Tanks and the New Battle for Life

From schoolchildren in Africa to waste treatment plants around the world, the importance of water storage has a far-reaching effect. Dr Gareth Evans looks at how tanks today are better equipped to ensure this effect is nothing but positive for the communities and operations that rely on them.

The association between the water industry and tanks is age old and over the centuries a range of materials has been pressed into service to provide storage facilities, each representing the pinnacle of available technology for its time. Against the present backdrop of increasing water quality standards and climatic necessity, it is scarcely surprising that innovation remains an important feature today, whether in terms of "traditional" applications or those that are more novel.

From the relatively water-rich nations of the world to those where drought and shortage are a constant fact of life, developments in tanks and water storage systems have a significant role to play in ensuring the security and safety of local supply.

Rainwater harvest

With many countries facing the twin – and apparently paradoxical – threats of drought and flooding in the wake of climate change, a growing interest has developed in rainwater harvesting (RWH) and sustainable urban drainage systems (SUDS).

In the UK alone the RWH market experienced triple-digit growth in the three years to 2008, when it topped £8m in value, and is predicted to continue in much the same vein in the four years to 2012, with continued steady growth expected to 2020.

"In the UK alone the RWH market experienced triple-digit growth in the three years to 2008, when it topped £8m in value."

Globally, the picture is even more encouraging for rainwater collection and storage systems manufacturers and vendors, according to recent estimates from BSRIA Worldwide Market Intelligence and consultants Mall. With market values of €28m suggested for France, €45m for Belgium, some €350m for Germany and Australia’s staggering €414m – half the worldwide total – despite the present economic downturn, long-term prospects for the industry look good.

Within the sector, pressures of footprint, durability and ease of connection have formed significant drivers on innovation in tanks and storage technology. In this respect, the likes of the new Carat tanks from Otto GRAF – Europe’s number-one supplier of rainwater harvesting solutions – provide an ideal example of how far things have come in recent years.

Constructed from Duralen – a proprietary material developed for the purpose – these revolutionary tanks offer high life expectancy and are available in sizes up to 6,500l, with the option of modular connection to accommodate larger storage demand. A unique twistable and extending turret vastly simplifies connection, while the tanks polish up their green credentials by being transportable in two halves, allowing ten times as many units to be carried on a single loader and thus slashing the transport carbon footprint of each.

On the other side of the world, one Australian company has begun to address the question of marrying the growing need for householders to store their own rainwater with the shrinking average size of new home plots.

With legislation and incentives in Queensland, New South Wales and Victoria already driving the uptake of rainwater harvesting, Stephen Cordell, founder of Azuratec, sees the time coming when "it will be compulsory to install rainwater tanks Australia-wide".

Facing the prospect of a conspicuous storage vessel in their backyard, householders have reacted favourably to the Azuratec alternative – an out-of-sight tank that fits within the slab forming the base of the building. Although it is slightly more expensive than a traditional tank, Cordell’s solution offers his fellow Victorians "water storage with no lifestyle cost".

Azuratec has not been alone in recognising that even when it comes to something as vital as water storage, space is still precious – a theme embraced by Sally Dominguez’s award-winning modular Rainwater HOG design, described as "visually discreet with unsurpassed functionality". Storing the largest possible volume in the smallest visible footprint looks set to remain a challenge for the sector.

Defy gravity

Space is a concern for the rest of the industry too. Conventional gravity settlement tanks in traditional water and wastewater treatment plants have largely plateaued in terms of performance, and the practical peak is lower than theoretical predictions would expect. To date, the inevitable corollary of this is that tank sizes must be increased – or flow rates reduced – to meet the ever more stringent quality standards that the industry faces.

However, particularly for tanks tasked with handling high sludge volume indices, a recently patented system pioneered in Northern Ireland may hold the key to defying such gravitational limitations.

"The world's best treatment process is only as good as the water storage system that supports it."

Designed to provide a slow and uniform cross-flow over the horizontal breadth and depth of the tank, the Voluflow system allows descending particles to thicken, while leaving the upper layers undisturbed by intermixing currents, in stark contrast to traditional arrangements. By avoiding the typical reverse flow patterns, turbulence, dead-zoning and intermingling of conventional tanks, the system – developed by Fostech – has demonstrated notable tolerance and stability, even to wide process variations.

Having initially emerged from work in other industry sectors, if the system lives up to its early promise for water, it could herald new standards for clarifier performance.

Storage and destratification

Ultimately, the world's best treatment process is only as good as the water storage system that supports it and stratification has long been identified as a problem in this respect, standing potable supplies being prone to contamination and the gradual loss of disinfectant residual. Additional onsite dosing is an obvious solution but in many cases the answer can be even more straightforward – simply keeping the water moving will prevent strata formation in the first place.

Pax Water Technologies, a Pax Scientific subsidiary, has focused on this niche market and developed the Pax Water mixer.

An active submersible unit, it can be retro-fitted in a matter of hours without the need to drain the tank, providing a mix-on-demand system to sidestep stratification, ensure even disinfectant distribution and eliminate nitrification – a common outcome with poorly mixed chloramines disinfection.

With thermal stratification inevitably becoming more pronounced when tanks sit in strong sunlight, the development of solar-powered mixer systems for remote, off-grid installations was an obvious next step, and one that Pax and SolarBee, another important player in the field, have already successfully taken.

"Roundabout Outdoor harnesses the power of African schoolchildren playing on a merry-go-round to pump water into local supply storage tanks."

Innovation can be child's play

Characterising technological innovation within the tank and storage space is difficult, beyond the obvious demands of public health and regulatory and environmental pressures. It remains the preserve of high-tech solutions, yet is ever ready to embrace the simplest of solutions.

On the one hand, recently published work by researchers at Duke University, Durham, North Carolina offers the promise of membranes, pipes and tanks coated with C60 buckyballs to prevent biofouling, on the other hand Roundabout Outdoor harnesses the power of African schoolchildren playing on a merry-go-round to pump water into local supply storage tanks. Advances such as these are poles apart but are nevertheless a unifying reflection of the two extremes of the sector’s innovative spectrum.

Between the recent G8 commitment to forging stronger partnerships in the developing world on water/wastewater and wider, more stringent regulatory demands and environmental pressures, tanks and storage appear destined to remain high on the industry’s agenda.

Storing water, it seems, has come of age.