CHAPTER 4 CASE STUDIES
4.5 COMPANY D
• a reduction in chemicals and metals to drain,
• a reduction in volume of effluent discharged, and
• a reduction in energy use.
Rinse
Figure 4.6: Flow diagram of main zinc line
Figure 4.7: Flow diagram ofscrewdriver plating process
4.5.2 Water Consumption
The factory had two sources of water, namely the municipality and the Umhlatuzana River, which runs behind the factory. The water from the municipality was used at the Chrome and RDI plants.
The water from the river was filtered and then pumped to the Zinc, Screwdriver and General Plating sections. Water was used mainly for rinsing purposes. Initially the company had one water meter, but water consumption was not monitored. The advantage of using the water from the river was that the company did not pay for the supply of this water, but was charged for it on discharge to sewer.
Prior to the implementation of waste minimisation the company consumed an average of 2 100 kL/month of water at a cost of approximately R 5 200 per month.
4.5.3 Chemical Consumption
The company used a variety of chemicals for its processes (see Appendix 3). A number of the chemicals have proprietary names, therefore the constituents are unknown. The company did not disclose the cost of their purchased chemicals.
4.5.4 Energy Consumption
The company was not a large consumer of energy. In the plating departments, energy was used mainly by a drying oven (for the drying of plated articles), and heated tanks. The rectifiers on site drew electricity constantly.
4.5.5 Wastes Generated Air Emissions
Cyanide and acid containing dust mists are the primary sources of air pollution.
Effluent
Rinsewater formed the largest proportion of the daily effluent generated by the company. Since rinsing serves to remove and dilute the dragout of chemicals and metals from baths, the effluent stream therefore became concentrated with these components. Copper, nickel, cadmium, silver and zinc were the metals that were expected in this stream. The effluent was treated with sodium metabisulphite (Na2S203 or MBS) for the reduction of hexavalent chrome to trivalent chrome, and sodium hypochlorite (NaOCI) to eliminate cyanide. Caustic soda or soda ash (Na2C03) was added to adjust the pH.
The quantity of effluent produced and the associated cost prior to implementing waste minimisation is shown in Table 4.5.
Table 4.5: Effluent Quantity and Costs Cost of effluent (R/kL)
Quantity ( kL/month)
Monitoring Charge (R/month)
Total cost (Rlmonth)
0.50 6692 431 3923
Effluent tariffs subsequently increased thus making the need for reducing the amount of effluent generated more urgent.
4.5.6 Waste Minimisation Programme
The company's waste minimisation programme began when it joined the club in June 1998. A commitment to implementing waste minimisation was made by the owner of the company, who was also the project champion. The company's main reasons for joining the club were to comply with new regulations, learn about waste minimisation, and achieve savings. However, the owner was initially sceptical of the benefits of waste minimisation and was therefore not very enthusiastic at the beginning. His views subsequently changed following the study tour to Denmark (discussed in Chapter 3, section 3.14). It was after this that serious effort was put into the waste minimisation programme.
Problems Identified
During the initial assessment, the following areas were chosen for investigation:
• Water consumption,
• Chemical consumption,
• Reducing metal concentrations to drain,
• Energy use, and
• Effluent control.
Water consumption was high throughout the factory. The company had one meter, which was not monitored regularly. There were no restrictions on the flowrate of water used for rinsing. Dosing of process tanks with chemicals took place manually. There was limited control over the use of chemicals, and as a result an excess of chemicals was being added to the tanks. This not only represented a waste in chemicals leading to higher consumptions than necessary, but also to increased metal concentrations in the effluent. Improved effluent control was important for the company to comply with new regulations. The company also spent between R 12 000 and R 15 OOO/mth on cleaning agents, which also led to effluent problems. While electricity consumption was not high at the company, it was wasted by the rectifiers on site, which drew electricity continuously even when not in use.
Waste Minimisation Options
Based on the problems identified, a total of twenty-five waste minimisation options were generated during the waste minimisation audit and by the owner of the company. As at November 2000, the following improvements were made by the company:
• Water Consumption:
The company installed a further seven water meters, and began monitoring water consumption daily. This enabled any problems to be identified immediately. Rinsewater flowrates were adjusted to the minimum rate required for adequate rinsing. Additional dragout tanks were installed in process lines. Counterflow rinsing was introduced to processes thus reducing the water requirements of the factory.
• Chemicals and Metals:
A storeman was employed to control the quantities of chemicals being added to baths. This resulted in a saving of 10 to 15 % in chemical costs. An automatic crane was installed over the chrome bath thus allowing the jobs to be lifted out of the bath and drained at a slower rate.
Process chemicals that were previously being carried over to the rinse water were being returned to the process bath. This resulted in a saving of 25 kg/month of Chrome. This in turn reduced the quantity of sodium metabisulphite required to treat the chrome in the effluent. The company managed to double the life of its cleaning solutions. Old cleaning solutions were being emptied into another tank, and new solutions were prepared. The jobs being processed were first dipped into the old cleaners and then the new ones. Increasing the drip times over process baths decreased losses of chemicals and metals through dragout on all lines. In addition, the orientation of jobs on jigs was improved to allow for maximum drainage of jobs, thus further reducing dragout, and saving chemicals.
The cyanide zinc plating process was replaced with the alkaline zinc process. This process costs the same to operate and eliminates cyanide from the effluent. The reduction of chemical consumption led directly to a reduction of metals going to drain, and the amount of chemicals required for effluent treatment.
• Energy Use:
Rescheduling of the oven used for drying of jobs resulted in a reduction in use from eight to three hours per day. The installation of thermostats on hot baths facilitated the monitoring of energy use and ensured that the baths operate at the minimum temperature required. Further reductions in energy use were achieved by switching off the rectifiers when not in use.
• Effluent Treatment
An effluent treatment plant was built on site to comply with legal discharge limits. Flocculants were added to aid in the settling of precipitated solids. Solids were effectively removed by filtration. Samples of effluent were taken by the local authority for compliance tests and the
effluent was either passed for discharge to sewer or not. Due to the reductions in water and chemical consumption, and chemical loss to drain, a smaller effluent plant was required to treat the factory's effluent.
4.4.7 Savings Achieved
The savings achieved from the implementation of the above options as at November 2000 is given in Table 4.6
Table 4.6: Savings Achieved Item
Water
Chemicals and Metals Energy
Effluent charges
Annual Saving (R/y) 59400 68400 33000 36500
.. ···_w·_ .. _ _ _ _ • _ _ .• _.,, __ •· •• __ ....••.•• " ... ____ ... ______ ._. __ .... ___ _
Total 197300
4.5.8 Environmental Benefits
Pay-back Irrunediate Irrunediate Irrunediate Irrunediate
Accompanying the substantial financial benefits to the company were also significant benefits to the environment. These were:
• a reduction in water consumption by 3 900 kL/mth,
• a reduction of 275 kg/y of chrome discharged to drain,
• a reduction in zinc metal to drain from 50 to 60 ppm to less than 25 ppm,
• the elimination of cyanide use,
• a reduction in the use of cadmium,
• a reduction in the volume of waste discharged, and
• a reduction in the use of electricity.