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CHAPTER 12

Service Reservoirs and Storage Capacity

Service Reservoirs & Storage

Specifies design of service reservoirs (ESR — elevated service reservoirs, GSR — ground service reservoirs, underground clear water reservoirs). Covers storage capacity calculation, structural design, hydraulic design, water quality maintenance.

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Key formulas

  • Storage capacity = Balancing storage + Fire storage + Emergency storage.
  • Balancing storage = (Maximum hourly demand × 4 hours) - (average hourly supply × 4 hours); typically 25-35% of daily demand.
  • Fire storage = Fire demand × duration (typically 4 hours for major hydrant flow).
  • Emergency storage = minimum 4-8 hours of average demand (for pipe breaks, power failures).
  • Total Storage typically = 33% of average daily demand (rule of thumb for balancing + fire).
  • ESR height (above GL): provides minimum residual pressure (17 m at ferrule) at farthest point; account for friction losses to that point.

Key values & thresholds

total storage pct of daily demand
33 (roughly 8-hour balancing)
balancing storage pct
25 - 35
fire storage hours of fire demand
4
emergency storage hours of avg demand
4 - 8
ESR height typical m
15 - 25
ESR diameter typical m
8 - 30 (by capacity)
ESR capacity typical ML
0.5 - 10
GSR capacity urban typical ML
5 - 50
free board above HWL m
0.6
dead storage below inlet m
0.5
retention time max hrs
24
reservoir wall concrete grade
M30 - M40
chemical resistance epoxy lining
required for potable storage

Clause-level requirements

  • Storage capacity = Balancing (25-35% of daily demand) + Fire (4-hour fire demand) + Emergency (4-8 hour supply). Rule of thumb: 33% of daily demand.
  • ESR (Elevated Service Reservoir): on staging for gravity distribution. Height provides 17 m residual at farthest ferrule after friction losses.
  • GSR (Ground Service Reservoir): at natural high points or fed by pumps. Lower cost than ESR; requires booster pumps to feed distribution.
  • Underground clear water reservoir (CWR): at treatment plant for contact time and balancing; covered to prevent contamination.
  • Reservoir geometry: circular (smallest surface area per volume) most common; rectangular for space constraints. Diameter:height ratio 2:1 to 3:1 for stability.
  • Hydraulic design: inlet and outlet on opposite sides; maximum retention < 24 hours to maintain chlorine residual; baffles for plug flow (optional).
  • Structural design: per IS 3370 (liquid retaining structures); M30-M40 concrete; crack width < 0.1 mm; waterproof internal surfaces.
  • Inlet/outlet arrangement: separate pipes with valves; overflow pipe 150 mm above HWL; scour outlet at bottom for cleaning.

Practitioner notes — what goes wrong in the field

  • Storage 33% of daily demand is rule of thumb — precise value depends on demand pattern and pumping schedule. Diurnal analysis gives specific value.
  • ESR staging height: determined by ground slope + required pressure at farthest point. For Delhi flat terrain, 15-20 m common; for Mumbai hilly, 5-15 m (already elevated by topography).
  • Multiple smaller ESRs vs one large ESR: multiple smaller ESRs preferred for redundancy and pressure zone management. One ESR per 2-5 km² service area.
  • Typical municipal ESRs: 0.5-10 ML capacity; GSRs 5-50 ML. Mumbai Master Balancing Reservoir (200 ML) is India's largest urban water storage.
  • Clear Water Reservoir at treatment plant: retention 30-60 min for chlorine contact + balancing for pumping. Typically 1-2 hours of plant output capacity.
  • Water quality in reservoirs: chlorine decay over retention time; avoid > 24 hour retention (chlorine drops below 0.2 mg/L, bacterial regrowth). Design for 8-12 hour turnover.
  • Structural design per IS 3370: strict crack width control to prevent leakage. M30 concrete minimum, waterproofing admixture, epoxy coating internal surfaces.
  • Overflow and scour: overflow pipe directed away from building; scour outlet 600 mm pipe for cleaning every 6-12 months. Silt accumulation 50-100 mm per year typical.
  • Access and ventilation: manhole 900 × 600 mm minimum; ventilation pipe with mosquito screen; lightning arrester on top. Inspection every 3 months.
  • Cleaning schedule: annual for urban reservoirs; bi-annual for rural. Empty, inspect internal coating, remove sediment, re-chlorinate before re-filling.
  • Modern construction: RCC in-situ most common; precast segments emerging for smaller (< 5 ML) reservoirs. GRP (glass-reinforced plastic) for very small (< 500 m³).
  • Cost: ESR ₹15-30 lakh per ML capacity (including staging); GSR ₹8-15 lakh per ML. Clear Water Reservoir at WTP ₹10-18 lakh per ML.

FAQs

What is service reservoir storage capacity?
Typically 33% of daily demand (rule of thumb) = Balancing (25-35%) + Fire (4-hr fire demand) + Emergency (4-8 hr avg demand). Precise value from diurnal demand pattern analysis.
What is ESR staging height?
Provides 17 m residual pressure at farthest ferrule after friction losses. For flat terrain Delhi: 15-20 m staging; hilly Mumbai: 5-15 m (terrain provides elevation). Calculated from HGL analysis.
What is IS 3370?
Indian Standard for design of liquid-retaining concrete structures (including water reservoirs). Specifies concrete grade (M30-M40), crack width control (< 0.1 mm), reinforcement, waterproofing, structural design methodology. Mandatory for all water reservoirs.
How many ESRs per city?
One per 2-5 km² service area. Multiple smaller ESRs (0.5-5 ML each) better than one large — provides redundancy, enables pressure zone management, reduces construction risk. Delhi has 200+ ESRs; Mumbai 150+.
What is retention time in reservoir?
Maximum 24 hours to maintain chlorine residual. Typical design 8-12 hours for good water quality. Above 24 hours: chlorine < 0.2 mg/L, bacterial regrowth, taste/odor issues. Smaller reservoirs or higher turnover prevent this.
What is balancing storage?
Storage to accommodate difference between steady production (from WTP) and variable demand (peak 2-3× average in morning/evening). Typical 25-35% of daily demand. Allows WTP/pumps to run at constant rate while meeting peak demand.
What is typical reservoir cost?
(2025 India rates) ESR ₹15-30 lakh per ML including staging; GSR ₹8-15 lakh per ML; CWR at WTP ₹10-18 lakh per ML. For 2 ML ESR in typical urban location: ₹30-60 lakh total project cost.

Calculator

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Service Reservoir Sizing (Balancing + Fire + Emergency)

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Size a service reservoir per CPHEEO Chapter 12 — balancing (25–35% of daily demand), fire (4-hour fire flow), and emergency storage (4–8 hr average).

Inputs
Average daily demandMLD
Balancing storage% of daily
Population servedpersons
For Kuichling fire demand
Fire durationhr
Emergency storagehr of avg
Outputs
Balancing storage
1,650m³
V_balance = daily demand × %/100
Fire storage
4,830m³
V_fire = 3182 × √(P/1000) × 60 × duration / 1000
Emergency storage
833m³
V_emerg = (daily / 24) × emerg hrs
Total storage required
7,313m³
Sum of balancing + fire + emergency
Total storage
7.31ML
CPHEEO Reference Values
Balancing storage25 – 35% of daily demand
Total storage rule≈ 33% of daily demand
Fire duration4 hours (standard)
Structural design codeIS 3370, M30–M40 concrete
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Cross-references

IS 3370 Liquid Retaining StructuresCPHEEO WS Chapter 11IS 456

Tags

service reservoiresrgsrclear water reservoirbalancing storagefire storageis 3370cpheeo
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Manual on Water Supply and Treatment · 3rd Edition (1999) with 2024 revision updates · Central Public Health and Environmental Engineering Organisation (CPHEEO), Ministry of Housing and Urban Affairs, Government of India.
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