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K. Svardal, H. Kroiss:
"Sewage treatment plants for winter use in Austrian mountains - a full case study";
Vortrag: TTL (technology-tourism-landscape) Conference, Wien; 03.11.2004 - 05.11.2004; in: "Proceedings TTL", Technische Universität Wien, (2004), S. 119 - 122.

Particularities of WWTP´s in winter tourism areas:

Wastewater treatment in winter tourism areas is a challenging task since sudden load increases - in the range of double to triple of the median load of the non-tourism season - are combined with
low wastewater temperatures. These two boundary conditions are especially critical for the nitrification process. The growth rate of nitrifiers at a temperature of 8 °C is only 0.25 d-1, which
means that even under optimum growth conditions the nitrifying population would only grow by approx. 20 % per day considering autotrophic decay. Of course, such optimum growth conditions
cannot be maintained within the entire aeration tank, subsequently there is simply no operational means to increase the nitrifying population to the increased load demands within the time span it
actually occurs.
Instead, operational precautions have to be taken in order to maximise the nitrifying population well in advance of the occurrence of the load step change. The nitrification capacity of an activated
sludge system is mainly determined by the nitrified load over the course of the SRT.

Basically two measures can be taken:
First, to operate the plant at maximum nitrification in the period before the load increase.
Second, to manage ammonium sources - i.e. storage of sludge dewatering filtrate water - and controlled dosing of the stored high ammonium load water over one SRT before the load increase.
The specific load situation has to be considered already in the plant design phase. Conventional single stage plants do not seem to be the most appropriate solution, since they need to be built
very large to be able to handle the increased load, which corresponds to aeration tank over capacities during the low loaded season.
Two stage plants can be considered a better choice for such load characteristics, but conventional two stage plant often encounter problems due to an insufficient substrate supply to the nitrifying
stage for nitrogen removal. Designing the plant for nitrification only during the high load season can lead to alkalinity (pH) related nitrification inhibition due to low buffer capacity of the
wastewater - which is the case in some of the main skiing regions in Austria.
A possible solution can be found in a special two stage concept - the Hybrid®-concept - which combines some of the advantages of single and two stage activated plants.

http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC04967313

http://www.landscape.tuwien.ac.at/ttl/docs/TTL%20Conference%20Proceedings.pdf