Wastewater Flashcards
Sewage (or wastewater):
the liquid containing contaminants that is generated by toilets, showers, sinks and industrial processes
Sewerage:
the whole system of sewers and pump stations, which convey sewage to a treatment plant
Sewer:
the pipeline which conveys sewage normally by gravity flow
Pressure main (or rising main)
a pipeline conveying pumped flow
Sewage Treatment Plant (STP):
or Wastewater Treatment Plant (WWTP)
where sewage is treated to the standard required for discharge to land or receiving waters
Effluent
the partially or fully treated sewage from septic tanks or a STP
Influent
the sewage entering a STP or partially treated sewage entering subsequent treatment processes.
Advantages of Conventional Gravity Sewerage (CGS):
- low operations and maintenance cost
- no equipment at house
- all sewage removed from site
Disadvantages of of Conventional Gravity Sewerage (CGS):
- higher capital cost
- greater disruption during construction
Advantages of Common Effluent Disposal (CED)
- smaller pipes, flatter grades
- reduced organic load at WWTP
- reduced peaking factors
Disadvantages of Common Effluent Disposal (CED)
- septic tanks must be maintained
- requires septage facility
Advantages of Variable Grade Sewers (VGS)
- smaller, shallower pipes
- reduced organic load at WWTP
- reduced peaking factors
Disadvantages of Variable Grade Sewers (VGS)
- may need higher maintenance
- pump and check valves required at low properties
- septic tanks must be maintained
- requires septage facility
Advantages of Septic Tank Effluent Pumping (STEP)
- smaller, shallower pipes
- reduced organic load at WWTP
- reduced peaking factors
- reduced infiltration
Disadvantages of Septic Tank Effluent Pumping (STEP)
- septic tanks must be maintained
- requires septage facility
- large number of pumps to run and maintain
- potential odour and corrosion issues
Advantages of Grinder Pumps (GP) (and pressure mains)
- smaller, shallower pipes
- reduced infiltration
- all sewage removed from site
Disadvantages of Grinder Pumps (GP) (and pressure mains)
- large number of pumps to run and maintain
- septic effluent
- potential odour and corrosion issues
Advantages of Vacuum Systems (VS)
- smaller, shallower pipes
- reduced infiltration
- all sewage removed from site
Disadvantages of Vacuum Systems (VS)
- unfamiliar technology
- vacuum pump discharge odour
- potential odour problem due to seasonal loading
WSP:
Waste Stabilisation Pond or Oxidation Pond
Why is there still a place for WSP in today’s waste management practices?
- use of solar and wind energy
- excellent disinfection if short circuiting is minimised
- potential to use new technologies to improve effluent quality
Aspects of best current design systems
- fine screening
- settlement zones for high solids wastes
- aerator assistance
- avoidance of localised over loading
- mixing calm water to bring algae to surface
- multiple ponds in series increases efficiency
- position inlet and outlets with flow against prevailing wind to reduce short circuiting
Improvement options for WSP
- WSP variations (anaerobic/aerobic, facultative ponds, multi maturation cells in series)
- trickling filters for ammonia reduction
- wetlands
- UV disinfection
- chemical dosing
- dissolved air flotation (DAF)
- sand filtration
- membrane filtration
Why do WWTP ponds have such good natural pathogen removal?
- sunlight
- dissolved oxygen
- temperature
- retention time
- pH
- predation
- settlement
Why is there a trend to deeper ponds (1.5-2.0m)?
To allow for more sludge accumulation and greater depth of overlying liquid to control odours
Creating a deeper pit near the inlet will:
store incoming solids and create ideal conditions for anaerobic digestion (if groundwater conditions allow)
How often is sludge removal needed?
Every 15-30 years depending on storage depth available
Removal of sludge via new pond:
- economic to construct an extra primary pond to allow a pond to be drained and sludge air-dried over summer. Sludge can be air dried to 50-60%DS and readily removed by excavator and trucks
Removal of sludge as slurry:
- removed as a slurry using a suction cutter dredge or by pumps mounted from a pontoon. Slurry can be dewatered using a portable belt press.
Sludge disposal location:
on-site landfill or regional landfill or forests
WSP are sustainable because:
- utilise green energy
- sunlight provides excellent disinfection
- multiple ponds-in-series or baffles reduce short circuiting
- add on processes can reduce pathogens
- sludge can be process using solar and air drying but extra space required
