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Phosphorus Recovery From Sewage Sludge Using the Aquacritox Supercritical Water
Aquacritox, supercritical water oxidation, sewage sludge, energy, phosphorus,
biological phosphorus removal
Sewage sludge is a potential source of energy. A kilogram of dried sewage sludge contains
approximately 20,000 BTU’s of energy. The challenge with releasing this energy is that sewage
sludge, when collected from the bottom of a clarifier is 99% water. In order to release the energy
present through gasification, pyrolysis or incineration, the water must be removed before oxidation
in air can occur. This requires thickening mechanical dewatering and thermal drying. Supercritical
water oxidation offers the potential to release all of the energy present in sludge, without first having
to dewater the sewage sludge.
Sewage sludge also contains phosphorus. Every person produces approximately 1.2kg/annum of
phosphorous. This is typically conveyed to a wastewater treatment plant. In the United Kingdom, the
annual phosphorus load from human sources which is present in wastewater is therefore
approximately 72,000 tonnes. This paper examines how the Aquacritox sludge to energy process
could be used in combination with Biological phosphorus removal & a fluidised bed phosphorus
recovery process to recover 70% of the total influent phosphorus load to the treatment plant. The
paper provides a simulated case study at a 500K PE Biological phosphorus removal plant illustrating
how this could be implemented. The paper also provides mass balance and cost estimates for the
The Aqua Critox® process is a patented Super Critical Water Oxidation process developed to
completely destroy the organic fraction of sewage sludge with no harmful emissions to air or water.
The process uses the unique properties of supercritical water to completely destroy through oxidation
all organic contaminants from sewage sludge.
The solids are mineralized within the Aqua Critox® process into sterile inert inorganic clay, carbon
dioxide and nitrogen. The inorganic clay material can be used as a construction material. This
inorganic clay like material can also be further processed to recover either or both phosphorous and
As the oxidation process is exothermic the process generates significant amounts of surplus heat
which can be recovered for heat applications or for the generation of electricity.
Because super critical water reactions do not necessitate the evaporation of water the process has a
high thermal efficiency.
Super critical water oxidation is able to achieve high organic destruction rates (99.99 %+) without
the requirement for expensive dewatering equipment upstream. Due to the unique properties of
super critical water, SCWO achieves greater destruction efficiencies than incineration or wet air
oxidation with the advantage in larger plants of excess electricity production over and above that
required to run the process i.e. positive energy balance. The Aqua Critox® Process
The Aqua Critox® process is a patented process that utilises water and oxygen at elevated
temperature and pressure to completely destroy the organic fraction of sewage sludge, with no
harmful emissions to air or water. The process is totally enclosed and can achieve destruction
efficiencies of close to 99.99%+ in less than 60secs residence time.
Both raw and digested sludge can be treated at 6% – 22 % dry solids, providing the option of
reducing dewatering requirements earlier in the treatment process.
A general arrangement for an Aquacritox Plant is provided as Figure 1
below. Figure 1
General Arrangement for Aquacritox Plant
Phosphorus is a non renewable resource for which there is no substitute. While it is possible to
substitute renewable energy for fossil fuels, no other mineral can take the place of phosphorus
Our ability to provide enough food to feed the human population is dependent on the use of artificial
fertilizers, which contain phosphorus, nitrogen and potassium. While nitrogen is abundant in the
atmosphere (- it just requires the use of large amounts of natural gas to capture it), phosphorus is
mined at just a handful of locations worldwide, primarily, the United States, China and Morocco.
(Hong et al. 2005
Today, Florida produces 25% of world's phosphate which makes the United
States the world's largest producer of phosphate rock.
While the timing for “Peak Phosphorus” may be fifty, or even one hundred, years out, as with peak
oil, it is not a question of if, but when. There are some indications that production has already peaked
in terms of the readily available resources while other estimates put this 20-30 years out as is
illustrated in Figure 2.
Estimates of Timeline for Peak in Phosphorus Production
This is leading to increased interest in phosphorus recovery in a re-usable form (Berg, U &
). This is particularly evident in Europe where rock phosphate deposits are
negligible. In 2006, 87% of the phosphorus used in EU fertilizers was from imported phosphorus,
with only 13% coming from mined resources.
Wastewater and sewage sludge represents a significant potential source of phosphorus. If all of the
phosphorus available in sewage sludge in the EU was recycled, this could provide for 28% of the
EU’s total phosphorus requirements. Sweden for example has meatment plants by 2015 (Levlin, E, 2003
The concentration of phosphorus in municipal wastewater is approximately 6-8mg/l with each person
contributing approximately 3.28 grams of Phosphorus per day. This is a consistent and predictable
source of phosphorus.
Every person produces approximately 1.2kg/annum of phosphorous. This is typically conveyed to a
wastewater treatment plant. In the United Kingdom, the annual phosphorus load from human sources
which is present in wastewater is therefore approximately 72,000 tonnes. This would be equivalent to
336,000 tonnes of Phosphorus pentoxide, which at a typical value of US$500 per tonne, would be
worth US$168M per annum. This does not include any additional phosphorus present in wastewater
from commercial or industrial sources.
Currently there are two ways in which this Phosphorus can leave a wastewater treatment plant: In the
treated wastewater, or in the sludge stream.
In the absence of specific phosphorus removal methods, approximately 75% of the phosphorus
which comes into the treatment plant in the raw wastewater leaves in the treated wastewater, and
approximately 25% leaves the plant in the biosolids or sludge stream. Where specific phosphorus
removal mechanisms are employed (Chemical / Biological), in excess of 85% phosphorus removal
can be achieved. In this scenario, 15% of the phosphorus which was present in the raw wastewater
leaves the plant in the treated wastewater and 85% leaves the plant in the sludge stream (where
chemical phosphorus dosing was employed it leaves the plant as an iron or alum precipitate, where
Biological phosphorus removal is used it leaves the plant as poly-phosphate within the biomass).
Currently the only method to recycle the phosphorus present in wastewater is by applying sludge to
land. In certain jurisdictions the application of biosolids to land is not practiced and this is leading to
interest in alternative methods of recovering and recycling phosphorus from sewage sludge. The Aquacritox Process as a Potential Means of Phosphorus Recovery
The Aquacritox process presents an opportunity to recover both energy and phosphorus from sewage
The Aquacritox process is a supercritical water oxidation process in which sludge is heated to
between 370o C and 500oC at 220bar of pressure in the presence of oxygen. All of the organic matter
is completely oxidized in an exothermic reaction. The energy released is used to generate heat and
electricity. The process produces an ash like product and a supernatant, which is essentially water
with COD levels less than 5mg/l.
Any soluble or Ortho-P which was present in the sludge is present as soluble Phosphorus in the
sludge supernatant. This phosphorus can then be readily precipitated as Calcium Phosphate or
Ammonium Magnesium Phosphate using crystallization processes such as the DHV Crystallactor
process or the Ostara PEARL process (Britton A. et al 2005).
Where required, phosphorus can also be recovered from the residual inert
solid stream. Phosphorus
is extracted by leaching the residuals with HCl, H2SO4 or NaOH. Leaching the residuals with 1%
NaOH solution at a temperature between 80-90oC for 75 to 90 minutes, can extract 65% to 90% of
the phosphorus. Extraction efficiencies are higher with acid. Once extracted, the phosphate can be
precipitated as Calcium Phosphate by the addition of lime.
Phosphorus Mass Balance at a Biological Phosphorus Removal Plant with and without
A mass balance study has been carried out to illustrate how an Aquacritox plant could be used at a
500,000 PE Biological Phosphorus Removal plant to treat the sludge and also recover the
phosphorus present. This is a simulated study based on data gathered at a number of actual biological
phosphorus removal wastewater facilities.
Two scenarios are examined. The first is Biological phosphorus removal with anaerobic digestion,
followed by sludge dewatering and sludge haulage off-site. The second is the same Biological
phosphorus removal plant, but with an Aquacritox system in place of the Anaerobic digestion and
sludge dewatering system.
Typical values for influent phosphorus loading have been used and a phosphorus removal efficiency
of 85% is assumed. Influent Parameters:
Final Effluent Phosphorus Concentration:
Scenario A: Biological Phosphorus removal with Anaerobic Digestion
In Scenario A, (Figure 3
) the total phosphorus mass loading into the plant is 750kg/day. There are
two ways in which phosphorus leaves the plant: the sludge stream and the treated wastewater. The
sludge stream will contain 85% of the phosphorus and the treated effluent will contain 15% of the
phosphorus. As with all Biological phosphorus removal plants with Anaerobic digestion, some of the
phosphorus taken up by the sludge is released in the Anaerobic Digester and is returned back to the
headworks of the plant with the sludge dewatering liquors. This ‘dead load’ of phosphorus which is
recycled would typically be in the region of 375kg/d.
Scenario A: Mass Balance for Bio-P Removal with Anaerobic Digestion at a
500K PE WWTP
Scenario A: P Mass Balance – Bio-P with AD (500K PE)
375 kg/d – Return P
Scenario B: Biological Phosphorous Removal with Aquacritox and phosphorus recovery
In Scenario B, (Figure 4) the Aquacritox process replaces the Anaerobic digester and sludge centrifuge. All of the energy present in the sludge is released and used to generate heat and power. The phosphorus which comes into the plant as Ortho-Phosphate is released from the sludge during the Aquacritox treatment step and is present in the supernatant. The expected phosphorus concentration in the supernatant in this scenario will be 847.8 mg/l. This is based on 250m3/day of sludge at 12% solids going through the Aquacritox plant. The mass of phosphorus present in the supernatant will be 585kg/day representing 78% of the total influent phosphorus load to the plant. It would typically be possible to recover at least 90% of this phosphorus from the supernatant using commercially available fluidised bed crystallisation
technologies. This would result in a recovery of 530 kg P per day, which would represent a
phosphorus recovery rate of 70%.
The soluble phosphorus present in the supernatant is an ideal feedstock stream for phosphorus
recovery in a fluidized bed crystallization reactor. There are a number of companies which have
commercially available fluidized bed crystallization systems such as the PEARL system offered by
Ostara Nutrient Recovery Technologies Inc and the Crystallactor developed by DHV. These systems
recover phosphorus as a precipitate such as Magnesium Ammonium Phosphate (struvite) or Calcium
Phosphate (Berg U. & Knoll G, 2005
) Figure 4
Scenario B: Mass Balance for Bio-P Removal with Aqua Crotix
® at a 500K PE
Scenario B: Mass Balance – Bio-P with Aquacritox
55 kg/d – Return P
Conclusions and Recommendations
This case study showed that sewage sludge can be successfully treated on an on-going basis
under stable operating conditions. The results confirm that complete oxidation of organic matter
is achievable. Analysis on the supernatant confirms that the water produced could be discharged
safely without further treatment. Analysis of the inorganic residue produced confirms that the
residual material is inert and is predominantly silica.
Berg, U & Schaum, C., 2005
Recovery of Phosphorus from Sewage Sludge and Sewage Sludge Ashes – Applications in
Germany & Northern Europe,
Ute Berg & Christian Shcaum
Ulusal Aritma, Camulari Sempozyumu, 23-25th March 2005 Hong et al, 2005
Study on the Recovery of Phosphorus from Waste-Activated Sludge Incinerator Ash
Kyung-Jin Hong, Noriya Tarutani Yoshitsune Shinya,
and Toshio Kajiuchi
Journal of Environmental Science and Health, 40:617–631, 2005 Levlin, E. , 2003
Phosphorus Recovery From Phosphate Rich Sidestreams In Wastewater Treatment Plants
E. Levlin and B. Hultman
Dep. of Land and Water Resources Engineering,
Royal Institute of Technology, S-100 44 Stockholm, Sweden
(E-mail: email@example.com;) Proceedings of a Polish-Swedish seminar, Gdansk, 23-25 March 2003.
optimisation of urban sanitation systems. E. Plaza, E. Levlin, B. Hultman, (Editors), TRITA-
LWR.REPORT 3004, ISSN 1650-8610, ISRN KTH/LWR/REPORT 3004-SE, ISBN 91-7283-
471-4. 2003 U. Berg & G. Knoll, 2005
P-RoC - Phosphorus Recovery from Wastewater by Crystallisation of Calcium Phosphate
U. Berg, G. Knoll, E. Kaschka, V. Kreutzer, D. Donnert, P.G. Weidler and R. Nüesch
IWA-WISA Conference 2005, Johannesburg, Süd-Afrika, 9. – 12. August, 2005
Britton, A. et al, 2005
Pilot-scale struvite recovery from anaerobic digester supernatant at an enhanced biological
phosphorus removal wastewater treatment plant
A Britton, F.A. Koch, D.S. Mavinic, A. Adnan, W.K. Oldham, and B. Udala
J. Environ. Eng. Sci. 4: 265–277 (2005)
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