Wastewater Collection And Treatment (Design Criteria)
Sewers
Average Sewage Flow
Normally about 80 to 90 percent of the per capita consumption of water becomes wastewater, considering 200 litre per capita water consumption and sewage flow of 80%, the per capita flow will be 160 litre/capita/day.
Peak Factor
Sewage flow does not remain uniform; it varies form time to time. Sewerage network will be designed for the peak sewage flow.
The peak factor will be calculated as follows:
Peak factor = 5.75
P0.2
Where P = Population in thousands.
The peak factor shall not be greater than 6 and not less than 2 in any case.
Flow Velocity
Minimum velocity in the sewer will be such that there should be no deposition in the sewer line. Minimum self cleansing velocity is 0.6 m/sec. under flowing full condition. Preferably it is taken as 0.75 m/sec for the designing of the system. Maximum velocity will not be greater than 2.4 m/sec.
Following Manning's equation will be used to determine the velocity in the sewer lines:
V=1/n (R2/3 S1/2)
Where;
V=Flow velocity in m/sec.
R=Hydraulic Radius of pipe in meters
S=Slope of the pipe
n=Manning's Coefficient of Roughness of the pipe
Sewer Capacity
The full carrying capacity of the pipe will be calculated as follows:
Q = AV
Where;
Q=Flow in m³/sec.
A=Cross-Sectional Area in m²
V=Flow velocity in m/sec.
Sewer Slopes
The minimum slope for a section of sewer will generally be based on the minimum velocity requirements.
Design Depth of Flow
Sewers will be designed to flow at 0.75 of full depth under peak flow conditions to provide requisite air gap under which condition the sewer will flow up to 90% capacity at peak flow. Thus the design flow will be calculated by multiplying peak flow with a factor of 1.12.
Determination of Pipe Sizes
Minimum sewer pipe size will be 200 mm except for house connections which will be 150 mm. All other pipe sizes will be determined from design flow calculations and velocity criteria.
Pipe Materials
The type of pipes to be used for sewerage system depends upon the following factors:
• Corrosion resistance
• Capital cost
• Local availability
• Ease of installation
• Efficiency of joints
• Load sustaining ability
• Useful life
The pipe materials mainly include unplasticised Polyvinyl Chloride (UPVC), Vitrified Clay (VC), Reinforced Cement Concrete (RCC), Asbestos Cement (AC) and High Density Polyethylene (HDPE) etc. All of these pipes are technically acceptable for use in sanitary drainage system although each material has its own particular merits for a given condition.
The market investigations carried out so far indicate that UPVC and HDPE pips are locally available. The cost comparison of the two pipes indicates that rates of both pipes are competitive and either of the two can be used. These two pipes are almost equally technically suitable but considering the growing trend of using HDPE pipes, HDPE pipes will be used for sewer lines. For house connections UPVC pipes will be used.
Depth of Cover to Sewers
In order to provide building connections, minimum earth cover over the pipes will be 1.0 m.
Trench Widths
Trench widths for lying of pipes of various sizes in the network are shown in table:
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Pipe Diameter (mm)
|
Trench Width (mm)
|
|
150
|
650
|
|
200
|
700
|
|
250
|
750
|
|
300
|
850
|
|
350
|
900
|
|
400
|
950
|
|
450
|
1050
|
|
500
|
1100
|
|
600
|
1250
|
|
700
|
1400
|
Bedding
Sand bedding will be used except where the pipes require additional support in the form of concrete surround as appropriate.
Location of Sewers
• Sewers will be generally located keeping in view the natural ground slopes in order to minimize the depth of excavation.
• Sewer will be positioned in accordance with the utility/service reservation requirements of the local Municipality.
Sewer Alignment
Sewer lengths between manholes will be laid at a uniform gradient and diameter and straight in plan.
Crossings of Other Utilities
Where the proposed sewers cross the existing utilities the sewer should be laid in such a way so as to avoid interference with these utilities. Sewers will be laid below water pipes wherever possible. If the water main underpasses any sewer line it will be protected by sleeving or concrete encasement at the crossing to minimize the risk of contamination of water supply.
Manholes Location
Manholes will be located according to conventional sewer network design i.e. at starting points, junctions between sewers (except building connections to sewers) and changes in direction and grade. Based on sewer size, the spacing between manholes will be as follows:
• 200 diameter< 50 metres
• 300 to 600 diameters for 50 to 80 metres
• 700 to 1200 diameter for 80 to 100 metres
Manhole Dimensions
For sewer lines up to 700 mm diameter manholes will have a circular chamber of 1.2 metre internal diameter. For large diameter pipes manhole chambers will be of 1.5 metre internal diameter.
Pipe Connections to Manholes
To allow for limited differential settlement between manholes and the connecting pipelines, there will be a flexible pipe joint located at the external face of the manhole and a second flexible joint approximately 750 mm from the face of the manhole
Adjustment for Height of Manholes
Manholes will be constructed with a minimum of two and maximum of three courses of concrete blocks between the manhole cover slab and manhole cover to allow for future adjustment of the top level to suit changes in final road or ground level but manhole neck will not exceed 750 mm.
Change in Sewer Diameter at Manholes
To minimize the risk of blockage in sewers, the diameter of the outgoing sewer must not be less than the diameter of the largest incoming sewer. The top of smaller sewers entering a manhole will normally be at the same level as that of the outgoing sewer.
Slope of Channel within Manhole
All manhole invert levels used in the sewer calculations will be the centre of the manhole and all distances and gradients will be calculated between centres of manholes. Where the incoming and outgoing pipes are of the same gradient and diameter the pipe gradient will be continued through the channel in manhole.
Drop Connection to Manhole
The drop connections to manholes will be provided if the difference in pipe invert elevation is greater than 600 mm otherwise no drop connection will be provided.
Manhole Material
Manholes will be of reinforcement cement concrete.
Building Connections
Cleanouts will be used for single building connections and these will be constructed just outside the boundary of each property served.
Inspection Chambers
Inspection chambers will be used for multiple building connections. Chambers will be constructed just outside the boundary of each property served and will be sufficiently deep to allow connection with the drain/sewer within the boundary at satisfactory gradients and to ensure that the connection to the sewer will have a minimum cover of 1.0 m. The diameter of inspection chambers will be 900 mm.
Connection Pipe Size and Grade
Connection pipes will be 150 mm diameter or greater depending on the population of the buildings and the available grade.
Connection to Sewer
• All connections to the main sewer made will be through Y r T fittings.
• Risers will be incorporated in the building connection where the depth of sewer exceeds 2.5 metres
Ventilation
• Ventilation of sewers is necessary to avoid the build up of noxious gasses and to minimize septic conditions.
• In developed areas sewers will naturally ventilate through the ventilation stacks provided as part of each building sanitary system. Therefore, there is no need of additional ventilation stacks.
Sewage Lift/Pump Station
Sewage Lift Station
Lift station will be provided where necessary. It will consist of a wet well and a dry well to house two centrifugal pumps, one in operation and the other standby. The pumps will operate automatically as a function of waste water level in the sump (wet well). Provision of ventilation and odour control system will be made.
Sewage Pump Station
The sewage pump station (if required) will pump the wastewater collected from the entire project area to the screening chamber of the treatment plant. This will be a complete pump house building with a wet well and dry well. This will also be equipped with centrifugal pumps. Provision of ventilation and odour control system will be made. These will also be operating automatically depending upon the water level. Positive suction head will be provided to the pumps, standing provision of pumps will be 50% of peak sewage flow.
SEWAGE TREATMENT PLANT
Process of Sewage Treatment
The proposed sewage treatment plant will be based on extended aeration, activated sludge process.
Capacity of Treatment Plant
The rated capacity of the plant will be average annual daily sewage flow. The flow will be worked out on the basis of per capita sewage flow and design population. The per capita domestic sewage flow will be 160 liter/day whereas the design population will be worked on the basis of adopted household size and number of plots in the project area.
Domestic Sewage Production
As discussed above, per capita domestic sewage production will be adopted as 160 liters/day; the day will be defined as 24 hrs.
Industrial Wastewater Production
The wastewater production hours will be considered as 8 hours in a day, from 8.00 am to 4.00 pm .
Sewage Flow Variations
The following sewage flows will be considered for the design of various treatment plant facilities mentioned below:
• Peak hour flow for Pumping equipment
• Peak hour domestic flow will be equal to peak factor x average daily flow.
• Maximum day flow for Sludge pumping system and Sedimentation tank
• Minimum hour flow for Low range of plant flow.
Influent Characteristics
The characteristics of domestic wastewater have been considered in the following ranges, the design parameters will be established after the sample testing of wastewater.
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Characteristic
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Concentration
|
|
- Biochemical Oxygen Demand (BODs)
- Suspended Solids (SS)
- Free Ammonia (as N)
- Total Nitrogen (as N)
- Total Phosphorous
- Design Temperature
|
200-400 mg/l
220-350 mg/l
25-50 mg/l
40-85 mg/l
8-15 mg/l
20 °C
|
The discharge from workshops etc. will be considered to be free from substances which will hinder the biological process or could not be removed through the process. Such substances will be removed at site by industry owners at their own expenses before discharging effluent into the proposed sewerage system. The substances which hinder the biological processes are classified as follows:
• Fats, oils and grease
• Priority pollutants
• Surfactants
Average sewage temperature for the coolest month will be adopted as design temperature which is taken as 20°C.
Effluent Characteristics
The sewage will be so treated that it meets the wastewater standards set by Ministry of Regional Municipalities and Environment according to situation like following are the major typical characteristics:
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Characteristic
|
Concentration
|
|
Biochemical Oxygen Demand (BOD5)
Suspended Solids (SS)
Nitrogen: Ammonia (as N)
Nitrate (as NO3)
Phosphorus (total as P)
Faecal Coliform
|
15 mg/l
15 mg/l
5 mg/l
50 mg/l
30 mg/l
200/100 ml
|
Screen and Screen Chamber
Mechanical screen will be provided to aim at safety of the pump and to remove solids that may retard treatment process and malfunctioning of equipment of treatment plant. The design criterion for screen chamber is given below:
• Velocity in approach channel : 0.5-1.0 m/sec
• Velocity through screen : 0.5-1.0 m/sec
• Average spacing between the bars : 20 mm
• Angle of inclination : 60° – 90°
• Head loss : 10 – 15 cm
• Vertical velocity component perpendicular: 0.15 m/sec to the screen section.
Oil Separator
An oil separator will be designed to remove the oil from wastewater which hinders the process of treatment. The oil will be trapped in a chamber from where it could be disposed off to appropriate site.
Aeration Tank / Reactor
Aeration tank based on extended aeration system (oxidation ditch) will be provided. Design of the tank will be based on following parameters:
• Hydraulic Retention Time
Hydraulic retention time of reactor will be taken as more than 20 hours and will be calculated as under:
Reactor Volume (m³)
HRT (days) = -------------------------------
Max. Daily flow (m³/day)
• Organic Loading Rate
The organic loading BOD rate will be less than 0.3 kg/m3. day.
Organic loading rate=BOD
V
Whereas
BOD=BOD in kg/day
V=Volume of reactor in m³
• Sludge Loading Rate
F/M ratio or sludge loading rate will be adopted as less than 0.15 and will be related as
[BOD] Q
F/M ratio = ---------- x ---
[MLSS] V
Whereas
[BOD] = Sewage BOD concentration in kg/ m³
[MLSS] = Biomass Concentration in reactor, 2-5 kg/ m³
Q = daily influent discharge in m³
V= volume of reactor in m³
Aeration System
Vertical type mechanical surface aerators will be provided in a rectangular aeration tank/reactor.
Clarifier
Circular RCC tank(s) will be provided for final settlement of suspended solids.
• Overflow Rate: 24 m³/m². day
• Solids Loading Rate: 10 kg/m². hr.
• Depth: 4 - 6 m.
Rapid Sand Filter
The effluent from final sedimentation tank will be passed through filters in order to further reduce the BOD and suspended solids. The filters will be designed at filtration rate of 5 m³/ m²/hr.
Disinfection of Treated Effluent
Disinfection of treated effluent will be carried out through chlorination which will be carried out through dry feed chlorinators and gas will be fed in treated effluent tank.
Treated Effluent Tank
Treated effluent will be stored in a tank. The tank will be designed for one day (annual average) retention time.
Disposal of Treated Effluent
The disinfected effluent will be utilized by the different users for irrigation purpose. In case, conveyance of treated effluent is not possible through gravity, pumps will be proposed to pump the treated effluent from storage tank.
Sludge Thickener/Holding Tank
Circular gravity thickener(s) will be provided for gravity settling of sludge. The basis of design is given as under:
• Solid loading rate: 30 kg/m2. day
• Sludge concentration :2 – 3% (i.e. 20–30 kg/m3)
• -Sludge production rate:0.4 – 0.6 kg/kg BOD removed
Sludge Drying Beds
Conventional sludge drying beds will be provided to dewater the stabilized sludge. The basis of design will be as under:
• Thickness of wet sludge : 250 mm
• Sludge retention time : 14 days
Sludge Disposal
The dewatered sludge through drying beds will be transported through vehicles and disposed off at landfill site; the site should be of sufficient capacity to store the sludge minimum for 100 days.
Interconnecting Pipe Material
Buried pipes will be of HDPE pipes, however exposed pipes will be of DI.
STRUCTURAL DESIGN
Code and Standards
• All reinforced concrete structures including water retaining structures shall be designed in accordance with the provisions of the latest editions of British Standards B.S. 8110, B.S 5337 and other relevant Standards and Codes of Practice.
• All material used shall conform to the latest British Standards.
• In addition, loadings, design procedures and material specifications may also fulfill the requirements of American Standards and Codes i.e. ACI/ANSI/ASCE/ASTM etc.
Material Strengths
• Concrete
The grade of concrete appropriate for use shall be selected from the preferred grades in BS 5328.
• Reinforcing Steel
All reinforcing steel to be used in reinforced concrete works shall conform to BS 4449 and BS 7295 having a minimum yield strength (fy) of 460 MPa (66,650 Psi).
Units and Design Loads
Units
The International System of Units (S.I. Units) shall be used for the project.
Design Loads
The structures should be so designed that adequate means exist to transmit the design ultimate dead, wind, and earthquake and imposed loads safely from the highest support level to the foundations. The characteristics load in each case should be the appropriate load as defined in and calculated in accordance with BS 6399.
Dead Loads
The dead loads on the structure will be computed from the unit weights of the materials. Following unit weights will be used for computing dead loads, unless otherwise specified.
|
Material
|
Unit Weight (kN/m³)
|
|
Reinforced Concrete
|
23.5
|
|
Plain Concrete
|
22.5
|
|
Water
|
9.8
|
|
Brick Masonry
|
18.8
|
|
Saturated Soil
|
21.1
|
|
Steel
|
76.9
|
|
Backfill Compacted
|
18.14
|
Live Loads
Live load at different buildings/locations shall vary according to the functional requirements.
Equipments/Machinery
Machine and equipment loads shall conform, to the requirements of the Manufacturer.
Wind Loads
The wind loads on the structures will be calculated using the following formula:
Wk = 0.613 Vs² N/m²
Where,
Vs = Design wind speed in m/sec = V S1 S2 S3
V = Basic wind speed in m/sec = 45 m/sec
S1 = Multiplying factor relating to topology
S2 = Multiplying factor relating to height above ground and wind Breaking
S3 = Multiplying factor related to life of structure.
Earthquake Loads
Acceleration coefficient for seismic loads shall be taken as recommended in the Geotechnical Investigation Report of the project and Highway Design Manual.
Temperature Effects
The temperature effects will be investigated against a maximum differential temperature of + 20 degree centigrade and included in the design. Unless otherwise specified, the maximum daily temperature shall be assumed as 55 °C. (According to the location of site)
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