Sedimentation behaviour of red mud—reply

Letters to the Editors

Sedunentafion behavrour of red mud--reply (Received 28 November 1978 accepted 5 December 1978)

sirs,

ott[l] puts forward three points concemmg my papert ng with integral hehaviour during sedimentabon of red mud nns (I) that the calculations of the series solution of the flow hon may be performed more expe&tlously than I inchcated, hat I incorrectly asserted that the dependence of sechmenn rate on some properties of the slurry had not previously noted, and (111)that the flow equation I present cannot be predictively “as the final cblution, must be known or d by trial and error Means of predicting Q-t would therebe most welcome” accept (I) and (II) and am grateful to Dr Scott for them I .. however, part company with him on Ius third point le approach represented by my paper[2] seeks to predrct the utant physical aspects of sedimentation in slurries in which bydrauhc condictivity, K, and the water potential, ‘I”, are defined functions of the water content, 9 (volume of r/unit volume of solid) It is concerned with the evolution of water content profiles, and the approach to equdibnum m : materials The rate of accumulation of water above the Ing solid is an integral consequence of these profiles le approach IS based on the proposItion that Darcy’s law ribes v, the volume flux of water relatrve to the partwles in 3nse to a space gradient of the total water potential, Cp For !rials where K(6) LSwell defined, this propositIon is unexIonable The one-dmmensional continuity statement m a coiate system m, defined in terms of the distribution of solid me, and with

Dr Scott is also incorrect in predicted Its calculation follows 9, the total potential of the water, presented more generally III [S], equtlibnum equation

asserting that Q.. cannot be dvectly from the definition of described in [2] The theory is but in essence results in the

-*=(y,-l)(M-m)

(2)

In this equation, yc is the sohd specific gravity, M IS the total volume of solid per unit area of sedimentation column, and m the cumulative volume of solid per unit area from the bottom of the column, z = 0 In terms of the space coordinate, z, M=

I

o=

(I+ a)-’ dz

(3)

Equation (2) prechcts 9 as a function of m in 0 G m =ZM Use of v(Q) then permits us to graph 6(m), and the use of the equation Z=

M I Ll

(1 + 4) dm

(4)

then permits us to calculate the eqmlibrium depth of sedimented solid Q.. IS simply the difference between the mttial slurry depth and2 It will be seen that, contrary to Scott’s assertion, there is a well-established physical basis for the flow equation m [2] and for ifs prechctwe use in sedimentation studies

(3, =-63, so unexceptionable The flow equation derived by combining vith Darcy’s law is therefore logically based on physically I principles Furthermore, transient water content profiles rved during constant-pressure and constant-rate filtration of mute are consonant with thrs flow equation [3, 41 be ffmd mechanics of sedimentation IS identical wrth that of tion for the slurries we have in mmd. and the problem is well d Its exploration in terms of the Lagrangan approach narized in [2] is thus lustdied ?= IS the total volume of water standing above the sedimented y at static equdibrium

D E SMILES hvrsron

of Envrronmental Mechanrcs, CSIRO

Canberra Australra

REFERENCES [I] [2] [3] (41 [5]

Scott K .I , Chetn Engng Scr 1978 33 793 Smiles D E , Chem Engng SCJ 1976 31 273 Smiles D E , Sorl Scr Sue Am J 1978 42 11 Smiles D E , Chem Engng SCI 1978 33 1355 Smiles D E , Separ SCI 1976 11 2

(Recewed for publrcatlon 30 November

. Sus,

a recent paper Wolfbauer et al [I] presented experimental on biodegradation in the tubular aeration basin and the R The results of their experiments show that the sub&rat concentrations are almost equal for a CSTR and a tubular tor This is in contradiction to the calculations which are d on the Monod kmetics The Monod lonetics predicted a me required for a CSTR much higher than for a plug flow tor On the other hand applying zero order kinetics according Juhrmann the same conversion would be predicted in the R and piug flow reactor The results of theu experiments show the model of Wuhrmann seems extremely well suited to ribe the kinetics COD removal

1978)

This is quite astonishing since in most papers published on kmetics of the activated sludge process, under steady conditions, the Monod model is used In my opinion the fact of equal exit concentrations for a CSTR and a tubular reactor may be explained by dynamics of microbial growth The experimental works of many authors have demonstrated that the specific growth rate p does not instantaneously adjust to changes in the limiting substrate concentration [2-8] Perret[B] has referred to this phenomenon as “growth-rate hysteresis” The lag in specific growth rate is not well understood, but could be caused by differences in physiological activities of bacteria In the experiments of Wolfbauer et al [I] a CSTR was in steady state but the tubular reactor was in unsteady state from

Letters

to the Edrtors

the pomt of vtew of condmons of mrcrobml growth The activated sludge pumped from a clardier to a tubular reactor was subjected to changes m the envrronment condtttons (step excttatrons on the substrate concentratton) Of course, the lag m specrfic growth rate should be taken mto account for calculattons of extt concentratrons m a tubular reactor

MAREK WOJCIK Technical and Agnculturol Academy Deportment of Technology and Chemrcoi 85-326 Bydgoszcz, Semrnarypw 3 Poland

Chem~ol En@neenng Scrence 1979 Vat 34 p 753

Enguaeenng

Pergamon Press

753 REF?SRJ?,NCES

[II Wotfbauer D, Klettner H and Moser F, Chem Engng Scr 1978 33 953 r21 Mateles R I , Ryu D Y and Yasuda T , Nature 1%5 208 263 Bloeng 1%7 9 [31 Galley J W and Bungay H R, Biofechnol 617 Bloeng 1968 10 [41 GllIey J W and Bungay H R , Blotechnol 99 (51 Storer F F and Gaudy A F, Enulr SCI Tech 1969 3 143 I61 Sundstrom D W , Klet H E and Brookman G T , Blotechnol Broeng 1976 18 1 Broeng 1977 19 1431 [71 Chase L M , Brotechnof [81 Elmaleh S and Ben Atm R , Chem Engng SCJ 1978 33 365 [9l Perret C J , J Gen Muzrobrol l%O 22 589

Pi-Wed III Great Bntam

(Recerved

for pubhcatwn

Dear Strs, WoJcrk (see above) has grven very mterestmg comments on the paper we have pubhshed 111thrs ~ourna![l], that would deserve thorough drscusslon At thrs moment only a few remarks on hrs contrtbutton shaJl be made It 1s selfevtdent, that the change tn the concentratron of the surroundmg substrate IS much more pronounced m case of a plug 0ow aeratron basm, than m a basm with strrred tank charactenshc. and that a hrgher mfluence of a lag phase ts to be expected m a plug flow basm However, d we assume the substrate concentratron to be gtven as BOD. COD or TOC. as m [l], then another effect IS much more effective. that hasbeen called “adsorptron” [2,3] It ISvery much enhancmg the ehmmahon of the substrate from the waste water tn the Brst tune after entertng the aeration bastn A lag phase tn BOD removal practrcally does not take place due to adsorption, a lag phase m oxygen consumptron accordmg to our experience 1s not eifectrve enough by far to explant the equal performance of a plug flow basm and a bastn with hrgh backmtxmg, this IS shown by the fact, that the maxrmum of the oxygen consumptton takes place very soon after mtxmg acttvated sludge and waste water Out of thrs reason the uuhzatron of the ktnetrc model of Wuhrmann[4] can be Justied as a tirst approxunation for m case of synthetrc waste waters the reahty Of a superpositron of the elunmatton reactrons has been proved[4]. and It gtves a reahsttc picture of the mfluence of restdence tnne drstrtbution on the steady state performance of the basm Of course the zero order reactrons Wuhrmann assumes can be regarded as Monod reactrons wtth very low values of K, A closer aproxunatron to reahty ISachreved by a model mcludmg ehnunatron by adsorptron[3], thrs model however at the moment IS formulated only for plug flow basms, we hope that we shah be able to extend thus type of model to basms with hrgher backnuxmg

15 March

1979)

We feel, that the mcorporatton of the dynamics of bacterial growth. especraily of the dynamtcs of adJustmg the specdic growth rate to the model should be done as a further refinement, modelltng however always IS a compromtse between the accuracy and the complexrty of the model We therefore would hke to conclude, that the kmetrc model of Wuhrmann is of course not the final or stngle answer to the problems of modelhng the activated sludge process, d avoids however serious mrstakes that occur when a Monod type model IS used and substrate 1s descnbcd by a gross parameter as BOD or COD, espectally when the performance of a contmuous plants IS to be calculated based on batch data The authors would hke to correct an erraneous cm&Ion of a paper by Joschek er al [5] Thts paper does not show a factor of 3 between kmettcs measurements m a batch reactor and a contmuous plant, but a factor of about 1 7,t e a divergence of about the thrted Instrtut for Chemical Fundamentals Technrcal Unruersrty Austno

Engmeenng of Grar

0

WOLFBAUER H KLETTNER F MOSER

REFERENCES [l] Wolfbauer 0 , Klettner H and Moser F , Chem Engrtg Scr 1978 33 953 [2] Theophrlou J , Wolfbauer 0 and Moser F , gwflwasserobwasser 1978 119 H 3 S 135/14O [3] Theophtlou J , Wolfbauer 0 and Moser F , gwflwasserobwosser tn press [4] Wuhrmann K and Beust F , Schwerz Zertschr Hydrologre 1958 xx [5] Joschek H , Dehler J , Koch W , Engelhardt H and Getger W , Chemre-Ing Techntk 1975 47(10) 422