12. SELF PURIFICATION OF NATURAL STREAMS

Transcription

1 1 Wastwatr Managmnt 12. SELF PURIFICATION OF NATURAL STREAMS Th slf purification of natural watr systms is a complx procss that oftn involvs physical, chmical, and biological procsss working simultanously. Th amount of dissolvd Oxygn (DO) in watr is on of th most commonly usd indicators of a rivr halth. As DO drops blow 4 or 5 mg/l th forms of lif that can surviv bgin to b rducd. A minimum of about 2.0 mg/l of dissolvd oxygn is rquird to maintain highr lif forms. A numbr of factors affct th amount of DO availabl in a rivr. Oxygn dmanding wasts rmov DO; plants add DO during day but rmov it at night; rspiration of organisms rmovs oxygn. In summr, rising tmpratur rducs solubility of oxygn, whil lowr flows rduc th rat at which oxygn ntrs th watr from atmosphr Factors Affcting Slf Purification 1. Dilution: Whn sufficint dilution watr is availabl in th rciving watr body, whr th wastwatr is dischargd, th DO lvl in th rciving stram may not rach to zro or critical DO du to availability of sufficint DO initially in th rivr watr bfor rciving discharg of wastwatr. 2. Currnt: Whn strong watr currnt is availabl, th dischargd wastwatr will b thoroughly mixd with stram watr prvnting dposition of solids. In small currnt, th solid mattr from th wastwatr will gt dpositd at th bd following dcomposition and rduction in DO. 3. Tmpratur: Th quantity of DO availabl in stram watr is mor in cold tmpratur than in hot tmpratur. Also, as th activity of microorganisms is mor at th highr tmpratur, hnc, th slf-purification will tak lss tim at hot tmpratur than in wintr. 4. Sunlight: Alga producs oxygn in prsnc of sunlight du to photosynthsis. Thrfor, sunlight hlps in purification of stram by adding oxygn through photosynthsis. 5. Rat of Oxidation: Du to oxidation of organic mattr dischargd in th rivr DO dpltion occurs. This rat is fastr at highr tmpratur and low at lowr tmpratur. Th rat of oxidation of organic mattr dpnds on th chmical composition of organic mattr.

2 2 Modul 12, Lctur Numbr- 15 M.M. Ghangrkar, IIT Kharagpur 12.2 Oxygn Sag Analysis Th oxygn sag or oxygn dficit in th stram at any point of tim during slf purification procss is th diffrnc btwn th saturation DO contnt and actual DO contnt at that tim. Oxygn dficit, D = Saturation DO Actual DO Th saturation DO valu for frsh watr dpnds upon th tmpratur and total dissolvd salts prsnt in it; and its valu varis from mg/l at 0 o C to 7.63 mg/l at 30 o C, and lowr DO at highr tmpraturs. Th DO in th stram may not b at saturation lvl and thr may b initial oxygn dficit D o. At this stag, whn th fflunt with initial BOD load L o, is dischargd in to stram, th DO contnt of th stram starts dplting and th oxygn dficit (D) incrass. Th variation of oxygn dficit (D) with th distanc along th stram, and hnc with th tim of flow from th point of pollution is dpictd by th Oxygn Sag Curv (Figur 12.1). Th major point in sag analysis is point of minimum DO, i.., maximum dficit. Th maximum or critical dficit (D c ) occurs at th inflxion points of th oxygn sag curv. Point of oxygn dmanding wast discharg Saturation DO 100 DO Contnt, % Do Oxygn Sag Curv Dt Dc Roxygnation Curv t tc Critical point Doxygnation Curv 0 Tim of flow in stram, t, days Figur 12.1 Doxygnation, roxygnation and oxygn sag curv 12.3 Doxygnation and Roxygnation Curvs Whn wastwatr is dischargd in to th stram, th DO lvl in th stram gos on dplting. This dpltion of DO contnt is known as doxygnation. Th rat of doxygnation dpnds upon th amount of organic mattr rmaining (L t ), to b oxidizd at any tim t, as wll as tmpratur (T) at which raction occurs. Th variation of dpltion of DO contnt of th stram with tim is dpictd by th doxygnation curv in th absnc of

3 3 Wastwatr Managmnt aration. Th ordinats blow th doxygnation curv (Figur 12.1) indicat th oxygn rmaining in th natural stram aftr satisfying th bio-chmical dmand of oxygn. Whn th DO contnt of th stram is gradually consumd du to BOD load, atmosphr supplis oxygn continuously to th watr, through th procss of r-aration or roxygnation, i.., along with doxygnation, r-aration is continuous procss. Th rat of roxygnation dpnds upon: i) Dpth of watr in th stram: mor for shallow dpth. ii) Vlocity of flow in th stram: lss for stagnant watr. iii) Oxygn dficit blow saturation DO: sinc solubility rat dpnds on diffrnc btwn saturation concntration and xisting concntration of DO. iv) Tmpratur of watr: solubility is lowr at highr tmpratur and also saturation concntration is lss at highr tmpratur Mathmatical analysis of Oxygn Sag Curv: Strtr Phlps quation Th analysis of oxygn sag curv can b asily don by suprimposing th rats of doxygnation and roxygnation as suggstd by th Strtr Phlps analysis. Th rat of chang in th DO dficit is th sum of th two ractions as xplaind blow: ddt/ dt = f ( doxygnation and roxygnation) OR ddt / dt = K Lt R Dt.(1) Whr, Dt = DO dficit at any tim t, Lt = amount of first stag BOD rmaining at any tim t K = BOD raction rat constant or doxygnation constant (to th bas ) R = Roxygnation constant (to th bas ) t = tim (in days) ddt/ dt = rat of chang of DO dficit Now, Lt = Lo K ' t. Whr, Lo = BOD rmaining at tim t = 0 Hnc,

4 4 Modul 12, Lctur Numbr- 15 M.M. Ghangrkar, IIT Kharagpur ddt dt = K' Lo. K ' t R' Dt.(2) or ddt K ' t + R' Dt = K' Lo..(3) dt This is first ordr first dgr diffrntial quation and solution of this quation is as undr Dt = K' Lo R' K' t R t R t [ ] K ' ' ' + Do. Changing bas of natural log to 10 th quation can b xprssd as: KLo Dt = R K K. t R. t R. t [ ] + Do.10.(4).(5) Whr, K = BOD raction rat constant, to th bas 10 R = Roxygnation constant to th bas 10 Do = Initial oxygn dficit at th point of wast discharg at tim t = o t = tim of travl in th stram from th point of discharg = x/u x = distanc along th stram u = stram vlocity This is Strtr-Phlps oxygn sag quation. Th graphical rprsntation of this quation is shown in Figur Point of wast discharg Saturation DO DO Concntration X Do Xc Dt Dc Critical point Distanc downstram, X Figur 12.2 Oxygn sag curv of Strtr-Phlps quation Not: Doxygnation and roxygnation occurs simultanously. Aftr critical point, th rat of r-aration is gratr than th doxygnation and aftr som distanc th DO will rach to original lvl and stram will not hav any ffct du to addition of wastwatr. At tim t=0 at x = 0.

5 5 Wastwatr Managmnt Dtrmination of Critical DO dficit (Dc) and distanc Xc Th valu of Dc can b obtaind by putting ddt/dt = 0 in quation 3, Hnc, OR Dc = Dc = K' Lo. R' K R Lo.10 K ' tc K. tc.(6).(7) Whr, tc is tim rquird to rach th critical point. Th valu of tc can b obtaind by diffrntiating quation 4 (or 5) with rspct to t and stting ddt/dt = 0 Thrfor, 1 R' Do( R' K' ) tc = log 1.(8) R' K' K' K' Lo OR 1 tc = log R K 10 R K ( R K ) Do 1 K. Lo.(9) Th distanc X c is givn by X c = tc. u Whr, u = vlocity of flow in th stram Th doxygnation constant K, is obtaind by laboratory tst or fild tsts, and varis with tmpratur as givn blow: K T = K 20 ( ) ( T θ 20).(10) Whr, θ varis with th tmpratur = in gnral or for 20 o C to 30 o C tmpratur, and for 4 o C to 20 o C K = 0.1 to 0.3 for municipal swag, bas 10, (0.23 to 0.70 for bas ) Th roxygnation constant R also varis with th tmpratur and can b xprssd as: ( ) ( T 1. 20) R T = R Whr, R /R = R = 0.15 to 0.20 for low vlocity larg stram = 0.20 to 0.30 for normal vlocity larg stram.(11)

6 6 Modul 12, Lctur Numbr- 15 M.M. Ghangrkar, IIT Kharagpur = 0.10 to 0.15 for laks and sluggish stram ( ) ( T 1. 20) R T = R (Pavy t al., 1985) Th ratio of R/K (or R /K ) is calld th slf purification constant f s and it is qual to 0.50 to 5.0. Exampl : 1 A city dischargs m 3 /day of swag into a rivr whos rat of flow is 0.7 m 3 /sc. Dtrmin D.O. dficit profil for 100 km from th following data: Rivr Swag fflunt from STP 5 day B.O.D. at 20 0 C = 3.4 mg/l 5 day B.O.D. at 20 0 C = 45 mg/l Tmpratur 23 0 C Tmpratur 26 0 C D.O. = 8.2 mg/l D.O. = 2.0 mg/l Vlocity of mix = 0.25 m/sc, R =0.4, K = 0.23 Solution Saturation valu of D.O. at C dg is 8.57 mg/l Rivr discharg = 0.7 m 3 /sc, Swag discharg = 20000/(24x 3600) = m 3 /sc... BOD of mix= = mg/l.. D.O. of mix.... = 6.66 mg/l.. Tmp. of mix.. = C.. Ultimat B.O.D. L t = L 0 (1 - ) = L 0 (1 -. ) L 0 = mg/l Initial D.O. dficit (D 0 ) = = 1.91 mg/l Doxygnation and roxygnation cofficints at C tmpratur K T = K 20 ( ) T-20 Hnc, K = 0.23 (1.047) = day -1

7 7 Wastwatr Managmnt R T = R 20 ( ) T-20 Hnc, R = 0.40 (1.016) = day -1 Critical tim t c = log (1- D ) L =.. log.. ( ) = days. Critical D.O. dficit, Dc = L 0. = = mg/l Distanc at which it occurs = L= vlocity x tim = (0.25 m/sc) x ( x 24 x 60 x 60 sc) = m = km Similarly tim rquird for mix to rach at 20 km distanc, t 20km = And DO dficit at 20 km can b calculatd using quation 4 K' Lo Dt = R' K' t R t R t [ ] K ' ' ' + Do. = day. Whr, K = d -1, R = d -1, Do = 1.91 mg/l and Lo = mg/l and t = day Hnc, DO dficit at 20 km = mg/l Similarly DO dficit at 40 km (i.. t = days) = mg/l and DO dficit at 80 km (i.., t = days) = mg/l and DO dficit at 100 km (i.., t = 4.63 days) = mg/l

8 8 Modul 12, Lctur Numbr- 15 M.M. Ghangrkar, IIT Kharagpur Th DO dficit at diffrnt points along lngth of rivr is as blow: Distanc in km Tim in days DO dficit, mg/l DO, mg/l Qustions 1. Explain factors affcting slf purification of natural strams. 2. What is roxygnation? What ar th factors that affct roxygnation? 3. Driv Strtr-Phlps quation for oxygn sag analysis. 4. A rivr is having discharg of 22 m 3 /s rcivs wastwatr discharg of 5 m 3 /s. Th initial DO of th rivr watr is 6.3 mg/l, and DO contnt in th wastwatr is 0.6 mg/l. Th fiv day BOD in th rivr watr is 3 mg/l, and th wastwatr addd to rivr has fiv day BOD of 130 mg/l. Considr saturation DO of 8.22 mg/l and doxygnation and roygnation constant valus of 0.1 and 0.3 pr day, rspctivly. Find critical DO dficit and DO in th rivr aftr on day. Th avrag vlocity of flow in th stram aftr mixing of wastwatr is 0.18 m/sc. 5. A municipal wastwatr tratmnt plant dischargs scondary fflunt to a rivr. Th worst condition occurs in th summr whn th tratd wastwatr in summr is found to hav a maximum flow rat of m 3 /day, a BOD 5 of 30 mg/l, dissolvd oxygn concntration of 1.5 mg/l and tmpratur of 25 o C. At upstram of th disposal point th minimum flow in th stram is 0.65 m 3 /sc with BOD 5 of 3.0 mg/l, dissolvd oxygn concntration of 7.0 mg/l and tmpratur of 22 o C. Th mixing of wastwatr and stram is almost instantanous at th point of disposal and vlocity of th mixtur is 0.2 m/sc. Th raration constant is stimatd to b 0.4 pr day at 20 o C tmpratur. Dtrmin dissolv oxygn profil for 100 km downstram of th rivr from th point of discharg.