Measurements in Water

1
Measurements in Water WastewaterOn completion
of this module you should be able to

• Have an understanding of the use of oxygen demand
  as an indicator of organic pollution in water
• Discuss the relevance of the BOD measurement and
  its limitations
• Compare the processes involved with other
  measurements
• Describe the effects of oxygen, temperature and
  pH in water and with microorganisms

2
Organic Measurements
There are many different sources, types and
complexities of aqueous organic matter. It is not
possible to quantitatively measure or determine
every organic contaminant

• Organics are carbohydrates, proteins, alcohols,
  acids and some lipids
• Concept of biodegradability and
  non-biodegradability
• Biodegradable organic materials are utilised as
  food by heterotrophic microbes
• Process of biodegradation uses DO and thus exerts
  an oxygen demand

3
Biochemical Oxygen Demand (BOD5)
Defined as the oxygen demand for a mixed
population of aerobic heterotrophic bacteria in
oxidising biodegradable organic carbon present in
the sample in 5 days at 20oC

• Free oxygen is used as the terminal electron
  acceptor
• (C,H,O),N,P,S O2 CO2 H2O NH4
  S2- PO43- energy
• BOD5 represents 60 -70 of complete oxidation

4
BOD5 (cont)

• Rate of biodegradation as a function of time can
  be described as a first order reaction i.e.
  dLt/dt -kLt
• BODt Lo(1 - e-kt)
• Reproducibility is ? 20 but reflects actual
  biodegradation
• In addition to organic carbon, reduced nitrogen
  i.e. ammonia can also be oxidised by nitrification

5
BOD time curve
6
How is the BOD5 value used?

• Quantifies the pollutant load
• Allows the comparison of waste streams
• Determines the efficiency of the wastewater
  treatment process
• Used as one of the criteria of discharge licence
  condition

7
BOD5 is widely used despite some limitations

• An active acclimated seed bacteria is required
• Toxic compounds will inhibit and invalidate
  results
• Presence of nitrifying bacteria will present
  false values
• Only readily biodegradable organics are measured
• Process is slow and takes 5 days
• Comparison of BOD5 values is valid for similar
  reaction constant rates
• Reproducibility of BOD5 test is poor

8
Nitrification
The conversion of ammonium to nitrate by
microbial action. Autotrophic bacteria is
involved in 2 stages. NH4 (3/2)O2
nitrosomonas NO2- H2O 2H
NO2- (1/2)O2 nitrobacter
NO3- _____________________________________________
___________ Overall NH4 2O2
NO3- 2H H2O

• It requires 4.6 mg/L of DO to oxidise 1 mg/L NH4
  - N
• Domestic wastewater typically contains 15 - 50
  mg/L of total nitrogen, which corresponds to a
  potential oxygen demand of 69 to 230 mg/L
• Exertion of nitrogenous BOD is considerably
  slower than carbonaceous BOD, as it depends on
  the number of nitrifying bacteria present

9
Carbonaeous and Nitrogenous Oxygen Demand
10
Chemical Oxygen Demand (COD)
The test uses a strong oxidising chemical agent
to completely oxidise organics

• The oxygen equivalent of the organic matter is
  determined by the amount of K2Cr2O7 used
• The organic matter is refluxed with K2Cr2O7 in
  boiling acid at 150oC in the presence of a
  catalyst (silver sulfate) for 2 h
• Organics Cr2O72- H CO2 H2O
  2Cr3

11
Chemical Oxygen Demand (COD)

• The amount of unreacted dichromate is determined
  by titration with ferrous ammonium sulfate
• The test takes 2 - 3 hours and reproducibility is
  ?10
• The test cannot discern between biodegradable and
  non-biodegradable carbon, consequently COD values
  will be higher than BOD5
• Test is widely used in evaluating industrial
  wastewater and in wastewater research

12
Total Organic Carbon (TOC)
TOC is an instrumental combustion technique in
which organic matter is volatilised at 1000oC to
CO2 which is then determined

• The method measures the carbon content and not
  the oxygen equivalent of the organic matter
• Test is rapid and uses small samples (20 ?L)
• Because of the small volume, extraneous organic
  particulate matter, algal cells can cause error

13
Other Common Measurements of Wastewater

• Total dissolved solids (TDS)
• Total suspended solids (TSS or NFR)
• Volatile residue
• Fixed residue

14
Dissolved Oxygen (DO)
An essential component for aquatic life, the
aesthetic quality of water and wastewater
treatment. Solubility is affected by

• Temperature, DO is 9.2 mg/L _at_ 20oC
• Total dissolved solids (Cl-)
• Pressure
• C's Cs (P - p)/(760 - p)

15
Redox
The utilisation of chemical energy in living
organisms (energy yielding reaction of cells)
involves oxidation - reduction reactions

• Oxidation is defined as the removal of electron/s
  from a substance (electron donor) to another
  (reactant). It can also be defined as the
  addition of molecular O2 or loss of hydrogen
• For any oxidation to occur, a subsequent
  Reduction must complement. Reduction is the
  reverse of oxidation i.e. a gain of electron/s or
  loss of O2 or the gain of hydrogen

16
Redox (cont)

• The energy source, which is the electron donor
  gives up one or more electrons, which are
  transferred to an electron acceptor. In this
  process the electron donor is oxidised and the
  electron acceptor is reduced
• Analogous to pH, concept of pE applies to redox
  processes
• Water with a high electron activity (low pE) is
  reducing, as in anaerobic digestion tanks
• Conversely, low electron activity (high pE) is
  oxidising, as in aerobic reaction tanks,
  chlorinated swimming pool

17
Redox (cont)

• One of the most common electron acceptors of
  living organisms is molecular oxygen
• (C,H,O),N,P,S O2 CO2 H2O NH4
  S2- PO43- energy
• The tendency of a compound to accept or release
  electrons is expressed quantitatively by its
  Reduction Potentional

18
Examples of Redox Reactions

• Combustion - compounds of carbon and also
  hydrogen are oxidised/burned by O2 in air to
  release energy and CO2 and H2O
• Respiration - living things obtain energy through
  respiration. O2 we breathe oxides
  carbon-containing compounds in our cells to
  produce energy, CO2 H2O
• Rusting - iron and steel left out in the open
  eventually rust in which iron is oxidised to a
  mixture of oxides
• Batteries - a voltaic cell is a device in which
  electricity is generated from a chemical reaction

19
Temperature
Chemical reactions and gas solubility are
effected by changes in temperature. Increasing
temperature has the following effects

• Increases conductivity
• Increases chemical reaction rates
• Increases biological reaction rates
• Increases species mortality rates
• Increases biological growth rates
• Decreases DO solubility

20
Microbial Growth and pH

• Hydrogen ion concentration influences the growth
  rate and limits growth of microorganisms
• Most bacteria have optimum growth rate at pH
  close to neutrality
• Changes in pH will result in shifts of species
  dominance
• Specific species will thrive in extreme pH values

21
End of Module 7