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Process Biochemistry Vol. 33, No. 2, pp. 199-203, 1998 © 1998 Elsevier Science Ltd

All rights reserved. Printed in Great Britain 01)32-9592/98 $19.0t) + 0.00

Surfactants in anaerobic digestion of salty cheese whey using upflow fixed film reactor for

improved biomethanation

Priti Patel and Datta Madamwar*

Post Graduate Department of Biosciences, Sardar Patel University, Vallabh Vidyanagar 388 120, Gujarat, India

(Received 3 January 1997; revised version received 5 July 1097; accepted 5 July 1997)

Abstract

In order to improve the anaerobic digestion process of salty cheese whey, the effects of various doses of surfactants: Tween 80, Triton-X-100, Tegopren 3022, sodium lauryl sulphate and Cetrimide, have been studied in an upflow fixed film bioreactor. Among these, sodium lauryl sulphate showed a 70% increase in gas production with a higher methane content (77%) and improved biodegradation. © 1998 Elsevier Science Ltd

Keywords: biomethanation, fixed film reactor, salty cheese whey, surfactant, energy, anaerobic digestion•

Introduction

There has always been a great deal of interest in anae- robic fermentation of industrial waste [1,2]. A large number of dairies in India dispose of their waste, especially cheese whey, into the environment in enor- mous quantities. Cheese whey, a by-product of cheese making, has a high organic content with a chemical oxygen demand (COD) of about 60-80 g li tre- ~, which leads to disposal problems. Anaerobic digestion of cheese whey offers a positive environmental impact since it combines waste stabilization with net fuel pro- duction and allows the use of the effluent as fertilizer. The major advantages of this process are low cost, high energy efficiency and process simplicity as compared to other waste treatment methods. However, in spite of waste reduction and energy potential, anaerobic diges- tion is not a popular process in the dairy industry. This is largely due to the problem of slow reaction rates which require long hydraulic retention time (HRT) and poor process stability using conventional reactors. The low growth rate of anaerobic micro-organisms has encouraged the development of high rate anaerobic bioreactors, in order to avoid the loss of micro-organ- isms in the effluent stream and to provide better

*To whom correspondence should be addressed.

199

process stability. Among the advanced technologies of anaerobic digestion, the fixed film reactor allows effi- cient digestion of high and low strength soluble waste at shorter HRT [3,4]. Our previous study has shown improved gas production with enriched methane content when salty cheese whey [with salt concentra- tions from 4 to 6% (w/v)] was used in a fixed film reactor at HRT of 2 days using charcoal as bedding material [5].

There is further growing interest in maximizing methane production from salty cheese whey, especially those with high Na + concentrations. The presence of high concentrations of sodium ions is detrimental to the performance of anaerobic bioreactors [6], and this problem was overcome by diluting salty cheese whey with total dairy waste with a low total solid content and by developing salt tolerant inocula. The mixing of total dairy waste with salty cheese whey reduces the salt content of the whey and supports the growth of salt tolerant methanogens. This dilution also provided an operational advantage due to the reduction in total solids. It has also been reported that surfactants increased the productivity of anaerobic digestion systems [7]. They show unusual catalysis of organic reactions and are known for the enhancement of bio- degradation of organic compounds [8-10]. An attempt has been made to determine the efffect of various sur-

200 P Patel & D. Madamwar

factants on anaerobic digestion of salty cheese whey using fixed film reactors.

Materials and methods

All chemicals used were of analytical grade. Cheese whey and dairy waste-water were collected from AMUL dairy (Cheese Manufacturing Unit) of Anand, India. The cheese whey had the following character- istics (w/v): lactose, 4.5-5%; protein, 0.6-0.7%; salt content (NaC1), 4-6%; total solids, 11-12%; volatile solids, 4.5-5.0%; COD, 6-8%; varying amounts of minerals and water soluble vitamins. The pH of the whey varied from 4-5 to 5.5.

The following surfactants were used in our study. Tegopren (T-3022), a speciality surfactant based on polyether-polymethyl siloxane copolymers, was obtained from Gold Schmidt AG, Essen, Germany. The following surfactants were obtained from the Central Drug House (CDH), India. Tween-80 (poly- ethylene sorbitane mono-oleate); Triton-X-100 (iso-octylphenoxy-polyethoxy-ethanol); sodium lauryl sulphate; Cetrimide (tetradecyl trimethyl ammonium bromide).

Reactor systems

Laboratory scale anaerobic upflow fixed film reactors consisted of a glass column with a void volume of ! litre, packing height 900 mm and packing volume 1.5 litre. Reactors were packed with charcoal as the support material having an average size of 5 x 5 x 5 ram. Biofilms were allowed to develop on the supporting materials for 60 days using effluent from another operating whey reactor after mixing with salty quick sand of coastal area, dug from about 60cm below the surface, as initial inoculum. This initial inoculum was slowly replaced by salty cheese whey diluted with total dairy waste-water (1:2 v/v) in order to obtain a final total solid (TS) of 3% (w/v) which was filtered through muslin cloth to remove flocs formed after adjusting the pH to 7.0 with lime. All reactors were operated at 37 + I°C. Steady state conditions were determined on the basis of constant COD of the effluent. All reactors were operated for 60 days after reaching the steady state condition. Surfactants were incorporated into the feed. The feed was pumped upwards continuously and the flow rate was adjusted with the help of a peristaltic pump (Gilson, Miniplus 3 model). The reactor was operated at an HRT of 2 days. This was found to be the most suitable from our earlier experience [5].

Experiments were carried out in quadruplicate for each surfactant and for each concentration.

Analytical methods

Gas production was measured by the displacement of acidified saturated salt solution making due correction for atmospheric pressure and temperature. Gas compo- sition was analysed with a chromatogram (Sigma) fitted with 2 m long stainless steel Porapak-R (80-100 mesh) column at 40°C and thermal conductivity detector. Nitrogen was used as a carrier gas at a flow rate of 40 ml/min. The temperatures of injector and detector were kept at 125°C. Fatty acids were analysed using gas chromatogram (Sigma) with a 10% FFAP column and a flame ionization detector. Column temperature was maintained at 180°C, whereas injector and detector temperatures were kept at 250°C. Nitrogen served as carrier gas. Identification and percentage of different fatty acids were based on a comparison of retention time and peak area of unknown with standard amount of each acid [11]. Feed and effluent samples were analysed for pH, volatile fatty acids (VFA), COD, total solids (TS) by standard procedures [12].

Results and discussion

The present investigations were confined to studies of the effects of various surfactants such as Tween-80, Tegopren-3022, sodium iauryl sulphate (SLS), Triton- X-100 and Cetrimide on anaerobic digestion of salty cheese whey using fixed film reactors. Steady state per- formance data are shown in Fig. 1. Increasing the con- centration of surfactants improved biomethanation providing maxima and minima at various parameters (Fig. 1). Amongst the various surfactants used, sodium lauryl sulphate showed the greatest effect with a maximum gas production of 5.7 litre litre- ~ of digester per day at 200 mg litre-1. The maximum enhancement of over 70% was achieved with the addition of 200 mg litre-1 SLS and gas production declined there- after. The gas was also higher in methane content in an SLS dosed reactor. Maximum enhancement of methane content from 69% in control reactor to 77% in SLS (200 mglitre-J) dosed reactor was achieved. The methane production, presented as litre of CH4 per gram of COD utilized, also improved by about 65% in SLS dosed reactors in comparison to control reactors with no surfactant, thus improving the fuel value.

One of the parameters which provides information about the process stability is the total volatile fatty acids [13,14[. SLS dosed (200mglitre -~) reactors showed the highest process stability as indicated by the lowest values of VFA. The data on the level of total volatile fatty acids present in the reactors stabilized with different doses of surfactants are given in Fig. 2. Average VFA concentrations ranged from 1.10glitre -I in the reactor with no surfactant to 0-520 g litre ~ in the SLS dosed reactor. These indicate that volatile fatty acids are consumed at a faster rate than in control experiments. The rate limiting step in

Surfactants in anaerobic digestion of salty cheese whey 2(11

methane fermentation often involves the degradation of fatty acids, and this is related to the efficiency of H2 utilization by methanogenic bacteria [15,16]. In the present study, it is clear that SLS dosing enhances the methane forming step of the digestion process. Thus the addition of SLS helps to maintain a low level of hydrogen by enhancing methane formation. Otherwise, the reactor would be stressed by accumulation of fatty acids, which in turn maintain low production of propionate and other reduced products [15,17]. This is further supported by the analysis of individual fatty acids. Propionate and butyrate were found to be the lowest in SLS dosed reactors, indicating a balance between the formation of fatty acids and their con- sumption (Fig. 2).

Process performance can also be judged by lower COD values indicating better biodegradation. COD values decreased with increasing dose of SLS, reaching maximum reduction at 200 mg SLS per litre. The COD was 3.0 g litre ~ in SLS dosed reactors (200 mg litre ~) in comparison to 7"5 g litre ~ in controls without sur-

factant. This indicates a trend of increased COD removal with increased dose of SLS, again showing higher bacterial efficiency in SLS dosed reactors. This in turn helped the biomethanation process.

The other surfactants, Tween-80, Triton-X-10(I and Tegopren-3022, also increased gas production with enriched methane content, indicating that surfactants in general enhance substrate conversion efficiency. An anionic surfactant like SLS exhibited greater effects on anaerobic digestion. Surfactants are especially noted for their wetting qualities and have unusual properties of micelle formation and show catalysis of organic reactions [18]. The addition of surfactant may lead to the formation of favourable active sites by forming micelles that enhance the coupling of sequential reac-

• tions for conversion of polymeric substances into soluble substances, fatty acids and finally into gases.

Orientation of the surfactant molecules at the solid- liquid interface could render the substances readily wettable by enzymes produced by bacteria, and there- fore the presence of surfactant may provide a morc

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favourable environment for the system. This will improve the digester performance and the quality of gas, giving a higher fuel value. However, the cationic surfactant, Cetrimide, was not found to be effective for the improvement of anaerobic digestion.

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Surfactants in anaerobic digestion of salty cheese whey 203

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