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Sewage water Charactors

The important parameters for assessing the characteristics of the sewage are temperature, pH, Colour and Odour, Total Suspended Solids (TSS), Total Dissolved Solids (TDS), Nitrogen, Phosphorous, Chlorides, Bio-chemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Faecal Coliforms and Toxic metals. Brief discussion of these characteristics is presented in the following sections.

  • Temperature

Temperature of wastewater influences the solubility of oxygen, which affects oxygen transfer capacity of aeration equipments and rate of biological activity. Normally the temperature of domestic wastewater is slightly higher than that of the water supply. Observation of temperature of wastewater is, therefore essential.

  • pH

The hydrogen ion concentration, more conveniently expressed as pH, is a valuable parameter in the operation of biological treatment units. The pH of fresh domestic wastewater is slightly higher than that of the water supply to the community i.e., 7.0 to 7.5. However, the onset of septic conditions of wastewater may lower the pH value, while the presence of industrial wastes may cause extreme fluctuations in pH.

  • Colour and odour

Fresh domestic wastewater has a slightly soapy and earthy odour and cloudy appearance depending upon its concentration. With passage of time, the wastewater becomes septic, dark in colour with a pronounced foul smell due to microbial activity.

  • Total suspended solids (TSS)

Though wastewater contains only about 0.1% solids, the nuisance caused by the solids cannot be overlooked, as 70% of them are highly decomposable organic matter. Concentration of the organic fraction of the solids is, therefore, necessary. The estimate of the total suspended solids (TSS), both organic and inorganic, gives a general picture of the load on wastewater treatment. The TSS is expressed in mg/L.

  • Total dissolved solids (TDS)

Total dissolved solids (TDS) are the concentration of dissolved solids of organic and inorganic nature. Dissolved inorganic fraction is to be considered when treated wastewater is used for irrigation or use of reclaimed wastewater for non-potable purpose is planned. The TDS is expressed in mg/L. Effluent with TDS value of > 2100 mg/ L is not suitable for irrigation.

  • Nitrogen

Nitrogen is an essential component of biological process. Its determination in wastewater is necessary for proper biological treatment and land irrigation of effluent. Generally domestic wastewater contains sufficient nitrogen, to take care of the needs of the biological treatment. The Nitrogen is expressed in mg/ L.

  • Phosphorus

Phosphorus is contributed to domestic wastewater from food residues containing phosphorus and their breakdown products. The use of increased quantities of synthetic detergents adds substantially to the phosphorus content of wastewater. Phosphorus, like nitrogen, is an essential nutrient for biological processes. Generally domestic wastewater contains adequate quantities of phosphorus needed for biological treatment. The phosphorus is expressed in mg/ L.

  • Chlorides

Concentration of chlorides in wastewater is slightly higher than the normal chloride content of water supply and is used as an index of the strength of the wastewater. Any abnormal increase should indicate discharge of chloride bearing wastes or saline groundwater infiltration. The chloride is expressed in mg/L.

  • Biochemical oxygen demand (BOD)

Bio-chemical oxygen demand (BOD) of wastewater is defined as the amount of oxygen required for the biological decomposition of biodegradable organic matter under aerobic conditions. The term biodegradable may be interpreted as the organic matter, which can serve as food for the bacteria, and energy is derived from its oxidation. Theoretically at 20°C an infinite time is required for complete biological oxidation of organic matter, but for all practical purposes, the reaction may be considered complete in 20 days. However, a 20-day period is too long to wait for results in most instances. It has been found by experience that a reasonably large percentage (70 to 80%) of the total BOD is exerted or oxidized in 5 days. Hence the standard BOD test is carried out, by incubating the sample with proper dilution in an incubator for a period of 5 days at 20°C.

  • Chemical oxygen demand

Chemical oxygen demand (COD) is defined as the amount of the oxygen required for chemical oxidation of both biodegradable and non-biodegradable organic substances. COD is expressed in mg/ L.

  • Toxic metals

Some heavy metals such as chromium, copper, cyanide etc., which are toxic to aquatic organisms may find their way into municipal sewerage through industrial discharges and present in significant concentrations in wastewater. Determination of these metals assumes importance if such wastewater is to be treated by biological process or disposed of in to stream or land.

  • Typical characteristics of wastewater

Table 1 lists the characteristics of domestic wastewater of different strengths. The strength of the domestic wastewater varies with the per capita water supply available. Since the per capita contribution of organic matter is constant (only a slight variation due to food habits) the strength of the wastewater depends on the per capita water available for dilution. Table 2 shows the per capita contribution of different characteristics of domestic wastewater.

Table 1 . Major constituents of typical domestic wastewater

Constituent/Parameter Concentration, mg/L
Strong Medium Weak
Total solids 1200 700 350
Dissolved solids (TDS) 850 500 250
Suspended solids 350 200 100
Nitrogen (as N) 85 40 20
Phosphorus (as P) 20 10 6
Chloride’ 100 50 30
Alkalinity (as CaCO3) 200 100 50
Grease 150 100 50
BOD5 300 200 100

Table 2. Per capita contributions of domestic wastewater characteristics

It Range of values contributed
in wastes(g/capita-day)
BOD, 5 days, 20°C 45 – 54
COD 1.5 to 1.9 x BOD,
Total solids 170 – 220
Suspended Solids 70 – 145
Grit (inorganic, 0.2 mm and above) 5 – 15
Alkalinity (as CaCO3) 20 – 30
Chlorides 4 – 8
Total nitrogen, as N 6 – 12
Total phosphorous, as P 0.6 – 4.5