Do not store greywater! Learning to use greywater can feel counter-intuitive because the rules for irrigating with fresh water do not apply. You can store fresh water or rainwater for later use, but greywater should be used as soon as possible after it’s produced. In pumped systems with a temporary surge tank greywater should not stay in it for more than 24 hours. Unlike rainwater and fresh water, greywater has nutrients and organic matter from soaps and dirt. As these start to decompose they use up the oxygen in the water, which begins to smell very bad. It is possible to treat the greywater for storage, but this is impractical in most situations because fresh greywater is generated on a daily basis.