The combined sewer and drains ventilation scheme appears to be satisfactory when properly implemented. The use of open gratings at manholes has been common since about 1834 but is now less favored due to offensive emanations. Investigations by Mawbey and De Courcy Meade concluded that vertical shafts alone are sufficient for proper air circulation, with an average upward current of 162 feet per minute and a downward current of 84-6 feet per minute. This chapter also discusses deodorising systems like the Reeves and Caink systems, which have not been widely adopted by local authorities due to their complexity and cost. Additionally, it covers the Shone and Ault System, an experimental method involving powerful suction fans for ventilating sewers and house drains, but this system was expensive and required constant supervision.
<Callout type="important" title="Key Findings">Vertical shafts are now preferred over open gratings due to their effectiveness in providing proper ventilation without causing nuisance.</Callout> The volume of sewage to be carried by the sewers is calculated based on dry weather flow, which may be assumed to equal the amount of water supply per day. The storm water calculation involves determining the time of concentration and using specific formulae for rainfall intensity. For example, a sewer serving an area with 400 acres and housing 20,000 persons would require a diameter of about 5 feet 6 inches to handle both dry weather flow and storm water efficiently.
<Callout type="risk" title="Ventilation Risks">Vertical shafts must be constructed properly to avoid nuisance by sewer air seeping into the surroundings.</Callout> The chapter concludes with details on gradients for sewers, self-cleansing velocities necessary to keep sewers free from deposits, and alternatives like pumping stations or sewage lifts when a gravitation system is not feasible.
Key Takeaways
- Vertical shafts are more effective than open gratings for sewer ventilation.
- Calculating the volume of sewage and storm water is crucial for proper sewer design.
- Self-cleansing velocities must be maintained to prevent deposits in sewers.
Practical Tips
- Use vertical shafts instead of open gratings to reduce offensive odors from sewers.
- Calculate the time of concentration accurately when estimating storm water volume.
- Ensure proper gradients and self-cleansing velocities for efficient sewer operation.
Warnings & Risks
- Improper construction of vent shafts can lead to nuisance by sewer air seeping into surroundings.
- Complex deodorising systems like Reeves or Caink are not widely adopted due to high costs and maintenance requirements.
Modern Application
While the historical methods for ventilating sewers and calculating sewage flow volumes have evolved, the principles remain relevant. Modern sanitation practices still rely on proper ventilation and efficient sewer design to prevent health hazards and nuisance issues.
Frequently Asked Questions
Q: What are the advantages of using vertical shafts over open gratings?
Vertical shafts provide better air circulation without causing offensive odors, making them more effective for sewer ventilation.
Q: How is storm water volume calculated in sewers?
Storm water volume is determined by calculating the time of concentration and using rainfall intensity formulae to estimate the amount of rainwater that needs to be carried by the sewers.
Q: What are some alternatives when a gravitation system for sewers is not feasible?
Alternatives include providing pumping stations or installing sewage lifts, such as Shone ejectors, to manage the flow of sewage in areas where natural gradients cannot be utilized.