Suitable types of biomass and their characteristics

Suitable types of biomass and their characteristics

Practically any kind of watery organic substance is suitable for anaerobic digestion. The agricultural residues and waste materials that can be used as substrate for biogas plants consist chiefly of:

- waste from animal husbandry, e.g. dung, urine, fodder residue and manure, .
- vegetable waste, e.g. straw, grass, garden residue, etc. (though such materials do not ferment well alone),
- household waste like night soil, garbage, wastewater, etc.

Solid and liquid agroindustrial waste materials, from slaughterhouses for example, and wastewater from sugar/starch processing are not gone into here, since small-scale biogas plants of simple design would not suffice in that connection (cf. chapter 6).

Waste from animal husbandry

Most simple biogas plants are "fueled" with manure (dung and urine), because such substrates usually ferment well and produce good biogas yields. Quantity and composition of manure are primarily dependent on:

- the amount of fodder eaten and its digestibility; on average, 40 - 80% of the organic content reappears as manure (cattle, for example, excrete approximately 1/3 of their fibrous fodder),

- quality of fodder utilization and the liveweight of the animals.

It is difficult to offer approximate excrement-yield values, because they are subject to wide variation. In the case of cattle, for example, the yield can amount to anywhere from 8 to 40 kg per head and day, depending on the strain in question and the housing intensity. Manure yields should therefore be either measured or calculated on a liveweight basis, since there is relatively good correlation between the two methods.

The quantities of manure listed in table 3.2 are only then fully available, if all of the anirnals are kept in stables all of the time and if the stables are designed for catching urine as well as dung (cf. chapter 3.3).

Thus, the stated values will be in need of correction in most cases. If cattle are only kept in night stables, only about 1/3 to 1/2 as much manure can be collected. For cattle stalls with litter, the total yields will include 2 - 3 kg litter per animal and day.

Table 3.2: Standard liveweight values of animal husbandry and average manure yields (dung and urine) as percentages of liveweight (Source: Kaltwasser 1980, Williamson and Payne 1980)

Species	Daily manure yield as % of liveweight		Fresh-manure solids		Liveweight (kg)
	dung	urine	TS (%)	VS (%)
Cattle	5	4-5	16	13	135 - 800
Buffalo	5	4-5	14	12	340-420
Pigs	2	3	16	12	30- 75
Sheep/goats	3	1 - 1.5	30	20	30 - 100
Chickens	4.5		25	17	1.5 - 2
Human	1	2	20	15	50- 80

Table 3.3: TS and VS-contents of green plants
(Source: Memento de l'agronome 1984)

Material	TS	VS
	(%)	(% of TS)
Rice straw	89	93
Wheat straw	82	94
Corn straw	80	91
Fresh grass	24	89
Water hyacinth	7	75
Bagasse	65	78
Vegetable residue	12	86

Vegetable waste

Crop residue and related waste such as straw, cornstalks, sugar-beet leaves, etc. are often used as fodder and sometimes processed into new products, e.g. straw rnats. Consequently, only such agricultural "waste" that is not intended for some other use or for composting should be considered.

Most green plants are well-suited for anaerobic fermentation. Their gas yields are high, usually above that of manure (cf. table 3.5). Wood and woody parts of plants resist anaerobic fermentation and should therefore not be used in biogas plants. Due to the poor flow properties of plant material and its tendency to form floating scum, it can only be used alone in a batch-type plant. In practice, however, batch plants are unpopular because of the need for intermittent charging and emptying.

In continuous-type family-size biogas plants, crop residue therefore should only be used as an addition to animal excrements. Any fibrous material like straw has to be chopped up to 2 - 6cm - and even that does not fully preclude scum formation.

Table 3.4: Digestion characteristics of animal-husbandry residues (Source: OEKOTOP)

Substrate	Scum formation/ sedimentation		Digestion	Recommended retention time (days)	Gas yield compared to cattle manure
Cattle manure	none	none	very stable	60- 80	100%
ditto, plus 10% straw	heavy	slight	very stable	60-100	120%
Pig manure	slight to heavy	heavy to slight	Danger of "tilting", i.e. acidification, at the beginning; slow run-up with cattle manure necessary	40 - 60	200%
ditto, plus 10% straw	heavy	slight	ditto	60 - 80	. . .
Chicken manure	slight to heavy	heavy	Slow run-upwith cattle manure advisable; danger of "tilting"	80	200%
Sheep/gcat manure manure	medium to heavy	none	stable	80-100	80%

Table 3.5: Mean gas yields from various types of agricultural biomass (Source: OEKOTOP, compiled from various sources)

Substrate	Gas-yield range (1/kg VS)	Average gas yield (1/kg VS)
Pig manure	340-550	450
Cow manure	150-350	250
Poultry manure	310-620	460
Horse manure	200-350	250
Sheep manure	100-310	200
Stable manure	175-320	225
Grain skew	180-320	250
Corn straw	350-480	410
Rice straw	170-280	220
Grass	280-550	410
Elephant grass	330-560	445
Bagasse	140-190	160
Vegetable residue	300-400	350
Water hyacinth	300-350	325
Algae	380-550	460
Sewage sludge	310-640	450

Table 3.6: C/N-ratios of varios substrates (Source: Barnett 1978)

Substrate	C/N
Urine	0.8
Cattle dung	10-20
Pig dung	9-13
Chicken manure	5-8
Sheep/goat dung	30
Human excrements	8
Grain straw	80-140
Corn straw	30-65
Fresh grass	12
Water hyacinth	20-30
Vegetable residue	35

Digestion characteristics and gas yields

As long as the total solids content of the substrate does not substantially exceed 10%, simple biogas plants can be expected to operate smoothly on a mixture of animal excrements and plant material (straw, fodder waste).

Manure from ruminants, particularly cattle, is very useful for starting the fermentation process, because it already contains the necessary methanogenic bacteria. On the other hand, the gas yield from cattle dung is lower than that obtained from chickens or pigs, since cattle draw a higher percentage of nutrients out of the fodder' and the leftover lignin complexes from high-fiber fodder are very resistant to anaerobic fermentation. Urine, with its low organic content, contributes little to the ultimate gas yield but substantially improves the fertilizing effect of the digested slurry and serves in diluting the substrate.

The carbon(C)/nitrogen(N)-ratio of animal and human excrements is normally favorable for the purposes of anaerobic fermentation (9 - 25:1), while that of plant material usually indicates an excessive carbon content.

In many cases, various substrates should be mixed together in order to ensure a favorable gas yield while stabilizing the fermentation process and promoting gas production. The following formulae can be used to calculate the C/N-ratio and total-solids content of a given mixture:

MC/N = [(C/N1 x Wl) + (C/N2 x W2) + . . . + (C/Nn x Wn)]/(W1 + W2 + . . . + Wn)

MTS = [(TSI x Wl) + (TS2 x W2) + . . . + (TSn x Wn)]/(W1 +W2 + ... + Wn)

MC/N = C/N-ratio of mixed substrate, MTS = TS-content of mixed substrate, C/N = C/N-ratio of individual substrate, W = weight of individual substrate, TS = TS-content of fresh material.