Welcome to the end of the second week of the course. In this week we started off by looking at the water safety framework of the World Health Organization, and then got into exploring some of the first processes in HWTS. Starting with sedimentation, and then looking at three different kinds of filtration: membrane filtration, ceramics, and biological filtration. The water safety framework of the World Health Organization consists of five different steps, starting with health based targets. And then moving into a system assessment, operational monitoring, and management and communication. These three together are called water safety plans. Finally, water safety plans are confirmed with verification monitoring. We saw how this concept of water safety plans, while normally applied to community scale systems, can also be useful in considering household water treatment. We also introduced a key metric which is the Log Reduction Values, LRV, which is just the base ten log of concentrations of something before and after treatment, usually pathogens. Finally, we looked at four different stages in household water treatment, and spent significant amounts of time looking at the first two stages, sedimentation and filtration. Sedimentation is frequently required as a pre-treatment step when the raw water contains high levels of turbidity, or suspended solids. In some cases, gravity settling alone is enough to remove particles but in other cases, enhanced settling is required through the additional of chemicals called coagulants which can either be naturally occurring, derived from plant materials or metallic salts, such as alum. Membrane filtration includes a wide range of different kinds of processes, from simple cloth filtration, to filtration through synthetic membranes, micro-filtration, ultra-filtration, nano-filtration, and reverse osmosis, having increasingly small pore sizes. Those pores are seen in comparison to the sizes of inorganic particles but also to microbes: bacteria, protozoa, and copepods, and those smallest of microbes, viruses. The smallest the tightest of the membranes like reverse osmosis can even remove dissolved compounds, even salts and some other small ions. However, these synthetic membranes that remove that, have very small pore sizes need increasingly high pressure to push water through the membranes. Ceramic filtration is similar to membrane filtration, except that instead of passing through a synthetic membrane, water is passed through ceramics made from clay and burn off material. We looked at physical electrostatic, and chemical mechanisms for removing microbes. And saw that ceramic filters have good removal of protozoa, pretty good removal of bacteria, but variable to low removal of viruses. However, they are very user friendly, and are a rapidly growing segment of the HWTS market. Biological filtration, such as in the biosand filter shown here, also realize on physical and electrostatic removal, but the principal mechanism is biological removal through the formation of an active biofilm called the Schmutzdecke. Biosand filters also, like ceramic filters, have good removal of protozoa, modest bacteria removal, and variable to low virus removal. However, again they are user friendly and easy to operate. A key message from this week then, is that household water treatment is very similar to conventional water treatment, in that it uses some of the same processes, each of which can involve multiple mechanisms in removing or destroying pathogens. We've looked at sedimentation, methods to reduce the particles and turbidity in raw water. Often as a pre-treatment step. And also filtration mechanisms involving physical and electrostatic mechanisms. As well as biological and chemical ones. However, filtration alone doesn't fully remove pathogens from water. And next week we'll look into more detail at actual disinfection processes.