Investigating the challenges of biogas provision in water limited environments through laboratory scale biodigesters

Figures & data

Figure 1. Set up of first two replicates of the dilution experiment.

Schematic showing initial 2 replicates of the dilution experiment.

Table 1. Operating conditions for the dilution experiments showing chemical oxygen demand (COD), total solids (TS), volatile solids (VS) and organic loading rate in terms of volatile solids (OLR_VS) and chemical oxygen demand (OLR_COD).

Reactor no.	Dung volume fed (mL/d)	Dung weight fed (g/d)	Water fed (mL/d)	TS fed (%)	TS fed (g/d)	VS fed (g/d)	COD of the feed (g/L)	OLRVS (kg VS/ m^3/d)	OLRCOD (kg COD/ m^3/d)
V200	8.00	7.62	0	10.63	0.81	0.65	190.76	3.26	7.63
V133	5.33	5.07	2.67	7.08	0.54	0.44	127.17	2.17	5.08
V100	4.00	3.81	4.00	5.31	0.40	0.32	95.38	1.63	3.82
V67	2.67	2.54	5.33	3.54	0.27	0.22	63.59	1.09	2.54

d = day, water is deionised.

Figure 2. Reduction of chemical oxygen demand $\left(P_{\mathrm{C}\mathrm{O}{\mathrm{D}}_{\mathrm{R}}}\right)$and volatile solids $\left(P_{\mathrm{V}{\mathrm{S}}_{\mathrm{R}}}\right)$ (%) over time. Error bar = standard error.

Line and dot charts showing chemical oxygen demand and volatile solids over time for each reactor group in the dilution experiment.

Figure 3. Mean values of pH over running time. Error bar = standard error.

Line and dot chart showing mean pH over the retention time for each reactor group.

Table 2. Mean and standard error chemical oxygen demand (COD) and volatile solids (VS) reduction in dilution experiment.

Reactor group	% COD reduction, $P_{\mathrm{C}\mathrm{O}{\mathrm{D}}_{\mathrm{R}}}$		% VS reduction, $P_{\mathrm{V}{\mathrm{S}}_{\mathrm{R}}}$		COD removal, $R_{\mathrm{C}\mathrm{O}{\mathrm{D}}_{\mathrm{R}}}$ (g/L/day)		pH
	Mean	SE	Mean	SE	Mean	SE	Mean	SE
V200	20.87	2.45	20.31	2.48	1.59	0.19	7.0	0.3
V133	7.59	3.07	12.25	1.48	0.39	0.16	6.5	0.2
V100	12.98	2.71	18.58	2.96	0.50	0.10	7.4	0.1
V67	6.23	7.77	17.23	6.19	0.16	0.20	7.5	0.2

Table 3. Properties of the feed used in the recycling experiments Note: Mean of 5–6 samples each (SE given in brackets). Dung: filtrate mixes based on theoretical values. COD = chemical oxygen demand, TS = total solids, VS = volatile solids.

Reactor phase	TS% RLD100	VS% RLD100	OLR VS kg/m^3/d RLD100	COD g/L RLD100	TS% RLD50	VS% RLD50	ORL VS kg/m^3/d RLD50	COD g/L RLD50
Phase 1	7.02 (0.04)	5.44 (0.03)	1.99 (N/A)	121.73 (5.57)	6.17 (0.03)	4.86 (0.02)	1.84 (N/A)	108.56 (5.04)
Phase 2	7.58 (0.09)	5.98 (0.06)	2.25 (N/A)	122.71 (5.69)	6.45 (0.06)	5.13 (0.04)	1.75 (N/A)	109.05 (5.10)
Phase 3	7.46 (0.19)	5.85 (0.13)	2.18 (N/A)	123.34 (6.19)	6.39 (0.06)	5.06 (0.07)	1.87 (N/A)	109.36 (5.35)

d = day.

Table 4. Mean volatile solids $\left(P_{\mathrm{V}{\mathrm{S}}_{\mathrm{R}}}\right)$ and chemical oxygen demand reduction $\left(P_{\mathrm{C}\mathrm{O}{\mathrm{D}}_{\mathrm{R}}}\right)$in the 3 recycling phases. Note: Mean values with standard error shown in brackets and significance refers to differences between phases

Reactors	Phase 1	Phase 2	Phase 3	Significant difference	P value	Whole run mean
Group RLD50% VS reduction	23.36 (0.85)	27.03 (1.06)	26.27 (0.69)	YES	0.02	25.61 (0.57)
Group RLD50% COD reduction	29.30 (4.00)	23.77 (1.36)	22.91 (2.21)	NO	0.77	24.53 (1.42)
Group RLD50 pH	7.50 (0.08)	7.64 (0.04)	7.46 (0.03)	YES	0.03	7.53 (0.03)
Group RLD100% VS reduction	29.14 (0.76)	31.52 (0.73)	29.25 (1.10)	NO	0.17	29.91 (0.56)
Group RLD100% COD reduction	36.02 (0.74)	27.94 (1.02)	25.03 (1.20)	YES	<0.01	28.26 (1.08)
Group RLD100 pH	7.58 (0.05)	7.65 (0.04)	7.58 (0.04)	NO	0.52	7.60 (0.02)

Figure 5. Mean volatile fatty acid levels (VFA) over running time of reactors.

Dot chart showing volatile fatty acid concentration in millimolar over the retention time of the recycling experiment.

Figure 4. Reduction of chemical oxygen demand $\left(P_{\mathrm{C}\mathrm{O}{\mathrm{D}}_{\mathrm{R}}}\right)$and volatile solids $\left(P_{\mathrm{V}{\mathrm{S}}_{\mathrm{R}}}\right)$ (%) over time with liquid digestate recycling. Error bars = standard error.

Line and dot chart showing the percentage volatile solid and chemical oxygen demand reduction over the retention time for both reactor groups.