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，世界人口将增至110亿。人类产生废物和废水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，这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，但这些技术在过去100年里几乎没有变化。活性污泥是使用最普遍的方法（按体积处理），但它是能源密集型的，占发达经济体电力消耗的3%。此外，世界上80%的废水进入未经处理的接收水。这项技术对一些人来说是昂贵和不可持续的，但对世界上大部分地区来说，这是根本负担不起的。在活性污泥工艺中，很大一部分（大约50%）的能量成本来自于需要将氧气通过大型污水池，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成二氧化碳，使其安全地释放到环境中。然而，废水中的有机物中含有能量。在活性污泥中，所有这些能量都流向了微生物，而作为工程师的我们无法获得这些能量。因此，虽然有效，但活性污泥法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方式，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，但由于这里没有氧气，细菌必须产生不同的废物，在经典厌氧消化的情况下产生甲烷，或者在生物电化学消化的情况下产生电子。在这种情况下，细菌只吸收废水中的部分能量，而我们作为工程师可以吸收其余的能量。厌氧消化也有大约100年的历史，被用于许多农场和工业废水流以及废水处理场产生的污泥。然而，它不能有效地处理稀释的废水，也不能在英国和其他国家典型的较低温度下有效。生物电化学系统（BES）是一项新开发的技术，它利用专门的细菌在电极上生长，并在它们消化废物时产生电流，本质上就像生物电池一样。BES技术已经被证明可以在低温下处理稀废水，但是它们的能源效率不高，高达90%的总输入能量丢失。其中一些能量会在细菌代谢时进入体内，但有些会以热量的形式散失。我假设，当这些细菌附着在生物膜的表面（比如电极上）一起生活时，产生的热量创造了一个局部温暖的环境，使细菌能够在低废水温度下生存和代谢。目前我们不知道有多少能量被加热，我们也没有能力精确地量化它。这笔拨款的目的是开发一个平台来进行这些关键的测量，以便我们能够设计和利用热能来改造废水处理。

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.