(642a) Recovery of Phosphorus From Sewage Sludge and Sewage Sludge Ashes With the Thermo-Reductive Recophos Process

The
principle of the RecoPhos process is the fractioned extraction of phosphorus
and heavy metals from sewage sludge and its ash at high temperatures under
reducing conditions. The RecoPhos process transforms a problematic waste into
several streams of secondary raw materials, leaving behind a minimum of solid
waste. The aim of the planned RecoPhos project is to develop a completely new
P-recycling process, which can avoid several of the known chemical-technical
problems that are still limiting the possible success of phosphorus recycling.

Introduction

There is no possibility to find a replacement for
the resource phosphorus. It is an essential element for all life forms and its
exploitable deposits are getting scarce quickly [1]. This and also the fact that
phosphate mining and refining has severe bad effects on the environment, like
enormous land use; and the production of vast amounts of dangerous wastes [2],
will make it inevitable in the near future to recycle phosphorus from every
suitable P-containing waste. Sewage sludge (coming from municipal wastewater
treatment) is considered to be one of the most important alternative Phosphorus
sources, as in this sludge all of the phosphorus discharged by humans is
concentrated and can, if properly collected and treated, be re-used completely.
Its inorganic dry matter (= ash content) has a phosphate content of
30 up to 70% of the content of phosphate rock produced from natural phosphate
ores [3]. In the project, the process will be developed using modeling and lab
scale experiments. Using these results, the basic design of a pilot scale plant
will be carried out, a market study will be performed and the environmental
impact of the process compared with the status quo will be evaluated in a Life
Cycle Assessment (LCA) study [4].

The RecoPhos process

The core
technique of the RecoPhos process, the so-called InduCarb reactor, works after
the same chemical principle as the Woehler process [5], producing amongst
others white phosphorus as a product.

The above
shown summary reaction runs in the inductive heated bed of carbon as
illustrated schematically in figure 1.

Figure  SEQ "Abbildung" \*Arabic 1: InduCarb reactor in the RecoPhos
process

RecoPhos
approaches the suitability of the following gas treatment aggregates very
similar as used in waste and sludge incineration plants:

• Quench (temperature > 280 °C) for the removal of low boiling heavy metals and compounds as dust, leaving the phosphorus gaseous;
• Quench (temperature < 100°C) for the recovery of white phosphorus from the off-gas stream;
• Scrubber for the removal of acidic compounds and Hg;
• Dry and wet electrostatic precipitator for the removal of dusts and aerosols;
• Activated carbon adsorber and cloth filter or activated carbon cloth filter in appropriate positions;
• Afterburner, if the production of phosphoric acid is wished.

To be able
to perform experiments, a fully operational continuous bench scale RecoPhos
plant will be designed and constructed in the course of the project. This
comprises the planning and basic engineering of the reactor as well as of periphery
like frequency converters, off-gas cleaning and automation technology.

Modeling and Simulation

The
mathematical modeling and simulation consists of two parts: thermodynamic
process simulation, and mathematical modeling of the inductive heated chemical
reacting packed bed of carbon particles. The design will be constantly enhanced
according to the experimental results. As a last step, using the most important
results, the basic design of a pilot scale will be created.

The modeling
of the InduCarb reactor also consists of two parts. At first the creation of a
macroscopic model is necessary to get an overview of the occurrence in the
reactor. Also it will be possible to get the dimensions for a reactor design.
The aim of the model calculation is to find the necessary parameters for the
dimensioning of the reactor. The correct choice of the reactor volume is
decisive for the attainable conversion in the process. Furthermore statements
relating to the design should be possible. The optimal height to diameter ratio
of the reactor is an important statement here. Furthermore, modes of operation
could be optimized in an existing reactor. Knowledge of the interactions
between the processes in the reactor gains a deeper insight into the process.
So you can make settings for targeted experiments.

The second
part is a mesoscopic model that will be developed by MUL adapting on an
existing transient, 2-dimensional heterogeneous model. The goal is to optimize
the heat input, the product quality and the energy efficiency.

All over it
is necessary to find out the key components and the reactions in the InduCarb
reactor. Also a reducing of the surface of the carbon bed during operation has
to be examined. Furthermore the input of additives should be checked (e.g. for
minimizing the consumption of the reduction agent carbon or for an alternative
energy supply to the reactor).

Summary

Recycling of Phosphorous from sewage sludge
provides a sustainable solution for production of this finite resource essential
for human nutrition. It will also be a solution to the environmental problems
of Phosphate rich effluents from waste water treatment plants. To this end, the
RecoPhos project employs the innovative InduCarb reactor design in which the
ashes from sludge incineration plants will trickle down an inductively heated
packed bed of Carbon. The reactions taking place inside the reactor for
production of high quality Phosphorous are based on the same principle as the
well-known Woehler process. The final design of the overall process will be
based on the results of modeling and simulation in conjunction with
experimental research at the pilot scale.

Acknowledgement

The
RecoPhos research project has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under
grant agreement no. 282856.

Literature

[1]       Cordell, D.; Drangert, J.-O.; White, S.
(2009). The story of phosphorus: Global

           food security and food for thought.
Global Environmental Change 19: 292-305

[2]        Sartorius C., von Horn J., Tettenborn F.
(2011) Phosphorus recovery from  

           Wastewater ? State-of-the-art and
future potential, International conference on

           Nutrient Recovery and Management
2011, January 9-12,
Miami, Florida, USA

[3]      
S. Donatello, D. Tong, C.R. Cheeseman. (2010). Production of technical
grade   

          
phosphoric acid from incinerator sewage sludge ash (ISSA). Waste

          
Management 30, 1634?1642

[4]        RecoPhos Grand Agreement, Annex I ?
Description of Work, 2011

[5]       Corbridge, D.E.C. (1995). Phosphorus: An
outline of its chemistry,

          
biochemistry and uses. Elsevier. pp. 556.