The biggest problem in cleaning up water pollution is that first, you have to identify what the pollutant(s) is/are. There are many different techniques available for removing the various types of water pollutants. Physical filtration can remove solid, insoluble material and some microorganisms such as bacteria. Biological filtration is used to break down many organic pollutants into simpler, less toxic chemicals. Adsorbent materials such as activated charcoal can be used to remove a wide range of organic pollutants. Absorbent materials such as mineral clays can be used to remove various pollutants, particularly oils. Synthetic materials called ion-exchange resins can be used to remove chemicals such as nitrates and phosphates. The complete list of techniques and materials used to clean up water pollution would fill up an entire book.