Kristina Stark,
Bengt Hultman and Erik Levlin
Dep. Land and Water Resources Engineering
 Royal Institute of Technology, Sweden
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INTRODUCTION
 Sludge management has been under
debate during the last years in Sweden due to e.g. difficulties to obtain
consensus on agricultural sewage sludge re-use. Expected requirements of
phosphorus recovery and restrictions of sludge disposal on landfill increase
the pressure to develop phosphorus recovery systems for wastewater treatment
plants.
Technology systems for recovery
of phosphorus as a product are presented in the poster. Systems with thermal
treatment and dissolution of phosphates from the sludge have about the same
chemical demand for phosphorus recovery. The demand for chemicals is much
dependent on the type of method for phosphorus removal (biological or
chemical). The systems vary in obtained phosphorus product.
A system is presented that
solves many problems with phosphorus removal and recovery related to chemical
and energy demands and process efficiency. Commercial systems are available
for phosphorus recovery by different process ways. The combination of ways
has advantages compared with the use of only one way.
A solution may be to use two step technology i.e. recovery of phosphorus by
two ways, one using biological technology and the rest of the phosphorus is
recovered by an advanced technology system. This technology may be suitable
for a wastewater treatment plant if the phosphorus recovery demands are high
(over 75%). Use of enhanced biological phosphorus removal and fractionation
of the sludge in two stages is advantageous both with respect to low
necessary chemical and energy demands and recovery efficiency.
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New Systems Technology
 A system is illustrated in figure 1 that solves many
problems with phosphorus removal and recovery related to chemical and energy
demands and process efficiency.
The process way A with activated sludge taken from the anaerobic zone in a
plant with biological phosphorus and nitrogen removal. A phosphate rich
stream is obtained from a thickener of the digested sludge. By use of
precipitation or crystallisation technique a phosphate product precipitation
can be obtained. Part of the phosphorus is still bound in the excess sludge
from the thickener. By use of process scheme B the excess sludge eventually
with post-precipitated sludge may be treated to recover additional phosphorus
and precipitation agents as in the BioCon system or the thermal way to
recover lime and phosphorus in lime. Chemically bound phosphorus may also be
recovered by process way C by adding chemicals and eventually heat for
phosphate release from excess sludge and post-precipitated sludge followed by
phosphorus recovery, for instance systems similar to KREPRO. In such systems
precipitation agents may be recovered and an organic stream is produced
containing easily biodegradable material as organic acids.
Commercial systems are available for phosphorus
recovery by different process way. The combination of ways has advantages
compared with the use of only one way.
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Two
Step Technology for Phosphorus Recovery
Two step
technology with phosphorus recovery is illustrated in figure 2. If the amount
of phosphorus in the sewage effluent is 5% and the amount of phosphorus in
the products from biological technology is 50%, then it will with a
phosphorus recycling demand of 75% only be needed 25% of the phosphorus
amount in the influent sewage to be recovered from the systems KREPRO, BioCon
or Aqua Reci. About 55% of the phosphorus in the digested sludge is needed to
be recovered from these processes. Two step technology can be suitable if
high demands are required for phosphorus recovery. This technology of system is built on how to recover phosphorus both
from the released phosphorus from the return sludge at the treatment in
separate digester and from digested sludge with some of the alternative
KREPRO, BioCon or Aqua Reci. Principle scheme of the system is shown in
figure 3. A phosphorus rich part stream is created by the separation of
anaerobic treated part flow of return sludge, and from treatment with methods
of solving of digested sludge, ash and rest product according to the methods
KREPRO, BioCon and Aqua Reci. An ammonium rich supernatant from the
dewatering of digested sludge and drying of sludge is received.
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ACKNOWLEDGEMENTS
The study has been supported by MISTRA, as a part of a research program
called Urban Water, and by Stockholm Water Co. Travel schoolarship from
Knut & Alice Wallenberg Foundation.
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CONCLUSION
Problems with phosphorus
recovery systems are e.g. that they need to take into account the presence of
iron and aluminium, which can bind phosphate and thereby make it more
difficult to obtain an effective phosphorus recovery. It should also be an
effective separation of the organic and inorganic material and the toxic
material to receive a clean phosphorus product.
Commercial systems are available
for phosphorus recovery by different process way. The combination of ways has
advantages compared with the use of only one way.
It is suggested that the wastewater plant can use one system or use two kinds
of phosphorus recovery methods. A system is presented that solves many
problems with phosphorus removal and recovery related to chemical and energy
demands and process efficiency.
Two step technology may be suitable for a wastewater
treatment plant if the phosphorus recovery demands are high with for instance
products from biological technology with 50% and the rest with some of the systems KREPRO,
BioCon or Aqua Reci.
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