Measurements in Water & Wastewater On completion of this module you should be able to:

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 Module 7

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 Module 7

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 Module 7

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 Module 7

5. BOD – time curve Module 7

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 Module 7

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 Module 7

8. Nitrification The conversion of ammonium to nitrate by microbial action. Autotrophic bacteria is involved in 2 stages. NH4+ + (3/2)O2nitrosomonas NO2- + H2O + 2H+ NO2- + (1/2)O2nitrobacter 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 Module 7

9. Carbonaeous and Nitrogenous Oxygen Demand Module 7

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+ Module 7

11. Chemical Oxygen Demand (COD) • The amount ofunreacted 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 Module 7

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 Module 7

13. Other Common Measurements of Wastewater • Total dissolved solids (TDS) • Total suspended solids (TSS or NFR) • Volatile residue • Fixed residue Module 7

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 @ 20oC • Total dissolved solids (Cl-) • Pressure • C's = Cs (P - p)/(760 - p) Module 7

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 Module 7

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 Module 7

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 Module 7

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 Module 7

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 Module 7

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 Module 7

21. End of Module 7 Module 7