BIOHEAT: Husbanding biological heat to transform wastewater treatment

世界人口目前为75亿，联合国预测，随着城市化进程的加快，到2100年，这一数字可能会上升到110亿。人类排泄物和废水的产生是生活中不可避免的后果。对其进行处理，使其能够安全地释放到环境中，对人类健康和我们所依赖的生态系统都至关重要。现有的有效技术能够处理城市地区产生的大量废水，但在过去的100年里，这些技术几乎没有改变。活性污泥法是使用最普遍的方法(按处理量计算)，但它是能源密集型的，在发达经济体中占电力消耗的3%[15]。此外，世界上80%的废水未经处理就进入接收水[16]。这项技术对一些人来说是昂贵的，也是不可持续的，但对世界上大部分地区来说，简单地说是负担不起的。活性污泥法中很大一部分(约50%)的能源成本来自于需要通过大型污水池鼓泡氧气，以便这些废物中的好氧细菌可以利用氧气将废物中的有机物消化为二氧化碳，从而使其安全地释放到环境中。然而，废水中的这些有机物中含有能量。在活性污泥中，所有这些能量都流向微生物，而我们作为工程师是无法获得这些能量的。因此，尽管活性污泥法是有效的，但它使用大量的能量来去除废水中的能量。如果我们要转向一种更可持续的废水处理形式，好氧活性污泥法需要被厌氧技术所取代。厌氧技术也使用自然产生的细菌来消化废物，但在这里，由于没有氧气，细菌必须产生一种不同的废物，在经典的厌氧消化中是甲烷，在生物电化学消化中是电子。在这种情况下，细菌只消耗废水中包含的部分能量，而我们作为工程师可以获取其余的能量。厌氧消化也已有100年的历史，用于许多农业和工业废水以及废水处理厂产生的污泥。然而，它在处理稀释的废水方面并不有效，而且在英国和其他国家典型的较低温度下也不有效。生物电化学系统(BES)是一项新开发的技术，它使用特殊的细菌在电极上生长，并在它们消化废物时产生电流，本质上就像一个生物电池。BES技术已被证明可以在低温下处理稀释废水，但它们的能量效率不高，高达90%的总输入能量丢失。这些能量中的一部分将在细菌新陈代谢时进入细菌，但另一些将以热量的形式损失。我假设，当这些细菌附着在生物膜的表面上时，例如在电极上，产生的热量正在创造一个局部的温暖环境，使细菌能够在较低的废水温度下生存和代谢。目前我们不知道有多少能量将被加热，也没有能力准确地量化它。这笔赠款的目的是开发一个进行这些关键测量的平台，以便我们能够设计和使用热能来改变废水处理。

The world's population stands at 7.5 billion and the UN predicts this could rise to 11 billion by 2100 with increasing urbanisation [13]. The production of human wastes and wastewaters in an unavoidable consequence of life. Treating this so it can be safely released to the environment is of paramount importance to both human health and the ecosystems we depend on. Effective technologies exist which are able to treat the large volumes of wastewater produced in urban areas, but these have changed little in the last 100 years. Activated sludge is the most prevalent method used (by volume treated) but it is energy intensive, accounting for as much as 3% of electricity consumption in developed economies [15]. Furthermore 80% of the world's wastewater goes into receiving waters untreated [16]. This technology is expensive and unsustainable for some, but for large parts of the world is simple unaffordable. A large proportion (roughly 50%) of the energetic costs in the activated sludge process comes from the need to bubble oxygen through the large tanks of sewage, such that the aerobic bacteria within these wastes can use the oxygen to digest the organic matter to carbon dioxide within the waste, making it safe to release to the environment. However there is energy contained within these organics in the wastewater. In activated sludge all this energy goes to the microorganisms, and we as engineers are unable to access it. Thus although effective, the activated sludge process uses substantial amounts of energy to get rid of the energy within the wastewater. If we are to move to a more sustainable form of wastewater treatment, the aerobic activated sludge process need to be replaced by an anaerobic technology. Anaerobic technologies also use naturally occurring bacteria to digest waste, but here as oxygen is not present the bacteria must produce a different waste, methane in the case of classical anaerobic digestion, or electrons in the case of Bioelectrochemical digestion. In this scenario the bacteria take only some of the energy contained in the wastewater, and we as engineers can take the rest. Anaerobic digestion has also been around for 100 years and is used on many farm and industrial waste streams as well as on the sludge produced by wastewater treatment sites. However it is not effective at treating wastewaters which are dilute, and is not effective at the lower temperatures which are typical of the UK and other countries. Bioelectrochemical systems (BES) are a newly developing technology that use specialised bacteria to grow on an electrode and produce currents as they digest the wastes, essentially acting like a biological battery. BES technologies have been shown to work with dilute wastewaters and at low temperatures, however they are not energetically efficient, with up to 90% of the total input energy going missing. Some of this energy will go to the bacteria as they metabolise, but some will be lost as heat. I hypothesise that when these bacteria live together attached to a surface in a biofilm, such as on an electrode, the heat generated is creating a localised warm environment allowing bacteria to survive and metabolise at low wastewater temperatures. Currently we do not know how much energy is going to heat, and nor do we have the ability to accurately quantify it. The aim of this grant is to develop a platform to make these critical measurements in order that we will then be able to engineer and husband the heat energy to transform wastewater treatment.