Wastewater treatment

• The purpose of waste water treatment is to remove contaminant from water so that the treated water can meet the acceptable quality standard. The quality standard usually depends whether the water will be reused or discharged into river.
• Methods of waste water treatment depends on composition of waste water and required quality for treated water. Treatment process are broadly classified as physical, chemical and biological treatments.
• Physical treatment methods utilizes physical separation of pollutant such as by filtration etc.
• Chemical treatment methods utilizes chemical characteristics of pollutant for purification. For eg. Coagulation etc
• Biological treatment methods utilizes biological characteristics of pollutants such as bacteria, viruses by purification.
• Other purpose of waste water treatment includes;
  • To reduce strength of sewage
  • To make waste water less offensive
  • To prevent public health from toxic effect of pollutant
  • To conserve nature

Steps of sewage treatment process:

I. Preliminary treatment of wastewater:

• The main objective of preliminary treatment is to remove gross solids (such as plastics, cloths, cans, dead body of animals etc), grits and fats from waste water.
• Some of the treatment technique applied for preliminary treatment purpose are;

i. Screening:

• Screening is used to remove gross solid waste like plastics, cloths, dead animals from waste water.
• For this purpose waste water is passed through a metal screen which consists of vertical or inclined steel bars usually set 5 cm apart.
• The removes gross solids are disposed by burning or composting.

ii. Grit removal:

• Grits are small, non-biodegradable particles which are heavier than suspended organic matters.
• Grits are removed by carefully regulating the flow velocity of sewage in grit removal tank

iii. Skimming:

• Skimming is the process of removal of fatty and oily material from sewage.
• In this method, sewage is placed in skimming tank and it is aerated from bottom so that fats and oils are collected at top of the liquid which are then removed by skimming.

II. Primary treatment of wastewater:

• After removal of gross solids, grits and fats, next step in treatment is removal of remaining suspended solids as much as possible.
• The main objective of primary treatment is to reduce strength of sewage by removing suspended materials.
• Some common technique applied for primary treatment of sewage are:

i. Sedimentation:

• Sedimentation tank is used for removal of suspended solids and some organic matters.
• There are different types of sedimentation tank.
• Common example is rectangular horizontal flow tank. In this tank sewage flow very slowly (1-2 feet/min) such that solids present is waste water settle at bottom.
• Settled solids are periodically removed by sludge scrapper.
• This technique removes about 90% of suspended solids and about 40% of organic matters from sewage.

ii. Mechanical flocculation:

• In this method sewage is paced in a flocculation tank, then sewage is rotated at an speed of 0.43m/sec with the help of rotating paddles
• While sewage rotates in circular motion, small size dissolved solids attached to each other to form large size solids and settles at the bottom which is then removed out.

iii. Chemical flocculation:

• In this method, sewage is placed in coagulation tank and then some precipitating agents such as alum is added.
• Alum forms precipitate of Al(OH)3, suspended solids attached to the precipitate such that size of precipitate increase gradually to settle down at bottom.

iv. Neutralization:

• If sewage is highly acidic or basic, it is neutralized by adding base or acid to facilitate growth of microorganisms during secondary treatment process.

III. Secondary treatment of waste water:

• In secondary treatment, dissolved or colloidal organic matters are present in sewage are removed by utilizing microorganisms. In this steps, microorganisms utilizes organic matter and converts them into inorganic minerals.
• Following changes occurs in sewage during secondary treatment;
  • Organic matter (carbon) is oxidized into CO2 and H2O
  • Organic nitrogen compounds are first converted into NH3 and then into NO3
  • Colloidal matters are coagulated or precipitated out.

• Thus main purpose of secondary treatment of sewage is to reduce BOD level.
• Various techniques are used in secondary treatment of sewage. Some of them are;

i. Trickling filter:

• Trickling filter consists of filtering bed, spraying arm and water collecting chamber.
• Filtering bed consists of well graded gravel, broken stone of size (40-150mm diameter).
• Effluent or sewage from primary treatment tank is sprayed uniformly over the filter bed. During filtration a gelatinous layer of bacteria, algae, protozoa and some fungi is produced on the surface of filter bed. This layer is called Zoogleal layer.
• As the water trickles through the filter bed, organic matter present in it are oxidized by microorganism of zoogleal layer.
• Although trickling filter is classified as aeration process of sewage treatment, it is facultative system. It is because aerobic bacteria lies on the top of the filter bed whereas anaerobic bacteria lies in middle or bottom of filter bed.
• Trickling filter can reduce BOD of sewage by about 65-85% depending on the rate of filtration.

ii. Oxidation ditch:

• Oxidation ditch consists of circular canal with inlet and outlet.
• In this method, sewage from primary treatment plant is placed in oxidation ditch and then it is agitated with the help of mechanical rotator and then left for a period of about 12-24 hours.
• During the period of oxidation, microorganism present in sewage oxidize the organic matter.
• Finally the sewage is removed from oxidation ditch through outlet for tertiary treatment.
• Oxidation pond or lagoon:
• Oxidation pond is also known as lagoon or reduced pond or stabilization pond.
• It is an aerobic method of sewage treatment technique.
• In this treatment method, sewage from primary treatment plant is placed in an oxidation pond and left there for 10-40 days.
• During this period in oxidation pond, microorganisms oxidize the organic matter present in sewage. Oxygen released by algae during photosynthesis is utilized by microorganism for oxidation of organic compounds. During oxidation CO2 and H2O are released which are utilized by algae for photosynthesis. Therefore there is mutually beneficial relationship between algae and bacteria.
• Some oxygen is also derived from atmosphere for oxidation because oxidation pond is open system.
• The oxidation pond remains aerobic during day time and first hours of night. During this period oxidation of organic compound (aerobic decomposition) takes place. During rest hours of night condition become anaerobic and anaerobic decomposition of organic compound takes place.
• Advantage of oxidation pond;
  • It is very simple and easy technique
  • Treated sewage can be utilized for irrigation
• Limitation of oxidation tank:
  • Holding time is very long (10-40 days)
  • It require large area
  • It creates bad odor. Furthermore it may become breading place for mosquitoes and other vectors
  • It is influenced by seasonal temperature. It is effective only in warm climate but not in cold and rainy season.

iii. Activated sludge system:

• Activated sludge system, consists of aeration tank, settling tank and sludge return system.
• At first sewage from primary treatment plant is mixed with sludge drawn from previous batch, which is known as activated sludge or return sludge.
• The activated sludge contains large number of microorganisms and serves as inoculum of microorganisms.
• After mixing of activated sludge, sewage is placed in aeration tank. In aeration tank. Sewage is continuously aerated for 6-8 hours. During this period microorganisms oxidizes the organic compounds to form CO2, H20 and NO3 etc.
• After oxidation, sewage is passed to settling tank and left undisturbed for 2-3 hours. Sludge settle to the bottom. This sludge is called activated sludge which is fully oxidized and is very offensive. This activated sludge can be used as inoculum for next batch of sewage.
• Most of the sludge is removed and some is returned to aeration tank for next round of treatment.
• By sludge digestion process, BOD of sewage is reduced by 5-15%.

iv. Septic tank:

• Septic tank is used for disposal of content of toilet where sewage system is not available for disposal.
• Septic tank is prepared under the ground.
• Sewage along with toilet content is placed into septic tank where heavier solid wastes settle down to from sludge whereas lighter solids including fats form layer on top of sewage called scum.
• In septic tank organic compounds in sewage is anaerobically digested by anaerobic microorganisms such as Methanogenic bacteria.
• After anaerobic decomposition, the sludge become stable and inoffensive whereas liquids in sewage percolates into soil from septic tank.

IV. Tertiary or final treatment of waste water:

• Tertiary treatment of waste water is final treatment process in which all the chemical and biological agents are completely removed from sewage before disposal into river.
• The main objectives of tertiary treatment process;

i. Removal of suspended solids:

• Suspended solids are removed by two methods:
  • Microstraining:
    • In this method, sewage is placed in rotating drum filter of pore size 25-35 µm and then drum is rotated,
    • During rotation, clear water comes out of drum and suspended solids remains inside drum.
  • Chemical coagulation and filtration:
    • Chemical coagulation and filtration:
      • In this method, precipitating agents such as alum is added in sewage. Fine suspended solids adsorbs to the surface of Al(OC)3 precipitate, finally precipitate with adsorbed solids are separated by filtration.

ii. Removal of dissolved solids like salts:

• Various technique are used for this purpose such as adsorption by activated carbon, reverse osmosis
  • Adsorption by activated carbon:
    • Dissolved solids can be removed by filtering the water through filter containing activated carbon particle.
  • Reverse osmosis:
    • Reverse osmosis removes dissolved solids like NaCl and microbial cells

iii. Removal of nitrate and phosphate

• If sewage after treatment is to be discharge into river, nitrate and phosphate should be removed from sewage before disposal. It is because nitrate and phosphate causes eutrophication.
• These plant nutrients are removed by biological process. At first sewage is placed in a tank containing nitrifying bacteria. These bacteria converts ammonium salt and nitrite into nitrate
• Then the sewage is placed into second tank containing denitrifying bacteria. These bacteria converts nitrate into Nitrogen gas that leaves the sewage.
• Phosphate is also removed by bacteria by microbial assimilation process.

iv. Killing of microorganisms

• Finally microorganisms in sewage are killed by disinfection like chlorination.

Sewage treatment; Process of wastewater treatment