Proximate and ultimate analysis (from January to December) of L. digitata are as shown in Table 3.1. The moisture content (MC) of L. digitata ranged from 80% to 92% with a peak in December and a through in August. Great variation was observed in the volatile solids; in August the value was three times higher (16%) than in December (5%). The Ash fraction was high in winter months and lowest in summer months. The carbon fraction peaked (37% of TS) in August due to the high concentration of storage carbohydrates (Adams et al., 2011a). Very low ammonia production in long term
digestion is expected due to the high carbon to nitrogen ratio in the peak season. The ash to volatile solids (A:V) ratio along with the C:N ratio is crucial in suggesting harvest time. Low values of the A:V ratio will yield higher methane productions per unit of collected mass of seaweed. August is suggested as the peak month for harvest as the seaweed has the highest organic content (82% of TS), lowest ash content (18% of TS), lowest A:V ratio (0.2) and suitable C:N ratio (32). These results are comparable with the previous studies conducted in the UK on the same seaweed species (Adams et al., 2011a; Schiener et al., 2015). However, Irish seaweed was found to fix slightly more carbon (37%) than British seaweed (36%) in the same peak harvesting month (Schiener et al., 2015). This may be attributed to the different climate and seawater conditions.
3.3.2 Biomethane yields of L. digitata
Seasonal variation in chemical composition in the substrate significantly influenced the BMP yield. Multiple comparisons results from one-way ANOVA indicated that seaweed harvested in summer significantly produced more biomethane than in winter (F=12.91, P < 0.002). BMP yields of L. digitata were highest in August with values of 327 ± 26 L CH4 kg VS-1 and lowest in April 203 ± 14 L CH4 kg VS-1 (Figure 3.1).
L. digitata has been previously reported as a potential biofuel feedstock due to easily biodegradable sugars laminarin (up to 30% TS) and mannitol (up to 25% TS) (Adams et al., 2011b). Adams et al. found the C:N ratio of L. digitata ranged from 10.9 in January to a peak of 31.9 in August harvested in the UK resulting in a BMP yield of
Table 3.1 Proximate and ultimate analysis of L. digitata round the year
Harvest Proximate Analysis Ultimate Analysis
MC % TS (%wwt) VS (%wwt) VS (% of TS) Ash (% of TS) A:V C % H % N % O % January 88.57 11.43 (0.44) 7.00 (0.41) 61.24 38.82 (1.25) 0.63 26.06 (0.38) 3.38 (0.06) 3.95 (0.02) 27.85 February 88.53 11.47 (0.05) 7.34 (0.18) 63.99 36.05 (1.76) 0.56 26.47 (0.16) 3.62 (0.09) 3.95 (0.09) 29.96 March 90.26 9.74 (0.02) 6.49 (0.14) 66.67 33.33 (1.37) 0.5 30.41 (0.90) 3.97 (0.11) 3.7 (0.06) 28.58 April 85.58 14.42 (0.22) 9.31 (0.18) 64.56 35.44 (1.02) 0.55 30.02 (0.23) 3.83 (0.05) 2.28 (0.04) 28.43 May 87.63 12.37 (0.09) 8.91 (0.07) 72.73 28.04 (0.43) 0.39 32.36 (0.18) 4.74 (0.08) 2.18 (0.05) 33.44 June 85.83 14.17 (0.04) 10.37 (0.13) 73.15 26.85 (1.11) 0.37 34.5 (0.77) 5.02 (0.03) 1.82 (0.05) 31.81 July 85.64 14.36 (0.03) 10.87 (0.04) 76.6 24.3 (0.44) 0.32 33.2 (0.05) 4.95 (0.09) 1.53 (0.10) 36.92 August 80.28 19.72 (0.11) 16.12 (0.04) 81.72 18.28 (0.27) 0.22 36.76 0.10) 5.54 (0.02) 1.14 (0.04) 38.28 September 80.54 19.46 (0.26) 15.67 (0.25) 80.51 19.49 (0.44) 0.24 36.62 (0.17) 5.3 (0.05) 0.93 (0.03) 39.37 October 84.2 15.8 (0.24) 11.92 (0.24) 75.42 24.56 (0.37) 0.32 33.45 (0.22) 4.71 (0.01) 1.22 (0.05) 36.55 November 84.81 15.19 (0.12) 11.44 (0.09) 75.29 24.71 (0.09) 0.33 37.18 (0.42) 4.98 (0.06) 1.53 (0.11) 31.6 December 91.61 8.39 (0.95) 5.26 (0.92) 59.59 37.58 (4.05) 0.64 30.82 (0.50) 4.05 (0.01) 3.4 (0.09) 21.32
254.14 ± 6.21 L CH4 kg VS-1 (Adams et al., 2011a). Due to variation in location and
season, different BMP yields were reported from various sites in Ireland. L. digitata collected in August (C:N ratio of 22.5) generated a BMP yield of 218.0 ± 4.1 L CH4 kg
VS-1 and a lower yield was recorded when harvested in May (184 L CH4 kg VS-1)
(Vanegas and Bartlett, 2013) and in January (103.3 ± 19.8 L CH4 kg VS-1) (Tedesco et
al., 2013). In this study, the BMP is higher than in previous studies (Table 3.2). It can be possibly due to the high C:N ratio (32), the low A:V ratio (0.2) (Table 3.1), the higher biodegradability (Table 3.2) and higher amount of storage carbohydrates. However, the high C:N ratio in September (40) slightly affected the gas yield, as the accumulated carbohydrates and the lack of proteins can result in an imbalance and a drop in pH and slight inhibition of methanogenesis (Wang et al., 2014).
Seaweed samples were washed with tap water and then kept in a vertical position for 15 minutes to remove the surface water. The biodegradability index (BI) is defined as the ratio of the BMP yield to the theoretical yield (Table 3.2). This value describes the efficiency of biodegradability of the substrate in the batch digestion. The highest
biodegradability index was observed in August (0.72) while the lowest was in December (0.44). Low biodegradability may be attributed to the low carbohydrate content and imbalances in the C/N ratio in winter. The average BMP of cellulose in these trials was 335.3 ± 13.2 L CH4 kg VS-1, corresponding to a biodegradability index of 0.81 ± 0.03.
This suggests a healthy inoculum condition and repeatable results between each BMP batch trials.
Table 3.2 Biomethane production over 12 months based on results of BMP analysis and theoretical analysis Harvest BMP yield (L CH4 kg VS-1) Harvest kg VS t-1 wwt Theoretical composition (CH4 %) Theoretical yield (L CH4 kg VS-1) Biodegradability index (BMP/theoretical) Specific yield (m3 CH 4 t-1wwt) January 237 ± 4 70 42 421 0.56 17 February 259 ± 5 73 46 406 0.64 19 March 265 ± 4 65 48 469 0.57 17 April 203 ± 14 93 50 462 0.44 19 May 263 ± 11 90 52 407 0.65 23 June 294 ± 1 104 53 493 0.60 30 July 303 ± 12 110 50 435 0.70 33 August 327 ± 26 161 54 452 0.72 53 September 303 ± 19 160 50 450 0.67 47 October 267 ± 7 120 51 428 0.62 32 November 256 ± 11 114 53 510 0.50 29 December 235 ± 10 50 53 537 0.44 12 wwt = wet weight
Figure 3.2 Salts accumulation in a batch reactor (summer to spring).
The salinity of seaweed was subtracted from the salinity of inoculum (6.9 ± 0.47 g/L) in order to present actual salinity increase in the batch digestion due to seaweed only.
Accumulation of salts can be a problem in the digestion of seaweed. It was observed that salinity was much higher in winter and spring as compared to summer and autumn (see Figure 3.2). A correlation between ash content and organic content (A:V) explains the tolerance level of salts during digestion. The salinity of inoculum and the seaweed was measured after completion of each batch (30 days). The salinity values were 11.0 ± 0.08 g/L, 10.4 ± 0.10 g/L, 9.8 ± 0.10 g/L and 9.6 ± 0.10 g/L in winter (January), spring (March), summer (June) and autumn (October) months, respectively (compared with the seawater salinity of 31 ± 0.90 g/L from the collection site). The salinity of seaweed was subtracted from the salinity of inoculum (6.9 ± 0.47 g/L) in order to present actual salinity increase in the batch digestion due to seaweed only. Salinity and A:V ratio had a significant effect on the gas yield during different seasons. Higher values of salinity and A:V led to lower values of gas production. It was observed that the optimum gas yield was obtained when salinity increased up to 30% (compared with inoculum salinity; in summer and autumn months). Lower yields were obtained when salinity increased by 60% or more in winter and spring. Excess levels of salinity level can adversely affect digestion performance. A low biomethane yield is expected at a high salinity level (Chen et al., 2008; Fang et al., 2011; Xia et al., 2016a).
Biomethane yields may be expressed based on per unit mass of volatile solid or per unit mass of wet weight (Figure 3.3). The yields per unit mass wet weight are more
understandable to the biogas developer and also combine the effects of proximate analysis (change in VS content as indicated in Table 3.1) and BMP results (expressed in Table 3.2 as L CH4 kg VS-1). The December BMP value is 72% of the August value
(Figure 3.3A) but it is only 22.6% of the yield when expressed per unit mass wet weight (Figure 3.3B). More significant results were obtained when ANOVA was performed on the basis of wet weight (F=5.75, P < 0.021) as compared to dry weight basis (F=4.90, P < 0.032).