Filtration + Separation - Elsevier Ltd

Filtration + Separation - Elsevier Ltd

Saving energy no uphill task with mountain water

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Even in regions where it is abundant, water for human consumption must be treated before it is fit for use. Water sources in areas with karsts, crevasses and fissured rocks are frequently plagued by high turbidity and bacterial content. In these circumstances, membrane filtration represents an efficient method of treatment.

In mountainous regions, for example, when using pressure-driven membrane filtering procedures, hydrostatic pre-pressuring that is already-available can be applied. Drinking water can be obtained in this way at an energy input of less than 0.05 kWh/m3, which will meet not just current, but also future, hygiene requirements.

One example looks at water from the mountainous regions of Southern Germany, Switzerland and Austria, which contains abundant sources of generally high-quality drinking water, usually able to be fed into the mains network by local water suppliers without any special treatment. Nonetheless, there are water-hygiene problems with some supplies relating to the revised european drinking-water directive.

Therefore, in fissured regions of Upper and Lower Franconia in Bavaria, as well as in the karst regions of the Alps and Franconian Jura, raw water for use as drinking water is frequently cloudy, turbid and micro-biologically laden.

In individual areas of Switzerland, up to 25% of routine drinking-water samples taken from the water mains are unsatisfactory because they contain faecal bacteria. Moreover, these samples were not taken in exceptional situations such as heavy rainfall, floods or heat/drought, but in following a scheduled testing plan that pays no special attention to weather conditions. In most cases, the water giving cause for complaint was even disinfected with chlorine or ultra-violet irradiation.

In the future, following the amended drinking-water directive in Germany, disinfection measures of this type to treat microbiologically-burdened raw water will be accepted only with appropriate initial pre-treatment.

And yet, even with a conventional multiple-stage treatment process, there is no certainty that bacteria and turbidity will be totally eliminated. For while sand and active carbon filters are relatively straightforward to operate, they will not yet solely make it possible to obtain drinking water that complies with required standards. Furthermore, additional precipitation and flocculation measures and/or oxidisation with ozone are required.
Flocculation in particular is based on a particularly complex chemical process, in which the chemicals administered on the basis of molecule size, load and load density form flakes with the constituents of the raw water, which can be held back in the next sand filter. It is often not possible among smaller suppliers to ensure optimal functioning of precipitation and flocculation, as well as the technically-similar demands of ozonisation, due to the shortage of trained staff.

Consequently the entire treatment chain can lose its efficacy. There is then an imperative to act under pressures from health authorities, who prefer greater controllability over foreign supplies in respect of small communities' own suppliers. The security of supply suffers again and again from a state of crisis under these pressures, and there is frequently heavy energy consumption inherent in shipping foreign water supplies.

Membrane filtration

The ultra-filtration procedure using membranes has been available for a number of years now, and facilitates very straightforward treatment in a one-step process when there are problems with turbidity peaks and high bacteria counts. Membranes form an absolute barrier to bacteria up to the size of viruses (20 mm, 0.02 µm). They ensure purification and disinfection at the same time, even if the raw water is exceptionally heavily burdened with particulate matter following rain, or when snow is melting.

Control of membrane filtration can be set up very easily, as the plant works essentially using the operating methods of actual filtration, rinsing down and periodic disinfection of the filter. Given properly-dimensioned equipment, chemical cleaning of the membrane filter is normally required only every few months and also takes place automatically.
Since the membranes used (with KTW authorisation) in the supply of drinking water are made of plastic, it is impossible to prevent a gradual ageing process. In order to be able to check whether the filter barrier is still intact during a lifetime – normally of six to eight years – a so-called integrity test is carried out. In this operation, the membranes are periodically sprayed with compressed air, and their pressure-retaining strength is tested over a set time-span.

Faulty hollow membrane fibres are then isolated or, in extreme cases, the module is replaced. The simplicity of this procedure, the compactness of the equipment and the low operating and maintenance costs render membrane filtration an attractive option to previously-familiar treatment procedures, especially for smaller suppliers.
In mountains, raw water is often under pre-pressure at the point where it is treated and/or stored. This raises the possibility of applying this pre-pressure for filtration, and in doing so reducing the bill for operating costs of a vital component (energy), while also securing environmental benefits. In this way, operating costs come down to levels that previously were only achievable with simple sterilisation procedures.

However, not all filtering procedures available on the market offer this facility.

Ultra-filtration procedures using pressure and suction drive

Two different procedures are offered to produce drinking water using ultra-filtration. These differ in one essential aspect:

• With pressure drive, the pre-pressure required for filtration is obtained with an input supply pump, and filtration then takes place in sealed tubular modules, which are usually assembled in the hollow fibre configuration (see figure 1). Filtration can then take place at the hollow fibres of the pressure-driven module from inside out or vice-versa, likewise in dead-end or cross-flow drive.

• With suction drive, membrane modules are immersed in a cell in the flow of raw water. A vacuum is applied at the filtrate outlet with the aid of a suction pump, which sucks the raw water into the hollow membrane fibres at the dead-end drive, from outside in. This procedure was originally developed for sewage-treatment purposes, where it proved its worth in aeration or further purification as a substitute for high-volume aeration basins.
Since Membratec is working with open containers that allow access to membrane modules for servicing and maintenance purposes, it is impossible with this system of immersed membranes to exploit existing pre-pressure from tapping water, or from an upstream artificial lake or dam for filtration purposes.

As well as addressing the more difficult operational handling with immersed ultra-filtration systems, we are also seeking to minimise the risk of blockages at the membrane surface by blowing air in below the membrane. In so doing, the blasts of air produce turbulence at the membrane surface, through which any blockages can be released. In addition, the hollow membrane fibres are displaced in a swinging motion, in which membranes rub up against each other and shear away any deposits on the membrane surface. Additional energy costs are incurred in this injection of air and the pendular movements produce high physical stresses on the hollow fibres.

Two examples will illustrate the above arguments. They are taken from the mountain regions of Switzerland, on the application of pressure-driven ultra-filtration modules when producing drinking water. (See case studies – opposite.)

Overall assessment

Ultra-filtration with membranes is applied frequently and successfully for the treatment of spring water in mountain regions, where the water is inadequately filtered underground (e.g. karst and crevasse water). Spring water is often held above human settlements and thus flows under pressure into high-level tanks. At five out of nine plants that currently produce drinking water from spring water, this pressure can be used for filtration purposes. Compared with procedures that function using immersed suction-driven membranes, pressure-driven ultra-filtration systems display a number of major advantages, which allow both planners and operators a greater degree of freedom, and can achieve high energy efficiency in operation.

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