Manejo psicológico de pacientes con distrofia muscular

In terms of total VFA yield, the 15-day HRT reactor had the highest production with 0.033 gVFA/gVSadded, followed by the 2-day HRT reactor and then the 10 and 1-day

reactors following behind that with similar amounts of VFA produced. Figure 4-12 below shows the largest yield coming from the 15-day HRT reactor on day 15.

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0 5 10 15 20 25 30 35 Time (days) V F A y ie ld ( g V F A /g V Aa d d e d ) 1 day HRT 2 day HRT 5 day HRT 10 day HRT 15 day HRT

Figure 4-12: Cumulative VFA yield (gVFA/VSadded) derived from the effluent of reactors

operated at different HRTs.

Decline in concentration indicating consumption

The increase on day 30 for the 10-day HRT reactor is much less than that achieved on day 20. The 5-day HRT reactor shows almost no increase on day 30. The reason for this can be seen more clearly in Figure 4-13 below, which shows the change in VFA concentrations in the reactors. The 10-day HRT reactor shows a general decrease in VFA concentration during its last 10-day cycle. The 5-day HRT reactor and 15-day reactor also show signs of VFA consumption occurring after day 27. The establishment of methanogenic or aerobic populations could explain this as discussed above. However the pH of all the reactors was typically under 6.00 which is at a level which is generally accepted in the literature as being inhibitory for methanogens (Hobson & Wheatly, 1993)

0 100 200 300 400 500 600 0 5 10 15 20 25 30 35 Time (days) V F A s ( m g /l ) 1 day HRT 2 day HRT 5 day HRT 10 day HRT 15 day HRT

Figure 4-13: VFA concentration in reactors operated at different HRTs.

If the reactors had been decanted on day 27, prior to the consumption of VFAs, the 15 day HRT reactor would have produced over 400 mg VFA and the 15 day HRT reactor would have had a significantly improved yield. Despite the consumption of VFAs, we can see that the longer HRT reactors are producing more VFAs that the 1- day HRT reactor even though the shorter HRT reactors appear to produce more soluble COD. The highest soluble COD yielding reactor (5-day HRT) was the lowest VFA yielding reactor while the lowest soluble COD yield (15-day HRT) also yielded the most VFA. An explanation of this could lie in the morphological phenotypes of

acidogens and hydrolysers. If acidogens are predominantly planktonic then they would be much more susceptible to washout that can occur at low HRTs. Short HRTs would not adversely affect hydrolysers if they are predominantly sessile and attached to the solids which are their food source. We might expect acidogens to be planktonic as their food source is soluble and therefore associated with the bulk liquid and not the solids. In a study of the hydrolysis of crystalline cellulose, Jensen

et al found that 25% of the bacterial population was attached and that the planktonic fraction dominated (Jensen et al., 2008). They noted that this was in contrast to rumen based studies where particle associated (sessile) bacteria are reported to range from 50% (Merry & McAllan, 1983) to 70% (Cheng et al., 1977). It was suggested that this was possibly due to the functional characteristics of the rumen, where soluble VFAs are removed thereby limiting the substrate available for planktonic organisms (Jensen et al., 2008). A later study by the same authors found that the rate for hydrolysis of cellulose was most strongly correlated with the concentration of sessile biomass rather than with the concentration of total or planktonic biomass (Jensen et al., 2009). This supports the proposition that hydrolysis should be less effected by changes in HRT than acidogenesis.

The implications of shorter HRTs favouring hydrolysis and longer HRTs favouring acidogenesis in leachbed systems are that it must be decided which regime would best favour methane production in the methanogenic digester. As acidogenesis can occur concurrently with methanogenesis in the digester and is not the rate-limiting step, it may be that a higher soluble organic content with reduced VFA content would result in a higher methane yield than a lower soluble organic content with increased VFAs. Opting for shorter HRTs could also help to prevent the establishment of a methanogenic population. With the same amount of influent coming in, a shorter HRT results in a smaller reactor vessel which in turn results in a higher organic loading rate. A higher OLR has been observed to result in a lowering of pH in acidogenic/hydrolytic reactors fed with unscreened dairy manure (Demirer & Chen, 2005). A lowering of pH could help to inhibit methanogens.

Limiting solids retention time to less than 25 days could also help to reduce the consumption of VFAs and soluble COD that occurs in the reactors, by not allowing a methanogenic population to establish. However, limiting the solids retention time

would result in a sacrifice of unhydrolysed solids that could contribute to soluble COD production and greater methane production downstream. Limiting the solids retention would also impact the on-farm operation of a leachbed system as solids removal would be required on a more frequent basis.

4.5. Rumen Dosing Experiment

Two rumen dosing experiments were carried to see if the addition of rumen contents would have any effect on gas production, VFA production, COD solubilisation or hydrolysis of cow manure sludge. The first experiment (RD1) compared cow manure sludge dosed with active rumen contents obtained from fistulated sheep to cow manure sludge dosed with autoclaved rumen contents. There was no discernable difference in VFA or COD extraction between the series. The results of experiment RD1 can be found in Appendix 5. The second experiment (RD2) was adjusted to ensure anaerobic conditions and had an additional control, which was cow sludge with no rumen addition. Experiment RD2 used a buffered solution mimicking cow saliva for dilutions. This solution was used to provide optimal chemical conditions for the rumen organisms. The buffered solution was sparged with CO2 to ensure

anaerobic conditions more favourable to rumen organisms. The headspace of the reactors was also flushed with CO2 after changing dilution liquid. The experiments

used a 1:1 dilution of sludge to buffer solution as this had been shown to be more favourable to both hydrolysis and acidogenesis in the dilution experiments (see section 4.2 above).