Introduction and Definitions Flashcards
Why do we need biological wastewater treatment
Micro-organisms (predominantly bacteria) can grow on polluting substances in wastewater and they consume the pollutants
What does the breakdown of polluting components lead to
They lead to the growth of new microorganisms:
- Source of C, N and P as building blocks for new cell biomass (anabolism)
- source of energy to build these building blocks to make new biomass (catabolism)
> energy comes from breakdown of polluting substances
> most energy inferred from aerobic break down
What is the use of the catabolic process in biological wastewater treatment
- Organic matter is broken down for ATP generation
- Redox reaction occurs where carbon gets oxidised and oxygen gets reduced
- The break down of pollutants breaks chemical bonds (in glucose) and releases this energy
- The released energy is stored in ‘energy-rich’ molecules known as ATP
- the breaking of the bonds lead to the electrons and hydrogen being shuttled to other molecules as the final electron acceptor:
> O2 in aerobic conditions which produces 38 ATP
> NO3- in anoxic conditions which produces 2-38 ATP
> CHO in anaerobic conditions which produces 2 ATP - therefore the more oxygen available the more ATP that can be produced
- the ATP molecules can afterwards be used to create new bonds to generate and build in the building blocks
What are the types of agents required to build oxygen atoms in the redox process
- a chemical substance (strong oxidant) -> COD
- a biochemical substance (activated sludge) -> BOD
General scheme for biological wastewater treatment
Raw sewage -> screening -> grit chamber -> primary clarifier -> aeration tank -> secondary clarifier (activated sludge returned to aeration tank or disposed) -> disinfectant -> discharge
What is the primary treatment process
- Wastewater removed from the sewer or factory has to be pretreated to remove coarse particles (e.g. sand), oils and fats
- Raw sewage -> screening -> grit chamber -> primary clarifier
What is the secondary treatment process
- Carbon, nitrogen and phosphorous (nutrients) are removed by the activated sludge in the biodegradation tanks
- Activated sludge is separated from the purified water in the sedimentation tanks
- The largest part of the settled activated sludge is returned to the biodegradation tanks (recycle sludge) and the excess activated sludge is removed
-aeration tank -> secondary clarifier (activated sludge returned to aeration tank or disposed)
What is the tertiary treatment process
- The purified water is discharged into receiving water bodies or is further treated to upgrade its quality
- disinfectant -> discharge
What is activated sludge
- Activated sludge is not a pure culture but a very (unknown) mix of:
> bacteria (morphologically dividable in floc and filament forming bacteria)
> protozoa e.g. ciliates, flagellates (which eat up bacteria) and amoebes
> metazoa e.g. rotifers, mematodes and worms - It is not an active culture
> if active there is too much growth and no need to create extra biomass
> When there is insufficient energy supply (starvation) oxidation of cell reserves occurs which leads to sludge mineralisation and lysis
What are the ideal demands for the activated sludge system
- establishment of a microbial community that breaks down completely and at a fair rate compared to the incoming waste
- little new biomass formed as possible
- settles well in the sedimentation tank
What are the basic reactions that occur in an activated sludge system
- Sorption of soluble, colloidal and suspended organics in or on sludge flocs
- Biodegradation of the organics with end products
> CO2, H2O and minerals
> new microbial biomass - Ingestion of bacteria etc. by protozoa or other predators
- Oxidation of ammonium to nitrite and nitrate by nitrifying bacteria
- When there is insufficient energy supply (starvation) oxidation of cell reserves occurs which leads to sludge mineralisation and lysis
What are the measures of organic pollution in an effluent
- Chemical Oxygen Demand
- Biological Oxygen Demand
- Total Organic Carbon
What is Chemical Oxygen Demand
The amount of oxygen required to oxidise organic carbon completely to CO2 by chemical means
- a strong oxidants is required such as K2Cr2O7
- the amount of dichromate oxygen used is determined and expressed as COD in mg O2/L
- for 1g of glucose we need 1.07g of COD to break it down (calcs)
- Oxygen to oxidise reduced nitrogen components to nitrate are not included
- Oxygen to oxidise reduced sulfur components to SO42- are not included
What is Biochemical Oxygen Demand
The amount of oxygen used by the non-photosynthetic micro-organisms at 20 degrees celcius to metabolise biologically degradable organic compounds
COD >= BOD
(graph)
What is the appropriate environmental conditions for measuring BOD
- neutral pH
- sufficiently large acclimatised microbial inoculum
- appropriate amounts of necessary mineral nutrients = N, P, Ca, Mg, Fe and S
- incubation in the dark -> avoiding interference with O2 producing organisms during daylight periods
- no nitrification due to the addition of inhibitors
What is the measurement process for BOD
- fill glass bottles with wastewater at different dilutions to make sure there is enough oxygen left at the end to measure
- measure DO conc at T = 0
- store bottles in the dark at 20 degrees C
- measure the conc again after 5 days
- calculate the difference between the two oxygen concentration values = BOD5
How can BOD me measured from COD and why
-COD is much faster (2 hrs)
- BOD5 = 0.65 x COD for municipal (non-industrial) wastewater if all COD is biodegradable (if not all biodegradable only relates to biodegradable part)
- Graph of biomass and time can be split into two phases
> 1-2 days of ‘feast; and growth on external substrate where 50% of COD is transformed to CO2 and energy and 50% is transformed into biomass; the amount of new biomass that is formed in those 2 days is 40% of the COD and is expressed in ‘dry weight’ of biomass
> the remaining 3 to 4 days the biomass is under starvation and there is no external substrate anymore so they start consuming themselves and there is a decline in biomass as oxygen consumptions is about 0.1 mg/LO2 per gram of dry weight of biomass per day
What is BODu
- after 20 days the ultimate BOD is measured
- this is when all material that is biologically degradable has been biodegraded (bCOD)
- BOD5 = COD x Fb (biodegradable fraction of COD) x 0.65 = bCOD x 0.65
What is TOC
- firstly acidification and purging of Inorganic Carbon (IC) measures the formed CO2
- then combustion of total carbon (TC) at 800 degrees c measuring the formed CO2
- TOC = TC - IC
- of the formed CO2 only the amount of C is counted as TOC mg C/l
What are the measures for suspended solids/activated sludge concentrations
- TS (total solids)
- SS (suspended solids)
What are the total solids
The portion of wastewater dried at 105 degrees C to constant weight
What are the suspended solids
- the amount of particulate matter present in a sample
-determined by separating the particulates out of the water by wither filtration or centrifugation and drying the residue at 105 degrees C to constant weight - expressed in dry weight
- TS >= SS
What are the types of suspended solids
- Volatile Suspended Solids
- Mixed Liquor Suspended Solids
- Mixed Liquor Volatile Suspended Solids
- Effluent Suspended Solids
Volatile Suspended Solids
- suspended solids, separated out of the sample, are dried and subsequently ashed at 600-650 degrees C
- the amount of ash is subtracted from the total amount of SS; VSS = SS -ash
- indicates the amount of activated sludge
