Assessment of surface water quality using Fuzzy Analytic Hierarchy Process (FAHP): A case study of Piyain River's sand and gravel quarry mining area in Jaflong, Sylhet

Abstract The aim of this study is to assess the quality of surface water of Piyain River adopting two different approaches. The first approach, determines the classification of water samples by the traditional water quality index, and the second one is focused on water classification analysis with the aid of the Mamdani Fuzzy Logic (MFL). For this case, the relative weight of water quality parameter is computed based on the Analytic Hierarchy Process (AHP). The AHP is a multiple criteria decision making procedure which provides a methodology to calibrate the numeric scale for the measurement of quantitative as well as qualitative performances. The relative importance of different attributes is given on a scale from 1 to 9. In fuzzy water quality assessment procedure, the trapezoidal membership functions are used because of its highest classification accuracy. Also, it can characterize the dynamic behavior of the water quality parameters. Twenty-one parameters are tested for the assessment of water quality. However, arsenic is not found in the water samples, so the rest twenty parameters is divided into three groups for utilizing the hierarchy by grouping some criteria. Total sixty-nine rules are used, three for each indicator and three for each partial group. In hydro-chemical analysis it is found that typical value of Sulfate (0.12–2.8 mg/L), Sodium (2–5.7 mg/L), Chloride (1.05–4.11 mg/L), Potassium (0.4–2.6 mg/L) is extremely lower thanstandard value where the standard for Bangladesh is 400 mg/L, 200 mg/L, 150–600 mg/L, 12 mg/L respectively. Specific Conductivity (45.7–135 μS/cm), Calcium (5.5–28 mg/L), Magnesium (1–24.5 mg/L) are below standard where the standard values are 500 μS/cm, 75 mg/L, 30–35 mg/L correspondingly. The measured value of pH (6.31–6.96) is between WHO and Bangladesh standard (6.5–8.5). Turbidity (3.5–99 NTU) is higher with respect to WHO guideline (5 NTU) &Bangladesh standard (10 NTU). The computed WQI value ranges from 19.84 to 154.21 whereas, FWQI1 and FWQI2 value ranges from 47.6 to 78.4 and from 51.8 to 80.7, respectively. It suggests that the surface water of the river exhibits good water condition. Besides, the quality of water sample from quarry area is found average to good. In a nutshell, FWQI2is more reliable than FWQI and WQI. Hence, it can be applied for monitoring water quality problem locally or regionally.