Olive oil mill effluent: Ageing effects on evaporation behaviour

Wat. Res. Vol. 25, No. 9, pp. 1157-1160, 1991 Printed in Great Britain. All rights reserved

0043-1354/91 $3.00+ 0.00 Copyright © 1991 PergamonPress pie

RESEARCH NOTE OLIVE OIL MILL EFFLUENT: AGEING EFFECTS ON EVAPORATION BEHAVIOUR MARIA CRISTINAANNESIN1 and FAUSTOGIRONI Department of Chemical Engineering, University of Rome "La Sapienza", Via Eudossiana 18, 00184 Roma, Italy (First received January 1990; accepted in revised form March 1991)

Abstract--Evaporation and distillation processes have been often used to treat wastewater from olive oil mill effluent: the effluent COD reduction varies widely with the characteristics of the waste. In this work we report some distillation tests on centrifuged olive oil mill effluent, performed in order to analyse the effect of storage time on the evaporation behaviour of this waste. Experimental data show that the ageing processes cause an increase in the concentrations of volatile compounds. According to a preliminary model such a behaviour can be described in terms of a chemical and a biochemical reaction between a few pseudo-compounds. Key words---distillation, mill effluent, olive oil, ageing time

NOMENCLATURE C C D ~

F= K= k= M

~

N= p= Ps ~

Q= T= t=

COD value (kg/m3) COD value pertaining to each component (kg/m3) distillate amount (kg) amount of sample to be distilled (kg) kinetic constant (d -t) vapour-liquid equilibrium ratio (dimensionless) universal constant in equation (3) normal boiling temperature (K) pressure (kPa) vapour pressure (kPa) constant in equation (4) absolute temperature (K) time (d)

Subscripts

A, B, S, R = pseudo-component j = pseudo-component W ~ water 1,2 = first and second reaction Superscripts

initial value before ageing O= initial value at the beginning of distillation oo~

tests infinite dilution lag value

Greek letters Or=

constant in equation (5) (K)

#= vaporization ratio (dimensionless) ?,= activity coefficient (dimensionless)

INTRODUCTION Disposal of olive mill effluent (OME) is a serious environmental problem due to its high organic loading; in the last few years several attempts to treat

wastewater from olive oil mills have been described in the technical literature (Bradley and Baruchello, 1980). In particular biological treatments have been suggested (Carreri et al., 1988; Shammas, 1984; Petruccioli et al., 1984), but they are not so attractive since the waste shows a low biodegradability and olive oil production is typically a seasonal process. These reasons account for the rising interest towards physical and chemical treatment processes (Amirante and Mongelli, 1982; Molinari and Drioli, 1988): in fact in Italy several pilot plants have been designed to treat O M E by evaporation and distillation processes, but there is disagreement about obtained C O D reduction results from technical reports. In our opinion, the large bias on the results is mainly due to the wide variability of O M E characteristics. In fact differences in extraction processes, in olive ripening, in storage time of olives before milling or storage time of wastewater before treatment, cause large differences in the concentration of volatile organic pollutants. Therefore a rational approach to the design of the O M E treatment process requires a previous characterization of wastewater in terms of pollutant concentrations and its physico-chemical behaviour. We have proposed (Annesini et al., 1983) to characterize O M E by grouping the u n k n o w n organic pollutants, with similar vapour liquid equilibrium behaviour, in classes each one of which is regarded as a pseudo-component. For each wastewater to be processed we have to account for a few pseudo-components (two or three) whose characteristics and concentrations can be evaluated from simple batch distillation tests.

1157

1158

Research Note

In this work we report a systematic study a b o u t the influence of storage time on wastewater characteristics. D u r i n g storage, m a n y chemical a n d biochemical reactions occur a n d large differences between fresh and aged wastewater are observed. In o u r work we describe the ageing p h e n o m e n a in terms of variation of p s e u d o - c o m p o n e n t concentrations a n d we propose a model o f two reactions to account for this process.

8

6I 2 E

•

P = 101.1

A

P =

El

P = 13.3 k P o

Materials OMEs were obtained from a commercial olive mill which operates with two subsequent centrifugation steps in order to separate oil, husk and water. The OME collected from the mill shows a mean value of SS equal to 37 kg/m 3, whereas, after solids separation, the supernatant has a COD value of about 50 kg/m 3.

~4

0,3 V o p o r i ZCIt. i o n

Method~" The wastewater obtained from the mill was split into two samples:

the first one was stored at -24~C: in this way chemical and biochemical reactions do not occur and samples to characterize not aged OME are available; ---the second one was stored at 2 0 + I°C in a tank connected to the atmosphere. At fixed times samples were withdrawn from the tank and stored at -24°C: in this way several different aged samples were obtained. Distillation tests were performed on samples centrifuged at 3500 rpm for 20 min in order to separate suspended solids and avoid their cracking during distillation. This reaction can produce light compounds which affect the distillation test. Batch distillation runs have been carried out with a glass apparatus where 400 cm 3 of OME were charged and small distillate fractions (20 cm 3) were collected at different times. A more detailed description of the apparatus and the experimental procedure is reported elsewhere (Annesini et al., 1983). Distillation tests were performed at 13.3, 26.6 and 101.1 kPa using OME with ageing times ranging from 0 to 61 days. Each sample and each distillate fraction was analysed to evaluate the COD value according to standard methods. Total COD value of samples with ageing times up to 61 days have been measured: the results are within __.10% of the COD value of not aged OME (50 kg/m3). R E S U L T S AND D I S C U S S I O N

The experimental results obtained with not aged O M E at different pressures are reported in Fig. 1 as a function of the vaporization ratio fl = D / F . In fact, in a previous work (Annesini et al., 1983) we showed that, if we represent the mixture with n pseudo-components, the total C O D of each distillate sample is given by: (1)

where c ° a n d kj represent the initial c o n c e n t r a t i o n a n d the v a p o u r - l i q u i d equilibrium ratio o f each pseudoc o m p o n e n t , respectively. Since the mixture is a very diluted aqueous solution, kj can be given as:

kj =

~,,~wp,j/P

2 6 , 6 kPo

a o

EXPERIMENTAL

C ( f l ) = E k j c ° ( 1 - fl)*~-'

kPo

(2)

0.6

0.9

rotio

Fig. 1. Distillation curves of not aged OME at different pressures.

where ~),~ is the activity coefficient o f j pseudo-comp o n e n t at infinite dilution in pure water, whereas p,j is the v a p o u r pressure of the same component. In e q u a t i o n (2) b o t h Psi a n d ?,j,~ are t e m p e r a t u r e dependent; for the v a p o u r pressure of each c o m p o n e n t , we used T r o u t o n ' s law and the C l a u s i u s - C l a p e y r o n e q u a t i o n to yield: lnp~j = M ( I - N j / T )

(3)

while, for infinite dilution activity coefficient, we assume: I n ''/ i,~w - Q// T.

(4)

F r o m the above equations we have: ki = [exp(M - ~ j T ) I / P .

(5)

In this e q u a t i o n we have the first p a r a m e t e r ( M ) which does not depend on the c o m p o n e n t a n d the second one (ej) which is characteristic of the j component. Substitution of e q u a t i o n (5) in e q u a t i o n (1) yields at each pressure the distillate C O D at different B values. This e q u a t i o n contains 2n + 1 parameters, i.e. c~(j = 1. . . . . n), ej ( j = 1 . . . . . n ) a n d M, which can be evaluated from the fitting of the experimental data. In this work, two volatile p s e u d o - c o m p o n e n t s are sufficient to describe the distillation behaviour, so five parameters have to be estimated from the fitting of the experimental data. The fitting parameters are reported in Table 1. These values show t h a t the first p s e u d o - c o m p o n e n t (A) is more volatile t h a n water a n d yields high C O D values in the first distillate samples; o n the contrary the second pseudo-comp o n e n t (B) is less volatile t h a n water a n d evaporates at the end of the test. T h e sum of the initial concentration of A a n d B is not equal to the total C O D c o n c e n t r a t i o n since some c o m p o u n d s with very low volatility are present which do not contribute to the

Research Note Table 1. Model parameters for olive oil mill effluent

1159 40

Fitting of not aged OME distillation curves

co(g/m3)

11SO

c o (g/m 3) ~,~

43150 6082

ct~ M*

30

~ ',

8951 23.5

E

•

t ~ 0

o

t - 26 d

0

o

t-54d

Fitting of aged OME distillation curves Kl(d -~) K2(d -I) a~

20

3.410 -3 4.810 2 6928

0 ~

0 u

*With temperatures in K and pressures in kPa. IO

COD of the distillate. Therefore we also consider a third non-volatile pseudo-component (S) showing a concentration equal to C O - c ° - c °. In Fig. 1 we compare the theoretical curves obtained with the parameters reported in Table 1, with experimental data at 13.3, 26.6 and 101.1 kPa. The figure shows that the agreement between experimental and calculated values is satisfactory at least for fl values smaller than 0.7; for fl > 0.7 the agreement is probably not so good for the production of volatile compounds due to the high temperatures obtained in the still high vaporization ratios. Batch distillation data of aged OME at 101.3 and 13.3 kPa are reported in Figs 2 and 3, respectively. These figures show that the ageing process causes large differences in distillation curves although, as previously reported, the total COD variations are in the range of experimental error. A rough analysis of the experimental data shows that: --first distillate samples show a COD value rising with ageing time. This trend indicates that aged OMEs have a concentration of volatile compounds higher than that of not aged ones; - - w i t h an ageing time greater than 40 days, we have a significant increase of the COD in the middle region of the distillation curves. This behaviour

40 1

U

__m

t "Od

',,

o

t

'~

/',

t -40d

V

t -61

~'~

" 26 d

d

0 VL 10

~' o -

0

0

0.3

0.6

0.9

V o p o r i z o t ion r o t i o

Fig. 3. Distillation curves of aged OME at 13.3 kPa. can be explained if we assume the presence, in aged OMEs, of another volatile pseudo-compound whose k-value is intermediate between that of light (A) and heavy (B) compounds. Of course, it is difficult to define the true reactions occurring during ageing of OME. Therefore, in order to describe the effects of ageing and to obtain a useful tool for process design, an approach which takes into account "fictitious" reactions between pseudo-components has been considered. Two reactions are required for a satisfactory fitting of the experimental data: - - t h e first reaction accounts for the COD increase in the first distillate samples. We assume that the heavy pseudo-component (B) degradates and produces the light compound (A): B--* A.

dcB/dt = - KIcB i

- \

20P

a .~_ a_ o__q _o _~ _o- 4>-o- 0-

We also assume that B degradation is a first order reaction with a kinetic constant K~:

!

30

o" "121,

~ V " " V"-V- -V_

re.m_ m

_

V

~

0.3 Voporizotion

~_e-m-

0.6

0 ~°~

0.9

rotio

Fig. 2. Distillation curves of aged OME at atmospheric pressures.

(6)

- - t h e second reaction accounts for the steep rise in the middle region of the distillation curves observed when ageing times are higher than 40 days. This phenomenon can be ascribed to biodegradation of the non-volatile pseudo-compound (S) to produce a pseudo-compound (R) with a volatility lower than that of A and higher than that of B. We also assume that this reaction occurs only when the ageing time is greater than a critical value "i (40 days). This hypothesis seems to be reasonable: in fact, it is well known (Bailey and Ollis, 1986) that batch biological processes show a lag phase where no increase in biomass concentration or metabolic activity are evident. The biodegradation reaction can be represented as: S~R.

1160

Research Note

Its rate is given by: (7)

dc~/dt = - K2c s.

Therefore an aged O M E can be described by means of two volatile pseudo-components (A and B) if the ageing time is smaller than the lag time 7, while three volatile compounds (A, B and R) are required for t > ~. Furthermore OMEs also contain a non-volatile pseudo-component (S) which does not contribute to the C O D of distillate. Equations (6) and (7) can be integrated finding the pseudo-component concentrations at different ageing times: cA = c~ + c~(1 - e x,,)

(8)

cB = c*e x,,

(9)

eR=0

t<7

C R = C * [ 1 - - e -K2"-~]

(10a) t~>t"

(10b)

where the asterisks refer to the concentration at the beginning of the ageing process. In order to fit distillation curves of an aged OME, we can use equation (1) again where the initial concentrations of pseudo-component cj0 coincide with the concentrations evaluated from equations (8), (9) and (10). Really the model contains some u n k n o w n parameters: the kinetic constants K~ a n d / ( 2 and the constant ctR which is required to calculate the equilibrium ratio of R from equation (5). Table 1 reports the fitted values of these parameters; in Figs 2 and 3 we compare the experimental data with the calculated distillation curves at different ageing times. The agreement seems quite satisfactory both at atmospheric and low pressure. CONCLUSIONS Experimental results on the distillation of aged O M E have shown that large variations in the concentration of volatile compounds occur during ageing,

whereas no significant variations of the overall COD have been detected. In this paper, the increase of volatile compound concentrations has been ascribed to a chemical and a biochemical degradation reaction; work is in progress in order to identify the true chemical alterations occurring in the waste and therefore to verify the interpretation suggested in this preliminary model. In any case, results reported in this study give useful information on the design of the disposal of such effluents. In particular obtained results indicate that a long storage time must be avoided if OMEs have to be treated by evaporation or distillation because the increase in the volatile pollutant concentration significantly reduces the equipment separation efficiency. Acknowledgement--This work was financially supported by

the Italian "Ministero della Universitfi e della Ricerca Scientifica". REFERENCES

Amirante P. and Mongelli C. (1982) Prove sperimentali di trattamento delle acque reflue di un oleificio con impianto di incenerimento. Riv. ital. sostanze grasse LIX. 295-300. Annesini M. C., Giona A. R., Gironi F. and Pochetti F. (1983) Treatment of olive oil wastes by distillation. Eft/. Wat. Treat. J. 23, 245-248. Bailey J. E. and Ollis D. F. (1986) Biochemical Engineering Fundamentals. McGraw-Hill, New York. Bradley R. M. and Barucbello L. (1980) Primary wastes in the olive oil industry. Effi. Wat. Treat. J. 20, 176-177. Carreri C., Balice V. and Rozzi A. (1988) Comparison of three anaerobic treatment processes on olive oil mill effluents. 2rid International Conference on Environmental Protection, S. Angelo d'Ischia, pp. 37-44. Molinari R. and Drioli E. (1988) Processi integrati di ultrafiltrazione e osmosi inversa nel trattamento delle acque reflue da frantoi oleari. Acqua-Aria 5, 579-588. Petruccioli M., Servili M., Montedoro G. F. and Federici F. (1988) Development of a recycle procedure for the utilization of vegetation waters in the olive-oil extraction process. Biotechnol. Lett. 10, 55-60. Shammas N. Kb. (1984) Olive oil extraction waste treatment in Lebanon. Effi. Wat. Treat. J. 24, 388-392.