Water Engineering: Membrane fouling for low energy advanced wastewater treatment

英国水务行业每天处理超过30亿立方米的污水，因此通过污水处理在保护野生动物和人类健康的水源安全方面发挥着重要作用。在欧盟内部，成员国被要求在2015年前达到水框架指令(WFD)中规定的一些新的、更严格的卫生决定因素目标。目前，污水处理主要由生物处理系统促进，通常设计为活性污泥法(ASP)。虽然对现有标准的处理是有效的，但运营公用事业公司和行业监管机构环境局都提出了担忧，即现有的ASP资产无法满足拟议的WFD标准。膜生物反应器(MBR)是一种膜分离与活性污泥法相结合的高级污水处理工艺。膜单元通常由公称直径在0.1至0.01微米范围内的孔组成，因此与传统的ASP相比，增强了颗粒的分离。此外，活性微生物的留存能力增强，使硝化作用更加强劲，因此有能力超过《世界污水排放标准》中提出的污水达标率。因此，MBR是增长最快的先进废水处理技术，全球年市场价值约为10亿美元。然而，在MBR工艺中运行膜所需的能量导致了比传统的ASP工艺明显更高的工艺能量需求。因此，这一限制限制了这项技术在城市污水处理中的应用，因为它与当前的监管和公用事业驱动因素相冲突，后者正寻求“改造废水处理以减少碳排放”(环境署报告，2009年)，并从长远来看转向碳中性废水处理。然而，满足更严格的废水同意的需求迫在眉睫，而且对稀缺水资源的需求增加进一步加剧了这一需求。MBR是应对这些挑战的不可或缺的技术。因此，该建议寻求从根本上减少与膜生物反应器中的膜操作相关的特定能量需求，以使这一关键技术能够被采用。在膜过滤过程中，颗粒在膜表面聚集，在膜表面形成集中的污染层。这种污垢随着时间的推移逐渐压缩，进一步限制了流动。膜能量需求来自于限制集中颗粒污染层积累所需的空气注入。克兰菲尔德最近的研究表明，通过操纵空气注入所施加的流体动力学，有可能重组污物层内的颗粒，使其更容易清除，从而将能源需求减少多达十分之一。理解这一关系背后的科学机制的关键是确定小颗粒(1微米)在这些污垢层中的作用，因为有人认为小颗粒代表临界污垢分数。虽然有方法可以在这种动态条件下测量污物层，但它们不够灵敏，无法检测亚微米尺寸范围的颗粒。因此，提出了一种新的反射光荧光直接观测方法，该方法将能够测量这一关键的粒子群。一旦建立，该方法将为这些复杂的污垢结构内的颗粒分布和颗粒传输提供定量证据。由此产生的证据将被用来在MBR内设计高度可逆的污垢层，消除关键的能量障碍，并使MBR作为一种降低碳排放的技术选项用于先进的环境保护。

The UK water industry treats over 3 billion m3 of sewage every day and so plays a major role in safe-guarding water sources for the protection of wildlife and human health through wastewater treatment. Within the EU, member states are required to meet a number of new, stricter sanitary determinant targets by 2015 which have been set out within the Water Framework Directive (WFD). Sewage treatment is currently predominantly facilitated by biological treatment systems, typically designed as activated sludge processes (ASPs). Whilst effective for treatment to existing standards, both operating utilities and the industry regulator, the Environment Agency, have raised concerns that the proposed WFD standards cannot be met by existing ASP assets. Membrane bioreactors (MBRs) are an advanced wastewater treatment process that couples membrane separation with the activated sludge process. The membrane units are typically comprised of pores with a nominal diameter in the range of 0.1 to 0.01 micrometres and thus enhance the separation of particles versus conventional ASP. Furthermore, the enhanced retention of active microbes enables more robust nitrification to be achieved and have therefore demonstrated a capability to exceed the proposed effluent compliance set out in the WFD.Consequently, MBRs represent the fastest growing advanced wastewater treatment technology with a global annual market value of around $1 Bn. However, the energy required to operate the membrane in the MBR process results in a markedly higher process energy demand than for conventional ASP technology. This constraint has therefore limited the uptake of this technology for municipal sewage treatment as it is in conflict with current regulatory and utility drivers which are seeking to, "Transform wastewater treatment to reduce carbon emissions" (Environment Agency Report, 2009), and in the long term move toward carbon neutral wastewater treatment. Nevertheless, the demand to meet stricter wastewater consents is imminent and is further exacerbated by the increased demand on scarce water resources. MBRs are an integral technology to fulfilling these challenges. This proposal therefore seeks to radically reduce the specific energy demand associated with membrane operation in MBR to enable uptake of this critical technology. During membrane filtration, particles accumulate at the membrane surface forming concentrated fouling layers at the membrane surface. This fouling layer gradually compresses with time, restricting flow further. The membrane energy demand arises from the air injection required to limit the accumulation of the concentrated particulate fouling layers. Recent studies at Cranfield have shown that by manipulating the hydrodynamics imposed by air injection, it is possible to restructure the particles within the foulant layer to make it more easy to remove, reducing the energy demand by up to ten times. Critical to understanding the scientific mechanism behind this relationship is in establishing the role of small particles (<1 micron) in these fouling layers as it is argued that small particles represent the critical fouling fraction. Whilst methodologies are available to measure foulant layers in such dynamic conditions, they are not sufficiently sensitive to detect particles in the sub-micron size range. Consequently, a novel Reflected Light Fluorescence Direct Observation method is proposed that will enable measurement of this critical group of particles. Once established, this method will provide quantitative evidence of particle distribution and particle transport within these complex fouling structures. The resultant evidence will be used to engineer highly reversible fouling layers within MBR, eliminating the critical energy barrier and enabling MBR utilisation as a reduced carbon technology option for advanced protection of the environment.