The introduction of biological nitrogen removal at many sewage treatme
nt plants in Sweden, particularly those located at coastal regions up
to Stockholm archipelagio in the east, is now proceeding primarily by
plant trial operation and construction works to be finalised in 1995/9
6 at the latest. The most characteristic problems with biological nitr
ogen removal in Sweden are the cold winter climate and the deficit of
an easily biodegradable carbon source for anoxic denitrification. This
report includes some R&D works from the sewage treatment plant in Hal
mstad on the west coast, including the concluding results of 2-years'
trial operation on nitrogen removal, The main problems have been to ev
aluate two different process alternatives during different climate con
ditions as well as different external carbon sources and compare such
a carbon source with an internal carbon source generated by anaerobic
enzymatic hydrolysis of primary or mixed raw sludge. Methanol has been
evaluated to be the best external carbon source with the lowest biolo
gical sludge production and little detrimental influence on the sludge
characteristics. The results of experimental works and cost calculati
ons on the acid phase of anaerobic sludge digestion under formation of
volatile organic acids (acetic acid etc.) showed that the costs for t
his carbon source with a modified sludge treatment scheme is competiti
ve with methanol from both a technical and economical point of view. D
osage of primary sludge as carbon source for anoxic predenitrification
has also given satisfactory results during the warmer period of the y
ear (June - November). This method is not applicable in Sweden during
the colder period of the year, because the nitrification will be depre
ssed due to a decrease of the sludge age. The two different process al
ternatives for biological nitrogen removal were conventional pre-denit
rification and a combined pre- and post-denitrification respectively.
The last-mentioned system is probably the best process alternative wit
h better operation flexibility and possibilities to a more far-reachin
g nitrogen removal. Both alternatives are recommended for practical op
eration in order to make it possible to switch over from one alternati
ve to the other with respect to e.g. the water temperature. Problems i
n connection with an expected, more stringent phosphorus limit of 0.3
mg P/l in the effluent are also discussed. A polishing step in additio
n to chemical pre-precipitation is required.