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


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Providing a closed system with no losses to effluent or waste is an objective

we probably all seek. However, to my knowledge perpetual motion does not exist,

and the elimination of all waste or effluent is not economically possible. At

some point waste is produced somewhere.

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.

Retro-fitting

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.

effluent purification

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.

Economical benefits

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

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