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Cleaner Technology Projects in Denmark 1996
Spreading of Sewage Waste
Spredning af spildevandsslam
Arbejdsrapport nr. 65, 1996, Miljøstyrelsen
The use of local authority sewage waste on agricultural areas has been debated during
the recent years. The discussion has focused on the contents of undesirable substances in
sewage waste in the agricultural production and on the possibilities of spreading the
sewage waste. Apart from that it has been discussed whether the sewage waste could be
equalised with the fertilisers which are normally used in the agricultural sector. This
project was conducted with the object of studying the possibilities of spreading local
authority sewage waste when applying different types of sludge and different spreading
techniques.
The tests included studies of the distribution along and across the direction of
motion of four representative types of sludge and ten different spreaders of different
makes and principles of construction.
The sludge types were selected according to dry matter content and drainage methods.
The dry matter content varied from 16 to 30 %. The drainage methods consisted of
centrifuging and pressing by means of a flat-belt separator or a plate filterpress.
The distribution along the direction of motion was determined for stationary
arrangements where the decreasing spreader and sludge weights were registered every 10
seconds in order to clarify the general emptying process. The distribution across the
direction of motion was determined by spreading the sludge over 0.50 x 0.50 m areas at a
total width of 28 m. The spreading area was covered eight times, so that the recorded
spreading pattern would correspond to the average spreading pattern of the entire load.
For both studies the desired rate of application was 20 t/ha at a working width of 12 m
and at a travelling speed of 5 km/h.
The coefficients of variation were calculated in both tests, and the findings were used
as a criterion for the accomplished work. Coefficients of variation less than 15 % are
considered to be satisfactory on equal terms with the demands made on the spreading of
farmyard manure and mineral fertiliser.
The results of the tests prove that, in general, the distribution along the direction
of motion is poor, however strongly dependent on the quality of sludge in question. It was
seen that for all combinations of spreaders and sludge types with a dry matter content
less than 20 %, the rate of application generally tended to decrease during the process of
emptying. In the beginning of the process the rate of application was from 100 to 200 % of
the average, whereas towards the end it was only 50%. The differences were smaller for
sludge types with a high dry matter content, but here the rate of application may be
affected by cloggings (bridgings), resulting in a very irregular emptying process.
After an alteration of the equipment, the average distribution across the direction of
motion proved satisfactory, and an acceptable working width could be attained. At the
beginning of the tests light-fluid sludge would leak from the machines unintentionally,
which would affect the pattern of distribution unfavourably.
The tests showed that the primary factor influencing the distribution of sewage sludge
was the sludge quality, and that there is a general need to upgrade the sludge to products
with a minimum dry matter content of 25 % and a homogenous structure which will permit an
even rate of application.
Author/ institution
Statens Husdyrsbrugsforsøg. Forskningscenter Bygholm.
This report is subsidised by the National Council for Recycling and Cleaner Production
ISSN no. 0908-9195
ISBN no. 87-7810-688-5
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