Case Study of Wastewater Treatment in a Starch Enterprise

Case Study of Wastewater Treatment in a Starch Enterprise

Overview

A starch company in Zhejiang employs an Anaerobic + A/O Process + Activated Carbon Adsorption Process for wastewater treatment. This company primarily produces carton adhesives and wheat starch, generating wastewater during production. Located in the Taihu Lake Basin, the company must comply with the State Council’s approval document *Guo Han [1998] No. 2, "The State Council's Reply on the 'Ninth Five-Year' Plan and 2010 Plan for Taihu Lake Water Pollution Prevention and Control"*. Wastewater must be treated to meet standards before discharge.

1. Design Water Quality, Quantity, and Treatment Requirements

Based on data provided by the enterprise, the influent water quality of the wastewater treatment station is:

• Flow rate: Q = 30 m³/d
• BOD₅: 5,000 mg/L
• pH: 0
• SS: 2,000 mg/L
• COD: 10,000 mg/L
• Chroma: 256 times

According to Taihu Basin requirements, the treated effluent must meet the Grade I standard of the *Integrated Wastewater Discharge Standard (GB 8978-1996)*:

• COD ≤ 100 mg/L
• BOD₅ ≤ 30 mg/L
• SS ≤ 70 mg/L
• pH: 6–9
• Chroma ≤ 50 times

2. Wastewater Treatment Process Flow

Based on the wastewater quantity, quality, current treatment practices for such wastewater in China, and pilot test results, the treatment process is determined as shown inFigure 1-1.

As illustrated in Figure 1-1, wastewater passes through a bar screen and flows into an equalization and pre-aeration tank. Alkali is added for neutralization, and moderate aeration ensures complete mixing. The wastewater is then pumped to a primary sedimentation tank to remove most suspended solids. It subsequently flows into an anaerobic tank and an A/O tank, where anaerobic, anoxic, and aerobic bacteria degrade most organic matter. The effluent from the aerobic tank flows into a secondary sedimentation tank for sludge-water separation. Depending on the water quality, the effluent may be further treated through activated carbon adsorption before metered discharge, or directly discharged after metering. Most of the sludge from the sedimentation tank is recycled to the A-stage of the A/O tank, while a small amount of excess sludge, along with sludge from the primary sedimentation tank, is transported after drying in sludge drying beds.

3. Main Structures and Design Parameters

1. Screen Chamber: Two bar screens with a spacing of 10 mm are installed in the inlet channel.
2. Equalization Tank: HRT = 6 h, plan dimensions 5.0 m × 2.0 m, effective depth 2.5 m, total depth 3.1 m, underground reinforced concrete structure.
3. Primary Sedimentation Tank: Surface loading rate 0.8 m³/(m²·h), HRT = 2 h, plan dimensions 2.5 m × 2.5 m, effective depth 1.6 m, total depth 4.0 m, above-ground reinforced concrete structure.
4. Anaerobic Tank: HRT = 3.0 d, effective volume = 90 m³, effective depth 4.0 m, total depth 4.5 m, equipped with filler, above-ground reinforced concrete structure.
5. A/O Tank: HRT = 36 h (A-stage: 8 h, O-stage: 28 h), plan dimensions 5.0 m × 2.6 m, effective depth 3.5 m, total depth 4.5 m, equipped with filler, above-ground reinforced concrete structure.
6. Secondary Sedimentation Tank: Vertical flow sedimentation tank, HRT = 2.0 h, surface loading rate 0.8 m³/(m²·h), plan dimensions 2.5 m × 2.5 m, effective depth 1.6 m, total depth 4.5 m, reinforced concrete structure.
7. Adsorption Tank: Plan dimensions 2.5 m × 2.5 m, effective depth 0.6 m, reinforced concrete structure.
8. Discharge Metering Well: Plan dimensions 1.0 m × 0.5 m, equipped with one electromagnetic flowmeter.
9. Sludge Drying Bed: Plan dimensions 5.0 m × 5.0 m, total depth 1.0 m, divided into two alternating cells, brick-concrete structure.
10. Control Building: Plan dimensions 6.6 m × 6.0 m, housing two blowers, one control cabinet, and one operation room.

4. Main Equipment

As listed in Table 1-1.

5. Operational Performance and Treatment Costs

Treatment efficiency is shown in Table 1-2. The total project investment is 324,250 CNY. Wastewater treatment costs are detailed in Table 1-3.

The annual operating cost of this wastewater treatment project is 45,100 CNY, with annual direct operating costs of 22,400 CNY. The unit treatment cost is 5.01 CNY/m³ of wastewater, and the unit direct treatment cost is 2.49 CNY/m³ of wastewater.