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 bacteria present in sewage include those bacteria which cause water-borne diseases such as cholera and typhoid.

·         The other microbes present in sewage are protozoa which cause  water-borne diseases like dysentery.

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. Raw sewage has millions of bacteria such as Bacillus subtilis, B. megaterium, P. fluorescens, Achromobacterspp, Microccoccus spp, bacteria of intestinal origin like E. coli, other coliforms, Proteus, Serratia etc. Potential pathogens like Streptococcus fecalis, Clostridium perfringens, pathogens like Vibrio cholerae, Salmonella typhi etc, Viruses like Polio virus, Hepatitis A and bacteriophages.

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.

Primary treatment (Physical)

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 significantly reduces the BOD (biochemical oxygen demand) which is the amount of oxygen required to oxidize total organic matter in the effluent. The BOD test measures the rate of uptake of oxygen by micro-organisms, 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:

§  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.

§  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.

§  Some common techniques applied 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 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 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.

Typical materials that are removed during primary treatment include

§  fats, oils, and greases (aka FOG)

§  sand, gravels and rocks (aka grit)

§  larger settleable solids including human waste

§  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;

     §  Fixed film system eg, Trickling filter

§  Suspended film system eg, Activated sludge system

§  Lagoon system eg, Oxidation pond

 

i.   Trickling filter:

Fixed Film System- where microorganisms grow on substrates such as rocks, sand or plastic and wastewater is spread over the substrate

§  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.

§  Basically, 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

§  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.

§  Newly constructed bed needs a few week to function- till the zoogleal layer is formed

§  Slime layer gets thicker occasionally sloughs off of the media surface and should be removed for efficient working of the system

 

 

ii. Activated sludge system:

•         Suspended Film Systems –        stir and suspend microorganisms in wastewater 

§  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 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.

§  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 is returned to aeration tank for next round of treatment.

§  By sludge digestion process, BOD of sewage is reduced by 5-15%.

                            

Advantages

Ø  Significant reduction in BOD - by 85% to 90%

Ø  Clear, colourless and odourless effluent

Ø  Reduction in intestinal pathogens

Ø  Requirement of little land

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.