Water footprint, product quality and cost control

Mark Bosley, Business Support Divisional Manager with Purite, looks at ways in which purification technology can deliver commercial returns for food and drinks producers.

Water is the single most commonly used raw material in the food and drinks sector, and yet according to a survey carried out by the Government funded Envirowise programme, well over half of all senior managers do not know how much their companies spend on water. Although the survey was designed to highlight the benefits of reducing water consumption and increasing water reuse, it served to highlight the fact that water is a finite resource and that its cost as a raw material is rising steadily. Indeed, some experts predict that the next major challenge, after the Carbon Footprint, will be the definition of a Water Footprint with which business users will need to comply.

Whatever the outcome, the basic point is clear: with the food and drinks sector in England and Wales using an estimated 157 million cubic metres of water annually, producers, if they are not doing so already, need to review their use of water. This makes sense commercially, to reduce, or at least control, costs at a time when operating margins are under pressure. It also makes sense if companies are to comply with growing legislative requirements, such as the latest abstraction licences that penalise high levels of water usage, and to meet commercial demands from customers, in particular the major supermarkets that increasingly expect their suppliers to conform to rigorous environmental and corporate social responsibilities.

The challenge is to minimise consumption of raw water without affecting product quality or increasing costs elsewhere in each business, especially in the use of energy.

Filter efficiencies and energy costs

Water purification or filtration is used throughout the process chain. This typically includes the treatment of raw water, normally abstracted from boreholes to remove iron, minerals and organic micro-pollutants; drawn from surface sources, to remove particulates and pathogenic micro-organisms; or from mains supplies, where chlorine or fluoride often have to be eliminated. Filtration will also be used downstream for removing colour and taste, and for clarification duties, and is increasingly being employed in recycling systems, for both process effluents and grey water.

Although different technologies are used throughout, few on their own offer the potential to produce major reductions in water consumption – at least, not without the need to make compromises elsewhere in the process chain or to adjust the mix of ingredients used in the finished product. In practice, water savings are likely to be incremental and will come from improvements in data gathering and analysis, more accurate process control techniques, such as the optimisation of spray bar functions, the replacement of faulty solenoid valves and the balancing of water circulation systems.

A number of the most commonly used purification systems do, however, have the capability to offer enhancements in operating and process efficiency, which can help both to save water through reduced waste and to minimise energy consumption.

For example, the latest membrane elements used in reverse osmosis systems provide high levels of flow at lower operating pressures. This allows pump speeds, and thus energy demand, to be significantly lowered. Further gains can be made if pumps are linked to variable speed drives, which enable the speed of each unit to be matched exactly to the output demands of the water treatment system.

Improvements in the efficiency of membrane elements have been achieved in several areas, most notably the development of extremely thin membrane materials, just 120 micron in thickness, based on polyamide thin film composites with non-woven polyester support webs. These are manufactured using advanced adhesive techniques, allowing the wound mesh space between layers and the layer to layer bonding areas to be reduced, without compromising the thickness of the feed spacer or affecting the mechanical or chemical properties of the membrane.

As a result, the feed pressure, compared with a traditional high rejection RO element, has been significantly reduced, with lower fouling potential and less pressure drop, while flow rates have increased several fold. This has also given the added benefit that operating life in many applications has been significantly extended, while the creation of a far higher active membrane surface area has provided designers with the option to reduce the total number of modules in larger reverse osmosis systems, thereby cutting capital and maintenance costs still further.

Energy consumption in the lab

For many food and drinks companies with access to onsite laboratories, it is worth noting that the use of traditional distillation units can add considerably to energy costs, requiring around 1kW of power for every litre of water produced.

Distillation also suffers from other drawbacks: it is relatively slow, making it difficult to produce water on demand, can only produce water to relatively low quality, and distilled water has to be used immediately or be carefully stored to ensure that it remains pure, as it can easily become contaminated with impurities such as atmospheric carbon dioxide. Stills used in hard water areas also require regular de-scaling and cleaning, which adds still further to overall costs.

By comparison, disposable deionisation or ion exchange cartridges connected directly to a mains water supply can produce purified water on demand, without the need for an electrical supply and with far less waste than a still. For higher volume requirements, compact, stand-alone reverse osmosis systems such as those available from Purite can produce high purity water from a mains supply. This is fed under pressure into a module containing a semi-permeable membrane that removes up to 98% of inorganic ions, plus virtually all colloids, micro-organisms, endotoxins and macromolecules. These systems have relatively low capital cost, and are inexpensive to operate and maintain, giving a fast return on investment.

Clearly, dedicated modern RO systems offer considerable benefits in terms of energy and, potentially, water savings in laboratories. The same also applies to process systems, especially if wastewater can effectively be recycled, or if a use can be found for grey water or rainwater.

Water reuse

Applications that use large volumes of raw water as an inherent part of the process, such as milk processing, vegetable washing, brewing and distillation of spirits, can make significant savings through water reuse. For example, according to figures from Envirowise, dairies in the UK typically use 1.3 litres of water for every litre of milk processed, primarily for steam production and cleaning in place (CIP) duties.

A large volume of this water can be purified using conventional methods of filtration, including dissolved air flotation, bioreactors and reverse osmosis to produce a waste water stream that is typically of a higher purity than that of mains water supplies and which can normally be produced at a lower cost per cubic metre. This water can then be used for machine washdown or similar duties.

One factor that does, however, need to be considered is the fact that the constituents of food wastewater are often difficult to predict due to the differences in biochemical oxygen demand (BOD) and pH in effluents from vegetable, fruit, and meat products and the seasonal nature of many food production and harvesting processes. For example, washing of vegetables generates water with high solids loading, dissolved organics and possibly surfactants, all of which will have an impact on the choice of filtration system and which may in some circumstances prevent reuse.

Pipework and maintenance

Finally, there are two areas that should not be overlooked: the design and construction of the treated water feed pipework; and the importance of regular maintenance to ensure that process equipment is operating at optimum levels and is free from leaks through which water may be lost.

The layout of water pipework systems can have a significant impact on system efficiency and energy consumption. Correct sizing of pipes is essential, especially in steam distribution and boiler systems, to ensure correct flow characteristics and minimise pressure drop, and thus the consumption of energy from pumps.

Additionally, oversized pipework will increase the cost of equipment and installation, and will cause a greater volume of condensate to form, due to the greater heat loss; in turn, this means that either more steam trapping is required or that wet steam is delivered to the point of use. Conversely, under sized pipes may deliver too low a pressure, with steam starvation, erosion, water hammer and noise, due to the increase in steam velocity.

Ultimately, it is in the interests of food and drinks producers, consumers and the water companies to work together to conserve water and, as importantly, to reduce the cost of water treatment and use. It’s important to recognise that there’s no magic bullet or single solution; instead, change has to take place incrementally throughout the supply and process chain, taking advantage of improvements in technology, such as those described above in the area of filtration, to drive long term efficiencies that add real value in terms of productivity and profitability.

Further information is available from Purite Ltd, Bandet Way, Thame, Oxon. OX9 3SJ. Tel: 01844 217141.

Email [email protected] Web: www.purite.com