removed through redox reaction with nitrite.

For breakpoint chlorination, COD removal rate reached 65.25% after 3.0 h reaction, but ammonia nitrogen removal was insignificant. This is because the high organic concentration in the wastewater consumes most of the HClO and ·OH generated from NaClO decomposition, which are preferentially used for organic pollutant oxidation, greatly reducing the ammonia nitrogen removal efficiency of breakpoint chlorination.

Electrochemical oxidation achieved simultaneous removal of both ammonia nitrogen and COD, with removal rates increasing gradually with reaction time. Compared with the other two chemical dosing methods, electrochemical oxidation allows flexible control of reaction rate and removal efficiency by adjusting current density and reaction time.

In terms of operating cost, the nitrite method costs 41.90 yuan/m³, breakpoint chlorination costs 119.76 yuan/m³, and electrochemical oxidation costs 55.30 yuan/m³. Comprehensive consideration of treatment effect and cost confirms that electrochemical oxidation is the most suitable process for this type of wastewater.

3.2 Effect of Initial pH

Initial pH is a critical parameter of electrochemical oxidation systems, affecting the formation of active species such as Cl₂, HClO and ClO⁻ during electrolysis. Tests were conducted at NaCl concentration of 6.0 g/L, current density of 20 mA/cm², electrode gap of 1 cm and electrolysis time of 3.0 h, with initial pH set at 6.0, 8.0 and 10.0.

Results show that both ammonia nitrogen and COD achieved the best removal efficiency at pH 8.0 (weak alkaline condition). When pH was 10.0 or 6.0, the removal rates decreased to varying degrees.

This phenomenon can be explained by the existing forms of active chlorine. Under acidic conditions, high H⁺ concentration inhibits the hydrolysis of Cl₂ to HClO, reducing the generation of oxidizing species. Under strongly alkaline conditions, excess OH⁻ reacts with HClO to form ClO₃⁻ with low oxidation activity, which is unfavorable for pollutant degradation. In addition, under weak alkaline conditions, ammonia nitrogen exists in equilibrium between non-ionic NH₃ and ionic NH₄⁺, with higher reaction activity.

Therefore, the optimal initial pH was determined as 8.0, which also matches the original pH of the wastewater, avoiding additional chemical consumption for pH adjustment.

3.3 Effect of Electrolyte Concentration

Electrolyte concentration directly determines the conductivity of the reaction system. Tests were conducted at initial pH of 8.0, current density of 20 mA/cm², electrode gap of 1 cm and electrolysis time of 3.0 h, with NaCl concentration set at 4.0, 6.0 and 8.0 g/L.

Results show that ammonia nitrogen and COD removal rates increase with rising electrolyte concentration. Higher Cl⁻ concentration promotes the chlorine evolution reaction on the anode, generating more ClO⁻ and other oxidizing species to degrade ammonia nitrogen and organic matter.

At NaCl concentration of 4.0 g/L, the amount of oxidizing species generated was insufficient, resulting in poor removal performance and low current efficiency. When NaCl concentration increased to 6.0 g/L, ammonia nitrogen removal rate increased significantly, while system voltage dropped from 3.20 V to 2.40 V, and energy consumption decreased from 92.16 kW·h/m³ to 69.12 kW·h/m³.

At NaCl concentration of 8.0 g/L, COD removal further improved, but excessive salinity will cause secondary pollution to subsequent treatment processes and accelerate equipment corrosion. Considering treatment efficiency, chemical cost and burden on subsequent processes, the optimal NaCl concentration was determined as 6.0 g/L.

3.4 Effect of Current Density

Current density determines the intensity of electron transfer during electrolysis, which is highly correlated with electrochemical oxidation efficiency. Tests were conducted at initial pH of 8.0, NaCl concentration of 6.0 g/L, electrode gap of 1 cm and electrolysis time of 3.0 h, with current density set at 10, 20 and 30 mA/cm².

Results show that COD and ammonia nitrogen removal rates increase with higher current density. At 10 mA/cm², the reaction rate was low, with only 63.08% ammonia nitrogen removal after 3.0 h. At 20 mA/cm², ammonia nitrogen removal reached 84.76%, and further improvement was observed at 30 mA/cm². Higher current density improves electron transfer efficiency and promotes HClO generation, thus enhancing pollutant degradation.

Notably, current density has a more significant impact on COD removal than on ammonia nitrogen removal. This is because ammonia nitrogen is mainly degraded by HClO, while organic matter can be oxidized by multiple active species. In chloride-containing systems, HClO is generated preferentially, so ammonia nitrogen removal is less sensitive to current density changes than COD removal.

However, higher current density also leads to higher energy consumption and faster temperature rise. After 3.0 h electrolysis at 10, 20 and 30 mA/cm², wastewater temperature rose from 20℃ to 38℃, 42℃ and 49℃ respectively. Excessively high current density accelerates Cl₂ generation, and elevated temperature reduces Cl₂ solubility in water, causing large amounts of unhydrolyzed Cl₂ to escape, reducing chlorine utilization rate and causing energy waste. Combining treatment efficiency and energy consumption, the optimal current density was selected as 20 mA/cm².

3.5 Effect of Electrode Gap

Electrode gap has significant influence on oxidation efficiency and energy consumption. Tests were conducted at initial pH of 8.0, current density of 20 mA/cm², NaCl concentration of 6.0 g/L and electrolysis time of 3.0 h, with electrode gap set at 1, 2 and 3 cm.

Results show that COD and ammonia nitrogen removal efficiencies decrease with larger electrode gap. The best ammonia nitrogen removal performance was achieved at 1 cm gap. Reduced electrode gap accelerates convection and diffusion mass transfer between electrodes, facilitating the enrichment of target pollutants on the anode surface and improving current efficiency.

Meanwhile, system voltage increases significantly with larger electrode gap, especially when the gap increases from 2 cm to 3 cm, leading to higher energy consumption and lower current efficiency. Therefore, the optimal electrode gap was determined as 1 cm.

4. Degradation Mechanism Analysis

For high-ammonia-nitrogen pharmaceutical wastewater treatment by electrochemical oxidation, COD reduction is achieved through both direct oxidation and indirect oxidation. Direct oxidation degrades organic pollutants directly on the anode surface, while indirect oxidation relies on strong oxidizing species such as ·OH and HClO generated during electrolysis.

For ammonia nitrogen removal, the dominant pathway is indirect oxidation mediated by active chlorine. The reaction mechanism is as follows:

1. Chloride ions are first adsorbed on the anode surface, lose one electron and form chlorine radicals (Cl·):
2. Chlorine radicals further react to form Cl₂:
   
   
3. Cl₂ hydrolyzes in water to generate HClO:
4. HClO reacts with ammonium ions stepwise to form chloramines, which are finally converted to nitrogen gas, completing ammonia nitrogen removal:
   
   
   
   

Different from breakpoint chlorination which requires continuous dosing of oxidant, electrochemical oxidation continuously generates active chlorine in situ, maintaining stable oxidation capacity and avoiding the safety risks of storage and transportation of large amounts of oxidizing chemicals.

5. Energy Consumption and Techno-Economic Analysis

Under the optimal operating conditions (initial pH 8.0, NaCl 6.0 g/L, current density 20 mA/cm², electrode gap 1 cm, electrolysis time 3.0 h), the system voltage remained stable at 2.40 V. The calculated operating energy consumption is 69.12 kW·h/m³. Based on industrial electricity price of 0.8 yuan/(kW·h), the electricity cost is 55.30 yuan/m³.

Cost comparison with other processes:

• Nitrite method: 41.90 yuan/m³ (including chemical and heating costs), but with poorer treatment effect and high requirements for chemical storage and transportation.
• Breakpoint chlorination: 119.76 yuan/m³ (mainly chemical cost), much higher than electrochemical oxidation.

In practical engineering application, operating energy consumption is the main factor restricting the large-scale promotion of electrochemical oxidation process. Therefore, it is recommended to combine electrochemical oxidation with biological treatment processes according to the required removal degree of ammonia nitrogen and COD. Electrochemical oxidation can be used as pretreatment to reduce toxicity and improve biodegradability, followed by biological treatment for deep purification, which can significantly reduce overall operating cost and optimize the economy of the whole process.

6. Engineering Application Insights

6.1 Applicable Scenarios

Electrochemical oxidation process is particularly suitable for the following scenarios:

• Pretreatment of high-ammonia-nitrogen and high-toxicity pharmaceutical wastewater, to remove ammonia nitrogen, reduce organic toxicity and improve biodegradability for subsequent biological treatment.
• Advanced treatment of pharmaceutical wastewater biochemical effluent, to ensure stable compliance of refractory COD and ammonia nitrogen.
• Small and medium-sized pharmaceutical production sites with large water quality fluctuation, where biological treatment is difficult to operate stably.
• Projects with strict requirements on chemical storage and management, to avoid safety risks of large-scale storage of oxidants and acid-base agents.

6.2 Key Design Considerations

• Pretreatment requirements: Suspended solids and grease in influent should be removed as much as possible before entering the electrochemical unit, to prevent electrode surface fouling and reduce current efficiency.
• Electrode selection: Dimensionally stable anodes (DSA) such as Ti/RuO₂-IrO₂ are recommended for chloride-containing systems, with high chlorine evolution catalytic activity and long service life.
• System corrosion protection: High chloride concentration will cause corrosion to equipment and pipelines. The reactor and pipelines should be made of corrosion-resistant materials such as titanium alloy, UPVC or PVDF.
• Tail gas treatment: A small amount of chlorine gas may escape during electrolysis. The reactor should be sealed and equipped with tail gas absorption device to ensure operational safety.
• Modular design: Adopt modular electrode assembly design, which is convenient for maintenance, replacement and capacity expansion according to actual treatment demand.

6.3 Operation and Maintenance Best Practices

• Adjust current density dynamically according to influent water quality and effluent requirements, avoid long-term operation at excessively high current density, to reduce energy consumption and extend electrode service life.
• Regularly clean the electrode surface to remove scale and pollutant deposits, maintain stable current efficiency.
• Monitor electrolyte concentration and pH in real time, supplement NaCl appropriately according to consumption, and maintain optimal reaction conditions.
• Establish regular electrode performance inspection mechanism, evaluate attenuation of current efficiency, and replace electrode modules in time to ensure stable treatment effect.

7. SYNERAQUA Technical Perspective

At SYNERAQUA, we recognize that electrochemical oxidation is a highly flexible and efficient advanced oxidation technology, especially suitable for the treatment of high-toxicity, high-salinity and refractory industrial wastewater such as pharmaceutical wastewater, where traditional biological processes are difficult to operate stably.

Our modular electrochemical oxidation systems adopt high-efficiency DSA electrode modules and intelligent power control system, which can be flexibly configured according to different wastewater quality and treatment requirements. Equipped with our self-developed intelligent operation platform, the system realizes real-time dynamic adjustment of current density, flow rate and electrolyte dosage based on influent load monitoring, improving current utilization rate by 15–20% compared with conventional fixed-parameter operation. We also provide full-cycle technical services including process scheme design, equipment supply, commissioning and operation guidance, helping industrial clients achieve stable compliance of wastewater treatment while optimizing total operating cost.

8. Conclusion
9. Electrochemical oxidation process has obvious applicability for high-ammonia-nitrogen pharmaceutical wastewater treatment, achieving simultaneous removal of ammonia nitrogen and COD. Compared with nitrite method and breakpoint chlorination, it has comprehensive advantages in treatment effect and operating cost.
10. Initial pH, electrolyte concentration, current density and electrode gap all have significant effects on treatment performance. Excessively high or low pH reduces removal efficiency; higher electrolyte concentration and current density enhance pollutant removal, while larger electrode gap reduces removal efficiency and increases energy consumption.
11. The optimal operating conditions are determined as: initial pH of 8.0, NaCl concentration of 6.0 g/L, current density of 20 mA/cm², electrode gap of 1 cm, and electrolysis time of 3.0 h. Under these conditions, ammonia nitrogen and COD removal rates reach 84.76% and 45.93% respectively, with energy consumption of 69.12 kW·h/m³ and electricity cost of 55.30 yuan/m³.
12. The ammonia nitrogen removal is mainly dominated by active chlorine mediated indirect oxidation, with final conversion to nitrogen gas. The process generates strong oxidizing species in situ, and has the advantages of flexible control, no need for large amount of chemical storage, and high operational safety.

FAQ

Q1: What is the core difference between electrochemical oxidation and breakpoint chlorination for ammonia nitrogen removal?
Breakpoint chlorination requires continuous dosing of sodium hypochlorite or liquid chlorine, which involves safety risks in storage, transportation and dosing, and the dosage is difficult to adjust dynamically. Electrochemical oxidation generates active chlorine oxidant in situ through electrolysis of chloride ions in water, which can flexibly adjust the oxidation capacity by controlling current density and reaction time. It also has direct oxidation effect on organic matter on the electrode surface, achieving better simultaneous removal of COD and ammonia nitrogen, and avoiding the management risk of large amounts of hazardous chemicals.

Q2: Why is weak alkaline condition optimal for electrochemical ammonia nitrogen removal?
The main oxidant for ammonia nitrogen removal is hypochlorous acid (HClO). Under acidic conditions, chlorine exists mainly in the form of Cl₂ gas which is easy to escape, reducing oxidation efficiency. Under strong alkaline conditions, HClO is converted to ClO⁻ with lower oxidation activity, and even generates chlorate with no oxidation effect. Under weak alkaline condition around pH 8, the proportion of HClO in active chlorine is the highest, and ammonia nitrogen also has high reaction activity, achieving the best removal efficiency.

Q3: How to reduce the high operating energy consumption of electrochemical oxidation process?
First, optimize process combination: use electrochemical oxidation as pretreatment to remove toxicity and part of ammonia nitrogen, then use biological process for deep purification, which can greatly reduce the cost compared with full electrochemical treatment. Second, optimize operating parameters: select appropriate current density and reaction time according to actual effluent requirements, avoid over-treatment. Third, improve electrode performance: adopt high-catalytic-activity DSA electrodes to improve current efficiency. Fourth, recover the waste heat generated during electrolysis for other production links, to improve comprehensive energy utilization rate.

Q4: What concentration range of ammonia nitrogen is suitable for electrochemical oxidation treatment?
Electrochemical oxidation is suitable for ammonia nitrogen concentration ranging from hundreds to several thousand mg/L. For ultra-high concentration ammonia nitrogen wastewater above 5000 mg/L, it is recommended to combine with stripping or chemical precipitation first to reduce the concentration to an appropriate range, then use electrochemical oxidation for deep treatment, to obtain better economy. For low-concentration ammonia nitrogen in advanced treatment stage, electrochemical oxidation can also achieve stable removal to below the standard, with advantages of small footprint and fast response.