Thursday 31 January 2019

Exercise problems

EXERCISE PROBLEMS
Design an aerated lagoon given the following data:
    • Wastewater flow = 12,400 cu.m/day
    • BOD5 =  300 mg/L
    • Population = 70,000 people
    • Detention period = 3 days
    • k' = 0.015 d-1 at 20℃
    • Y=0.5
    • Kd = 0.07 d-1 [BOD5 basis]
    • Use ideal  complete-mixing model
    • Lagoon is proposed to be followed by another treatment unit
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Design a flow through aerated lagoon to treat a wastewater flow of 3800 cu.m per day. The treated liquid is to be held in a settling basin with a 2 day retention time before discharge. Total biological solids produced are equal to computed volatile solids divided by 0.8. Assume: 
  • Influent suspended solids (non biologically degraded) = 200 mg/L; 
  • Influent soluble BOD5 = 200 mg/L; 
  • Effluent suspended solids after settling = 20 mg/L; 
  • Kinetic coefficients:
    •  Y = 0.65; 
    • Ks = 100 mg/L; 
    • 𝞵max = 6.0 d-1; 
    • Kd = 6.07 d-1; 
  • Lagoon depth = 3m; 
  • Design mean cell retention time = 4 days
  1. On the basis of mean cell retention time, determine the surface area of the lagoon. 
  2. Estimate the soluble effluent BOD5 measured at the lagoon outlet
  3. Estimate the concentration of biological solids produced
  4. Estimate the suspended solids in the lagoon effluent before settling
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Design a facultative aerated lagoon  to serve 40,000 people given the following data:
  • Sewage flow = 180 LPCD
  • Raw BOD5 = 50 g/person/day
  • BOD5 should not exceed 40 mg/L in winter
  • Average temperature of air in winter = 18℃
  • Average temperature of air in summer = 37℃
  • KL = 0.7 per day and Kd = 1.2 per day at 20℃
  • Coliforms in sewage - 10^7 per 100 ml
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Design a flow-through type aerated lagoon without recycling for the following data:
  • Volume of wastewater to be treated = 3.0 MLD 
  • Influent suspended solids (non-biodegradeable) = 150 mg/L
  • Effluent suspended solids = 50 mg/L
  • Influent soluble BOD5 = 150 mg/L
  • Effluent soluble BOD5 = 150 mg/L
  • Summer air temperature = 35℃
  • Winter air temperature = 25℃
  • Wastewater temperature = 27℃
  • Mean cell residence time (ϴc) = 3 days
  • Water depth in the lagoon = 3.5m
  • Elevation of plant = 600m
Assumed data:
  • Proportionality constant (f) = 0.5
  • Oxygen concentration in aerated lagoon = 1.5 mg/L
  • First order soluble BOD5 rate constant, Kbase e = 2.5 d-1 at 20℃
  • Mixed Liquor Volatile Suspended Solids, X = 0.8 * MLSS (MLSS = Mixed Liquor Suspended Solids)
  • Aeration constants α = 0.85 and β = 1.0
Temperature coefficient = 1.06
Kinetic coefficients:
                                Y = 0.6
                                K = 6.0 d-1
                                Ks = 90 mg/L
                                Kd = 0.06 d-1
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Determine the aerated lagoon size and power requirements to serve 50,000 people assuming sewage generation rate = 250 LPCD, influent BOD5 = 300 mg/L and effluent BOD = 20 mg/L.
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Design a facultative pond in a waste stabilization pond to treat 3.5MLD of wastewater which has a design loading of 440 kg BOD/ha-d. The design temperature is 30C.
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Tuesday 29 January 2019

References

References
  1. Wastewater Engineering - treatment, disposal and reuse - Metcalf and Eddy
  2. Wastewater treatment for pollution control - Soli J Arceivala
  3. Environmental Engineering - Kiely Gerard
  4. Environmental Engineering: Sewage disposal and airpollution engineering - S.K.Garg
  5. Environmental Engineering - Keiley
  6. Environmental Engineering- A design approach - Sincero & Sincero
  7. Unit Operations and Processes in Environmental Engineering                  - Tom D. Reynolds
  8. Introduction to Environmental Engineering - P. Aarne Vesilind
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Design considerations for aerated lagoons

DESIGN CONSIDERATIONS


  • The following factors  should be kept in mind when designing an aerated lagoon
    1. The desired quality of effluent is based on concentration of solids to be maintained in the effluent
    2. BOD5 or COD removal gives the efficiency of substrate conversion
    3. Quantity of oxygen to be supplied
    4. Power needed for mixing and supply of oxygen
    5. If the temperature variations between summer and winter are significant, the effect of temperature should be considered.
    6. The design criteria required for designing an aeration system are:
      1. Detention time: Suspended growth aerated lagoons are designed on the basis of Hydraulic Retention Time (HRT) and Mean.Cell Residence Time (mCRT) or Solids Retention Time,𝜃c (SRT). Typical design value of SRT or HRT for suspended growth aerated lagoon is ~ 3 to 6 days while for facultative aerated lagoon is ~ 4 to 10 days.
      2. Oxygen requirement: The oxygen required for oxidising organic solids varies from 0.7 to 1.4 kg of oxygen per kg of BOD5 removed. Oxygenation capacity of aerators varies from 1.85 to 2.0 kg O2 per kW of power delivered under standard conditions. Oxygenation capacity of aerator at field conditions is given by the equation:                                               N = [Ns (Cs - CL) * 1.024^(T-20a)] / 9.2
      3. Power requirement: Power required for mixing the contents of the aerated basin varies from 0.8 to 1.0 kW/1000 cu.m of basin volume. The power required to keep bio-solids in suspension is 1.5 to 1.75 kW/1000 cu.m and the power required to keep ALL solids under suspension is 15 to 18 kW/1000 kW/1000 cu.m
    7. The assumptions for the design of an aeration system are:
      1. Oxygen content = 2302%
      2. Diffuser efficiency = 30 to 50% (or as per manufacturer specifications) 
      3. Field Oxygen Transfer Efficiency = 50% (or as per manufacturer specifications) 
      4. Air weight (density) = 1.201 kg/cu.m for aerobic reactors
    8. Criteria adopted in design of aerated lagoons:
      1. mCRT = 3 to 6 days without recycle for domestic wastewaters and 10 to 30 days with recycle for domestic wastewater
      2. Oxygen requirement = 0.7 to 1.4 * BOD5 removed, kg/d
      3. Solids concentration in lagoon  = 30 to 300 mg/L for aerobic flow through type                                                              = 30 - 150 mg/L for facultative type and                                                                        = 4000 to 5000 mg/L for extended aeration type
      4. Hydraulic detention time = 2 to 10 days for aerobic flow through type  
                                                                   = 3 to 20 days for for facultative type and                              
                                                                   = 0.2 to 7 days for extended aeration type
                           5. Depth of lagoon = 2 to 5m
                           6. Power required for oxygen supply = 1 to 8 HP/1000 cu.m of basin volume
                           7. Oxygen transfer capacity of surface aerators = 1.85 to 2.0 kg O2/kW h at standard     
                                                                                                      conditions
                9. Lagoon surface area is assumed rectangular in the ratio 1.75 - 1.95:1 (Length:Width)

               10.Dispersion number (D/UL) = 0.2 to 1.0 for rectangular or long lagoons and 2 to 4 and above 
                                                                 for squarish lagoons
               11. BOD5 removal efficiency = k * t  where k = 0.776
               12. %BOD removal efficiency is determined using the graph showing wehner-wilhelm equationc   
                      for substrate removal efficiency based on dispersed flow model
               13. Sludge accumulated is determined using a cleaning interval of 3 years
               14. Dispersion number (D/UL) generally lies between 0.1 and 0.4
                       if (D/UL) ~ 0.2 or less the reactor is a plug flow reactor
                       if (D/UL) ~ 3.0 to 4.0 the reactor is well mixed or is approaching complete mixing
 USEFUL FORMULAE:
Power = Work/Time and the units of power is W (watt)
Watt = Joules/second
Horse Power (HP) = 750 W
Power = Force * Displacement / Time = Force * Velocity
V = Q * t
Hydraulic loading or surface loading rate or Overflow rate = flow (m3/d) / surface area (m2)
Weir overflow rate = flow rate / total weir length
Organic loading = Applied kg of BOD per day / Volume of tank
Mean Cell Residence Time (mCRT) or Solids Retention Time (SRT) = 
Biomass in reactor / Biomas removed from reactor
N = [Ns (Cs - CL) * 1.024^(T-20a)] / 9.2
N = Oxygen transferred under field conditions in kg O2/h
Ns = Oxygen transfer capacity under standard conditions in kg O2/h
Cs = Dissolved Oxygen saturation value for sewage under operating temperature
CL = Operating Dissolved Oxygen level in aeration tank 1 to 2 mg/l
T = Temperature in oC
a = Correction factor for oxygen transfer for sewage (0.8 to 0.85)
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Monday 7 January 2019

IMPORTANT QUESTIONS


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EFFLUENT DISPOSAL

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DESIGN OF WASTEWATER IRRIGATION SYSTEMS

DESIGN OF WASTEWATER IRRIGATION SYSTEMS

  • The enormous increase in the generation of waste water on account of rapid growth of industrialization and urbanization has posed serious threat on human and natural resources.
  • Wastewater has become a significant issue in urban areas especially in developing countries.
  • Land treatment of wastewater which is economical among other conventional methods which is viable solution for the treatment and disposal of water particularly for the developing countries such as India.
The following techniques of disposal of wastewater on land are discussed in detail below.

 Land disposal systems are used for:
  • disposal of pre-treated municipal effluents
  • not used extensively due to large land requirement and this reason is aggravated by code-required setbacks (municipality requirements)
  • not used frequently due to requirement of significant pre-treatment before application
  •  
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WASTE STABILIZATION PONDS

WASTE STABILIZATION PONDS
  1. Types of ponds
  2. Factors affecting pond ecosystem
  3. Design of aerobic and anaerobic stabilization ponds
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Aerated lagoons

AERATED LAGOONS

Aerated lagoons have been successfully used for the treatment of domestic or industrial wastewater
Aerated lagoons
An aerated lagoon is a wastewater treatment plant in which mechanical or diffused-air aeration is used to artificially supplement the oxygen supply. Aerated lagoons are of the following four types:
  1. -Facultative aerobic lagoons
  2. -Aerobic, flow-through lagoons
  3. -Dual-powered lagoons
  4. -Aerobic lagoons with recycling of solids
Aerated lagoons 
Aerated lagoons have great depth resulting in lesser surface area than waste stabilization ponds.They are relatively shallow earthen basins varying in depth from 2 to 5m and provided with mechanical aerators on floats or fixed platforms. The mechanical aerators are used to provide oxygen for the biological treatment of wastewater and also to keep the suspended solids in suspension.Sometimes diffused aeration is also used.

Facultative aerated lagoons 
In this case, the power input per unit volume is enough only for diffusing the required amount of oxygen in the wastewater but not for maintaining all solids in suspension. Therefore, the solids settle down and undergo anaerobic decomposition. These ponds exhibit partly aerobic and partly anaerobic activity. Hence, they are called facultative. They are also referred to as aerobic lagoons. They remove 70 to 90% of the BOD load.

Aerobic lagoons with recycling of solids are similar to (Extended aeration activated sludge process) aerobic flow-through plants. The power level is high to keep all suspensed solids under suspension. Solids are prevented from escaping out with effluent. Hence the concentration of solids is high. Arrangements are incorporated  for settling and recycling of solids. High BOD removal, as high as 95 to 98% can be acheved in these lagoons.In these lagoons, an earthen lined basin is used in place of a RCC reactor basin.The Hydraulic Retention Time is longer than that used in conventional extended aeration process. The aeration requirement for these lagoons should be higher than those for all other types of lagoons. In these type of lagoons, most of the solids are maintained in suspension.The aerobic lagoon with recycling of solids is the same as the activated sludge process.

Aerobic flow through dual power aerated lagoons
In case of aerobic, flow-through type lagoons, the power supplied is sufficient to diffuse enough oxygen but also keep all suspended solids in suspension. They are similar to activated sludge aeration tanks. There is no settlements of solids in these types of aerated lagoons. The aeration period is 2 to 5 days. The wastewater leaves along with the suspended solids. The BOD removal is around 50 - 60 %. This process is normally followed by facultative aerated lagoons or algal ponds or converted into aerobic lagoons with recycling of solids.

Dual powered aerobic lagoons are a special case of aerobic, flow-through lagoons where the first lagoon is an aerobic flow through lagoon followed by a facultative aerated lagoon in series. These lagoons are given the name dual powered since power maintained in first lagoon is different from that in subsequent lagoons. Detention time in both lagoons may be different. 1 to 3 days in aerated lagoons and 3 to 4 days in subsequent lagoons is adopted to meet desired final effluent quality.


Aerobic lagoons with recycling of solids
This type of lagoon is essentially an extended aeration activated sludge process. In these cases, an earthen basin is used instead of a RCC reactor basin.The Hydraulic Retention Time (HRT) is longer that that used in conventional extended aeration process.The aeration requirement for an aerobic lagoon with recycle is higher than that required by the aerobic flow through lagoons so that all the solids are maintained under suspension.

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PLANNING DOMESTIC WASTEWATER TREATMENT

PLANNING DOMESTIC WASTEWATER TREATMENT

In order to guarantee availability of water, it is essential for the local government to plan and build water supply schemes that supply water to the community in sufficient quantity that is potable in terms of quality. Water which is safe and potent for drinking is called wholesome water. A lot of money is needed for planning, designing and executing a wastewater treatment system. A sewerage system should be properly and skillfully planned and designed in order to efficiently remove sewage to the point of disposal and avoid health hazards to the population. A sewerage system requires enormous capital investment as well as recurring repair, maintenance and operational (RMO) expenses. In order to plan a wastewater treatment plant, the topography of the region plays a vital role. The plant should be located at the lowest point thereby reducing the installation of pumps and other energy consuming equipment. It should be located away from the community to reduce odour problems and minimise spread of vector borne diseases. The wastewater treatment facility should be located close to land with porous soil for disposal or should be close to a water body for economic disposal of treated waste in accordance with local rules and regulations.
  • Outline of unit processes
Wastewater can be treated by physical, chemical or biological methods. Based on the method used to treat the waste, the treatment methods are classified as unit processes and unit operations. The types of treatment used to remove contaminants by addition of chemicals or use of biological mass or microbial activities are known as unit processes. If the treatment involves use of physical forces, it is known as unit process.  A few important treatment methods that fall in this category are:

  1. Screening-- removes floating materials
  2. Mixing-- mixing the contents of wastewater for various purposes
  3. Flocculation-- coalescing of particles to form bigger aggregate flocs
  4. Sedimentation-- removal of solids or biomass by gravity settling
  5. Flotation-- removal of solids by floating them on the liquid
  6. Elutriation-- washing of sludge to remove alkalinity
  7. Filtration-- removal of suspended solids by filtering them from liquid
  8. Heat transfer and drying-- removal of water or moisture from sludge
    • Different types of treatment methods
    Based on the type of agent used, unit processes are further classified as:
    1. Chemical unit process: These processes involve removal of contaminants by adding chemicals. Processes of this type are:
      1. Chemical neutralization - involves control or adjustment of system pH
      2. Chemical coagulation - involves removal of colloidal particles by chemical destabilization and flocculation
      3. Chemical precipitation - involves enhancement in removal of suspended solids, phosphorus, heavy metals and BOD in specific system conditions
      4. Chemical oxidation - involves removal of grease, ammonia, BOD, COD and odour
      5. Chemical disinfection - involves killing pathogens in influent and treated effluents
    Note: Chemical processes are not normally provided in conventional treatment of domestic wastewater due to high cost of chemicals. 
    • Major treatment methods that fall under biological unit process are:
      1. Suspended growth process and
      2. Attached growth process
    Special cases involving removal of excessive nutrients and liquid or gaseous impurities the following advanced treatment methods are used:
    1. Biological nitrification-denitrification
    2. Reverse osmosis
    3. Ion-exchange and
    4. Ultrafiltration.
    • Primary treatment
      • The primary treatment system is preceded with a preliminary treatment system for the removal of floating substances and large inorganic matter causing operational problems in primary and secondary units in the treatment system. The preliminary treatment system consists of:
        1. Sump and pump unit
        2. Approach channel
        3. Screen chamber
        4. Grit chamber and
        5. Skimming tank (oil and grease traps)
    The sump and pump unit and approach channels are NOT treatment units. They can be called - 'holding and conveying' units as they are primarily involved in collection, lift an conveyance of wastewater.

    • Screening - Screening is the first operation in a sewage treatment plant. It consists of screens of several sizes to trap and remove floating matter. The floating material if not removed, have the potential to choke pipes and adversely affect sewage pumps. Thus the main purpose of providing screens is to protect the equipment from damage due to floating material. Screens are preferably placed before the grit chambers or may be accommodated in the body of the grit chamber. Screens may be coarse, medium or fine. Material collected on screens is removed using rakes. Screens may be classified as fixed screens or movable screens.

    • Neutralization - Neutralization consists of neutralizing the excessive acidity or alkalinity of wastewater by adding alkali or acid respectively. This takes place in either the equalisation tank or a separate neutralisation tank.

    • Equalization - Equalisation consists of holding wastewater for a pre-determined time in a continuously mixed basin in order to produce wastewater of uniform characteristics. This is necessary if the characteristics of wastewater varies throughout the day.
    • Flocculation - When certain chemicals (Ex - coagulants like alum) are added to water, the tiny suspended colloidal particles are trapped in a gel like precipitate formed by the reaction between water and the coagulant. This is called a 'floc'. Tiny flocs come and bind together (coalesce) forming large flocs that settle at the bottom. This process is called flocculation.
    • Sedimentation - This is one of the oldest method of removing suspended particles in water. It is based on the principle that suspended particles settle to the bottom when water is 'stationary'. However, water cannot be made to stand still in a treatment facility and hence, in order to achieve the desired effect, the velocity of water is reduced and the distance of travel from the inlet to the outlet is increased. Thus the particles settle to the bottom of the sedimentation tank from where they can be removed.
    • Floation - Floatation consists of creation of fine air bubbles in the tank by introduction of air into the tank from the bottom. The rising air bubbles get attached to the fine suspended particles, increasing their buoyancy and lift them to the surface of the liquid and thus they canbe removed by skimmimg.

    • Nitrification and Denitrification Systems - Nitrification is the biological oxidation of ammonia to nitrite and subsequently to nitrate. This involves a type of bacteria called nitrosomonas.

    • Environmental impact and other considerations in planning treatment facilities
      • Wastewater treatment plants require large space
      • After significant treatment, the effluent from  the treatment plant is discharged on land or into near-by water body causing pollution.
      • The life-cycle environmental load of the treatment facility should analysed
      • The construction and operational phases of the treatment facility should be given due importance
      • Damage to ecosystem quality should be assessed including ecotoxic emissions along with the combined effect of eutrophication and acidification
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    INDEX

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