Determination of water quality parameters
Three wastewater samples were collected at a time from the wastewater disposal canal of the NBSM at Natore: one at the head end of the canal near the mill, one at 1.5 km and the other at 6 km downstream from the canal head end. These samples were designated by WW-1, WW-2 and WW-3, respectively. One freshwater sample, collected from a hand tubewell at the NBSM complex, was designated by FW. The water samples were collected three times during the mill operating period (mid-November to mid-March) of 2010−2011. The samples were collected in 500 ml inert and opaque plastic bottles that were washed with distilled water first and with the water to be sampled immediately before collection of the samples. The temperature of the wastewater at the three locations of the canal was measured with a digital thermometer. After collection, the water samples were transferred to laboratory and stored at 4 °C for analysis within 8−10 h of collection. Nitrate-nitrogen (NO3-N), nitrite-nitrogen (NO2-N), ammonia-nitrogen (NH3-N), phosphorous (P), boron (B), zinc (Zn), manganese (Mn) and iron (Fe) contents of the freshwater and wastewater samples were measured by a DR/890 colorimeter (Hach Co., USA). The pH and electrical conductivity of the samples were measured by an EC/pH meter. The biological oxygen demand (BOD), which is the amount of oxygen required by microorganisms to stabilize biologically decomposable organic matter under aerobic condition, indicates the amount of biologically degradable organic matter in the wastewater. The BOD of the wastewater samples after 5 days (recognized standard known as BOD5) was measured with BOD sensors and incubator (VELP SCIENTIFICA, Italy). The chemical oxygen demand (COD) measures the amount of oxygen required for oxidation of organic compounds present in wastewater by means of chemical reactions involving oxidizing substances such as potassium dichromate and potassium permanganate. The wastewater samples were shaken vigorously and the required quantity of each water sample was mixed with a COD reagent in separate vials. The vials were heated at 200°C for 2 h in a COD reactor for digestion. The samples were cooled at room temperature and their COD’s were measured by colorimetric method.
Determination of soil properties
Soil samples, both wastewater affected and unaffected, were collected from two locations of the sugar mill area: West Baiddanathpur village (site 1) and East Baiddanathpur village (site 2). The site 1 was approximately half a kilometer downstream and the site 2 was 1.5 km downstream from the head end of the wastewater disposal canal. The samples were transferred to the laboratory, dried in air and sieved through a 2-mm square mesh sieve. Sub-samples were prepared from these samples that were stored in polyethylene bags for analysis. Three repacked (disturbed) samples for each location were prepared in core samplers of 5 cm diameter and 5 cm height. Three undisturbed soil samples were also collected from each location in the core samplers. The fractions of sand, silt and clay in the soils were determined by Hydrometer Method. By plotting the percentage values of these fractions on the Marshall’s triangular co-ordinate, the textural classes of the soils were determined. The EC and pH of the soils were determined by measuring them in the saturation extract of the soils by a combine EC/pH meter at 25o C. The saturated hydraulic conductivity (Ks) of the six core samples (3 undisturbed and 3 disturbed) was determined by Constant Head Method; Ks was calculated by Ks = V / tiA where, Q is the flow rate of water through the soil, V is the volume of water collected in time t, A is the cross-sectional area of the soil core, i = h / L is the hydraulic gradient, ?h is the difference in hydraulic head under which water flows through the sample and L is the length of the core sample. Accounting for the volume of water that drained from a saturated soil core by gravity in 48 h the gravitational water was calculated from the ratio of the volume of water drained to the volume of soil and expressed as a percentage. The volume of the soil was calculated from the diameter and height of the core sampler. The water retained by the soil was determined by the difference in weight of the sample after the gravitational water was drained out and that of the oven dry sample. The field capacity was then calculated from the ratio of the volume of water retained to the volume of the soil and expressed as a percentage. After measuring the gravitational water and field capacity, the porosity was calculated by adding the gravitational water and field capacity with the assumption of full saturation of the soil samples. The bulk density was calculated by the ratio of the mass of soil to the total volume of the soil. Soil-water retentions in the experimental soils were determined by using a Sand Box (Eijkelkamp, Agrisearch Equipment, The Netherlands) for low suction (<100 cm of water) and a pressure plate apparatus (Soil Moisture Equipment Corp., Santa Barbara, Ca., USA.) for high suctions (100 − 10000 cm of water). The total nitrogen, organic carbon, available phosphorus and sulphur, and exchangeable sodium and potassium contents of the soils were determined in the Humboldt Soil Testing Laboratory of the Department of Soil Science, Bangladesh Agricultural University, Mymensingh.