Mr. Schulz, of the Weihenstephan Agricultural Engineering Association in
Freising, near Munich, presented a paper emphasizing how biogas
technology can benefit municipalities, given current environmental
problems. Calculations relating to the avoidance of pollution, e.g. by
reducing emissions of methane and ammonia, the use of digested sludge as
a substitute for chemical fertilizers, and on reduction of ground water
pollution are becoming increasingly important. For the individual
farmer, biogas meant less work, said Mr. Schulz. For example, digested
sludge reduced the capability of weed seeds to germinate, at the same
time causing plants to be far healthier than conventional fertilizers.
Professor A. Gosch, of Friedberg Agricultural College in Giessen,
presented a pioneering co-fermentation concept. Nutrients are added to
substrates to be fermented to improve the yields of biogas plants. Gosch
recommended an extremely economical method which is also environmentally
compatible: lawn cuttings, maize silage, used cooking fat, sour whey,
brewery waste, household waste, can be used as additives. In one plant,
even fat from a deep fryer was processed - a waste product that poses a
problem for restaurants in particular. Professor Gosch explained that
the bacteria had to be “weaned“ to the new substances gradually.
Instead of composting household waste, which involves heavy costs for
municipalities, this alternative waste disposal plan proposes that
farmers should be paid to dispose of them. As a result, the energy yield
of biogas plants would be increased and the nutrients would enrich the
(biological) fertilizer. Farmers in Baden-Wurttemberg have put forward a
proposal for small, decentralized plants with capacities of 1, 000
tonnes p. a. to process the biowaste of communities with 700 to 15, 000
inhabitants. The financial benefits are said to be appreciable, both for
the municipality and for the plant operator. (Professor Gosch is
offering two publications on co-fermentation at cost price. Address:
Wiesenstraáe 14, D-6000 GieBen, FRG)
The remarks on plant engineering in connection with solid manure
technology reflected recent findings concerning the profitability and
energy yield of plants with steel tanks and concrete slurry pits. The
use of solid manure and bedding straw is the most far-reaching recent
chance in the technology for agriculture.
Solid manure technology
Due to the higher percentage of organic dry matter in the substrate the
gas yield can be tripled as compared to the yield from the more liquid
dung urine substrate. To achieve this, however, the solid components
must be thoroughly pulverized with a powerful stirrer, first in the
influent collecting tank and then in the digester.
Today, there are two types of plant which optimize gas production using
a stirrer - the concrete pit plant (biogas storage plant) and the steel
tank plant (through-flow type). With the concrete pit plant, a concrete
liquid manure tank with a concrete cover is expanded to convert it into
a biogas plant. Storage and digestion take place in the tank. The gas
produced in the digesting chamber is collected in the chamber itself, in
a bag made of plastic sheet.
As a variant of this, open manure pits can be covered with double
plastic sheet. The outer, fabric-reinforced sheet keeps its shape
stable, while the lower sheet rises and falls depending on gas
production and consumption. By combining the liquid manure with the
digestion chamber the plant is more compact than a steel tank plant, and
can be built with less labour and materials. The onIy d is advantages
are those resulting from mixing fresh substrate and fermented material
from the digesting chamber. Thus, the quality of the biofertilizer and
compatibility with (vegetable) plants are reduced, while the odour is
In addition, the storage plant method leads to considerable fluctuations
in gas production over the year. In practice, this problem is
counteracted by using co-generative motors, excess power being fed into
the grid. Since 1991, the utility companies have bought this power at
75% of their resale price.
On the other hand, the steel tank plant has proved to be a reliable type
of through-flow biogas plant for 30 years now, even with problematic
types of liquid manure. The heart of the plant is a horizontal steel
tank- normally a used heating oil tank - with a paddle type stirrer. The
gas is stored in the tank. The only disadvantages of this type of plant
are the weather protection and the space requirement.
Fuel for block-type thermal power stations
The south-west German division of the Technical Surveillance Association
(TUV) has so far laid down only minimum technical requirements. These
include in particular safety devices to control blowback, overpressure
and vacuum and automatic gas flow interruption during motor operation.
However, regulations for lowpressure operation are much more urgently
needed, because according to the TUV expert unjustified or excessive
demands are already pushing investment costs up.
Fig. 1 Storage-type biogas plant with double-skin pit cover and
swivel-mounted gaslight stirrer.
Biogas as a fuel for block-type thermal power stations represents the
ther best possible way of utilizing the energy thus produced. The
outlook for co-generation has become all the more interesting since
legislation was passed in Germany ensuring that electricity fed into the
grid from private producers is paid for.
Internal combustion engines normally generate far more heat than
electrical power, but if combined with a gas storage tank it would be
possible to conclude agreements with power station operators to provide
them with electricity in particular to help cover peak loads. The heat
given off by the engine can be used to heat the plant and residential
Many different technical solutions have been evolved for co-generation.
Various models were presented by different speakers. While the Fiat
Totem (Total Energy Module) is commonly used in digester gas technology,
one could just as easily imagine using a Chevrolet engine for other
In tests, a converted turbo diesel engine attained a level of efficiency
in electricity generation 40% higher than that of conventional
block-type thermal power stations. A low-cost variant of a co-generation
“kit“ might also be suitable for developing countries, as was shown
during the presentation.
Model calcurations presented at the meeting indicated that the
technology would be profitable only with a 25% government subsidy - at
today's prices! On the other hand, the calculations did not take into
account the future benefit of the plant if it is used for disposal of
biowaste, with a higher energy yield and production of higherquality
The heart of the plant is a horizontal steel tank with a paddle-type
stirrer. Plants of this type have proved highly reliable, one having
been in use for 30 years without giving any problems.
In particular, it copes well with floating scum, sediment layers and
congestions, even with problematic manures such as liquefied solid
manure with a high straw content and pig manure, which tends to form
sediment layers. Nor are there any problems with gas-tightness or
Energy consumption for mixing is very low. When the liquid manure is
pumped in daily, the same quantity of fermented material overflows into
the manure storage tank at the other end. The elongated shape of the
tank helps to prevent loss of unfermented manure.
Used heating oil tanks with a capacity of 50 to 100 m3 are normally
They are between 2.5 m and 2.9 m in diameter end from 12 m to 14.5 m
long. If necessary several tanks can be combined.
Through-Flow-Type Biogas Plant
The plant is insulated on the outside with 20 cm thick Rockwool A
digester volume of 16 - 20 m3 is required per cubic metre of liquid
pumped in daily The plant takes up more space than a storage-type plant
and requires weather protection, but the gas is stored in the same way.