JOURNAL OF FERMENTATION AND BIOENGINEERING

Vol. 83, No. 5, 502-504. 1997

Biomethanation of Salty Cheese Whey Using an Anaerobic Rotating Biological Contact Reactor

CHIRAG PATEL AND DATTA MADAMWAR*

Post-Graduate Department of Biosciences, Sardar Pate1 University, Vallabh Vidyanagar 388 120, Gujarat, India

Received 1 July 1996/Accepted 7 January 1997

The anaerobic treatment of salty cheese whey was investigated using horizontal and vertical shaft types of anaerobic rotating biological contact (ARRC) reactors to determine the effects of the hydraulic retention time (HRT) and the type of rope used to form the fixed-film structure on the rotating discs. Optimal performance was obtained with the vertical shaft-type ARRC reactor operated at 3-d HRT and 37’C, using cotton rope to form the t&d-film structure. Maximum gas production of 4.1 ZAdigester . d with a methane content of 74% and COD reduction of 85% were achieved.

[Key words: biomethanation, salty cheese whey, anaerobic rotating biological contact reactor]

In India, cheese whey, a by-product of cheese produc- tion, is being generated in enormous quantities as a result of increased production of cheese. The whey has a high chemical oxygen demand (COD) of about 60 to 8Og/l, which very often causes a problem of disposal. However, it also represents a potential energy source, and its anaerobic digestion offers an excellent approach in terms of both energy conservation and pollution con- trol.

In spite of several advantages, the anaerobic digestion process has not gained popularity in the dairy industry, largely due to the problem of the slow reaction, which requires a longer hydraulic retention time (HRT) and exhibits poor process stability in a conventional reactor. In order to solve these problems and to develop a better methanogenic process there is growing interest in de- veloping a more suitable high-rate anaerobic bioreactor for the generation of methane from acidic cheese whey, especially one which has a high Na+ concentration. A considerable amount of information is available on the processing of sweet cheese whey (l-2), but there is very little on the use of salty whey. The presence of a high concentration of sodium ions have been found to be detrimental to the operation of anaerobic reactors (3). This problem could be overcome by diluting cheese whey with total dairy waste having a low total solid content and by developing a salt-tolerant inoculum. 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 would also provide an operational advantage due to the reduction in the total solids. However, this needs to be studied in detail.

We report here on the performance of an anaerobic rotating biological contact reactor (ARBC) handling salty cheese whey diluted with total dairy wastewater and the development of a special inoculum for the reactor.

Salty cheese whey and total dairy wastewater were collected from the AMUL Dairy of Anand, India. Two ARBC reactors were constructed using metallic sheeting, each having a void volume of 5 I (Fig. 1A). One of the two reactors was fitted with a vertical shaft and the other with a horizontal one. The reactors were packed with a fixed-film structure made up of a series of discs

* Corresponding author.

(6 plates) mounted on the vertical/horizontal shaft. Discs were formed using various types of rope (cotton/nylon/ coconut/asbestos) 5 mm in thickness by tying the rope onto hooks fixed on concentric metallic rings (Fig. 1B). The specifications of the ARBC reactors are given in Table 1.

A biofilm was allowed to develop on the fixed-film discs for 30 d using effluent from another operating whey reactor after mixing it with salty quick-sand from a coastal area (taken about 60cm below the surface) as the initial inoculum. The initial inoculum was slowly replaced by fresh diluted salty cheese whey (salty cheese whey was diluted with total dairy wastewater to obtain final total solids [TS] of 3% [w/v]). Salty cheese whey and total dairy waste in the proportion 1 : 2 was normal- ly found to be suitable for obtaining TS of 3% (w/v) with a NaCl concentration of 1.5 to 2% (w/v), which was filtered through a muslin cloth to remove flocks which formed after adjusting the pH to 7 with lime. Both the reactors were operated at 37 + 1 “C. Steady-state conditions were achieved on the basis of constant gas production, and constant COD of the effluent. All the reactors were run for 45 d after reaching the steady state. The feed was pumped continuously by adjusting the flow rate with the aid of a peristaltic pump. The reac- tors could be operated at a desired HRT. Experiments were repeated four times and the average data were recorded with standard deviations.

Gas was collected and measured from the displace- ment of acidified saturated salt solution, making due correction for atmospheric pressure and temperature. The gas composition was analyzed using a Sigma gas/liquid chromatograph (Model MI 404) equipped with a 2-m long Porapak-R (80-100 mesh) column at 40°C and a thermal conductivity detector using nitrogen as the car- rier gas with a flow rate of 40ml/min. The temperatures of the detector and the injector were 125°C. The feed and effluent samples were routinely analyzed for pH, volatile fatty acids (VFA), COD, total solids (TS), and volatile solids (VS), as per standard procedures (4). Vari- ous types of fatty acids were analyzed using the same chromatograph as above with a 10% FFAP column and flame ionization detector. The column temperature was maintained at 170°C and the injector and detector tem- peratures at 250°C. Nitrogen served as a carrier gas.

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FIG. 1. Anaerobic rotating biological contact reactors. (A) 1, Vertical shaft type; 2, horizontal shaft type. (B) Sectional views of impellers: 1, vertical shaft type; 2, horizontal shaft type.

Identification and percentages of different fatty acids were based on comparison of the retention time and peak area of unknowns with standard amounts of each acid.

The present study was carried out to ascertain the most appropriate type of rope for the fixed-film struc- ture, and to standardize the conditions to optimize the efficiency of the ARBC reactor. Table 1 summarizes the steady-state performances of the vertical and horizontal shaft types of ARBC reactors operated at 37”C, using coconut rope to form the fixed-film structure for biofilm formation, under different hydraulic retention (HRT) times. In general, better process performance was achieved by the vertical ARBC, in which the rotating fixed-film discs were completely immersed in the fluid, giving them better contact with the substrate. When the HRT was increased from 1 to 5 d, there was a gradual decrease in gas production expressed as liters of gas produced per liter of digester per day. This is mainly because the organic load was decreased with the increase in HRT. However, presenting the same data as liters of gas produced per g of TS loaded shows a spurt in gas production with a higher methane content when the HRT was increased from 1 to 3 d. Further increases in

NOTES 503

TABLE 1. Specifications of anaerobic biological contact reactors

Specification Horizontal Vertical

Total volume in I 6.0 6.0 Void volume in I 5.0 5.0 Length/depth in cm 28 15 Diameter in cm 18 29 Impeller: (a) Diameter in cm 14 23 (b) Length in cm 28 12 (c) No. of discs 6 6 (d) No. of paddles on each disc 2 3 (e) No. of hooks on each paddle 6 4 (f) Total no. of hooks 72 72 (g) Distance between two hooks in cm 4.5 2.5 (h) Total length of rope in m 4.5 5.0 (i) Thickness of rope in cm 0.5 0.5 (j ) Volume occupied by impeller in I 0.25 0.40

the HRT did not increase the gas production propor- tionate to the total solids intake. The best results in terms of gas production and its methane content were obtained when the ARBC reactors were operated at 3-d HRT.

Process stability, as evidenced by lower volatile fatty acids, is one of the sensitive parameters (5) in anaerobic digesters. Table 2 shows data on the levels of total vola- tile fatty acids present in the reactors operated at differ- ent HRTs. The concentration of volatile fatty acids was generally found to decrease with an increase in HRT. The maximum process stability occurred with a lower concentration of volatile fatty acids (5), and this was achieved at 3-d HRT, indicating a proper balance be- tween the formation of acids and their consumption. At 3-d HRT, the levels of propionate and butyrate were low (Table 2). This, in turn, favours the methane-forming step of the digestion process, which is the slowest and most rate-limiting step.

The process performance can be also supported by lower COD values, which are indicative of greater bio- degradation (6). The COD values were reduced considera- bly when the HRT was increased from 1 to 3 d; the im- provement in the percentage of COD removal suggested that the biodegradability was increased by increasing the HRT up to 3 d due to better bacterial efficiency (7). However, further increases in the HRT gave only negligi- ble increases in the percentage of COD removal.

Having ascertained the optimum HRT for the best process performance and maximum gas production with enriched methane content, experiments were carried out to determine the most suitable supporting material for the fixed-film structure, using coconut, asbestos, nylon, or cotton rope as the support material. The highest total gas production was obtained in the reactor having a fixed-film structure of cotton rope (4.1 I/I digester. d in vertical and 3.7 I/I digester .d in horizontal), followed by asbestos (3.8 1/l digester .d in vertical and 3.4 f/l digester. d in horizontal) and coconut (3.3 IN digester. d in vertical and 3.0 l/l digester.d in horizontal) and the lowest in the reactor containing a nylon rope fixed-film structure (3.0 l/i digester .d in vertical and 2.7 IN digester.d in horizontal). Cotton rope seems to provide a better surface for the attachment of methanogens and other anaerobic bacteria, resulting in good biofilm de- velopment on the support medium. However, being cellu- losic in nature, the cotton undergoes changes over a period of time and has to be replaced every 6 months.

504 PATEL AND MADAMWAR J. FERMENT. BIOENG.,

TABLE 2. Methane production from salty cheese whey in anaerobic rotating biological contact reactors operated at different hydraulic retention times

Hydraulic Type of retention Gas production Gas production Methane Volatile fatty acids (g/l) COD removal pH

time reactor (l/I digester. d) (Ng TS . d) (%) Total Propionate Butyrate (%)

1 H 4.3 k0.23 0.14 66 2.10 0.92 0.65 65t2.38 6.5 V 4.92024 0.16 67 1.95 0.85 0.63 67k2.94 6.6

2 H 3.3kO.12 0.22 70 1.80 0.70 0.47 72kl.92 6.7 V 3.8f0.13 0.25 72 1.65 0.82 0.43 14k2.54 6.6

3 H 3.OkO.10 0.30 73 1.30 0.36 0.32 77k2.07 6.9 V 3.3kO.12 0.33 73 1.18 0.30 0.28 78k2.54 7.0

4 H 2.5k0.13 0.33 72 1.41 0.45 0.38 79k2.78 6.9 V 2.7i0.21 0.36 71 1.28 0.40 0.35 8Ok3.16 6.8

5 H 2.1 kO.19 0.35 73 1.38 0.66 0.55 80t3.16 6.7 V 2.3t0.18 0.38 71 1.30 0.61 0.51 8Ok3.16 6.8

COD of influent: 30 g/l. Operating temperature: 37°C. Fixed-film structure: coconut rope. Rotating speed of impeller: 6 rpm. Reactors were onerated for 45 d after reaching steadv-state conditions (steady state was reached in 30 d). H, Horizontal; V, vertical.

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The methane content was found to be highest in the cotton rope ARBC reactors (74%), followed in descend- ing order by asbestos (74%), coconut (73%), and nylon rope (70%). The COD values were also lowest in the cot- ton rope reactors, indicating greater biodegradation and thus better process performance. Data on COD removal under steady-state conditions, which, again, was highest in with cotton rope (86%). The cotton rope reactors also gave the lowest values of volatile fatty acids (1.0 g/c), suggesting a better process stability. Thus, the overall best performance was exhibited by the ARBC reactors with a cotton rope fixed-film structure on the rotating discs.

The pH remained largely unaffected by changes in the type of fixed-film structure on the rotating discs, being between 6.5 and 7.0 in all the reactor configurations. The speed of impeller rotation had a marginal effect on the overall process performance. Rotation of the discs at 6 rpm gave the highest gas production with reduced vola- tile fatty acid concentrations and low COD values, in- dicating a high process performance and process stabil- ity. Thus, the ARBC reactor, especially the vertical type, can be expolited for biomethanation of salty cheese whey.

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