Separate Sewers | Civil Engineering Optional Notes for UPSC PDF Download

Separate Sewers

• Separate sewers involve collecting municipal wastewaters (such as blackwater from toilets, greywater, and industrial wastewater) and surface run-off (rainwater and stormwater) separately.
• This separation helps prevent sewer system and treatment station overflows during rainy periods and avoids mixing relatively clean surface run-off with pollutants from municipal wastewater.
• The design of sewers and treatment stations should focus on the volume of wastewater only, allowing for potential reuse of surface run-off and rainwater after simplified treatment for purposes like landscaping or agriculture.

Basic Design Principles of Sewerage Systems

• Precipitation
• Blackwater
• Faecal Sludge
• Greywater
• Brownwater
• Urine or Yellowwater
• Feces
• Non-biodegradable Wastewater

Basic Design Principles:

• Sewerage systems are categorized as combined sewerage (conventional sewers) and separate sewerage.
• Combined sewerage carries both surface run-off and wastewater, while separate sewerage carries them separately.
• Recent trends favor separate systems to manage sewage and surface run-off in distinct systems.
• Separate systems help avoid issues like combined sewer overflow during heavy rainfalls.
• By controlling surface run-off separately, flooding risks in low-lying areas can be minimized.
• During flooding, rainwater, not sewage, can enter basements, safeguarding against unsanitary conditions.

Sewer Separation and Its Impacts

• Separation ensures sewage doesn't enter the environment but goes to treatment plants through a closed system.
• Stormwater can flow to detention basins and watercourses.
• However, surface run-off can still enter wastewater sewerage pipes due to unsealed joints or illegal connections.
• Conversely, wastewater may wrongly enter stormwater systems.
• Sewer separation eliminates combined sewer overflows.
• Modern systems have separate sanitary sewers and stormwater drains.
• Challenges include disruption, cost, and increased pollutant discharge from untreated run-off.
• Inadequate separation can lead to issues like basement flooding and sewer overflows.
• Urban stormwater drains in cities can collect various pollutants from different sources.
• Regular inspections are crucial to prevent unexpected pollution from entering the drainage system.
• Pollutant loads in stormwater vary based on urban structure, materials, rainfall, and sewer processes.
• Careful planning of the sewerage network is essential to address these issues.

Tools like rainfall modeling, sewer treatment modeling, and river quality modeling aid in planning.

Costs Considerations and Operation & Maintenance in Wastewater Management

• Costs Considerations: When building a separated sewer system, the construction costs may be higher compared to a combined system because two distinct networks are needed. These networks must be maintained and operated separately. Moreover, replacing a combined system with a separated one can be very expensive.
• Operation and Maintenance (O&M): O&M is a crucial aspect of wastewater management that influences the choice of technology. Many wastewater treatment projects fail post-construction due to inadequate O&M. Annually, O&M costs for treatment and sewage collection are typically similar to the depreciation on the initial investment. O&M necessitates:
• Thorough planning by specialized engineers.
• Qualified and trained staff dedicated to their roles.
• An extensive system with spare parts and utilities for O&M.
• A maintenance schedule, team, and facilities for repairs.
• A management approach that ensures reliable service with minimal disruptions.
• A significant yearly budget solely for O&M and service enhancement.

Maintenance Policy

• Maintenance can be corrective or preventive.
• Corrective maintenance involves repairs after breakdowns, leading to service interruptions.
• Ideally, preventive maintenance replaces parts at the end of their lifespan, aiding optimal budgeting and minimal service impact.
• Operational and maintenance (O&M) requirements are crucial in technology selection.

Health Aspect

• Combined sewer systems offer high hygiene and comfort levels.
• Separated systems enhance safety by transporting sewage directly to treatment plants, averting environmental overflow.
• Availability of sufficient water is essential for waste transportation in sewers.
• Systems are viable in urban areas with adequate resources for implementation and maintenance.
• Separated sewers are useful in areas prone to heavy rainfall, like monsoon regions.
• Stormwater runoff management is crucial during heavy rainfall periods.
• Professional expertise is needed for the planning, construction, and maintenance of separated systems.
• Preferable construction of separated systems helps prevent overflow, limits wastewater pollution, and enables surface water reuse.