Acidity:Can water be acidic in taste? Most natural water, domestic wastewater and many industrial wastewater are buffered by a carbon dioxide-bicarbonate system. Acid waters are of concern because of their corrosive characteristics and the expense involved in removing or controlling the corrosion-producing substances. Mineral acids are measured by titration to a pH of about 3.7.
Alkalinity:When will the water be alkaline in taste? The alkalinity of natural water is primarily due to the salts of weak acids. Although, weak or strong bases may also contribute. Natural water contains appreciable amounts of carbonate and hydroxide
alkalinity. Higher alkaline waters are usually unpalatable. Alkalinity is measured volumetrically by titration with N/50 or 0.020 NH2SO4.
Hardness:Water is more often hard. Do you agree? Hardness is caused by metallic ions that are capable of reacting with soap to form a precipitate. Calcium bicarbonate, magnesium sulfate, strontium chloride, ferrous nitrate and manganese silicate are the major
sources for hardness in wastewater. Hardness is determined using ethylene-di-amine tetra acetic acid (EDTA) or its sodium salts as the titrating agent.
Chloride:Chloride is a major contributor to the ‘total dissolved solids’ in water/wastewater. The chloride content of water/wastewater increases as its mineral content increases. Chlorides at a concentration above 1000 mg/l give a salty taste, which is objectionable to many people. Chloride concentration of wastewater is estimated by Mohr’s method using silver nitrate with potassium chromate as an indicator.
Biochemical Oxygen Demand (BOD):The strength of wastewater is judged by BOD. This is defined as the amount of oxygen required by bacteria while stabilizing the organics in wastewater under aerobic conditions, at a particular time and temperature. This can be referred as BOD5, which accounts for 70% of the total BOD. The measurement of BOD is based on the principle: determination of dissolved oxygen content of water/wastewater on the first day and dissolved oxygen content on the fifth day (’5′ in BOD5 indicates this). The difference in dissolved oxygen concentrations between first day and fifth day is expressed as BOD of wastewater.
Chemical Oxygen Demand (COD):What does COD of wastewater mean? This reflects the concentration of organic compounds present in wastewater. This measures the total quantity of oxygen required for oxidation of organics into carbon dioxide and water. The oxidation of organics in wastewater is carried out by the action of strong oxidizing agents. Generally, acidified potassium dichromate is used as an oxidizing agent for the determination of COD. Silver sulfate is used as the catalyst for the oxidation of organics in wastewater during the determination of COD. Mercuric sulfate is added to control the interference of chloride in the estimation of COD. The method consists of adding a known concentration of potassium dichromate (added with silver sulfate and mercuric sulfate) into wastewater containing organic compounds to be oxidized in the heating condition. After oxidation, the excess potassium dichromate is back titrated with ferrous ammonium sulfate.
Importance of COD:Estimation of COD expresses the total concentration of organics present in the waste water. This measures approximately the theoretical oxygen demand of wastewater. The determination accounts for about 95% of the organic concentration in wastewater. This forms about 1.43 times the BOD of wastewater. BOD to COD ratio reveals the treatability of wastewater. If the ratio of BOD/COD is above 0.5, the wastewater is considered to be highly biodegradable. If the ratio is less than 0.3, the wastewater is deemed to undergo a chemical treatment before the routine biological treatment.
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