LARGE SCALE SEWAGE TREATMENT

LARGE SCALE SEWAGE TREATMENT

• Sewage treatment on a large scale of populations of city is known as Large Scale Sewage Treatment.
• In Cities, Sewage and Garbage are generated in massive amount per day which is treated by Large Scale Sewage Treatment plants regularly.
• The principal objective of Wastewater treatment is generally to allow Human and Industrial effluents to be disposed of without danger to human health or unacceptable damage to the natural environment.
• Overall processes of conventional Large Scale Sewage Treatment plant can be divided into three steps:

1)    Primary treatment (Physical method) - Physical separation of Insoluble materials to lower Biological Oxygen Demand (BOD)

2)  Secondary treatment (Biological method) - Microbial decomposition of Organic materials in the Effluents.

3)  Tertiary treatment (Chemical method) - Chemical removal of inorganic nutrients and pathogenic microbes.

4)    Sludge Treatment (Anaerobic digestion of Sludge)

Large Scale Waste Water Treatment

1) PRIMARY TREATMENT

• Primary treatment method is a Physical method used to remove Solid wastes from sewage.
• During Primary treatment, Wastewater is pumped into Settling tank where Heavier material solids sink to the bottom (as Sludge) and Lighter weight solids float to the top surface (Sedimentation).
• After Alum and other Coagulating agent is added as a Flocculating agent, the Sludge is removed, and the partially clarified water is further treated.
• Primary treatment removes 25 % to 35 % of the BOD in the water.
• The Sludge is removed on either a Continuous or an Intermittent basis, and the Effluent (the liquid flowing out) then undergoes Secondary treatment.

 2) SECONDARY TREATMENT

• Secondary treatment is also called Biological treatment or Microbial degradation. The microbial activities may be Aerobic or Anaerobic.
• By this Secondary treatment process, about 90 – 95 % of the BOD and many pathogens are removed.
• Reduction of BOD by 90 – 95 % is achieved through

ü  Mineralization of small fraction of Organic matter

ü  Conversion of large proportion to Removable solids.

•         There are several means by which BOD can be reduced in Secondary treatment. They are

a)     Oxidation ponds

b)    Trickling filter

c)     Activated sludge process

d)    Rotating Biological Contactor (RBC)

a) Oxidation Ponds

Oxidation Pond

• The Oxidation ponds (also called Lagoons or Stabilization ponds) permit the growth of Algal forms on Waste water effluent.
• Oxidation pond is used for Secondary treatment in Rural areas or Industrial sectors.
• The Organic materials are degraded by Heterotrophic bacteria (microorganisms that use Organic carbon as food) into Simpler forms that in turn support the growth of Algae. Algae use these nutrients to increase their biomass.
• Oxygen evolved by Algae after Photosynthesis maintains the Oxygen deficit created by Heterotrophs.
• The algae growing in oxidation ponds are Chlorella pyrenoidosa, Chlorella ellipsoides, Scenedesmus acutus, Scenedesmus quadricauda and Spirulina platensis.
• Secondary treatment through Oxidation ponds is the Aerobic sewage treatment device.
• The efficiency of Oxidation process can be improved by constructing Shallow ponds.

b) Trickling Filter

Trickling Filter

• Aerobic Secondary Treatment also can be carried out with a Trickling filter.
• The waste effluent (Sewage) is sprayed over Pile of rocks (Stone bed about 2 m deep and Individual rocks are 5 – 10 cm in dm). A Revolving sprinkler (Distribution arm) is suspended over a Stone bed distributes the liquid sewage over it. The Microbial biofilms have developed on the Stone bed, and the microbial community degrades the Organic waste (about 80 to 85 %) and reduce the BOD of effluent.
• The Stone bed is usually coated with Slimy bacterial growth mainly by Zooglea ramigera and other Slime producing bacteria. The Slime is colonized by the Heterotrophic microorgan­isms. Example - Bacteria (Beggiatoa alba, Sphaerotilus natons, Achromobacter sp., Pseudomonas sp. & Flavobacterium sp.), Fungi, Nematodes and Protozoa.
• Trickling filter based Secondary treatment is completely an Aerobic process and the microorganisms get air through Porous bed.
• A newly constructed bed needs a few weeks to function efficiently unless the Zoogleal film is coated over it.

c) Activated Sludge Process

• Activated sludge systems are quite efficient: they remove 75 – 95 % of the BOD from sewage.
• Activated Sludge Process treatment contains two main steps. They are

                 i) Aerobic oxidation

                ii) Settling-out process

Activated Sludge System

i) Aerobic oxidation

• In the aeration tanks of an Activated sludge system, Air or pure Oxygen is passed (Aeration) along with Agitation through the Effluent from the Primary treatment.
• The name Activated sludge process is derived from the practice of adding some of the Sludge from a previous batch to the Incoming sewage. This inoculum is termed Activated sludge because it contains large numbers of Aerobic sewage-metabolizing microbes.
• The activity of these Aerobic microorganisms oxidizes much of the sewage Organic matter into Carbon-di-oxide and Water.
• The important members of this microbial community are species of Zooglea ramigera bacteria, which form bacteria-containing masses in the Aeration tanks called Floc or Sludge granules.
• The other microorganisms found in Activated sludge flocs are the Heterotrophs such as Gram nega­tive rods (Escherichia coli, Enterobacter sp., Pseudomonas sp., Achromobacter sp. and Flavobacterium sp.), Arthrobacter sp., Corynebacterium sp., Mycobacterium sp., Sphaerotilus sp., Large filamentous bacteria, Some filamentous fungi, Yeasts and Pro­tozoa.
• Soluble organic matter in the sewage is incorporated into the Floc and its microorganisms.

ii) Settling – out process

• Aeration is discontinued after 4 to 8 hours, and the contents of the tank are transferred to a Settling tank, where the floc settles out, removing much of the organic matter. These solids are subsequently treated in an Anaerobic sludge digester, which will be described shortly.
• Final, the clear effluent is Disinfected and Discharged.
• Probably more Organic matter is removed by this Settling-out process than by the relatively Short-term Aerobic oxidation by Microbes.

Bulking in Activated sludge system

• Occasionally, the Sludge will float rather than settle out. This phenomenon is called Bulking.
• When Bulking happens, the Organic matter in the Floc flows out with the discharge effluent, resulting in local pollution.
• The Bulking in sludge is caused by mas­sive development of Filamentous bacteria (Sphaerotilus sp., Baggiatoa sp., Thiothrix sp. & Nocardia sp.) and Filamentous fungi (Cephalosporium sp., Cladosporium sp. & Geotrichium sp.).

Advantages of Activated sludge system

• Significant reduction in BOD and suspended solids.
• Reduction in Intestinal pathogens.
• Requirement of little land area for construction.
• No need of high dilution of final effluent.

d) Rotating Biological Contactor

• Another biofilm-based design for secondary sewage treatment is the Rotating Biological Contactor system.
• Rotating Biological Contactor is a series of disks several feet in diameter, mounted on a shaft. The disks rotate slowly, with their lower 40 % submerged in wastewater.
• Rotation provides Aeration and contact between the Microbial biofilm on the disks and the wastewater. The rotation also tends to cause the accumulated biofilm to slough off when it becomes too thick. This is about the equivalent of floc accumulation in Activated sludge system.

3) TERTIARY TREATMENT

• The effluent from Secondary treatment contains only 5 % to 20 % of the original quantity of Organic matter. So, it requires Tertiary treatment (Chemical method).
• Tertiary treatment is an extremely costly process that involves Physical and Chemical methods. Fine sand and Activated Charcoal are used in Filtration. Various Flocculating chemicals precipitate Phosphates and particulate matter.
• Finally, the Wastewater from Secondary treatment is Disinfected, usually by Chlorination, after which the Wastewater is either released into Rivers or the Ocean or, in some states used to Irrigate agricultural crops.
• The Chlorine-containing effluent, when released into streams and lakes, can react to produce Carcinogenic compounds that may enter the Food chain or be ingested directly by humans in their drinking water. It would be safer to remove the Chlorine before releasing the effluent, but this is rarely done today, although the cost is not great.
• Ultraviolet lights are now replacing Chlorination as the final treatment of Effluent. They destroy microbes without adding Carcinogens to our streams and waters. Likewise, especially in Europe, the treatment of effluent with Ozone is replacing Chlorination. Ozone generators are simple and not very costly, and they do not add Carcinogens to natural waterways.
• Instead of Chlorination, some communities remove nitrates, phosphates, and any remaining BOD or microorganisms from the water by passing it over Fine Sand Filters and Activated Charcoal Filters. Nitrate is converted to Ammonia and discharged into the air (removes roughly 50 % of the Nitrogen content), whereas Phosphorus is precipitated using Lime or Alum (removes 70 % of the Phosphorus content).
• Tertiary treatment is expensive; therefore, it is not employed unless very necessary.

ANAEROBIC SLUDGE DIGESTION

• Sludge accumulates in Primary sedimentation tanks, Activated sludge process and in Trickling filter Secondary treatments. For further treatment, these sludges are often pumped to Anaerobic sludge digesters.
• The process of Sludge digestion is carried out in large tanks (Anaerobic Sludge Digesters) from which oxygen is almost completely excluded (Anaerobic).
• Anaerobic Sludge Digesters are large fermentation tanks designed to operate anaerobically with continuous supply of untreated sludge and removal of final, stabilized sludge product.
• An Anaerobic Sludge Digester is designed to encourage the growth of Anaerobic bacteria, especially Methane-producing bacteria that decrease these organic solids by degrading them to soluble substances and gases, mostly Methane (60 – 70 %) and Carbon-di-oxide (20 – 30 %).

Anaerobic Sludge Digesters

Steps involved in Anaerobic Sludge Digestion

• Sludge is digested anaerobically in three steps. They are

a)     Step – 1: Fermentation

b)    Step – 2: Acetogenic reaction

c)     Step – 3: Methanogenesis

a) Step – 1: Fermentation

• In the Fermentation step (Anaerobic fermentation), Anaerobic microbes ferment Organic materials in Sludge to produce Carbon-di-oxide and Organic acids (Butyrate, Propionate, Lactate and Succinate).
• Anaerobic bacteria involved in this step are Bacteroides sp., Clostridium sp., Peptococcus sp., Peptostreptococcus sp., Eubacterium sp. and Lactobacillus sp.

b) Step – 2: Acetogenic reaction

• In the Acetogenic reaction step, the Organic acids (Butyrate, Propionate, Lactate and Succinate) are metabolized to form Hydrogen and Carbon-di-oxide, as well as Simpler organic acids such as Acetic acid.
• Acetogenic bacteria involved in this reaction are Syntrophomonas sp., Syntrophobacter sp. and Acetobacterium sp.

c) Step – 3: Methanogenesis

• Finally in Methanogenesis step, the Simpler organic acids like Acetic acid, Hydrogen and Carbon-di-oxide are converted to Methane gas.
• The products produced during Acetogenesis are utilized as substrate by Methanogenic bacteria.

ü  Acetate is used to produce Methane and Carbon-di-oxide by Acetobacterium sp., Methanosarcina sp. and Methanothrix sp.

ü  Hydrogen and Bicarbonates are used to produce Methane by several bacteria like Methanothrix sp., Methanobrevibacter sp., Methanomicrobium sp., Methanogenium sp., Methanobacterium sp., Methanococcus sp. and Methanospirillum sp.

•      A critical balance between Oxidants and Reductants is maintained during Methanogenic processes.
•        Any leftover Sludge is then dried for use as Landfill or Fertilizer.