Case Study on Food Industry Wastewater Treatment

Case Study on Food Industry Wastewater Treatment

Overview

A slaughterhouse in Zhejiang primarily engages in pig slaughtering and related meat processing. The production wastewater, mainly from slaughtering activities, exhibits high COD, significant color, and contains substantial amounts of oils, suspended solids such as pig hair, and large fluctuations in water quality. Typical pH ranges from 5.0 to 7.5, and the wastewater may also contain parasite eggs. The daily discharge is approximately 160 m³/d.

 

After pretreatment, the wastewater passes through two contact oxidation tanks in series with an intermediate sedimentation tank, achieving high treatment efficiency. The BOD removal rate of the two-stage contact oxidation process can reach 95%.

 

During renovation, a high-load filter was adopted. A pre-aeration tank was added between the primary sedimentation tank and the first-stage biological filter. The effluent from this tank is diluted with recycled treated water before entering the filter. The process generates significant foam, necessitating 1 hour of pre-aeration before treatment.

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I: Design Water Quality, Quantity, and Treatment Requirements

The designed water quality and quantity are listed in Table 1-1.

 

• The treated effluent must meet the first-grade standards of the *"Discharge Standard for Water Pollutants in the Meat Processing Industry" (GB 13457-1992)*:
• COD ≤ 80 mg/L
• BOD₅ ≤ 30 mg/L
• SS ≤ 60 mg/L
• pH 6.0–8.5
• Animal and vegetable oils ≤ 15 mg/L
• NH₄⁺-N ≤ 15 mg/L
• Coliform group ≤ 5000 counts/L

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 II: Wastewater Treatment Process

a:Process Flow

The wastewater treatment process is shown in Figure 1-1.

 

b: Process Description

Wastewater from the plant passes through a bar screen and hair skimmer to remove large suspended impurities such as meat fragments and pig hair. It then enters a primary sedimentation and oil separation tank for natural sedimentation to remove sand and oils before flowing into the equalization tank. The wastewater is pumped from the equalization tank to an anaerobic tank for anaerobic digestion and then flows by gravity into the contact oxidation tank. After aerobic biochemical treatment, the wastewater enters a secondary sedimentation tank for sludge-water separation. The supernatant is disinfected and discharged after meeting standards.

Sludge from the secondary sedimentation tank is recirculated to the anaerobic and contact oxidation tanks. A small amount of excess sludge, along with sludge from the primary sedimentation and oil separation tank, is discharged into a sludge storage tank for periodic transport and use as fertilizer.

 

c: Process Characteristics

• Anaerobic plus aerobic treatment, a mature process.
• Membrane-based treatment ensures stable efficiency and reduced sludge production.
• The system offers operational flexibility to adapt to changes in water quality and quantity.

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III: Main Structures and Design Parameters

a: Bar Screen and Hair Skimming Pit:

Removes suspended impurities. Underground brick-concrete structure, plan dimensions 4.0 m × 1.5 m, depth 1.5 m.

 b: Primary Sedimentation and Oil Separation Tank:

Removes oils and sand. Plan dimensions 4.0 m × 1.5 m, effective depth 1.5 m, total depth 1.8 m. Underground reinforced concrete structure.

c: Equalization Tank:

HRT = 24 h. Plan dimensions 7.0 m × 8.0 m, effective depth 3.0 m, total depth 3.2 m. Underground reinforced concrete structure.

d: Anaerobic Tank:

HRT = 48 h. Plan dimensions 14.0 m × 5.5 m, effective depth 4.5 m, total depth 5.0 m. Above-ground reinforced concrete structure, integrated with the contact oxidation tank.

 e: Contact Oxidation Tank:

HRT = 24 h. Plan dimensions 10.5 m × 3.5 m, effective depth 4.2 m, total depth 5.0 m. Equipped with filler media. Above-ground reinforced concrete structure, integrated with the anaerobic tank.

f: Secondary Sedimentation Tank:

Surface loading rate 0.65 m³/(m²·h). Plan dimensions 3.5 m × 3.5 m, effective depth 5.0 m. Above-ground reinforced concrete structure, integrated with the contact oxidation tank.

g: Sludge Tank:

Temporary sludge storage. Plan dimensions 7.5 m × 1.0 m, total depth 3.2 m. Underground reinforced concrete structure.

h: Integrated Equipment Room:

Plan dimensions 4.0 m × 5.0 m. Above-ground brick-concrete structure.

I: Clear Water Tank and Discharge Metering Well:

Clear water tank HRT = 1.5 h, plan dimensions 2.5 m × 13.0 m, total depth 1.0 m. Discharge metering well plan dimensions 2.5 m × 1.0 m, total depth 1.0 m. Standardized discharge metering outlet.

 j: Standard Discharge Outlet:

Equipped with standardized metering devices.

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IV: Main Equipment

Main equipment is listed in Table 1-2.

 

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 V: Operational Status

The wastewater treatment project was designed in August 2001, with construction, equipment and pipeline installation, and commissioning completed the same year. The project passed compliance acceptance and commenced operation. All indicators met design standards, ensuring compliant wastewater discharge.

The total investment for the wastewater treatment project was 618,000 CNY, including:

• Equipment investment: 284,500 CNY
• Civil engineering costs: 273,700 CNY
• Other expenses: 59,800 CNY

Wastewater treatment costs are listed in Table 1-3.

 

The annual operating cost of the project is 99,960 CNY, with annual direct operating costs at 62,880 CNY. The unit treatment cost is 2.08 CNY/m³ of wastewater, and the unit direct treatment cost is 1.31 CNY/m³ of wastewater.