Stirring things up: Do surface mixers in drinking water reservoirs improve water quality?

清洁水是一种重要的资源，也是社会和工业面临的重大世界性挑战。英国和世界各地的饮用水水库在夏季会变得氧气枯竭，因为太阳能加热的增加会形成不同的温度层。水库中这种强烈的温度分层对包括氧气和营养物质在内的溶解物质的垂直运输起到了障碍作用。英国所有主要的自来水公司都使用工程干预措施，试图减轻水库中低氧对成本和水质的影响。低氧除了对鱼类和其他水生生物致命外，还会导致滋生滋生藻类，导致水华爆发。除了这些水华造成的恶臭的“豌豆汤”状态，使水库不适合娱乐和作为一种景观特征，进入饮用水处理厂的藻类物质与氯化过程相互作用，产生致癌化合物。这种分层也为蓝藻的生长创造了理想的条件，蓝藻已经进化到能够调节它们在水柱中的浮力和位置；蓝藻毒素是饮用水中味道和气味问题的关键因素之一。缺乏氧气还会导致海底沉积物中的锰释放到水中。像铁一样，锰在氧化时会变色(在这种情况下是黑色的)，这就使得饮用水供应不适合饮用，直到使用昂贵的处理过程来去除锰。为了解决这个问题，自来水公司采用了水库分层系统，将水混合在一起，这样氧气就可以从表面向下混合。有几种去分层的方法；英国水务公司最常用的是气泡羽流，但最近公用事业公司一直在对地面混合器进行新的资本投资，这种混合器的作用是将水从表面推到底部，在水库中创造循环。然而，这些去层化系统并不完善，公用事业公司仍然存在水质问题，必须通过水处理过程来处理。因此，更好地了解和优化分层系统可以改善源水质量，减轻饮用水厂的处理负荷，并在需要较少混合的情况下减少自来水公司的能源消耗。该项目的总体目标是改善依赖分层来维持水质的供水水库的管理。为此，提出了一项有两个目标的实地活动：1.通过地面搅拌器系统和其他干预措施，在实地评估水库中混合和水质的空间分布。2.通过模型研究确定水质的变化是由于搅拌器还是由于气候控制的自然变化。这些目标将通过现场测量和Durleigh水库的建模来实现。2015年，杜利的气泡分层系统被表面分层系统取代。通过在杜利的长期测量，将在整个夏季对搅拌机的性能进行评估。通过对没有混合器的水库进行建模，水质的变化可以归因于干预或气候强迫的自然变异性。通过这些目标，我们将产生可用于更好地了解人工混合水库的水动力学和水质的知识，以期在未来改善我们的水资源管理。

Clean water is an important resource and a crucial worldwide challenge for both society and industry. Drinking water reservoirs in the UK and around the world become depleted in oxygen in the summer, when layers of different temperature form due to increased solar heating. This strong temperature stratification in the reservoir acts as a barrier to vertical transport of dissolved substances including oxygen and nutrients. All major water utilities in Great Britain use engineering interventions to attempt to mitigate the cost and water quality impacts of low oxygen in their reservoirs.Besides being lethal for fish and other aquatic organisms, low oxygen leads to the growth of nuisance algae, leading to blooms. In addition to the malodourous "pea soup" state that these blooms create, making the reservoir unappealing for recreation and as a landscape feature, the algal matter that enters drinking water treatment plants interacts with the chlorination process to create carcinogenic compounds. The stratification also creates ideal conditions for the growth of cyanobacteria, who have evolved to be able to regulate their buoyancy and location in the water column; cyanobacterial toxins are one of the key contributors to taste and odour problems in drinking water. Lack of oxygen also causes manganese in the bottom sediments to be released into the water. Like iron, manganese turns colour when oxidised (black in this case), which then makes drinking water supplies unsuitable for consumption until expensive treatment processes are utilised to remove the manganese. To deal with this problem, water utilities employ reservoir destratification systems that mix the water so that oxygen can mix downwards from the surface. There are several methods of destratification; bubble plumes have most commonly been used by water utilities in the UK, but recently utilities have been making new capital investments in surface mixers, which act by pushing water from the surface down to the bottom, creating circulation in the reservoir. Yet these destratification systems are imperfect, and utilities still suffer from water quality problems that must be dealt with through the water treatment process. Thus better understanding and optimization of destratification systems can lead to improved source water quality, lighter treatment load at drinking water plants, and a decrease in energy use by water utilities if less mixing is required.The overarching goal of this project is to improve the management of water supply reservoirs that depend upon destratification for maintaining water quality. To this end, a field campaign is proposed that has two aims:1. Assess in the field the spatial distribution of mixing and water quality in a reservoir with a surface mixer system and other interventions. 2. Determine through a modelling study if changes in water quality are due to the mixer or to natural variation in climate controls.These aims will be achieved through combined field measurements and modelling of Durleigh Reservoir. In 2015, Durleigh's bubble destratification system was replaced with a surface destratification system. Through long-term measurements in Durleigh, the performance of the mixer will be assessed throughout the summer. By modelling the reservoir without the mixer, changes in water quality can be attributed to the intervention or to natural variability in climate forcing. Through these aims, we will generate knowledge that can be used to better understand the hydrodynamics and water quality of reservoirs with artificial mixing, with a future aim of improving management of our water resources.