Recent Advances in Urban Planning and Construction
Chemical precipitation of phosphorus on a wastewater treatment pilot
plant
GHEORGHE BADEA, MARIUS - DANIEL ROMAN, IOAN GIURCA, CĂLIN OVIDIU
SAFIRESCU, IOAN AŞCHILEAN, DAN MUREŞAN
Building Services Engineering Department, Technical University of Cluj-Napoca
21 Decembrie Boulevard 1989, no. 128-130, 400604, Cluj - Napoca
ROMÂNIA
[email protected], http://instalatii.utcluj.ro/departamente.php
Abstract: Chemical precipitation is used to remove the inorganic forms of phosphate by the addition of a
coagulant and a mixing of wastewater and coagulant. During the monitoring period of experimental studies on
the pilot plant for the chemical reduction of phosphorus were used different chemicals, as follows: ferric
chloride, polyaluminium chloride, ferric sulfate and aluminium sulfate. Throughout the experimental studies
the wastewater quality indicators such as: chemical oxygen demand (COD) and ammonium compounds were
investigated in order to confirm the results according to NTPA 001/2005 for these parameters. The chemical
removal of the phosphorus has led to best results for polyaluminium chloride (PAX 16) with residual
concentrations of total phosphorus up to 1 mg/l, according to normative NTPA 001/2005.
Key-Words: phosphor, wastewater, chemical precipitation, ammonium, nitrification, denitrification, optimal
dose.
methods are based on the transfer of soluble
phosphorus to a solid phase and complemented by
solid-liquid separation [2].
According to the principles of removing
phosphorus from wastewater, methods of removal
may be divided in three groups: chemical
precipitation and adsorption of phosphorus,
biological and combined.
Phosphorus removal by chemical precipitation is
at present the best known process of those
mentioned above and is widely used even if its
relatively high costs. During recent years progress
has been made to understand the mechanisms of
biological phosphorus removal and to develop
the design procedures. Absence of proper
mathematical models of the process, however,
makes application of biological phosphorus
removal difficult without primary laboratory
investigations [3].
Chemical precipitation is used to remove the
inorganic forms of phosphate by the addition of a
coagulant and a mixing of wastewater and
coagulant. The multivalent metal ions most
commonly used are aluminium and iron [1].
1 Introduction
Surface waters contain certain level of phosphorus
in various compounds, which is an important
constituent of living organisms. In natural
conditions the phosphorus concentration in water is
balanced so accessible mass of this constituent is
close to the requirements of the ecological system.
The general purpose of phosphorus removal is to
eliminate the excess phosphorus content from
wastewater discharged to receiving waters and
then to utilize this excluded phosphorus load in
the way which is the most proper for the
natural phosphorus cycle in nature.
This policy should prevent surface waters
against eutrophication-related problems.
Phosphorus removal in most cases is required
today; usually a concentration of 1–2 mg/l has to
be maintained.
Excess phosphorous is removed by simultaneous
precipitation, frequently in combination with
enhanced biological phosphorous removal. With a
mixing tank upstream of the biological reactor for
nitrification–denitrification with a retention time of
15 to a maximum of 30 min.
Phosphorus is present in wastewater usually in
soluble form. Only about 15% of total phosphorus
contained in settleable particles may be removed
by primary sedimentation with no metal salt
addition. After to this fact traditional removal
ISBN: 978-960-474-352-0
2 Problem Formulation
2.1 Pilot wastewater treatment plant
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Recent Advances in Urban Planning and Construction
FeCl3 + PO43− ↔ FePO4 ↓ +3Cl − (2)
55.85 + (35.45g · 3) + 30.97 + (16g · 4) ↔
55.85 + 30.97g + (16g · 4) + (35.45g · 3) ↔
162.2g + 94.97g ↔ 150.82g + 106.35g
Pilot installation is composed from a tank divided
by partition walls and hydraulic systems: zones of
denitrification and nitrification.
FeCl3/ P=162.2g/30.97g = 5.2
The molar ratio of Fe/ P is 1/1, the weight ratio
Fe/P is 1.8 / 1 and the FeCl3/ P is 5.2 / 1. The
reaction between ferric sulfate and phosphate is:
Fe2 ( SO4 )3 + 2 PO43− ↔ 2 FePO4 ↓ +3SO42 − (3)
Precipitation with ferric chloride tests followed
the total phosphorus removal at different doses for
P-total value was determined initially from influent,
determining the optimal dose to a mass ratio equal
to or greater than that established by the reaction
stoichiometry (see equation 2). According to this
report reactions FeCl3: P = 5.2.
Ferric chloride solution was used for precipitate
a concentration of 8.2 % (initial stock solution was
diluted 41 % in order to be able to fit in field of the
dosing pump) and a density of 1.44 g/cm³.
Dosing began with an excess of 20 %, following
50 % and 75 % of this report so that reports of mass
between ferric chloride and P - total were: 6.4, 7.8
and 9.1.
Tests ferric sulfate precipitation followed total
phosphorus removal at different doses for P-total
value was determined from influent, determining the
optimal dose to a mass ratio equal to or greater than
that established by the reaction stoichiometry (see
equation 3). According to this reaction Fe2(SO4)3: P
= 6.4. Reports of mass between ferric chloride and P
- total were: 7.36, 9.6 and 10.8. Solution of ferric
sulfate dosage used was a solution of 42 %
concentration and density 1.53g/cm³.
Fig.1 Pilot installation
Pilot plant description by zones : 1- anaerobic zone;
2- anoxic zone; 3- anoxic zone; 4- aerobic zone; 5anoxic zone; 6- aerobic zone; 7- membrane; 8- DO
reduction, 9-overflow.
2.2 Experimental determinations
On the pilot installation were followed:
- influent and effluent quality variations;
- efficiencies in removing organic substances;
- efficiencies in removing nitrogen based
compounds;
- removal of phosphorus by precipitation with
different chemical reagents.
During the operation of the treatment plant were
made on the quality of influent and effluent
measurements and analyzes were performed for the
following indicators:
- temperature and pH (SR-ISO 10523:2009) [6];
- chemical oxigen (COD, SR-ISO 6060:1996);
- ammonium, (photometric method), Hach DR
3800;
- orthophosphates, (photometric method), cuvette
tests, Hach DR 3800.
2.2.2 Precipitation
aluminium
The basic reaction is:
phosphorus
Al 3+ + H n PO4n −3 ↔ AlPO4 + nH +
with
(4)
Aluminium ions react with phosphate ions to
form aluminium phosphate:
2.2.1 Precipitation of phosphorus with iron
The basic reaction is:
Al 3+ + PO43− ↔ AlPO4 ↓
Fe 3+ + H n PO4n −3 ↔ FePO4 + nH +
(1)
Iron salts used for chemical precipitation of
phosphorus were ferric chloride and ferric sulfate.
Basic reaction between ferric chloride and
phosphate is:
ISBN: 978-960-474-352-0
of
(5)
Reacting one mole of aluminium to one mole of
phosphate and the weight ratio Al / P is 27/31, so as
0.87/1. Typically, aluminium is used in the
precipitation of phosphorous in the form of hydrated
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Recent Advances in Urban Planning and Construction
Table 1, 2 presents the values for this three
parameters: COD, NH4+ and P-total for influent and
effluent and different types of chemical reagents
used for the precipitation of phosphorus.
aluminium sulfate. The reaction of aluminium
sulfate phosphate is as follows:
Al2 ( SO4 ) 3 ⋅ 18H 2O + 2 PO43 − →
2 AlPO4 ↓ +3SO42 − + 18H 2O
(6)
(26.98g ·2) + (32.07g ·3) + (16g ·12) ·28g + (16g
·18) + (30.97g ·2) + (16g ·8) ↔
(26.98g ·2) + (30.97g ·2) + (16g·8) + (32.07g ·3) +
(16g ·12) + (16g ·18)
342.17g + 316g + 189.94g ↔
243.9g + 288.21g + 316g
658.17 g/61.94 g = 10.62 g
Table 1 Parameter values for influent and effluent
Precipitation with aluminium sulfate tests
followed total phosphorus removal at different doses
for P-total value was determined initially from
influent, determining the optimal dose to a mass
ratio equal to or greater than that established by the
reaction Al2(SO4)3·18H2O:P=10.62:1 concentration
of hydrated aluminium sulfate solution with 18
molecules of water was 10 %.
2.2.3 Precipitation of phosphorus
polyaluminium chloride (PAX 16)
with
Precipitation with polyaluminium chloride (PAX
16) tests followed total phosphorus removal at
different doses for P-total value was determined
initially from influent, determining the optimal dose
to a mass ratio equal to or greater than that
determined by reaction stoichiometry. PAX product
16 contains a proportion of 8.2 % Al, (see reacting
5) aluminium reacts with 1 mol per mole of
phosphate leading to a mass ratio of Al: P = 0.87: 1.
Dosing was performed for the corresponding
mass reports Al: P = 0.87, 1.05, 1.3. (Product PAX
16 had a density of 1.33 g / cm ³).
3 Problem Solution
Analysis methods used for the determination of
ammonium, chemical oxygen demand and P-total,
are presented as follows:
NH4+- method of determination for this indicator
was ammonium Hach Lange cuvette test method LCK 302, LCK 303; area I measure 47-130 mg / L
NH-N and 2-47 mg / L - N. equipment: Hach DR
3800.
COD - method of determination for this indicator
was the standard method as SR - ISO 6060: 1996.
[5].
P-total - Hach Lange cuvette test LCK 348method measuring range 0.5 -5.5 mg/l PO – P;
equipment: Hach DR 3800.
ISBN: 978-960-474-352-0
Operating data were studied to evaluate chemical
phosphorus precipitation at the pilot plant and the
relationships between wastewater conditions (e.g
phosphorus concentration), chemical dose were
examined graphically and with correlation and
linear regression.
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Recent Advances in Urban Planning and Construction
Table 2 Parameter values for influent and effluent
Fig. 3 Evolution of the ammonium concentration in
the influent and effluent
Test results for precipitation of phosphorus on
the pilot plant shown in fig. 4, 5, 6, 7 describe
variation of parameter P-total for influent and
effluent after adding chemical reagents: ferric
chloride, ferric sulfate, polyaluminium chloride
(PAX 16) and aluminium sulfate. From this
graphics just (PAX 16) has a high efficiency
according to NTPA 001 – 2005.
Fig. 4 Evolution of the P-total concentration in the
influent and effluent, precipitated with ferric
chloride (FeCl3)
Fig. 2 Evolution of the COD concentration in the
influent and effluent
ISBN: 978-960-474-352-0
Fig. 5 Evolution of the P-total concentration in the
influent and effluent, precipitated with ferric sulfate
Fe2(SO4)3
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Recent Advances in Urban Planning and Construction
Fig. 8 Optimal dose of precipitating with reagent for
removal of P-total
Fig. 6 Evolution of the P-total concentration in the
influent and effluent, precipitated with
polyaluminium chloride (PAX 16)
From graphic can be seen it is a maximum
efficiency to reduce P-total for polyaluminium
chloride (PAX 16) (Hahn, 1992). The dose
corresponding to a reduction of 100 % is the value
of 9.6 g PAX16 / g P total. Values of P - total from
the effluent after precipitation with PAX 16 were
within in the limits imposed by normative NTPA
001-2005.
Experimental tests for other reagents: ferric
sulfate, aluminium sulfate and ferric chloride
resulted in an average efficiency of 70 % and
achieved a value of 78 % for ferric chloride,
corresponding to a dose of 7.8 g FeCl3
to precipitate one gram of P (this value corresponds
to an addition of 50 % of the stoichiometric value)
(Droste,1997).
Fig. 7 Evolution of the P-total concentration in the
influent and effluent, precipitated with aluminium
sulfate Al2(SO4)3
References:
[1] ATV-DVWK-A 131E, Dimensioning of single
stage activated sludge plants, 2000, pp.26-28.
[2] Balmér P. and Hultman B., Control of
phosphorus discharges: present situation and
trends, Hydrobiologia, Vol. 170. 1988, pp.305319.
[3] E. Plaza, E. Levlin, B. Hultman, Phosphorus
removal from wastewater, A literature review,
Stokholm, 1997, pp. 9-12.
[4] *** NTPA 001/2005, Normative establishing
the polluants limits for urban and industrial
wastewater when discharged into natural
receivers.
[5] *** SR ISO 6060:1996, Determination of
chemical oxygen demand (COD).
[6] *** SR ISO 10523:2009, Water quality.
Determination of pH.
[7] ***
Urban
Waste
Water,
Directive
91/271/EEC.
4 Conclusion
During the monitoring period, the precipitation tests
followed some steps:
- sampling of raw water (influent) and treated
wastewater (effluent);
- determination of quality indicators for influent
and effluent: COD, ammonium and total
phosphorus. The experimental results are presented
in table 1, 2, also the evolution of this parameters
in graphics. Influent has high organic load COD =
713 mgO2/l (in normative COD = 500 mgO2/l) and
ammonium content exceeding the limits from
normative NTPA 001 - 2005: 30 mg/l [4], [7].
In fig. 8 can be observed the optimal doses of
coagulation reagents and also the efficiently
reduction for the P-total with different chemical
precipitations as follows: ferric chloride,
polyaluminium chloride, ferric sulfate and
aluminium sulfate.
ISBN: 978-960-474-352-0
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