What is Sewage Flow Treatment to control Water Pollution?

(Part -I)

By

Dr. Sharad N. Pachpute

 

Introduction to Polluted water

• all kinds of impurities are floated out with water which is called sewage, before reaching to natural water storage such as channels, lakes, rivers and oceans. The concentration of impurities found in wastewater depends on the type of industry.
• Water is said to be polluted or contaminated when it contains enough harmful  impurities and cannot be used for further use like drinking, farming, swimming, and fishing
• Water is said to be polluted when it contains a source of contamination. After the use of water from manmade activities, chemical and process industries, water can contain organic or inorganic particles. silts and harmful chemicals. Such polluted water can be called as Sewage water.
• The untreated wastewater is called as influent and effluent for treated water 
• Sewage causes the pollution of natural water bodies like rivers, lakes and wells of freshwater. Hence, wastewater treatment has been important for many cities.
• Water pollution can be controlled by following mechanical, biological and chemical treatment of sewage

 

 How Do Waste Water Treatment Plants Function?

• Wastewater treatment is essential for the purification of sewage received from cities and industry
• To minimize the burden on natural water  resources, wastewater treatment plants are essential  cities and colonies
• Industrial wastewater or sewage treatment plants purify water and recycle it for further use again
• Both Primary and secondary treatments are the most commonly used techniques for cleaning wastewater.

• Primary treatment is essential for removal of suspended solid objects made from organic and inorganic materials. Secondary treatment improves the purity of the water after primary treatment
• This water is disinfected to remove bad odour and it also helps to kill pathogenic bacteria
• Influent: untreated wastewater
• Effluent: treated wastewater

• The outline of wastewater treatment is shown below. As per-flow rate of wastewater and concentration of solids, different treatment mechanical and chemical treatments are provided.

 Primary treatment

• Primary treatment of waste water is carried out by using screens, grit chamber, and sedimentation tank to get quality water which is free from waste and bad bacteria.
• Before getting of clean water, the sewage passes through metallic screens for removal of large floating objects which can block or damage pipes or sewage equipment.
• After the screening process, water is allowed to falls into a grit section to separate where small particles from the water. It set cinders, stones, and sand at the bottom region.
• After removal of big and small objects, sewage water can contain suspended solid particles the organic and inorganic materials.

• Approximately 25-50 % of the incoming bio-chemical oxygen (BOD), 50-70% of the total suspended solid objects, 60-70% oil and grease are removed by primary treatment.
• Some organic compound made from nitrogen and phosphorous, heavy metals carried out with solid objects are thrown out during primary sedimentation process
• Primary treatment is a must for cleaning of any waste water or sewage. The water obtained after primary treatment is not consumed by humans but it can be suitable for irrigation.

• Sectional View of primary Clarifier

  

• Primary Grit Chamber or Clarifier

• Scum Removal from Primary Grit Chamber

3.1 Grit Chamber

• A grit chamber is a concrete tank designed to reduce the velocity of sewage water and remove grit materials such as suspended particles (clinkers, eggshells, bone chips) that will settle out of the water.
• Grit can cause damage to pumps and other plant equipment by wear and tear.
• Grit material is non-putrescible solids that have a higher hydraulic subsidence value compared to organic solids. The specific gravity ( the ratio of density of solid/ density water) of grit solids is usually in the range of 2.4 -3.0.
• The composition of grit material depends on following conditions.

1. 1. Types of inlets and catch basins
   2. Climatic conditions
   3. Types of street surfaces
   4. The flow rate of stormwater
   5. Construction and condition of the sewer plant
   6. Type of ground and groundwater
   7. Source of wastewater either from industry or urban areas

 

 Importance of Grit Chamber

1. Grit particles cause abrasion and wear of surfaces.  Hence, a grit chamber is essential to minimize the damage of mechanical equipment (agitator, pumps and pipe) in a sewage plant.
2. To reduce maintenance or operating cost due to the frequency cleaning of particle deposition of grit matters in ducting.
3. To avoid blocking of pipelines and channels by grit deposition

 Types of Grit Chambers

Major types of grit chambers are given below:

  a) Horizontal flow Grit Chamber

• • The shape of the grit chamber can be rectangular or square
  • Velocity controlled V-shaped long grit channels
  • Entry and exit of sewage flows are kept on opposite sides

  b) Vortex Grit Chamber

• • Vortex type cone and the centrifugal action of sewage pull down the grit to the bottom
  • Sewage flows in the tangential direction to create a vortex of fluid flow. This will help to funnel the grit towards the centre, and hence be drawn down at the bottom chamber.
  • An agitator or stirrer is used to keep the grit materials in suspension.

• • Flow pattern for vortex grit chamber

 c) Aerated Grit Chamber

• • Compressed air is supplied for the decomposition of organic matters in sewage
  • These grit chambers have a spiral-flow aeration tank installed with air-diffusion tubes placed on one side of the tank.
  • As the sewage enters into the grit chamber the shape of the chamber and the air makes the sewage flow in a helical pattern

• Aerated grit chambers with airlift pumps are shown below.
• Major design parameters are width to minimum detention time for peak flow rate, height ratio, airflow rate and grit quality in sewage

 

Based on the quantity and quality of grit material handling, pressure drop (head loss) requirements, space, topography and capital and maintenance cost, sewage plant can be two types

• Manually Cleaned sewage plant
  • Suitable for small capacity
  • Mechanization is not affordable
• Mechanically Cleaned sewage plant
  • Suitable for large-sized Sewage treatment plant (flows capacity of more than 10 MLD
  • Mechanized grit removal plants are effective for bigger cities and industries

 Design of Sewage Flow inside the Grit Chamber

• In primary treatment, the grit chambers work well when as a sedimentation tank separates heavier inorganic materials (specific gravity > 2.65) and push flow forward with light organic materials.
• The low velocity of sewage in grit chamber may cause settling of lighter organic matter. High velocity of sewage flow can cause the settlement of the silt and grit present in the water water. For proper sedimentation process, a certain scouring velocity should be maintained in the grit chamber. The critical velocity for scouring flow is calculated using the Schield’s formula:

V_crit = 3 to 4.5 (g(S_s – 1)d)^1/2

• In the horizontal grit chamber, the velocity of sewage flow generally varies  from 10 to 30 cm/s  for high flow rate design
• A similar velocity is kept at all variations of sewage flow rate to ascertain that mainly organic solid particles and not the grit matter which  is scoured from the bottom region of the chamber.

Design and selection of grid chamber depend on the following parameters

• Dimension of grid chamber
• The flow rate of sewage
• Viscosity of sewage
• Particle size and its distribution
• Particle removal efficiency

Sedimentation Process

• To remove minute particles from the water is allowed to pass through the sedimentation tank. In this tank, water flow is reduced and solids particles are settled at the bottom of the tank. The solid collected is called as raw primary biosolids f(sludge) that is taken out of the tank by pumping.
• Primary treatment of wastewater is not able to meet the desired quality of water Hence, further treatment is essential to fulfill the needs of cities and industries.

 Secondary treatment

• This method used trickling filters and an activated sludge process to remove more than 80% of suspended waste materials (of the organic and inorganic) from the water.
• The trickling filter is around 10 feet and a deep bed of stones through which sewage flows. After trickling filter, partially treated wastewater flows to another sedimentation tank to remove bacteria
• An activated sludge process can be used instead of a trickling filter. This process is quick and provides earlier result compared to the tricking filter
• After the primary sedimentation process, sewage transfers into an aeration tank and blends with air for some hours. Sludge is added with bacteria to break down the organic matter in sewage. Similar to the trickling filter, partially treated waste water is sent to another sedimentation tank for further process.
• At the end of the secondary stage, the effluent (treated wastewater) obtained from the sedimentation tank is disinfected using chlorine or chemicals to kill harmful bacteria
• The odor of effluent is removed before being discharged for further utility

• The objectives of secondary treatment is to further clarify the effluent from primary treatment to remove organic and inorganic suspended particles
• Aerobic biological treatment is carried out in the presence of oxygen by aerobic microorganisms (bacteria) that metabolize the organic matter in the wastewater, thereby producing more microorganisms and inorganic end products (CO₂, NH₃ , and H₂0)
• Several aerobic biological treatments are necessary for secondary treatment differing primarily in the manner in which oxygen is supplied to the microorganisms and in the rate which organisms metabolize the organic mater
• Secondary treatment is essential to remove dissolved and suspended biological water-born bacteria using water-borne micro-organisms.

Tertiary treatment

• This is the final stage of the wastewater filtration process where the effluent (treated wastewater) from the secondary treatment is processed further for its best quality before being discharged to the environment
• In this treatment, the effluent is treated again to get good quality before releasing to the environment.
• The tertiary treatment reduces harmful chemicals like ammoniacal nitrogen and phosphorous.
• It is important to ensure a very high quality of the effluents which should be cleaner than lake or river water.

Sludge treatment

• The sludge is the solid waste that is obtained during the treatment of waste water. These solid wastes are disposed of safely.
• The sludge is dehydrated before being disposed of for agriculture soil amendment.

• Sludge can be used for biogas treatment for agriculture use

 

Scope of CFD Modeling for Sewage Flow Treatment

• Sewage flow consists of water, suspended dirt particles, and heavy animal oils. This is the problem of multiphase flow.
• CFD modeling of sewage flow treatment can be carried out using the slurry flow by selecting appropriate numerical models for multiphase flow.
• Basic CFD modeling needs to be studied to a selection of numerical schemes. Then you must the density  and viscosity of slurry in numerical simulations
• CFD Analysis of Slurry Flow, we must know the following boundary conditions 
  • Geometry or ducting and valves, primary and secondary treatment tanks
  • The mass flow rate of sewage flow
  • Concentration and  size of suspended particles
  • The average velocity of suspended particles
• Models used for Sewage flow
  • Turbulent models
  • Multiphase models: Discrete Phase Model, Discrete Element Method, Eulerian-Eulerian approach for a higher concentration of particles (more than 20% per unit mixture volume

   Click: Slurry Flow and Its CFD Modeling

 

 

Conclusion

• Sewage treatment is carried out significantly in primary treatment to remove suspended solid particles and oil layers
• Secondary and tertiary treatment is useful for further biological purification and removal of harmful salts
• The design of sewage treatment depends on the amount of wastewater collected, size of primary tanks
• CFD simulations will help to find the concentration of particles in sewage flow or slurry. It is a problem of multiphase flow considering a particle-laden flow.