Dissolved metals, thermal stratification, low oxygen levels, algae blooms – water authorities face many challenges in maintaining safe source water reservoirs. Water authorities and water managers can now destratify their source water reservoirs using a low cost and low energy solution.
There is an ever increasing pressure to on water managers around the world to provide safe, clean drinking water. However now with the added pressure of cost minimisation and rationalisation of authorities, there is a need to reduce the cost of water treatment and improve the environment through a reduction in process and of chemical compounds introduction. This highlights the tension of management of the resource while at the same time; improving and growing the revenue base. Water managers have been driven by the need to treat water with better technology and improved process; however the significant savings in improving the raw water supply have largely been overlooked.
A growing number of authorities around the globe, in all diverse climates are realising that artificial ‘top to bottom’ circulation has significant measurable short term and long term triple bottom line benefits in the water treatment process. The value of improving, or at-least maintaining, the quality of the water at the source, otherwise referred to as ‘Source Water Management’, often provides a reduction in final water treatment costs while at the same time improving the aquatic environment.
Because of the benefits of destratification, the question has started to shift from ‘IF’ an authority should circulate (destratify) a reservoir, to ‘HOW’ should the reservoir be circulated?
Temperature and Dissolved Oxygen (DO)
Raw water reservoirs around the world and in every climate (hot or cold) stratify; that is the temperature of the water changes with depth. The level of stratification potential may differ dependent upon a number of factors: depth, location, age of the dam, amount of vegetable matter in the dam etc. Stratification reduces or prevents natural convection circulation across the water column. Consequently the oxygen transfer at the atmospheric interface only affects the water close to the surface. While the water at the surface, close to the atmospheric interface is rich in oxygen, the level of oxygen reduces with depth. In some reservoirs,the lowest parts of the water body can be anoxic, and this water is far more difficult and expensive to treat.
This is also true in relation to temperature. As with oxygen, the temperature of the surface water of a reservoir (lake, dam, pond) is often much warmer than the water at the bottom. This too is related to the depth of a body of water and the temperature can vary by as much as 10°C to 15°C. Most reservoirs naturally “invert or turnover” and cleanse themselves in autumn and spring. During this temperature driven process, low oxygen bottom waters mix with well-oxygenated surface waters. This circulating action breathes new life into bottom areas of these reservoirs, raising oxygen levels, lowering nutrient levels and providing a refuge for fish and other aquatic life. In many cases, urbanisation, agricultural development and other demands on the watershed cause increases in dissolved nutrients and oxygen demand and nature’s two turnovers per year simply isn’t enough to cope. The aquatic system is left with insufficient dissolved oxygen to support a healthy environment all year round. At some point, something must give. Fish kills, muck/sludge accumulation, odour, algae growth and declining fisheries occur.
From a water treatment perspective, annual, biannual (or even more times in some locations) uncontrolled reservoir inversion creates significant water treatment challenges (chemical, biological, environmental) and cost (treatment and management) but these can be avoided.
Where natural inversion brings poor quality water from the bottom to the surface and creates poor quality water for an extended period, top down aeration takes the good , highly oxygenated water on the surface and pushes it down to the bottom. Controlled convection circulation within a reservoir mimics Nature’s turnovers but increases the turnover rate to as much as several times per day. This top-to-bottom circulation increases dissolved oxygen levels and can prevent water from stratifying; creating stable temperatures from top to bottom throughout the entire year. This “artificial top-down circulation” improves fish and aquatic habitats by expanding the healthy areas in which they can live, while minimising the potential environments for algae to grow and flourish.
A Reducing Bottom Environment
Stratification prevents the natural (top-down) convection circulation. As a result the water body becomes a reducing environment due to chemical and biological breakdown towards the bottom of the reservoir. In this reducing environment, metal salts and some other heavy metals i.e. positive ions (P, Ca, Fe, Mn, Cu, Arsenic, beryllium, chromium etc) become soluble and are present in the water which then requires chemical dosing and filtration for removal at the Water Treatment Plant to make it safe for drinking. In this reducing environment, fish and other marine creatures cannot thrive or survive due to a low dissolved oxygenated water environment, resulting in fish kills and other marine environmental issues. By increasing the DO in the entire reservoir, these solubilised ions are oxygenated and fall out of solution and fall to the bottom.
Stratification can create a competitive environment for toxic algae species, such as Cyanobacteria or Blue Green Algae (BGA). These come with several deadly taxonomy (e.g. Anabaena, Mycrocystis, and Cylindrospermopsis) potentially producing toxins in elevated concentrations. This then leads to public health concerns and additional and dangerous water treatment costs. Stratification can also promote stagnation which leads to high concentrations of bacteria and pathogens such as Cryptosporidium which is a protozoan pathogen of the Phylum Apicomplexa and causes a diarrheal illness (and death in some cases), called cryptosporidiosis.
Circulation and destratification
Destratification has been recognised for at least 50 years, as a way to minimise the cost of final water treatment and better public health outcomes. Only in the last 15-20 years however, has a holistic approach to water treatment been a focus, and ‘Source Water Management’ or reservoir management has come to the fore. Source Water Management has resulted in a significant reduction in the costs of water treatment, plus a significant improvement to the health of the resource. This has resulted in a desire to reverse the environmental degradation of aged and aging reservoirs. This in turn provides risk minimisation to the asset owners, improves water quality and public health and at a lower cost.
Historically, destratification has been attempted using high energy, low efficiency compressed air circulation known as bubble plumes. This highly inefficient method of destratification has had some success however it is now possible to produce high efficiency, low energy simulated natural convection (top-down) circulation. This form of circulation has reduced the cost of destratification water reservoirs by over 80% while drastically improving water quality and performance outcomes.
For 20 years, WEARS Australia has designed and developed Source Water solutions which provide these benefits to water bodies of all sizes. The ResMix system is a floating axial pump that is designed to destratify reservoirs by pumping highly oxygenated water down through the thermocline that causes stratification and significantly change the temperature and oxygen profile of reservoirs. Our ResMix systems have been used domestically in Australia across the world, in places like the United Kingdom, United States of America and the Middle East and can be designed suit a combination of water requirements.
The ResMix system can be operated from mains power, but in remote locations solar panels can provide the small power needs. What more and more people are realising is that top down water circulation of reservoirs provides improved outcomes for the environment and for the community in general by providing better, less expensive but highest quality drinking water.
Contact us to find out more about how WEARS Australia can assist with problems caused by destratification.