12. Waste Water Treatment Flashcards
What is waste water?
- Essentially anthing that goes down a drain becomes waste water
- So, rainwater, water from industrial activies, water from commercial activities, water from homes, …
Why do we need to treat waste water (4)
- Because it can contain chemical contaminants, that can cause harm to ecosystems and humans
- Because it contains N, P and that causes eutrophication
- Because addition of organic material will decrease the amount of oxygen in water
- If it isn’t treated properly it just goes back into a water body for other people to consume.
What contaminants can be found in waste water? (6)
- Viruses
- Parasites
- Bacteria
- Fecal coliform
- Urban and agricultural run off
- Food and livestock runoff
Pathogens found in waste water (6)
- Fecal Bacteria, Viruses etc.
- Salmonella sp.: typhoid fever, gastrointestinal (GI) problems
- Shigella sp.: dysentery
- Escherichia coli: most strains are harmless, some cause GI problems
- Vibrio cholerae: cholera
- Coliform bacteria: indicator of harmful bacteria
How many people lack access to safe drinking water?
1.1 billion
How much of the 4 billion yearly cases of diarrheal disease result from contaminates water and inadequate sanitation and hygene?
88%
How many people die from diarrheal diseases every year?
1.8 million
What is DO?
Dissolved oxygen in water. In ppm
What is a good dissolved oxygen (DO) level for good water quality?
8-9 ppm
What is BOD?
Biochemical oxygen demand. It is the amount of oxygen required to degrade whatever amount of organic matter you put in the water.
How is the DO (dissolved oxygen) affected when BOD (biochemical oxygen demand) increases?
DO decreases drastically
What is the consequence of a drastic decrease of DO following the increase of BOD?
- A lot of the biological organisms will die bc of the lack of oxygen
- This will reduce the BOD
- Which will give the DO the ability to recover
What is the biochemical oxygen demand? (BOD)
Measures the amount of O2 (mg/L) required for the aerobic degradation of organic material in a water sample
BOD5
- The BOD5 value gives an index of the pollution potential of an organic pollutant
- The higher the BOD5, the more polluted the water is
5 is for 5 days
The BOD test (BOD5) : 6 steps
- Fill a 300ml airtight bottle with dilution (P) pf wastewater sample
- A commercial microbial “seed” culture may be added to ensure that sufficient live cells are present to consume the O2
- Measure the initial DO (D1) and seal bottle
- Incubate for 5 days at 20 degrees celcius in the dark
- Measure the final DO (D2)
- Calculate BOD as follows: BOD = D1 - D2 / P
What is inlcuded when we measure BOD this way? (2)
- O2 required for inorganic oxidation
- O2 required by nitrifers
Usefulness of calculating BOD (3)
- Provides an estimation of waste loading to treatment plants, required for proper design of a treatment plant
○ You need to understand what is coming in
○ You can adjust the parameters of a treatment plant based on what is coming in - Provides an evaluation of the efficiency (drop in BOD) of a treatment plant → try to reach municipal / provincial regulations / targets
○ i.e. ~ 25 ppm BOD - Prediction of the effect of effluent release on DO in the receiving stream; however, lab conditions may not match those in the receiving stream, making precise predictions difficult
○ If you know what the BOD is, then you have some kind of idea how the ecosystem is going to react to the ecosystem
Chemical Oxygen Demand (COD)
The amount of O2 consumed in the complete oxidation of organic matter
When is the COD higher than the BOD
The COD will be higher than the BOD if biologically recalcitrant organic compounds are present
How do we calculate COD?
- The reaction is carried out under acidic conditions and employs a strong oxidizing agent (i.e., potassium dichromate) to oxidize organic compounds to CO2
- Dichromate does not oxidize ammonium to nitrate (analogous to microbial nitrification)
Impacts of sewage (2)
- Depletion of oxygen in the water body
- Spread of pathogens through the water
3 Objectives of Municipal-Water Treatment
- Removal/reduction of nutrients, i.e. N, P
- Removal/inactivation of pathogenic microbes
- Reduce organic C content… leafing to the reduction of BOD
Therefore …. protect receiving ecosystems from nutrient overload and protect humanity from wastewater pathogens
What are the four treatments for waste water?
- preliminary
- primary
- secondary
- tertiary
Which treatments are physical or chemical processes?
Preliminary, primary, tertiary
Which treatment is a biological process
Secondary
Preliminary treatment (4)
- To protect equipment and downstream processes
- Consists of screens that remove solid debris, such as sticks, dead animals …
- Bar screen (large debris removed to landfill), grit chamber, mesh screen
- Sometimes it involves pre-aeration and flow equalization
Primary treatment (4)
- Wastewater flow is slowed down and suspended solids settle to the bottom by gravity
- Add flocculants to aid solids and colloid settling as well as some phosphate-removal (like Alum and FeCl3)
- Settling (primary clarifier) → sludge or bio-solids
- Primary treatment can remove >90% of organic matter
Primary treatment
Wastewater clarifier
- Clarifier/sedimentation tank (slow water flow)
- Skim off grease, foam, from surface
- Settled material (sludge) removed from bottom, dried and sent to landfill or anaerobic digester
- Clear effluent (raw sewage) flows over top edge of weir and may go on to secondary treatment
Secondary treatment
What are the three thing used to do secondary treatment?
- Activated sludge
- Trickling filter
- Sludge digestor
Secondary treatment
What are the 3 microbial processes occuring during seconday treatment?
- Nitrification NH4 –> NO2 –> NO3
- Removal of pathogens
- Removal of nutrients (BOD) as biomass i.e., reduce remaining organic C.
Secondary treatment
Activated Sludge Process (AS)
What kind of process is it? Aerobic or anaerobic?
Aerobic
Secondary treatment
Activated Sludge Process (AS)
- Can reduce organics (BOD) 90% in 4-8 hours
- Requires lots of O2, fast
- N, P, C are converted to microbial biomass (flocs), thus go from dissolved to solid form
- Formation of flocs is an important part of the AS process
○ Finely dispersed aggregates of cells and organic matter - Following the aeration step, the flocs settle out of solution, removing BOD
- A key element of the AS process is the recycling of a portion of the settled floc (this is the “activated” sludge)
Secondary treatment
Sequence Batch Reactor (SBR)
4 cyclic steps
- fill
- react
- settle
- decant
Secondary treatment
4 things that secondary treatment will do
- decreasing BOD of the waste water
- decreasing NH4+ (ammonium)
- removing pathogens
- removing nutrients (BOD) as biomass
Secondary treatment
Decreasing the BOD of the wastewater
Addition of oxygen through aeration to the liquor encourages the multiplication of aerobic bacteria and they consume the nutrients, i.e., dissolved organic matter, thereby reducing the BOD
Secondary treatment
Decreasing NH4+ concentration (4)
- NH4 + is toxic and present at high concentrations in municipal wastewater
- Nitrification is a major process that occurs in activated sludge and trickling filters
○ NH4 + + O2 → NO2 - Nitrosomonas sp.
○ NO2 - + O2 → NO3 - Nitrobacter sp. - NO3 - assimilated into biomass by other species
- Denitrification NO3 - → NO2 - →→ N2 may occur during the settling stage when some anaerobic zones develop at the bottom of the tank.
Secondary treatment
Removal of pathogens (3)
- Trapped in floc (activated sludge) or biofilm (trickling filters)
- Consumed by predators
- Don’t grow (wrong conditions)
Secondary treatment
Removal of nutrients (BOD) as biomass (4)
- After the AS aeration tank, wastewater moves to settling tanks
- Settling of floc (sludge) leaves cleaner water to flow out (Effluent should appear clear after this step)
- Some of the sludge is recycled to the aeration tanks (activated sludge) but most is discarded or sent to an anaerobic digester
- In trickling filters, biomass is removed by periodic backwashing
Secondary treatment
Microbiology of flocs in activated sludge wastewater treatment (4)
- Filamentous Nocardia spp. help form flocs with Zoogloea, Flavobavterium, and Pseudoms
- Requires a proper ratio of filamentous to single cell microbes to form optimal floc structure that will sink/sediment and not float
- Bacteria are preued on by ciliates, amoebas, rotifers, nematodes
- Serve a crucial function in maintaining proper floc structure which gives optimal sedimentation
Secondary treatment
Bulking
- Major problem during settling of floc most often caused by an over-abundance of filamentous organisms in the sewage population
- Can happen in any biological wastewater treatment process (aerobic or anaerobic, municipal or industrial) that involves settling and can be caused by other mechanisms (slime, gas)
- Filamentous organisms hold the flocs apart, decreasing their density and thus preventing settling
- Too much biomass escapes with the effluent, increasing its BOD
- High BOD in effluent → pollution of receiving waters
Secondary treatment
Causes of bulking (4)
- Nutrients (esp industrial systems)
- Flooding
- Seasonal changes (i.e., temperature effects)
- Toxic chemical influx
- pH changes
Secondary treatment
What causes floating sludge bulk? (3)
- Hydrophobic / porous organisms → EPS
- Denitrification → gas
- Fats, oils and grease
Secondary treatment
How to control bulking? (5)
- Find causes and reverse if possible
- Bulking can be controlled by predation by ciliated protozoans on the filamentous bacteria
- Chemical amendments may help to control bulking species or promote settling
- Chlorination treatment
- Re-aeration rate increased
Secondary treatment
Trickling Filter
Is it an aerobic or anaerobic process?
aerobic
Secondary treatment
Trickling Filter (6)
- Relies on formation of a biofilm food web on the surface of the 2 m deep loose gravel
- Apply highly aerated sewage spray (donʼt flood!)
- Requires periodic backwash, dispose of sludge
- Same principles as activated sludge (removal of pollutants as biomass, nitrification, trapped pathogens)
- As wastewater flows through filter, nutrients are absorbed by microbes in biofilms
- Cleaner effluent flows out of bottom
Secondary treatment
Biofilms in a trickling filter (5)
- Mostly algae and fungi, but also bacteria and protozoa
- Stuck together by Polysaccharides (EPS)
- Food web Micro-habitats/environments
- Micro-channels
Secondary treatment
Sludge digestor
What type of process is it aerobic or anaerobic?
Anaerobic
Secondary treatment
Sludge digester (4)
- Anaerobic and slow, similar to a septic tank
- Usually done in batches in extremely huge tanks (because it is slow - need big volume)
- Tanks big and expensive, most treatment plants don’t have them (send sludge to a plant that does)
- “Anaerobic contact process” is fastest, flow-through, but still very slow
Secondary treatment
How to speed up sludge digesters? (3)
- Mix tanks, and add heat
- Recycle “ripe” sludge
- Burn natural gas produced, to heat and power system
Secondary treatment
Sludge digester PROS (4)
- Generation of CH4 as additional source of energy
- Reduction of organic matter
- The remaining sludge can be used as soil fertilizer
- Reduces production of landfill CH4 (greenhouse gas)
Secondary treatment
Sludge digestor CONS (4)
- Accumulation of heavy metals and contaminants in sludge
- Expensive
- Skilled manpower for design, construction
- Difficult to maintain optimal reaction anaerobic conditions
Secondary treatment
Microbial process in anaerobic digestion in sludge digestors (2)
Denitrification:
* NO3 - → → → N2
Fermentation & methanogenesis
* Biomass/organic matter converted to gases CO2, CH4 (methanogenesis), H2S which are vented
* Fermentation also yields heat
* Effluent contains organic acids (acetate, butyrate, propionate) and recalcitrant organic compounds
* Effluent can be released, returned to an aerobic 2nd treatment, or go to 3rd treatment
Secondary treatment
What happens to the sludge in sludge digestors? (6)
- Digester breaks down input sludge to simple components and residual sludge
- Kills/destroys pathogens
- Residual sludge is “stabilized”, not pathogenic, not smelly i.e., similar to “dirt” (humics, grit, etc) \
- Good fertilizer except for heavy metal content
- Residual sludge de-watered and usually land-filled
- Effluent (high BOD) goes back into sewage treatment system
Secondary treatment
What happens to the effluent from secondary treatment? (3)
- Release with or without disinfection, or
- Second round of secondary treatment (i.e., effluent from sludge digester can go to activated sludge system, effluent from activated sludge can go to a trickling filter), or
- Send to tertiary treatment
Tertiary treatment
Tertiary Treatment (5)
- Removal of specific compounds
- Not always necessary
- Removal of PO4 via precipitation ( only if high [P] is a specific problem ; 1st and 2nd treatments only remove 30% of P )
- Final clarifier
- Charcoal filters
Tertiary treatment
Charcoal filters (4)
- In big tanks, like a grain silo or in a sand-gravel filter as used for drinking water purification
- Remove organic compounds recalcitrant to biodegradation
- Often in specific industrial applications
- Re-use charcoal after burning, to destroy organics
Tertiary treatment
Advantages of sewage ponds / lagoons (3)
- simplicity (low tech)
- low contruction costs
- low maintenance costs
Tertiary treatment
Sewage Pond system (4)
- Aerobic and anaerobic zones
- Reduces BOD by 75 to 95%
- Requires a ~ 7 to 50 day retention time
- Used in rural and small communities
Tertiary treatment
Typical Surface-Aerate Lagoon (3)
- Transfer air into the basins required by the biological oxidation reactions
- Provide mixing required for dispersing the air and for optimizing contact between contacting the reactants (i.e., oxygen, wastewater and microbes)
- Aeration not as sufficient as in activated sludge systems but ….
→ 80 to 90% removal of BOD with retention times of 1 to 10 days
Tertiary Treatment
Septic Tanks (4)
- Anaerobic degradation of waste
- Similar to sludge digestion
- Effluent dispersed into well-drained soil for consumption by aerobic bacteria
- Sludge must be periodically removed
Tertiary Treatment
Tertiary Treatment
Artificial wetlands
- Wastewater treatment plants are impractical for purifying runoffs from large agricultural operations.
- Much of our current water supply is already filtered and purified by natural wetlands, such as the Florida Everglades.
- Some agricultural operations are building artificial wetlands to replace treatment plants.
- Only works for small municipalities
