Water collection

Water collection

1. sewer systems

   • Combined sewer systems
     • carries both foulwater and stormwater
     • only one pipe is required and its capacity needs to be little more than that of a storm sewer system
     • disadvantage is that large quantities of polluted water (a mixture of stormwater and foulwater) must be bypassed to watercourses during storm events to avoid overloading sewage treatment works
   • separate systems
     • higher initial capital costs
     • foulwater is conveyed to the treatment works for purification before discharge to a watercourse, while the stormwater is discharged directly to a receiving water through the storm sewer system
     • necessary to have at least one open vent
       • The vent pipe ensures that the pressure within the domestic drainage system does not fall below atmospheric pressure and so risk water being siphoned from sanitary fixtures.
       • In addition, the vent pipe assists in ventilating the public sewer and prevents foul air from the sewer entering the internal building foul drainage system.

2. Design considerations

   • normally flow partially full and therefore rely on pipe gradient to convey the wastewater along the pipe
     • sewers must be adequate in size and gradient, so that they will carry the maximum (design) flow without being surcharged
   • minimum cleansing velocities
     • to prevent the deposition of solids in the sewer system at low flows
   • If adequate gradient is not available due to ground topography, the wastewater is normally collected by gravitational flow to some convenient low-point, from where it is pumped
   • material must be selected so as to be capable of withstanding external loading
   • selection of pipe material
     • wastewater must not be aggressive to the wall of the sewer
   • manhole covers every 100- 150m

3. waste water flows

   • dry weather flow (DWF) is approximately equal to the flow of drinking water supplied to an area
     • 225 litres/head/day
   • designed to cater for 3 DWF
   • combined sewer systems
     • much lower potential for polluting watercourses
     • foul water very diluted
     • 6 DWF
       • to ensure that, under storm conditions, the most polluting component of the wastewater (frequently termed the ‘first foul flush’) is transported to a treatment works
       • The remaining stormwater flow (which could be large as 40 DWF) is generally ‘offloaded’ from the sewer system by means of storm overflows to adjacent watercourses
       • Under storm conditions, about 3 DWF of the incoming 6 DWF is temporarily stored on site in balancing tanks until the storm abates

4. storm sewer design

   • influencing factors
     • rainfall
     • catchment
     • sewer network
   • starting size of pipe
     • 150 mm in diameter
   • flow velocities
     • not too high to be abbrasive
       • max 3.5 m/s
     • not too low so solids are deposited
       • 0.75m/s
     • 3DWF
       • 675 litres/head/day

5. Pumping stations

   • dry well suction pump
     • installed in a dry chamber or well, adjacent to the wet-well (also referred to as the pump sump)
     • wet-well serves to equalise the incoming flow, which is always variable
     • advantage
       • configuration is the relatively greater facility for inspection and maintenance
   • submerisable pump
     • Both the pump and the motor are housed in a waterproof casing
       • submerged in the liquid being pumped
     • disadvantage
       • maintenance of the pump-motor assembly is a more disagreeable task

6. storm overflows

   • frequently result in watercourses being polluted by untreated sewage
   • storm flow characteristics
     • should not come into operation until the prescribed flow is being passed to treatment
     • flow to treatment should not increase significantly as the amount of overflowed storm sewage increases
     • maximum amount of polluting matter should be retained in the forward flow to treatment
     • chamber should be so designed as to minimise turbulence and risk of blockage
     • hould be self-cleansing and require the minimum of maintenance