Biological methanation in fermenters of municipal WWTP

2010 - 2014


Contact Person:

Ao.Univ.Prof. Dipl.-Ing. Dr.techn. Matthias Zessner

Univ.Prof. Dipl.-Ing. Dr.techn. Helmut Rechberger

Dip.-Ing. lukas Egle

Brief Description

Phosphorus (P) is an essential and at the same time finite resource, whereas Austria owns no P-
deposits and P-containing fertilizers have to be imported. At the same time municipal waste water can
be seen as a potential but often unused P-resource. Due to potential environmental risks (heavy
metals, organic pollutant and germs) application of sewage sludge or composts from sewage sludge to
agriculture is not widely used in Austria. Internationally several technological approaches have been
developed within the last years to enable recovery of P from different streams from a waste water
treatment plant (WWTP). Within this study these technologies will be comparatively assessed in
respect to technical, environmental and economic criteria using a methodology specifically developed
for this purpose. Therefore this study can serve as a starting point on which considerations an
optimization of P-management in the future can be based. This study show, that P-recycling
technologies from return water from sludge thickening and dewatering can be easily implemented and
economically operated under certain preconditions. Further on, these technologies produce a product
with very little contamination and in addition for some technologies with very good plant availability. As
return water from sludge thickening and dewatering contains only a small part of P entering the
WWTP, the potential P-recovery with these technologies is low. Therefore, these technologies are not
qualified to serve as an essential element of a strategy that aims at high rates of P-recovery from
waste water. Worth mentioning for these technologies are the operational advantages as avoidance of
incrustation and reduction of nutrient backcharge within the WWTP, which could support an
implementation under certain circumstance. The results for technologies for P-recovery from sewage
sludge show, that they are either far away from an economic operation (wet-chemical approaches) or
are technically hard to control (wet-oxidation approaches) or a prognosis on their future recycling
potential is actually not possible due to missing confirmed information (metallurgic approaches). It
should be noted that the resource demand does not corresponds with the recycling potential, both
related to the technology and on the national context. For the metallurgic approach further research is
necessary, whereas the potential with regard on a future implementation is clearly given. The most
promising technologies in order to achieve high P-recycling rates are technologies with recovery from
sewage sludge ashes. As prerequisite for implementation, incineration of sewage sludge without
mixing with other substrates poor in P is required. The required structures as e.g. mono-incineration
plants in Austria would have to be expanded. The available technological approaches for recovery
range from (i) wet-chemical approaches with very good or moderate heavy metal depollution and
comparably lower recycling potential, via (ii) thermo-chemical approaches with systematic heavy metal
depollution, high recycling potential but restricted plant availability of the recycling product to (iii)
approaches with the aim of an improvement of the plant availability of the ash but without specific
heavy metal depollution. Furthermore the integration of sewage sludge ash to existing industrial
processes as fertilizer industry and P4-generation is feasible. Which of the available technologies for
treatment of sewage sludge ash should be favored depends on the demands on economic factors,
heavy metal removal and plant availability. A big advantage of a strategy with incineration of the
sewage sludge is the possibility for co-incineration and P-recovery with other P-rich substances as
meat and bone meal and the possibility of a temporary storage of the P-rich ash, from where P could
be recovered later on (“Austrian Phosphorus Mine”).