Filtering Water Samples

Filtering involves forcing water through a membrane filled with tiny holes, about 25 microns across (¼ of a human hair). The filter removes suspended solids from the water, and in fact, the weight of the solids caught in the filter is one common analysis parameter: total suspended solids (TSS). Such solids can be virtually invisible to the naked eye, but not dissolved, so they'll eventually settle out if the sample is left undisturbed. Polluted coalmine water can have tiny bits of iron hydroxide (a.k.a. yellow boy) suspended in it. In a water sample fixed with acid but not filtered in the field, this iron hydroxide will dissolve into the water before it arrives at the laboratory, which will give incorrect test results for metals. Alternatively, in a sample that is not acidified, dissolved oxygen can cause dissolved ferrous iron to oxygenate into ferric iron en route to the lab. Unless the insoluble ferric iron is filtered out at sampling time, the lab can't determine how much ferrous vs. ferric iron the water actually contained when it was sampled. Ferrous iron oxygenation also consumes acid, so certain laboratory acidity or pH measurements will be wrong.

Although filtering water samples eliminates these errors to some extent, it requires special equipment and training. Many volunteer groups leave filtering to the analysis laboratory, accepting any inaccuracies that creep in beforehand. Ultimately, the decision to filter relies on a group’s capacity and requirements, and it should be discussed with the laboratory that receives the water samples.

All waste generated by LaMotte test procedures (not including bacteria test) may be poured down the drain with the water running to dilute it. The only time the waste may not be disposed of in this manner is if the waste generated is from 30 or more tests for a single parameter. For example, if 30 students all use the LaMotte test kit to test the dissolved oxygen levels of a sample, the waste cannot be poured down the drain. A waste management facility should be contacted to determine the correct method of disposal. But if 10 students test for pH, 10 students test for nitrates and 10 students test for dissolved oxygen, the waste generated by this group can be disposed of down the drain with the water running to dilute it. If the tests are done in the field, carry a waste container with a lid along with the kits. Pour all the waste into the container and save it for appropriate disposal at a later time. Before using a kit, be sure to read its instruction manual and all related safety precautions. Also, review the material safety data sheets (MSDS) for the chemicals included with the kits.

Note: These SOPs do not provide information on the use of any specific chemical kits or meters. The program recommends that whatever type of kit you choose to use, always follow the manufactures instructions and recommendations for its use and care. If you are using a meter; these must be calibrated at regular intervals and always before using them in the field. When submitting chemical data to WV Save Our Streams always describe the kit you are using. The description should include the manufacturer, kit-type, range, model number etc.