Organic Matter, DO, BOD, COD and Modelling Flashcards
What elements is organic matter made up from?
C, H, O together with some N, P and S
What percentage of suspended/filterable solids in wastewater is organic matter?
Suspended: 75%
Filterable: 40%
Where does organic matter in wastewater come from?
Animals and plants/organic synthesis
What compounds is organic matter typically made up from?
Proteins, carbohydrates, fats and oils, urea (fresh wastewater only), synthetic organic molecules (simple to v complex)
Name 2 ways to measure organic matter
- If one substance, can isolate and measure
2. If a mixture, could measure total organic carbon
What type of organic matter are we interested in measuring in wastewater?
Biodegradable
What information lets us exploit microorganisms for the measurement of biodegradable organic matter?
That they pull oxygen from the local environment to degrade organic substances, and this amount is in direct proportion to the amount of organic matter metabolised.
In what 2 terms are organic matter most commonly measured in?
BOD and COD
Where do we want the degradation of organic matter to occur and why?
In a wastewater treatment plant do it does not happen in the environment
How does oxygen get into water?
Diffusion from wind/waves, photosynthesis from green plants, algae and cyanobacteria
How is oxygen removed from water?
Respiration and oxygen-consuming processes
Give the overall equation for photosynthesis
Water + CO2 + nutrients > organic matter + O2
What is the step of digesting food that consumes the oxygen?
The production of ATP
What mg/L of DO are you looking for for healthy fish in ‘good’ surface water?
> 6 mg/L
What 4 things does DO depend on and why?
- Temperature (colder = more oxygen)
- Altitude (higher up = less due to partial pressure)
- Salinity (salty water has less DO because the chlorides etc. react with the oxygen)
- Stream structure
Define DO
The amount of molecular oxygen dissolved in water
Define oxygen-demanding material
Anything that can be oxidised by water dissolved oxygen, usually including organic matter and ammonia
Give the equation for organic carbon breaking down
organic C + O2 > CO2
Give the equation for ammonium ions breaking down
NH4+ > NO2- > NO3-
Why are high oxygen levels needed?
For healthy stream ecology
At what point does low DO become an aesthetic problem?
<1 mg/L
How much DO does carp need?
3 mg/L
How much DO does trout need?
5-8 mh/L
Define oxygen demand
The amount of oxygen required to oxidise a waste
What are the 3 representations of DO?
- Theoretical (the amount of oxygen required for microbes to degrade wastes - most useful to assess the strength of an organic pollutant)
- Chemical oxygen demand (the amount of oxygen required for chemically oxidising the waste)
- Biochemical oxygen demand
What does oxygen demand measure?
It is an indirect measurement of the amount of organic (or carbon containing) impurities in water
Define theoretical oxygen demand
The amount of oxygen required to oxidise a known compound completely to CO2 and H20
What other compound can ThOD be used for?
Ammonia
How do you calculate ThoD?
- Write and balance the chemical oxidation reaction with the end products of CO2 and H20
- Calculate the ThOD based on the mass of contamination in the water
Define COD
The EQUIVALENT amount of oxygen required to oxidise any organic matter in a water sample by means of a strong chemical oxidising agent
What does COD use instead of bacteria to break down the organic matter?
Using a strong oxidising agent (acidic solution or dichromate)
When is COD usually employed?
When looking at only one fraction of wastewater - particulate or soluble
How do you use a COD test?
Put your sample into the COD vial containing the oxidising agent (usually 5 ml) and there will be a colour change that you can then measure
Define BOD
The amount of oxygen used by microorganisms in the BIOCHEMICAL oxidation of organic matter
What 4 reasons do we want to measure BOD?
- Determine waste load arriving at a treatment plant
- To assess the affect of the discharge of the waste water into the environment
- To assess the efficiency of a wastewater treatment plant/particular process step (BOD removal)
- To control plant processes
How is a BOD test carried out?
Liquid containing organic waste (either full strength or diluted) is placed in a bottle. The DO is measured and the bottle sealed. The DO of the sample is measured again after the incubation period. The difference in the values represents the oxygen used by microbes in degrading the waste.
Why is a typical BOD over 5 days?
It is a historical convention that says that the longest amount of time waste water takes to get to the UK sea is 5 days.
What is ultimate BOD?
The BOD for 20-30 days/total BOD
What are the two stages of decomposition in a BOD test?
- Carbonaceous stage (CBOD) - oxygen demand only from carbon containing compounds
- Nitrogenous stage (NBOD) - represents a combined CBOD and NOD demand
Draw the graph representing the 2 stages of BOD decomposition (BOD against time)
Drawing 5
Why are there two stages of decomposition in BOD?
Because different microorganisms degrade the CBOD and the NBOD and the ones that degrade NBOD tend to take longer to grow (around 6 days)
How can we make sure that we are only measuring CBOD?
Must inhibit denitrifiers (by adding TCMP)
Give the 2 step and overall reaction for nitrification
Drawing 6
Under what conditions does BOD incubation occur?
20 deg C, in the dark, excess of essential nutrients (N, P, K, Fe, etc)
What might you have to do with a BOD in prep depending on the concentration and water characteristics?
Dilute it, add nutrients or seed with bacteria to remove toxins
What are the 2 rules that must be adhered to when measuring BOD?
- There must be at least 2 mg/L of oxygen used over the course of the experiment
- At least 1 mg/L of oxygen must remain in the final sample
What can BODu be used for?
Modelling DO regimes in rivers and estuaries
How does BOD analysis change with temperature?
The higher the temperature of incubation, the higher the BOD
What must you do to a wastewater with a very high waste concentration?
Dilute it - almost always required
What happens if you do not have enough microbes in the BOD sample?
Seed the dilution water
What is typically used to seed bacteria into a BOD sample?
Supernatant from settled raw sewage elsewhere on the plant, but will vary by facility, best used immediately on collection (can refrigerate for up to 36 hrs)
What should be the theoretical difference in DO concentrations in the blank control over the incubation period?
0 if water but can vary slightly, if over 0.2 mg/L then cannot be used
What is a typical domestic waste BOD, and a typically very high BOD?
Domestic: 200-600
High: 20,000 (agricultural wastes)
List 5 interferences to BOD analysis
- Dependent on biological activity and impacted by conditions and components which inhibit their growth (eg. Cl, pH, mineral acids, heavy metals)
- Pretreatment may be required (Cl removal by sodium sulphite, pH neutralisation)
- Neutralised and dechlorinates samples must be seeded for the BOD test
- Must be incubated in the darg to prevent photosynthesis and growth of algae
- Residue build up on the bottles - acid cleaned every time
What are the 4 limitations of a BOD test?
- Pre-treatment is required when dealing with toxic wastes
- Only biodegradable organics are measured
- A high concentration of active, acclimated seed bacteria is required
- A long period of time is required to obtain results (5 days)
What is assumed about the rate of oxidation?
That it is a pseudo-first order process and no re-aeration occurs
Draw a graph of the relationship between oxygen consumption and time
Drawing 7
Give 4 advantages of COD over BOD
- Assay time (3 hrs/15 min vs 5 days)
- Fewer manipulations of the sample
- Oxidises a wider range of chemical compounds
- 7 day preservation of the sample (sulphuric acid < pH 2) vs 48 hrs @ 4 deg C
Give 2 disadvantages of COD vs BOD
- Not directly applicable to the 5-day BOD results
2. Not all of the measured COD can be degraded biologically, so cannot be used to assess biological treatment
What is the ratio BOD/COD used to indicate and values should you expect?
Treatability of wastewater
>0.5 - easily treatable
<0.3 difficult to degrade/toxic compounds (may require seeding)
Give the BOD/COD value of domestic water, after primary settling and industrial wastewater
Domestic: 0.3-0.8
After primary settling: 0.4-0.6
Industrial wastewater: >0.3
Define pollution load
Volume x concentration
Why is pollution load used?
To quantify the amount (mass) of a particular pollutant and is useful for the design of WWTP
What is population equivalent?
A theoretical unit that is employed to express pollution loading (in terms of BOD) where 1 population equivalent = ‘the appropriate load from one person’
What is the standard 5-day BOD for one person?
60g
Why might population equivalent actually reflect the population of a community?
If there is a lot of commercial/industry effluent
What considerations need to be made when deciding how to manage wastewater?
- Assess the impact of the liquid effluent
2. How far will it travel and affect other areas?
What happens to BOD when wastewater and clean water streams mix?
The BOD is spread over the whole volume as in conventional mass balance
What does stream/river merging assume?
That it is in steady state and mixing occurs simultaneously
Which two properties of wastewater and a stream can be combined using mass balance?
The DO content and the BODu
How is initial dissolved oxygen deficit calculated?
By subtracting the initial dissolved oxygen from the saturation DO concentration
Draw a diagram showing the concentration of DO, BOD and types of organisms as time and distance of flow of a wastewater travels away from it’s source,
Drawing 8
How does knowing the oxygen content at varying distances down from a point source help?
It allows you to predict where the issues are going to happen and communities may get upset
What is the difference between lab and environment measurements?
Lab - closed system therefore no inputs and no re-aeration
Environment - open sstem therefore re-aeration occurs
How can you deal with the differences in lab and environment through modelling?
The use of the Streeter-Phelps oxygen sag curve that models the river as a plug-flow reactor where oxygen is depleted by BOD exertion and gained through re-aeration
Draw the Street-Phelps oxygen sag graph
Drawing 9
What are the 8 limitations of the Streeter-Phelps equation?
- Considers single point of discharge (most rivers have multiple, including non-point source)
- One oxygen sink (BOD, doesn’t count photosynthesis or Benthic oxygen demand - the amount of oxygen used by river sediments by microorganisms aerobically degrading earlier)
- One reaeration source (surface aeration)
- It is an underestimation
