The Effect of Filtering Water through Charcoal and Sand on Nitrogen Content
Lindsey Babcock and Lelah Allen
Because clean water is a necessity for life on this planet, we tested whether water filtered through charcoal would have fewer impurities than water filtered through sand, specifically looking at nitrogen content. There were visible differences between the two filtering methods, as the water filtered through the sand exhibited more turbidity than the charcoal filtered group as well as the control water directly from the river. There was a significant difference of nitrogen content between the sand and the charcoal filtered groups (P = 0.01).
Keywords: Nitrogen, water, filtration, hydrologic cycle
Water is a necessary element for life on this planet. We need water to drink and to grow food; however, we also use water in industry and in transportation that causes pollution. The internal structure of water is extremely complex as most water also contains particles of other substances. Water is considered to be the universal solvent, because it can weather even very hard rock and dissolve many substances. The continuing water cycle brings water through physical changes. The chemical process where water dissolves and carries away minerals and salts in the soil is called leaching. Water picks up materials it comes into contact with and is changed by the chemical compounds that are created. When weakly acidic rain falls, it reacts with materials in rocks and soils causing more substances to be added to it. Water travels via gravity, and the mineral quality of the water in streams changes with the seasons. This weakly acidic water flows through streams, soil, and rocks to rivers, lakes and seas. In the spring when snows melt and the soils thaw, minerals from the decomposition of the previous fall are released and the mineral content is higher. In the fall, as the frost kills the leaves, it also stops the transpiration process and mineral content in the water changes because the movement of water between the soil and air also changes. Processes in water are much slower, and the concentrations of reacting materials are therefore much lower (Renn 1968).
Plants and animals that live in the water work together to reduce the organic content of the water. Plant roots can help maintain a balance of oxygen to penetrate the substrate and promote nitrogen reduction. Nitrogen changes during bacterial decay; from decaying plant or animal material it changes into inorganic chemical ammonia (Verhagen et al. 1993). Denitrification is the process by which organisms convert nitrite (NO3) to Nitrogen gas (N2). Most denitrifying bacteria are heterotrophic. Bacteria in the water slowly change ammonia to nitrite and then to nitrate. These relative concentrations of nitrite and ammonia can help determine the proximity of the waste discharge. Nitrates above 200ppm can be deadly (Verhagen et al. 1993).
Scientists have noticed an increase in nitrate in water sources; this is due to more nitrogen being introduced. Nitrate is a byproduct of nitrogen. The increase of nitrogen is due to an increase in fertilizers being used and animals and humans’ fecal matter. Plants do not absorb nitrate, and this leaves nitrate in the soil to seep into the water supply. This is very concerning because scientist are unsure of the health risks associated with nitrates (Ward et al. 2005).
J. Lane Notter (1878) talks about how charcoal is able to absorb matter when water is filtered through it, while sand is a good filter it does not do much actual purification. Charcoal filters are more readily used for filtration (Notter 1878). This lead us to our hypothesis that the charcoal filter will be better at removing nitrate than the sand filter will.
On October 25, 2011 at 0800, a gallon of water was extracted from the shoreline of the Fox River in Waukesha, Wisconsin. The river had no particular odor, but it did exhibit high turbidity. On October 27, 2011, the water was filtered through paper coffee filters, and then 100 mL of water was filtered again through one of two mediums: charcoal or sand. The charcoal was broken down to a texture more comparable to sand. Filters were measured at one half cup of material, however, because sand and charcoal have very different densities, the filters were of varying weight. The average weight of the sand before filtering the water was 100 g, while the average charcoal filter weight was 37 g. Paper coffee filters were attached to 500 mL beakers and the sand or charcoal to be used as filters was added. The water was allowed to strain for 10 minutes. Samples of the river water were tested for nitrogen content using the LaMotte Nitrate N Phosphate test kit (Model NPL code 3119), and were then compared to the colorimetric scale. This same procedure was repeated for the filtered samples. A 1-tailed independent t-test was used. Data was analyzed using Excel Microsoft 2010.
The sand filter was significantly greater than the charcoal filter (Fig. 1, P = 0.01) at reducing nitrate levels in water from the Fox River in Waukesha, WI. The mean of the nitrate from the sand filter was 2.2 ppm of nitrate with a standard deviation of 0.44. The mean of the nitrate from the charcoal filter was 3.7 ppm of nitrate with a standard deviation of 0.96. The mean for the control was 2.6 ppm of nitrate with a standard deviation of 1.1.
Figure 1: Mean (+/- S.D.) of nitrate (ppm) in the water from the Fox River in Waukesha, WI after filtering with the charcoal filter, the sand filter, and the control (plain coffee filter).
Our data did not support the hypothesis that the charcoal filter would filter better than the sand filter. We found that the sand filter was a significantly better filter than the charcoal. This may be because it took longer for the water to filtrate through the sand because of the finer particles. How well a filter is able to do its job can depend on the amount of water and what the water contains (Notter 1878). The charcoal was much coarser and allowed the water to travel through faster which may have left more nitrate in the water. This does not however explain why the charcoal filter has more nitrate than the control. We believe this may be due to the fact we used barbeque charcoal that may have contained additives that would increase the nitrate level. If we did this experiment again we would use a natural charcoal. We would also want the sand and charcoal to be a comparable coarseness.
Notter. 1878. The Purification Of Water By Filtration. The British Medical Journal. 2(928). 556-557. Retrieved from JSTOR.
Renn, C.E. 1968. A Study of Water Quality. LaMotte Company, Chestertown, Maryland USA
Verhagen, F., Duyts, H. and Laanbroek, H. 1993. Competition for ammonium between nitrifying
and heterotrophic bacteria in contiguously percolated soil columns. Applied
Environmental. Microbiology 58: 3303
Ward, M., eKok, T., Levallois, P., Brender, J., Gulis, G., Nolan, B. T., and VanDerslice, J., 2005. Workgroup Report: Drinking-Water Nitrate and Health-Recent Findings and Research Needs. Environmental Health Perspectives, 113(11) 1607-1614. Retrieved JSTOR.