Tannins in drinking water are caused by natural decaying of organic matter. Leaves or pine needles in the gutters are generally the cause in a rainwater collection system. A faint yellowing of water generally occurs at .5 parts per million or PPMs with .5 – 2.0 PPMs looking like the color of ginger ale, and 3.0 – 5.0 PPMs would take on the appearance of dark tea. The tannins may cause a yellow color of the water, yellow staining on fixtures, and yellow staining in laundry.
Although aesthetically displeasing, tannins generally pose no real health risk, but may affect the performance of the UV system by not allowing the UV light to penetrate thoroughly through the water column.
Therefore, it is very important to minimize tannins before storage of collected rainwater. Organic matter must be kept from accumulating in gutters, down spouts, and screen baskets. Gutter screens are an effective method of prevention of organics in gutters and down spouts. Both gutter screens and screen baskets should be inspected and cleaned as needed to prevent tannins from occurring. Since the discoloration is in solution, removal can be difficult and expensive. Ozone, ion exchange, and activated carbon can help with removal of tannins to some degree. If you do end up with tannins in your water, it is best to drain the cisterns, rinse and start collection again, however the best solution is diligence in maintenance.