Chemical characteristics of sewage and their testing


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Chemical characteristics of sewage and their testing

  2. 2. The Chemical Characteristics of sewage helps in indicating: the stage of sewage decomposition, its strength, and extent and type of treatment required for making it safe. Therefore, chemical analysis is carried out, which includes test for determining:  total solids, suspended solids and settleable solids;  pH value;  chloride content;  nitrogen content;  presence of fats, greases and oils;  sulphides, sulphates and H2S gas;  dissolved oxygen;  chemical oxygen demand(COD);  bio-chemical oxygen demand (BOD)
  3. 3.  Solids present in sewage may be in any of these forms:  Suspended / non-filterable solids  Dissolved solids  Colloidal solids / filterable solids  Settleable solids  Further, the solids in sewage comprise of both:  Organic solids (45%)  Inorganic solids (55%)
  4. 4. TS: Total Solid TSS: Total Suspended Solids VSS: Volatile Suspended Solids FSS: Fixed Suspended Solids TDS: Total Dissolved Solids VDS: Volatile Dissolved Solids FDS: Fixed Dissolved Solids TVS: Total Volatile Solids TFS: Total Fixed Solids
  5. 5. It’s determination is important because efficiency of some of the treatment methods depends upon the availability of suitable pH value. It can be measured using potentiometer.  Chlorides found in sewage is derived from kitchen wastes, urinary discharges, industries  It can be measured by titrating the waste water(i.e. sewage) with standard silver nitrate solution using potassium chromate as indicator.
  6. 6.  Nitrogen may occur in one of the following forms:  Free ammonia, called ammonia nitrogen;  Albuminoid nitrogen, called organic nitrogen;  Nitrites and  Nitrates.  Organic nitrogen can be measured by adding strong alkaline solution of KMnO4 to already boiled water sample & again boiling the same. The Ammonia gas thus liberated is measured which gives the quantity of organic nitrogen. The sum total of ammonia nitrogen is called kjeldahl nitrogen.  Nitrates and nitrites are measured by colour matching techniques.
  7. 7.  They are derived in sewage from the discharge of animals and vegetable matter, or from the garages, kitchens of hotels and restaurants, etc.  Such matter form scum on top of the sedimentation tanks, clogs the void of filter media and affects the diffusion of oxygen, thus interfering with the normal treatment methods.  For estimating their amount, a sample of sewage is evaporated, the residue solids left is mixed with ether and poured off, and again evaporated leaving behind oils and fats as residue, which can be weighed.
  8. 8.  They are formed due to the decomposition of various sulphur containing substances in sewage.  In aerobic digestion of sewage, the aerobic and faculative bacteria oxidizes sulphur & its compounds to initially form sulphides, which ultimately breakdown to sulphates.  in anaerobic digestion, the anaerobic and facultative bacteria reduce the sulphur and its compounds to sulphides, with evolution of H2S along with methane, carbon dioxide.
  9. 9. The actual quantity of DO is governed by: •Solubility of oxygen • Temperature • Purity of water • It decreases as temperature increases  It is also determined by winklers method.
  10. 10. It is the measured amount of oxygen needed to chemically oxidize the organic matter using a strong oxidizing agent like potassium dichromate under acidic condition. It can be used to measure both biodegradable and non biodegradable organic matter.
  11. 11. It is defined as the amount of oxygen required by the microorganisms (mostly bacteria) to carry out decomposition of biodegradable organic matter under aerobic conditions. •The BOD test is widely used to determine the pollution strength of domestic and industrial wastes in terms of the oxygen that they will require if discharged into natural watercourses. It is the one of the most important test in stream pollution control activities. •This test is of prime importance in regulatory work and in studies designed to evaluate the purification capacity of receiving bodies of water. •It is also useful in design of wastewater treatment plant and also to measure the efficiency of some treatment processes. •of some treatment processes.
  12. 12. •The sample is first diluted with a known volume of aerated water. The initial D.O of the diluted sample is measured. The diluted sample is then incubated for 5 days at 20 0C. The D.O. of the diluted sample after the incubation period is found out. The difference between the initial D.O. of the diluted sample after the incubation period is found out. The difference between the initial D.O value and the final D.O. Value will indicate the oxygen consumed by the sewage sample in aerobic decomposition in 5 days. The BOD in mg/lit or ppm is then calculated by using the equation: •BOD= [D.O. consumed in the test by the diluted sample x ( Volume of the diluted sample/ Volume of undiluted sewage sample)]
  13. 13. Following are the limitation of BOD Test: •It measures only the biodegradable organic matter. •Time duration of the test is very long i.e. 5 days, so if quick results are needed it is not useful. •Pretreatment is needed if the sample contains toxic waste. •Nitrifying bacteria can cause interferences and could give higher results. To avoid them proper care must be taken. •It is essential, to have high concentration of active bacteria present in the sample.
  14. 14. 1. Compute a) the theoretical oxygen demand and b) organic carbon concentration of a water that contains: glucose(200 mg/l)and benzene (25 mg/l)? 2. HINT: Yt=L [1-(10)-KD.t] KD = 2.61 x (m/c) L = 1/ (2.3 KD C3)
  15. 15. THANK YOU...!!!