The Greywater Guerrillas |
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SAND FILTERSSand filtration is commonly used for drinking water purification, specifically removal of suspended solids and bacteria. Sand filters can range from household to municipal scale and have been used to treat drinking water for hundreds of years. As water flows through the sand particles are trapped and nutrients absorbed by microorganisms living on the sand. Slow sand filtration is a good, low-maintenance, electricity-free option for treating springwater or surface water with high levels of bacteria or suspended solids. Rainwater doesn't need treatment unless it becomes contaminated after collection.Many internet sites recommend using sand filters to filter greywater for either irrigation or toilet flushing. One study showed that sand filters did not clean greywater sufficiently to meet the health requirements for re-use inside the house for toilet flushing. In most cases greywater for irrigation does not need treatment. So while a sand filter installed after a surge tank would remove some bacteria and suspended solids, we haven't encountered many of these systems in operation, and don't recommend them in this application because of the periodic cleaning sand filters require. In 2001 we experimented with the incorporation of a sand filter into a greywater treatment system. The greywater flowed from a shower, through a surge tank, slowly through a sand filter, and then into a constructed wetland. We installed the sand filter experimentally for extra treatment, because the residents were concerned about using greywater directly on their small garden plot of greens. The sand filter markedly lowered turbidity, and the combination of wetland and sand filter cleaned the water more than enough for surface irrigation. After three years of intermittent use, the sand filter became clogged. The household residents switched the system to the sewer, didn't clean it, and ended up not using the system for a whole year. They then replaced the sand and the system has since worked fine for two years. This case study is a clear example of the benefits of ultra-low maintenance systems, and a warning against over engineering. We and other greywater designers have noticed again and again that even simple maintenance tasks such as emptying a woodchip biofilter or replacing sand in a sand filter get neglected or forgotten even by avid gardeners and greywater recyclers. So we offer the following advice: Design the simplest system you can, with the fewest moving parts and maintenance tasks. Remember that greywater needs thorough treatment only if you are going to discharge it into a stream or pond, or put it through a drip irrigation system. Consider redesigning your irrigation system to one of the passive options discussed previously. If you absolutely must have a drip irrigation system, or if you have installed a wetland to treat pond water and still require more treatment, read on. In our experience, sand filters should only be used after a constructed wetland, as a final "polishing" stage. Most effective treatment is acheived with a two-stage wetland: a reedbed followed by a pond. The reedbed environment filters and settles most particulate matter, microbes consume and adsorb bacteria, and plants uptake nutrients. These processes continue in the pond system, where water becomes remarkably clear. In our experience the effluent from such systems is suitable for discharge to surface or groundwater, although more testing is needed to determine how many nutrients remain in the water. Nutrients are an asset in irrigation system, so the only remaining challenge is to remove particulate matter that can clog drip emmitters. Here, after wetland treatment, is where a sand filter can save work: given the low suspended sediment levels, a sand filter can function for several years without maintenance, where an in-line filter screen may need to be changed every several months. For more information on sand filters, see http://catas1.org/eng/water.htm#4c |
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