Project Overview
Project Introduction
A certain group in Jiangsu is a comprehensive national-level enterprise ranked among the top 500 township enterprises, with operations spanning textiles, dyeing and finishing, chemicals, machinery, building materials, and trade. In 2003, the company commissioned Donghua University to design an integrated wastewater treatment plant with a daily capacity of 15,000 tons. After more than three years of operation, the plant met the Grade I standards of China's Integrated Wastewater Discharge Standard (GB8978-1996).
However, in 2007, a large-scale cyanobacterial bloom occurred in Taihu Lake, severely impacting normal production and daily life in the Taihu Basin. In response to the State Council's requirements for strengthening water environmental protection in the region, wastewater treatment plants within the Taihu Basin were mandated to enhance effluent quality, particularly in terms of nitrogen and phosphorus removal. Jiangsu Province subsequently issued the *DB32/1072-2007* standard for major water pollutant discharges from urban wastewater treatment plants and key industrial sectors in the Taihu region, which took effect on January 1, 2008.
The requirements of *DB32/1072-2007* are detailed in Tables 2-5 and 2-6.
Under this standard, treated effluent from the textile dyeing and finishing industry must meet the following limits:
- Chemical oxygen demand (COD): 50 mg/L
- Ammonia nitrogen (NH₃-N): 5 (8) mg/L
- Total nitrogen (TN): 15 mg/L
- Total phosphorus (TP): 0.5 mg/L
For the textile dyeing and finishing industry-a heavily polluting sector with high water consumption-this represents an extremely stringent standard. Many enterprises faced significant challenges, with some even confronting survival crises. Consequently, numerous textile and dyeing enterprises in Jiangsu's Taihu Basin initiated upgrades to existing wastewater treatment plants or constructed new facilities to comply with the new regulations.
Energy-Saving and Emission-Reduction Design Plan
Given current energy-saving and emission-reduction requirements, merely retrofitting wastewater treatment plants is insufficient. Upgrades must incorporate source control, cleaner production practices, and comprehensive resource utilization. Based on the group's specific conditions, this plan proposes the following measures:
① Wool Textile Wastewater: Improve the recovery rate of lanolin from high-concentration, high-turbidity wool-scouring wastewater to reduce the burden on wastewater treatment.
② Water Reuse: Recycle purified wool-scouring wastewater to reduce freshwater consumption, steam usage, and chemical dosing, achieving integrated water and thermal energy utilization for energy savings and emission reduction.
③ Desizing Wastewater Source Control: Recover relatively pure polyvinyl alcohol (PVA) sizing agents from desizing wastewater as raw materials. Concentrated composite sizing agents can be repurposed as fuel for thermal energy. The separated liquid from recovery can be reused in desizing baths, reducing alkali consumption, energy use, and pollutant discharge.
④ Enhanced Biofilter Performance: Reoptimize biofilter design parameters to ensure effluent meets the stringent Grade 1A standards of Jiangsu's *DB32/1072-2007* (see Table 2-6).
Design Scale and Objectives
Design Water Volume
According to the group's development plan, the focus lies on the dyeing and finishing (bleaching and dyeing workshop). Based on the existing three production lines, three additional production lines will be expanded in 2004, ultimately achieving the goal of ten production lines.
Total water volume: Q(total) = 15,000 m³/d, Q = 625 m³/h.
Breakdown:
- Fine wool textile wastewater: 3,000 t/d
- Knitting and dyeing wastewater (pure cotton, cotton-polyester wide-width): 2,000 t/d
- Dyeing and printing wastewater (pure cotton, cotton-polyester wide-width): 9,000 t/d
- Wool scouring wastewater (Australian wool): 400 t/d
- Domestic sewage: 600 t/d
Influent Water Quality
The Phase I project of the group primarily receives four types of wastewater: fine wool textile wastewater, dyeing and printing wastewater, knitting and dyeing wastewater, and domestic sewage. The water quality characteristics of these four wastewater streams are detailed in Table 2-7.
Treatment Objectives
The wastewater treatment plant complies with the Grade I standards of China's Integrated Wastewater Discharge Standard (GB8978-1996, see Table 2-8). For certain specific indicators, the stricter discharge limits outlined in Jiangsu Province's *DB32/1072-2007* (Major Water Pollutant Discharge Limits for Urban Wastewater Treatment Plants and Key Industrial Sectors in the Taihu Lake Region) are adopted (see Table 2-9).
These discharge standards were finalized upon approval by the environmental protection authorities.
Treatment Process Research
Treatment Process Study
The Phase I project of the group primarily handles textile dyeing and finishing wastewater. Based on long-term research by Donghua University on various textile dyeing and finishing wastewater qualities, it was identified that the group's fine wool textile wastewater includes wool scouring wastewater. Although the extraction of lanolin from the first, second, and third scouring tanks significantly reduces CODcr and BOD, the final mixed wastewater still maintains a CODcr level exceeding 3.2×10³ mg/L. After pretreatment via chemical coagulation flotation or sedimentation, approximately 90% of COD can be removed, reducing CODcr to around 4,000 mg/L. However, when this wastewater mixes with fine wool textile wastewater, the CODcr increases from 600–800 mg/L to 1,200–1,500 mg/L.
The dyeing and finishing wastewater primarily consists of desizing wastewater with CODcr levels as high as several thousand to tens of thousands of milligrams per liter, along with elevated pH and poorly biodegradable PVA. According to data provided by the client, the dyeing and printing wastewater exhibits CODcr levels above 2,500 mg/L. In contrast, mercerizing wastewater has a relatively low CODcr (200–400 mg/L) due to alkali recovery, resulting in an overall pH of approximately 11 for the group's dyeing and finishing wastewater.
The original design complied with China's Integrated Wastewater Discharge Standard (GB8978-1996), which set the CODcr discharge limit at 100 mg/L. However, the new Jiangsu Provincial standard requires reducing CODcr from 100 mg/L to 60 mg/L. Achieving an additional 40 mg/L reduction in CODcr for textile dyeing wastewater under low-substrate conditions is highly challenging. By comparison, municipal wastewater treatment plants only need to reduce CODcr from 60 mg/L to 50 mg/L under the same standard-a mere 10 mg/L decrease-and municipal wastewater is far more biodegradable. Therefore, merely retrofitting the existing wastewater treatment plant is insufficient. Source control must be prioritized, and advanced treatment processes must be enhanced (e.g., redesigning biofilter parameters).
The group's energy-saving, emission-reduction, and wastewater treatment retrofit plan includes the following key measures:
① Wool Textile Wastewater: Improve lanolin recovery rates from high-concentration, high-turbidity wool scouring wastewater to reduce the treatment burden. By optimizing centrifugal separation parameters, lanolin recovery can be increased from 40–45% to 45–50%. Strict control of grease and sand content in scouring tanks (Table 2-10) further enhances recovery efficiency.
② Water Reuse: Recycle purified wool scouring wastewater to reduce freshwater consumption, steam usage, and chemical dosing, achieving integrated water and thermal energy utilization. For instance, sand-removed water from the first scouring tank can be reused in the same tank, increasing lanolin concentration for easier extraction and reducing centrifugal separation energy costs.
③ Desizing Wastewater Source Control: Recover relatively pure PVA sizing agents via ceramic membrane technology for reuse as raw materials. For composite sizing agents, high-temperature (90°C) ceramic membrane concentration allows recovery as fuel. The separated clear liquid can be recycled into desizing baths, reducing alkali (caustic soda) and steam consumption while significantly cutting pollutant discharge.
④ Biofilter Optimization: Upgrade biofilter media from traditional ceramic particles to cost-effective, high-porosity lightweight materials, increasing adsorption filtration rates to 6 m/h. This reduces effluent CODcr from 80 mg/L to below 60 mg/L, ensuring compliance with the new standards.
Treatment Process Flow
The retrofitted process retains the original workflow (Figure 2-1) but operates under improved influent conditions: CODcr concentrations and pH are significantly lower, eliminating the need for acid dosing and reducing operational costs.
- Wool Scouring Wastewater: After resource recovery, CODcr drops from 4,000 mg/L to 2,000 mg/L (50% reduction), allowing fewer aeration blowers to operate.
- Desizing Wastewater: Recycling 300–500 t/d of high-concentration desizing wastewater reduces the CODcr of 9,000 t/d dyeing wastewater from 2,500 mg/L to 1,200 mg/L (over 50% reduction).
Treatment Efficiency Forecast
Post-retrofit treatment efficiencies for each unit are summarized in Table 2-11.
Project Investment Estimate
The investment breakdown is provided in Table 2-12.
Economic Benefit Analysis
① For wool textile wastewater, particularly high-concentration, high-turbidity wool scouring wastewater, improving lanolin recovery rate reduces the burden on wastewater treatment. By optimizing technical parameters of the existing wool scouring wastewater recycling process, the centrifugal separation lanolin recovery rate can be increased from 40-45% to 45-50%. With lanolin priced at 12,000 yuan/ton and the factory originally producing 50-60 tons of lanolin monthly, the 5-10% increase in recovery rate translates to an additional 3-6 tons of lanolin recovered per month. Assuming a monthly increase of 5 tons, annual lanolin sales revenue would increase by approximately 720,000 yuan.
② Increasing the reuse of purified wool scouring wastewater in the treatment process reduces production water consumption, steam usage, and chemical dosage, achieving comprehensive utilization of water and thermal energy for energy conservation and emission reduction. By recycling sand-removed water from the first scouring tank back to the same tank, the lanolin concentration increases, improving extraction efficiency while saving energy for centrifugal separation. In terms of steam savings, the recycling of scouring liquor can reduce thermal energy consumption. The factory's average monthly steam consumption is 726 tons, with water usage at 5,100 tons. With a 70% wastewater reuse rate, 3,570 tons of scouring wastewater can be recycled monthly. The scouring water temperature ranges from 45-58°C, and the process upgrade is expected to save 2 tons of steam daily. At 148 yuan per ton of steam, this amounts to monthly savings of 60 tons (8,880 yuan) and annual steam cost reduction of 106,700 yuan. Daily reuse of 100 tons of scouring wastewater saves 3.8 yuan per ton (2.2 yuan for water + 1.6 yuan for discharge fees), resulting in annual water savings of 33,000 tons and additional benefits of 125,400 yuan.
③ For desizing wastewater, implementing source control by recovering relatively pure PVA sizing agents as raw materials and utilizing concentrated composite sizing agents as fuel achieves thermal energy recovery. The clarified liquid from the separation process can be reused in desizing baths, saving chemicals (alkali) and energy while significantly reducing pollutant discharge. The factory has two production lines for desizing, with alkali consumption of 2 tons per 10,000 meters of fabric. Monthly production exceeds 3 million meters, requiring over 600 tons of alkali. At 1,000 yuan/ton, alkali usage can be reduced by 20-50%. Assuming 20% savings, this translates to monthly alkali cost reduction of 120,000 yuan and annual raw material savings of 1.44 million yuan.
Feasibility Analysis of Energy Conservation, Emission Reduction, and Compliance Discharge
Through process improvements and wastewater pretreatment measures, the influent CODcr concentration can be reduced from 1,800 mg/L to 1,200 mg/L (30% reduction), alleviating the load on the wastewater treatment system and creating conditions for further optimization of effluent quality. To meet the new discharge standards, upgrades to the wastewater treatment system are necessary. By adjusting the design parameters of the biological filter, the final effluent CODcr concentration can be reduced by 25%, achieving the goal of total CODcr emission reduction with significant environmental benefits.
The system also achieves emission reduction and water conservation through wastewater recycling, alleviating water resource scarcity while reducing corporate water costs.
The process improvements substantially reduce pollution at the source, recover resources, enable efficient material utilization and resource regeneration, promote cleaner production and circular economy practices for the enterprise, delivering strong social benefits while generating considerable economic returns.