Chapter 5 Sludge treatment

Transcription

1 Chapter 5 Sluge treatent 5.1 Sluge efinition: Sluge is ae of soli aterials separate fro the water line uring wastewater treatent. In aition to solis, sluge contains a high percent of water. 5.2 Sluge sources: In wastewater treatent sluge is classifie into the following types: A. Grit sluge: It is all the solis collecte in the grit reoval chaber, because grit can be easily raine an is relatively stable in ters of biological activity, it ose not nee treatent an is generally ispose irectly to lanfills. ١

2 B. Priary or raw sluge: Priary sluge is the sluge taken fro the botto of the priary g settling tanks. It contains fro -8% solis (1% 10,000 ).his L sluge is copose of settleable raw solis. Alost 7% of the priary sluge is organic. C. Seconary Sluge: his sluge consists of icroorganiss an inert aterials that has been waste fro the seconary treatent processes. hus 9% of this sluge is organic this sluge contains fro 0.8 to 2% g solis ( i. e. 8, ). L ٢

3 5. Sluge quantities: A. Priary sluge: he quantity of priary sluge is calculate using the following equation: M E ss (5.1) p Q in Where, M p ass of priary sluge, kg kg ss= suspene solis in the influent, E = efficiency of priary seientation tank taken fro (Figure (1)) Q in = influent flow to priary seientation tank,. ٣

4 kg o convert the sluge quantity fro to : M p Q p (5.2) X p Q p = flow rate of priary sluge,. kg X p =concentration of solis in priary sluge,. X S (5.) p sluge where, k kg sluge sluge ensity, ( usually taken as water 1000 unless given S = solis fraction in sluge expresse as ecial fraction (i.e = 5%). ٤

5 Exaple 5.1:- Deterine the quantity of priary sluge both in kg an /, given the following:- SS = 700 g/l Q in = 2000 /hr S = 5% ry solis in the sluge. D = 6 (iaeter of priary seientation tank) ٥

6 Q inf P.S A. S.S Q eff sluge sluge 1 St hickener Q thick Sluge igester Q thick Sluge 2 n hickener Sluge Q ew Dewatering ٦

7 Solution:- a) fin the overflow rate to the priary seientation tank:- Qin O/ F A 2 2 D *6 2 A hr O/ F hr 1018 b) Fin the seientation efficiency (E):- fro figure (1), E = 54% ٧

8 c) Calculate sluge quantity:- M P = E * SS * Q in g 10 L kg Kg SS L 10 g 24hr Qin hr Kg M P M P QP X X Q P P P Kg SL S Kg 6 50 Kg Kg Kg ٨

9 b) Seconary sluge quantities:- Seconary sluge quantities were iscusse previously in the suspene growth treatent:- kg XV Px sluge prouction Px Qw, sluge prouction in X r For ore etails on these quantities return to chapters an 4. c 5.4 sluge treatent:- he ain goal of sluge treatent is to convert it to an inert soli prouct that can be safely ispose in the environent or reuse for useful purposes. he ain sluge treatent steps area. Sluge thickening. b. Sluge stabilization. c. Sluge ewatering. In the following section, a etaile iscussion of these treatent steps is given. ٩

10 5.5 Sluge thickening:- he ain goal of sluge thickening is to reuce the sluge volue by reoving as uch as possible of the water content of sluge, leaing to the increase of the solis content of sluge. hickening is achieve by the following three ethos:- a) Gravity thickening:- Gravity thickening is achieve in circular tanks siilar to seientation tanks. Sluge is allowe to settle an copact. he thickene sluge is withrawn fro the botto of the thickener. o iprove thickening soe cheicals are ae, this process is calle sluge conitioning. Sluge conitioning is iscusse below. ١٠

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13 b) Flotation thickeners:- For sluges with low weights, gravity thickeners are not efficient. In this case, the sluge floats on the surface an than reove by scu reoval ars. c) Mechanical thickening:- Mechanical thickeners inclue centrifuges, rotary ras an filter belts. he centrifuges an rotary rus use the centrifugal force to separate solis fro liquis. Filter belts are squeezing achines in which the sluge is inserte between two oving belts resulting in the separation of solis fro liquis. ١٣

14 5.6 Sluge conitioning:- Sluge conitioning is a pretreatent step to increase the efficiency of sluge thickening an ewatering of sluge. he two ost coon ethos of sluge conitioning are:- a) Cheical conitioning:- Aition of lie Ca (OH) 2. Aition of ferric chlorie Fe cl Aition of alu AL 2 (SO 4 ). Aition of organic polyers. hese cheicals are coagulants as they neutralize an estabilize the electrically charge particles in the water an allow the to settle efficiently. ١٤

15 Organic polyers replace Fe cl an AL 2 (SO 4 ) salts in orer to overcoe the ajor probles involve with these inorganic cheicals. Avantages of organic polyers over inorganic cheicals are:- Dosage is aroun 10 ties lower than that of inorganic cheicals (typical osage is 5-10 kg/ ton sluge ) Cost saving obtaine in capital cost an running cost. reate sluge ose not contain inorganic cheical such as Fe + an AL +. b) Heat treatent:- Heat treatent is a conitioning process that involves heating of sluge for short perios of tie uner pressure. By heat treatent, the structure of sluge flocks is altere to liberate ore water fro particles. he high capital cost of equipent liit the use of this etho to sall treatent plants only. ١٥

16 5.7 Design of gravity thickeners:- Priary an seconary sluge are either treate in separate thickeners or in one coon thickener. he following table gives the esign criteria for gravity thickeners accoring to the sluge source:- Sluge source Influent S (%) Expecte uner flow S (%) - Separate sluges PS..... F..... RBC..... WAS Cobine Sluges PS + WAS PS + F PS + RBC Mass loaing kg/ Note: PS = priary sluge., F = rickling filter sluge, RBC = Rotating biological contactor sluge, WAS = Waste activate sluge. ١٦

17 - he area of the thickener is eterine using the ass loaing criteria accoring to the sluge source as shown in the above table. - he epth of the thickener is eterine as follows. As shown in Figure (4), the thickener epth is ivie to the following regions:- صورة ص 10/ 5 h 1 = free boar epth above sluge surface, typically taken as 0.6. h 2 = clear liqui zone, typically taken as 1.0. h = settling zone, typically taken as 1.5. h 4 = hickening zone, it is a sluge storage region an have to be calculate as shown later. ١٧

18 So, the thickener sie wall epth (H) is calculate as :- H = h 1 + h 2 + h + h 4 he epth at the center of the thickener is calculate by aing (H) to h 5 :- h 5, thickener iaeter. 16 o calculate (h 4 ), the thickening zone height, we assue that this zone has a volue enough to store one ay thickene sluge. So the total solis interring the thickener will accuulate in this zone but the concentration of solis in the thickene zone will be uch ore than that in the sluge interring the thickener so, to calculate (h 4 ):- V A. h V A V Z Z Z ( ( X ( ( X ) ) in out 4 Volue Surface ) ) in ( M out ( ) SL of area in ) slugein of t ( S ) the out the thickener, thickening zone, 2 ١٨

19 ( ) in = ass of sluge ry solis accuulate in the thickening zone, (kg) Kg (M ) in = ass of sluge ry solis interring the thickener per ay, t = tie, ays. (X ) out = concentration of solis in sluge in the thickening zone an in the thickening zone an in the thickene sluge coing out of the thickener, (kg/ ) ( SL ) = ensity of thickene sluge, (kg/ ) (S ) out = concentration of solis in the thieone sluge expresse as ecial fraction. Cobine equations (5.4) an (5.6) an solve for h 4 : ( ) in h4...(5.9) A ( X ) out ١٩

20 Exaple (5.2):- 0Design a gravity thickener to treat a cobine priary an activate sluge. he following ata is given:- Priary sluge:- kg QP 500 /, SL 1100, S 5% Seconary sluge (activate sluge):- kg Qs 150 /, SL 1040, S 1% hickener esign criteria:- - L S = solis loaing rate = 50 kg/ 2. (assue) - hickene sluge concentration (S ) out = 7% ( SL ) = ensity of thickene sluge = 1120 kg/ ٢٠

21 Solution:- 1) calculate (M ) in, ( ) in :- (M ) in = M P + M S kg M P QP X P Qp SL S kg kg M S Qs X s QS. sl. S kg kg kg ( M ) in kg ( ) in ( M ) in * t kg ( note : t 1 always for the esign of thickeners) 2) calculate the area of the thickener "A ":- ( M ) in kg / 2 A L 50 kg /. S kg ٢١

22 - Fin the iaeter:- 4 4 A ( not o. k) (note:- the axiu iaeter for thickeners is 20) Use two thickeners:- 581 Area of each thickener = = o. k ) calculate the epth of each thickener:- ٢٢

23 h 4 ( X ) A out ( t ( X ) in ( ) sl out ) ( S out kg kg ( ) in 1450kg kg h4 0.64, say kg ) h5 1.19, say sie wall epth:- H = h 1 + h 2 + h + h 4 = =.75 Depth at the center = = 4.95 = H C ٢٣

24 4)Culcalate the reuction of sluge volue:- ( M ) out / 2 hicken isch arg e Q ( X ) 78.4 Qp QS ( Qw) Reove Q n ( QW ) Reove % reuction QP QS ( ) n out 185 / ( 185) / % ( ) 2 / 5.8 sluge stabilization:- Sluge stabilization is the process in which the bioegraable organic atter is converte to non-bioegraable (or inert) atter. he ain purpose of this process is to prouce a sluge that will not unergo any further ecoposition when ispose to the environent (i.e stable sluge). If sluge is ispose without stabilization it will create ba oor an health hazars. ٢٤

25 he ost coon etho use for sluge stabilization is the anaerobic igestion. his process is achieve in a close anaerobic tank calle igester. Figure (2) shows a typical igester cross section. he igester shown is a cylinrical tank with a conical shape botto. It is an anaerobic copletely ixe reactor. he cover of the igester is a floating cover to give flexibility for gas accuulation an to prevent explosion. he sluge in the igester is heate to 5 o C to give the best igestion efficiency. he igeste organic atter is converte into gases such as ethane (CH 4 ), carbonioxie (CO 2 ) an (H 2 o). he prouce ethane is collecte an use for energy prouction ٢٥

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27 Cross section through a typical stanar rate igester 5.9 Solis reuction in igesters:- he solis in the sluge interring to the igester is classifie as follows:- صورة ص 5 17/ Fro this classification, we unerstan that the solis reuction will be perfore only on the bioegraable portion of the organic atter, while the inorganic an the non-bioegraable portions will leave the igester without reuction Anaerobic igester esign:- he volue of the igester is eterine base on the sluge etention tie ( c ) using the following relation:- V.. (5.10) Q in c an ( c ) is selecte in the range of ays base on the percent of organic atter estruction. he following equation is use to calculate the percent reuction as a function of ( c ):- R s = 1.7 ln (( c )) (5.11) ٢٧

28 Where:- R s = percent Reuction of organic atter,% c = sluge etention tie, ays. he iaeter of the igester is typically is the range he siewall epth shoul not be less than 8 because of ixing ifficulties in shallow tank, an axiu siewall epth is 15. he floor of the igester is usually conical with a botto slope of 1:6. he rate of gas prouction in igesters is typically taken as 0.5 gas for each "kg" organic atter estroye. he percent of ethane is approxiately 65% of the total gas prouction. he prouce gas is collecte fro the top of the igester an passe through a water tank in which CO 2 an H 2 S an other gases are issolve while CH 4 (ethane) is collecte a gain an store in a cylinrical tank calle gas holer. hen, ethane is use for power generation that can be use in the wastewater treatent to operate the electrical equipents. Methane is 60-70% of the total gas prouce ٢٨

29 Exaple (5.):- Anaerobic igester is to be esigne to treat a thickene sluge having the following ata:- Q = 70 /, S = 7%, M = kg/, S.G = 1.12 = 20 ays, 70% of M is organic, S.G = 1.05 after igestion. c 1) Calculate the require igester volue:- * V = Q. c = (70 /) * 20 ay = 7400 his volue ose not inclue the volue of the conical volue at the botto. Assue the siewall height ٢٩

30 H = 8, So the surface area As A S V H 925* not. ok So we can either increase the epth "H" to a value in the range of 8-15, or we can use two igesters. Let us first try to increase H to 15: So As * so let us use 2 igesters. not. ok, ٣٠

31 H = 8, So the surface area A S V H 70 So Q 185 / ( for eaeh igester) 2 V Q. (185 / )* assu H 15 A s c * , say18 ٣١

32 0.5 free bear H h Slope = 1:6 ٣٢

33 if the igesters is neee to be lower than 15 we can use or 4 igesters to get saller igesters. Correct "H" for = 18 :- 2 2 A s V 700 H 14.5 A a a (0.5) as freeboar between the axiu sluge level an the floating cover Calculate the conical part epth:- 18 h ٣٣

34 2. Calculate the efficiency of solis reuction:- R 1.7 ln 18.9 s c 1.7ln %. Calculate the organic atter weight: M 1450 kg / ( for each igester) 2 M organic atter 70% kg / o ( M iaert ) Raw inert atter 0% kg / 4. Calculate the weight of the estroye organic atters. M est = 60% * organic atter = 0.60 * kg/ (for one igester) = 610 kg/ (M inert ) = = 4068 kg/ (inert atter) Calculate the total inert aterials leaving the igester(m):- M ( M ) ( M ) inert Raw inert 8427 kg ٣٤

35 5. Calculate the gas prouction:- Gas = 0.5 /kg x M est 0.5 kg 610 kg 052 Methane 60% fro each igester Note: this volue is at stanar conitions i.e. = 20 0 C, P = 1 atospheric. It is usually store in gas holers (or gas storage tanks) at a pressure larger than the atospheric pressure to reuce the volue of gas holers for exaple, if the gas is store at P = 2 at the volue V1P 1 181*1 V2 916 so we can size the gas holer. P 2 2 ٣٥

36 6. Calculate the percent reuction of total solis by the igester:- M est % R % M Calculate the concentrations of solis after igestion:- M 8427 kg / kg X 45.6 Q 185 / S X se kg kg % 1.05 ٣٦

37 8. Mass an flow balance for the igester:- M 8427 kg/ Q 185 / X 45.6 kg/ S 4.% se 1050 kg/ igester M se 1450 kg/ Q 185 / X 78 kg/ S 7% 1120 kg/ ٣٧

38 5.10 Sluge e watering:- Dewatering is a physical process use to reuce to water content of sluge after stabilization. Several ethos are use for ewatering as follows:- A) Natural ewatering:- Natural ewatering epens on evaporation an percolation. Sluge rying bes are the ost coon exaple of natural ewatering. Evaporation Sluge Fine san Gravel Perforate pipe Sluge rying bes ٣٨

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40 B) Mechanical ewatering:- Mechanical ewatering ethos are ivie to three ain categories:- a) Vacuu filtration:- Water is reove uner applie vacuu through a porous eia that retains solis an allow water to pass. b) Pressure filtration:- Water is reove by applying presue (squeezing). Belt filter process is the ost coon exaple on this etho. Sluge Dewatere sluge Squeeze water Sluge oposal ٤٠

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42 ) Centrifugation:- Solis are separate fro liqui by centritugal forces. Soli bowl centrifuge is an exaple on this etho. Mass an flow balance is perfore on ewatering units as follows:- M S X Q Dewatering unit M S X Q Note:- "S" after ewatering in the range of 25 50%. ٤٢

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