FLOW SEGREGATION OPTIONS TO REDUCE EFFLUENT TREATMENT PLANT RUNNING COSTS

Transcription

1 Proceedings of the International Conference on Chemical Engineering 2011 ICChE2011, December, Dhaka, Bangladesh FLOW SEGREGATION OPTIONS TO REDUCE EFFLUENT TREATMENT PLANT RUNNING COSTS Mohidus S. Khan* 1 Deptartment of Chemistry, McGill University, Montreal, Canada H3A 2A7 Alexandra E.V. Evans* International Water Management Institute, 127 Sunil Mawatha, Pelawatte, Battaramulla, Sri Lanka Matthew Chadwick* Stockholm Environment Institute, University of York, UK. Textile production is the largest manufacturing sector in Bangladesh. However, the water consumed and water pollution caused by textile dyeing processes, severely damage the local environment. To reduce the pollution levels a proper effluent treatment plant (ETP) should be installed and efficiently operated to treat the wastewater before it is discharged. High operating costs and lack of technical expertise are major bottlenecks to this. It is further complicated because wastewater from textile dyeing units is rarely uniform through time, due to different dyeing sequences and shades, and can be broadly classified into two types: highly polluted and less polluted. These two streams can be separated at the dyeing machine outlet and treated according to their characteristics. This paper suggests how that could be achieved. We collected wastewater from a semi-automated knit fabric dyeing plant with a maximum daily production capacity of 10 tonnes. The factory has an ETP with physico-chemical and biological treatment units. We analyzed ETP running cost and found major cost heads. We collected wastewater from different stages for different dye shades, analyzed different pollution indicating parameters and found that 31 per cent of wastewater is highly polluted and 69 per cent is less polluted. By separating less polluted wastewater streams and treating them using biological treatment units at controlled ph followed by sedimentation and filtration steps, the factory can reduce chemical consumption in ETP operation and can reduce the running cost by up to 35 per cent (approximately $42,500 per year). Given the total number of textile manufacturing units in Bangladesh, implementing flow segregation could have a considerable impact on the number of ETPs introduced and operated, and thus on the environment. 1. INTRODUCTION Textile production is the largest manufacturing sector in Bangladesh with over 4,500 Ready Made Garment (RMG) units and 3.5 million workers (The Finanacial Express, 2010). In spite of having a positive impact on the Bangladesh economy, the unplanned growth of small and medium scale industrial activities has negative consequences. A textile dyeing industry produces a large volume of effluent each day, which contains unused or partially used organic compounds, strong colour, dissolved solids, high chemical oxygen demand () and biological oxygen demand (BOD) (Ali et al., 2005, Khan et al., 2009, Khan et al., 2006a, Khan et al., 2006b). According to the Bangladesh Environment Conservation Act (ECA 1995) and Environmental Conservation Rules (ECR 1997) textile dyeing industries are categorized as Red industries, and must treat and monitor the wastewater quality to ensure that it conforms to national discharge quality standards (Huq, 2003). To reduce the pollution level a proper effluent treatment plant (ETP) should be installed and efficiently operated to treat the wastewater before it is discharged. However, high installation and operating costs, and lack of technical expertise are major bottlenecks to this. Akthtaruzzaman et al. showed that, the yearly operating cost of an ETP can be higher than the ETP installation cost (excluding land price) (Akhtaruzzaman et al., 2006). Therefore, many industry owners prefer to 187

2 install window-dressing ETP to satisfy international buyers and law enforcement forces but rarely operate them except for show (Khan et al., 2006b). Typically an ETP for a textile industry would consist of physical, chemical and biological treatment (Lash and Kominek, 1980) units to ensure the best performance. A combination of physico-chemical and biological units are most commonly used in textile dyeing industries in Bangladesh (Ali et. al, 2005), although pure physico-chemical plants are also used due to land constraint (Khan et al., 2009). Regardless of the ETP type, most of the textile industries face ETP operational problems due to lack of technical expertise in waste management. Wastewater from textile dyeing units is rarely uniform through time due to different dyeing sequences and shades. The textile dyeing sequence involves several process baths (also known as dyeing stages), such as scouring, neutralization, dyeing, acid wash, softening and rinsing. Wastewater discharged from different stages has different characteristics; therefore, on the basis of pollution load, textile dyeing effluent streams can be broadly classified into: a) highly polluted and b) less polluted effluent streams. These two streams can be separated at the dyeing machine outlet and treated according to their characteristics. This can reduce ETP running costs. In this study we analyzed ETP running costs to identify major cost-heads. We also collected wastewater from a semi-automated knit fabric dyeing plant with a maximum daily production capacity of 10 tonnes which is split between dark shades, medium shades, light shades and bleached white. Wastewater samples were collected from different stages for different dye shades and analyzed for different pollution indicating parameters to identify highly polluted and less polluted effluent streams. Using material balance technique, we calculated the cumulative pollution load of highly polluted and less polluted effluent streams of different shade dyeing. Finally, we analyzed possible cost saving options using effluent flow segregation technique. 2. PROJECT APPROACH In order to address the issues of increasing water pollution from textile industries and inadequate treatment, the project Managing Pollution from Small- and Medium-Scale Industries in Bangladesh was initiated. It was funded by the European Union (EU) Asia Pro Eco Programme and the Department for International Development (DFID, UK) Knowledge and Research Programme, between 2003 and The aim of the project was to reduce pollution while maintaining the profitability of the industries and thereby ensure the incomes of the employees as well as the livelihoods of those who depended on the natural resources that were being impacted. The activities involved cleaner production and improved wastewater treatment. The project was implemented by the Stockholm Environment Institute (SEI), the Bangladesh Centre for Advanced Studies (BCAS) and the University of Leeds, UK. In this component of the project, the team worked with a composite textile industry producing knit fabric to collect effluent samples from each stage of dyeing process. The purpose of the work was three-fold: 1. To analyze the operating cost of an existing ETP to identify the major cost heads. 2. To develop pollution load database for the stages for different dyeing shades and identify highly polluted and less polluted effluent streams. 3. To help textile industries to reduce ETP operating costs by treating highly polluted and less polluted effluent streams according to their characteristics. 3. SAMPLE FACTORY The factory in which the research was conducted undertakes knitting, dyeing and sewing of cotton (usually referred to as a composite textile industry). The dyeing unit of a composite textile industry contributes the major part of the total effluent produced from the manufacturing process. The industry used semi-automated winch dyeing machines with a daily dyeing capacity of 10 tonnes which was split between dark colours (35 per cent), medium shades (15 per cent), light shades (30 per cent) and bleached whites (20 per cent). Winch dyeing involves various stages or unit processes, which include pre-treatment of the fabric, dyeing of the fabric and post-treatment of dyed fabric (Khan et al., 2009; Eckenfelder, 2003). The factory had an existing ETP of 35m 3 /hr capacity with a provision of future expansion. The existing ETP had physico-chemical and biological units as shown in Fig SAMPLE COLLECTION Wastewater samples were collected from winch machine outlets after every stage of the dyeing process. Three sets of samples of process liquor were collected each time the dye bath was emptied (at the beginning, the middle and the end of 188

3 discharging process) and then mixed uniformly to get a representative sample. The samples were preserved below 4 C and transported to the analytical laboratory as quickly as possible to minimize the effect of spontaneous chemical reactions and microbial activity in the samples. Sample testing was undertaken at the Bangladesh University of Engineering and Technology (BUET) and started within 24hrs of collection. 5. MATERIAL BALANCE TECHNIQUE For each stage of dyeing, the following parameters were assessed: ph, total dissolved solid (TDS), total suspended solid (), 5-day biochemical oxygen demand ( ), chemical oxygen demand () and sulphate ( ). All parameters were measured in mg per litre except ph. The effluent quantity discharged from each stage was measured in litres. To calculate the weighted average of any specific parameter, the effluent volume (litre) discharged in a single stage is multiplied by the concentration of that particular parameter in the corresponding effluent water; the result gives the total quantity of that parameter (mg) present in the corresponding stage effluent water. Summing these values for each stage in the dying process gives a cumulative value for that parameter, which can be divided by the total volume of effluent. Dividing the cumulative quantity (mg) by cumulative effluent water generated (litre) gives the weighted average concentration present in the effluent generated by the specific dye shade dyeing. Taking TDS as an example: = Σ([TDS concentration in single stage effluent] [corresponding effluent volume]) Σ(effluent volume generated in single stage) = Σ([mg/L] [L]) Σ(L) = mg L Calculating the average of ph is a little different. At first, the ph values were converted into [H + ] and [OH - ] ions; the cumulative [H + ] and [OH - ] ions were balanced and then the balanced ions were reconverted into ph value. Fig 1: Block diagram of the existing ETP 6. RESULTS AND DISCUSSION 6.1 Running Cost Analysis We calculated the daily running cost of the existing ETP. This included all chemical and electricity consumption but not labour costs. According to the industry management, the treatment cost of one tone, i.e. 1m 3 of effluent water was about BDT The major heads of the operating cost were: a) chemical consumption for physic-chemical units (i.e. coagulation - lime and ferrous sulphate; flocculation -polyelectrolyte), b) chemical consumptions for neutralization units (i.e. hydrochloric acid), c) chemical consumption for biological units (i.e. urea and di-ammonium-phosphate), and d) electricity consumption for all units. From ETP running cost analysis we found that the chemical consumption for physico-chemical units is the major cost head which is 68 per cent of the total running cost but the chemical consumption for biological units was only 1 per cent of the total running cost. 6.2 Effluent Characteristics Analysis Effluent water samples for the dark shade dyeing process were collected from a 400 kg knit fabric dyeing batch. Different stages of dark shade dyeing and their pollution loads are given in Table 1. Pretreatment of fabric included scouring, hot wash with detergent, cold wash and neutralization with acetic acid. In the dyeing stage, electrolyte (globar salt), alkali and other chemicals were added along with different dyes. Post-treatment of dyed fabric required lots of water for rinsing, a cold wash, a hot wash with soaping agent, neutralization with acid, washing with fixing agent and washing with softener. Different volumes of wastewater were generated from each stage (Table 1). Different stages of the medium shade dyeing are shown in Table 2. For medium shade dyeing, effluent water samples were collected from a 189 kg knit fabric dyeing batch. Pre-treatment stages for the fabric were scouring, hot wash with detergent, neutralization with acid wash and wash with enzyme solution. Like, dark shade dyeing, electrolyte, alkali, other chemicals and different colour dyes were used in the medium shade dyeing stage. Post-treatment of dyed fabric required lots of rinsing, an acid wash, a hot wash with soaping agent and a wash with softener (Table 2). 189

4 Wastewater samples for the light shade dyeing process were collected from a 600 kg knit fabric dyeing batch. Pollution indicating parameters and volume of wastewater generated in light shade dyeing process are shown in Table 3. The processing stages were similar to those for medium shade dyeing, except the enzyme wash. Bleached white fabric processing stages were: scouring, wash with soaping agents, neutralization with acid and wash with softeners (Table 4). Wastewater samples for bleached white fabric processing were taken from a 286 Kg knit fabric batch. Fig 2: Breakdown for running cost for a physicchemical and biological ETP 6.3. Identifying Dyeing Stages Generating Highly Polluted and Less Polluted Effluent Water By analyzing the pollution load of different dyeing stages those generating highly polluted effluent streams were identified and are shaded in Table 1 to 4. For dark, medium and light shade dyeing, stages like scouring, hot wash with soaping agent, neutralization or acid wash, enzyme wash, dyeing and softening generate highly polluted effluent streams. Effluent water generated from the rest of the dyeing stages can be considered less polluted due to low,,, and colour. Considering the dyeing plant with a maximum daily production capacity of 10 tonnes which is split between 35 per cent dark shades, 15 per cent medium shades, 30 per cent light shades and 20 per cent bleached white, the composite characteristics of highly polluted effluent streams and less polluted effluent streams, using the material balance approach, are enlisted in Table Flow Segregation Options Table 5 shows that effluent water generated from less polluted stages is 69 per cent of total effluent water generated from the industry. These have a low / ratio, which indicates that the effluent is more amenable biological treatment and need not be chemically treated. Table 6 shows the typical removal rates of pollution by major ETP units. An optimally operated biological treatment unit can remove 50 to 90 per cent of and from effluent [Table 6 (Khan et al., 2009, Tchobanoglous et al., 2003, Crites and Tchobanoglous, 1998)]. Therefore, a well performing biological treatment at controlled ph followed by sedimentation and filtration steps can treat less polluted effluent to within Bangladesh legislation requirements, which is < 150mg/L and < 150mg/L. The highly polluted effluent can be treated using existing physico-chemical units followed by biological treatment units. A well run physico-chemical unit can remove per cent of total (Khan et al., 2009, Tchobanoglous et al., 2003, Crites and Tchobanoglous, 1998) and in combination with a biological treatment unit can remove >95 per cent or the pollution load ETP Running Cost Savings Using Flow Segregation It has been shown that by segregating the flow and treating the two streams differently (i.e. only biological treatment for less polluted streams) the Government of Bangladesh Environmental Quality Standards (GoB 1997) can be met. More importantly for the factories, this can also reduce ETP running costs by reducing chemical inputs and energy use. Using flow segregation option, physico-chemical units will only treat 31 per cent of total effluent water. In simplified approach, it can be predicted that using flow segregation options running cost at physico-chemical units can be reduced upto 69 per cent, which is about 47 per cent (= 69% of 68%, or, ) of total running cost. ETP Running cost correlating with pollution load may give rather accurate cost saving estimation. Consider a single pollution indicating parameter, such as, to make a cost saving assessment on basis of pollution load. is one of the major pollution indicating parameters. If we segregate the effluent water streams as highly polluted and less polluted (Table 5), then the physico-chemical units have to deal with about 57 per cent [= ( ) 100 / ( )] of total load (see Table 5), which is 43 per cent less than before. To simplify the calculation, we assume the chemical dosing in physico-chemical units is a linear function of load; therefore, flow segregation can reduce the chemical consumption of physico-chemical units by up to 43 per cent. From Fig 2, the cost of chemicals consumed in the physico-chemical unit is 68 per cent of the total ETP running cost; hence, flow segregation option can save more than 29 per cent (= 43% of 68% or ) of the total running cost. Considering chemical dosing for physico-chemical units is linearly proportional to any other pollution indicating parameters, such as, sulphate or TDS, flow segregation can reduce the running cost 22, 35 and 29 per cent, respectively. 190

5 Chemical consumption for biological units is only 1 per cent of total ETP running cost; therefore, any additional demand of nutrients for micro-organisms will not increase the ETP running cost significantly. Doubling the chemical consumption cost in biological units will only increase the ETP running cost by 1 per cent. Acid consumption at neutralization unit may reduce due to less volume effluent water at physico-chemical units; however, the reduced amount can be balanced due to possible high demand to control ph for biological units. Hence, chemical consumption at neutralization units may not fluctuate considerably. Considering a safety factor at neutralization unit and biological units, flow segregation can reduce the operating cost by about 20 to 35 per cent, which is BDT 4.3 to 7.5 per m 3 of effluent treated. When running at maximum production capacity, the industry produces 1,277 m 3 of effluent per day. The flow segregation option can save ETP operating costs of about BDT 5,500 to 9,500 per day. This number becomes more significant when we consider on yearly basis (340 working days a year): BDT 1.87 to 3.25 million taka a year, which is equivalent to USD $24,500 to $42,500 (1 USD ~ 76 BDT). However, the above cost assessment does not consider any design modification cost of the ETP plant. To perform flow segregation, the industry has to install: a) separate channels to carry highly polluted and less polluted effluent streams to the treatment plant, b) separate equalization tanks (or to split the existing equalization tank into two chambers), and c) ph controlling units before chemical dosing and biological reactors to ensure an optimum treatment environment. Moreover, the industry management has to train production workers to discharge effluent from dyeing units according to pollution strength. Most of the necessary modifications are engineering modifications; the modification and worker training cost can be recovered in few months from the operating cost savings. It is also important for the industries to conduct jar test to accurately calculate chemical consumptions and cost saving. Table 1: Characteristics of Dark Shade Dyeing Stages of a Semi-Automated Winch Dyeing Machine Amount of fabric: 400 Kg Stages Volume of Scouring Hot wash Cold Wash Neutralization Dyeing Rinsing Cold Wash Hot Wash with Soaping Agent Rinsing Rinsing Neutralization Rinsing Wash with Fixing Agent Softening Table 2: Characteristics of Medium Shade Dyeing Stages of a Semi-Automated Winch Dyeing Machine Amount of fabric: 189 Kg Stages Volume of Scouring Hot wash Neutralization (Acid wash) Wash with Enzyme solution Dyeing Rinsing Rinsing Acid wash Rinsing Hot wash with soaping agent Hot wash Rinsing Softening

6 Table 3: Characteristics of Light Shade Dyeing Stages of a Semi-Automated Winch Dyeing Machine Amount of fabric: 600 Kg Stages Volume of Scouring Hot wash Neutralization (Acid wash) Dyeing Rinsing Acid wash (50 0 C) Rinsing Hot wash Rinsing Softening Table 4: Characteristics of Bleached White Fabric Processing Stages of a Semi-Automated Winch Dyeing Machine Amount of fabric: 286 Kg Stages Volume of Scouring Soaping Neutralization Softener Wash Table 5: Composite Characteristics of Effluent Generated in 24 Hours Daily Capacity of Dyeing (maximum): per cent Dark Shade, 15 per cent Medium Shade, 30 per cent Light Shad, 20 per cent White Parameters Unit Effluent from all stages Highly polluted effluent streams Less polluted effluent streams BD Standards (Huq, 2003) Volume of water (m 3 ) ton Percentage of total volume per cent ph* TDS* mg/l * mg/l * mg/l ** * mg/l : * Sulphate* mg/l *Weighted averages **BOD limit of 150 mg/l implies only with physico-chemical processing Table 6: Typical removal rates of pollution load by major ETP units (Khan et al., 2009, Tchobanoglous et al., 2003, Crites and Tchobanoglous, 1998) Component Constituent Removal rate (%) Fine screens 5-55 Physico- Chemical: BOD Depending on type and detention time Biological: Trickling filters 0-90 Activated sludge BOD and Final effluent quality 10 mg/l Anaerobic CONCLUSION This study has shown that there is both an economic and environmental advantage to managing treatment for textile dyeing wastewater according to its chemical strengths. If industries have access to laboratories they can relatively easily identify the least polluting loads that could bypass some of the treatment steps. They would then have to implement physical changes to the ETP and wastewater channels and changes in operating practice i.e., how factory floor workers disposed of waste baths and ETP operators managed the system. This requires a willingness on the part of the factory owners/managers and floor 192

7 staff. The managers have the incentive of reduced costs but the incentives are not so obvious for staff. That said, the change in practice is minor e.g. pulling a lever and therefore need not be onerous. The calculated cost savings of up to 35 per cent of the cost of treatment could easily cover physical alterations and training. Two problems can be envisaged. One is that at present factories do not regularly operate their ETPs so this saving is only theoretical. Buyers or authorities must encourage or enforce operation and suggesting ways of bringing down costs to within more acceptable ranges may appeal to textile manufacturers. The second problem is that factory workers may accidently release highly polluted wastes into the less polluted stream. It can be envisaged that this would cause spikes in the pollution load leaving the factory but on the grand scale is immaterial because treatment is currently so limited and because one or two accidental releases would be well diluted by the less polluted effluent streams. 8. ACKNOWLEDGEMENT The authors would like to thank Dr J. Knapp, Leeds University, UK and Dr M. Sharif, BCAS for their valuable technical suggestions in the project activities. 9. REFERENCES 1. Akhtaruzzaman, M., Clemett, A., Knapp, J., Mahmood, M. and Ahmed, S. (2006), Choosing an Effluent Treatment Plant, Dhaka. 2. Ali, M.S., Ahmed, S. and Khan, M.S. (2005), Characteristics and Treatment Process of Wastewater in a Nylon Fabric Dyeing Plant, Journal of Chemical Engineering, IEB ChE, 23, pp Crites, R. and Tchobanoglous, G. (1998), Small and Decentralized Wastewater Management Systems. McGraw-Hill College. 4. Eckenfelder, W.W. (2000), Industrial Water Pollution Control, 3rd ed., McGraw-Hill, New York. 5. Huq, M.E. (2003), A Compilation of Environment Laws Administrated By the Department of Environment, Department of Environment and Bangladesh Environmental Management Project (BEMP), Dhaka. 6. Khan, M.S., Ahmed, S., Evans, A.E.V. and Chadwick, M. (2009), Methodology for Performance Analysis of Textile Effluent Treatment Plants in Bangladesh, Chemical Engineering Research Bulletin, 13, pp Khan, M.S., Knapp, J., Clemett, A. and Chadwick, M. (2006), Improving Effluent Treatment and Management, in: M. Chadwick (Ed.), Managing Pollution from Small-Scale Industries in Bangladesh, Research for Development, Department for International Development (DFID), UK, pp Khan, M.S., Knapp, J., Clemett, A., Chadwick, M. and Mahmood, M. (2006), Managing and Monitoring Effluent treatment plants, Dhaka. 9. Lash, D.L. and Kominek, E.G. (1980), Primary-Waste-Treatment Method, in: Cavaseno, V. (Ed.), Industrial Wastewater and Solid Waste Engineering, McGraw-Hill, New York, pp Tchobanoglous, G., Burton, F.L. and Stensel, H.D. (2003), Wastewater Engineering: Treatment and Reuse, 4th ed., McGraw-Hill, New York. 11. The Financial Express (2010), Full blown RMG violence at Ashulia, Dhaka. 193