Water quality which is not met the standard can cause disturbance and health problems. Water
that contain high hardness can lead to soap wastefulness, forming precipitation and cookware
crust, and become one of the risk factors for kidney failure. Various ways can be applied to reduce water hardness, one of which is filtration using ion exchange media. The ion exchange
media used in this study is cation resin. The purpose of this study was to determine the effect of
cation resin thickness variation at filter on the decrease of well water hardness. This research
was an experiment with pre-test post-test with control group design, and was conducted in January 2018, located in Pereng hamlet of, Bumirejo, Lendah, Kulon Progo Regency. The data
were analysed using one way anova. The average water hardness before processing was 460
mg/l, while the mean decrease in the control group was 42,66 mg/l; in P1 group was 197,33 mg
/l, in P2 group was 197,33 mg/l, and in P3 group was 197,33 mg/l. The results of data analysis
showed that the thickness variations of cation resin influence the decrease of well water hardness in the control group, and P1, P2, and P3 groups (p-value <0,001). The most effective cation resin thickness is the P3 group, i.e. filter with 11 cm thickness cation resin.