Oxygen Demanding Materials

Transcription

1 Experimental Oxygen Demanding Materials Disslved Oxygen Minimum cncentratin is required fr the survival f higher aquatic life Significant discharges f rganic wastes may depress the D.O. cncentratins in receiving waters micrbially-mediated xidatin each state has established ambient disslved xygen standards Anther use f D.O. is the assessment f xidatin state in grundwaters and sediments als a very imprtant parameter in bilgical treatment prcesses indicates when aerbic and anaerbic rganisms will predminate used t assess the adequacy f xygen transfer systems indicates the suitability fr the grwth f such sensitive rganisms such as the nitrifying bacteria. used in the assessment f the strength f a wastewater thrugh the Bichemical Oxygen Demand (BOD) Oxygen Demand is a measure f the amunt f reduced rganic and inrganic matter in a water. It relates t xygen cnsumptin in a river r lake as a result f a pllutin discharge. Measured in several ways BOD - Bichemical Oxygen Demand COD - Chemical Oxygen Demand TOC-Ttal Organic Carbn ThOD - Theretical Oxygen Demand =Carbnaceus demand + Nitrgenus Demand Theretical Oxygen Demand This is the ttal amunt f xygen required t cmpletely xidize a knwn cmpund t CO and HO. It is a theretical calculatin that depends n simple stichimetric principles. It can nly be calculated n cmpunds f knwn cmpsitin. If N is present then it gets xidized t NH3. NH3 can be further xidized t nitrate. Every mle f NH3 requires mles f xygen t get xidized t nitrate. Chemical Oxygen Demand (COD) This test determines the amunt f xygen needed chemically t xidize rganics in water r wastewater. The xidizing agent is ptassium dichrmate (KCrO7) in an acid slutin (typically 50% HSO4) and in the presence f a Silver Sulfate (AgSO4) catalyst. This test is attractive because it takes nly hurs. The majr disadvantage is that sme rganics like detergents and armatics may nt be easily xidized. COD s are always higher than BOD values. Fr municipal wastewater, COD 1.6BOD5. Bilgical Oxygen Demand BOD is a measure f the xygen used by aerbic micrrganisms while degrading and rganic pllutant. Briefly, the BOD test emplys a bacterial seed t catalyze the xidatin f 300 ml f full-strength r diluted wastewater. The standard test is run in the dark fr five days at 0 C in a glass bttle. This is knwn as the 5 day BOD r BOD5. The xygen cnsumptin is easily measured by DO readings. The xygen cncentratin in the BOD bttle typically fllws the pattern as shwn belw:

2 L0 Oxygen Cncentrati n mg/l Lt L-Lt = BODt Time The figure indicates that initially xygen depletin is rapid because f high cncentratins f rganic matter present. As the cncentratin f rganic matter decreases s des the rate f xygen cnsumptin. It has been nted that the curve in the abve figure is mathematically dented by a first rder reactin. Therefre, dl kl dt Integrating Lt L dl L k t 0 dt yields L t L e kt Where L0 = xygen cncentratin at time 0 Lt = xygen cncentratin at time t Since BOD is defined as the xygen used, the ttal xygen used in time t = L0 - Lt Therefre, BODt = L0 - L e kt = L0 (1- kt e ) L0 is ften referred t as the ultimate BOD BOD rate depends n nature f waste ability f micrrganisms t degrade the waste T 0 C Temperature- Crrectins t k can be made as fllws kt k = fr temp between 4 and 0 0 C C and fr temp between 0 and 30 C. Nitrgenus Oxygen Demand Oxygen cnsumptin due t carbn is called carbnaceus bilgical xygen demand (CBOD) and xygen cnsumptin due t nitrgen xidatin is called nitrgenus bilgical xygen demand (NBOD). Bacteria that xidize rganic cmpunds typically cnvert the nitrgen in these cmpunds t ammnia. The ammnia is further bidegraded by nitrifying bacteria (tw kinds) in tw steps: Nitrsmn as NH O NO 1 NO The verall reactin is NH3 + O = HNO3 + HO H O Nitrbacte r NO3 O H

3 SAMPLE PROBLEMS 1. The BOD5 f a sewage sample is 00 mg/l. What is the ultimate BODu at 0 C if k0 = 0.16 day -1? What is the BOD5 and BODu f the sample at 30 C if k30 is 0.5 day -1? Lt = L (1-e- kt ) Given Lt = 00 L=? k0 = 0.16 day -1 t= 5 days 00 = L(1-e -.16*5 ) r L = mg/l = BODu Nte BODu is nt a functin f temperature. Therefre same BODu will be used at 30 C BOD5 =Lt = (1 e -.5*5 ) = mg/l. The BOD3 f a wastewater is 10 mg/l while the BOD7 f the wastewater is 17 mg/l. What is the value f the ultimate BOD (BODu) in mg/l and the bidegradatin rate cnstant k in day -1? Als calculate the value f the 5 day BOD. Assume temperature f 0 C. The bidegradatin rate f a wastewater at 0 C is 0.1 day -1 while the rate is 0.4 day -1 at 30 C. What is the value f O if the fllwing relatinship hlds k1= ko T1-T? 3. Dr. Jahan s class is preparing a harmless ptin fr engineering students fr a Hallween event. The rganic chemical being used is ttally harmless, nn txic, highly bidegradable and readily xidized chemically. The chemical frmula f the cmpund is C10H0 and the cncentratin prepared is 150 mg/l. What are the THOD, BOD and COD f the sample? Justify the values f the BODu and COD values. Assume BODu is 0.85*COD. If the bidegradatin rate k is 0.5day-1 what is the BOD5 in mg/l?

4 PROBLEMS FORM TEXTBOOK

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