WASTEWATER TREATMENT
Large scale Wastewater treatment
Sewage is wastewater released by homes, industries agricultural fields and other human activities. It contains large amounts of organic matter and before disposal, sewage should be treated in sewage treatment plants. The purpose of waste water treatment is to remove contaminants from water so that the treated water can meet the acceptable quality standards for reuse or for being discharged into river.
Composition of Sewage
Typical raw sewage has 99.9% water and 0.1% organic and inorganic solids. The following components make the sewage:
· The organic impurities present in sewage are human faeces, animal wastes (like animal dung), urea (as urine), oil, fruits and vegetable wastes, pesticides, herbicides, etc.
· The inorganic impurities present in sewage are nitrates, phosphates and metals.
· The nutrients present in sewage are nitrogen and phosphorus.
· The microbes present in sewage include bacteria, protozoa which cause water-borne diseases and others
Wastewaters from different sources accumulate in sewage, chemical composition vary depending upon the sources. Domestic sewage contain human wastes and wastewaters from personal washing, food preparation, laundry and washing of kitchen utensils. Relatively low in solids and more than 99% water. Industrial sewage contain organic compounds from sugar factories, paper mills, breweries, slaughter houses etc. and inorganic wastes from mines and metal industries. Microbial flora in sewage include algae, bacteria, fungi, protozoa, viruses etc.
Methods of waste water treatment depends on composition of waste water and required quality for treated water.
The purpose of waste water treatment include;
§ To reduce strength of sewage
§ To make waste water less offensive
§ To prevent public health hazards from toxic effect of pollutants
Thus, sewage treatment involves a more complex set of procedures according to the volume and composition of organic matter in the water.
Sewage treatment is carried out in three stages.
These treatment steps basically involve physical removal of large and small particles. Initially, floating debris is removed by filtration and then the grit are removed by straining/sedimentation. All solids that settle form the primary sludge, and the supernatant forms the effluent. The effluent from the primary settling tank is taken for secondary treatment.
Secondary treatment (Biological)
The primary effluent is subjected to biological treatment by aerobic microbes. While growing, the microbes degrade organic compounds in the effluent and reduces the BOD (biochemical oxygen demand). BOD is the amount of oxygen required to oxidize organic matter. The greater the BOD of waste water, more is its polluting potential.
The effluent is then passed into a settling tank where the sediment or sludge is collected and later digested anaerobically. During digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide which form biogas. The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.
Tertiary treatment (Chemical)
Here, the fluid from the secondary treatment process is subjected to chemical treatments to remove phosphates and nitrates that might cause eutrophication or pollution. The ions are precipitated by adding calcium or iron, and the ammonia is released by oxidizing it to nitrate in the nitrification process or by stripping. Adsorption to activated charcoal is also used to remove many organic pollutants.
During each stage of sewage treatment, the solids that sediment are collected as sludge which is burned or buried in landfills. Or, it can be treated in an anaerobic sludge‐digesting tank where the microorganisms break down the organic matter of proteins, lipids, and cellulose into smaller substances such as organic acids, alcohols, and simpler compounds. Methane gas is produced in the sludge tank, and it can be burned as a fuel to operate the waste treatment facility. The remaining sludge is incinerated or buried in a landfill.
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/straining:
§ Screening/straining is used to remove solid waste like plastics, cloths, dead animals from waste water. For this purpose, waste water is passed through a metal screens/strainers which allows water to pass and retain solid matter.
§ The removed solids are disposed by burning or composting.
II. Primary treatment of wastewater:
§ After removal of gross solids, grits and fats, next step in treatment is sedimentation
i. Sedimentation:
§ Sedimentation tank is used for removal of suspended solids and some organic matters.
§ Settled solids are periodically removed -this technique removes about 90% of suspended solids and about 40% of organic matters from sewage.
Typical materials that are removed during primary treatment include
§ fats, oils, and greases
§ sand, gravels and rocks
§ larger settleable solids
§ floating materials
After primary treatment the effluent is taken for secondary/ biological treatment and the sludge is carried to sludge treatment plant
III. Secondary treatment of waste water:
§ In secondary treatment, dissolved or colloidal organic matters are present in sewage are removed by microorganisms. In this steps, microorganisms (mainly, bacteria and protozoa) utilize organic matter and converts them into inorganic minerals. These are utilized to supply their carbon and energy needs
§ Removal of 90-95% BOD and many pathogens occur during this process
§ The 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;
Trickling filter, Activated sludge system, Oxidation pond
i. Trickling filter:
§ Trickling filter consists of filtering bed, spraying arm and water collecting chamber.
§ Filtering bed consists of a bed of porous material made of crushed stone/gravel/slag/coke/ limestone chip/synthetic material
§ Effluent or sewage from primary treatment tank is sprayed uniformly over the filter bed.
§ The set up is thus, a pile of rocks over which waste water is sprinkled with the revolving sprinkle and it slowly trickles down-Sewage aerated during the spraying process
- As the water slowly trickles down the rock bed, a gelatinous layer of bacteria, algae, protozoa and some fungi is produced on the surface of filter bed. This layer is called Zoogleal layer.
- Filter bed coated with slimy bacterial growth –Zooglea ramigera- and other slime producers. Slime colonised by other bacteria like Pseudomonas, Beggiatoa, Flavobacterium etc, fungi, nematodes and protozoa.
§ As the water trickles through the filter bed, organic matter present in it are oxidized by microorganisms of zoogleal layer.
§ A stationary culture of microorganisms - continuous supply of nutrients from the sewage which is degraded into simpler end products
`§ The sewage recirculated repeatedly till the sufficient BOD reduction is achieved
§ Sewage is aerated by the circulation of air through the porous bed
§ Trickling filter can reduce BOD of sewage by about 65-85%
§ Newly constructed bed needs a few week to function- till the zoogleal layer is formed
§ Slime layer gets thicker occasionally and should be removed for efficient working of the system
ii. Activated sludge system
§ Activated sludge system, consists of aeration tank, settling tank and sludge return system
§ Activated Sludge Process (ASP) is a widely used biological and aerobic treatment
§ At first incoming sewage from primary treatment plant is mixed with sludge drawn from a 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.
§ Aeration leads to the formation of flocs, which are the masses of bacteria associated with fungal filaments to form mesh like structures. Flocs actively degrade the sewage since they contain large numbers of metabolizing bacteria with yeasts/fungi/protozoa
§ After oxidation, sewage is passed to settling tank and left undisturbed for 2-3 hours. Flocs/Sludge settle to the bottom. This sludge is called activated sludge, it can be used as inoculum for next batch of sewage.
§ Heterotrophs like E. coli, Enterobacter, Pseudumonas, Flavobacetrium, Zooglea, large filamentous bacteria, filamentous fungi, yeasts and protozoa are present in the flocs. Slime producers secrete slime which holds the flocs together
§ Settled sludge removed periodically to avoid bulking of sludge by rapid development of filamentous bacteria/fungi
§ Most of the sludge is removed and some returned to aeration tank for next round of treatment.
§ By sludge digestion process, BOD of sewage is reduced by 90%.
Advantages
Ø Significant reduction in BOD - by 85% to 90%
Ø Clear, colourless and odourless effluent
Ø Reduction in intestinal pathogens
Disadvantage
Ø Foam formation due to the presence of some detergents in sewage (ABS)
iii. Oxidation ponds:
§ Lagoons/stabilization ponds
§ Aerobic Secondary treatment method in rural areas/industrial sectors
§ 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.
§ Organic materials degraded by heterotrophic bacteria (aerobic heterotrophic zone) into simpler forms which in turn allow algal growth (photic zone)
§ 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 (Chlorella, Scenedesmus, Spirulina species) 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.
- Finally the sewage is removed from oxidation ditch through outlet for tertiary treatment.
Advantages
Ø It is very simple and easy technique
Ø Treated sewage can be utilized for irrigation
Disadvantages
Ø Holding time is very long (10-40 days)
Ø It require large area
Ø It creates bad odor. It may become breading place for mosquitoes and other vectors
Ø It is influenced by seasonal temperature
Tertiary treatment (Chemical)
Here, the fluid from the secondary treatment process is subjected to chemical treatments to remove phosphates and nitrates that might cause eutrophication or pollution. The ions are precipitated by adding calcium or iron, and the ammonia is released by oxidizing it to nitrate in the nitrification process or by stripping. Adsorption to activated charcoal is also used to remove many organic pollutants.
Single dwelling units /Household wastewater treatment
Used for small scale/domestic waste water treatment. Household wastes are treated in single units eg., Cesspools, Septic tank, lagoons
Cesspool
• Temporary arrangement where house sewage is discharged into a tank- in cases, where a well-defined sewage management system is absent
• Cesspools are constructed underground- it has walls of cylindrical rings with pores with an opening near ground level
• Wastewater enters through the opening/inlet- liquids escape into the surrounding soil and the solid matter retained in the tank
• Bottom of the cesspool opens to the underground; suspended solid matter settles down and form sludge at the bottom
• Water passes out through the pores in the walls to the surrounding soil
• Organic materials are digested by primarily aerobic and then anaerobic bacteria
• After a time, the crevices become filled with soil and other solid matter, and the leeching process is interfered with, so occasional cleaning to be done (with strong acids)
Septic tank
- The septic system is a waste treatment facility on a small scale.
- In a septic tank, household sewage is digested by anaerobic bacteria, and solids settle to the bottom of the tank. The water seeps out and enters the soil, where bacteria complete the breakdown processes.
- Septic tank is prepared under the ground- Small rectangular chambers just below ground level
- 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. Liquid effluent is taken through pipes in distribution box and discarded underground
- Sedimentation as well as biodegradation of sedimented sludge
- Effective for small settings- however effluent should be further treated before disposal since there is no complete removal of pathogens
- Drainage prevented from entering drinking water supply
- Should be located away from drinking water source
- Should be de-sludged at regular intervals
- Two compartment setup preferred now; more effective sedimentation
Oxidation ponds
• Useful if comparatively large area of land is available
• small ponds/lagoons
• Organic materials undergo aerobic digestion whereas sediments undergo anaerobic digestion
Imhoff tank
• Designed by Karl Imhoff – German Engineer
• A chamber for receiving and processing of sewage
• Clarification of sewage occurs by simple settling and sedimentation, followed by anaerobic digestion of the sludge
• There is an upper chamber for sedimentation, collected solids slide down into a lower chamber and is digested
• The two chambers are unconnected, with sewage flows in to the upper sedimentation chamber and no flow of sewage in the lower digestion chamber
• Sludge is collected in the lower chamber where it undergoes anaerobic digestion
• The lower chamber has separate biogas vents and pipes for the removal of digested sludge, typically after 6-9 months of digestion.
• It is basically a two-story septic tank- retains the septic tank's simplicity while eliminating many of its drawbacks, such as mixing of fresh sewage and septic sludge in the same chamber.
• Imhoff cone: used in drinking water treatment facilities
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