Water Quality - Condensed Version 1999

Transcription

1 Water Quality - Condensed Version Chemical Characteristics: Organic Matter CHONPS + oxygen and nutrients -----> new bacterial cells + carbon dioxide (CO 2 ) + water + (aerobic bacteria) ammonium (NH 4+ ) + sulphate (SO 4-2 )+ phosphate (PO 4-3 ) C - carbon O - oxygen H - hydrogen N - nitrogen S - sulphur P - phosphorous ORGANIC MATTER Measurement of Organic Matter Variety of tests have been developed to measure organic matter content of water:! COD! TOC! ThOD! BOD (CBOD and NBOD) Chemical Oxygen Demand (COD)! chemical test that measures chemical oxygen demand of natural waters, municipal wastewaters and industrial wastes! indicates amount of equivalent oxygen required to oxidize organic matter to CO 2, H 2 O, ammonium (NH 4+ ), phosphate (PO 4-3 ), sulfate (SO 4-2 ) etc! cannot differentiate between biologically oxidizeable and inert organic matter! rate constant not available, need to do both COD and BOD to determine non-biodegradable fraction! certain inorganic compounds can interfere with test! test reaction is as follows: C a H b O c + Cr 2 O H > Cr +3 + CO 2 + H 2 O heat & catalyst organic dichromate matter titrate against Cr +3 " potassium dichromate and a catalyst is added to the sample and heated " dichromate reacts with the organic matter " COD is the difference in dichromate or the amount of Cr +3 generated! takes about three hours! possible to correlate COD and BOD readings on a site specific basis (not reliable) 359-Stu-99B.wpd B-1

2 Water Quality - Condensed Version 1999 Total Organic Carbon (TOC)! instrumental TOC test! sample is evaporated and oxidized catalytically to carbon dioxide! amount of CO 2 released is measured Theoretical Oxygen Demand (ThOD)! if chemical formula is known, the ThOD can be calculated using stoichiometry C a H b O c N n P p S s + XO > X 1 CO 2 + X 2 H 2 O +X 3 NH 4 + +X 4 PO X 5 SO 4-2 and NH O 2 ---> NO H + + H 2 O (2 stage process) Carbonaceous Biochemical Oxygen Demand (CBOD)! defined as the amount of oxygen required for the complete biological decomposition of organic mater under aerobic conditions at a standardized temperature and time! most organic materials are biodegradable, and they are utilized for food for naturally occurring microorganisms within a reasonable length of time! in dissolved form they consist of starches, fats, proteins, alcohols, acids, aldehydes and esters! microbial utilization of dissolved organics can be accompanied by oxidation (addition of oxygen to or the deletion of hydrogen from elements of the organic molecule) or by reduction (addition of hydrogen to or deletion of oxygen from elements of the organic molecule) Nitrogenous Biochemical Oxygen Demand (NBOD)! stoichiometric amount of O 2 required to oxidize NH 4+ to NO 3 -! usually occurs at 5 to 8 days; to ensure this does not occur for BOD 5, inhibitor like trichloromethyl pyridine (2-chloro-6-pyridine) is put into samples! oxidization occurs by autotrophic bacteria and the use nitrogen compounds as their energy source and CO 2 as the carbon source.! Occurs in two steps (i) oxidation of ammonia nitrogen to nitrite by the bacteria of genera (NH 4+ to NO 2- ) by nitrosomonas (nitrosococcus, nitrosospira, nitrosocystis and nitrosogluea) NH O 2 ---nitrosomonas--> NO H 2 O + 2H + (ii) oxidation of nitrite to the most oxidized state of nitrogen, nitrate, by the bacteria of genera; nitrobacter (commonly referred to), nitrospira and nitrocystis NO O 2 ---nitrobacter---> NO Stu-99B.wpd B-2

3 Water Quality - Condensed Version 1999 Figure 1: BOD Model 359-Stu_99B.wpd B-3

4 Modelling BOD If a closed system is observed long enough, nearly all the degradable organic matter including that in cell tissue will be converted into CO 2, NH 4 and H 2 O. In doing so the ultimate BOD will be approached, provided that the nitrification reaction is suppressed. Assuming that the rate of oxidation of organic matter at any instant is proportional to the amount of oxidizable matter present, the first order reaction is: or where L t = oxygen equivalent to organics at time t, mg/l t = time in days k = decay rate in 1/d rearranging and integrating gives: where L o = total oxygen equivalent of the organics at time t = 0, mg/l However, the oxygen equivalent remaining is not of importance, rather the amount of oxygen consumed, i.e. BOD. Therefore, or BOD t = L o - L t BOD t = L o - L o e -kt BOD t = L o (1 - e -kt ) where BOD t = BOD at any time t, which is the carbonaceous oxygen demand equation. BOD u = ultimate BOD, mg/l k = decay constant, d -1 t = elapsed time, d Similar expressions can be derived for nitrogen and sulfur F00B.wpd B-4

5 Determination of BOD Data! collect sample! dilute and place in BOD bottle (volume = 300 ml) different wastes consume different amounts of DO, thus samples must be diluted accordingly, i.e., raw wastewater diluted more than secondary effluent dilution factor (DF) = Bottle Volume/Sample Volume! measure oxygen consumption before and after incubation! preliminary experiments required to determine the correct DF, if not DO depletion will be outside the optimum range; BOD value only valid when: O 2 depletion < 90% O 2 depletion > 30%! calculate BOD using the following equation (as in Lab Manual) BOD = [(D 1 - D 2 ) - (B 1 -B 2 )f] x DF where BOD D 1 D 2 B 1 B 2 f DF = BOD for sample after t days incubation, mg/l = DO of diluted sample after 15 minutes, mg/l = DO of diluted sample after t days, mg/l = DO of seeded dilution blank after 15 minutes, mg/l = DO of seeded dilution blank after t days, mg/l = indicates how much oxygen that the seeded dilution water will consume; (volume of seeded dilution/volume of bottle) = Dilution Factor (bottle volume/sample volume) Example: BOD Determination using Dilution Water without seeding a Sample Sample Volume DF DO i DO f O 2 Dep % BOD A a 1237 B C D E b 44 value not valid as O 2 depletion < 30% b value not valid as O 2 depletion > 90% Thus, sample BOD = [ ]/3 = ! reporting of BOD values (only when reporting value, not for calculations) > 100 nearest < 100, nearest 5 < 10 nearest F00B.wpd B-5

6 Evaluation of Coefficients Various ways exist to determine k and BOD u. The method we will use is the Thomas method Water Sewage and Works, Vol. 97 pg. 123, Thomas recognized the similarity of the series expansions for 1-e -kt and kt[1+(kt/6)] -3 and the BOD equation and developed the approximate formula: linearizing gives (1) (2) Plotting (t/bod) 1/3 vs t gives a straight line, where (3) and (4) Solving for BOD u and k gives the necessary coefficients. Example: BOD Coefficient Determination Time (1) d Corrected Time (2) d Measured BOD (3) mg/l (t/bod) 1/3 (4) Plot Measured BOD vs Time as shown on Figure 1; Columns (3) vs (1) 3590-F00B.wpd B-6

7 2. If lag time exists in Figure 1, subtract it from Time (Column 1) to obtain Corrected Time (Column 2). 3. Calculate Column (4) using Corrected Time and Measured BOD 4. Plot Column (4) vs Column (2) as shown in Figure 2 5. From Figure 2: intercept = 0.22 slope = Figure 1: Raw BOD vs Time 6. Manipulate Eqs. 10 and 11 to solve simultaneously for k and BOD u gives: k = /d BOD u = 319 mg/l 7. Using the above coefficients gives the governing BOD t equation, Figure 2: (t/bod)^0.33 vs Corrected Time! typical values for k and BOD u for default temperature of 20 C water type k, 1/d BOD u, mg/l tap water < surface waters weak municipal wastewater strong municipal wastewater treated effluent ! temperature strongly influences reaction rate; higher the temp, greater the decay temperature changes are accounted for using the van't Hoff-Arrhenius relationship k T1 = k T2 (T1-T2) where is for 4 < T 20 C is for T > 20 C 3590-F00B.wpd B-7

8 ! time usually 5 d no magic number but has been adopted for a number of reasons " flow time to open sea for British Rivers did not exceed 5 days " temperature taken at 20 C because river temps did not exceed 18.3 C 5 day is usually 2/3 of BOD u Australia takes BOD 7 as the norm! dilution water contains the follwoing: nutrients " magnesium sulphate (MgSO 4 ) " calcium chloride (CaCl 2 ) " ferric chloride (FeCl 3 ) all four do not add to oxygen consumption " phosphate buffer nitrification inhibitor if required seed; depends on type of wastewater! automated method AER-O-FLO 3 minute BOD " online readings on a sub-sample " needs to be calibrated with BOD 5 readings " approximately 2h maintenance on probe " company claims that it corrleates within 95% for values between 2 and 5000 mg/l colour metric version EXAMPLE - BOD in River Raw wastewater is generated at a rate of 0.04 m 3 /s, with a BOD 5 of 120 mg/l (k 20 of /d). If physical-chemical treatment is used to treat the wastewater, which reduces BOD loading by 65%, how much of the organic matter remains in the river 3 d downstream. What is the BOD 5 value of the sample 3 days downstream from the outfall? The river temperature is 10 C, flowrate is 0.1 m 3 /s, with an upstream of outfall BODu of 5 mg/l (k 10 = /d) F00B.wpd B-8

9 Sources of Organics (natural and man made)! Natural consist of decaying products of organic solids, i.e. weeds, leaves, trees combinations of carbon, hydrogen, oxygen, phosphorous and sulfur " sulfur cycle (see Fig. 3.7 in course text) principal natural organic compounds in wastewater are proteins (40-60%), carbohydrates (25-50%) and lipids(10%)! Synthetic Organic Compounds: since 1940 over 100,000 organic compounds have been synthesized major concern because of the large variety of chemicals that were and still are being produced some have found their way into water, causing concern for health, treatment and ecology! Surfactants soaps or synthetic detergents that break down grease and oil replaced the non-degradable alkyl-benzene-sulfonate (ABS) with linear-alkyl-sulfonate (LAS) biological ones are beneficial! Pesticides and Agricultural Chemicals: used to control diseases and pest organisms major component of non-point source contamination toxic to most aquatic organisms and many are known or suspected carcinogens! Cleaning Solvents: also known or suspected carcinogens found in aquifers PCE (used as a septic tank cleaner in New Jersey) improper disposal practices! Trihalomethanes (THMs): probable carcinogen created when chlorine used to treat water and wastewater chlorine reacts with a precursor to form a trihalomethane " chloroform - CHCl 3 " bromodichloromethane - CHBrCl 2 " bromoform - CHBr 3 " dibromochloromethane - CHBrCl ozone suggested as a disinfectant 3590-F00B.wpd B-9