Design Criteria

• Design Period
• Sewer Network
  • Peak Factor
  • Ground Water Infiltration & Leakage
  • Minimum & Maximum Veloicty of Flow
  • Depth of Flow
  • Recommended Slope for Minimum Velocity
  • Pipe Material Options for Gravity Sewer
  • Minimum Size of Sewers
  • Bedding Options/Design
  • Trench Section-Minimum Cover
  • Manhole Size-Depth-Type
  • Spacing of Manhole
  • Size of Manhole
  • Cover Frame
  • Drop Manhole
• Pumping Main and Pumping Hours
  • Pumping Hours
  • Flow Formula
  • Hazen William Coefficients
  • Rising Main Material and Size
• Sewage Pumping Station
  • Type-Construction-Accessories
  • Location
  • Screen and Overflow
  • Wet Well
  • Type of Sewage Pumps and Configuration
• Sewage Treatment Plant
  • Type
  • Centralized or Decentralized
  • Screening
  • Sedimentation
  • Grit Removal
  • Settling Velocities and Surface Overflow Rates
  • Detention Period-Surface Loading-Weir Loading
  • Sizeing/Up Flow-Horizontal Flow
  • Secondary Sewage Treatment
  • Effluent Quality Standards
  • Land Requirement

Design Period

The design period is different from planning horizon year. Table 1.1 CPHEEO Manual  gives design period for different components of sewerage system and sewage treatment.  STP should be designed in phases, refer clarification in table 1.1.  First phase of STP should be for about 10 years but land of STP should be taken for 30 years period.

Design Period of Sewerage Components

Sl. No	Design Component	Design Period	Remarks
1	Land Acquisition for STP, SPS, sewers etc	30 Years	Land acquisition in future difficult
2	Sewer network (laterals, Trunk mains, Outfall et)c	30 Years	Replacement difficult and costly
3	Pumping mains	30 Years	Cost may be economical
4	Pumping Stations-Civil Work	30 Years
5	Pumping Machinery	15 Years	Life of pumping machinery is 15 years
6	Sewage Treatment Plants	30 Years	The construction shall be modular in phased manner as actual population less than design population and in Indian cities initially flows are much less due to connectivity problems.
7	Effluent disposal and utilization	30 Years	Provision of design capacities in the initial stages itself is economical

Sewer Network

Para 3.2.4, of manual stipulate that generally 80% of the water supply may be expected to reach the sewers unless there is data available to the contrary. However sewers should be designed to minimum waste water flow of 100 litres per capita per day. Industries, commercial buildings etc who often use water other than the municipal supply and may discharge their liquid waste into the sanitary sewers. Such quantities should be worked out separately.

Input Requirement	Data Source	Remarks/Design criteria
Quantity of water supply used by consumers other than that from Municipal/PHED system	From ULB and Public

Peak Factor
Peak factor is ratio of maximum flow at any time to average flow and depends on contributary population. Para 3.2.5 of Manual stipulate peak factor for different contributary population. The minimum flow may vary from 1/2 to 1/3 of average flow.

Sl. No	Contributing Population	Peak Factor
1	Up to 20,000	3
2	20,000 – 50,000	2.5
3	50,000 – 75,000	2.25
4	Above 75,000	2

Ground Water Infiltration and leakage

Some quantity of ground water or subsoil water may infiltrate into sewers through defective joints, broken pipes etc. This is significant when water table is high and head of ground water is more than the head of sewage in sewers. Some quantity of sewage may leak out from defective joints and defective pipes when head of sewage is more in sewers than head of ground water outside. Infiltration and leakage mainly depends on quality of construction and water table levels. Infiltration can be considered, Para 3.2.7 Manual, 5000-50000 liters per day per hectare or 500-5000 liters per day per km length of sewers or 250-500 liters per day per manhole for sewers laid below ground water level.

Minimum and maximum Velocity of flow A velocity of 0.6 mps would be required to transport sand particles of 0.09 mm size with a specific gravity of 2.65. Hence a minimum velocity of 0.6 mps, Para 3.4.3 Manual, for present peak flow and 0.8 mps at design peak flow is recommended for sanitary sewers. Thus the sewers are designed on the assumption that although silting might occur at minimum flow, it would be flushed out during peak flows. Erosion of sewers is caused by sand and other gritty material in the sewer and also by excessive velocity. Velocity in a sewer is recommended not to exceed 3 mps.

Depth of flow
The closed sewers should not run full, otherwise the pressure will rise above or fall below the atmospheric pressure and condition of open channel flow will cease to exist. Also from consideration of ventilation sewers should not be designed to run full. In case of circular sewers, table 3.6 of Manual(Hydraulic properties of Circular sections), the velocity is maximum at 0.8 full and is 1.14 times the velocity at full flow. The discharge at 0.8 full is 0.98 times the discharge at full flow. Therefore the maximum flow depth should be 0.8 full at ultimate peak flow for all pipe diameters.

Constant (n)				Variable (n)
d/D	v/V	q/Q	Nd/n	v/V	q/Q
1.000	1.000	1.000	1.000	1.000	1.000
0.9	1.12	1.07	1.07	1.06	1.02
0.8	1.14	0.97	1.14	1	0.89
0.7	1.12	0.84	1.18	0.95	0.71
0.6	1.07	0.67	1.21	0.89	0.56
0.5	1	0.5	1.24	0.81	0.41
0.4	0.9	0.34	1.27	0.71	0.27
0.3	0.78	0.2	1.28	0.61	0.15
0.2	0.62	0.09	1.27	0.49	0.07
0.1	0.4	0.02	1.22	0.33	0.02
Where : D = Full Depth of flow(internal dia), d Actual Depth of flow, V = Velocity at full depth, v = Velocity at depth ‘d’, n = Manning’s coefficient at full depth, nd = Manning’s coefficient at depth ‘d’, Q = Discharge at full depth, q = Discharge at depth ‘d’

Recommended Slopes for Minimum Velocity

For sewers running partially full, for a given flow and slope, velocity is little influenced by pipe diameter. As such for present peak flows up to 30 lps, the recommended slopes for minimum velocity are given in Table 3.7 of Manual. These slopes would ensure minimum velocity of 0.6 mps in the early years.

Sl. No	Present Peak Flow in LPS	Slope per 1000
1	2	6
2	3	4
3	5	3.1
4	10	2
5	15	1.3
6	20	1.2
7	30	1

Pipe Material Option for Gravity Sewer
Brickwork is used for large diameters as sewers can be constructed in any shape. However now it is not common. Concrete pipes are commonly used now as these can be manufactured to any reasonable strength and laying is easy and jointing is leak proof. However these pipes are subject to corrosion where acid discharges are carried or where velocities are not sufficient to prevent septic conditions or where the soil is highly acidic or contains excessive sulphates. Only high alumina cement concrete should be used when it is exposed to corrosive sewage or industrial wastes. Salt Glazed Stoneware Pipes are mostly manufactured in sizes 80-1000 mm but sizes greater than 380 mm are generally not used due to economic considerations. The length of these pipes is 60 cm, 75 cm and 90 cm. These pipes are good for corrosion resistance and erosion resistance. However due to less length, more joints, difficulty in jointing, requirement of special bedding and less compressive strength of pipes manufactured in India; use of these pipes is reducing in India. AC pipes cannot stand high superimposed loads, subject to corrosion from acids in sewage and high sulphate soils, require special bedding and weak against erosion where high velocities are encountered; as such use of AC pipe is not prevalent. Cast iron, DI and steel pipes are not used due to high cost. uPVC pipes are manufactured in sizes 75, 90,110, 140, 160,250,290 and 315 mm outer dia. uPVC pipes are smooth, light, easy to joint and leak proof joint. Rates are also low. These days these pipes are used for making connection from house to sewer but not prevalent in street sewers. HDPE pipes are available up to 630 mm dia. The pipes are recently development in India but are costlier than RCC pipes and uPVC pipes. The welded joints are leak proof and as such some people have started using these pipes.GRP pipes are widely used in other countries where corrosion resistant pipes are required at reasonable rates. Characteristics of various pipes  for gravity sewers makes comparison for different pipe options. 'Characteristics of metallic, PSC and GRP pipes' and 'Characteristics of plastic, DI and GI pipes' can be referred to adjudge suitability of pipe material option for particular use.

Input Requirement	Data Source	Remarks/Design criteria
Rates of Pipe for supply and transportation, Schedule of Rates, Places where pipes are manufactured,	Pipe rates be taken from Manufacturers	Drains are to carry only rain water

Pumping Main and Pumping Hours

Pumping Hours
Find out the actual availability of power in the city and based on it adopt pumping hours. More pumping hours give lower pipe size and hence cost saving. Future expected improvement in power supply may also be considered.

 

Hydraulic design is done using Hazen-Williams formula as given below:

V = 4.567 X 10-3  CD^0.63 * S^0.54 and

Q = 1.292 X 10-5 CD^2.63 *S^0.54

Where, Q - Discharge in m3/hr;

V - Velocity of flow in m/sec;

d - diameter of pipe in mm;

C - Hazen-Williams Co-efficient;

S - Slope of Hydraulic Gradient;

‘C’ Value adopted used in the hydraulic design has been taken 140 for DI pipes with cement mortar lining inside.

Hazen-William Coefficients
S. No	Conduit Material	Recommended ‘C’ Values
		New pipes	Design
1	Concrete (RCC & PSC) with socket & spigot joints	150	120
2	Asbestos cement	150	120
3	Plastic pipes	150	120
4	Cast iron	130	100
5	Steel welded joints	140	100
6	Steel, welded joints lined with cement or bituminous enamel	140	120

Rising Main material and size
Rising mains (pressure mains or force mains) are provided to carry sewage to higher elevations. It is generally provided to convey sewage from Sewage Pumping Stations to a higher level inlet chamber of nearby sewer or Sewage Treatment Plants. Characteristics of different pipes for pumping main help to choose between pipe options for particular site. The size of the main should be determined considering initial cost and capitalized O & M cost for different sizes as per method given in Water Supply Manual. It is designed according to the following considerations:

Input Requirement	Data Source	Remarks/Design criteria
Rates of different type of Pipe

Sewage Pumping Station

Type, construction and Accessories
Earlier the pumping stations used to be rectangular with dry and wet wells adjacent to each other or circular with central dry well and peripheral wet well or circular with a dividing wall to separate the dry and wet wells and with centrifugal pumping sets. Now wet well (no dry well) with submersible pumps are more prevalent. The construction is of RCC. Sulphate resistant cement is used in corrosive soils. Provision of flow measurement, adequate ventilation, safety equipments, pump lifting arrangements shall be made.

Input Requirement	Data Source	Remarks/Design criteria
Cost of different types of Pumps, efficiency etc	Pump Manufacturers

Location
At places, where depth of sewer becomes too deep and it is difficult to lay sewer at such depths, sewage-pumping station is required to lift the sewage to nearby manhole or to the STP, from where it will flow by gravity.

Screens and Overflow.

All SPS will be provided with coarse screens before the wet well with clear opening of 40-50 mm between the bars for the manually cleaned type and 25 mm for the mechanical type. The screening units shall always be provided in duplicate. It is also  provided with a bye bass on the upstream side, to avoid overflow of the screen channel in case of sudden power failure. Drainage facility shall also be provided in the individual screen channels to empty these channels for maintenance purposes.

Input Requirement	Data Source	Remarks/Design criteria
Cost of coarse screens, options available

Wet Well

The sewer line will discharge the sewage into a wet well. The capacity of wet well/sump should be such that deposition of solids is avoided and sewage does not turn septic. The capacity should not be too low to require frequent on-off of pumping sets. The capacity of the wet well is to be so kept that with any combination of inflow and pumping, the cycle of operation for each pump will not be less than 5 minutes and the maximum detention time in the wet well will not exceed 30 minutes of average flow. The high water level in sump well will not exceed invert level of lowest incoming pipe.

Types of Sewage Pumps and Configuration

Non clog submersible pumps are proposed in all Pumping stations as per availability in the Indian market. Pumping units are designed to handle suitably peak, average and low-flow from connected sewers. The capacity of pumps shall be adequate to meet the peak rate of flow with 50% standby. The general practice is to provide 3 pumps for a small capacity pumping station comprising 1 pump of 1 DWF, 1 of 2 DWF and third of 3 DWF capacity. For large capacity pumping station, 5 pumps are usually provided, comprising 2 of ½ DWF, 2 of 1 DWF and 1 of 3 DWF capacity, including standby.

Sewage Treatment Plant

Tool for Design of Waste Stabilization Ponds

Tool for Design of Extended Aeration STP

Tool for area calculation of STP with extended Aeration process

Characteristics of different STP processes

Tool for finding Life Cycle Costs of different processes of STP

Types
The removal of contaminants from sewage is brought by a sequential combination of various physical unit operations and chemical and biological unit processes. The physical unit operations include screening, grit removal and sedimentation. The biological processes is broadly classified as i) suspended growth processes, both aerobic and anaerobic, including activated sludge process, extended aeration, lagooning, nitrification, de-nitrification and anaerobic digestion and ii) attached growth processes such as aerobic and anaerobic filter processes. The treatment plants are also called primary treatment if treatment is limited to physical process, secondary treatment when treatment is up to biological treatment and tertiary treatment when treatment goes beyond biological treatment to meet some further requirement of effluent quality.

Centralized or Decentralized
The treatment plants can be centralized or decentralized. In case of decentralized more number of smaller capacity plants are constructed. This system decreases cost of sewerage system but increases initial and O & M cost of STPs.

Screening
Screen is a device with openings generally of uniform size for removing bigger suspended or floating matter in sewage and are placed before SPS and STPs. Coarse screenings have opening of 75-100 mm for SPS and 50 mm for STPs. Medium screens have opening of 20-50 mm and fine screens have openings less than 20 mm. Coarse screens are manually cleaned but medium and fine screens are mechanically cleaned. Fine screens may be of the drum or disc or mat type. Quantity of screenings is about 0.0015 cum/ml with screen size of 10 cm and 0.015 cum/ml in case of 2.5 cm.

Sedimentation
For primary sedimentation tanks, both, surface overflow rate and detention period (Hydraulic Residence Time) are important design criteria as the solids to be settled are flocculent in nature and undergo flocculation. The major design parameters for secondary settling tanks designed to remove bio-flocculated solids are solid loading rate or solid flux as well as surface over flow rate. The overflow rates for settling tanks as per Manual. The smaller values in the range are applicable for plants less than 5 MLD.

Type of Settling	Overflow Rate cum/sq m. d		Solid loading		Depth	Detention time
			Kg/sq m. d
	Average	Peak	Average	Peak	m	hr
A. Primary Settling
Primary settling only	25-30	50-60			2.5-3.5	2-2.5
Primary settling followed by   secondary treatment	35-50	80-120			2.5-3.5	2-2.5
Primary settling with activated sludge return	25-35	50-60			3.5-4.5	2-2.5
B. Secondary Settling
Secondary Settling for trickling filter	15-25	40-50	70-120	190	2.5-3.5	1.5-2.0
Secondary Settling for activated sludge (excluding extended aeration)	15-35	40-50	70-140	210	3.5-4.5	1.5-2.0
Secondary Settling for extended aeration	15-Aug	25-35	25-120	170	3.5-4.5	1.5-2.0

Grit Removal

Grit consists of coarse particles of sand, ash and clinkers, egg shells, bone chips and many inert materials of inorganic in nature. For STPs more than 10 MLD mechanical grit cleaning system should be provided. In case of manually cleaned grit chambers at least two units should be provided and all mechanically cleaned units should be provided with a manually cleaned unit to act as a bypass. The grit content is 0.05 to 0.15 cum/ml for domestic sewage. The minimum size of grit to be removed is 0.2 mm although 0.15 mm is preferred where ash content is more. Detention period of 60 seconds can be taken.The settling velocities and surface over flow rates for ideal grit chamber at 10 degree centigrade.

 

Settling velocities and Surface Overflow Rates for Ideal Grit Chambers

Rectangular		Arch-type		Circular
Depth	Size	Depth	Size	Depth	Size
< 900	900 x 800
900 – 2,500	1,200 x 900			900 – 1,650	900
		>2,500	1,400 x 900	1,650 – 2,300	1,200
				2,300 – 9,000	1,500
				9,000 – 14,000	1,800

Input Requirement	Data Source	Remarks/Design criteria
Options and cost of grit removal systems

Detention Period/Surface loading/ Weir loading
The detention period of 2 to 2.5 hours for primary settling tank and 1.5 to 2 hours for secondary settling tank will produce optimum results. Longer detention period may affect the tank performance adversely due to setting in of septic conditions particularly in tropical climates. Longer detention period in secondary settling tanks may result in denitrification which adversely affects the settling efficiency. For all primary, intermediate and secondary settling tanks, except in the case of secondary tanks for activated sludge process, weir loading of the order of 125 cum/d.m. for average flows is recommended. For secondary settling tanks in activated sludge or its modifications, the weir loading is around 185 cum/d.m. The depth in vertical flow tanks may be 2.0 m excluding hoppers.

Sizing/Upflow-horizontal flow
Up flow tanks have been used for sewage sedimentation but horizontal flow types are more popular. Diameter of circular tanks vary widely from 3 to 60 m although common range is 12 to 30 m. Rectangular tanks, maximum length and widths of 90 and 30 m with length to width ratio of 1.5 to 7.5 and length to depth ratios of 5 to 25 are recommended.

Secondary Sewage Treatment

Many processes which are broadly classified as aerobic or anaerobic and as attached growth or suspended growth are prevalent for secondary treatment of sewage. The characteristics and salient features of these processes. The life cycle cost and NPV should be worked out to decide technology for STP.

Effluent Quality Standards
The sewage after treatment may be disposed either into a water body such as lake, stream, river, estuary and ocean or onto land. It may also be utilized for industrial reuse or reclaimed sewage effluent in cooling systems, boiler feed, process water, reuse in agriculture and horticulture, watering of lawns, golf courses, ground water recharge or for preventing saline water intrusion in coastal areas. The effluent should fulfill statutory requirements laid down by pollution control boards for disposal in water bodies and for irrigation. The General Standards for Discharge of Environment Pollutants notified by Ministry of Environment & Forest (MoEF), GoI in May 1993.

Effluent Disposal Standards
Parameters	Standards for disposal in
	Inland Surface water	Land for irrigation
BOD (mg/l) 5days 200C	30	100
TSS (mg/l)	100	200
COD(mg/l)	250	-
pH	5.5-9.0	5.5-9.0
Sulphides (mg/l as S)	2		-
Total Chromium (mg/l as Cr)	2		-
Faecal Coli form MPN/100 ml
Desirable	1,000
Maximum. Permissible	10,000

Land Requirement

Requirement of land for different processes of treatment is given below:

Activated Sludge/Trickling Filter   -	0.5 acre/MLD
Aerated Lagoon     -	1.2 acre/MLD
Stabilisation Pond    -	2.5 acre/MLD
UASB      -	0.42 acre/MLD
Extended Aeration    -	0.35 acre/MLD

Input Requirement	Data Source	Remarks/Design criteria
Cost of Land	Patwari, local enquiry