Comparison of technology, costs and environmental benefit of wastewater treatment plants in mountainous areas in the alps

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Due to the extreme climatic situation the feasibility of biological wastewater treatment in mountainous regions often is questioned. In the course of the Life-project 3 refuges operated also during the winter or spring have been investigated according to the temperature aspect - the Rastkogelhütte and the Meisner Haus (already existing treatment plants) and the Essen/Rostocker Hütte (new WWTP). In general the activity of micro-organisms is significantly reduced at decreasing temperatures. This dependency is described by an exponential function: R(T °C) = R(20 °C) * EXP(kT * (T - 20)). The reaction rate R at the temperature T is calculated from the reaction rate at 20 °C by the temperature coefficient kT. In the case of heterotrophic (carbon eliminating) bacteria kT = 0.069, i.e. elimination rates at 10 °C are exactly 50 % less than at 20 °C. The temperature effect on nitrifying organisms is even stronger with a decrease of the elimination rate of  63 % (kT = 0.098 according to ASM1). At low wastewater temperatures (T < 8 °C) properly designed WWTPs achieve at least carbon elimination. Measurement data of treatment plants listed above indicate advanced nitrogen elimination even at very low temperatures. The following figure shows the case study Rastkogelhütte with no significant correlation between the wastewater temperature and the treatment efficiency. Obviously the influence of the current load and other operating conditions dominates the influence of temperature.

Fig. : Annual profiles of wastewater temperature and nitrogen elimination of the WWTP Rastkogelhütte (sampling 1999 - 2001).

In case of plant operation during the winter season specific measures need to be taken: For supervision and maintenance at any time accessibility despite snow should be provided by the construction of a building around the reactors or most important shafts or manholes. Additionally thermal insulation should prevent extreme cooling of the water. In the course of the Life-project no biofilm treatment works have been installed at refuges with winter operation. In general the activity of the large biofilm surfaces is endangered by cold air or frost respectively. Due to the higher specific energy of water in comparison with air activated sludge systems are less temperature sensitive.

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