Requisitos

Biogas production

The addition of maize to the digestion of cattle slurry appeared to have a significant impact on the volumetric methane yield where the addition of a small quantity of maize caused the volumetric methane yield to increase by over 200% (Table 7.7). The addition of maize was also shown to be

important when the digesters were fed with poor quality cattle slurry, as shown in Chapter 2 in the case of the winter cattle slurry. The difference between the batches of cattle slurry, shown in Table 10.1 makes it difficult to compare the different working conditions tested but the summary in Table 10.2 gives an approximate idea to which conditions would provide the farmer/digester operator with the greatest volumetric methane yield.

Table 10.2: Comparison of the average volumetric and specific methane yield from each co-digestion trial tested in this research following no recirculation. Additionally, the improvement to the volumetric methane yield when compared to the corresponding control is shown. The bold values represent the greatest values while the bold italic values represent the lowest values. ammonia and therefore the alkalinity of the system. This appeared to allow maize to be digested at higher loading rates and shorter retention times without a clear decline in the specific methane yield. The importance of the buffering capacity of the cattle slurry was suggested by Angelidaki and Ellegaard (2003) and was shown in the digestion of food waste (Alvarez and Lindén, 2008).

In addition, this benefit was highlighted when the stability of the digestion of OFMSW was improved during the initial stages of the process (Capela et al., 2007). This strong support does suggest that the main contribution of cattle slurry to the digestion of energy crop was the buffering capacity. The addition of maize to the cattle slurry was also shown to have a positive influence on the digestion process and this was clearly shown in Trial 2 (Chapter 6). This trial

suggested that the addition of maize to cattle slurry played an important role when the anaerobic digester was fed with cattle slurry with low digestibility. This ‗dilution‘ impact was highlighted in Chapter 6 and supports research that looked at dilution as a means of improving the digestion of cattle slurry (Vedrenne et al., 2008). This highlights that both co-substrates can bring benefits to the system and the benefits do not necessarily all come from the introduction of cattle slurry.

Increasing the proportion of energy crops in the co-digestion process appears to produce contradictory results. The research by Nordberg et al., (2005) demonstrated no clear difference in the specific methane yield as the amount of energy crops increased from 45 to 72% while Lehtomäki et al., (2006) indicated that increasing energy crops from 30% to 40% resulted in a decline in the specific methane yield. Explanations for the decline after 30% were not given and from the performance data no clear difference can be observed. The trend shown in the initial two digestion trials of this current research was that of an increase in the volumetric methane yield with an increase in the proportion of energy crops. This supports the previous research but unlike Lehtomäki et al., (2006) it was possible for the maize proportion to increase to 75%

without a significant decline in the specific methane yield.

The benefit of increasing the ammonia concentration and alkalinity was shown when comparing the digestion of maize alone and with cattle slurry (Chapter 7). These parameters could be responsible for the difference in the energy crop proportion achieved by this research and Lehtomäki et al., (2006). In Lehtomäki et al., (2006) the digestion of grass and cattle slurry at a 40:60 ratio had a ammonia concentration of 0.5 g l^-1 while the digestion of maize and cattle slurry at a 75:25 ratio had a concentration between 2 and 2.5 g l^-1. The one piece of research that did touch upon the impact of increasing the proportion of maize when co-digested with cattle slurry was Mähnert et al., (2006) and this can be directly compared to digestion trial 2 (Chapter 6) as both followed a similar process. In trial 2 a loading rate of 4 g VS l^-1 d^-1 was tested with a three different proportions, ranging from 25 to 75% for both batches of cattle slurries. As the maize proportion increased the specific and volumetric methane yields both showed an increase, supporting the trend shown by Mähnert et al., (2006).

In the literature reviewed in this research, the majority of the co-digestion trials focused on loading rates ranging from 1 to 4 g VS l^-1 d^-1 with only Nordberg and Edström (1997) testing a co-digestion mixture of 6 g VS l^-1 d^-1. The impact of increasing the loading rate was tested in Chapter 7 and it was shown that the loading rate could increase to 6 g VS l^-1 d^-1 with no clear

decline of methane yield during the trial. Application to the Hill model (Husain., 1998) suggested that the maximum load for the co-digestion process, with co-substrates at equal proportions, was 6 g VS l^-1 d^-1; this is 4 g VS l^-1 d^-1 lower than the suggested maximum loading rate for dairy cattle slurry (Figure 2.2). The literature review highlighted the fact that minimal research has been undertaken on the impact of increasing the loading rate on a fixed proportion of co-substrate. This gap in the research was addressed by the trials reported in Chapter 7 which provided a detailed study on the impact that increasing the load had on both the mono-digestion of the substrates and when they were co-digested in equal proportions. Both Lehtomäki et al.

(2006) and Mähnert et al. (2007) highlighted that increasing the loading rate resulted in a decline in the specific biogas yield. The specific methane yields given in Chapter 7 were only shown to decline by 0.02 l g^-1 VS in comparison to an increase in the volumetric methane of 0.59 l l^-1 d^-1 suggesting that, unlike the work by Lehtimäki et al., (2006), increasing the load past 3 g VS l^-1 d^-1 can be beneficial in terms of the methane output.

Digestate

In addition to the production of biogas, the impact that co-digestion had on the characteristics of the digestate was also investigated as this has often been ignored or forgotten in previous co-digestion research. Section 2.5 of the literature review focused on the application of the digestate and it highlighted that a number of characteristics changed upon digestion including the reduction in methane emitted after application (Amon et al., 2006a, Clemens et al., 2006, Weiske et al., 2006).

This was supported by the residual methane yield trials undertaken on the digestate produced by the mono-digestion trials in Chapter 7; a decline of 24% and 36% was shown by the cattle slurry and maize respectively. The impact that co-digestion, with no recirculation, had on the residual methane yield was not tested but the impact that solids and liquid recirculation had was investigated in Chapter 9. In both cases the post-digestion methane production was lower than that produced during digestion, 50% and 32%. The impact that the addition of maize had on the residual methane yield was not touched upon in this research but the impact of the addition of a co-substrate was highlighted by Clemens et al., (2006) where the addition of potato starch to cattle slurry was shown to lower post-digestion emissions.

In addition to the release of methane the application of cattle slurry to the land has been associated with the emission of ammonia. Clemens et al., (2006) showed that emissions of ammonia related to an increase of ammonia that occurred upon digestion. An increase in ammonia was shown in all trials when the cattle slurry was digested but the addition of maize resulted in a reduction in

concentration suggesting that the addition of maize could be having a positive influence on the emissions.

An important characteristic highlighted by the literature review was the dry matter content and the positive influence that digestion had by reducing this content (Johnson et al.,2005, Smith et al., 2001a, Thompson and Meisinger.,2002, Misselbrook et al.,2005). This was repeated in all the trials tested which also highlighted that the addition of maize reversed this, with an increase in dry matter content as the quantity of maize increased. The negative impact of digesting cattle slurry was the increase in the pH which, as shown by Sommer et al., (1993), can result in an increase in the loss of ammonia during storage. The impact of the addition of maize was to reduce the pH but only by one unit in the first two trials. The range of impacts that the addition of maize has on the digestate, both positive (ph, ammonia, VFA content) and negative (dry matter), could be counterbalancing each other. For example, the increase in the dry matter may lead to an increase in emissions and a reduction of nutrients reaching the crops but the decline in pH and ammonia may be contributing to a reduction of ammonia emissions. The actual impact that the addition of maize can have on associated emissions is not known but opens up to interesting future work.

In an attempt to determine how the different scenarios effect the total emissions produced by the farm a GHG emissions calculator (Salter, pers comm 2010) was applied and the results are presented in Table 10.3. This uses the case study of a large dairy farm with the following constraints:

 192.5 hectare total area

o 162.5 hectares for grazing (based on a stock density of 2 livestock units per hectare) o 22 hectares for maize silage (fodder for cattle)

 325 Cattle herd

o 150 followers (such as bulls, calves) o 175 dairy

 Cattle are housed indoors for 50% of the year

 Farm has an on-site CHP unit

The model assumes that the farm remained as a dairy farm with the herd size and supporting land remaining constant; it was therefore assumed that the farmer would rent or buy additional land for the maize required by the digester. Table 10.3 gives the associated GHG emissions produced and

saved by different working conditions; these conditions relate to those tested in this research, as summarised by Table 10.1 and Table 10.2

Table 10.3: GHG emissions

NB: Assumes a methane potential of 0.18 and 0.35 l g^-1 VS for cattle slurry and maize respectively

The GHG emissions considered here include the emissions produced from the fuel required by the farm and the emissions saved by exporting the energy produced from the digester, in terms of carbon dioxide equivalents (CO2 eq). The emission balance of the farm, in the absence of a digester, was calculated to be 341 tonnes CO2 eq; comparing this value with the emission balance produced by introducing a digester indicates an emission savings at all scenarios. The emissions from the energy required by the farm did increase as the maize proportion increased however; this was shown to be counterbalanced by the increase in the emissions saved by exporting the energy produced by the digester. It was the 25% cattle slurry proportion that produced the greatest emissions saved resulting in the total emissions being just 48 tonnes CO_{2 eq}., 208 tonnes CO₂ eq lower than that of a equivalent loading rate but with a cattle slurry proportion of 65%.

VFAs and phenols are the main contributors odour and these have been shown to be reduced during digestion (Powers et al., 1999) When compared to cattle slurry, which is commonly applied directly to land, it was shown in all the trials the concentration of VFA‘s declined suggesting a potential decline in odour. Upon the addition of maize, no significant difference was shown with the

exception of the trial testing the winter cattle slurry (Chapter 6) where the addition of maize resulted in a decline in VFA‘s suggesting a beneficial impact on the odour.

The heavy metal contents of the digestates produced in digestion trial 2 (Chapter 6: winter cattle slurry) and digestion trial 3 (Chapter 7) were tested and compared to the upper limits specified in PAS110 (Table 2.3). From the comparison it was shown that chromium, nickel and zinc all exceeded the upper limits indicating that application of the digestate would result in non-compliance (British Standards Institute, 2008). The PAS 110 limits are expressed in terms of mg kg^-1 DM so compliance of the digestate is less likely to be achieved as digestion reduces the DM content, increasing the concentration. A more suitable approach would be to determine the quantity of heavy metals applied to a field from the raw cattle slurry and the digestate; this is shown in Table 10.4. This comparison is based on the application of the slurry/digestate to a 22 hectare field growing maize with a soil nitrogen supply index of 1. The quantity of slurry/digestate applied was calculated based on recommended fertiliser inputs of 80, 85 and 205 kg ha^-1N, P2O5 and K2O (Defra, 2000). The data used for the raw cattle slurry was the average of the five batches used in this research.

Table 10.4: Quantity of selected heavy metals to a 28 hectare field growing maize assuming limits of 80, 60 and 250 kg^-1 for nitrogen, phosphorous and potassium. The digestates relate to that produced in digestion trial 2 (winter) and in digestion trial 3 (co-digestion trial).

OLR Cattle slurry

Digestate required (based on limit for

P₂O

Cadmium Chromium Copper Nickel Zinc

kg m^-3 % t (ww) t (DM) kg

Despite the concentrations showing non-compliance with the upper limit, chromium, nickel and zinc all showed lower or equal quantities to that applied from the raw cattle slurry. This indicates that applying limits in terms of the dry matter content to the use of digestate as a fertiliser may not be sensible and may not give an accurate picture of the actual amount of metals applied to the field.

10.3 Synergy (Objective 5)

Literature on co-digestion has resulted in a number of studies suggesting that synergy between the substrates may occur (Machmuller et al., 2007, Lehtimäki et al., 2006) However research by Mähnert et al., (2006) suggests that it is possible to calculate the biogas yield of a co-digested mix by the sum of the individual substrates. In the first long term trial undertaken in this research there was evidence of potential synergy but subsequent trials suggested that this could have been due to the inaccuracy of measuring the predicted methane yield and/or the impact of washout. Research by a number of different authors highlighted the difference between digesting substrates under batch and long term trials (Callaghan etal., (1998,1999)) suggesting that quoting the presence of synergy based on comparison of batch and long term trials could be brought into doubt (Lehtimäki et al., 2006, Chapter 5, this research). One strength of this research is that it attempted to increase the accuracy in the calculation of the predicted yield by comparing trials that followed the same operational conditions. The comparison between the mono and co-digestion trials in Chapter 7 provided support for the statement given by Mähnert et al., (2006) as the calculated combination of the maize and cattle slurry methane yields, determined by long term mono-digestion trials, gave a yield that was more or less equal to that produced by co-digestion of the two substrates.