Chemical characteristics of sewage/Waste water/total solid, suspended solid and Dissolved solid and Settleble solid/pH values/BOD and COD/Biochemical Oxygen Demand/Per capita Load/Chemical Oxygen Demand/Nitrogen/Dissolved Oxygen/DO/Sanitary Engineering/Environmental Engineering

Chemical  characteristics of Sewage

Chemical characteristics of sewage indicate the sewage pollution extent and the type of treatment required. The basic chemical characteristics with test for determining are:

a. Total solid, Suspended and Dissolved solid and Settleble Solid:

Present in small amount (0.1%), solid can be in four different forms. These solids can be  both  organic  and  inorganic  matter.  Organic  matter  consist  of  carbohydrate, fat, nitrogenous compounds etc. and inorganic matter consist of minerals and salts.

Total solid is the sum of all kind of solid.

The  estimation  of  suspended  solids,  both  organic  and  inorganic,  gives  a  general  picture  of  the load  on  sedimentation  and  grit  removal  system  during  sewage  treatment.  Dissolved  inorganic fraction is to be considered when sewage is used for land irrigation or any other reuse is planned.

Suspended solid:

solids whose specific gravity are lighter and remains in floating stage. 

Dissolved solid: those solids that are in dissolved stage in waste water. 

Settleble  solids:  Those  portions  of  solids  (organic  or  inorganic)  matter  which  settles  out  if sewage is allowed to remain undisturbed for a period of 2 hours.

b. pH Values:

pH  of  sewage  represents  the  –ve  logarithm  of  concentration  of  hydrogen  ion. It is  a valuable parameter in the operation of biological units. The pH of the fresh sewage is slightly more than the water supplied to the community. However, decomposition of organic matter may lower the pH, while the presence of industrial wastewater may produce extreme fluctuations. Generally the pH of raw sewage is in the range 5.5 to 8.0.

ORGANIC MATTER

Heterogenous  mixture  of  various  organic  compounds, Main  component  carbohydrate,  protein and fats. Organic matter are determined by two method.

Direct method: This involves determination of total organic carbon. 

Indirect method: BOD5, COD and ultimate BOD

BOD and COD

Organic compounds present in sewage are of particular interest for environmental engineering. A large variety of microorganisms (that may be  present in the sewage or in the receiving water body)  interact  with  the  organic  material  by  using  it  as  an  energy  or  material  source.  The utilization  of  the  organic  material  by  microorganisms  is  called  metabolism.  To  describe  the metabolism of microorganisms and oxidation of organic material, it is necessary to characterize quantitatively concentration of organic matter in different forms. In view of the enormous variety of organic compounds in sewage it is totally unpractical to determine these individually. Thus a parameter must be used that characterizes a property that all these have in common.  In practice two  properties  of  almost  all  organic  compounds  can be  used:

(1)  organic  compound  can  be oxidized; and 

(2) organic compounds contain organic carbon. 

In environmental engineering there are two standard tests based on the oxidation of organic material:

1) The Biochemical Oxygen Demand (BOD) and  Chemical Oxygen Demand (COD) tests. 

In  both  tests,  the  organic  material  concentration is  measured  during  the  test.  The  essential differences  between  the  COD  and  the  BOD  tests  are  in  the  oxidant  utilized  and  the  operational conditions  imposed  during  the  test  such  as  biochemical  oxidation  and  chemical  oxidation.  The other  method  for  measuring  organic  material  is  the development  of  the  Total  Organic  Carbon (TOC) test as an alternative to quantify the concentration of the organic material.  

Biochemical Oxygen Demand (BOD):

The BOD of the sewage is the amount of oxygen required for the biochemical decomposition of biodegradable  organic  matter  under  aerobic  conditions.  The  oxygen  consumed  in  the  process  is related to the  amount of  decomposable organic matter. The  general range  of BOD observed for raw sewage is 100 to 400 mg/L

PER CAPITA LOAD

Chemical Oxygen Demand(COD)

The  COD  gives  the  measure  of  the  oxygen  required  for chemical  oxidation.  It  does  not  differentiate  between  biological  oxidisable  and  nonoxidisable material.  However,  the  ratio  of  the  COD  to  BOD  does  not  change  significantly  for  particular waste and hence this test could be used conveniently for interpreting performance efficiencies of the treatment units. In general, the COD of raw sewage at various places is reported to be in the range  200  to  700  mg/L.  In  COD  test,  the  oxidation  of  organic  matter  is  essentially  complete within two hours, whereas, biochemical oxidation of organic matter takes several weeks. In case of wastewaters  with a large range of organic compounds, an extra difficulty in using BOD as a quantitative parameter is that the rate of oxidation of organic compounds depends on the nature and  size  of  its  molecules.  Smaller  molecules  are  readily  available  for  use  by  bacteria,  but  large molecules  and  colloidal  and  suspended  matters  can  only  be  metabolized  after  preparatory  steps of  hydrolysis.  It  is  therefore  not  possible  to  establish  a  general  relationship  between  the experimental  five-day  BOD  and  the  ultimate  BOD  of  a  sample,  i.e.,  the  oxygen  consumption after  several  weeks.  For  sewage  (with  k=0.23  d-1  at  20oC)  the  BOD5  is  0.68  times  of  ultimate BOD,  and  ultimate  BOD  is  87%  of  the  COD.  Hence,  the  COD  /BOD  ratio  for  the  sewage  is around 1.7.

C. Nitrogen

The  presence  of  nitrogen  in  sewage  indicates  the  presence  of  organic  matter,  the  principal nitrogen compounds in domestic sewage are proteins, amines, amino acids, and urea. The various form of  nitrogen in sewage are 

i. Free ammonia 

ii. Albuminoidal nitrogen called organic nitrogen 

iii. Nitrites 

iv. Nitrates  

Ammonia  nitrogen  in  sewage  results  from  the  bacterial  decomposition  of  these  organic constituents  and  indicates  the  very  first  state  of decomposition.  Nitrogen  being  an  essential component  of  biological  protoplasm,  its  concentration  is  important  for  proper  functioning  of biological  treatment  systems  and  disposal  on  land. Generally,  the  domestic  sewage  contains sufficient  nitrogen,  to  take  care  of  the  needs  of  the  biological  treatment.  For  industrial wastewater if sufficient nitrogen is not present it is required to be added externally. 

Albuminoid   nitrogen   indicates   the   quantity   of   nitrogen   present   in   sewage   before   the decomposition of the organic matter is started. 

Nitrites indicate the presence of partly decomposed organic matter. 

Nitrate indicates the presence of fully oxidized organic matter. 

The test on nitrogen confirms the decomposition stage of organic matter. For example if nitrite is present  then  it  indicates  incomplete  decomposition however  if  nitrate  is  present  it  shows  that sewage  is  well  oxidized.  It  is  however  important  to  limit  the  concentration  of  nitrate  before disposal as it may cause nitrate poisoning. 

In infant it nitrate rich water causes blue baby disease. Nitrogen compound leads to a phenomenon of eutrophication.

In the process of conversion of ammonia to nitrite and nitrate, oxygen are consumed this reduces DO content. 

Free ammonia is measured by boiling of sewage and measuring the gas. 

Albuminoid nitrogen is measure by adding  potassium permanganate to the boiled sewage. If  unboilded  sample  is  used  to  add  potassium  permanganate,  it  gives  the  sum  of  ammonia nitrogen and organic nitrogen which is known as kjedahl nitrogen. 

Nitrite is measured by color matching method by adding sulphonilic acid and napthamine. Nitrates  is  measured  by  adding  phenol  di  sulphonic acid  and  potassium  hudroxide.  Generally nitrogen  content  in  the  untreated  sewage  is  observed  to  be  in  the  range  of  20  to  50  mg/L measured as TKN or 45 ppm nitrate.

d. Phosphorus:

Phosphorus  is  contributing  to  domestic  sewage  from food  residues  containing  phosphorus  and their   breakdown   products.   The   use   of   increased   quantities   of   synthetic   detergents   adds substantially to the phosphorus content of sewage. Phosphorus is also an essential nutrient for the biological processes. The concentration of phosphorus in domestic sewage is generally adequate to  support  aerobic  biological  wastewater  treatment.  However,  it  will  be  matter  of  concerned when the treated effluent is to be reused.  It is essential for microorganism growth but in excess amount it leads to eutrophication.The concentration of PO4 in raw sewage is generally observed in the range of 5 to 10 mg/L. 

e. Cloridshe: 

Municipal  water  contains  large  quantity  of  chlorides,  derived  form  the  kitchen  wastes,  human feaces  and  urinary  discharges  etc.  The  normal  chloride  content  is  120  ppm  however  in  water supplies it is accepted to 250ppm.  

Concentration  of  chlorides  in  sewage  is  greater  than  the  normal  chloride  content  of  water supply. The chloride concentration in excess than the water supplied can be used as an index of the strength of the sewage. The daily contribution of chloride averages to about 8 gm per person. Based  on  an  average  sewage  flow  of  150  LPCD,  this  would  result  in  the  chloride  content  of sewage  being  50  mg/L  higher  than  that  of  the  water supplied.  Any  abnormal  increase  should indicate discharge of chloride bearing wastes or saline groundwater infiltration, the latter adding to the sulphates as well, which may lead to excessive generation of hydrogen sulphide. 

Dissolved oxygen DO 

DO  content  represents  the  amount  of  oxygen  in  dissolved  state  in  sewage.  It  is  an  important parameter that indicates the purity of water and hence waste water. In case of sewage, if it is too polluted, a situation may arise when it has no DO content, in such cases it becomes important to increase  the  DO  content  of  sewage  by  aeration.  Presence  of  DO  content  facilitates  aerobic decomposition of waste water. After treatment, it is important that sewage contain at least 4 ppm DO  content  before  disposing  to  the  receiving  water value.  4  ppm  DO  content  is  required  to maintain the aquatic life of the water bodies. It is determined by wrinklers method in lab. 

Oil and grease:  These are fraction of organic matter that are soluble in hexane. It constitutes about 10 % of total organic matter.