Danh gia kha nang chju tai cua nguon tiep nhan nu^oc thai tren song Thi Vai

Tuyen tap Cd)ng trinh Hoi nghj Khoa hoc

Ca hoc Thiiy khi Todn qudc, Phan Thiet, 2008 37

Danh gia kha nang chju tai cua nguon tiep nhan nu^oc thai tren song Thi Vai

Nguyen Tat Dac

ViiJn Quy Hoach Thiiy lai mien Nam

Tom tdi: Bdo cdo nay ncu mdt cdch ddnh gid kha ndng tiep nhdn nude thdi cua cdc sdng kenh chia dnh hudng cua thuy trieu. Trudng hcjp cu thd cho sdng Thi Vai dircrc xem xet.

Assessment of receiving capability of waste water discharging to the Thi Vai River

Abstract: It Is proposed In this report a way to assess receiving capabilm' of waste water discharging to rivers affected hy tide. The Thi Vai River Is a case In consideration

1. Gidi thieu c h u n g :

Thi Vai la sdng kha sau dd ra bicn d eira sdng Cai mep. Thugng ngudn hi mot sd sudi nhd va dugc ndi vdi sdng Ddng Nai bang sdng Ddng Mdn. Ddng Mdn la sdng nho va nong, lai CO cdng Ba Ky each sdng Ddng Nai gan 17km, chieu rdng cdng hien tai la 4,5m. Khi trieu len nude sdng Thi vai va Dong Nai cd the chay qua nhau, nhung khi trieu rut cd nhung diem bi can, vi the cd thdi diem 2 sdng chay qua lai nhung luu lugng trao doi rat nho. Song Thi vai cung dugc ndi vdi mang kenh rach ciia viing Duycn hai, Tp Hd Chi Minh nhd rach Tac Hdng va sdng Go gia. Trong nhirng tinh loan trude day Thi Vai thudng duac xem la mdt sdng cut, chiu anh hudng chii yeu ciia che dp ban nhat trieu lir Vung tau. Trong nhung nam gan day tren dpc bd sdng Thj vai, vdi chieu dai khoang 40km. da xay dung mdi nhieu khu cdng nghiep, cang, dan cu nhu khu Go Dau, Phii My, My xuan, Cai mep, Nhon trach... Mdt sd nha may nhu VeDan, che bien thuc pham, super-phdt phat, hoa chat... dang xa ra sdng Thi Vai mot khdi lugng Idn nude thai ed nong cac chat ban rat cao, gay 6 nhiem nang cho ngudn nude va cac ngudn thuy hai san cua sdng nay. De cd hire tranh tdng quat cho cdng tac danh gia va qui hoach xir li d nhiem ngudn nude, mot chuyen dc tinh tdan thiiy lire va d nhicm cho sdng nay da dugc thuc hien, trong do npi dung danh gia kha nang tu lam sach ciia sdng nay. Ddi vdi sdng khdng cluu anh huong cua trieu each danh gia theo phuong phap ap dung cho cac sdng ciia Chau Au cd the sir dung de tham khao. Tuy nhien ddi vdi cac sdng chiu anh hudng trieu nhu d Vict nam can co sir thay ddi nhat dinh nhu duoc trinh bay dudi day.

Doi vdi sdng Thj Vai, nude thai san xuat va sinh boat lir cac khu cdng nghiep va dan cu dpc sdng dugc xa true tiep vao sdng anh huong rat Idn ddi vdi chat lugng nude sdng va cac kenh sdng khac cd ddng chay lien thdng vdi sdng Thj vai do che dp thiiy trieu.

Tieu chuin Viet Nam vl Mdi trudng ciing dua ra ndng dp eho phep ddi vdi cac chat d nhicm trong nude thai cdng nghicp (TCVN 5945 - 1995). Tuy nhien do luu lugng nude thai lang rdt nhanh ciing vdi ndng do cac chSt d nhicm trong nude thai cao lai chua dugc xir ly dat chudn lam cho tai lugng chit d nhiem dd xuong kenh rach trong khu vuc vugt qua kha nang tu lam sach cua he thong kenh rach. Can khong che khdng chi vc ndng dp chat d

Ddnh gid khd ndng chiu tdi cU(^

38 nsiidn Hep nhdn nude thdi tren sdns ThjJ-^

nhiem cd trong nude thai ma cdn vl tai lugng cho phep thai ra kenh rach. Day la giai p'^^P cho viec xac dinh kha nang tiip nhan nguon thai cua he thdng kenh rach trong khu v u c

D I diinh gia kha nang tu lam sach cua timg con sdng trong [I] da dua ra khai mem hang sd tu lam sach ciia timg con sdng. Hang s6 nay ty le thuan vdi tdc do ddng chay va ty le nghich vdi dp sau tnmg binh cua mat cat ngang. Trong viing trieu khi ddng chay ddi hudng (dirng im roi ddi chieu) vdi van toe x5p xi bang khdng, do dd hang so tu lam sach xdp xi khdng. Day la thdi dilm nude ban thai vao sdng hSu nhu dirng tai chd ma khdng bi pha loang. Thdi diem ndng do bin duoc pha loang ll)i da khi d()ng chay dat gia tri cue dai (Idn nhat liic trilu len, hoac Idn nhiit khi trieu rul). Hinh 2 cho giii trj max va binh quiin doc sdng Thi vai. Hinh ve cho thiiy ed nhung dpan cd kha nang tu liim sach tdt nhung cd nhirng dgan kha nang tu lam sach kcm, Vi thi khi xa nude thai nen chpn nhung dgan cd kha nang tu lam sach tdt hon. Tat nhien thco tieu chuan ncu trong [1] thi kha nang tu lam sach cua sdng Thj vai rat kem.

2. C a c nguon va phan bo cac nguon doc song Thj vai (hinh 1):

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Hinh 1: Sa dd cdc ngudn xu thai dpc sdng Thi Vai Vi tri cdc diem xa doc idng Thi Vdi tirc6ng Bd Ky ra biiyi

^Y l^^ ^^^^ 7 ^ ^ ^ ' ^ ^ 23.11 24.61 28 11 53.61 Kxn . ^ ' ! 1 \ \ \ \ 1 Bi^n Cong DanCu Cnghuep VeDan My Xuan My Xuan M v X u a n Phu My KhuCN

BaKy Nh.Trach NhTrach G O D I U 2A+2B IB lA Cai Mep

Nguyen Tdt Dac 39

Theo hien trang va qui hoach phat trien eho den nam 2020, dpc hai bd sdng Th; vai se cd 4 nhdm ngudn thai cbii yeu :

- Khu Gd dau : Ngudn thai chii yeu cua kliu nay la nha may Vcdan, Luu lugng nude thai mdt ngiiy dem la 1544m3 nude thai cd ndng dp BOD rk cao (194 mg/1), ngoai ra cd nude thai ciia khoang 10,000 dan, cang Gd dau, mdt sd nha may nhd nhu phan xuong PS ciia cong ty Marubeni xa ra sudi Sao, nguon xa cua nha may super- phdt phat. Cac nguon thai nay each cira sdng Cai mep gan 30km.

- Khu My xuan ; Gdm cac khu My xuan A, My xuan A2, B2. BI. Nude thai tren 15,000 m3 trong mdt nga\' dC-m vdi ndng dp BOD lir 360 den 620mg/L.,

- Khu Phu My-Ciii Mep: Gdm cang, nha may dien vii mot viii ngiin dim. Mot ngay dcm lugng nude thai sc tren 10.()00m3 vdi ndng dp BOD tir 1 10 den 36()mg/L - Khu Nhon Trach: Hien tai nude thai cd 17.000m3/ngay dem vdi nong dp BOD cd

150mg/L, va den 2010 nude thai cd 5O.O0Om3/ngay dcm vdi ndng BOD binh quan xap lOOmg/L.

Ngoai ra cdn cd mdt sd nha may nhd va dan cu phan bd dpc song.

Hinh 2-M:LX va binh quirm hang so tu larn sach doc song Tlti ^'ai thang 3-4_5/2iji:i5

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CN Whan Trach '

M y X u i n B f l g j ba Go Oia \

'- " • - r . •-• J ' JL • - - r - ' -• - : T T kho-ang c,3ch (Km) t d h a luu cong Ba Ky toi ng,a ba Go Gia

3. Danh gia kha nang t y lam sach

^i hilu them vl kha nang chju tai ciia tirng con sdng hay tirng dpan song ta can phai hieu CO che tu lam sach ra sao.

Khi cac chirt thai ban huu ca trong nude thai dugc xa vao song kenh se bi phan buy chii ylu bang con dudng sinh hoa va bj pha loang trong ddng chay. Cac vi sinh vat se su dung oxy hda tan trong nude d l thuc hien phan iing oxy hda lam phan hiiy eac chat huu ca, Nhu vay nlu lugng oxy trong nude cang nhieu thi kha nang tu lam sach cang tot. Lugng oxy hda tan trong nude dugc san sinh tir cac phan irng quang hgp ciia ddng thuc vat thiiy

40

Ddnh gid khd ndng chiu W' '^"^

ngudn tiep nhdn niiAc thdi tren sdns ThU-^

sinh va phu thudc viio lugng oxy tham vao nude tir khdng khi. Ddng chay cang manh thi lugng oxy tham vao cang nhieu. Sdng cang sau thi ciing khd tham oxy vao khap mat cat ngang so vdi sdng ndng. Mat khiic ddng chiiy ciing manh thi kha nang lam tham oxy tir khdng khi va pha loang nong dp chat thai ban cang cao. Nhu vay ca che va kha nang tu lam sach ciia con sdng phu thudc manh vao chi do thuy van thuy krc ciia chinh con sdng dd.

Hau het cac sdng kenh a Viet nam dIu chiu anh huong cua thiiy trieu; nhirng luc ddng chay ddi chieu thi van tdc gdn nhu bang khdng, nude thai ban vao ddng chay hau nhu khdng bi pha loang, vi thi tai cac thdi dilm nay dp bin dat gia tri cue dai. De cd the hinh dung vc lai lugng t()i da cho mdt ngudn thai la xci mdt nguon thai lai dicm XIco luu lugng q, ndng dp C ; ngudn thai tiep thco tai dicm X2 cd km lugng ql, ndng do L, cac ngudn nay dugc thai vao mdt dpan sdng thco so dd sau (hinh 3):

q c

•

Q L C l

Q2,(b2

ql L Q3, C3

04,C4

— •

• •

XI

Hinh 3: Cdc ngudn xd gia dinh doc sdng

Trong dd Ql, CI tuong irng la luu lugng va ndng dp ciia nude sdng trude dilm xa, cdn Q2, C2 la luu lugng va ndng dp nude sdng ngay sau diem xa. NIU ggi C, la ndng dp tdi han cho phep (theo tieu chuan nao dd, vi du TCVN 5945 - 1995) thi tieu chuan phai tuan thiila : C2 < C,. va C4 < C,

Gia sir rang ddng chay trong sdng va nude thai thco hudng mui ten. va nude thai va nude sdng dugc xao trdn hoan tdan thi sau khi xao trdn ndng do nude sdng tai sau dilm thai (tai XI) dugc tinh bang cdng thirc sau (coi q ^ 0)

C2

qC + Q\.Cl ci + Q\

C.^Vl

1 +

^1

C + a C l 1 + a vdi a = QI/q ; DodilukienC2 <C,

Ta cd:

^ C + a C l

C, > - : - Hay C <{l + a)C, ~a C\ C<C. l + a ( l - — )

C

^1

(1) (1) la cong thuc ngugc co ban de tinh ndng dd cho phep cua ngudn thai. Tuy nhien trong (I) CO mat 2 tham so la CI va a = Ql/q, Gia su ndng dd nude sdng trude dilm thai la CI khdng vugt qua hay bang C, khi dd tir (1) suy ra

; ' > . j ; 7

Nguyen Tdt Ddc 41

C < ( l + a ) C , - a C , = C, (2) Tir (2) ta suy ra rang ncu ndng dp nude sdng trude ngudn xa da dat chuSn thi ndng trong ngudn xa khdng dugc vugt ndng dp chuan cho phcp vii khdng phu thudc vao luu lugng nude sdng (tire la khong phu thudc vao gia tri ciia he sd a ). Ta xet mot so trudng hpp ciia ty sd P = Cl/C, de xem gia tri cua C tinh tir cong thue (1) ra sao. Luu y rang nong dp cho phcp C phu thudc viio 2 tham sd a vii p ma tham sd a=Q 1/q lai phu thudc vao chi do dong chay. Bang 1 dudi day cho gia tri C/Ct la ty sd ndng dp ed the cho phep ciia nude thai so vdi nong dp lieu chuan. Cac gia tri trong bang nay phu thupc vao nong dp nude sdng trude ngudn xa va phu thudc \ao che dp dong chay so vdi luu lugng xa (tham sd a=Ql/q).

Cot dau ticn trong bang la ty sd Cl/Ct, luc la ndng dp nude sdng trude dicm xa so vdi ndng dp lieu chuan. Hang dau ticn trong bang la ly sd cua luu lugng ddng chay so vdi luu lugng ngudn xa.

Bang 1: Ndng dp cho phcp trong ngudn thai so vdi nong dd cho phcp eiia nude sdng

CI/ST---^.^/

1.2 1.1 1 0 9 0 8 0 7 0.6 0.5 04 0.3 0.2

0.1 0.98 0.99 1 1 01 1.G2 1 03 1 04 1 05 1 06 1 07 1 08

1 0.8 Q.9

1 1.1 1.2 1.3 14 15 1.6 1.7 18

2 0.6 0.8 1 1,2 14 1.6 1.8 2 2 2 2.4 2 6

3 0.4 0.7 1 1.3 1.6 1.9 2.2 2.5 2 8 3.1 3.4

4 0.2 0.6 1 1.4 18 2.2 2.6 3 34 3.8 4 2

5 0 0.5

1 1.5

2 2 5

3 3.5

4 4.5

5 10 -1 0 1 2 3 4 5 6 7 8 9

50 -9 -4 1 6 11 16 21 26 31 36 41

100 -19 -9

1 11 21 31 41 51 61 71 81

200 -39 -19 1

21 41 61 81 101 121 141 161

300 -59 -29 1

31 61 91 121 151 181 211 241

400 -79 -39 1 41 81 121 161 201 241 281 321

500 -99 -49 1

51 101 151 201 251 301 351 401

800 -159 -79

1 81 161 241 321 401 481 561 641

1000 -199 -99

1 101 201 301 401 501 601 701 801

Chang ban khi ndng dp nude siing trude diem thai CI bang 0.8 ndng dp cho phcp (P = Cl/C,=0.8) va luu luong ddng chay gap 50 lin (a=Ql/q=50) luu lugng xa thi ndng dp cho phcp trong ngudn xa cd the gap 11 lan nong dp cho phcp cua nude song, neu gap 100 lan (a=Ql/q=100) thi nong dp cho phep trong ngudn xa cd the gap 21 lan nong dp cho phep ciia nude sdng. Rd rang kha nang tai va pha loang phu thudc manh vao che dp thiiy van thiiy luc ciia con sdng.

Tir bang nay ta rut ra cac nhan xet sau day:

+ N I U nong dp nude song da dat tdi ning dp ban tdi han cho phcp thi ndng dp trong nguon thai cung khdng dugc vugt qua nong dp eho phcp trong nude sdng (ddng td vang trong bang linh tren), khdng phu thudc ddng chay trong song Idn hay nhd.

+ Khi do ban trong sdng cdn dudi gidi ban cho phcp thi ddng chay cang manh thi ndng dd bin cho phcp trong nguon thai cang cao. Vi thi khuyen cao nen xa nude thai vao thdi dilm khi ddng chay manh. Chang ndng dp BOD cho phep trong nude sdng la lOmg/L;

Ndng dd BOD nude sdng trude dilm xa la 8mg/L (CI/Ct=0.8); gia sir ngudn thai vdi luu lugng 0.1m3/s thi khi ddng trilu Idn ban 10m3/s (a =100), thi tir bang tren ndng do cho phep trong nguon xa cd thi gap 21 ISn ndng dp tieu chuan, cd nghia la 210mg/L. Vdi ddng trilu cao ban thi nong dp trong nguin thai cd thi cao ban (ncu dong tricu la 20m3/s thi ndng dp cd the la 41 Omg/L)

Ddnh gid khd ndng chiu tdi cu^

42 nsudn Hep nhdn nirdrc thdi tren sdns ThjASi + Vice pha loang trude khi ihai: Nhilu ngudi cho rang lay nude sach pha loang " " " ^ dp ban trude khi thai cd ve nhu de chap nhan, tuy nhien theo bang tren viec pha loang se dan tdi luu lugng nude thai tang, khi dd he so a, va he qua la nong dd cho phcp cung gia'^^- Vi du sau day minh hpa dilu miy: ISy vi du nhu neu tren, gia su ddng trieu van la I0m3/s, luu lugng xa bdn la 0.1m3/s thi ndng dp cho phep trong ngudn thai la 210mg,/L; gia su thuc hien sir pha loang de ndng dp trong ngudn thai giam di mdt nira va khi dd luu lugng thai tang Icn glp ddi la 0.2m3/s; ddi chilu^'di bang t^rcn he sd a = 50 do dd ndng dd cho phcp trong ngudn thai chi cdn la 11 Omg/L. Tir dd kliuyin cao rang cin phai xem xet ca luu^lugng xa va ndng do trong ngudn xa \'a nen khuyin cao hoac bat budc xa vao nhirng thdi diem khi ddng chay manh.

Do\ vdi cae dilm khac, chang han diem X2, vice tinh toan cimg tuong tu, chi luu y rang tir XI tdi X2 khdng cd ngudn nao ca, chiit thai bi pha Inang thco ddng chay, vi C2 <

C, nen chac chan C4 < C, va lai ap dung cac tinh toan nhu iren. Lim y : 0 2 = Q l + q ; 0 3 = Q 2 (bo qua su thay ddi do trir giua XI,X2); 0 4 = 0 3 + q l

Ddi vdi sdng Thj Vai luu lugng xa thai va ndng dp tuang irng tai cac dicm thai nhu sau:

Diem thai Luu luong thai (m3/s) None dp BOD trong ngudn

Dim cu Nhan Trach 0.022 220 Cong nghiep Nhon Trach 0.576 87 Gd DSu-Ve Dan 0.0225 194

M y x u a n 2 A - 2 B 0,0359 161 My Xuan IB 0.0849 620 My Xuan 1A 0.0562 620 Phu My 4 0.2513 110 Cai Mep 0.1623 361 Gia sir ndng dp BOD trong nude song tdi da la 1 Omg/L, thi:

Khu dan cu Nhan trach chi dugc thai khi dong chay tren 5m3/s

Khu cdng nghicp Nhon Trach chi dugc thai khi ddng chay Ircn 50m3/s

Khu Gd Dau-Ve Dan, My Xuan 2A-2B chi dugc thai khi ddng chay tren 10m3/s Khu My Xuan lA, IB cd ndng dp BOD trong ngudn rat cao. vi ihl chi dugc xa thai khi ddng chay gap tu 500 din 800 lan luu luong cua neuon thai, tire la ddng chay tren 60m3/s.

Khu Phil My 4 chi dugc xa khi dong chay Idn han 30m3/s Kim Cai mep chi dugc .xa khi dong chay Idn han 60m3/s.

Khu dan cu Nhon Trach nam dudi cdng Ba Ky vdi luu lugng toi da cd 20m3/s, vi thi cdn xem xet thdi dicm xa.

Cac diem xa cdn lai neu xa vao cac thdi dilm cd ddng chay Idn han 60m3/s thi cd thi bao dam do pha loang, Tuy nhien do ed nliirng thdi dilm khdng xa, lugng nude thai bi lich

Nguyen Tdt Ddc

₄₃

tu va lam tang luu lugng xa. Cac sd lieu luu lugng xa neu tren la tinh trung binh, vi thi ddi vdi toan bd sdng Thi vai nen xa vao cac thdi diem cd ddng chay Idn hon 100m3/s.

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Tai lieu tham khao

[IJ WHO, "Assessment of sources of Air, Water, and Land Pollution ". Part II. Geneva, 1993,