1

Chuyn : M HNH CHT LNG NC

A. MC CH Nhm trang b cho hc vin cc kin thc c bn:

- L thuyt c s v cc qu trnh c bn chuyn ho cc cht nhim trong ngun nc, cc nguyn l c bn xy dng m hnh cht lng nc.

- Cc m hnh cht lng nc v phm vi ng dng trong thc tin. B. YU CU Hc vin cn c k nng v ngn ng lp trnh, ton ng dng v hc cc mn c s v khoa hc mi trng: ho l, thu lc, sinh thi mi trng, ho mi trng, nhim mi trng. C. NI DUNG

Chng 1. Cht lng nc v nh gi cht lng ngun nc Chng 2. C s l thuyt MHCLN Chng 3. M hnh lan truyn v chuyn ho cc cht nhim trong dng

chy

Chng 4. Thit lp m hnh cht lng nc, cc m hnh DO&BOD

Chng 5. Gii thiu cc phn mn m phng cht lng ngun nc

M hnh Qual 2E

D. TI LIU THAM KHO [1]. Jerald L. Schnoor. Environmental Modeling. John Wiley & Sons. Inc, 1996. [2]. A.James. An Introduction to water quality modeling. John Wiley & Sons. Inc, 1996. [3]. Mervin D.Palmer. Water quality modeling. A guide to effective practice. WB, 2001. [4]. Linfield C.Brow and Thomas O.Branwel. The enhanced stream water quality

Qual2E and Qual2E-uncas: Documentation and user manual. EPA, 1987. [5]. Steve C. Chapra. Sufrace water-quality modeling. Mc Graw - Hill, 1997.

1

Chng 1

Cht lng ngun nc & nh gi cht lng

ngun nc 1.1. Ngun nuc v phn loi ngun nc 1.1.1. S hnh thnh cht lng v thnh phn tnh cht ngun nc

Cc yu t v cc qu trnh hnh thnh thnh phn ho hc ca nc thin nhin bao gm hai nhm: cc yu t tc ng trc tip v cc yu t iu khin cc qu trnh hnh thnh cht lng nc din ra trong dng chy

Cc yu t tc ng trc tip Khong vt, th nhng, sinh vt v con ngi. Cc yu

t ny tc ng lm cho nng cc cht trong nc tng ln v gim i.

Qa trnh khong vt ho: din ra rt phc tp, ph thuc vo c im ca thnh phn khong vt: nhan thch hiu nc, k nc v ngm nc..Cc loi mui: NaCl, CaCO3, CaSO42-...

Khong vt phong ho: allluminoSilicat (nhm silic) chim phn ln trong ln v tri t phong ho chuyn vo nc. Khong vt st: thnh phn chnh ca nhan thch.

Th nhng (t trng) : khc vi thnh phn khong vt, th nhng ngoi cc thnh phn v c (90-95%) trong t trng cn c cc thnh phn hu c v hu c khong vt. Thnh phn hu c c ngun gc: sn phm phn hu gc ng vt, thc vt v sn phm ca cc qu trnh sinh ho trong t.

S xm nhp vo mi trng nc ph thuc vo cc yu t kh tng thu vn, a hnh, lng ma v cng ma.

Sinh vt v con ngi Cc sinh vt c vai tr rt a dng v rng ri. Thc hin

cc chu trnh sinh -a-ho: iu chnh cn bng sinh thi, to

2

nng sut sinh hc s cp (to, ph du...) v cc cht hu c ban u (vi khun c nh m)...

Cc hot ng pht trin gy nhim ngun nc Cc yu t iu khin Cc yu t iu khin bao gm : kh hu, a hnh, ch

thu vn, s pht trin ca h thc vt thu sinh. Kh hu: nh hng trc tip n lu lng v nng cc

cht, nhit nh hng n cc phn ng ho hc, sinh hc...

a hnh: nh hng gin tip n cc qu trnh khong ho, xi mn v ra tri b mt.

Ch thu vn: thnh phn ca nc, nng cc cht ho hc trong nc ph thuc vo dng chy. Chiu di dng chy, din tch lu vc.

Qa trnh hnh thnh cht lng nc Qa trnh khuch tn: l qu trnh dch chuyn cc cht ho

tan, phn tn trong nc do nh hng ca gradient nng . Tun th theo nh lut Fick

Qa trnh chuyn khi do khuch tn i lu. Vn chuyn (ti cc cht trong dng chy, s xo trn).

Cc qu trnh vn chuyn cc cht vo trong ngun nc Thu phn: phn ng trao i gia nc v cc loi khong cht. Ho tan:ph hu cu trc mng tnh th ca cc loi mui

v phn ly thnh cc dng ion Cc qu trnh tch cc vt cht khi ngun nc Bao gm cc qa trnh lng: do t trng, nng vt gii

hn bo ho, qa trnh hp ph, qu trnh keo t, cc qu trnh phn ng gia cc hp cht v cc qu trnh sinh thi cht lng nc.

3

3. Thnh phn v tnh cht ca nc thin nhin Cc ion ho tan Nuc l mt dung mi rt tt ho tan hu ht cc loi

khong cht v c, cc axit, baz v cc mui v c. Cc ion ch yu trong nc l cc ion ca cc loi mui khong, Cl-, SO42-, HCO3-, CO32- v cc ion kim loi Na+, K+,Ca2+, Mg2+,Mn2+ ... chim khong 90-95% trong nc ngt v trong cc ngun nc khong >99% trong tng s cc cht ho tan.

Hm lng cc ion ho tan ph thuc vo c im kh hu, a mo v v tr ca thu vc.

c im hnh thnh cc ion ho tan ca cc dng chy do cc nhn t ch o quyt nh: lng nc ma, bc hi v qu trnh phong ha.

Cc cht kh ho tan Hu ht tt c cc cht kh (tr CH4) u c kh nng ho

tan hoc phn ng vi nc. Thnh phn cc cht kh ph thuc vo iu kin t nhin ca ngun nc.

Cc qu trnh hnh thnh cht kh trong nc t nhin: ho tan t kh quyn (O2, N2, CO2, cc loi kh tr...) sn phm t cc qu trnh sinh ho (H2S,CH4,N2,CO2...) v qu trnh bin i trong khong cht c sn trong nc ngm.

Nng cc cht kh ho tan tun th theo nh lut Herry

Cc cht rn Phn loi theo t trng: lng c d>10-5m v l lng. Theo kch thc: lc c d >10-6m v khng lc c, cc

hp cht keo d = 10-6-10-9m v dng ho tan d

4

Thnh phn sinh hc Thnh phn v mt c th sng ph thuc vo : thnh

phn ho hc ca ngun nc, ch thu vn, a hnh ni c tr, kh hu

Cc loi thu sinh vt trong nc: vi khun, nm, siu vi trng, to, nguyn sinh ng vt, ng vt a bo, ng vt c xng, nhuyn th...

Cc hnh thc sng trong ngun nc rt a dng: dng ph du (plankton, phytoplankton, macroplanton); c, sinh vt sng bm, sinh vt y...

Vi khun ng vai tr quan trng trong vic phn hu cc cht hu c, h tr qu trnh t lm sch ngun nc c ngha to ln v mt sinh thi. Cc loi vi khun chia thnh hai loi : t dng (heterophic) v d dng (autotrophic). Phn loi ngun nc Theo mc ch s dng c chia thnh cc loai ngun nc:

cp cho sinh hot, v cc mc ch khc nh gii tr, tip xc vi ngun nc v nui trng cc loi thu sn.

Theo mn thng theo nng mui trong ngun nc c chia thnh nc ngt, nc l v nc mn.

Theo v tr ngun nc chia thnh cc ngun nc mt (sng, sui, ao, h...) nc ngm.

1.2.Cht lng ngun nc v nh gi cht lng ngun nc

1.2.1. Cht lng ngun nc 1.2.2. nh gi cht lng ngun nc Cc ch tiu nh gi cht lng ngun nc

Cc ch tiu vt l Nhit Mu sc Mi, v c.

5

Cc ch tiu ho hc Cc cht l lng, phn tn nh... Cc cht kh ho tan Cc ion ho tan...

Cc ch tiu sinh hc Vi trng, vi khun gy bnh. 1.3. Cc ngun gy nhim mi trng nc.

1.3.1. Nc thi sinh hot t cc khu dn c

Ngun gc c im ngun thi, ch thi Tnh cht, thnh phn

1.3.2. Nc thi cng nghip

Ngun gc c im ngun thi, ch thi Tnh cht, thnh phn

1.3.3. Nc ma chy trn

1.3.4. Cc hot ng t tu thuyn

Cc ngun khc: nc ma, cc vng x l cht thi rn...

1.2. M hnh cht lng nc, ng dung trong cng

tac quan ly CLNN

MHCLN l cc phn mn tnh ton cc ch tiu phn nh

cht lng ngun nc. Cc ch tiu bao gm: cc ch

tiu vt l, ho hc v thnh phn sinh hc ca ngun

6

nc trn c s gii cc phng trnh ton hc m t mi

quan h gia cc ch tiu phn nh cht lng nc cng

nh cc qu trnh c lin quan n n.

1.2.1. Lch s pht trin

M hnh cht lng nc l mt trong nhng cng c qun l

cht lng ngun nc mt cch tng qut v ton din, mang

li hiu qu kinh t cao. Trong nhng nm gn y c ng

dng rt rng ri trong cc lnh vc : d bo nhim, nh gi

xu th bin i cht lng nc, khai thc s dng hp l ngun

nc v lm c s khoa hc cho vic bo v tng hp ngun

nc.

Trong gn mt th k, t m hnh n gin u tin cho n

nay s pht trin MHCLN c th c tm tt theo cc giai on

sau :

Giai on u th k 20

M hnh cht lng nc u tin c Streeter-Phelps

thit lp 1925, m phng s thay i cc cht hu c BOD & D

( thiu ht oxy) vng h lu cc ngun thi im trn dng

chy sng Ohio. M hnh c thit lp da trn c s cc gi

thit : dng chy n nh, s phn hy cc cht hu c theo phn

ng bc nht v s thiu ht oxy trong dng chy l do s phn

hy cc cht hu c.

7

Trong nhng nm ca thp k 30 -50, kt hp cc kt qu

nghin cu l thuyt v qu trnh xo trn, khuch tn ri vt

cht trong dng chy ca Taylor, Eder v cc phng php tnh

ton s lan truyn cht trn dng chy, Cc tc gi c gng nng

cao tin cy bng vic xem xt ng thi nh hng ca qu

trnh khuch tn ri n qu trnh lan truyn cc cht nhim

trong dng chy. Cc nghin cu tp trung vo cc mi quan h

gia s thay i gi tr BOD&D trn cc dng chy vi cc ch

thy lc khc nhau.

Tuy nhin, trong giai on ny do s hn ch ca phng

php tnh, cng c tnh ton cng nh cc iu kin thc nghim

trong dng chy nn cc MHCLN ch yu tp trung gii quyt

cc vn t ra trong cc dng chy n gin knh, sng vi

iu kin n nh, mt chiu. Kt qu t c trong giai on

ny l cc cng thc thc nghim xc nh hng s tc ho

tan oxy, cc s liu thng k v hng s tc phn hu cc cht

hu c trong cc dng chy c ch thu lc khc nhau.

Giai on thp k 60

Trong giai on ny, cng vi cng c tnh ton mi (my

tnh in t) cc phng php tnh ton c hon thin, cc

phng php s gii bi ton thy lc, bi ton lan truyn cht

trong dng chy tr nn quen thuc. Cc MHCLN c pht

8

trin tnh ton vi bi ton nhiu chiu hn v x l cc vn

m trc y khi gii quyt cn gp rt nhiu kh khn. tin

cy ca m hnh cng c nng cao hn.

Cc vn c quan tm trong giai on ny l p dng vo

tnh ton trong thc tin cc vn nh cp trn nhng cc

m hnh gii quyt cc bi ton nhiu chiu hn v cc vn

phc tp hn. tin cy ca m hnh c nng cao do b sung

thm cc qu trnh c nh hng n s phn b nng cc

cht hu c trong dng chy :

- Qa trnh lng cc cht l lng, phn tn nh trong qu trnh

lan truyn.

- Qa trnh gii phng cc cht t lp bn y do s c st ca

dng chy vi lp bn y.

- Qu trnh quang hp v h hp ca h thc vt thu sinh

Cc MHCLN c pht trin rt a dng. Nu nh trc y

cc m hnh thun ty nh gi nhng tc ng ca ngun thi

im n cht lng nc sng, cc m hnh cp n s

lan truyn trong dng chy ca cc sng rng, vng ca sng.

Trong cc ng dng vo thc tin, cc nghin cu xc nh

cc s liu thc nghim v hng s tc ho tan, h s chuyn

ho cc cht trong dng chy. Vi cc lu vc c ch thu

9

lc tin cy ca kt qu tnh ton m phng cn nhiu hn ch

.

Giai on thp k 70

Thp k 70, vi s hon thin cc phng php nghin cu

thc nghim xc nh s phn tn vt cht trong dng chy cc

MHCLN pht trin a dng hn. cp n vai tr ca qu

trnh t lm sch ca ngun nc (kh nng chuyn ha cc cht

bn ca h ng thc vt), cc nghin cu tp trung thm vo

cc yu t nh hng n qu trnh ph dng ngun nc. S

chuyn ha cc cht nhim trong chui thc n. S tch ly cc

cht nhim trong cc sinh vt tiu th. Bc u, cc nghin

cu ch dng li cc nghin cu s ph dng ca cc h cha

nc. i vi dng chy vn ny cng c cp n,

tuy nhin khi trin khai ng dng cn rt nhiu kh khn trong

vic nh gi v hiu chnh m hnh.

Giai on thp k 80 n nay

Cui nhng nm 80 tr li y, Cc MHCLN tp trung

nghin cu mi quan h gia cc qu trnh sinh thi -cht lng

nc trong dng chy. Cc m hnh c thit lp di dng

n gin hn nhng tin cy cao hn.

Thomann v Mueller (1987) m hnh ho cc nh hng

ca mi quan h gia cc loi ph du thc vt vi cc cht dinh

10

dng trong dng chy n cht lng nc sng. Cc cht dinh

dng c a vo dng chy di dng cc ngun thi im.

Law v Chalup (1990) xy dng MHCLN trn c s s

pht trin ca qu trnh quang hp v h hp ca to. iu ny

c Bowie b sung vo m hnh Qual 2E (1993). Di Toro v

Fitzpatrick (1993) pht trin, b sung thm mi quan h gia cc

sinh vt ln tiu th (sinh vt tiu th bc I) s chuyn ha v

tch ly cc cht dinh dng.

Hin ti, hng pht trin ca MHCLN l nghin cu s

tch ly cc cht hu c bn vng trong chui thc n, s tch

ly cc cht c trong cc c th sng. M phng phn b nng

cc cht nhim trn cc dng chy phc tp nh s lan

truyn cc cht nhim t cc ngun thi im, cc ngun thi

phn tn, cc ngun thi pht sinh thm trong qu trnh chuyn

ha cc cht nhim.

Cc p dng thc tin, c trin khai rng cc dng

chy c ch phc tp nh cc dng chy sng rng, ca sng,

cc vng, vnh v cc vng bin ven b.

1.2.2. Phn loi MHCLN v phm vi ng dng

MHCLN l cc phn mn tnh ton cc ch tiu phn nh

cht lng ngun nc nh : cc ch tiu vt l, ho hc v

thnh phn sinh hc ca ngun nc trn c s gii cc phng

11

trnh ton hc m t mi quan h gia cc ch tiu phn nh

cht lng nc cng nh cc qu trnh c lin quan n n.

Ty thuc i tng nghin cu, cch tip cn, cc gi thit khi

thit lp, cc thng s v mi quan h gia cc qu trnh khi thit

lp phng trnh v cc phng php s c p dng tnh

ton nn mi m hnh c nhng th mnh v hn ch khc nhau.

Trn c s mi quan h gia cc qu trnh, cc yu t hnh

thnh v nh hng n cht lng ngun nc, cc MHCLN

c chia thnh hai loi : m hnh tnh ton s lan truyn,

phn b cc cht nhim trong dng chy v m hnh m

phng s hnh thnh cht lng nc v xu th bin i cht

lng ngun nc.

M hnh tnh ton s lan truyn, phn b cc cht

nhim trong dng chy

M phng s bin i cc ch tiu cht lng nc theo

thi gian trong khng gian ca dng chy. Vic thit lp m hnh

da trn c s gii phng trnh ti v ti-phn tn cc cht

nhim trong dng chy. Cc yu t c trng v dng chy c

xc nh t cc m hnh thy lc, cc s liu thng k hoc o

thc nghim nh cc m hnh QualI, II; Stream I, II... Loi m

hnh ny c u im v hn ch sau :

12

- Cho kt qu nhanh v s lan truyn, phn b cc cht t

cc ngun thi n cht lng nc. Trn c s cho

php chng ta nh gi tc ng ban u cng nh nhng

nh hng lu di n cht lng ngun nc.

- tin cy cao, d s dng do i hi t cc s liu u

vo.

- p dng rng ri trong vic nh gi tc ng ca cc

hot ng pht trin, d bo xu th bin i cht lng

ngun nc.

Nhc im l : cha xem xt n cc yu t hnh thnh

cht lng ngun nc, vic tnh ton m phng trong cc

khong thi gian ngn v c bit nh gi nh hng ca

cc ngun thi phn tn, cc s c mi trng n cht

lng ngun nc cn gp nhiu kh khn.

M hnh m phng s hnh thnh cht lng ngun nc

M phng s hnh thnh cc ngun gy nhim (cc ngun

thi v ti lng cc cht thi) v s thay i cht lng nc

theo thi gian v khng gian. Thit lp trn c s ghp ni cc

m hnh thy lc vi m hnh lan truyn cht nhim trong

dng chy nh WSHMM, MIKE SYSTEM... So vi loi m

hnh trn c cc u im v hn ch sau :

13

- M t mt cch tng qut v ton din hn v cht lng

ngun nc cng nh cc yu t nh hng n cc ch tiu

cht lng nc.

- Xem xt, nh gi c mc tc ng ca cc cht

nhim t cc ngun khng im n cht lng ngun

nc. Cc cht nhim c ngun gc t cc hot ng

nng nghip, t cc khu vc th v tp trung dn c

c a vo dng chy theo nc ma chy trn.

- i hi mt lng rt ln v ng b cc thng tin ban

u nh : cc s liu a hnh lu vc, cc s liu v thy

vn, dng chy...v khi lng tnh ton rt ln v phc

tp.

- nh gi, hiu chnh m hnh cng nh chun ho cc h

s ...gp nhiu kh khn trong thc t v i hi mt

khong thi gian di do vic d bo cc thng tin ban u

c tin cy thp.

Tu thuc i tng c th, mc ch nghin cu nh gi

m cc loi m hnh k trn c thit lp, tnh ton m t cc

qu trnh trong khng gian 1 hoc 2,3 chiu.

1

CHNG 2. C S L THUYT CA M HNH

CHT LNG NC

2.1. Cc phng trnh c bn M t s xo trn v lan truyn cc cht nhim trong dng chy da trn c s l

thuyt ca qu trnh khuch tn ri. L thuyt ny c tha nhn rng ri trn th

gii. Phng trnh vi phn c bn m t qu trnh lan truyn v khuch tn ri vt cht

trong dng chy da trn cc gi thit c bn sau :

- Cht lng khng nn.

-Tr s Reynold ln khng xt n hiu ng ca qu trnh khuch tn phn

t.

-S lng cc cht gii phng trong mt n v thi gian trn mt n v th tch l

rt nh v b qua nh hng ca chng n cu trc ca dng chy.

Trn c s nh lut bo ton khi lng, phng trnh vi phn m t qu trnh xo

trn, lan truyn v chuyn ha cc cht nhim trong cht lng l tng-phng trnh ti

-khuch tn nh sau :

( ) 0=+

+

+

++

++

SFzCD

zyCD

yxCD

xzCu

yCu

xCu

tC

zyxzyx (2.1)

Trong :

Dx, Dy, Dz - H s khuch tn phn t, m2/s.

ux, uy, uz - Vn tc dng chy theo cc phng x,y,z, m/s.

F(S) - S hng c trng cho qu trnh chuyn ho cc cht nhim bi

cc qu trnh vt l, ho hc v sinh hc din ra trong dng chy.

Phng trnh (2.1) l phng trnh l thuyt na kinh nghim m t qu trnh ti

v khuch tn i lu vt cht trong dng chy. Khi p dng phng trnh vi phn (2.1)

gii bi ton xc nh s xo trn vt cht trong dng chy ri trong ng ng, Taylor

(1954) t cc nghin cu thc nghim kin ngh s dng h s khuch tn tch phn c

th xem xt nh hng ca trng vn tc trn tit din ca mt ct t n qu trnh

khuch tn ri vt cht.

2

T cc nghin cu trn, phng trnh vi phn (2.1) m t s khuch tn ri vt cht

trong dng chy c vit li l :

( )SFzC

zyC

yxC

xxCu

xCu

xCu

tC

zyxzyx +++=+++ )()()(

(2.2)

Trong :

x; y; Z - H s khuch tn ri tch phn ti im ang xt, h s xo trn ri theo cc phng x,y,z.

T cc phng trnh (2.1), (2.2) cho thy s lan truyn cc cht trong dng chy

ri trong mi trng hp ph thuc vo vn tc dng chy ti im ang xt.

Trong dng chy t nhin, trn knh, sng... s xo trn, khuch tn ca cc cht

ti mt im lun lun chu nh hng ca trng vn tc theo phng ngang v phng

thng ng, lm r s khc bit ny so vi dng chy trong ng thng s dng h s

phn tn ri. Phng trnh 2.2 c vit li l :

( )SFzCE

yCE

xCE

xCu

xCu

xCu

tC

zyxzyx +++=+++ 22

2

2

2

2

(2.3)

Trong :

Ex Ey Ez - H s (phn tn ri) khuch tn ri vt cht theo phng x,y,z ti

mt ct.

Nh vy, qu trnh phn tn ri vt cht trong dng chy l t hp ca qu trnh

khuch tn ri vt cht trong trng vn tc dng chy c hng v vn tc khc nhau.

2.2.H s khuch tn ri Nh phn tch trn, s khuch tn vt cht trong dng chy l qu trnh

khuch tn phn t qua mng v khuch tn i lu. H s khuch tn ri chu nh hng ca s xo trn gia cc lp dng chy ri c vn tc khc nhau qua mt ct ngang /2,10,11/. Cc phng xo trn ca nc thi vi nc sng ti mt im trn mt ct ngang dng chy c th hin H.2

3

Trong thc t, cc h s khuch tn ri thng c xc nh gi tr trung bnh vi tit din mt ct t ca dng chy, khng ph thuc vo ta im tnh ton v ln ca n ph thuc vo cc yu t thy lc ca dng chy ti mt ct.

H s khuch tn ri theo phng thng ng z H s khuch tn ri theo chiu thng ng ti mt im trn mt ct ngang c

xc nh theo cng thc :

z kd u zdzd

=

1 (2.4)

Vi : u g d S = : vn tc trt (vn tc ng lc) ca dng chy ti mt ct. z : ta im tnh ton, m. k : h s ri Von Karman (k 0,40 ). d : chiu su trung bnh ca dng chy ti mt ct. S : dc y ca dng chy, m/m. g : gia tc trng trng, m/s2.

Gi Z l h s khuch tn ri trung bnh cho mt ct. = d zz dzd 0

1 (2.4) Jobson v Sayer (1970) ; Csanady (1976) bng cc nghin cu thc nghim xc

nh h s khuch tn ri v xut cng thc kinh nghim v h s khuch tn ri z ti mt mt ct ngang ca dng chy.

z du= 0 067, (2.5)

H s xo trn theo phng ngang y Fischer (1967,1969) bng thc nghim xc nh h s khuch tn ri v a ra

cng thc tnh h s khuch tn ri y ti mt im trn mt ct ngang ca dng chy. y d u= 0 1 5, (2.6)

Hnh.2 Cc phng xo trn ca dng chy.

4

Lau v Krishnapan (1977) bng cc thc nghim trong cc dng chy t nhin trn knh, sng c cc ch chy khc nhau, kin ngh mc sai s ca h s y .

Dng chy trong knh thng hnh thang : y du= 0 15 50%. (2.7) Sng c ch chy m : y d u= 0 6 5 0 %, (2.8)

Sng hnh dng cong, khc khy : = uRdu

cy 2

32

25 (2.9) Trong :

u :vn tc trung bnh ti mt ct ngang dng chy, L.T-1 Rc :bn knh thy lc, L

d : chiu su trung bnh, L. T cng thc (2.5) v (2.6) d dng nhn thy y z 10 nh vy qu trnh khuch

tn ri theo phng ngang ca dng chy ln hn rt nhiu so vi qu trnh khuch tn ri theo phng thng ng.

H s khuch tn ri x Tng t nh vic xc nh z v y , Elder (1959) bng thc nghim xc nh

h s khuch tn ri ti mt im trn mt ct ngang theo chiu dng chyx .

= dux 93,5 (2.10) So snh x; y v z cho thy : x >> y >> z nh vy qu trnh xo trn v pha

long cc cht nhim din ra ch yu theo theo chiu ca dng chy. Hay ni mt cch khc s xo trn, khuch tn ri cht nhim ti mt im no ti mt mt im no trn ct ngang dng chy chu nh hng ch yu vo trng vn tc ca dng chy ti mt ct ngang .

H s phn tn dc dng chy Ex Khi xem xt nh hng ca trng vn tc ti mt ct ngang ln s khuch tn dc

theo chiu dng chy Fischer (1967) kin ngh thay th h s khuch tn ri x bng h s phn tn dc dng chy E . H s ny c xc nh bng cch chia nh mt ct dng chy ra thnh nhiu n v nh c th xt c nh hng ca trng vn tc ti mt ct ln s xo trn v phn tn vt cht.

( ) =yw

y

y

ix ddydydyuduu

AE

00 0

11 (2.11)

Vi :

w : chiu rng ca dng chy ti mt ct ang xt, L ui : vn tc trung bnh qua n v mt ct i ,L.T-1 u : vn tc trung bnh trn ton b mt ct, L.T-1 d : chiu su ca mt ct n v th i , L

5

y du= 0 6, h s khuch tn ri theo phng ngang gia tit din n v mt ct th i v i-1.

Tch phn (2.11) ta c h s phn tn theo chiu dng chy ti mt ct ngang ca dng chy.

T cc s liu thc nghim Fisher xut cng thc kinh nghim tnh h s phn tn dc Ex ti mt ct ngang dng chy.

= duuWE x

22011.0 (2.12)

2.2. S chuyn ho cc cht trong dng chy Ga thit rng, cc cht xm nhp vo dng chy knh, sng qua nc thi v cc

ngun thi c kch thc nh (so vi dng chy sng) c th coi nh l cc ngun

im. Do trong nc thi c nng cc cht cao hn trong ngun nc nn s xut hin

xu th cn bng nng bng cch vn chuyn cc cht t nc thi sang nc sng.

Qa trnh vn chuyn cht c xy ra bng hai cch :

-Qu trnh vn chuyn cht do dng chy (advection)

-Qu trnh khuch tn cc cht do dng chy ri (turbulent disfusion).

Ngoi ra, tu thuc ch thu lc ca dng chy, tnh cht thnh phn cc cht

bn m nng cc cht nhim trong dng chy cn ph thuc cc qu trnh : cc qu

trnh vt l (hp th, hp ph, lng, bay hi..), ho hc (s phn hu, tng tc ho hc...)

v sinh hc (tch t sinh hc, phn hu, chuyn ho...)

S chuyn ha cc cht hu c trong dng chy l tng hp cc qu trnh thu

ng hc, thu ho, sinh hc ...din ra trong ngun nc. Di gc sinh thi, y l

kh nng m bo s t n nh ca h sinh thi khi c tc ng t bn ngoi. Qa trnh

chuyn ha -phn hu cc cht hu c nh cc loi vi sinh vt...

S chuyn ha cc cht hu c trong dng chy c lin quan n tt c cc qu

trnh sinh thi trong h sinh thi dng chy sng. Cc qa trnh sinh thi, mi quan h

gia cc qu trnh din ra trong dng chy khi tip nhn cc cht thi t cc ngun bn

ngoi c m t hnh 2.2

6

T s hnh 2.1 cho thy s thay i nng cc cht hu c d phn hy

trong dng chy c lin quan n cc qu trnh : vt l, ha hc v sinh hc din ra trong

dng chy. Cc qu trnh phn hy sinh hc, chuyn ha u c lin quan n cc qu

trnh sinh thi din ra trong dng chy : s pht trin ca thc vt nc, v cc qu trnh

c lin quan khc ...

2.2.1.Cc qu trnh sinh thi - cht lng nc trong dng chy

2.2.1.To

S pht trin ca to trong dng chy sng

S pht trin ca to trong sng chu nh hng ca cc yu t kh tng, thy

vn, thnh phn ha hc v cc qu trnh sinh hc din ra trong dng chy.

Cu trc chu trnh pht trin ca to nh sau : sn xut bc I chuyn ha cc cht

dinh dng dng v c thnh sinh khi ca to; sn xut bc II ca cc loi ng vt bc

II ni i km vi vic tiu th to. Cht v bi tit s gii phng cc cht hu c dng

Thc vt

Oxy ho tan Bn y

BOD

Cht thi t bn ngoi

Chu trnh Nit

To

Quang hp

H hp

Quang hp

H hp

Cht v phn hu

Lm thong Vn ni Lng

Cht v phn hu

Sinh trng

Hnh 2.1. S cc qu trnh chuyn ho cc cht hu c trong dng chy

7

cht vn v dng ha tan. Cc con ng ti to s chuyn cc cht hu c thnh cc

dng v c l cc cht cn thit cho sn xut bc I.

S pht trin ca to trong nc c biu th bng cng thc sau :

( ) ( )0 A tpp tC C e = (2.13) Trong :

( )pC t -Sinh khi ca to thi gian t (mg/l). ( )0pC -Sinh khi ca to thi im to(mg/l).

A -H s tc pht trin ca to (ngy-1). t -Thi gian (ngy)

ng hc ca qu trnh pht trin ca to ng hc ca qu trnh pht trin ca to lin quan n s chuyn ha cc cht hu c

trong dng chy sng. Phng trnh tng qut

( ) 1dA A Adt d

= (2.14) Trong :

t -Thi gian (ngy)

-H s sinh trng ca to (ngy-1) -H s h hp ca to ph thuc vo nhit , h hp ni sinh v h

hp do qu trnh trao i cht (ngy-1).

1 -H s lng, ph thuc nhit (ngy-1) d -Chiu su ct nc (m)

Mi quan h gia s pht trin to v nng cc cht dinh dng

Cc yu t ch yu nh hng n s pht trin ca to trong dng chy sng bao

gm : nhit , nh sng, cc cht dinh dng, s mt i, s lng ca to, s km hm c

ch qu trnh quang hp v hnh thi dng chy cng nh vn tc dng chy.

nh hng ca cc cht dinh dng nit, pht pho v cng chiu sng i vi

s pht trin ca to c biu th di dng sau :

( ) ( ) ( )max f N F P f L = -Hm ca cc cht dinh dng c biu th qua biu thc ca Michaelis-Menton:

8

( )N

Nf NK N

= +

( )P

Pf PK P

= +

Trong :

N -Nng ca nit v c trong nc, NO3-

P - Nng ca PO43- trong nc, mg/l

kN,kP - Hng s Michaelis ca Nit, Pht pho

max -Tc sinh trng ln nht ca to -nh hng ca nh sng n s sinh trng ca to c th xc nh bng cc biu

thc theo cc hm Monod, Smith v Steel.

Theo hm Monod : ( ) ZZI Z

If LK I

= +

Trong :

Iz - Cng nh sng su z (E/m2s) KI - Cng chiu sng cho s sinh trng ca to t 50% tc ln

nht.

Theo hm Smith 2 2

( ) ZZI Z

If LK I

= +

Trong :

KI -Cng chiu sng cho s sinh trng ca to t 70% tc ln

nht.

Theo hm Steel (1 )

( )Z

s

IIZ

Zs

If L eI

=

Trong :

Iz -Cng nh sng cho s sinh trng ca to l cc i (=max)

Trong tt c cc trng hp trn, cng chiu sng theo su u tun theo

nh lut Beer Lambert vi :

ZZ SI I e =

9

Trong :

IS -Cng nh sng trn b mt.

Z -Chiu su ct nc.

nh hng ca nhit

S ph thuc vo nhit ca tc pht trin ca to c xem xt bng phng

trnh ca ONeill

1 max

max opt

T TXT Tmax

Tmax opt

T Tf XeT T

=

Vi ( ) 20.52 11 1400

w wX

+ + =

( )10 10ln ; 1.85max optw Q T T Q= = Tmax v Topt l nhit gy cht v nhit pht trin ti u ca to. Vi to lc,

cc gi tr ln lt l 45oC v 27oC.

2.2.1.2.Chu trnh nit trong ngun nc v qa trnh nitrat ha Chu trnh tun hon nit

Nit cng vi pht pho v cc bon l cc thnh phn dinh dng ch yu nh

hng n s sn xut trong thy vc nc. Tn ti trong nc di mt s dng nh

Nit hu c, nit amn, nitrit, nitrat...Chu trnh nit trong nc c m t trong hnh

2.2.

Qa trnh nitrat ha

Qa trnh amn ha cc hp cht hu c c cha nit, nh ur CO(NH2)2, nhm

amin...do t cc ngun thi a vo dng chy c thc hin bi cc vi sinh vt gy thi

ra nh cc loi Pseudomonas Flucrecens, P.aerugisa, Protens-Vulgarie...theo cc phn

ng thy phn sau.

( )2 2NH CO H O+ ( )2 2NH CO H O+ ( )2 2NH CO H O+ ( )2 2NH CO H O+

10

Sau trong nc xy ra qu trnh nitrat ha, chuyn ha amomonia thnh nitrat.

y l qu trnh hai giai on c thc hin bi cc vi sinh vt t dng nh

Nitrosomonas, nitrobacter chng s dng cc bon v c (CO2) l ngun ccbon.

Cc phn ng c trng cho qu trnh ny c biu th bng cc phng trnh sau

NH4+ + O2 2 2NO H H O ++ + 2 2NO O + 3NO

Qa trnh oxy ha nitrit thnh nitrat thng din ra rt nhanh hn nhiu so vi qu

trnh nitrat ha. Phn ng ca qu trnh c th c vit li l.

4 2NH O+ + 3 2NO H O H ++ +

S chuyn ha NH4+ thnh NO3- i km vi vic tiu th mt lng ln oxy ha

tan, v vy qu trnh ny c nh hng n cn bng oxy trong dng chy.

Trong dng chy sng ngoi ra cn mt qu trnh quan trng na l qu trnh

tng tc trao i gia nit trong dng chy v nit trong cc lp bn y. qu trnh

ny c th hin hnh trn.

Nit hu c NH4-N NO2-N N03-N Thy phn

Nitro- somonas

Nitro- bacter

Ngun thi Nit hu c

Ngun thi NH4+

oxy N2 oxy Ngun thi NO3-

Nit thc vt Nit ng vt

Cht

Kh Nitrat

Hnh2.3.Chu trnh nit trong ngun nc sng

11

Cc yu t nh hng n qu trnh

Qa trnh nitrat ha trong sng ph thuc vo cc yu t mi trng. Cc yu t

nh hng ch yu n qu trnh ny l nng cc cht nn NH4-N, NO2-N, oxy ha tan

cng nh cc iu kin nhit , pH cho s pht trin ca cc loi vi sinh vt tham gia

qu trnh ny.

Mi quan h ca cc yu t n qu trnh nitrat ho c biu th bng cng thc

ca Michaelis-Mentons

( )4 2

4 22

4 2

, , ...maxN NNH NO

NH NO f T O BODk NH k NO

= + +

Trong :

N -H s tc pht trin ca vi sinh vt nitrat maxN -H s tc pht trin ln nht ca vi sinh vt nitrat 4 2,NH NO -Nng ca NH4 v NO2

4 2,NH NOk k -Hng s Michaelis ca NH4 v NO2

( )2, , ...f T O BOD -Hm xt n s ph thuc ca qu trnh nitrat ha vo nhit , oxy ha tan, nng cc cht hu c theo BOD v vn tc dng chy.

Cc yu t nh hng ch yu n qu trnh nitrat ha trong dng chy l pH,

nhit , oxy ha tan, cc cht c cng nh hnh thi ca thy vc nc, chuyn ng

ri v nh sng.

nh hng ca pH

Hiu sut ti u ca cc vi sinh vt tham gia qu trnh nitrat ha t c khi gi

tr pH nm trong khong t 7.5 n 8.5

Trong cc ngun nc sng nghin cu, gi tr pH thng nm trong khong gi

tr kim yu. Khi qu trnh quang hp din ra mnh s lm tng gi tr pH, trong khong

thi gian rt ngn gi tr pH c th t c cc gi tr trn 9. Theo Hubber (1984) th

hiu sut ca qu trnh nitrat ha s b gim xung r rt khi gi tr pH cao xut hin trong

mt on ngn.

nh hng ca nhit i vi qu trnh nitrat ha

12

Tc pht trin ca cc vi sinh vt nitrat chu nh hng rt ln bi nhit .

Nhit ti u l 20-30oC. nhit nh hn 5oC hoc nhit ln hn 45oC qu trnh

xy ra rt yu.

Hm lng oxy ha tan thp nh hng n tc ca qu trnh c th biu th

bng quan h Warwick.

nh hng ca lng oxy ha tan n qu trnh nitrat ha

Hm lng oxy thp nh hng n tc ca qu trnh nitrat ha. S ph thuc

ca tc ca qu trnh nittrat ha vo hm lng oxy ha tan c biu th bng phng

trnh phn ng ng hc enzzym ca Michaelis-Mentens

maxDO DODO

DOk DO

= +

Trong :

DO -H s nh hng ca oxy n qu trnh nitrat, mg/l/ngy. maxDO -H s nh hng ln nht ca oxy n qu trnh nitrat ha,

(mg/l/ngy)

DOk -Hng s Michaelis ca oxy, DOk =2mg/l (EPA, 1975)

Trong trng hp nng oxy ha tan qu nh c th ly 0DO = nh hng ca nng cht hu c BOD i vi qu trnh nitrat ha

S ph thuc ca qu trnh nitrat ha vo gi tr BOD c biu th bng phng

trnh Curtis,1994

5 5

1.2 0.035BOD BOD = nh hng ca cc cht c ch, cc cht hu c d phn hy v nh sng

Cc cht km hm nh fomat, glucose lm tng tc pht trin ca nitrosomonas

(Bock, 1980) cn cc cht pyruvat, acetat lm tng tc pht trin ca nitrobacter.

Formiat, acetat, glucose, pepton c tc dng km hm qu trnh oxy ha cc hp cht

amn.

nh hng ca hnh thi dng chy v vn tc dng chy

13

Qa trnh nitrat ha xy ra theo hai cch do cc vi sinh vt l lng cun theo dng

chy v cc vi sinh vt dnh bm. Hiu sut ca qu trnh ph tuc ch yu vo cc loi vi

sinh vt nitrat ha c dng thc vt hoc kh nng gn kt vo cc ht l lng, cc gi th

c sn trong dng chy.

ng hc ca chu trnh nit trong dng chy

Qu trnh thy phn

1 3 4org

org org

dNA N N

dt = (2.16)

Trong :

orgN -Nng cc hp cht hu c cha Nit, mg-N/l.

1 -H s t l nit trong sinh khi to, mg-N/mg-A. -H s h hp ca to, ngy-1

4 -H s lng ca cc hp cht hu c, ngy-1 3 -H s thy phn ca cc hp cht hu c, ngy-1 A -Nng sinh khi to, mg-A/l

Qa trnh ammn ha

343 1 4 1org

dNH N NH F Adt d

+

+= (2.17)

Trong :

4NH+ -Nng cc hp cht nit dng ammn, mg-N/l.

1 -Hng s tc ca qu trnh oxy ha 4NH + sang 2NO 3 -Ngun 4NH + trong lp bn y F -H s t l 4NH + tiu th do to

-H s tc sinh trng ca to, ngy-1 Qa trnh Nitrit ha

14

21 4 2 2

dNO NH NOdt

+ = (2.18 )

Trong :

4NH+ - Nng cc hp cht nit dng ammn, mg-N/l

2NO - Nng cc hp cht nit dng nitrit, mg-N/l

1 -Hng s tc ca qu trnh oxy ha 4NH + sang 2NO , ngy-1 2 - Hng s tc ca qu trnh oxy ha 2NO sang 3NO , ngy-1

Qa trnh Nitrat ha

3 2 2 1(1 )dNO NO F A

dt

= (2.19)

Trong :

(1 )F -H s t l 3NO tiu th do to 2.2.1.3. S phn hy cc cht hu c

Qu trnh phn hu sinh ho cc cht hu c

Tc phn hy sinh ha cc cht hu c trong dng chy ph thuc vo tc

pht trin cc vi sinh vt, s pht trin ca cc vi sinh vt t l vi tc pht trin ln

nht maxB v ph thuc vo nng cht nn. Qa trnh ny c biu th bng phng trnh ng hc Monod.

maxB B

s

Sk S

= + (2.20)

.maxB Bs

dS S BBdt Y Y K S

= = +

BdB Bdt

= Trong : S -Nng cht nn

B -H s tc pht trin ca vi khun Y -Sn lng vi khun

B -Nng sinh khi vi khun

ks -Hng s Michaelis

15

i vi nhiu cht nn, ks c gi tr c 10-1 g/ml. Nu gi tr ny ln hn ng k so vi S , s bin mt ca cht nn l qu trnh bc mt theo c B v S.

ng hc ca qu trnh phn hy cht hu c

Gi thit tc phn hy cc cht hu c tun theo quy lut ca phn ng bc

mt. Qa trnh phn hy cc cht hu c trong dng chy c m t bng phng trnh.

ddL k Ldt

= (2.21)

Trong :

kd - Hng s tc phn hy cc cht hu c, ngy-1

L -Nng cc cht hu c theo BOD, mg/l

Hng s ny thay i theo thnh phn cc cht hu c. Hng s c gi tr cng ln,

tc phn hy cng nhanh.

S ph thuc vo nhit T ca hng s tc phn hy cc cht hu c c

xc nh bng cng thc Van't Hoft - Arrhenius : ( )k kd d T= 20 20 (2.22)

Trong :

kd -Hng s tc tiu th oxy -phn ng phn hy cc cht hu c (T-1)

kd,20 -Hng s tc tiu th oxy nhit 20oC.

-H s thc nghim ( )048,1 nh hng ca dng chy n s thay i nng cc cht hu c

Trong dng chy t nhin, s thay i nng cc cht hu c ph thuc vo ch

thy lc ca dng chy (vn tc dng chy, ch xo trn khi nc thi vi nc

sng) c nh hng ca yu t dng chy n s thay i nng cc cht hu c

trong dng chy. Do cc qu trnh nh : thay i nng do s keo t, lng cc cht hu

c dng phn tn nh.

S khi m hnh s thay i nng cc cht hu c theo BOD trong dng

chy

BOD Phn hy hiu kh

Lng

Cc ngun thi im, phn tn

16

ng hc ca qu trnh

( )1 3dL k k Ldt = + (2.23 ) Trong :

L -Nng cc cht hu c theo BOD (mg/l)

1k ,kd -Hng s tc phn hy cc cht hu c (ngy-1)

3k , ks -H s tc gim cc cht hu c do qu trnh lng (ngy-1).

H s tc chuyn ho cc cht hu c trong dng chy(kr=kd+ks=k1+k3)

2.2.1.4.Cn bng oxy trong dng chy

Cn bng oxy ha tan

Cn bng oxy trong dng chy sng l kh nng cha v qu trnh ha tan oxy t

khng kh qua b mt thong ca dng chy.

Hnh. 2.5.S cn bng oxy trong ngun nc

oxy ha tan

Trao i oxy t nhin

Nitrat ha Kh nitrat ha

Nhu cu oxy do h hp

Nhu cu oxy ca bn y

Nhu cu oxy phn hy sinh ha

B sung do qu trnh quang hp

17

Cc qu trnh nh hng n nng oxy ha tan

Cc qu trnh tiu th oxy trong dng chy bao gm : qu trnh nitrat ha v kh

nitrat ha, qu trnh h hp ca h thc vt nc, qu trnh oxy ha cc cht lp bn

y v s phn hy cc hp cht hu c.

Cc qu trnh b sung lng oxy ha tan bao gm : qu trnh ha tan v khuch tn

oxy t mi trng khng kh qua b mt thong, qu trnh quang hp ca h thc vt

nc.

Nh vy, t cc phn tch cc qu trnh sinh ha trong dng chy ta c phng

trnh tng qut m t cn bng oxy ha tan trong dng chy :

( ) ( ) ( ) ( )42 3 4 1 5 1 4 6 2 2BH kdDO k DO DO A k L NH NOdt d + = + (2.25) Trong :

k2 (ka) - Hng s tc khuch tn oxy qua b mt thong, ngy-1

k1(kd) - Hng s tc phn hy cc cht hu c, ngy-1

A - Nng sinh khi to, mgA/l

k4 - Nhu cu oxy ca qu trnh h hp trong lp cn y,

mg/m2.ng.

DO - Nng oxy ha tan, mg/l

DOBH - Nng oxy ha tan trng thi bo ha, mg/l

L - Nng cht hu c theo BOD, mg/l

d - Chiu su ct nc trung bnh, m

3 - H s tc sn xut oxy do qu trnh quang hp ca mt n v sinh khi to, mgO/mgA.

4 - H s tc tiu th oxy ca qu trnh h hp mt n v sinh khi to, mgO/mgA.

5 - Hng s tc tiu th oxy khi oxy ha mt n v NH4+ (mgO/mgN)

18

6 - Hng s tc tiu th oxy khi oxy ha mt n v NO2- (mgO/mgN)

- Hng s tc sinh trng ca to 1 - Hng s tc ca qu trnh oxy ha 4NH + sang 2NO ,ngy-1

2 - Hng s tc ca qu trnh oxy ha 2NO sang 3NO , ngy-1 - H s h hp ca to, ngy-1

Qa trnh khuch tn oxy qua b mt thong

Qa trnh ha tan oxy t khng kh vo nc sng c biu th bng h s ka. H

s ny ph thuc vo mc thiu ht oxy trong dng chy, tc ha tan theo nh lut

Henry. Cc yu t ph thuc bao gm : p sut, nhit , din tch b mt thong v

mn...

Trong dng chy sng, ph thuc vo s chuyn ng ca khi dng chy, nhit

mi trng khng kh, nhit nc v s c mt ca cc cht hot tnh b mt trong

ngun nc sng v c xc nh bng cc cng thc thc nghim i vi cc dng

chy c cc ch thy lc khc nhau.

Cc cng thc thc nghim v hng s tc thng kh (reaeration) trong dng

chy thng c biu th theo tc dng chy v chiu su ct nc hoc s phn tn

cc cht v chiu su ct nc.

Cc cng thc thc nghim Hm ca tc dng chy v chiu su ct nc.

O'Connor v Dobbin's k uHa

= 12 90 5

1 5

, ,, (2.26)

Owen - Edwards - Gibbs k uHa

= 230 75

1 75

,

, (2.27)

Churchill - Elmore - Buckingham k uHa

= 111 67, (2.28)

Trong :

ka -Gi tr logarit t nhin -hng s tc ti ha tan oxy(ngy-1)

:u -Vn tc trung bnh dng chy, m/s

H -Chiu su trung bnh, m.

19

Q -Lu lng ca dng chy, m3/s

t -Thi gian, ngy-1.

Hm ca tc dng chy, s phn tn v chiu su ct nc.

5.05.1

5.0

xa EHuk = (2.29)

Chng 3

THIT LP V LA CHN M HNH

CHT LNG NC 3.1. Cc bc thc hin xy dng m hnh cht lng nc 3.1.1.Bc u

- Mc ch ca vic xy dng m hnh, vn t ra nghin cu l vn

g, tnh cp thit ca vn cn nghin cu.

- Vn nghin cu: ci g s chuyn bin, thay i v s xy ra trong dng

chy m chng ta nghin cu.

- Kh nng cho php chng ta nghin cu. Cc yu t cn xc nh cng

nh kh nng v iu kin cho php chng ta c th xc nh c...

- Cc vn chnh cn xc nh: nht, t trng, dng chy, h s chuyn

ho hay l cc qu trnh ph dng ho cn xem xt...Cc vn cn i su

nghin cu, cc vn cn gii hn.

3.1.2. Bc tip theo

- Cn nhc la chn m hnh nghin cu : m hnh vt l hay m hnh

s (numericl model).

M hnh vt l: tn km do vic phi xy dng mt m hnh thu nh,

chi ph vn hnh v hiu chnh cc thng s m phng

iu kin t nhin ...tn km v i hi cc nhm

chuyn gia c kinh nghim lu nm trong lnh vc ny.

M hnh s: cn nhc la chn gia m hnh 1D,2D v 3D. Khong

thi gian tnh ton vi cc h s. Trung bnh trong mt

chu k thi gian hay tc thi.

M hnh 1 chiu (1D) : kinh t tho mn yu cu. Vic tnh ton n gin v c

th tnh tay. Gip chng ta c th xc nh nhanh v c th s b xc nh c

cc cc h s trong qu trnh kho st v o c thc nghim hiu chnh m

hnh.

Cc loi m hnh ny rt ph bin do s liu u vo n gin hn rt nhiu.

Thch hp cho cc nghin cu ban u.

Khi vn nghin cu tr nn phc tp hn rt nhiu, m hnh 1D khng

p ng c yu cu ca vn t ra th trn c s m hnh 1D c th d dng

pht trin ln 2D, 3D.

Nu cm thy s dng ri th c th dng li y. Hay ni mt cch

khc m hnh mt chiu l m hnh c s ca m hnh cht lng nc. c bit

i vi cc dng chy sng, khi khng c s phn vng trong dng chy theo cc

mt ct ngang. Cn theo chiu thng ng y th thng c th b qua do chiu

su thng nh hn rt nhiu so vi chiu rng.

Cc m hnh 2,3D: kt qu cho chi tit hn, tuy nhin khi xy dng thng

rt kh xc nh cc h s...Khi s dng i hi rt nhiu cc hiu bit su v c

th v vn m chng ta nghin cu. Cha chc cho chng ta cc kt qu nghin

cu tt hn nhng chc chn cho chng ta hnh nh s p hn v d nhn hn...

3.2. La chn m hnh cht lng nc 3.2.1. La chn m hnh Vic la chn m hnh cht lng nc : cn xem xt m hnh no l thch

hp cho i tng m chng ta nghin cu da trn c s l thuyt.

Thng vn ny c ngi khc la chn vn ny gip chng ta

ri. l cc m hnh s dng rt rng ri gn hn 100 nm ca lch s pht

trin MHCLN.

Tuy nhin cn lu : trong cc m hnh cht lng nc s dng rng ri th

cc h s, cc sai s trong iu kin thc t ca h c hiu chnh trong iu

kin c th ca h.

Nh vy, chng c s tha nhn trong con mt ca rt nhiu ngi (nhng

ngi tha nhn m hnh). Nhng khi p dng vo iu kin thc t ca chng ta

s xut hin cc vn :

-Kinh ph y mua ton b chng trnh tnh ton.

-Cc d liu, cc h s c th thu thp v xc nh trong iu kin c th

trn i tng ca bn hay khng. Nu khng c s xy ra vn l : i tng

nghin cu ca bn mt phn s trong iu kin ca a phng nghin cu v

mt phn s nm ti a im ca ngi xy dng chng trnh ny.

Qa trnh lan truyn cht

D liu Chui s

ng dng s b

Kim tra,so snh vi cc s liu

Hiu chnh

p dng vo tnh ton

Kim tra

Sa cha

Chui s liu

Chui s liu

Chui s liu

M hnh so snh

Pht trin l

thuy

Phng

php s

L thuyt

M hnh mi Tng qut tnh

ton

D bo cht lng nc

Cht lng nc thc t

Hnh 3.1. S thit lp m hnh cht lng nc

Vn ny, c th v d nh mt chic t c th l rt thch hp, rt tt trong

iu kin n i nhng n c th tr thnh khng c gi tr trong iu kin nhit

i. Khng c sn phm thng mi no m c th p dng cho mi i tng.

Nh vy khi mun s dng mt chng trnh c sn cn thit phi c hiu

rt su v cc gii hn, cc iu kin bin, cc h s ca chng trnh c th ci

tin v hiu chnh cn thit.

iu kin tin quyt y l: bn phi xy dng v la chn cc h s

trong iu kin c th ca bn.

3.2.2. S pht trin l thuyt ca vn Vic u tin cn pht trin l thuyt ca vn ca bn vi i tng c

th m cn xem xt v nh gi. Pht trin l thuyt cc vn : phng trnh

chuyn ng, phng trnh lin tc v phng trnh lan truyn cht.

Quan im ca Albert Einstein: mi m hnh cn phi tho mn yu cu

ca thc tin v cng n gin cng tt, tr phi khng th n gin hn.

3.2.3. Kim nh v hiu chnh Bng cc chui s liu thc o.

V d: m hnh tnh ton s lan truyn cc cht t cc ngun thi im vo dng

chy, iu kin chn la l ma h. Cc chui s liu c kim nh, cc h s s

thay i so vi iu kin cc ma khc s khng cho chng ta gi tr xp x tt

hiu chnh.

Sau khi s dng cc chui s liu hiu chnh cc h s s thay i. Khi

hiu chnh cn la chn mt vi h s n nh hiu chnh cc h s khc v sau

tin hnh hiu chnh cc h s c nh hoc hiu chnh tt c cc h s ng

thi.

Cc iu kin cn lm r khi hiu chnh:

- iu kin bin v ti trng.

- iu kin ban u.

Cc qu trnh c trng cho i tng (qu trnh m chng ta cn m t v lm

r). Cc cht hu c : qu trnh vt l, ho hc hay sinh hc trong dng chy.

Cc h s cn kim nh: ng hc, h s, cc h s c trng cho qu trnh

chuyn ho cc cht trong dng chy.

3.3. Cc phng php s tnh ton s lan truyn cc cht nhim trong

dng chy 3.3.1.Cc phng php s trong nghin cu m hnh thy lc

Trong t nhin dng chy thng khng n nh, cc yu t thy lc

ti mt mt ct thay i theo thi gian (chuyn ng khng dng). Chuyn ng

khng n nh c chia thnh hai loi: chuyn ng khng n nh thay i gp,

v chuyn ng khng n nh thay i chm dn.

Phng trnh c bn ca dng chy l tp hp h thng hai phng trnh

o hm ring phi tuyn c cc h s bin i dng Hyperbolic. Phng php s

gii loi phng trnh ny gp mt s tr ngi trong vic t cc iu kin bin.

Vic tch phn h phng trnh ny gp nhiu kh khn, ch c th tm

c nghim trong mt vi trng hp c bit nh knh c tit din hnh ch

nht, y nm ngang v b qua sc cn. Cc iu kin ny rt khc xa so vi iu

kin thc t.

Hin nay, cc phng php s c s dng tnh gn ng cho dng n

nh thay i chm dn bao gm : phng php gii tch, phng php ng c

trng v phng php sai phn hu hn.

Phng php tch phn ton hc cht ch c s dng tm nghim gii

tch ca h phng trnh trn. Theo phng php ny, cn c nhng gi thit

a h phng trnh Saint-Vennnant v dng n gin, ri tch phn cc phng

trnh ny vi cc gi thit l mt ct sng hnh ch nht hay lng tr, dc bng

khng hoc bng hng s, nhm c nh, b qua sc cn...

Nh cc gi thit trn, cc phng trnh vi phn o hm ring phi tuyn c

bin i thnh phng trnh vi phn o hm ring tuyn tnh. gii c cc

bi ton dng khng n nh.

Phng php sai phn hu hn

Phng php sai phn l bin i t h phng trnh vi phn o hm ring

Saint-Vennant thnh h phng trnh i s phi tuyn do tnh cht phi tuyn ca

h ng trnh Saint-Venannt. Sau thc hin vic tuyn tnh ha h phng trnh

i s phi tuyn v gii h phng trnh thu nhn c cc gi tr v cc yu t

thy lc cn bit trn cc nt li tnh ton.

Phng php sai phn gii bi ton thy ng lc mt chiu c cc

tc gi nghin cu, nh J.A.Cung, C.Lai, V.Bellos, F.Ionescu...vi cc phng

php sai phn theo cc s khc nhau.

C hai phng php sai phn : phng php sai phn theo s n v

phng php sai phn theo s hin. Mi mt phng php c nhng mt mnh

khc nhau.

Phng php sai phn s hin

Nu sau php sai phn h hai phng trnh i s vi hai n s v c th

gii ngay c hai n s hoc gi tr ca hm n ti cc nt li c xc nh

mt cch ring l khng cn cc phng trnh ti cc nt li khc th s sai

phn c gi l s hin. Cc s sai phn : tam gic cn ngc, tam gic

cn thun, s hnh thoi trung tm...u im ca s hin l s n gin ca

vic tnh ton v lp chng trnh. Nhng l khng cho chng ta tnh ton vi cc

bc thi gian ln, v s sai phn n nh gi tr bc thi gian b hn ch

bi iu kin CFL(Courant-Friedric-Levi) dn ti vic la chn thi gian tnh ton

b khng thch hp cho cc bi ton yu cu tnh ton vi cc chu k di.

Phng php sai phn s n

Trong trng hp sau khi sai phn, s hm n ln hn hai th phi p dng

sai phn cho mt lp cc im ri rc. Cng vi cc iu kin bin sau khi tuyn

tnh ha ta c h phng trnh i s tuyn tnh ng kn. Gii h phng trnh

i s ny ta c nghim mt lot cc im m chng ta cn tnh ton. Vi s

n vic tnh ton cc gi tr hm n ti mt nt no i hi phi gii ng thi

mt h phng trnh ca tt c cc nt cng mt thi im, cn bc thi gian

c chn theo quan im ca chnh xc ch khng phi theo tnh n nh. Cc

s n tr nn rt thun tin cho vic tnh ton dng chy khng n nh trong

mng knh sng c lng dn phc tp, nhc im l vic gii h phng trnh ti

mi bc thi gian cn phi tnh lp, c khi lng tnh ton ln, i hi b nh

v thi gian tnh ton ln hn.

Vi s n vic tnh ton cc gi tr hm n ti mt nt no i hi

phi gii ng thi mt h phng trnh ca tt c cc nt cng mt thi im,

cn bc thi gian c chn theo quan im ca chnh xc ch khng phi

theo tnh n nh. Cc s n tr nn rt thun tin cho vic tnh ton dng chy

khng n nh trong mng knh sng c lng dn phc tp, nhc im l vic

gii h phng trnh ti mi bc thi gian cn phi tnh lp, c khi lng tnh

ton ln, i hi b nh v thi gian tnh ton ln hn.

3.3.2. Phng php s gii bi ton lan truyn cht

Cc phng php gii tch hu nh khng th p dng tm nghim tng

qut ca bi ton Ti-khuch tn trong trng hp tng qut. Ch c p dng

trong trng hp vi gi thit dng chy vi ch n nh...Cch gii ph bin

l cc phng php s : phng php sai phn hu hn, phng php phn t hu

hn. Hai phng php ny c nhng u v hn ch khc nhau.

Phng trnh ti-khuch tn l phng trnh dng Parabolic, c gii bng mt

s phng php s quen thuc, nh phng php sai phn hu hn, phng php

phn t hu hn, phng php ng c trng. Cc phng php sai phn cho

bi ton loi ny : phng php Lax-Richardson, phng php Upwin, phng

php Leonard, phng php Duford-Frankel, phng php Lax-Wendroff, phng

php Crank-Nicolson, s Brian-Stone, phng php sai phn theo hng,

phng php sai phn trung tm.

3.4.Cc m hnh BOD & D (DO) trong dng chy 3.4.1. Phng trnh c bn

Phng trnh c bn m t s lan truyn v phn b vt cht trong dng

chy c gi l phng trnh ti-phn tn vt cht trong dng chy. Trn c s

nh lut bo ton khi lng ta c :

)(sFxCE

zyCE

yxCE

xxCu

xCu

xCu

tC

xyxzyx +

+

+

=+++

(1.1)

Trong :

C -Nng cc cht trong dng chy, mg/l.

ux,uy,uz -Thnh phn vn tc trong dng chy, m/s.

Ex, Ey, Ez -H s phn tn ri vt cht trong dng chy, m2/s.

F(s) -Hm s m t s thay i cc cht trong dng chy do

cc qu trnh vt l, ha hc v sinh hc.

Vi gi thit, b qua s ti do vn tc dng chy, qu trnh khuch tn theo

cc phng oy, oz t phng trnh (1.1) ta c phng trnh m t qu trnh lan

truyn-khuch tn cc cht hu c ti mt im trong dng chy.

)(sFxCE

xxCu

tC

xx +

=+

(1.2)

Trong cc nghin cu ban u cng nh ng dng cc MHCLN cho cc

dng chy sng rng, ca sng thng c xem xt trong khng gian mt chiu.

3.4.2. Phng trnh c in Streeter-Phelps

Cc nghin cu v s lan truyn, chuyn ha cc cht hu c trong dng

chy u da trn c s phng trnh c in Streeter-Phelps vi cc gi thit sau:

-Tc phn hu cc cht hu c tun theo quy lut phn ng bc nht.

-S thiu ht oxy ho tan (D) trong dng chy do s phn hu cc cht hu

c.

-Ch lan truyn cht trong dng chy n nh.

Nm 1925, Streeter v Phelps thit lp phng trnh ton hc m t qu

trnh tiu th oxy do s phn hy cc cht hu c trong dng chy.

Phng trnh Streeter-Phelps :

u dLdx

k Ld= (1.3a)

DkLkdxdDu ad += (1.3b)

Trong iu kin n nh, gii h phng trnh (1.3a,b) bng phng php

tch phn cho kt qu v phn b nng cc cht hu c v thiu ht oxy ho

tan (BOD &D) vng h lu ngun thi.

uxk

o

d

eLL = (1.4a)

+= u

xku

xk

da

oduxk

o

ada

eekk

LkeDD (1.4b)

Trong :

u -Vn tc trung bnh ca dng chy, m/s.

D0 - thiu ht oxy, mg/l.Ti im c to x=0 (im xo

trn nc thi vi nc sng).

D - thiu ht oxy, mg/l.

L0 - Nng cc cht hu c theo BOD, mg/l. Ti im c ta

x=0 (im xo trn nc thi vi nc sng).

L - Nng cc cht hu c theo BOD, mg/l.

kd - Hng s tc tiu th oxy do s phn hu hp cht hu

c, ngy-1.

ka - Hng s tc ha tan oxy qua b mt thong, ngy-1.

Bi tp :

1. tnh ton, v ng cong BOD v DO hoc D trong dng chy n nh.

3.4.3. Cc nghin cu pht trin trn c s phng trnh Streeter-Phelps

M hnh Streeter-Phelps (1.3a,b &1.4ab) ch k n cc yu t tiu th oxy

do qu trnh h hp ca vi khun hiu kh trong tng nc v ha tan oxy qua b

mt thong khi trong nc c s thiu ht oxy ha tan. Khi p dng cho cc i

tng khc nhau cho thy, tin cy ca m hnh cn thp do cha cp n cc

yu t nh hng n phn b BOD & DO trong dng chy nh : qu trnh quang

hp v h hp ca h thc vt nc, h hp trong tng cn y, cc qu trnh

ammn ho, nitrit, nitrat...cng nh nh hng ca dng chy n s lan

truyn cc cht trong dng chy nh : qu trnh lng cc cht l lng, s phn tn

cc cht do dng chy ri, nh hng ca mn v gi...

Nhm mc ch hon thin hn, nng cao tin cy ca m hnh Streeter-

Phelps i vi cc dng chy trong t nhin, cc tc gi OConnor, Dobbins,

Thomat, Camps, Di Toro...bng cc nghin cu thc nghim b sung thm cc

qu trnh sinh thi-cht lng nc nhm mc ch m t mt cch tng qut, y

cc yu t c lin quan n phn b, thay i BOD&DO trn dng chy.

Qa trnh h hp ca lp bn y

Gi thit dng chy c lp nc nng vi s tch lu nhiu cc cht hu c

y dng chy, t cc phng trnh (1.3a,b) ta c:

HSDk

xDu

tD

a =+

(1.5)

Phn b BOD & D trong iu kin n nh l (1.4) v:

+

=u

xkHk

Su

xkDD a

a

ao exp1exp (1.6)

Trong :

S - Nhu cu oxy ca lp cn y, mgO/m2.ngy.

H - Chiu su lp nc, m.

Qa trnh oxyho cc hp cht cha nit

B sung thm qu trnh tiu th oxy do qu trnh oxy ho cc hp cht hu

c cha nit (qu trnh nitrat ho), t (1.3a,b) ta c:

NkDkdxdDu na = (4.5)

thiu ht oxy D c xc nh theo:

+

=

uxk

uxk

kkNk

uxk

DD anna

onao expexpexp (4.6)

Trong :

N - Nng cc hp cht cha Nit theo NBOD, mg/l

kn -Hng s tc tiu th oxy do qu trnh nitrat ha, ngy-1.

Qa trnh quang hp-h hp

Cc phng trnh ton hc m t s thiu ht oxy do s phn hu cc cht

hu c v qu trnh quang hp-h hp ca h thc vt nc.

tba PRDkxDu

tD )( =

+ (1.9)

S thiu ht oxy trong iu kin n nh :

+

=u

xkk

PRu

xkDD a

a

ao exp1exp

Trong :

P - Sn phm s cp ca qu trnh quang hp, mg/l.ngy

R - Lng oxy tiu hao do qu trnh h hp, mg/l.ngy

Cho v d tnh ton

Qa trnh lng cc cht l lng

Ga thit, trong qu trnh lan truyn mt phn cc cht hu c dng l lng,

phn tn lng ng, trong trng hp ny, s lan truyn cc cht hu c c m t

theo cc phng trnh sau:

LkdxdLu r= vi kr = kd + ks (3.7)

DkLkdxdDu ad = (3.8)

Tch phn phng trnh (3.7) v th vo (3.8) ta c:

= uxkLL rO exp (3.9)

+

=u

xku

xkkk

Lku

xkDD ar

ra

Odao expexpexp (3.10)

Trong :

kr -H s tc chuyn ho cc cht hu c trong dng chy,

ngy-1.

ks -H s tc thay i nng cc cht hu c do cc qu

trnh lng ng, hp th cc cht hu c trong dng chy,

ngy-1.

Qa trnh phn tn vt cht trong dng chy

Cc sng rng v vng ca sng vi s nh hng ca gi, dng triu c ch

xo trn ht sc phc tp vic m phng cn xem xt v nh gi ng thi

nh hng ca cc qu trnh chnh ln s phn tn cc cht nhim trong dng

chy. Cc qu trnh chnh bao gm : s phn tn ri vt cht do ch xo trn

phc tp. Cc phng trnh c bn ca qu trnh :

Lkx

xExLu

tL

r=

+

2

2

(1.10)

DkLkxDE

xDu

tD

ar +=

+

2

2

(1.11)

Phn b BOD & D trong iu kin n nh :

+

+= 2

2

411

2exp

41

uEk

Eux

uEk

Q

WL dd

( ) ( ) ( )

= 2211

12

exp112

exp1 mE

uxm

mE

uxmQkk

WkD

da

d

Vi 214

1u

Ekm d+= ; 224

1u

Ekm a+=

Trong :

Q - Lu lng dng chy, m3/s.

W - Ti trng thi ca cc cht nhim t ngun thi, mg/s.

E - H s phn tn, m2/s.

Cho v d tnh ton

Phng trnh tng qut

M hnh tng qut cho dng chy knh, sng, sng rng v ca sng.

LA

QdtdL

xLE

xL

AQ

tL w++

=+

2

2

(1.4)

DOA

Qdt

dDOxDOE

xDO

AQ

tDO w++

=+

2

2

(1.5)

Vi :

( )LkkdtdL

sd +=

( ) ( )PRHSNkLkDODOk

dtdDO

ndBHa =

Trong :

Q - Lu lng dng chy sng, m3/s.

Qw - Lu lng dng chy gia nhp c cha cht nhim, m3/s.

L - Nng cc cht hu c theo BOD, mg/l.

kr - H s tc chuyn ha cc cht hu c trn on sng,

ngy-1.

ka - Hng s tc phn hy cc cht hu c, ngy-1.

ks - H s tc lng cc cht hu c trong dng chy, ngy-1.

kd - Hng s tc phn hy cc cht hu c, ngy-1.

DOBH - Nng oxy ha tan trng thi bo ha, mg/l

DO - Nng oxy ha tan, mg/l.

kn - Hng s tc phn hy cc hp cht cha Nit ngy-1

N - Nng cc hp cht cha Nit theo NBOD, mg/l

P - Sn phm s cp ca qu trnh quang hp, mg/l.ngy

R - Lng oxy tiu hao do qu trnh h hp, mg/l.ngy

S - Nhu cu oxy ca lp cn y, mg/m.ngy.

H - Chiu su trung bnh ca dng chy, m.

Ex - H s phn tn dc theo chiu dng chy, m2/s

Cc phng trnh (1.4), (1.5) m t mt cch tng qut v phn nh mt

cch y nht cc qu trnh c lin quan n s thay i ca BOD&DO trong

dng chy. T cc phng trnh tng qut trn, ty thuc tng hon cnh c th,

mc ch nghin cu m la chn m hnh thch hp i vi cc i tng nghin

cu khc nhau. Vic la chn m hnh cho mt i tng nghin cu c th, da

trn c s c im : ch thy lc dng chy, cc qu trnh sinh thi cht lng

nc din ra trong dng chy. Cc m hnh cht lng nc cho cc dng chy khc

nhau bao gm : knh, mng thot nc thi th, dng chy sui, sng nh, sng

rng v ca sng.

1

Chng 4

XY DNG M HNH LAN TRUYN V CHUYN HA CC

CHT NHIM TRONG DNG CHY 4.1. Phng trnh s lan truen cht trong dng chy

Cc m hnh cht lng nc trong dng chy thc cht l cc phng trnh ton

hc c dng m t cc qu trnh xo trn, pha long v chuyn ha cc cht trong

dng chy da trn c s nh lut bo ton khi lng ca cc cht nhim m chng ta

ang xt trong dng chy. Thit lp m hnh vn chuyn-khuch tn cc cht trong dng

chy da trn c s cc nguyn l c bn l : s thay i theo thi gian ca cc cht hu

c trong mt n v th tch V cho trc nm trong dng chy do cc nguyn nhn sau :

Trao i khi lng do qu trnh khuch tn vt cht qua din tch mt ct ngang. l qu trnh dch chuyn ca vt cht di tc ng ca gradient nng . Qa

trnh ny tun th theo nh lut Fick.

S thay i, chuyn ha cc cht trong dng chy do cc qu trnh chuyn ha sinh ha v trao i vt cht trong dng chy. C s l cc phn ng trao i ion, cc

phn ng oxy ha-kh, cc qu trnh sinh a ha, s thy phn cc cht, cc qu

trnh vt l, ha hc v sinh hc.

Xt mt n v th tch V trong dng chy (hnh2.1), gi thit rng s xo trn trong

h thng l l tng, trn c s nh lut bo ton khi lng ta c :

S tch ly vt cht = (lng vt cht ti vo +lng vt cht khuch tn vo)

-(lng vt cht ti ra +lng vt cht khuch tn ra)

phn ng chuyn ha

Hnh. 2.1S cn bng vt cht trong mt n v th tch

QC )( CCQ +

xcEA

)(

xc

xcEA

+

2

Hay

( ) kCVxC

xCEACCQ

xCEAQC

tCV

+

++

+=

(2.1)

kCVxxC

xEAx

xCQ

tCV

+

=

Trong :

E- H s phn tn dc dng chy, L2.T-1

xAV = A - Din tch mt ct t, L2

k -Hng s tc phn hy, T-1

Chia hai v cho xAV = v vi gi thit rng trong khong thi gian ang xt 0=

tV v khi 0x ti ....) ta c phng trinh vi phn mt chiu m phng s thay i

nng cc cht nhim trn on sng tnh ton.

kCxC

xE

xC

AQ

tC

x

+

= (2.2)

Hay :

kCxCE

xCu

tC

x =

+

2

2

(2.3)

Trong dng chy t nhin, cc ga (Q,A,x) l cc hm s thay i lin tc theo thi

gian cng nh khng gian theo chiu dng chy. Vi gi thit rng trn on sng chng

ta ang xt c s b sung thm hoc ly bt i cc cht bi mt nguyn nhn c hc no

( ngun thi, hoc cc im ly nc...) t (2.7) ta c phng trnh tng qut m t s

lan truyn cc cht nhim trn dng chy m chng ta cn xem xt v tnh ton.

( )( ) ( )[ ]

( ) ( ) ( ) dtdC

xCtxAtxE

xtxAxtxCtxQ

txAtC

+

= ,,

,1,,

,1 (2.4)

Trong :

dtdC -S thay i nng cc cht nhim theo thi gian do cc qu

trnh vt l, ha hc v sinh hc trong dng chy.

E -H s phn tn dc theo chiu dng chy,m2/s.

Q -Lu lng dng chy, m3/s.

3

A -Din tch mt ct t, m2.

T phng trnh (2.8) trn cc on dng chy sng, ca sng phng trnh ton

hc m t qu trnh lan truyn cht trong dng chy mt chiu l

kACxCAE

xxAC

tAC

x

=+

(2.5)

Trong :

C - Nng cc cht nhim (mg/l)

k - Hng s tc chuyn ha cc cht nhim trong qu trnh lan

truyn (ngy-1).

A - Din tch mt ct t (m2) .

4.2.Tnh ton s lan truyn cc cht nhim trong dng chy sng C s ton hc ca m hnh m phng cht lng nc gm hai bi ton :

Bi ton thy ng lc mt chiu vn chuyn vt cht cc cht nhim da trn c

s phng trnh bo ton khi lng v ng lng v bi ton chuyn ha cc cht

nhim trong dng chy da trn c s phng trnh cn bng vt cht. Cc gi thit khi

thit lp m hnh ton hc s lan truyn cc cht hu c d phn hy sinh hc theo BOD

&DO c coi nh l mt h thng vn chuyn, ti -khuch tn mt chiu cc phn ng

sinh ha xy ra trong thy vc dng chy l phn ng bc mt.

2.1.Cc phng trnh ton ca m hnh

Bi ton thy ng hc mt chiu

Cc phng trnh thy ng hc mt chiu l cc phng trnh vi phn o hm ring

phi tuyn m t cc qu trnh chy trong knh h do Saint-Vennant xut. Da trn c

s nh lut bo ton khi lng v ng lng da trn c s mt s gi thit sau :

Trong khun kh l thuyt nc nng, coi p lc phn b l thy tnh, tc p lc tng tuyn tnh vi chiu su ct nc.

Mt nc l hng s, khng ph thuc vo nng vt cht v mui.T dn n s bo ton khi lng v th tch tng ng.

nh hng ca ma st v qu trnh chy ri c th biu th Lc cn ca y sng l nh v c th b qua.

Bi ton thy ng lc mt chiu trong dng chy c vit nh sau :

4

At

Qt

q

Qt x

QA

gA zx

gAQ Q

K

+ =

+ + + =

2

2 0 (2.27)

Vi K ARn

=2

3

Trong :

A -Din tch mt ct t, L2.

T - Thi gian,T.

Q - Lu lng dng chy, L3.T-1.

g - Gia tc trng trng, L.T-2.

q - Lu lng dng gia nhp v ly i trn mmt n v chiu di dng

chy, L3.L-1T-1

n - H s maning's.

R - Bn knh thu lc, L

Bi ton lan truyn cht hu c trong dng chy 2

2x bsC C C dCu E Lt x x dt

+ = + (2.28)

Trong :

Ex -H s phn tn dc dng chy (m2/s)

L -Nng cht hu c theo BOD (mg/l)

LBS -Nng cc cht hu c theo BOD trong dng gia nhp q(mg/l)

2.2.Tnh ton s lan truyn cht trong dng chy

Phng php s bi ton dng chy khng dng mt chiu

Gii h phng trnh Saint-Venant bng phng php sai phn hu hn s n 4

im Preissman.

5

Hm lin tc ( , )f x t , cc o hm theo thi gian t v theo khng gian x ca

hm ( , )f x t c xp x bng cc biu thc sai phn theo s n 4 im Preisman trong

hnh 2.6 nh sau :

1 1

1 1( ) ( )2

n n n nj j j jf f f ff

t t

+ ++ ++ + (2.29)

1 11 1( ) (1 )( )

n n n nj j j jf f f ff

x x

+ ++ + + (2.30)

Trong :

njf -Ga tr ca f ti im (x, t=n)

,t x -Bc thi gian v kch thc ca mt li sai phn -Trng s cn bng dao ng trong khong t 0.5 -1.0

S n 4 im Preisman c cc u im sau :

-Lun n nh vi trng s > 0.5

(n)

(n +1)

xj-1 jx

t

J -1 J+1 xj

fjn

fjn+1 fj+1

n+1

fj+1n

t

Hnh 2.6.Li tnh cho s sai phn 4 im Preismann

6

-Xp x tt theo nh lut bo ton.

-Cho kt qu ng thi ca hai bin s ti mt im ng thi ca li tnh.

Cc nghin cu cho thy trng s ng mt vai tr rt quan trng trong quan im tnh ton. Khi < 0.5 th s l khng n nh v iu kin. Khi = 0.5 th s n nh khng bn vng v cho chnh xc bc hai. > 0.5 th s n nh v iu kin, tc c th tnh ton vi bt k t l x t

no. Trong thc t tnh ton dng khng n nh trong dng chy sng, s tnh n

nh v iu kin thng chn =0.7 S Preismann cho php thit lp li tnh mn do vi bc li x khng u

theo chiu khng gian dng chy trong khi chnh xc ca php xp x khng b nh

hng. Hai bin s Q, Z c tnh ng thi ti cng mt im ca li tnh do d

dng kim nghim v hiu chnh m hnh tnh ton.

Bc thi gian tnh ton t l mt trong nhng thng s quan trng ca bi ton. V cc m hnh dng phng php sai phn n nn v nguyn tc khng hn ch bi iu

kin Courant-Friedrich-Levy nh vi cc s hin.

Sai phn ha h phng trnh Saint-Vennant cho dng chy khng n nh mt

chiu ta c :

1 11 1( ) ( )(1 )

n n n nj j j j

j j

Q Q Q QQx x x

+ ++ + +

1 11 1

2 2

n n n nj j j jQ Q Q QQ

t t t

+ ++ + +

1 2 1 2 2 221 11 1 11 1

( ) ( ) ( ) ( )(1 )n n n nj j j jn n n n

j j j j j j

Q Q Q QQx A x A A x A A

+ ++ ++ + ++ +

+

1 11 1(1 )

n n n nj j j j

j j

Z Z Z ZZx x x

+ ++ + +

Trong : 1j jx x x+ =

1 11 1

2 2

n n n nj j j jZ Z Z ZZ

t t t

+ ++ + +

7

Th cc cng thc trn cho h phng trnh Saint-Vennant ta c h phng trnh

tuyn tnh vit cho mt on sng j= 1j jx x x+ = bt k trn dng chy, ta c : 1 1

1 1

00

i i i i

i i i i

A Z B Z C Q D Q GA Z B Z C Q D Q G

+ +

+ +

+ + + + = + + + + = (2.31)

Trong :

'

, , ,, , , ,

A B C DA B C D G -Cc h s ca h phng trnh c nu trong ph lc II

1,i iZ Z+ -Mc tng mc nc thi im i+1 v thi im i 1,i iQ Q+ - Mc tng lu lng thi im i+1 v thi im i

Nh vy :

-Vi 1 on sng ta c 2 phng trnh vi 4 n s l lu lng v mc nc hai

thi im lin tip i v i+1.

-Vi n on sng ta c 2n phng trnh vi 2n+2 n s.

-on sng u tin v cui cng cc n s lun lun c xc nh (iu kin

bin). Nh vy s n ca h l (2n+2)-2

Phng php gii h phng trnh c dng l phng php kh ui Gauss

a h phng trnh v dng ma trn 3 ng cho tnh ton cc n.

Bi tp 1. Xy dng h phng trnh sai phn trn

Bi tan lan truyn cht nhim trong dng chy Sau khi xc nh c cc yu t c trng v dng chy : lu lng, vn tc v

din tch mt ct t ca on sng cn tnh ton, xc nh hay gii bi ton lan truyn

cht trn dng chy. Vic tnh ton ty thuc mc ch, yu cu bi ton lan truyn cht

c gii cho hai trng hp tng ng vi hai trng thi ca dng chy : trng thi n

nh v trng thi ng lc.

Bi ton n nh

Vi gi thit ch dng chy n nh, hoc c coi nh l n nh trong khong

thi gian (chu k triu) m chng ta cn xem xt, phng trnh vi phn 2.9 c vit cho

nng cc cht hu c trong dng chy nh sau :

8

022

=+

LkxLE

xLu d (2.32)

022

=++

DkLkxDE

xDu ad (2.33)

Trong :

u - Vn tc trung bnh ca dng chy trong mt chu k triu,m/s.

E - H s phn tn dc dng chy trn on sng tnh ton,m2/s.

Gii phng trnh vi phn bc 2 (2.32) v (2.33) bng phng php tch phn vi

cc iu kin bin ta c kt qu : 2.35 , 2.36 v 2.37

L = 0 ti x = - v L = Lo ti x = 0 ta c :

++= 24112exp u

EkE

uxLL dO

L=0 ti x = + v L = Lo ti x = 0 ta c :

+= 24112exp u

EkE

uxLL dO

Nng cht hu c ban u Lo c xc nh trn c s cn bng vt cht ti

on sng c ngun thi. Vi cc gi thit v l lun tng t cn bng vt cht trn on

sng hnh 2.1 ta c :

241 uEkQ

WLd

o

+= (2.36)

Tng t vi s thiu ht oxy ta c :

( ) ( ) ( )

= 2211

12

exp112

exp1 mE

uxm

mE

uxmQkk

WkD

da

d (2.37)

Vi 214

1u

Ekm d+= ; 224

1u

Ekm a+=

Trong :

ka - Hng s tc ha tan oxy, ngy-1

kd - Hng s tc phn hy cc chp cht hu c, ngy-1

Q - Lu lng dng chy,m3/s

9

D - thiu ht oxy trong dng chy,mg/l.

L - Nng cc cht hu c theo BOD,mg/l

E - H s phn tn cc cht nhim trn on sng,m2/s.

W -Tc pht thi cht nhim t cc ngun thi vo on sng,

mg/s.

V d tnh ton 1. Cc bi ton ca phng trnh c in Streeter-Phelps

2. Bi ton b sung thm qu trnh phn tn cc cht nhim trong dng

chy ri.

Bi ton ng lc Vic tnh ton s lan truyn cc cht nhim trn chiu di dng chy hoc mt

mng li sng da trn c s chia nh thnh cc on sng c chiu di khc nhau.

Trong mi on sng cc thng s v cc yu t thy lc c coi nh l khng i theo

thi gian. Phng php s gii bi ton lan truyn cht l phng php sai phn hu hn.

Phng php s bi ton lan truyn cht

Gii phng trnh bng phng php sai phn hu hn theo s sai phn 6 im

Brian -Stone

(n)

(n +1)

xj-1 jx

t

J -1 J+1 xj

Cjn

Cjn+1 Cj+1

n+1

Cj+1n

t

Hnh 2.7.Li tnh theo s n s sai phn Brian - Stone

Cj-1n+1

Cj-1n

10

Theo s sai phn hnh 2.7 ta c :

1 1 11 1 1 1

1 1 2 1( ) ( ) ( )6 3 6

n n n n n nj j j j j j

C C C C C C Ct t

+ + + + +

+ + + + + (2.38) 1 11 1 1 1( ) ( )1

2

n n n nj j j jC C C CC

x x x

+ ++ + +

(2.39)

1 1 121 1 1 1

2 2 2

2 212

n n n n n nj j j j j jC C C C C CC

x x x

+ + ++ + + + +

(2.40)

Sai phn ha cc s hng ca phng trnh ti-khuch tn theo s li sai phn

6 im (phng php xp x Crank-Nicholson) ta c :

+

+

++

++++

+

+

tCACA

tCACA

tCACA

tAC ij

ij

ij

ij

ij

ij

ij

ij

ij

ij

ij

ij

1111

1111

1111

11

61

32

61

(2.41)

+

+++

+

+

+

++

++

++

xCAUCAU

xCAUCAU

xUAC ij

ij

ij

ij

ij

ij

ij

ij

ij

ij

ij

ij 111111

11

11

11

11

11

11

21

21

(2.42)

( ){ } { }( )1 1 1 1 1 1 1 11 1 12 11 1 ( ) ( ( ) ( ) (2 2 i i i i i i i ij j j j j j jjCEA EA EA C C EA EA C Cx x x + + + + + + + ++ + + + ( ){ } { }( )1 1 1 12 11 1 ( ) ( ( ) ( ) (2 2 i i i i i i i ij j j j j j jjEA EA C C EA EA C Cx + + + + + + + (2.43)

S hng phn ng chuyn ha :

( ) ( )

+

+

2

1 ij

ij ACACkkAC (2.44)

11

Thay cc s hng sau khi sai phn 2.41, 2.42, 2.43, 2.44 vo phng trnh ti-phn

tn v t v = f( A,E,u )ti thi im i+1; =f(A,E) ti im j+1 v =f(A,E,u ) ti thi im i. Phng trnh sai phn c n gin li l :

ij

ij

ij

ij

ii

ij

ij CCC = ++++++ 11111111 (2.45)

Trong :

, ( , , )f A E U = ti thi im i+1 ( , )f A E = ti thi im i+1 ( , , )f A E C = ti thi im i

Nh vy :

-Vi 3 im tng ng vi 2 on sng ta c 1 phng trnh vi 3 n s l

nng cc cht nhim thi im n= i+1.

-Vi n im tng ng vi n-1 on sng ta c (n-2) phng trnh vi n n s.

-im u tin v im cui cng cc n s lun lun c xc nh (iu

kin bin v iu kin ban u). Nh vy s n ca h l (n-2) n s.

Phng php gii h phng trnh c dng l phng php kh ui

Gauss a h phng trnh v dng ma trn 3 ng cho tnh ton cc n.

V d tnh ton 1. on ca sng c chiu di 4000m, c chia thnh 4 on bng nhau (5

im tnh ton).

Vn tc trung bnh trn cc on sng l 1,33m/s.

H s phn tn Ex = 666m2/s.

iu kin bin:

Bin thng lu: C=1 vi mi thi im

Bin h lu: C=0 vi mi thi im

iu kin ban u: C1,1= C2,1= C3,1= 1

C4,1= C5,1= 0

Tnh nng ti cc im sau khong thi gian t = 500s Kt qu

C22=C2,2 = 49/54

12

C23=C3,2 = 4/9

C42=C4,2 = 5/54

4. Trnh t thit lp m hnh cht lng nc Trnh t tin hnh xy dng m hnh cht lng nc cho dng chy sng Hng

theo cht hu c d b phn hy sinh hc c tin hnh theo cc bc sau :

M hnh thy lc

-Xc nh cc yu t nh hng n ch thy lc ca dng chy.

-Phn on dng chy, xc nh cc im cn tnh ton trn li tnh sai phn.

-Gii h phng trnh, tnh ton cc thng s :

Lu lng dng chy trung bnh ti cc on sng cn tnh ton.

Vn tc dng chy trung bnh trn cc on sng. Cc on sng chu nh

hng ca triu vn tc c ly theo gi tr trung bnh cho mt chu k triu.

Din tch mt ct t c xc nh bng cc s liu thc o lm c s cho

cc s liu hiu chnh m hnh thu lc.

M hnh lan truyn cht d phn hy sinh hc

-Xc nh cc c trng c bn ca qu trnh lan truyn cht trn cc on sng

cn tnh ton. Cc yu t c trng c bn bao gm :

H s phn tn trn cc vng sng c ch thu lc c trng : dng chy

n nh tng i, dng chy thay i chm dn v dng chy chu nh hng ca

triu.

Hng s tc phn hu, chuyn ho cc cht hu c trong dng chy.

-Xc nh v tr cc ngun thi, nng cc cht hu c b sung trn cc on

sng.

- Gii phng trnh vi phn bng phng php sai phn.

S tnh ton tng qut bi ton lan truyn cc cht nhim trong dng chy

c trnh by trn s hnh 2.8

13

Bi tp

V d n gin v on sng c 4 on. Yu cu tnh ton trong trng thi n nh, ng lc.

M hnh thy lc Cc s liu thng k

Nhp s liu Q,u,A

Cc s liu ti trng, h s phn tn

Gii phng trnh ti-phn tn

Tnh ton ngun thi, nng , tc phn hy

Kt qu

S sai phn

CHNG 5. CC PHN MM TNH TON, M PHNG CHT LNG NC PH BIN

Vi cc mc ch nghin cu, m phng trn cc i tng khc nhau nn

cc MHCLN rt phong ph v a dng. Theo hng dn ca ngn hng th gii

(WB) trong lnh vc ngn nga v gim thiu nhim i vi cc d n pht trin

v cc ng dng trong thc tin trn th gii cng nh nc ta trong vi nm gn

y, thng s dng cc phn mn sau tnh ton m phng cht lng nc :

M hnh HSPF (Hydrological Simulation Program Fortran (USEPA) (1984)

M phng trong khng gian 2 chiu trng thi ng lc vi cc thng s

cht lng nc: cc cht ho tan, SS, DO, cc cht dinh dng v cc loi vi

khun ch th. D bo xu th thay i cht lng nc trong dng chy sau cc

trn ma v cc thng tin v vic thu nc cc knh.

M hnh SWMM (Storm Water Management Model )

Pht trin trn c s m hnh HSPF, tnh ton xu th bin i cht lng

nc cho c mt lu vc sng...vi cc ngun thi khng im. M hnh SWMM

l m hnh 1 chiu vi trng thi ng lc m phng s chy trn nc ma qua

cc vng t nng nghip v cc khu vc th vi cc thng tin v dng chy

ca cc h thng thu gom nc.

M hnh WAPS (USEPA)

Ghp ni m hnh thu lc (DYNHYD) vi m hnh lan truyn cht

(WAPS), m phng s lan truyn v chuyn ha cc cht nhim trong dng

chy. Ty theo mc ch, s liu u vo v cc thng tin c s v cc qu trnh

chuyn ha cc cht trong dng chy, c th s dng tnh ton cc dng n

gin, ci tin hay phc tp.

H thng MIKE

Trong nhng nm 1990, vin thy lc an mch thit lp h thng m

hnh cht lng nc cho knh, sng. H thng ny c th tnh ton s lan truyn

cht nhim trong dng chy t cc ngun khc nhau vo cc lu vc khc nhau.

Ty thuc i tng nghin cu, yu cu tnh ton cc thng s cht lng nc

trong dng chy sng, ca sng, h hay bin m p dng cc phin bn khc nhau

nh MIKE 11, MIKE 21, MIKE 3, MIKE SHE, MIKE MOUSE v MIKE BASIN.

M hnh WQRRS (Water quality for River )

Ghp ni m hnh Qual II vi m hnh tnh ton s lan truyn cht nhim trong

cc h cha nc. Tnh ton 1, 2 chiu cho h thng sng - h trng thi ng

lc vi s liu u ra l cht lng nc sng lm s liu u vo cho m hnh

cht lng nc h.

M HNH QUAL2E V QUAL2E-UNCAS (USEPA) 4/1999

M hnh QUAL I c xy dng trn c s phng trnh vi phn bc nht

do Streeter-Phelps lp nn. Phng trnh l hm ti lng cc cht hu c, tc

phn hy v tc tiu th oxy. M hnh c s dng d bo nng cc

cht hu c BOD v DO h lu ngun thi vo dng chy sng. QUAL I thng

c s dng cho dng chy n nh mt chiu.

QUAL II (1970) l s ci tin t QUAL I m phng s lan truyn cc cht

nhim trong dng chy mt v hai chiu i vi dng chy n nh h lu ngun

thi. S ci tin c th p dng i vi dng chy sng rng (c cp thm s

phn tn cc cht nhim trong dng chy). i vi cc trng hp khng n

nh c pht trin t cc trng thi n nh vi vic tnh ton theo thi gian

bng cch tnh lp li nhiu ln. nh gi, Xem xt nhy bng cch phn tch

sai s theo m phng Monte Carlo

Cc thng s cht lng nc m phng bao gm : chlorophyll-a, DO,

BOD, cc cht dinh dng, cc cht bo ton v khng bo ton, cc vi khun ch

th. c p dng xc nh mc tc ng ca cc cht bn ca ngun im

v ngun khng im i vi cht lng nc trong dng chy.

QUAL2E - Uncas (1985) nng cao chnh xc ca m hnh vi s b sung

thm vic xem xt nhy ca cc s liu u vo.

Hn ch khng xem xt s lan truyn ca cc thng s kim loi nng v cc

cht phn tn nh du, m...Tuy nhin y l mt trong nhng m hnh c s

dng ph bin nht, do nhu cu s liu u vo t.

Mo.Dau.pdfC1.pdfC2.pdfC3.pdfC4.pdfC5.pdf