Reducing effluent discharge
Developing a closed system for the treatment of industrial wastewater is a worthy but unrealistic goal. John Smith at Water Management looks at some more viable alternatives
If one was to design a surface finishing plant from scratch a number of features could be incorporated to reduce waste and effluent. However the initial capital costs of the installation will be higher than a conventional design.
The process could be designed to maximise the benefits of various technologies:
- metal recovery from dedicated streams by concentration, ion exchange or deposition,
- effluent elimination by the use of multi-tank counterflow rinsing back into the process tank combined with evaporation. This may result in production problems due to the build-up of impurities,
- recycling of high-quality rinse waters via ion exchange or membrane processes,
- batch treatment of waste solutions combined with liquids/solids separation
and polishing technologies.
With an existing site, which may have evolved over several years, we are faced with quite a different scenario and may have to deal with:
mixed processes and effluent streams,
- fixed equipment and processes, the alteration of which will increase the cost of implementing recycle schemes.
For instance, what would be the impact of collecting all the rinse waters and passing them through a de-ionisation process? If dragout reduction and contamination of the recycled rinse had not been considered, the cost of running the Demin plant could outweigh the potential savings made by water and effluent recycling.
With an existing surface finishing plant or process, a number of factors could be considered. The first stage when investigating the cost/benefit of recycling is to reduce the volume of rinse/wash water to a minimum, as this will have a major impact on reducing the capital required for any recycled wastewater treatment plant.
With regard to water flow, consider how low can you go. Consideration of multi-stage counterflow rinses may have a major engineering and downtime impact, as may the installation of static dragout baths. Both produce a concentrated effluent and, depending on the process, may cause problems in the effluent treatment stage.
Having completed the first stage and achieved a reduction in rinse/wash water flow rates, the second stage is to identify water flow and quality required at specific locations. This analysis will have a major impact on the processes employed, operating cost, capital spend and space required. It will also indicate whether a phased approach to recycling would be advantageous in order to spread capital spend and reduce operating costs.
There are various processes that can be employed in recycling. However, it is unwise to do away with the existing effluent plant as there will be some effluent to dispose of. Unless a duplex recycling plant is installed, downtime will also occur as a result of maintenance, regeneration, back washing or even breakdown.
Processes used will be dependent on the volumes and water qualities required. Unless you want to re-circulate final effluent direct from the clarifier outlet, and risk the inclusion of suspended solids, the simplest form of treatment is filtration. This would not only act as the first stage of recycle treatment, but coupled with polishing precipitation could reduce effluent total metal levels to less than 1ppm.
The most cost-effective installation is probably a back-washable sand filter,
the installed cost of which can be in the order of:
£2,000.00/m³/h at 1-2m³/h
£1,300.00/m³/h at 20-30m³/h
Dependant on the amount of grey water which can be recycled, this represents
a potential for rapid payback as well as major improvements in final effluent
quality. For example, 5m³/h effluent with a potential to recycle 2m³/h
to non-critical applications operating 16h/ day, five days per week. Cost of
filter = £9,000.00. Reduction in water and effluent costs = £9,200.00.
For water that is free from all suspended solids, finer filters can be installed subject to requirements.
Foaming can become a problem with recycled grey water. This can be overcome by modifying the effluent treatment process and/or the installation of an activated carbon filter for the removal of organics. This would also be required if further purification processes are to be installed in order to protect ion exchange resins or membranes from organic fouling and oxidation by chlorine.
Having decided on the size of filter and the volume of grey water which can be used, there are a number of other options which can be considered. However, unless the plant is operating in excess of 100-120h/per week, payback on further purification processes will probably be in excess of two-and-a-half years.
Subject to the nature of the dissolved salts, gasses, organics, etc, it may be possible to provide chemical treatment to correct the water quality. For example, by pH adjustment, removal of dissolved gases, etc.
If the ionic loading (conductivity) is low, ion exchange may be considered. The configuration of the plant process will depend on the qualities of water required. However, some careful monitoring and calculations will need to be done relating to specific site conditions to arrive at an accurate cost/benefit analysis that reflects the variability of effluent content. These plants require chemical regenerants, use mains water for regeneration and require regenerant effluent to be treated.
The third option is the utilisation of membrane processes. The advantages are that regenerant chemicals are eliminated. These are continuous processes and are not greatly affected by changes in ionic loading. The specific membrane process selected will again be driven by the water quality required.
Membrane processes produce two streams from the incoming filtered effluent. The permeate (treated water stream) can be up to 80% of the input flow and, dependant on the specific process employed, can produce demin/distilled water quality.
The concentrate, which equals 20%-35% of the input, contains 95% of all the
impurities at approximately 3-5 times the initial effluent concentration. This
can be returned to the effluent treatment or preferably polished, filtered and
discharged to drain.
Another interesting application for membrane processes is in concentration for metal recovery. If after nickel (Ni) plating, a flowing rinse containing 600ppm Ni is passed through a reverse osmosis (RO) plant, the Ni content in the concentrate stream increases to 3,256ppm. The concentrate can then be used either directly back into the plating solution or taken to Ni recovery. However, because the concentrate is only 20% of the rinse flow, the recovery plant will probably need to be smaller and with the higher Ni content, may work more efficiently. In addition, the permeate stream can be recycled as high-quality water.
Perpetual motion does not exist as, if we recycle, we will use water on filter back washes and demin regenerations. We will also need to remove water to drain, to avoid the build-up of soluble components such as organics, either by continuous bleed or via reverse osmosis concentrate.
It is possible to economically achieve an 80-90% reduction in water use and
effluent discharge for plants operating at 100h/per week. However, careful analysis
of requirements will need be carried out in order to determine the most appropriate
and cost-effective processes. Therefore, plant manufacturers, who have an interest
in selling their own equipment, may not offer the best and most unbiased advice