1. Bibliographic Information

1.1. Title

The central topic of the paper is the engineering design and commissioning of a slaughterhouse wastewater treatment project.

1.2. Authors

The authors are Zhao Jiajia, Gong Nanjun, and Liu Yumeng. Their affiliation is the Hubei Research Institute of Ecological and Environmental Science, located in Wuhan, China. Zhao Jiajia is further identified as the corresponding author, born in 1986, with a master's degree and serving as an engineer. Her primary work involves research and consulting design in water ecological management and water pollution control.

1.3. Journal/Conference

The paper was published in Water & Wastewater Engineering (《给水排水》, pinyin: Gěishuǐ Páishuǐ). This journal is a reputable publication in the field of water supply and drainage engineering in China, indicating that the research has undergone peer review and is considered relevant and reliable within the industry.

1.4. Publication Year

The publication year is 2025 (Vol. 51, No. 6), with a submission date of 2024-09-04.

1.5. Abstract

The paper describes a wastewater treatment process for slaughterhouse wastewater employing a sequence of coarse grille - vibration screen - grease trap tank - regulating tank - air flotation machine - two-stage MBBR tank - secondary sedimentation tank - disinfecting tank - two-stage constructed wetland. A key design feature is the extended hydraulic retention time (HRT) in the regulating tank, which effectively manages the contradiction between intermittent wastewater discharge and the continuous operation required by biological treatment units. The two-stage MBBR and two-stage constructed wetlands are highlighted for their effectiveness in pollutant removal. The final treated effluent consistently meets the first-level emission indicators specified for livestock slaughtering and processing in Table 3 of "The discharge standard of water pollutants for meat-processing Industry" (GB 13457-92).

1.6. Original Source Link

The original source link is /files/papers/691dc4008106199151d7e970/paper.pdf. This appears to be a local file path or an internal repository link, indicating the paper is available in PDF format. Its publication status is officially published in the specified journal.

2. Executive Summary

2.1. Background & Motivation

The core problem addressed by this paper is the effective and compliant treatment of slaughterhouse wastewater. This problem is significant due to several factors:

• High Pollutant Load: Slaughterhouse wastewater is characterized by high concentrations of organic matter (COD, BOD5), suspended solids (SS), oils and greases, and ammonia nitrogen, primarily from blood, meat scraps, hair, and animal waste.

• Intermittent Discharge: The wastewater discharge from slaughterhouses is highly intermittent, often concentrated during specific operational hours (e.g., early morning slaughtering), which poses a challenge for continuous biological treatment processes that require stable influent conditions.

• Strict Discharge Standards: The treated effluent must meet stringent environmental discharge standards, such as the first-level emission indicators specified in $GB 13457-92$, to prevent environmental pollution of receiving water bodies.

• Operational Challenges: Specific operational issues like foaming in physical-chemical treatment units (e.g., air flotation) can impede treatment efficiency and lead to environmental hygiene problems.

  Existing research has explored various combinations of physical, chemical, and biological methods for slaughterhouse wastewater. However, effectively integrating these to handle intermittent, high-strength wastewater while ensuring stable compliance with strict standards, and addressing practical operational issues like foaming, remains a challenge.

The paper's entry point and innovative idea lie in proposing a comprehensive, multi-stage combined treatment process that specifically addresses the intermittent nature of discharge and operational problems like foaming, while leveraging the high-efficiency removal capabilities of MBBR and the ecological benefits of constructed wetlands to ensure stable and high-quality effluent.

2.2. Main Contributions / Findings

The primary contributions and findings of this paper are:

• Comprehensive Process Design: Proposing and validating a robust combined treatment process (coarse grille - vibration screen - grease trap tank - regulating tank - air flotation machine - two-stage MBBR tank - secondary sedimentation tank - disinfecting tank - two-stage constructed wetland) specifically tailored for high-strength, intermittent slaughterhouse wastewater.

• Effective Flow Equalization: Demonstrating that designing a regulating tank with a significantly longer hydraulic retention time (HRT) (e.g., 29.87 hours in this project) successfully resolves the conflict between intermittent wastewater discharge and the need for continuous, stable operation of downstream biological units.

• High-Efficiency Pollutant Removal: Highlighting the effectiveness of the two-stage MBBR system (integrating both anoxic and aerobic stages with suspended carriers) and the two-stage constructed wetlands in achieving high removal efficiencies for COD, BOD5, SS, NH3-N, and Oil & Grease. The MBBR benefits from high biomass concentration and strong shock load resistance, while the constructed wetlands provide further polishing and ecological benefits.

• Practical Problem Solving: Addressing and solving a common operational issue in air flotation units – severe foaming caused by proteins, blood, and detergents – through the design of a specialized anti-foaming overflow cover, which improves treatment efficiency and site hygiene.

• Stable Compliance with Stringent Standards: Achieving an effluent quality that consistently surpasses the first-level emission indicators of $GB 13457-92$ after a 3-month commissioning period, confirming the process's reliability and stability.

• Economic Viability: Providing an analysis of the investment and operating costs, demonstrating that while the unit water investment might be slightly higher due to the small treatment scale, the overall operating cost is manageable.

  These findings collectively solve the problem of effectively treating complex slaughterhouse wastewater, ensuring environmental compliance, and addressing practical operational challenges, thereby offering a valuable engineering solution.

3. Prerequisite Knowledge & Related Work

3.1. Foundational Concepts

To understand this paper, a foundational understanding of various wastewater treatment technologies and parameters is essential for a beginner.

• Slaughterhouse Wastewater: This refers to the wastewater generated during the slaughtering and processing of animals. It is typically characterized by high concentrations of organic matter (blood, gut content, meat scraps), suspended solids (hair, skin particles), fats, oils, and grease (FOG), and nutrients (nitrogen, phosphorus). It often has a reddish color and a strong odor.
• Intermittent Discharge: This describes a wastewater flow pattern where discharge occurs in bursts or at specific times, rather than continuously throughout the day. Slaughterhouses often operate during specific shifts, leading to periods of high flow and concentration followed by periods of low or no flow.
• COD (Chemical Oxygen Demand): A measure of the total amount of oxygen required to oxidize all organic and inorganic compounds in water chemically. It is a common indicator of organic pollution.
• BOD5 (Biochemical Oxygen Demand in 5 days): A measure of the amount of oxygen consumed by microorganisms in water as they decompose organic matter over a 5-day period. It indicates the biodegradability of organic pollutants.
• SS (Suspended Solids): Solid particles that are suspended in water and can be removed by filtration. In slaughterhouse wastewater, SS often includes hair, skin, meat scraps, and coagulated proteins. High SS can lead to sludge accumulation and hinder treatment processes.
• NH3-N (Ammonia Nitrogen): A form of nitrogen present in wastewater, typically originating from the decomposition of proteins and urea. High concentrations can cause eutrophication and be toxic to aquatic life.
• Oil and Grease: Fats, oils, and greases present in wastewater, primarily from animal fats. These can cause blockages in pipes, interfere with biological treatment, and create environmental pollution (e.g., surface slicks).
• Coliforms: A group of bacteria that are indicators of fecal contamination in water. Their presence suggests the potential presence of pathogenic (disease-causing) microorganisms.
• Hydraulic Retention Time (HRT): The average length of time that a soluble compound remains in a tank or reactor. For a given tank volume and flow rate, HRT is calculated as Volume / Flow Rate. Longer HRT allows more time for processes like equalization or biological reactions.
• B/C Ratio (BOD/COD Ratio): The ratio of BOD5 to COD. It indicates the biodegradability of the wastewater. A ratio greater than 0.45 generally suggests that the wastewater is readily biodegradable and suitable for biological treatment.
• Coarse Grille: A physical pretreatment unit consisting of parallel bars or screens used to remove large solid objects (e.g., animal parts, large scraps) from wastewater, protecting downstream equipment.
• Vibration Screen: A fine screening device that uses vibrations to separate smaller suspended solids (e.g., hair, fine meat particles) from wastewater.
• Grease Trap Tank (Oil-water Separator): A tank designed to separate fats, oils, and greases from wastewater through gravity differential. Lighter FOG floats to the surface and is skimmed off, while heavier solids settle to the bottom.
• Regulating Tank (Equalization Tank): A tank used to balance variations in wastewater flow rate and pollutant concentration over time. It stores intermittent flows and releases a more constant flow to subsequent treatment units, ensuring stable operation.
• Air Flotation Machine (Dissolved Air Flotation, DAF): A physical-chemical treatment process that removes suspended solids, oil, and grease by introducing fine air bubbles into the wastewater. These bubbles attach to the particles, causing them to float to the surface as a scum layer, which is then removed.
• Coagulants (e.g., PAC - Poly Aluminum Chloride): Chemicals added to wastewater to destabilize small suspended and colloidal particles, causing them to clump together into larger flocs.
• Flocculants (e.g., PAM - Polyacrylamide): Chemicals that enhance the aggregation of flocs formed by coagulants, making them larger and heavier for easier separation (e.g., by settling or flotation).
• MBBR (Moving Bed Biofilm Reactor): A biological wastewater treatment process that utilizes numerous small plastic carriers (media) within the reactor. These carriers provide a large protected surface area for microorganisms (biofilm) to grow and thrive. The carriers are kept in suspension by aeration or mixing, allowing for high biomass concentration and efficient organic and nutrient removal. It combines advantages of both activated sludge and biofilm processes.
  • Biofilm: A layer of microorganisms attached to a surface (in MBBR, the carriers) and encased in an extracellular polymeric substance (EPS).
  • Anoxic Zone: A zone in biological treatment where oxygen is absent, but nitrate is present. Denitrifying bacteria convert nitrate to nitrogen gas, removing nitrogen from the wastewater.
  • Aerobic Zone: A zone where dissolved oxygen is supplied, allowing aerobic bacteria to break down organic matter and nitrify ammonia.
• Secondary Sedimentation Tank: A clarifier located after biological treatment (like MBBR) where biological flocs (sludge) settle out by gravity, separating treated water from the biomass.
• Disinfecting Tank: A tank where treated wastewater is disinfected to kill or inactivate pathogenic microorganisms before discharge.
• Sodium Hypochlorite (NaClO): A common disinfectant used in wastewater treatment, effective in killing bacteria and viruses.
• Constructed Wetland: An engineered system that mimics natural wetlands to treat wastewater. It uses natural processes involving vegetation, soil, and microbial activity to remove pollutants.
  • Horizontal Subsurface Flow (HSSF) Wetland: A type of constructed wetland where wastewater flows horizontally beneath the surface of the gravel or soil media, allowing contact with plant roots and microorganisms.
  • Emergent Plants (e.g., Reeds): Plants that grow in water but have stems and leaves that emerge above the water surface. Their roots provide surface area for microbial growth, transfer oxygen to the rhizosphere, and absorb nutrients.
• Sludge Storage Tank: A tank used to temporarily store the concentrated sludge generated from various treatment units (e.g., sedimentation tanks, grease traps) before dewatering.
• Screw Press (Dewatering Machine): Mechanical equipment used to remove water from sludge, reducing its volume and facilitating disposal.

3.2. Previous Works

The paper references a few previous works in the context of slaughterhouse wastewater treatment:

• [1] Liu Yunpeng et al. (2019) describe the engineering application of an air flotation - UASB - A2O process for slaughterhouse wastewater treatment.

• [2] Liu Shijun et al. (2018) discuss an air flotation - UASB - contact oxidation combined process for slaughterhouse wastewater.

  These works highlight common approaches that combine physical-chemical pretreatment (air flotation) with biological treatment (UASB, A2O, contact oxidation).

• UASB (Upflow Anaerobic Sludge Blanket): An anaerobic biological treatment process where wastewater flows upwards through a blanket of anaerobic granular sludge. It is effective for high-strength organic wastewater but typically requires a post-treatment for further purification.

• A2O (Anaerobic-Anoxic-Oxic): A biological nutrient removal process that involves anaerobic, anoxic, and aerobic zones in series to remove organic matter, nitrogen, and phosphorus.

• Contact Oxidation: A biological treatment process that uses a submerged fixed-film reactor, where microorganisms grow on media and come into contact with wastewater and air.

  The paper also cites:

• [5] Zhang Tie and Zhu Xiaoyun (2008) on the research and application of MBBR in refinery wastewater treatment, providing general background on MBBR technology.

• [6] Zhang Jinfu et al. (2021) on case studies of hog slaughtering enterprise wastewater treatment projects, used for economic comparison of unit water investment.

3.3. Technological Evolution

Slaughterhouse wastewater treatment has evolved from basic physical separation to more complex and integrated systems. Initially, focus was on removing gross solids and FOG through screens and grease traps. As environmental standards tightened and understanding of biological processes grew, biological treatment became central.

• Early Stages: Primarily physical screening and grease traps for gross pollutant removal.

• Introduction of Chemical Treatment: Coagulation-flocculation and air flotation were introduced to enhance the removal of SS and FOG and reduce the load on biological units.

• Anaerobic Treatment: UASB and other anaerobic reactors gained prominence for high-strength organic wastewater, offering energy recovery (biogas) and efficient COD removal at lower energy costs.

• Aerobic Treatment: Activated sludge processes, A2O for nutrient removal, and MBBR emerged as robust options for further COD, BOD5, and NH3-N removal. MBBR specifically offers the advantage of high biomass concentration and resistance to shock loads due to its biofilm carriers.

• Advanced Treatment/Polishing: With increasingly strict discharge limits, advanced treatment steps like filtration, disinfection, and constructed wetlands are employed for final polishing, removal of residual pollutants, and pathogen inactivation.

  This paper's work fits into the current state of technological evolution by combining established and advanced methods: leveraging robust physical-chemical pretreatment (coarse grille, vibration screen, grease trap, air flotation), sophisticated biological treatment (two-stage MBBR for high-efficiency organic and nutrient removal), and a natural polishing step (two-stage constructed wetland), while specifically designing the regulating tank to address the intermittent nature characteristic of this industry.

3.4. Differentiation Analysis

Compared to the main methods in related work, the core differences and innovations of this paper's approach are:

• Emphasis on Intermittent Flow Management: Unlike many systems that assume relatively constant flow, this paper explicitly tackles the intermittent discharge characteristic of slaughterhouses by designing a regulating tank with a significantly longer HRT (nearly 30 hours for the recent phase). This ensures a stable and continuous feed to the sensitive biological units, which is crucial for their optimal performance. The cited UASB and A2O processes typically benefit from flow equalization, but the explicit design for prolonged HRT is a key differentiator here.

• Two-Stage MBBR for Enhanced Performance: While MBBR is a known technology, the use of a two-stage MBBR system (with separate anoxic and aerobic sections in each stage, and independent sedimentation/return) is designed for more efficient and robust pollutant removal. This allows for specialized microbial communities in each stage, higher overall treatment capacity, and better adaptation to varying loads compared to single-stage or simpler biological reactors like UASB followed by a single A2O or contact oxidation unit. The MBBR system combines the advantages of both biofilm and activated sludge processes.

• Integrated Natural Polishing: The inclusion of two-stage constructed wetlands as a final polishing step is a significant feature. While disinfection is common, constructed wetlands provide an eco-friendly, low-cost, and sustainable method for further reducing residual COD, BOD5, SS, and NH3-N, and can also enhance the aesthetic appeal of the treatment facility. This goes beyond typical chemical disinfection alone.

• Addressing Practical Operational Challenges: The development of patented solutions (e.g., anti-foaming overflow cover for the air flotation machine) to overcome specific operational problems encountered during commissioning demonstrates a highly practical and innovative aspect of this engineering project, which is often not detailed in theoretical process descriptions.

• Meeting Stringent Standards with Robustness: The combination of these technologies aims to achieve first-level emission standards under $GB 13457-92$ consistently, which implies a robust design capable of handling the inherent variability and high pollutant loads of slaughterhouse wastewater.

  In essence, while previous works combine elements like air flotation and biological units, this paper's strength lies in its comprehensive, multi-barrier approach, specific design considerations for intermittent flow and foaming, and the synergistic use of MBBR and constructed wetlands for high-performance and environmentally sound treatment.

4. Methodology

4.1. Principles

The core idea of the method used in this paper is to apply a multi-stage, comprehensive treatment train that combines physical, chemical, biological, and natural purification processes. This approach is designed to effectively remove a wide range of pollutants found in slaughterhouse wastewater, from large solids and fats to soluble organic matter, nutrients, and pathogens, ensuring the final effluent meets stringent environmental discharge standards. The theoretical basis is rooted in leveraging the strengths of each unit operation:

• Physical pretreatment: Removes coarse solids and separates oil/grease, reducing the load on subsequent units.
• Flow and quality equalization: Stabilizes influent conditions for biological processes.
• Chemical-physical treatment (Air flotation): Enhances the removal of suspended solids and residual oil/grease.
• Biological treatment (MBBR): Utilizes diverse microbial communities to break down organic matter and remove nitrogen efficiently.
• Sedimentation: Separates biomass from treated water.
• Disinfection: Eliminates pathogens.
• Natural purification (Constructed wetland): Provides final polishing and ecological benefits.

4.2. Core Methodology In-depth (Layer by Layer)

The wastewater treatment process is a sequential flow through several interconnected units. The overall process flow is depicted in the following figure (Figure 1 from the original paper):

该图像是示意图1，展示了屠宰废水处理的工艺流程。该流程包括粗格栅、振动筛网、隔油池、调节池、气浮机、一级MBBR池、二级MBBR池、二沉池和接触消毒池等，最终通过两级人工湿地达到排放标准。

图1工艺流程 Fig.1Wastewater treatment process

The main structures and equipment parameters are detailed in Table 2. This table will be used to describe the specific design of each unit.

The treatment process proceeds as follows:

1. Coarse Grille and Vibration Screen (粗格栅 - 振动筛网):

   • Purpose: The initial step to remove large suspended solids, such as animal hair, skin, bones, and other debris. This protects downstream pumps and equipment from clogging and reduces the overall suspended solids load.
   • Mechanism: Wastewater first passes through a coarse grille to capture larger items. Subsequently, a vibration screen with finer mesh further separates smaller suspended particles like fine hair and meat scraps.
   • Design (from Table 2):
     • One underground reinforced concrete structure.
     • Dimensions: $3.2 \mathrm{~m} \times 2.2 \mathrm{~m} \times 2.2 \mathrm{~m}$.
     • Two channels, $2.2 \mathrm{~m}$ deep, $800 \mathrm{~mm}$ wide.
     • Manual coarse grille with bar spacing of $5 \mathrm{~mm}$, installed at a $60^\circ$ angle.
     • One manual cleaning unit for initial operation (近期).
     • Followed by a vibration screen with a screw conveyor for removing hair and meat scraps.

2. Grease Trap Tank (隔油池):

   • Purpose: To separate and remove fats, oils, and greases (FOG) from the wastewater. High FOG content can interfere with biological processes and cause blockages.
   • Mechanism: Utilizes gravity separation. Oil and grease, being lighter than water, float to the surface, while heavier solids settle to the bottom. The accumulated oil/grease is skimmed off, and settled sludge is removed.
   • Design (from Table 2):
     • One tank divided into 2 sets, each set having 2 compartments.
     • Single set effective area: $7.28 \mathrm{~m}^2$.
     • Designed surface loading for initial operation (近期): $0.77 \mathrm{~m}/(\mathrm{m}^2 \cdot \mathrm{h})$.
     • Equipped with a sludge hopper at the bottom and oil skimming at the top.
     • Pneumatic diaphragm pumps are used for sludge discharge.

3. Regulating Tank (调节池):

   • Purpose: To equalize the highly variable flow rate and pollutant concentration of slaughterhouse wastewater, ensuring a stable and continuous influent to downstream biological treatment units. This is critical for processes that require consistent conditions.
   • Mechanism: The tank acts as a buffer, storing wastewater during peak discharge periods and releasing it at a constant rate during low-flow periods. The long HRT allows for some preliminary anaerobic degradation and homogenization of water quality.
   • Design (from Table 2):
     • One tank with a designed effective volume of $168.12 \mathrm{~m}^3$.
     • Designed HRT for initial operation (近期): $29.87 \mathrm{~h}$. This unusually long HRT is specifically highlighted as a key feature to handle the intermittent discharge of slaughterhouse wastewater and ensure continuous operation of the biological treatment.
     • The tank is covered (池顶加盖) and has a longitudinal slope at the bottom towards a sedimentation trough equipped with a sludge discharge pipe.
     • Wastewater is lifted from the tank top to the air flotation machine using self-priming pumps.

4. Air Flotation Machine (气浮机):

   • Purpose: To further remove suspended solids, fine particles, residual oil and grease, and some organic matter, especially those that are difficult to settle or float by gravity alone.
   • Mechanism: This unit is a vortex air flotation machine. It operates by dissolving air under pressure into a portion of the treated effluent, which is then recycled and mixed with the incoming wastewater. When the pressure is released, tiny air bubbles form and attach to suspended particles and FOG, carrying them to the surface to form a scum layer, which is then scraped off. Coagulants (PAC) and flocculants (PAM) are typically added upstream to enhance the aggregation of particles.
   • Design (from Table 2):
     • One set of vortex air flotation machine, made of internally and externally anti-corrosion carbon steel.
     • Treatment capacity: $12.5 \mathrm{~m}^3/\mathrm{h}$.
     • Equipped with an air compressor, dissolved air release device, skimmer, and control system.
     • Designed HRT: $20 \mathrm{~min}$.
     • A dosing area is set upstream for adding coagulant (PAC) and flocculant (PAM) for pollutant reduction and phosphorus removal.
   • Commissioning Note (from 4.1): During initial debugging, severe foaming occurred due to proteins, blood, and detergents in the wastewater. This problem was solved by designing and implementing a patented anti-foaming overflow cover for the air flotation equipment (patent ZL 2022 2 162381.8 [4]), which automates foam elimination and guides overflow recovery. After this modification, the unit operated effectively even without PAC and PAM for oil and animal hair removal.

5. Two-Stage MBBR Tank (两级MBBR池) - (Stage I MBBR + Vertical Sedimentation Tank and Stage II MBBR + Secondary Sedimentation Tank):

   • Purpose: The core biological treatment unit designed for efficient removal of organic pollutants (COD, BOD5) and ammonia nitrogen (NH3-N). The MBBR process combines the advantages of activated sludge (high treatment efficiency) and biofilm reactors (resistance to shock loads, high biomass concentration).
   • Mechanism: The MBBR tanks contain suspended plastic carriers that provide a large surface area for microorganisms to attach and grow as a biofilm. Wastewater flows through these tanks, and the biofilm degrades pollutants. The two-stage design allows for specialized treatment conditions (e.g., separate anoxic and aerobic zones in each stage) and optimizes removal efficiency.
   • Design (from Table 2):
     • One structure, divided into 2 sets, consisting of Stage I MBBR, vertical sedimentation tank, and Stage II MBBR, built side-by-side with shared walls. Initially, one set operates, with the other for future expansion.
     • Stage I MBBR:
       • Designed effective water depth: $4 \mathrm{~m}$.
       • Effective volume: $123.2 \mathrm{~m}^3$.
       • Designed total HRT: $21.91 \mathrm{~h}$ (Anoxic section: $7.47 \mathrm{~h}$, Aerobic section: $14.44 \mathrm{~h}$).
       • Suspended filler addition ratio: $1/2$ (presumably by volume of tank) and $1/5$ (perhaps different types or sections).
     • Vertical Sedimentation Tank (after Stage I MBBR):
       • Designed HRT: $4.57 \mathrm{~h}$.
       • Surface loading: $0.46 \mathrm{~m}/(\mathrm{m}^2 \cdot \mathrm{h})$.
       • Effective water depth: $2.1 \mathrm{~m}$.
       • Sludge hopper height: $1.5 \mathrm{~m}$.
       • Equipped with 2 dry sludge pumps (1 operating, 1 standby).
     • Stage II MBBR:
       • Designed effective water depth: $3.7 \mathrm{~m}$.
       • Effective volume: $66.0 \mathrm{~m}^3$.
       • Designed total HRT: $11.74 \mathrm{~h}$ (Anoxic section: $2.30 \mathrm{~h}$, Aerobic section: $9.44 \mathrm{~h}$).
       • Suspended filler addition ratio: $1/3$ and $1/6$.
   • Commissioning Note (from 4.1):
     • MBBR Activation: The MBBR tanks were filled to one-third of their effective water depth, then seeded with $5 \mathrm{~t}$ of activated sludge (approx. 80% moisture content) from a nearby wastewater treatment plant.
     • Muffled Aeration: An initial 8-hour muffled aeration (闷曝, implying aeration without influent to cultivate sludge) was performed.
     • Gradual Feeding: This was followed by a sequence of 1.0 h influent - 8 h muffled aeration - 1.5 h influent - 8 h muffled aeration - 2.0 h influent - 8 h muffled aeration, gradually increasing the feeding time.
     • Sludge Recirculation: Sludge from the vertical sedimentation tank hopper is continuously recirculated back to the anoxic tank of the Stage I MBBR with a 100% recirculation ratio.
     • Stage II MBBR Maintenance: If the sludge activity in the Stage II MBBR decreases, sludge from the vertical sedimentation tank and secondary sedimentation tank is recirculated to its anoxic tank for 1-2 hours to supplement active sludge.
     • Flocculant Use: Small amounts of PAM are added to the vertical sedimentation tank and secondary sedimentation tank to enhance sludge settling, leveraging the flocculation properties of activated sludge.
     • Biofilm Advantage: The paper emphasizes that the biofilm on suspended carriers can reach $100 \sim 200 \mathrm{~\mu m}$ thickness, providing a high biomass concentration. Under aeration and water flow disturbance, this biofilm effectively removes pollutants through physical adsorption, microbial metabolism, and endocytosis, leading to significantly improved degradation efficiency compared to suspended growth processes [5].

6. Secondary Sedimentation Tank (二沉池):

   • Purpose: To separate the biologically treated water from the biomass (sludge) that has grown in the MBBR tanks. This ensures clear effluent for subsequent treatment steps.
   • Mechanism: Clarification occurs by gravity settling of the heavier biological flocs.
   • Design (from Table 2):
     • One reinforced concrete structure, divided into 2 sets (1 for initial, 1 for future).
     • Tank dimensions: $6.8 \mathrm{~m} \times 3.6 \mathrm{~m} \times 4.0 \mathrm{~m}$.
     • Designed HRT: $3.20 \mathrm{~h}$.
     • Designed surface loading: $0.63 \mathrm{~m}/(\mathrm{m}^2 \cdot \mathrm{h})$.
     • Designed effective water depth: $2.0 \mathrm{~m}$.
     • Sludge hopper height: $1.2 \mathrm{~m}$.
     • Equipped with 2 dry sludge pumps (1 operating, 1 standby).

7. Disinfecting Tank (接触消毒池):

   • Purpose: To inactivate or kill any remaining pathogenic microorganisms in the treated water before discharge, ensuring public health safety.
   • Mechanism: Sodium hypochlorite solution (10% effective ingredient) is added to the water, and a contact time is provided to allow the disinfectant to act.
   • Design (from Table 2):
     • One reinforced concrete structure.
     • Tank dimensions: $3.6 \mathrm{~m} \times 3.6 \mathrm{~m} \times 2.5 \mathrm{~m}$.
     • Disinfection uses sodium hypochlorite solution with 10% effective ingredient.
     • Designed HRT for initial operation (近期): $1.48 \mathrm{~h}$.

8. Two-Stage Constructed Wetland (两级人工湿地):

   • Purpose: To provide a final, natural polishing step for the treated wastewater, removing residual pollutants (e.g., nutrients, trace organics) and enhancing the overall water quality. It also contributes to environmental beautification.
   • Mechanism: These are horizontal subsurface flow wetlands, meaning wastewater flows horizontally through a porous medium (crushed stone) beneath the surface. Pollutant removal occurs through physical filtration, chemical precipitation, adsorption, and biological degradation by microorganisms attached to the media and plant roots. Emergent plants (e.g., reeds) absorb nutrients and facilitate oxygen transfer to the root zone.
   • Design (from Table 2):
     • Two constructed wetlands connected in series (串联).
     • Horizontal subsurface flow type.
     • Effective area: $420 \mathrm{~m}^2$.
     • Designed surface loading for initial operation (近期): $0.015 \mathrm{~m}/(\mathrm{m}^2 \cdot \mathrm{h})$.
     • Internal filling: $0.7 \mathrm{~m}$ thick crushed stone media, divided into three layers:
       • Layer 1: $\phi 5 \sim 10 \mathrm{~mm}$ (grain size), $300 \mathrm{~mm}$ thick.
       • Layer 2: $\phi 10 \sim 20 \mathrm{~mm}$ (grain size), $200 \mathrm{~mm}$ thick.
       • Layer 3: $\phi 20 \sim 30 \mathrm{~mm}$ (grain size), $200 \mathrm{~mm}$ thick.
     • Lined with $2 \mathrm{~mm}$ thick HDPE impermeable geomembrane at the bottom.
     • Upper layer planted with emergent plants (reeds), with a planting spacing of $400 \mathrm{~mm}$.

9. Sludge Storage Tank (污泥储池):

   • Purpose: To temporarily store the sludge generated from the various sedimentation and separation units (e.g., grease trap, MBBR sedimentation, secondary sedimentation).

   • Design (from Table 2):

     • One reinforced concrete structure.
     • Tank dimensions: $\phi 3.0 \mathrm{~m} \times 3.3 \mathrm{~m}$ (diameter $\times$ height).
     • Designed HRT for initial operation (近期): $3.1 \mathrm{~h}$.

   • Sludge Dewatering: The concentrated sludge from this tank is then dewatered by a screw press to achieve a moisture content of less than 80% before external transportation for disposal.

     The following are the results from Table 2 of the original paper:

     处理单元	主要设备参数
     粗格栅十振动筛网	1座，地下钢筋混凝土结构，池体尺寸3.2 m×2.2 m×2.2m，两条格栅渠，渠深2.2m，渠宽800 mm,人工粗格栅栅条间隙5 mm,安装角度60°,近期1台人工清渣，后设振动筛网，配套螺旋输送机除皮毛、肉沫等
     隔油池	1座分2组,每组分2格，单组有效面积7.28 m2，设计近期表面负荷0.77 m/(m2·h)，池体下部设污泥斗，上部隔油,排泥用气动隔膜泵
     调节池	1座，设计有效容积168.12m3,设计近期停留时间29.87h,池顶加盖，池底设纵向坡度坡向沉淀槽，槽内设排泥管，出水由池顶自吸泵提升至气浮机
     气浮机	1套涡凹气浮机,内外防腐碳钢材质,处理水量12.5m3/h,配套空压机、溶气释放器、刮渣机及控制系统，设计停留时间20 min，前端设加药区，投加混凝剂(PAC)和助凝剂(PAM),降污除磷
     两级MBBR 池	1座分2组，由一级 MBBR池十竖流沉淀池十二级MBBR 池 2组并列共壁合建;一级MBBR 设计有效水深4m,有效容积123.2m3,设计总停留时间21.91 h(缺氧段7.47 h,好氧段 14.44 h)，悬浮填料投加比例1/2、1/5;竖流沉淀池设计停留时间4.57h,表面负荷0.46 m/(m2·h)，有效水深2.1m,污泥斗高1.5m,设2台干式排泥泵,1用1备；二级 MBBR 设计有效水深3.7m，有效容积66.0 m3,总设计停留时间11.74h(缺氧段 2.30 h,好氧段9.44h),悬浮填料投加比例1/3、1/6
     二沉池	1座分2组(近期1组、远期1组),钢筋混凝土结构,池体尺寸6.8m×3.6m×4.0m,设计停留时间3.20h,设计表面负荷0.63m/(m2·h),设计有效水深2.0m,池底污泥斗高1.2m,设2台干式排泥泵,1用1备
     接触消毒池	1座,钢筋混凝土结构，池体尺寸3.6 m×3.6 m×2.5 m，用有效成分为10%的次氯酸钠溶液消毒，设计近期停留时间1.48 h
     两级人工湿地	2 座串联，水平潜流形式，有效面积420m2，设计近期表面负荷0.015m/(m2·h),内铺0.7m厚碎石填料分三层(𝜑5～10 mm厚300 mm、φ10～20 mm厚 200 mm、φ20～30 mm厚 200 mm),下铺2 mm厚 HDPE防渗土工膜，上层种挺水植物芦苇,种植间距400mm
     污泥储池	1座,钢筋混凝土结构,池体尺寸φ3.0m×3.3 m，设计近期停留时间3.1h

5. Experimental Setup

5.1. Datasets

The wastewater treated in this project is slaughterhouse wastewater, specifically from a livestock slaughtering and processing facility. The paper defines the design influent water quality (i.e., the characteristics of the raw wastewater) and the required effluent water quality (i.e., the target discharge standards).

The following are the results from Table 1 of the original paper:

• Source: The wastewater originates from slaughtering operations, cleaning of production equipment and floors, and employee domestic sewage within the slaughterhouse.

• Characteristics:

  • High COD (up to $3000 \mathrm{~mg/L}$) and BOD5 (up to $1600 \mathrm{~mg/L}$), indicating a high organic load.
  • High SS (up to $900 \mathrm{~mg/L}$), attributed to animal hair, bone fragments, meat scraps, blood, etc.
  • High Oil and Grease (up to $700 \mathrm{~mg/L}$).
  • Significant NH3-N (up to $300 \mathrm{~mg/L}$).
  • The B/C ratio is calculated as $1600/3000 \approx 0.53$, which is greater than 0.45, indicating good biodegradability and suitability for biological treatment.

• Design Flow Rate: The design flow rate is $135 \mathrm{~m}^3/\mathrm{d}$ (average $6 \mathrm{~m}^3/\mathrm{h}$) for the near term and $270 \mathrm{~m}^3/\mathrm{d}$ (average $12 \mathrm{~m}^3/\mathrm{h}$) for the long term.

• Discharge Standard: The treated effluent must meet the first-level emission indicators for livestock slaughtering and processing as per Table 3 of "The discharge standard of water pollutants for meat-processing Industry" (GB 13457-92).

  No concrete example of a data sample (e.g., a specific batch analysis report) is provided, but the range of pollutant concentrations comprehensively characterizes the influent.

5.2. Evaluation Metrics

The effectiveness of the wastewater treatment process is evaluated based on the removal efficiency of key pollutant parameters and the final effluent quality's compliance with national standards. The metrics used are:

1. COD (Chemical Oxygen Demand):

   • Conceptual Definition: Measures the amount of oxygen required to chemically oxidize organic and inorganic pollutants in water. It's a broad indicator of pollution load.
   • Formula: No specific formula for COD itself, as it's a measured concentration.
   • Symbol Explanation: $C_{\text{COD}}$ represents the concentration of Chemical Oxygen Demand.

2. BOD5 (Biochemical Oxygen Demand in 5 days):

   • Conceptual Definition: Quantifies the amount of oxygen consumed by microorganisms to biologically decompose organic matter in water over a 5-day period at $20^\circ\mathrm{C}$. It reflects the biodegradable organic content.
   • Formula: No specific formula for BOD5 itself, as it's a measured concentration.
   • Symbol Explanation: $C_{\text{BOD5}}$ represents the concentration of Biochemical Oxygen Demand over 5 days.

3. SS (Suspended Solids):

   • Conceptual Definition: Measures the concentration of solid particles suspended in water, which can be removed by filtration. High SS can lead to turbidity and sludge accumulation.
   • Formula: No specific formula for SS itself, as it's a measured concentration.
   • Symbol Explanation: $C_{\text{SS}}$ represents the concentration of Suspended Solids.

4. NH3-N (Ammonia Nitrogen):

   • Conceptual Definition: Represents the concentration of nitrogen in the form of ammonia and ammonium ions. It's a key nutrient pollutant that can cause eutrophication and be toxic.
   • Formula: No specific formula for NH3-N itself, as it's a measured concentration.
   • Symbol Explanation: $C_{\text{NH3-N}}$ represents the concentration of Ammonia Nitrogen.

5. 动植物油 (Oil and Grease):

   • Conceptual Definition: Measures the concentration of fats, oils, and greases originating from animal and vegetable sources. These are difficult to treat and can cause environmental problems.
   • Formula: No specific formula for Oil and Grease itself, as it's a measured concentration.
   • Symbol Explanation: C_{\text{Oil & Grease}} represents the concentration of Oil and Grease.

6. 大肠菌群 (Coliforms):

   • Conceptual Definition: A group of bacteria whose presence indicates potential fecal contamination and the possible presence of disease-causing pathogens. Measured as counts per liter.
   • Formula: No specific formula for Coliforms itself, as it's a measured count.
   • Symbol Explanation: $N_{\text{Coliforms}}$ represents the number of Coliforms per liter.

7. 去除率 (Removal Efficiency):

   • Conceptual Definition: The percentage reduction in the concentration of a pollutant after a treatment step or the entire treatment process.
   • Mathematical Formula: $ \text{Removal Efficiency} (%)= \frac{C_{\text{in}} - C_{\text{out}}}{C_{\text{in}}} \times 100% $
   • Symbol Explanation:
     • $C_{\text{in}}$: Influent concentration of the pollutant.
     • $C_{\text{out}}$: Effluent concentration of the pollutant.

5.3. Baselines

The paper's method is primarily compared against the following baselines:

• National Discharge Standard: The most critical baseline is the first-level emission indicators set for livestock slaughtering and processing in Table 3 of "The discharge standard of water pollutants for meat-processing Industry" (GB 13457-92). The objective is to ensure the final effluent water quality meets or surpasses these stringent standards.

• Unit Water Investment Comparison: The paper also implicitly compares its unit water investment (investment per cubic meter of treated water) against similar slaughterhouse wastewater treatment projects found in the literature, specifically referencing Zhang J.F. et al. [6]. This provides an economic benchmark.

  These baselines are representative because they define the regulatory requirements and economic feasibility against which any successful wastewater treatment project must be judged.

6. Results & Analysis

6.1. Core Results Analysis

After individual unit debugging and linked debugging, the activated sludge was cultivated and acclimated. The wastewater flow was gradually increased over three months until the effluent quality consistently met the discharge standards.

The average values for the effluent quality indicators were:

• COD: $57 \mathrm{~mg/L}$

• BOD5: $14 \mathrm{~mg/L}$

• SS: $23 \mathrm{~mg/L}$

• NH3-N: $10 \mathrm{~mg/L}$

• Oil & Grease: $8 \mathrm{~mg/L}$

  These results far exceeded the first-level emission indicators set for livestock slaughtering and processing in Table 3 of "The discharge standard of water pollutants for meat-processing Industry" (GB 13457-92). This strongly validates the effectiveness of the proposed method in achieving high-quality effluent.

The air flotation machine debugging was particularly challenging due to severe foaming from proteins, blood, and detergents. The initial addition of coagulants and flocculants exacerbated this. The problem was resolved by designing and implementing a patented anti-foaming overflow cover [4]. This innovative solution allowed the air flotation machine to effectively remove oil and animal hair even without the continuous addition of coagulants and flocculants in subsequent operations.

The two-stage MBBR system was successfully debugged by seeding with activated sludge and gradually increasing influent flow. The continuous recirculation of sludge from the vertical sedimentation tank to the anoxic tank of Stage I MBBR (at 100% recirculation ratio) and supplementary sludge return to Stage II MBBR (when activity decreased) ensured robust biological activity. The use of small amounts of PAM in the sedimentation tanks improved settling efficiency. The MBBR's high biomass concentration due to biofilm growth on carriers contributed significantly to the high pollutant removal rates.

6.2. Data Presentation (Tables)

The following are the results from Table 1 of the original paper:

The following are the results from Table 3 of the original paper, showing the influent and effluent water quality for each process unit and the overall removal rates:

Analysis of Unit Performance:

• Physical Pretreatment (Coarse Grille + Vibration Screen): Achieved 10-20% removal for COD, BOD5, and SS, and 5% for Oil & Grease. This initial step is effective in removing large, easily separable solids and helps reduce the load on subsequent units.
• Grease Trap Tank: Showed good removal for Oil & Grease (30%) and SS (35%), demonstrating its effectiveness in separating fats and suspended solids.
• Regulating Tank: While primarily for flow and quality equalization, it also contributed to pollutant reduction (10% for COD, BOD5, SS, and 5% for NH3-N, Oil & Grease), likely due to some anaerobic degradation during its long HRT.
• Air Flotation Machine: Achieved a significant 90% removal for Oil & Grease and 60% for SS, as well as 30-40% for COD, BOD5, and NH3-N. This highlights its crucial role in physical-chemical pretreatment, especially for fatty wastewater. The high Oil & Grease removal is particularly notable given the initial foaming issues, indicating the success of the anti-foaming solution.
• Stage I MBBR + Vertical Sedimentation Tank: This biological stage achieved very high removal efficiencies: 90% for COD, 95% for BOD5, 70% for SS, 85% for NH3-N, and 70% for Oil & Grease. This demonstrates the excellent performance of the MBBR in degrading organic matter and nitrifying ammonia.
• Stage II MBBR + Secondary Sedimentation Tank: Provided further significant removal, with 40-50% for COD, BOD5, SS, NH3-N, and 30% for Oil & Grease. This shows the benefit of the two-stage approach for enhanced purification.
• Two-Stage Constructed Wetland: As the final polishing step, it further reduced COD and BOD5 by 30%, NH3-N by 25%, and SS and Oil & Grease by 10%. This indicates its effectiveness in removing residual pollutants and improving effluent quality, especially for COD, BOD5, and NH3-N.

Overall Performance: The total removal rates are impressive:

• COD: 98.1%

• BOD5: 99.1%

• SS: 97.5%

• NH3-N: 96.8%

• Oil & Grease: 98.8%

  The final effluent quality (average values: COD 57 mg/L, BOD5 14 mg/L, SS 23 mg/L, NH3-N 10 mg/L, Oil & Grease 8 mg/L) consistently met and significantly surpassed the stringent first-level emission standards.

6.3. Ablation Studies / Parameter Analysis

• Air Flotation Foaming Solution: The paper effectively describes an ablation study (though not explicitly named as such) regarding the air flotation unit. Initially, PAC and PAM were used, leading to severe foaming. By ceasing PAC/PAM addition and instead implementing a patented anti-foaming overflow cover [4], the foaming issue was resolved, and the unit still achieved excellent removal of oil and animal hair. This demonstrates that the physical design improvement was more effective than chemical dosing for this specific operational challenge.
• MBBR Sludge Cultivation: The detailed commissioning process for the MBBR (seeding, muffled aeration, gradual feeding, continuous sludge recirculation, and supplementary return) highlights the careful parameter tuning and operational strategy required to cultivate robust active sludge and achieve optimal performance. The ability to control sludge concentration and cultivate advantageous microbial species in stages is crucial for its high removal efficiency.
• Investment and Operating Costs:
  • Total Investment: The total investment for the project was $211.47 \mathrm{~ten\ thousand\ RMB}$ (2.1147 million RMB), comprising $154.61 \mathrm{~ten\ thousand\ RMB}$ for civil engineering and $56.86 \mathrm{~ten\ thousand\ RMB}$ for equipment purchase and installation.
  • Unit Water Investment: The unit water investment was calculated as $7832.2 \mathrm{~RMB/\mathrm{m}^3}$.
    • Comparison: The paper notes that similar slaughterhouse wastewater treatment projects mentioned by Zhang J.F. et al. [6] had unit water investments of $3166.8 \mathrm{~RMB/\mathrm{m}^3}$, $6237.5 \mathrm{~RMB/\mathrm{m}^3}$, and $8427.7 \mathrm{~RMB/\mathrm{m}^3}$. The current project's unit water investment is slightly higher due to its relatively smaller treatment capacity.
  • Operating Costs (per unit of water):

    • Electricity: Daily electricity consumption was approximately $300 \mathrm{~kW \cdot h/d}$. With a local electricity price of $0.87 \mathrm{~RMB/(kW \cdot h)}$, the electricity cost per unit of water (for $135 \mathrm{~m}^3/\mathrm{d}$) is $1.93 \mathrm{~RMB/\mathrm{m}^3}$.

    • Chemicals:

      • PAM: Dosed at $20 \mathrm{~mg/L}$ with a market price of $20,000 \mathrm{~RMB/t}$.
      • Sodium hypochlorite: Dosed at $10 \mathrm{~mg/L}$ with a market price of $800 \mathrm{~RMB/t}$.
      • Total chemical cost per unit of water: $0.41 \mathrm{~RMB/\mathrm{m}^3}$.

    • Labor: Two operators with a monthly salary of $3000 \mathrm{~RMB/(month \cdot person)}$.

      • Total labor cost per unit of water: $1.48 \mathrm{~RMB/\mathrm{m}^3}$.

    • Total Operating Cost: The combined total operating cost per unit of water is $3.82 \mathrm{~RMB/\mathrm{m}^3}$.

      This detailed breakdown of costs provides valuable economic insights into the project's feasibility and operational expenses.

7. Conclusion & Reflections

7.1. Conclusion Summary

This paper successfully presents the engineering design and commissioning of a slaughterhouse wastewater treatment project using a comprehensive process combining coarse grille - vibration screen - grease trap tank - regulating tank - air flotation machine - two-stage MBBR tank - secondary sedimentation tank - disinfecting tank - two-stage constructed wetland. The core findings demonstrate the feasibility and high efficiency of this integrated approach. Key elements contributing to its success include the long hydraulic retention time in the regulating tank to manage intermittent discharge, the robust pollutant removal capability of the two-stage MBBR (which merges the advantages of biofilm and activated sludge processes), and the final polishing effect of the two-stage constructed wetlands. The project successfully addressed practical operational challenges, such as foaming in the air flotation unit, through innovative solutions. Ultimately, the treated effluent consistently and significantly exceeds the first-level emission indicators specified in $GB 13457-92$, proving the reliability and environmental compliance of the system.

7.2. Limitations & Future Work

The paper does not explicitly state limitations of the proposed system. Instead, it highlights specific innovations developed during the project that address operational challenges, which can be seen as overcoming previous limitations or common problems in the field. These innovations include:

• Patented Anti-foaming Overflow Cover: The design and implementation of "An anti-foaming overflow cover for air flotation equipment" (ZL 2022 2 162381.8 [4]) to solve the severe foaming issue caused by proteins, blood, and detergents in slaughterhouse wastewater. This suggests that without such an innovation, foaming would be a significant operational limitation for air flotation in this context.

• Patented Anti-clogging Inlet/Outlet Device for MBBR: The paper mentions the design of "An inlet and outlet device for MBBR tanks to prevent clogging of suspended fillers" (ZL 2022 2 1545294.2 [3]), indicating that filler clogging can be a potential issue in MBBR systems, which was addressed proactively.

  These points suggest that the authors have actively identified and mitigated potential weaknesses or common operational challenges, rather than leaving them as limitations. Future work could involve:

• Long-term performance monitoring: While 3 months of stable operation is good, longer-term data would further validate the system's robustness and efficiency under seasonal variations or changes in raw wastewater characteristics.

• Energy optimization: Further investigating energy consumption, especially in aeration for the MBBR and pumping, to identify areas for optimization.

• Resource recovery: Exploring potential for resource recovery from the wastewater or sludge, such as biogas production from high-strength organic fractions or nutrient recovery.

• Transferability to other industries: Assessing the applicability and necessary modifications of this integrated process for other high-strength organic industrial wastewaters.

7.3. Personal Insights & Critique

This paper provides a robust and practical engineering solution for a challenging wastewater stream. The comprehensive, multi-barrier treatment train is well-conceived, leveraging established technologies (air flotation, MBBR) with natural polishing (constructed wetlands).

Key Strengths:

• Practicality: The paper focuses heavily on practical implementation and problem-solving, as evidenced by the detailed commissioning notes and the development of patented solutions for real-world issues like air flotation foaming. This makes the work highly valuable for practitioners.
• Intermittent Flow Management: The explicit design of a regulating tank with a very long HRT is a critical and well-executed strategy to handle the inherent intermittent discharge of slaughterhouse wastewater, which is often a downfall for purely biological systems.
• Efficiency: The high overall removal efficiencies and the ability to significantly exceed stringent discharge standards are impressive, demonstrating the system's effectiveness.
• Sustainable Elements: The incorporation of constructed wetlands for final polishing adds a sustainable and ecologically friendly dimension, contributing to both water quality improvement and potentially site aesthetics.
• Cost Analysis: Including both investment and operating costs provides a realistic economic perspective, although the comparison of unit water investment could benefit from more detailed context on the comparable projects (e.g., treatment complexity, specific discharge standards).

Potential Areas for Improvement/Further Consideration:

• Details on MBBR Fillers: While the paper mentions filler addition ratios, more specifics on the type, material, surface area, and fill ratio (volume of carriers to reactor volume) of the MBBR carriers would be beneficial for replication or deeper analysis.

• Sludge Production and Management: While sludge dewatering is mentioned, a more detailed breakdown of sludge volume, characteristics, and final disposal methods (beyond "external transportation") would be valuable, as sludge management is a significant aspect of wastewater treatment.

• Long-term Monitoring Data: The paper presents commissioning and initial stable operation results. Long-term performance data (e.g., over a year) would provide greater confidence in the system's resilience to seasonal changes, temperature fluctuations, and potential variations in raw wastewater characteristics.

• Energy Footprint: While electricity cost is mentioned, a more granular analysis of energy consumption by individual units (e.g., aeration in MBBR, pumping) could pinpoint specific areas for future energy efficiency improvements.

  Overall, this paper serves as an excellent case study in applied environmental engineering, demonstrating a well-designed, robust, and operationally intelligent solution for a challenging industrial wastewater problem. Its methods and conclusions could be particularly applicable to other industries facing high-strength, intermittently discharged organic wastewaters.