Reengineering of sand dams to reduce water scarcity and meet Millennium Development Goals

世界上有7.48亿人无法获得改善的饮用水来源，其中43%生活在撒哈拉以南非洲(世卫组织和联合国儿童基金会，2012年)。他们需要走很长一段路去取水，在这段时间里，他们既不在学校，也不赚钱。他们喝的水可能被污染得很厉害，以至于他们生病了。有很多技术可以把干净的水带到人们的家附近。一种选择，尤其是在肯尼亚，是沙坝。沙坝是在季节性河流上建造的不透水的混凝土结构，目的是在暴雨期间将水和泥沙(沙)困在后面。在旱季，水被储存在沙粒之间的空间中，以便通过井进行提取。在没有积水的情况下，蚊子等水传播媒介的威胁较少(Hut等人，Lasage等人，2008)。但是，到目前为止，还没有关于沙坝取水质量的研究。有一种假设认为水质受到沙子的保护，但这一假设尚未得到检验。我们知道，当水通过干净的沙子时，病原体可以通过生物过程去除--这一慢速砂滤(SSF)原理被用于传统的水处理。因此，在这项研究中，我们正在检验这样一个假设，即沙坝中的水不仅不会受到污染，而且在穿过沙子时其质量会得到改善。这一假设将通过现场测量、实验室实验和计算机模拟的组合方案进行验证。如果沙坝的水质被证明具有可接受的微生物质量，那么它就可以安全地饮用，而不需要任何进一步的处理，对社区和为他们服务的机构来说，这是一个更有吸引力的供水选择。人们主要关注沙坝清除污染物的效果：1.我们知道，当水通过细沙时，比通过粗沙时，可以去除更多的病原体。然而，沙坝挡住了粗沙。工程化SSFS中去除病原体的生物过程主要发生在沙土的上部2 cm处。然而，沙坝中的这一层在暴雨期间会受到干扰，因此生物过程可能不会有效地工作。在下雨的时候，大坝积水的浑浊度很高。这意味着它有很多细小的颗粒。这些可能会改变生物过程。总体而言，通过这项研究，我们将全面测量沙坝后面的水的质量，确定颗粒大小或浑浊是否会影响水质，从而提出如何更好地设计沙坝以最大化水质。现场测量将得到英国慈善机构卓越发展的支持，该机构自1985年以来已在肯尼亚修建了325座沙坝。我们将随机选择他们的三个沙坝，并在季节性暴雨后安装压力计。压力计是一种管子，管子上有孔，这样管子里的水位就能反映出被困沙子里的水位。每天都会测量水位。水质测试样本将每周从这些管道中采集。将测量一系列水质参数，包括引起腹泻的细菌大肠菌群和浊度。下一阶段将进行实验室实验，复制洪水事件结束后沙坝内的条件。柱子将填充不同粒度的沙子，然后用与现场测量的大肠菌群计数和浊度相似的水饱和。在四个月的时间里，将测量从柱子底部逐渐去除的水中的大肠菌群数量，以确定去除病原体的效率。最后阶段将利用实验室和现场工作的结果建立沙坝系统的计算机模型。这些都可以用来了解颗粒大小、沙坝大小和进水水质等因素的影响，从而对沙坝设计提出建议。

There are 748 million people in the world who do not have access to improved sources of drinking water and 43% of these live in sub Saharan Africa (WHO and UNICEF 2012). They face long walks to fetch water, during which time they are not in school or earning money. The water they drink may be so contaminated it makes them ill. There are lots of technologies that can bring clean water close to people's homes. One option, particularly in Kenya, is sand dams. Sand dams are impermeable concrete structures constructed across seasonal rivers in order to trap both water and sediment (sand) behind them during rain storms. The water is stored in the spaces between the sand grains for abstraction using a well during the dry season. With no standing water, there are fewer threats from water-borne vectors like mosquitoes (Hut et al 2008, Lasage et al 2008).However, there are to date no studies on the quality of water removed from sand dams. There is an assumption that the water quality is protected by the sand, but this has not been tested. We know that pathogens are removed by biological processes when water is passed through clean sand - this principle of slow sand filtration (SSF) is used in conventional water treatment. So in this research we are testing the hypothesis that water in a sand dam is not only protected from contamination, but its quality is improved as it passes through the sand. This hypothesis will be tested through a combined programme of field measurements, laboratory experiments and computer modelling. If sand dam water quality can be demonstrated to be of acceptable microbiological quality then it can be drunk safely without any further treatment and it is a more attractive water supply option to communities and agencies serving them.There are principle concerns about how well sand dams will remove contaminants:1. We know that more pathogens are removed as water passes through fine sand than coarse sand. However, sand dams trap coarse sand.2. The biological processes that remove pathogens in engineered SSFs happen mainly in the upper 2cm of the sand. However, this layer in sand dams is disturbed during the rain storms so the biological processes may not work effectively.3. During rain events the water trapped by the dam has high turbidity. This means it has a lot of fine particles. These may change biological processes.Overall, through this research we will comprehensively measure the quality of water trapped behind sand dams, determine whether grain size or turbidity affect water quality and thus propose how sand dams can be better designed to maximise water quality.The field measurements will be supported by Excellent Development, a UK based charity who have built 325 sand dams in Kenya since 1985. We will randomly select three of their sand dams and install piezometers after the seasonal rain storms. A piezometer is a tube with holes in it such that the water level in the tube reflects the water level in the trapped sand. Water levels will be measured daily. Samples for water quality testing will be collected from these tubes weekly. A range of water quality parameters will be measured including coliforms, which are the bacteria that cause diarrhoea, and turbidity.The next stage will be to do lab experiments that replicate conditions within sand dams from the end of the flood events. Columns will be filled with sand of different grain sizes, and then saturated with water with similar coliform counts and turbidity to that measured in the field. The coliform count of water removed gradually from the bottom of the column over a four month period will be measured to establish the efficiency of pathogen removal.The final stage will be to create computer models of the sand dam system using the results of the lab and field work. These can be used to understand the effects of factors like grain size, sand dam size and input water quality so that recommendations on sand dam design can be made.