M HNH BOD/DO N GiN M HNH STREETER - PHELPS

Dng chy vo

Bin trng thiB sung Suy gim

Dng chy ra

Th tch nc6/3/2006 BUI TA LONG, TSKH 1

Ni dungo o o o o t bi ton Cc khi nim c bn C s l lun ca m hnh Streeter - Phelps Cc cng thc tnh ton i km Mt s bi ton ng dng

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i tng cn xem xt (cn xy dng m hnh)Cho: 1. S lng ngun im v ngun khng im 2. c trng ca on sng, h Cn tm: 1. Cht lng nc ca khc sng xem xt

o M hnh gip a ra tnh ton s. o M hnh c gip nh gi, d bo nh hng do hot ng sn xut ca con ngi ln mi trng.

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Mt s khi nim v k hiuo BOD (Biochemical Oxygen Demand) nhu cu oxy sinh ha ton phn (mg/l); o BOD i din cho nhng thnh phn c th phn hy sinh hc. Nu c oxy, qu trnh phn hy sinh hc s i hi mt lng oxy tng ng vi lng gim BOD; o BOD5 Nhu cu oxy sinh ha sau thi gian 5 ngy (mg/l); o COD (Chemical Oxygen Demand ) nhu cu oxy ha hc (mg/l); o DO (dissolved oxygen) Nng oxy ha tan (mg/l);6/3/2006 BUI TA LONG, TSKH 5

Thut ng, c s php lo T in mi trng v pht trin bn vng Anh Vit, NXB KH & KT. o ISO 6107-3:1993: TCVN 5982-1995. Cht lng nc, thut ng - phn 3. o ISO 6107 6:1980: TCVN 5985 1995- Cht lng nc, thut ng- phn 6.

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Cht hu co Cc cht hu c (organic wastes): cc cht thi c ngun gc t cc sinh vt sng hoc cht. Khi c a vo ngun nc, cc cht hu c s lm cho cc vi sinh vt hiu kh pht trin. Cc vi sinh vt ny s tiu th oxy lm cho lng oxy ho tan s gim xung cho nn c s dn dn bin mt. Khi phn ln lng oxy ho tan gim i thi cc vi sinh vt k kh s bin i cc hp cht c cha lu hunh thnh H2S lm cho nc c mi ...

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Cht d phn hu sinh hco Cht d phn hu sinh hc (readily biodegradable substances): cht c th b phn hu sinh hc n mt mc nht nh no theo cc php th nh i vi kh nng phn hu sinh hc hon ton. o S phn hu sinh hc hon ton: s phn hu sinh hc dn n s v c ho hon ton. o Phn hu bc nht: s phn hu cu trc phn t ca mt cht n mc d loi b mt tnh cht c trng no .

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Hng s tc phn hu K1o Hng s tc phn hu cc cht hu c k1 l i lng c trng cho tc ca phn ng phn hu cc cht hu c d phn hu sinh hc do cc vi sinh vt hiu kh trong dng chy c xc nh trong iu kin tnh trong phng th nghim.dLt = - K1 .Lt dt

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Hin tng np kh (oxy t khng kh xm nhp vo nc mt)o Np kh (reaeration) l qu trnh vt l (chuyn ha khi lng) ca oxy t kh quyn vo khi nc. o s dng cc m hnh cht lng nc sng cn thit phi xy dng phng php tnh ton h s tc ha tan oxy qua mt thong (ngy -1).6/3/2006 BUI TA LONG, TSKH 10

Cc bc cn thit xy dng m hnh1. Quyt nh la chn loi m hnh 2. Xy dng cc m hnh l lun (Conceptual Model(s)) 3. Lm n gin ha cc gi thit 4. Vit phng trnh chuyn ng (Governing Equations) 5. Gii ra kt qu s 6. ng dng trn my tnh

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D , Lt , C t

q

D0 ,T

K1 , K N , K a , R a6/3/2006 BUI TA LONG, TSKH

dD = K 1 Lt - K a D dt 12

M hnh cht lng nc trong dng chy n gin c c trng bio M hnh dng chy, su ca nc, nng BOD, DO o Bin thi gian v khng gian o Mt chiu, mt ct ngang nhn gi tr trung bnh (cc bin ch thay i theo dc sng) o Hy tm Q(x,t), H(x,t), BOD(x,t), DO(x,t)

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M hnh BOD/DO n gino Loi m hnh ny lin quan n nng oxy trong sng v sui o M hnh cht lng nc u tin xem xt mi quan h BOD/DO trong mt h thng sng c pht trin bi Streeter Phelps nm 1925.6/3/2006 BUI TA LONG, TSKH 14

Mt s gi thit lm n gin ha cho m hnho o o o o o o Khng c s khuch tn, mt l hng s Nhit khng i v c coi l bit Tt c BOD c chuyn ha vi cng mt tc Khng din ra qu trnh quang hp/h hp Tc phn ng c bc 1 Tc lng ng oxy l hng s theo thi gian Tt c cc h s tc khc l khng i theo khng gian v thi gian

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Cc gi thit tr gip lm n gin ha m hnh Streeter Phelpso Ch c mt ngun nhim tn ti. o Ti trng nhim khng i c thi ra mt im cho trc. o Sng khng c nhnh. o Vn tc dng chy khng i. o Mt ct ngang dng sng coi nh khng i. o S khuch tn to iu kin cho nng BOD v DO coi nh ng u trong mt ct ca sng. o S phn hy sinh hc din ra trong sng c bc nht v ngoi ra khng c s tham gia ca cc qu trnh khc.6/3/2006 BUI TA LONG, TSKH 16

Phng trnh Streeter - PhelpsdD = K 1 Lt - K a D dtD = Cs Ct Cs = nng oxy bo ha Ct = nng oxy thi im t Lt = nng cht hu c, c o bng BOD thi im t o K1 = h s tc phn hy cc cht hu c hay hng s tc tiu th oxy do qu trnh phn hy cc cht hu c (ngy-1) o Ka = h s tc ha tan oxy qua mt thong (ngy-1) o o o o6/3/2006 BUI TA LONG, TSKH 17

Phng php tnh trong thc to Lm th no tnh c K1 v Lt? o Lm sao tnh ton nhng tc ng ca nhit ? o Lm sao c lng s np kh (reaeration)? Gi tr ca K1, Ka, Lt v No c a ra cho mt s trng hp c trng Bng sau y. KN l hng s tc ca s nitrat ha: NH+4 + 2O2 NO3- + H2O + 2H+

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Cch tnh K1, KNNo l nng ammonium v L0 l nng cht hu c o bng BOD. K1 v KN ph thuc vo nhit T: tnh K1 KT (ti 200C) 1.05 tnh KN 1.06 1.08

T K 1 (T ) = K1 20 0 C K T - 20

T K N (T ) = K N 20 0 C K T - 20

(

(

)

)

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Gi tr c trng K1, KN, No v Lo (20oC) (Bng A)K1 Nc thi th Nc thi th x l c hc Nc thi th x l sinh hc Nc ung Nc sng6/3/2006

KN 0.15 0.20 0.10 0.25

No 80 120 70 120

Lo 150 250 75 - 150

0.35 0.40 0.35

0.10 0.25

0.05 0.20

60 120

10 80

0.05 0.10 0.05 0.15

0.05 0.05 0.10BUI TA LONG, TSKH

01 02

01 0-520

Cch tnh Kao Ka ph thuc vo nhit , vn tc dng chy v su dng chy nh c ra trong nhng phng trnh di y: 2 .26 v K a ( 20 0 C ) = (ngy -1 ) 2 H 3 2 .26 v q (T - 20 ) K a (T ) = e (ngy -1 ) H 2/3

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Mt s k hiu dng trong cc phng trnho Ka (20oC)= h s np kh (tc ha tan oxy qua mt thong) 200C; o Ka(T)= h s np kh (tc ha tan oxy qua mt thong) T0C; o v = vn tc trung bnh ca dng chy (m/s) o H = su (m) o = hng s = 0.0240oC-1, 5oC