The flow chart shows the construction and function of a simple water filter.
Overall, the process consists of two main stages. In the first stage, impure water goes through three filtering layers - sand, charcoal and gravel from top to bottom - inside the filter tank. In the second stage, clean water is kept in the storage tank and made ready for drinking.
Initially, water collected from natural environment contains many pollutants such as dirt, waste plastics and industrial chemicals. After being poured into the filter tank, it first goes through a layer of sand, which traps unwanted solid particles due to gravity. As the water flows down, it passes through a layer of charcoal where harmful chemicals are absorbed, and unpleasant odors and tastes are removed. Subsequently, water flows through a gravel layer connected to an outlet pipe. While gravel has limited purification ability, it is essential for supporting the upper layers, allowing smooth water flow and protecting the outlet from getting blocked.
In the second stage, purified water is pumped up to a large, sealed drinking water storage tank. A tap is installed near the bottom of the storage tank making it easy to access and control the flow of drinking water.
推荐一些保守的写法（如果你不太清楚每层的作用，那就只能写一下正确的废话了）：
1）The filter tank contains three layers arranged from top to bottom: sand, charcoal, and gravel. Water passes through each of these layers in sequence, which contributes to the overall purification process.
2）The exact role of each layer is not shown, but they seem to be part of the general filtering process.
