Lecture 3 Activated sludge and lagoons

Transcription

1 Lectue Activated sludge and lagoons

2 ACTIVATED SLUDGE PROCESS Activated sludge pocess is used duing seconday teatment of wastewate. Activated sludge is a mixtue of bacteia, fungi, potozoa and otifes maintained in suspension by aeation and mixing []. In this pocess, a biomass of aeobic oganisms is gown in lage aeated basins. These oganisms beakdown the waste and use it as thei food to gow themselves. Activated sludge pocesses etun settled sludge to the aeation basins in ode to maintain the ight amount of oganisms to handle the incoming "food". Activated sludge pocesses have emoval efficiencies in the ange (95-98%) than tickling filtes (80-85%). [2] ORKING OF ACTIVATED SLUDGE SYSTEM [] A pimay settle (o pimay claifie) may be intoduced to emove pat of the suspended solids pesent in the influent and this educes the oganic load to the activated sludge system. The biological eacto o aeation tank is filled with a mixtue of activated sludge and influent, known as mixed liquo. It is necessay to maintain cetain mixed liquo suspended solid (MLSS) in the aeated tank maintain good emoval efficiency. The aeation equipment tansfes the oxygen necessay fo the oxidation of oganic mateial into the eacto, while simultaneously intoducing enough tubulence to keep the sludge flocs in suspension. The continuous intoduction of new influent esults in a continuous dischage of mixed liquo to the seconday settle whee sepaation of solids and liquid takes place. The liquid leaves the system as teated effluent, wheeas some pat of the sludge is eciculated to the aeation tank called as etun sludge and est of sludge is taken fo anaeobic digestion. DESIGNING OF ACTIVATED SLUDGE SYSTEM Suppose, is the flow ate of influent (m /d), is the flow ate of waste sludge (m /d), is the flow ate of etun activated sludge (m /d), V is the volume of aeation tank (m ), S 0 is the influent soluble substate concentation (BOD g/m ), S is the effluent soluble substate concentation (BOD g/m ), X o is the concentation of biomass in influent (g VSS/m ), X R is the concentation of biomass in etun line fom claifie (g VSS/m ), X is the concentation of

3 biomass in sludge dain (g VSS/m ) and X e is the concentationn of biomass in effluent (g VSS/m ) [4]. VSS stands fo volatile suspended solids. Figue 4... Activated sludge pocesss (a) Equations used fo design of aeation tank VX C X C 0 Y S S VX k d (b) Mass balance aound claifie X Fo X e =0 X X e X X X X X X X Recycle atio =

4 Poblem 4..: An activated-sludge system is to be used fo seconday teatment of 5,000 m /d of municipal wastewate. Afte pimay claification, the BOD is 70 mg/l, and it is desied to have not moe than 25 mg/l of soluble BOD in the effluent. A completely mixed eacto is to be used, and pilot-plant analysis has established the following values: hydaulic detention time θ C ( )=0 d yield coefficient (Y)=0.5 kg/kg, k d =0.05 d -. Assuming an MLSS concentation of 4500 mg/l and an undeflow concentation of 2,000 mg/l fom the seconday claifie, detemine () the volume of the eacto, (2) the mass and volume of solids that must be wasted each day, and () the ecycle atio. Solution: Given that =0,000 m /d, θ C =0 d Using C V=6 m VX Using C =0 X Y S0 S VX k d 5, 000 m /d 0.7 kg/m kg/m 0. d 0.05 d V 4.5 kg/m - X = kg/d If the concentation of solids in the undeflow is 2,000 mg/l kg/d 60.4 m /d 2 kg/m Fo X e =0 X X 5,000 m / d 4.5 kg/m kg/d X X 2 kg/m 4.5 kg/m ,000 Recycle atio m /d

5 PONDS AND LAGOONS Othe than activated sludge pocesses, ponds and lagoons ae most common suspendedcultue biological systems used fo the teatment of wastewate. A wastewate pond, altenatively known as a stabilization pond, oxidation pond, and sewage lagoon, consists of a lage, shallow eathen basin in which wastewate is etained long enough fo natual puification pocesses. Classification of lagoons is based on degee of mechanical mixing povided. Aeobic lagoon: The eacto is called an aeobic lagoon, when sufficient enegy is supplied to keep the entie contents, including the sewage solids, mixed and aeated. To meet suspendedsolids effluent standads, solids ae emoved fom the effluent coming fom an aeobic lagoon. Facultative lagoon: In facultative lagoon, only enough enegy is supplied to mix the liquid potion of the lagoon, solids settle to the bottom in aeas of low velocity gadients and poceed to degade anaeobically and this pocess is diffeent fom facultative pond only in the method by which oxygen is supplied. Facultative lagoons ae assumed to be completely mixed eactos without biomass ecycle [5]. Aeobic lagoons with solid ecycle: The aeobic lagoon with solids ecycle is same as extended aeation activated-sludge pocess, but an eathen (typically lined) basin is used in place of a einfoced-concete eacto basin. It is necessay that the aeation equiement fo an aeobic lagoon with ecycle must be highe than the values fo an aeobic flow-though lagoon to maintain the solids in suspension. DESIGN OF LAGOONS Pocess design consideations fo flow-though lagoons [6] BOD emoval Effluent chaacteistics Tempeatue effect Oxygen equiement Enegy equiement fo mixing Solids sepaation

6 Applying mass balance on lagoon given in abovee figue BOD in = BOD out + BOD co S 0 S S k V k 0 hee, S/S 0 =faction of soluble BOD emaining, k=eaction ate coefficient (d - ), θ=hydaulic detention time (d - ), V= eacto volume (m ), and = flow ate (m /d). If seveal eactos ae aanged in seies, the effluent of one pond becomes the influent to the next. A substate balance witten acoss a seies of n eactos esults in following equation: Sn S0 k n k k S V ks T 20 T-20 n onsumed A wide ange of values fo k is available in the liteatue. Althoughh many vaiables elating to both the eacto and wastewate affect the value of k, wate tempeatue affects it most significantly. k value at any tempeatue can be find out by following equation: hee, k 20 = eaction ate constant at 20 C (anges fom 0..2 to.0) and =tempeatue coefficient anges fom.0 to.2. Poblem 4..2: astewate flow fom a small community aveages 400 m /d duing the winte and m /d duing the summe. The aveage tempeatue of the coldest month is 0 C, and the aveage tempeatue of the wamest month is 0 C. The aveage BOD 5 is 200 mg/l with

7 70% being soluble. The eaction coefficient k is 0.2 d - at 20 C, and the value of tempeatue coefficient is.06. Pepae a peliminay design fo a facultative pond teatment system fo the community to emove 90% of the soluble BOD. a) Find volume of facultative lagoon to emove 90% of the soluble of BOD. b) Find the dimensions of thee squae lagoons in seies with depth.5 m. Solution: (a) Estimation of ate constants at given tempeatue k d Summe: k d inte: (b) Estimation of volume of lagoon Summe: inte: S 20 S0 k V= m V 0.4V V 400 V=29062 m (c) Estimation of dimensions of thee squae lagoons in seies, S 0,S,S 2,S V i V i V i 2 S S n 0 V i k n n Summe : V i V i m inte: V i Vi m

8 REFERENCES [] [2] 8%20-%20Activated%20Sludge.pdf [] [4] [5] Peavy, H. S., Rowe, D. R., Tchobanoglous, G. Envionmental Engineeing, McGaw-Hill, 985. [6] Tchobanoglous, G., buton, F. L., stensel, H.D. astewate Engineeing: Teatment and Reuse-Metcalf&Eddy, Inc., Tata McGaw-Hill, 200.