Important points related to Environmental engineering

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Important questions related to Environmental engineering

    • The gases which are generally evolved during anaerobic decomposition of sewage are CO2+ NH3+ H2S + CH4
    • The deviation from the ideal flow of wastewater in the sedimentation tank causes short circuiting. It results in variation in the detention period in the top and bottom layer of wastewater in the tank.


    • The septic tank has long detention period of 12 to 36 hours.
    • The detention period for a sedimentation tank without coagulation is 4 hours, whereas with coagulation is 2 – 2.5 hours.


    • Oxidation ponds have a long detention period between 10 to 20 days.
    • The average time required by water to pass through the settling tank is called ‘flowing through period’ which is always less than the detention period.


    • Ppm( parts per million) can be written as milligram per litre (mg/L). This measurement is the mass of a contaminates per unit volume of water.
    • Overflow rate = Q/A where Q is discharge rate in litre per day, A is horizontal area in m2


    • Phenolphthalein indicator turns water into pink color, if the pH is more than 8.3
    • Disinfection is a process which is done to kill microorganism present in the water after the filtration process.


    • When there is no recirculation of treated sewage in high rate trickling, the recirculation
      factor is unity.
    • The dose of lime used in the excess lime treatment is 10-20ppm and the excess lime can be removed by the process of re-carbonation.


    • pH of sea water varies from 7.5 to 8.4
    • Addition of KMnO4 to water imparts pink color. It not only kills the bacteria, but also helps in oxidizing the taste producing organic matter.

BOD/COD ratio:

BODultimate deals with oxidation of biodegradable organics and COD deals with oxidation of biodegradable organics(BOs) and non-biodegradable organics(NBOs). The difference will represent NBOs. BODu/COD ratio will be less than or equals to 1. Hence more the number NBOs less will be the BOD/COD ratio and vice versa. It is considered that for fully biodegradable wastewater the ratio lies between 0.92 to 1.0.

Biological Oxygen Demand(BOD):– The amount of oxygen required to oxidize(loss of electron) or decompose the organic matter present in sample of wastewater by the action microorganisms under aerobic conditions at standard temperature(i.e.20˚) for 5 days. BOD helps in

    • determining strength of sewage


    • BOD also tells the fresh water required for efficient disposal of wastewater by dilution process and


    • size of unit installation for treatment of wastewater.

Chemical Oxygen Demand(COD):–The amount of oxygen required to oxidize biodegradable and non biodegradable matters in the presence of chemicals(such as potassium dichromate) only for 3 hours as compared to 5 days of BOD test. When there is lack of time we generally considered COD test for the control and operation of treatment plants.

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Arbaj Demrot is the founder of VideRime Online Learning, a leading engineering website. He did his BE Civil and M.Tech Structure from RGPV University, Bhopal and has been working as an Assistant Professor in a reputed college.

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