Wastewater Treatment Flashcards
What are the two sources of pollution?
Point sources - (e.g. industrial discharge)
Non-point sources (e.g. agricultural runoff)
What types of pollutants to domestic waste water systems contain?
Organic matter
Pathogenic matter
Toxic substances (e.g. cosmetics, detergents, medicines)
What are the three treatments and the three stages involved in wastewater treatment?
Physical, chemical and biological treatments
Primary, secondary and tertiary
Describe the primary treatment of waste water
Physical removal of large objects and suspended solids
Physical process
Waste is passed through a series of screens (metal bars) to remove objects (screening)
Waste is settled in tanks to remove sand and grit
Then passes to a series of sedimentation tanks to allow suspended organic particles to settle to bottom of the tank (primary clarification)
What are the outputs from primary treatment?
Solid Fraction
- Large objects = landfill/incineration
- Sand and Grit = landfill
- Suspended organic particles (primary sludge) = disposal to land/water or further treatment
Liquid Fraction
- Remaining wastewater = disposal to land/water or further treatment
What are 2 methods of secondary treatment of wastewater and what is the basic principal?
Percolating/trickiling filters
Activated sludge process
All based on same principal: aerobic microorganisms are used to breakdown organic matter in the wastewater
What organic compounds are broken down and into what?
Carbon, Hydrogen, Oxygen, Phosphorus, Nitrogen and Sulphur
into
CO2, Carbonates, water, Phosphates, Nitrates and Sulphates
How do percolating/trickling filters work?
- Wastewater passed slowly through ponds of filtering media (2m deep and 2-2000m^2 surface area)
- Filtering media consists of large chips of (25-50mm) limestone, carbon or plastic
- Chips provide a large surface area on which a film of microorganisms grow
- Drains at bottom of filter bed collects filtered wastewater and provides source of air
- Large airspaces present between chips
- Additional air may also be pumped into system
How does a percolating/trickling filter break down waste?
Bacteria breaks down soluble organic compounds
Protozoans and worms feed on particulate organic matter and bacteria (prevent clogging)
What happens after treatment in the percolating/trickling filter?
Outflowing drain passes to secondary settling tank
In settling tank solid sludge (organic matter generated by microbial digestion) is separated from liquid effluent
What is the activated sludge method?
- Flows through aerated tank
- Continuous flow of sewage is passed through tank
- Complete replacement of tanks contents takes a few hours
- Tank is vigorously aerated
- Microorganisms in the tank break down organic matter
- Digested effluent is passed to secondary settling tank
- Liquid effluent is disposed of or passed to tertiary treatment
- Some sludge is returned to aeration tank for further breakdown
What organisms breakdown activated sludge?
Bactera, fungi, protozoans and rotifers (narrower range of organisms than P/T filter
Protozoans particularly beneficial;
- reduce turbidity
- reduce bacterial numbers in effluent
- reduce BOD, COD (chemical oxygen demand), organic nitrogen and suspended solids in effluent
What is the difference between the PT filter and the AS methods?
Both equally effective
AS is cheaper to install and requires less land
Less insects and odours with AS
AS is more expensive to operate and requires stricter monitoring
What are the outputs from secondary treatments?
Solid fraction
- Secondary sludge = disposal to land/water or further treatment
Liquid fraction
- Remaining wastewater = disposal to land/water or further treatment
What are the methods of tertiary treatments?
- Further biological treatment
- modified trickling filter (high rate filter)
- modified AS system
- Ion exchange to remove phosphate and other inorganic ions and potential toxins
- Further sedimentation
- Constructed wetlands
- Disinfection or UV treatment to remove pathogens
- Filtration (sand or activated Carbon)
What is sewage sludge?
Sewage treatment generates a substantial amount of organic rich sludge
Contains 98% water, 2% organic and inorganic particulate matter, potentially toxic substances (e.g. heavy metals, pharmaceuticals) and various pathogens
What are the methods of sludge disposal?
Dumping at sea Sludge spread on drying beds and condensed, then spread on agricultural land or landfilled Composting Anaerobic digestion Incineration
What are the problems with sludge disposal?
Pathogenic organisms and toxins
Run-off to waterways
Health risk to people living near disposal sites
How does anaerobic digestion work?
Sewage sludge broken down biologically by anaerobic digestion
Takes place in large, heated tanks sited above or below grown level - oxygen free environment
Anaerobic microorganisms breakdown organic matter
Methane rich gas (65% methane and 35% CO2) is produced which can be used to generate heat or electricity or can be upgraded to natural gas
Digested sludge waste is separated into liquid and solid components
Liquid component can be used as fertiliser
Solids may be utilised as a soil conditioner or further processed to produce organic compost
What is municipal effluent?
One of the main sources of pollution into aquatic environment
Very complex
Domestic and industrial sources
Contains both traditional pollutants (e.g. PCBs, Heacy metals, pesticides) and more novel contaminants (e.g. endocrine disruptors, nanoparticls, microplastics, pharmaceuticals)
Major source of organic pollution
What are the main sources of ORGANIC pollution?
Sewage
Agricultural waste (slurry, milk waste etc.)
Food processing waste
Waste from manufacturing of natural textiles and paper
Aquaculture waste
What is the global situation with regards to organic pollution?
Wei River (China) >800milliontonnes of wastewater annually
650 mt enters Mediterranean per year
1.1 m L raw sewage enters river Ganges (India) per minute
Many areas the majority of sewage entering waterways is untreated
- East Asia 89%
- Latin America/Carribean 86%
- West and Central Africa 80%
What effects can organic pollution have on the biota?
Changes the physical and chemical aquatic environment
- Dissolved oxygen levels decrease
- Suspended solids and silt increase
- Toxic waste products of decomposition (e.g. ammonia, acetic acid) increase
Organisms which cannot tolerate these changed conditions disappear
Name some pollution tolerant species and pollution sensitive species
Tolerant - chironomidae - platyhelminthes - leeches Sensitive - mayfly nymph - stone fly nymph - caddis fly larvae
