PHOSPHATE RECOVERY FOLLOWING BIOLOGICAL AEROBIC
TREATMENT OF PIG SLURRY
M.L. DAUMER*, F. BELINE,F GUIZIOU and J. MARTINEZ
CEMAGREF, Livestock and Municipal Wastes Management Reseach Unit, 17 av. de Cucillé,
CS 64427, 35044 Rennes Cedex, France
* marie-line.daumer@cemagref.fr
ABSTRACT
Total phosphorus and dissolved ortho-phosphate was monitored in three different farm-scale biological aerobic treatment
plants. For each step of treatment, a mass balance was established. Samples of raw slurry, solid products, aerated slurry,
liquid effluent and sludge were analysed for their concentration in total phosphorus. Liquid products were analysed for their
dissolved ortho-phosphate content. Three major streams result from the treatment system : separate solids, sludge and liquid
effluent which represent 5% , 15-40% and 75-83% of the raw slurry, respectively. A mechanical separation step prior
aeration allowed to export 25-30% of total phosphorus as organic fertiliser. A large amount of total phosphorus (e.g. 60-70%)
was located in sludge. Phosphorus remaining in liquid effluent is about 15-25% .Raw slurry separation and sufficient aeration
allowed to concentrate phosphorus in sludge. Insufficient aeration led to release phosphorus as dissolved ortho-phosphates
within liquid effluent. Improvements of biological aerobic treatment to enhance phosphorus removal and/or recycling ability
are considered.
Key words : Phosphorus, pig slurry, biological aerobic treatment,
INTRODUCTION
Slurry and manure are traditionally used as fertiliser for landspreading in agriculture,
providing important source of organic matter. When manure is spread in excess of the crop
nitrogen and phosphorus requirements or in inappropriate soil and weather conditions,
pollution of surface waters and aquifers can result (nitrate leaching, eutrophication) [1, 2]. In
countries with intensive livestock production systems, and particularly pig farming,
considerable amounts of pig slurry should be sent for purification through treatment process
which aim to reduce and prevent such environmental effects. In this context, several treatment
strategies for livestock effluent have been defined and recommended in order to prevent
nitrate pollution. For pig slurry, biological aerobic treatment and subsequent nitrogen removal
by nitrification and denitrification is seen as an alternative to nitrogen surplus [3, 4]. This
aeration technique is becoming popular among pig farmers. Evolution of French regulation
[5] led to develop treatment of pig slurry. Thirty full-scale farm plants were working on a
routine basis in 1999. This number is now rising up to several hundred units in Brittany
(Western France).
Such treatment remove up to 75% of total nitrogen of raw slurry but does not affect
other elements like phosphorus, potassium, copper and zinc [6]. In this context, if the
management of co-products of the biological treatment is based on nitrogen crop
requirement, it results in over application of phosphorus on agricultural land (up to 850 kg.ha1
) [7]. Transfer of P from soils by runoff could occur in sufficient quantities to contribute to
eutrophication of both inland and coastal water [8].
To limit such potential pollution, French Water Agency recommends that, for farmers
using aerobic treatment, the application of treated slurry to land would be calculated on a
maximum amount of P applied of 110 kg P.ha-1.year-1. In other European country like
Netherlands, P landspreading is more limited and adapted to crop need (e.g. 48 kgP.ha-1).
Daumer et al. page 1
Consequently, implementation of this limit requires a land surface equivalent or superior to
the land needed for nitrogen spreading without treatment.
Recovery of P in a form suitable for crops and easy to handle as fertiliser offers an
opportunity to better control the application of P and reduce overall P use in agricultural
system by recycling it. This requires to concentrate this element in a solid form available for
storage and transport.
Actually, raw pig slurry separation is widely used and allows to obtain a product with
total solid equal to 30-35% which contains between 15-25 % of initial total phosphorus. Only
a few biological treatment plants, equipped with a vacuum filtration system for sludge,
recover and export 80-85% of initial total phosphorus. Sludge is concentrated, mixed with
others wastes, dried and exported like organic fertiliser. However, 15-25% of total
phosphorus, mainly as soluble phosphorus, is remaining in liquid effluent and is spread on
local area [9]. This form of phosphorus is of particular concern for the receiving water bodies
because it is easily dissolved by runoff [10] and immediately available for uptake by algae
and aquatic plants.
In this context, a better characterisation and understanding of the evolution of different
forms of phosphorus during aerobic treatment of pig slurry may help us to identify further
treatment strategies to enhance phosphorus recovery, to evaluate environmental impact of coproducts and optimise management of the livestock effluents.
The aim of this study is to assess the impact of three different type of treatment plant,
running on a full-scale operation in Brittany, on i) distribution, ii) evolution and iii) recovery
of total phosphorus. Moreover, dissolved phosphorus, especially ortho-phosphates were
measured in liquid co-products in two units.
MATERIALS AND METHODS
Plant design and operation
The three treatment units monitored were located near Lamballe (Brittany, France).
For each of them, the main production characteristics of the pig farms are given in table 1.
Table 1.
Saws (total)
Productive saws
Weaned pigs in a year
Post-wean places
Pigs in a year
Fattening places
Farm units characteristics
1
363
286
7200
960
4500
1200
2
392
346
7752
1800
6266
1592
3
420
346
8332
1226
7986
2230
The first (unit 1) was designed to treat 19 m3 of slurry.d-1. Raw slurry is collected and
mixed in a reception pit (340 m3) with alternative mixing. Every hour, raw slurry (0.4-0.6 m3)
is pumped up to the aeration reactor (965 m3) which is equipped with two suction aerators of
22 kW each. Each aerator runs alternately during 15-20 min. Between each aeration
sequence, there is an anoxic phase (15-20 min) . To maintain a constant level in the reactor,
aerated slurry is discharged by gravity to a decantation tank. Sludge is pumped every 12 h
during 3 min and sent to a storage tank (1500 m3). Liquid phase is transferred by gravity to a
lagoon.
Daumer et al. page 2
The second (unit 2) was designed to treat 18 m3 of slurry.d-1. Raw slurry is collected
and mixed in a reception pit (300 m3) with alternative mixing. Every 3 hours, raw slurry is
pumped up and mechanically separated by a press-auger. Separated solids are weighed and
stored in a shed. Separated slurry is transferred by gravity to the aeration reactor (680 m 3)
which is equipped with two surface aerators (18,5 kW each). The first one is controlled by a
timer (1h/cycle), while the second has an authorised period and runs only when ORP is low.
To maintain a constant level in the reactor, aerated slurry is automatically discharged to a
decantation tank. Liquid phase is transferred by gravity to a lagoon. Sludge is stored in a
separate tank (1770 m3).
The third (unit 3) was designed to treat 16 m3.d-1 of raw slurry. Raw slurry is collected
and mixed in a reception pit with continuous mixing. Slurry is then mechanically separated by
a press-auger. Separated solids are weighed and stored in a shed. Liquid phase is settled in a
decantation tank and the separated slurry is transferred by gravity to a buffer tank while
sludge is sent back to the press-auger separator. Hydraulic residence time in this separation
equipment is 5 days. Once a day, at the beginning of anoxic stage, separated slurry is pumped
up to the aeration reactor (960 m3) which is equipped with fine bubble diffusers supplied by
two compressors (9kW each). The first one is controlled by a timer (20h.d-1), while the second
has an authorised period during the night and runs only when dissolved oxygen was below 1
mgO2.l-1. To maintain a constant level in the reactor, aerated slurry is automatically pumped,
mixed with a polymer and sent to a vacuum sieve-belt separator. Liquid phase is transferred
by gravity into a 0,5 m3 holding tank and was pumped and sent to a lagoon, sludge was kept
in the same shed than separated solids. Schedule of this sophisticated unit is presented in
figure 1.
The three plants were equipped with weighing machine and flow meters to calculate
the co-products flows. pH, temperature and oxydo-reduction potential (ORP), especially to
follow anoxic and aerobic stages during treatment, were continuously monitored.
3
9
1
1
5
Figure 1.
2
1
9
5
4
2
10
0
3
6
7
7
1
4
6
8
8
Biological aerobic treatment plant (unit 3) : 1 : Reception pit ; 2 : mixer ; 3 : press-auger ; 4 :
decantation tank ; 5 : shed ; 6 : buffer tank ; 7 : biological reactor ; 8 : fine bubble diffusers ; 9 :
vacuum sieve-belt ; 10 : lagoon.
Daumer et al. page 3
Sampling:
Weekly during 8 consecutive weeks, raw slurry (RS) was sampled in the reception pit,
through stirring of at least 30 min. Solid phase (SP) and separated slurry (SS) samples were
taken directly from press-auger. Aerated slurry (AS) was sampled in the reactor while slurry
was mechanically stirred, during aerobic stage for unit 1 and 2, or during anaerobic stage for
unit 3. Sludge (S) was sampled just before the storage tank for the unit 1. For unit 2,
decantation tank is used as intermediate storage tank, so it was not possible to sample sludge
just after separation. Sludge was then obtained from a laboratory centrifugation. In unit 3,
sludge was sampled directly from the sieve-belt separator. Liquid effluent (LE) was sampled
between the decantation tank or between the sieve-belt separator and the lagoon.
For dissolved ortho-P evolution, an analysis was done on liquid products sampled as
above. Another sample was taken, the same day, in the lagoon as far as possible from
receiving point, it was called spread effluent (SE). Only unit 1 and 2 were studied.
Analysis :
Samples are prepared as required by French standard NF U 42-090. Analysis of total
solids (TS), total suspended solids (TSS), volatile solids (VS), volatile suspended solids
(VSS), total Kjeldahl nitrogen (TKN), ammoniacal nitrogen (NH4) were performed using
current laboratory methods. Nitrite, nitrates and dissolved ortho-P were measured by ionic
chromatography .
Total phosphorus (total-P) was analysed with spectrophotometric method (430 nm)
after dry ashing and dissolution in nitric and chlorhydric acid following by a colouring
reaction with ammonium molybdate vanadate. These analysis were performed by the
Laboratoire de Développement et d’Analyses (LDA) located in Ploufragan (22, France) .
RESULTS
The loss of nitrogen confirms that more than 60%-70% of initial nitrogen was
transformed as nitrogen gaseous forms during biological aerobic treatment. Consequently,
removal of 49% and 90% of the total and soluble COD were observed respectively.
Mass balance and distribution of co-products
A mass balance was established at the different stage of the treatment by sampling
each input and output flow (Figure 2). On a weigh basis, mass balance was closed to 100%
(min : 94 ; max : 113%). Similar results were obtained for the total solids (TS) except for unit
3. For this unit, recovery of total solid is around 75%. Similar discrepancies observed for
other compounds may be explained by natural sedimentation at the bottom of the biological
reactor for this unit.
The mass of liquid effluent varied from 75 % of the initial product for the unit 1, to 83
% for the unit 3. With a mechanical separation following the biological reactor (unit 3),
sludge represented 10% of the mass of raw material while this phase represents 37 % with
decantation only (unit 1 and 2). Because of sludge separation in laboratory, data for the unit 2
are not presented but were probably closed to the result obtain for the unit 1. For unit 2 and 3,
separated solid from press-auger represent about 5% of initial mass of slurry.
Daumer et al. page 4
Recovery (%)
Unit 1
Unit 3
120
100
80
60
40
20
0
Mass
recovery
Figure 2.
Unit 2
Separation of Biological Separation of
raw slurry
reactor
aerated
slurry
Mass balance at the different steps of the treatment
Distribution of phosphorus
Distribution of total-P in these co-products is illustrated by Figure 3. A large amount
of total phosphorus was located in the sludge (60-70%), while 25-30% was found in separated
solids. Phosphorus remaining in liquid effluent was about 15 % and 25 % for unit with and
without separation of raw slurry, respectively.
%PRS
150
S
LE
SP
100
50
0
Unit 1
Figure 3.
Unit 2
Unit 3
Distribution of total-P in co-products
Table 2 shows phosphorus concentration in each co-product. Press-auger allows to
increase almost fivefold the concentration of total-P in the solid product. Mechanical
separation following aeration allows to increase fourfold the concentration of phosphorus in
solid product compare with decantation only.
Table 2.
Raw slurry
Separated phase
Aerated Slurry
Sludge
Liquid effluent
Total-P concentration in products (kgP.t-1)
Unit 1
1.22
1.1
1.96
0,39
Unit 2
1.09
5.41
1.1
4.37
0.22
Unit 3
0.96
5.85
0.9
6.55
0.13
Daumer et al. page 5
Evolution of dissolved Ortho-P during the treatment :
Figure 4 describes evolution of dissolved ortho-P and total-P in the liquid fraction
during the treatment process.
Other forms
Ortho-P
1,5
-1
1
g.l
Reception
Pit
0,5
0
RS
Unit 1
Unit 1
Unit 2
Unit 2
SS
RS
g.l-1
1,5
1
0,5
0
Unit 2
1,5
g.l
-1
Reactor
0
AS
LE
g.l
Lagoon
Unit 2
Unit 1
Unit 2
Unit 1
Unit 2
0,4
0,2
0
SE
-1
0,6
g.l
Fields
Unit 1
0,6
-1
Sludge
storage tank
1
0,5
0,4
0,2
0
Figure 4.
Evolution of total P and dissolved ortho-P during biological aerobic treatment of pig slurry.
At the beginning of the treatment total-P and dissolved ortho-P are present in the same
proportion in the two farms. Concentration of total-P is 1,2 kgP.t-1 for the unit 1 and 1,1 kgP.t1
for the unit 2, among which about 10% of dissolved ortho-P.
Fraction of dissolved ortho-P in slurry has increased threefold with the mechanical
separation (10 to 30 %). Part of dissolved ortho-P increased fourfold between input and output
of the bilogical reactor in unit 1 (10 to 40 %). In opposite, it is twice divided in the unit 2 (30
to 15 %). Evolution during storage in the lagoon is similar for the two units, dissolved ortho-P
is reduced by an half during this anaerobic stage.
Table 3 presents ortho-P concentration in co-products.
Daumer et al. page 6
Table 3.
Ortho-P concentration in liquid products (kgP.t-1) ;
Raw slurry
Separated slurry
Aerated slurry
Liquid effluent
Spread Effluent
Unit 1
0.128
0.441
0.262
0.108
Unit 2
0.113
0.303
0.167
0.200
0.098
DISCUSSION
Phosphorus content of raw slurry was about 1 kgP.t-1 and is in agreement with data
given in the literature [7, 8, 11].
Phosphorus recovery by press-auger (25-30%) was consistent with previous results on
separation efficiency (e.g. 7-33%) [12, 13]. Only 7% of separation efficiency was obtained by
Pieters et al. [14] with the same equipment, probably due to the low content of TS of the
slurry used in their experiment.
Total nitrogen and total-P concentrations in solid product were 7.3 kgN.t-1 and 5.41
-1
kgP.t respectively for unit 1 and 8.2 kgN.t-1 and 5.85 kgP.t-1 respectively for unit 3. Nitrogen
to phosphorus ratio is lower than crop requirements, but this product can be used as
phosphorus amendment.
For unit 3, separated solid from the first step is mixed with sludge obtained by the
mechanical separation which follows biological treatment. Concentration in this mixed
product, estimated from concentration and mass of each product is about 6.5 kgP.t-1.
Phosphorus recovery obtained by mixing sludge and separated solid is about 80-85%. At the
present time, this product is mixed with other waste, dried and used as fertiliser. No data are
available on this process, so it is not possible to assess this process in term of quality of
product reuse and economic sustainability.
In unit 1 and 2, the low dry matter of the sludge obtained after decantation of aerated
slurry did not allow to export this product. Also this sludge is locally spread. The spreading of
this product, led to amount of phosphorus widely higher than crop requirements. In this
context, the improvement of P recovery seems to be required.
Concerning the P remaining in liquid effluent which represent about 15%-25%, the
total amount is applied locally. Spreading this product in respect with agricultural
requirements needs large area ( 2-6 ha for 1000 m3 of liquid effluent). At the present time,
there is no regulation. French water agency recommend to spread only during raining
deficiency period which is not always achievable in countries like Brittany. Due to the local
meteorology, liquid effluent is often spread, when the lagoon is full, on a small area just
around the treatment unit. This management, imposed by technical constraint of automatic
irrigation, could led to runoff of phosphorus.
New strategies to enhance phosphorus recovery from products of biological aerobic
treatment of slurry have to be developed. In this purpose, other equipment like decanter
centrifuge could improve the phosphorus recovery, up to 80% of phosphorus with suitable
technology. However, this technique doesn’t allow recovery of soluble phosphorus. Specific
energy consumption for this equipment is twice than for a press-auger [12].
Phosphorus recovery by accumulation in sludge in treatment of waste water process
has been widely reviewed by Mino et al [15]. Enhanced biological phosphorus removal
(EBPR) is achieved by successive anaerobic and aerobic step. During anaerobic stage, when
no electron acceptors (like oxygen and nitrate) are present, the phosphorus removing bacteria
take up less fatty acid such as acetic acid and store them as polyhydroxy-alkalanoates. The
Daumer et al. page 7
energy for this process is derived from glycogen conversion and the stored polyphosphates by
hydrolysis and excretion of phosphate. Following this stage, in the aerobic period, oxygen is
used as an electron acceptor for the oxidation of stored polyhydroxyalkalanoates, which leads
to phosphate uptake for polyphosphate regeneration [16].
Biological aerobic treatment of pig slurry process alternate aeration and anoxic stage to allow
nitrification and denitrification of ammoniacal nitrogen. Although biological phosphorus
removal in aerobical treatment of pig slurry has never been described, the difference between
unit 1 and unit 2 for the evolution of soluble phosphorus in biological reactor could be partly
explain by this process. The main difference between the two units is due to aeration. The
insufficient aeration observed for the unit 1is well correlated with the ORP, often comprised
between - 350mV and 0 mV even during aeration stage, and oxidised forms of nitrogen
present is mainly as nitrites. Aeration and anoxic stages are very short and metabolism cannot
be established. For example, nitratation, which is the second phase of nitrification does not
occurs. Moreover, nitrites could be implied in phosphorus uptake inhibition [17]. In opposite,
the minimum ORP of unit 2 varies from -300mV (anaerobic conditions) to –50mV (anoxic
conditions), in relation with DCO brought by raw slurry, without aeration. It rises 200 mV
when aerators were running. Anaerobic or anoxic and aerobic stages are clearly defined.
Metabolism of similar nature to EBPR could be induced. However , further trials are needed
to confirm this assumption.
From these observations, two opposite strategies for recovery phosphorus during
aerobic treatment could be taken under consideration :
i) concentrate phosphorus in sludge which will be strongly separated and exported, if
a sustainable market is developed.
ii) release phosphorus under a soluble form in liquid effluent and recover it as a useful
form.
Results obtained concerning evolution of dissolved phosphorus concentrations for
unit 1 and 2 show that aeration strategy could strongly influence concentration of dissolved
ortho-P. NH4 and PO4 concentrations in liquid effluent are 8.6 x 10-3 and 6.5 x 10-3 mol.l-1
respectively for unit 1 and 7.1 x 10-4 and 6.5 x 10-3 mol.l-1 respectively for unit 2. This
correspond to a NH4: PO4 ratio of approximately 1.3 : 1 for unit 1 and 0.1 : 1 for unit 3.
Therefore, it is possible to precipitate phosphorus as struvite, obtained if magnesium,
ammonium and phosphate are in equal (stoichiometric) proportions in a pH range from 7 to
11 (9 being the pH of minimum solubility) [18]. This form of phosphorus can be use directly
as a slow-release fertiliser and would therefore be directly used in the farm or sold as a
valuable waste by-product . This process has been tested to remove phosphorus from
anaerobic swine lagoon effluent [18], but, due to the high ammonium concentration of, raw
slurry, it seems not to be a valuable approach as regard to the high cost for chemicals
(Magnesium and phosphate) and operation [12]. In our case, effluent is denitrified and
phosphate would not be a limiting factor.
Precipitation of potassic struvite is another way possible. This process has been
studied for anaerobic swine lagoon effluent and denitrifies calf manure [19]. It is used to
remove P from a denitrified calf-manure in a farm-scale treatment plant in Netherlands [18].
In this case, waste has been nitrified and denitrified, potassic struvite (KMgPO4) is formed.
Potassium concentration in liquid effluent from biological treatment of pig slurry is generally
about 5 x 10-2 mol.l-1 [9] . It could replace ammonium for struvite precipitation and be
removed from liquid effluent, in which high concentration is another environmental concern.
However, magnesium and potassium were not quantified in this first study, therefore
magnesium was the limiting constituent and had been added in process to remove phosphorus
from anaerobic swine lagoon effluent and denitrified calf manure.
Daumer et al. page 8
Another way to improve P recovery is to precipitate the remaining phosphoric
compounds with lime milk. About 35 to 40 kg CaO per m3 of slurry has to be added to the
liquid effluent obtained by aerobic biological treatment. After precipitation, phosphorus
concentration in the clarified liquid can be less than 2 mg.l-1 [12]
CONCLUSIONS
This preliminary study allowed us to evaluate biological treatment of slurry in term of
phosphorus recovery and environmental risk. Phosphorus distribution in final co-products
depends on the evolution of different forms, dissolved or not, during treatment. This evolution
could be induce by aeration strategies. Sufficient aeration allows to concentrate phosphorus in
sludge, while insufficient aeration led to release soluble phosphorus. As a consequence of
these results, two strategies for P recovery seems possible :
i)
increase P uptake during aeration and improve sludge separation.
ii)
increase P release and develop specific treatment .
If potassic struvite could be formed, this approach would become a mean to recover
potassium too.
Further studies are necessary to confirm these results and get better knowledge of
phosphorus transformation during aerobic treatment of pig slurry to define which treatment
strategy is the most suitable and thus, optimise recovery of phosphorus under a valuable form.
ACKNOWLEDGEMENTS
This project was co-funded by the Water Agency and the Regional Council of Brittany. We are grateful
to the manufacturers and farmers for access to the plants.
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