Characteristics of sewage

I. Physical characteristics of sewage:

1. Temperature:

• Temperature of sewage depends upon season. However temperature is slightly higher than that of ground water.
• High temperature of sewage is due to evolution of heat during decomposition of organic matter in sewage.

2. Color:

• Color of sewage indicates its strength and age.
• Fresh domestic sewage is grey in color but septic sewage is dark in color
• When industrial effluent is mixed it give characteristic color to sewage

3. Odor:

• Fresh domestic sewage is almost odorless.
• Septic or stale sewage is putrid in odor which is due to generation of H2S during anaerobic decomposition of organic matters.
• When industrial effluent is mixed, it give characteristics odor to sewage

4. Turbidity:

• Sewage is highly turbid.
• Turbidity of sewage is due to dissolved substances, colloidal matters, suspended solids and microbial cells.

 II. Chemical characteristics of sewage:

1. Organic matter:

• In general sewage contains large amount of organic matters. However amount of organic matter depends on types and condition of sewage.
• Organic matter in sewage may be found in the form of dissolved substances, colloidal matter, suspended or sedimented form.

2. Chloride:

• Human beings discharge large amount (8-15gm/day) of chloride in the form of NaCl, especially through urine and sweat. So domestic sewage from toilet and bathroom contains higher level of chloride.
• Sulfite:
• In sewage sulfite in the form of H2S (hydrogen sulfite) is generated during anaerobic decomposition of organic matters by anaerobic bacteria.
• H2S gives putrid odor to sewage.

3. Biological oxygen demand (BOD):

• Sewage usually have high BOD due to presence of large amount of organic matters.
• Value of BOD ranges from 100mg/ltr for very dilute sewage to 600mg/ltr or more for concentrated sewage containing industrial effluent mix.

4. Dissolved oxygen (DO):

• Due to high level of microbial cells and biodegradable organic matters, sewage have very low level of dissolved oxygen.
• In some sewage, DO is completely absent.
• Level of Do depends on age and condition of sewage. Low level DO is also due to lower solubility of oxygen in sewage. Oxygen is only 95% soluble in sewage than in pure water).

5. pH:

• sewage is slightly alkaline in pH

6. Nitrogen:

• In sewage nitrogen is found in variety of form like organic nitrogen, ammonia, nitrite, nitrate etc
• Fresh sewage mainly contains organic nitrogen and very little inorganic form of nitrogen. On the other hand organic septic sewage contains high inorganic nitrogen and low organic nitrogen.
• In sewage nitrite never accumulate in concentration greater than 1mg/ltr because it is intermediate product during conversion of ammonia into nitrate (NO3).
• In sewage treatment plant, NH3 and NO2 are finally converted to NO3.

7. Oxidation-Reduction (O-R) potential:

• Oxidation-Reduction potential indicates energy state of sewage in terms of its oxidizing or reducing potential.
• O-R potential is very valuable index to monitor sewage treatment plant.
• In aerobic treatment process like tripling filters, positive OR potential of about +2—to +600 is needed. In anaerobic treatment process like sludge digestion, negative OR potential of about -100 to -200 is needed.

III. Biological characteristics of sewage:

1. Bacteria:

• Two types of bacteria are found in sewage.
  • Intestinal bacteria:
    • Non-pathogenic intestinal bacteria are normal flora of gastointestinal tract of human and animals and enter into sewage together with stool. Examples; faecal coliform, faecal streptococci, Clostridium perfingens, et
    • Pathogenic intestinal bacteria such as Salmonella, Shigella, Vibrio cholera, Yersenia enterocolitica etc enter into sewage through stool of patients.
  • Real sewage bacteria
    • The natural habitat of these bacteria is sewage.
    • Both aerobic as well as anaerobic are found in sewage.
    • Aerobic bacteria play important role in oxidation of organic matter during aerobic process.
    • Common anaerobic bacteria includes;
      • Clostridium sporogens
      • Bifidobacterium
      • Peptococcus
      • Methanogenic bacteria like Methanobacterium, methanosarcina
    • Common aerobic bacteria includes;
      • Zeoglea remigera
      • Noacrdia
      • Flavobacterium
      • Achromobacter
      • Nitrosomonas
    • ** Zeoglea remigera is the main organism found in trickling filter.

2. Algae:

• Some algae found in sewage includes Chlorella phormidum, Ulothrix etc
• Algae are used in trickling filter in sewage treatment plant

3. Fungi:

• Fungi like Fusarium and Sporotricum are found in sewage which play important role in trickling filter.

4. Virus:

• Some viruses causing human disease such as Poliovirus, Rotavirus, Hepatitis A and E etc are found in sewage which get access through stool of patients.

5. Protozoa:

• Some protozoa that cause disease of intestinal tract enter into sewage together with stool of patient.
• Examples: Entamoeba histolytica, Giardia, Balantidium coli etc are pathogenic protozoa
• Few protozoa such as Vorticella and Opercularia are found in trickling filter.

Physical, chemical and Biological characteristics of sewage