CHAPTER THREE EFFECT OF TOTAL SOLIDS ON ANAEROBIC DIGESTION

Transcription

1 CHAPTER THREE EFFECT OF TOTAL SOLIDS ON ANAEROBIC DIGESTION

2 3. 1 BACKGROUND Methane production by anaerobic fermentation involves a series of successive biochemical reactions carried out anaerobically by different types of bacteria and thereby transforming varied orgnic materials into methane and carbon-di-oxide. The efficiency of the biochemical reactions is influenced by several factors such as ph, temperature, substrate concentration and the retention time, etc. Based on many studies which have been carried out in this laboratory (unpublished work of SRERI) and elsewhere Hughes (1979), Padmanabhan (1985), and Chawala (1986) to see how these parametres affect the rate of biogas production, it can be generalized that a ph range of , a 39

3 mesophilic temperature of C and retention times of 30 to 60 days favors maximum output of biogas production from different types of biomass, however, the optimum substrate concentration would not be the same for all types of biomass. In the cattle dung digestion, presence of 7-9 per cent total solids is considered optimum for biogas production. Acharya (1958) studied the effect of total solids concentration of cattle dung from 5 to 12 per cent and found that the maximum rate of gas production was at 8 per cent total solids. Jain and Kumar (1986) reported, cattle dung to water ratio of 3:2 to be the optimum for feeding the biogas plants. Bousfield et al. (1979) found that at 35 C gas producton increased by 40 per cent when concentrations of solids of cattle dung increased from 5 to 10 -per cent. Ranade et a/. (1990) reported good yield of biogas from the dung slurry containing 14 per cent total solids and recommended 2:1 dilution of the dung in water. Wajcik (1979) studied the effect of water content on biogas production from dairy cow manure and straw with substrate concentration of 10 per cent to 40 per cent and showed that the rate of biogas production was significantly influenced at a total solids content above 32.5 per cent. High solids affect hydrolysis reaction, i.e. the acid forming mechanisms and finally methane production. Ghosh et al. (1979) studied the batch digestion of materials such as cattle dung, water hyacinth, algae, bagasse and the rice husk at different concentrations in the range of 1 per cent to 10 per cent total solids and found that 40

4 the gas production was increased with increase in total solids concentration. Pillai et al. (1983) have reported that, in semicontinuous digestion of water hyacinth, biogas production increases linearly with the increase in organic loading. In an upf1ow-anaerobic sludge blanketreactor it was observed that the biogas production increases linearly with increasing faecal concentration (Final report of Upflow Anaerobic Sludge Blanket : low cost sanitation research project in Bandung, Indonesia, 1988). In the case of sewage sludge digestion, Maiina and Miholis (1968) and Sawyer and Grumbling (1960) have recommended an optimum solids concentration in the digester from 6 to 8 per cent. 3.2 EX PERIMENTAL The total solids present in fresh, cattle dung usually vary between per cent; in the fresh water hyacinth from 5 to 8 per cent, whereas, in the night soil it varies from 20 to 24 per cent. In light of several and varied research reports it was necessary for the purpose of present investigation to examine the effect of substrate concentration on the biogas production from cattle dung, water hyacinth and night soil. Therefore, the anaerobic digestions were carried out (batch scale) in 1 1 Erlenmey- er flasks containing seeded biomass substrate in a range A 41

5 of concentrations and sufficient water to make a total volume of 500 ml, (Please see Photograph No. 1). After 45 days of digestion the pararaetres such as T.S., T.V.S., C.O.D. and methane content were determined as per the methods described in Chapter II. * 3.3 RESULTS AND DISCUSSION The experimental data are presented in the following tables and figures : TABLE 3.1 EFFECT OF TOTAL SOLIDS CONCENTRATION ON ANAEROBIC DIGESTION Biomass Total Volume ph : Cattle dung : 500 m1. : 7.3 ; (Fresh) 9 Per cent Total Biogas Biogas Per Per Per total produced produc- cent cent Cent sol ids Cl ) C.O.D. T.V.S. (dry Reduced Reuced weight) tion (1/kg fresh weight) Methane Content

6 ^ Digestion (Days) Fig. 3.1 ; Effect of Total Solids on Cattle Dung Digestionvo(l)uouonpojdssBoia bjoj.43

7 Although, the total biogas production was observed to be increased with increase in totol solids concentration of cattle dung (Fig 3.1), the levels of biodegradation however, were n early the same at all the concentrations of \ total solids tested (Table 3.1) ranging from 8.6 to 16 per cent in terms of the parameters such as, the efficiency of biogas production (20.0 to /kg. fresh weight). C.O.D. reduction (40.5 to 43.8 per cent) and T.V.S. reduction (21.0 to 22.4 per cent). However, the methane content was observed to be the highest (60 per cent) in the case of T.S. of 12 per cent. The data suggest that the current practice of mixing C.D. with water in 1:1 ratio before being fed to the biogas digester should be revised. By increasing the total solids concentration in the biogas digester without affecting the efficiency of the biogas production, it would be possible to decrease the size of the biogas digester and consequently its cost. Anaerobic digestion of cattle dung, with the higher solids, however, may create problem of choking of inlet and outlet pipes of the biogas plant, nevertheless, such problem could be circumvented by introducing suitable changes in the design of the digester in order to keep the influent and effluent flows unhindered. The results of the anaerobic digestion of water hyacinth show that, more the organic loading at the start, more is the amount of biogas obtained (Table 3.2 and Fig 3.2). However, all the parameters such as the efficiency 44

8 TABLE 3.2 EFFECT OF TOTAL SOLIDS CONCENTRATION ON ANAEROBIC DIGESTION Biomass : Water hyacinth (Dry powder) Total Volume : 500 ml ph 7.2 Per cent total sol ids (dry weight) Total Biogas produced ( 1 ) Biogas production (1/kg dry weight) Per cent C.O.D. Reduced Per cent T.V.S. Reuced Per Cent Methane Content / f < i 45

9 Total Biogas Production (I) 00 (0 fo 01 to Digestion (Days) Fig. 3.2 : Effect of Total Solids on Water Hyacinth Digestion o CM ro 01 o CO CO 01 O <* 46

10 1 TABLE 3.3 EFFECT OF TOTAL SOLIDS CONCENTRATION ON ANAEROBIC DIGESTION Biomass : Night Soil (Dry Powder) Total Volume 500 ml. ph : 7.2 Per cent Total Biogas total produced solids (1) (dry weight) Biogas production (1/kg dry weight Per cent C 0 D. Reduced Per cent T. V. S. Reuced Per Cent Methane Content o, s* 47

11 Digestjon (Days) Fig. 3.3 Effect of Total Solids on Night Soil Digestion 4% 8% -*7 12% -Q- 16%COCDCM(I)uojionpojdseBoiaieioj.48

12 of biogas production, C.O.D. and T.V.S. reduction are affected severely with increase in total solids from 8 to 16 per cent. The efficiency of biogas production went down from /kg to 73.5 (1/kg. >, whereas, C.O.D. and T.V.S. reduction decreased from 49.9 to 27.2 per cent and from 42.2 to" 24.4 per cent respectively (Table 3.2). It seems that, for the efficient production of biogas from water hyacinth the substrate concentration of 8 per cent or less would be ideal. The experimental data on night soil digestion (Table 3.3) indicate that, values of both the C.O.D. and T.V.S. reduction were the highest at a substrate concentration of 4 per cent (58.1 and 64.8 per cent respectively). Although, the total biogas production was observed to be increased with increase in total solids concentration (Fig 3.3) the efficiency of biogas production, and also the methane content, however, peaks at a substrate concentration of 12 per cent ( /kg, 59.7 per cent) with a reduction in the C.O.D. value by about 50 per cent. The slurry with a total solids concentration of four per cent would be roughly equivalent to a slurry of night soil derived from a person using two litres of water for ablution. Therefore, to achieve optimum T.V.S. and C.O.D. reduction and also to satisfy the health and hygiene point of view, recommendation to limit the consumption of water i to about 2 1 per person using biogas plant attached-toi1et block would be fully justifiable. 49