Download Biological Removal of Phosphorus from Wastewater and Recovery

REPORT
on
Biological Phosphorus Removal Technology
Submitted to
Professor Dr. Wonyong Choi
By
Sudan Raj Panthi
Advanced Remediation & Treatment
(ART) Lab
School of Environmental Science and Engineering
POSTECH
2005
Biological-Phosphorus Removal
-Sudan Raj Panthi
INTRODUCTION
Water quality is heavily influenced by the availability of nutrients, particularly nitrates
and phosphates. Although an excess of these two nutrients leads to eutrophication of the
waters and, in the worse case, to the disappearance of the aquatic ecosystems, being the
limiting nutrient as far as fresh water eutrophication is concerned phosphorus is usually
the element of greatest concern in sewage treatment. In many jurisdictions there are strict
limits set for maximum allowable phosphorus discharge; a 1 mg/L or low concentration
in treated effluent is frequently specified. A typical activated sludge process can serve to
remove a portion of the phosphorus, typically about 20%, by incorporation with in the
insoluble biomass that is subsequently removed in the organic sludge. However, because
the influent phosphorus concentration is of the order of 8-10 mg/L, the treated water still
contains phosphorus in excess of allowed, 1 mg/L or lower concentration. Hence an
advanced and efficient phosphorus removal unit is needed in addition to the conventional
waste water treatment plant with activated sludge process.
In the past, the elements necessary for plant and animal growth were recycled in
primitive agricultural communities. Crops were consumed by animals and man close to
their place of production. The resulting animal and human manures, as well as crop
wastes, were then applied to cultivated land, returning the nutrients to the same land. In
order to obtain high crop yields and to produce enough food for growing world
populations, intensive modern agriculture requires large quantities of nutrients in a form
readily available to plants: mineral fertilizers. The phosphorus cycle has thus largely been
replaced by a linear throughput system: phosphates are extracted from a non-renewable
resource (phosphate rock), pass through crops, animals and man, and end up either in
landfill (of raw or incinerated sewage sludge) or in rivers and the sea (if sewage or animal
wastes are not adequately treated). Comparatively little is restored to agricultural land.
Phosphates from fertilizers and manure may also build up in agricultural soil, in some
circumstances towards or beyond saturation levels. Part of the applied phosphates may
tend to run off into surface water rather than being retained in the soil and crops.
Forms of Phosphorus
Phosphorus exists in water in either a particulate phase or a dissolved phase. Particulate
matter includes living and dead plankton, precipitates of phosphorus, phosphorus
adsorbed to particulates, and amorphous phosphorus. The dissolved phase includes
inorganic phosphorus and organic phosphorus. Phosphorus in typical domestic
wastewaters is usually found in the form partially protonated phosphate species (HPO42or H2PO4- ). Phosphates can be in inorganic form or organic form. Organic phosphate is
phosphate that is formed primarily by biological processes and bound to plant or animal
tissue. They are contributed to sewage mainly by body waste and food residues.
Inorganic phosphate is phosphate that is not associated with organic material. Types of
inorganic phosphate include orthophosphate and polyphosphates. Orthophosphate is the
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most stable kind of phosphate, and is the form used by plants. Orthophosphate is
produced by natural processes and is found in sewage. Polyphosphates (also known as
metaphosphates or condensed phosphates) are strong complexing agents for some metal
ions. In water, polyphosphates are unstable and will eventually convert to orthophosphate.
PHOSPHORUS REMOVAL TECHNNOLOGIES
Development of technologies for phosphorus removal started in the 1950s in response to
the issue of eutrophication. Removal was initially achieved by chemical precipitation but
now chemical removal process are being replaced by biological process as chemical
processes have as disadvantage that they are not selective. The chemical precipitate
accumulates in the sludge, leading to extra costs for the treatment of the excess sludge.
Moreover, since the sludge content in a treatment system is limited to a maximum
amount, large treatment reactors are required in order to maintain the same amount of
biological sludge. Since wastewater contains many different ions, these will always
disturb the processes for chemical phosphorus-removal and recovery too. Further more in
chemical process the considerable amount of the salts (usually chloride) remains in the
water, resulting in increased salinity of surface waters. Biological processes can be highly
selective, and can achieve easily low concentrations. Disadvantage is that the phosphate
is only concentrated in the biomass. If the phosphate is set free from the biomass in a
small volume a concentrated phosphorus solution is obtained where physical-chemical
techniques can be used for recovery. A combination of a biological process for
concentrating the phosphates and a physical-chemical process for recovery seems
therefore to be the best option for phosphorus recovery. Short description of all these
processes is given as follow;
CHEMICAL PROCESS
In the chemical treatment systems chemicals are added to precipitate the Phosphorus
from the wastewater. Chemicals typically used for Phosphorus removal include metal
salts such as aluminum sulfate (alum), ferric chloride and calcium hydroxides. As well as
being an effective coagulant for removal of the turbidity associated with various colloids,
alum is able to react with phosphorus in order to precipitate out the highly insoluble
aluminium phosphate. Usually the reaction can be shown simply as;
Al3+(aq) + PO43-(aq) => AlPO4(s)
(1)
When aluminium ion is added directly to a low alkalinity waste stream containing
phosphorus, some direct reaction of the type shown here may occur but under the
mechanisms of acid/base and complexation reactions. However, depending on the pH
aluminium and iron form other complexes, particularly with organic ligands, or
precipitate as Al(OH)3. Nevertheless the solid still is able to remove phosphorus by way
of specific adsorption on the active surface of the freshly formed floc; a possible reaction
involves displacement of hydroxide ions by the partially protonated phosphate species:
Al(OH)3 + HPO42-(aq) + H2O => AlOH(HPO4).H2O(s)+ 2OH-(aq)
(2)
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Following the surface reaction, phosphate slowly migrates to the interior parts of the
aluminium colloid, leaving fresh surface to continue, to adsorb more phosphate. As these
processes continue, the gel like hydrous aluminium oxide gradually converts more
completely into aluminium hydroxyphosphate or aluminium phosphate, although
complete conversion may not be effected during the lifetime of the precipitate in a
sewage treatment plant. During the recycle process, the precipitated alum is still capable
of reacting with phosphorus by above reaction thus removing more phosphate species
from solution. The mechanism of use of ferric chloride is same as aluminium salt but the
calcium hydroxide requires that the sewage mixed liquor pH be raised to at least 9.0 in
order to ensure complete precipitation. In the case of use of calcium hydroxide by raising
pH, final pH should be adjusted by lowering it to a more acceptable value, usually
between pH 6.5 and 7.5.
Owing to the low costs of chemical addition, the ease and stability of such methods, and
the lack of feeling for biological processes among civil engineers, chemical phosphate
removal has long been the dominant treatment process. In most countries where
phosphate removal is required, however, biological phosphate removal has become the
preferred process, eventually supplemented by chemical precipitation.
BIOLOGICAL PROCESS
Introduction of the biological process
All kind of organisms need energy, carbon source and some nutrients for their growth
and maintenance. Organisms that use energy from light, like plants are termed
phototrophs, and those obtaining energy from chemical reaction are termed chemoterms.
Autotrophs use inorganic carbon, such as CO2, for cell synthesis, while heterotrophs use
organic compounds for cell synthesis. Apart from Carbon the common nutrient required
in a biological process are O, H, N, S, P, K, Ca, Mg, and Fe as relatively large amounts
and called macronutrients and trace elements like Mn, Zn, Co, Mo, Ni, Cu are required in
a relatively very low amounts and called micronutrients. Generally for all leaving
organisms carbon source is considered as food and it is converted to glucose and
subsequently CO2, water and energy and the process is called as “Internal cellular
respiration” or simply “Respiration”. There are three stages of respiration “Glycolysis”,
the “Krebs Cycle” and “Electron transport system”. In glycolysis the series of reactions
resulting in the break down of the six-carbon “glucose” to two molecules of the threecarbon compound “pyruvate”. Before entering the krebs cycle, pyruvate combines with a
compound called coenzyme A to form “Acetyl coenzyme A”, which then enters to krebs
cycle and form carbondioxide and hydrogen atoms. The cycle itself generates very little
energy for the cell but the hydrogen atoms that enter the electron transport system, where
they later generate energy in the form of ATP (Adenosine triphosphate). ATP give energy
to cell by hydrolyzing to ADP(Adenosine diphosphate) and release inorganic phosphorus
(Pi) or gain and store energy by phosphorylation of ADP to ATP.
Principle and Process Design
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Heterotrophs bacteria of the genus Acinatobactor are particularly known for their ability
to assimilate phosphorus during their growth. The biological phosphorus removal process
begins with waste water moving into basins where anaerobic conditions are mentioned.
In this basin, there is no BOD oxidation, but due to high mixed-liquor biomass level,
causes the release of acetate and other fermentation products by way of fermentation
reaction. The fermentation products like Volatile fatty-acids (VFA) /HAc are substrates
that are favoured by Acinetobactor and other phosphorus accumulating bacteria, and they
stimulate growth of these species compared with other microorganisms in general
population. Thus the anaerobic conditions provide an environment that results in
population selection and growth of the population of phosphorus accumulating bacteria.
Without the oxygen-free phase, such organisms would be present in only very small
amounts in the activated sludge. Therefore in the anaerobic phase substrate is taken up
and stored in their cells as polyhydroxyalkanoates (PHA) by phosphorus-accumulating
bacteria, this result in phosphate release into the liquid phase. The basic scheme of the
biological phosphate removal process is shown in Fig. 1.
Anaerobic Basin
- No BOD oxidation
- PHB (PHA) Polymerization
- Poly Phosphate hydrolysis
- Pi release
Influent
Aerobic Basin
- BOD oxidation
- Biomass synthesis and
decay
-PHB(Poly hydroxybutyrate)
Utilization
- Poly Phosphate
accumulation
To waste
Effluent
Clarifier
Fig. 1. Typical phosphorus removal Plants
Upon entry into the aerobic zone, the selected microorganisms efficiently take up the
soluble phosphorus from the waste water stream and accumulate approximately 12%
phosphorus on a dry-weight basis, compared to 1% - 3% for normal bacteria. In several
cases the ratio of chemical oxygen demand (COD) to phosphate in the influent is too low
to produce enough biomass for polyphosphate storage. The polyphosphate-accumulating
bacteria have a clear competitive advantage in that they can accumulate the fatty acid
substrate in the cells without the need of an external electron acceptor. When the sludge
becomes aerated (or nitrate is available) other heterotrophic organisms have no substrate
while the polyphosphate-accumulating bacteria can grow at the expense of their stored
substrate, poly hydrobutyrate(PHB). Depending on the system nitrification process may
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or may not occur. If there is provision of nitrification too with direct O2 supply NO3- is
generated and ample electron acceptors are available that make it possible to other
heterotrophic bacteria to oxidized electron donors, gain energy, and grow. The mixed
liquor existing the aerobic basin is settled in clarifier and most of it is recycled back to
the head of the process, where it mixes with the influent and enters the anaerobic basin.
This recycle ensures that all of the biomass experiences alternating anaerobic and aerobic
conditions. The wasting sludge is considered to rich in poly phosphate and proceeds to
treatment.
Combination of chemical and biological phosphate removal
As mentioned above, biological phosphate removal is heavily dependent on the formation
of polyphosphate accumulating bacteria. Removal of phosphate by chemical and
biological methods requires a well-controlled addition of chemicals. If the addition of
chemicals is too high, the phosphate will be fixed as a precipitate and not be available for
the bacteria to form polyphosphate. If this polymer is absent the polyphosphateaccumulating bacteria cannot accumulate the substrate under anaerobic conditions and
thereby lose their competitive advantage over normal heterotrophic bacteria.
Key Issues in biological phosphorus process
Culture of Representative Species
Although based on work by different groups biological Phosphate removal has become a
full grown technology which can be applied on large scale, at this moment however, still
no pure culture representative for the biological Phosphorus-removal process has been
isolated (Van Loosdrecht et al. 1997)1. When micro-organisms are in the aerobic period
usually polyphosphate is accumulated as excess and the term "luxury uptake" or
"overplus effect" is used the context of biological Phosphorus-removal. In these cases the
phosphate-uptake is a kind of transient response and not part of the normal metabolism.
The polyphosphate accumulated by these organisms is only used as phosphorus reserve.
The organisms involved in biological P-removal, however, accumulate polyphosphate as
part of their normal physiology, and moreover can use the stored polyphosphate as
energy storage (Van Groenestijn et al. 1989)2. In the past Acinetobactor was identified as
only one genus of phosphorus accumulating bacteria, but recently phosphorus
accumulating bacteria are found among the Pseudomonas, Arthrobactor, Nocardia,
Beyerinkia, Ozotobacter, Aeromonas, Microlunatus and others too (Rittamann, B.E. and
McCarty P.L 2001)3.
1 Van Loosdrecht MCM, Hooijmans CM, Brdjanovic D, Heijnen J (1997) Biological phosphate removal processes;
Appl Microbiol Biotechnol 48:289-296.
2
Van Groenestijn JW, Bentvelsen MMA, Deinema MH, Zehnder AJB (1989) Polyphosphate degrading enzymes in
Acinetobacter spp. and activated sludge. Appl. Environm. Microbiol. 55:219-223.
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Rittamann, B.E. and McCarty P.L (2001) Environmental Biotechnology: Principles and Applications. McGRAWHILL International Editions. pp 539-544.
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Physiology of microorganisms
The physiology of the microorganisms involve in biological phosphorus removal process
has risen by different researchers differently. However many researchers are working in
this field but a widely accepted model has to be come. Mino T, Van Loosdrecht MCM,
Heijnen JJ (1997) have reviewed many research papers which is summarized later.
Performance and nitrogen removal combination
To improve the phosphorus removal performance or to achieve a low and consistent
effluent standard and to recover phosphorus from phosphate rich sludge many chemical
aided units are being design. For ideal phosphorus removal system, electron donors
(COD) and acceptors (O2 or NO3-) should not present simultaneously in an aerobic phase.
In practice biological phosphorus removal processes are often combined with nitrogen
removal systems (Nitri. / denitri.). Therefore, this can easily lead to introduction of nitrate
in the anaerobic phase. So there are many types ( Table 1.) of biological phosphorus
removal process associated with biological nitrogen removal activities.
Table 1. Summery of few Biological Phosphorus removal Designs.
Phosphorus
Development
Nutrient removal
Design type
Removal
efficiency
year
Status and Process summery
Technology
Full
Plug flow
1-Aerobic basin
Phostrip process
1965
scale
configuration. Bio P
1-Anaerobic
removal combine with thickener and P
chemical precipitation stripper
1- Precipitation unit.
1- Clarifier
Modified
Bradenpho
1974
Full
scale
N removal-93%
P removal-65% or less
Sludge recycle to
aerobic tank
Phoredox
1976
Full
scale
University of Cape 1983
Town (UCT) type
Full
scale
Pre-denitrification
P and N removal
process
Sludge recycle to
aerobic tank
Minimizing the
recycling NO3 to the
anaerobic tank
Further reduce NO3
recycle.
Modified UCT
1990
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Anerobic (1h)anoxic(2.7h)aerobic(4.7h)anoxic(2.2h)aerobic(1h)-clarifier
Anerobic (1h)anoxic(2.7h)aerobic(4.7h)-and
clarifier in series
Anerobic -anoxicaerobic-and clarifier
in series
Anoxic basin
divided into two
compartments
Review of Research papers:
“Biological Mechanism of Acetate uptake mediated by carbohydrate
consumption in excess Phosphorus removal systems” in “Water Research”
(1988) 22(5):565-570;
(V. Arun, T. Mino, and T. Matsuo)
“Biological Phosphate removal process” in “Applied microbiology and
biotechnology” (1997) 48:289-296;
(M.C.M. van Loosdrecht, C.M. Hooijamans, D. Brdjanovic and J.J. heijnen)
“Microbiology and biochemistry of the enhanced biological phosphate
removal process” in “Water Research (1997)32(11) 3193-3207;
(T Mino, M.C.M. van Loosdrecht, and J.J. Heijnen)
In these review papers the authors have described the historical development of
biological phosphorus removal process and conclude the most important microbiological
mechanism in phosphorus removal systems.
PHA
Simple aerobic
cell-respiration
process
Poly P
ATP=ADP + Pi + energy
VFA
Reducing
Power
a. Anaerobic condition
b. Aerobic condition
Fig. 2 Metabolic processes of organisms involved in biological phosphorus removal.
Figure 2 gives a schematic representation of biochemical model. According to this model
under anaerobic conditions the bacteria use stored poly-phosphate as energy source for
ATP production with the aid of the enzyme Poly-P:AMP-phosphotransferase. ATP is
used for the uptake of volatile fatty acids (VFA) and subsequent formation of
polyhydroxyalkanoates (PHA). The conversion of the acetate to PHA requires reducing
power, because PHA is more reduced compound than acetate, and this is gain by
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anaerobic degradation of intracellularly stored glycogen to acetyl-coenzyme (acetyl-CoA)
(Mino Model).
When oxygen and nitrate or nitrite are present in the absence of substrate, PHA is used as
substrate. Under these conditions the bacteria not only produce new biomass but also
restore the storage pools of polyphosphate and glycogen. This leads to a net uptake of
phosphate in the overall process. If substrate as well as electron acceptors are present the
substrate is predominantly converted into PHA instead of being used for growth. The
presence of three storage polymers in the cells (PHA, Glycogen and polyphosphate)
makes the microbiology of the bio-P organisms extremely complex. A further
complicating factor is that the growth of the cells is not directly coupled to substrate
oxydation, as in normal microbiological processe. Growth results from the difference
between PHA consumption rate and PHA use for glycogen and polyphosphate formation.
Fig. 3 Schematic representation of a biological phosphorus-removal process. The graph gives a schematic
representation of the change in concentrations in a plug-type of process.
A basic scheme for a biological phosphate removal process is shown in Fig. 3. In the
anaerobic phase substrate is taken up by polyphosphate-accumulating bacteria, this result
in phosphate release into the liquid phase. In the cells the VFA substrate is stored as
polyhydroxyalkanaates (PHA) as carbon and electron source. The declining glycogen
concentration in biomass support the “Mino Model” described above. In the aerobic
phase the organisms grow and accumulate phosphate in the cells. Owing to a net growth
of cells, the phosphate is removed. The polyphosphate-accumulating bacteria have a clear
competitive advantage in that they can accumulate the fatty acid substrate in the cells
without the need of an external electron acceptor. When the sludge becomes aerated (or
nitrate is available) other heterotrophic organisms have no substrate while the
polyphosphate-accumulating bacteria can grow at the expense of their stored substrate.
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“Effect of Nitrate on Phosphorus Release in Biological Phosphorus
Removal Systems” in “Water Science and Technology (1994)30(6) 263-269;
(T. Kuba, A. Wachtmeister, M.C.M. van Loosdrecht, and J.J. Heijnen)
In conventional anaerobic phosphorus removal systems with long aerobic retention time,
nitrate is formed in the aerobic zone by nitrification. Many researchers reported that
transfer of nitrate into the anaerobic zone inhibits the phosphorus release and
subsequently deteriorate the Bio-P removal systems. In this paper the authors have
studied the effect of nitrate in the aerobic phase on the phosphate release, especially a
possible role of denitifying phosphorus removal bacteria (DPB).
Experimental set –up:







A 3.5-L bench anaerobic-anoxic reactor.
Operation in a 6-h cycle mode, four cycles per day, at 2, 3.5, and 0.5 hours for the
anaerobic phase, anoxic phase, and settling period, respectively.
The enriched DPB sludge (300 ml) for the reactor was taken from A2 SBR
(Anaerobic/Anoxic Sequencing Batch Reactor) and A/O (Anaerobic/Oxic) SBR.
HRT and SRT of the reactor were maintained at 24 hrs and 20 days, respectively.
The raw wastewater was of synthetic origin with 400 mg/L COD (Containing
HAc) and 15 mg/L of T–P, respectively.
0.1 l (117 mmol/l) nitrate was added to the anoxic reactor at constant rate during
first 110 mins.
Control pH at 7.0 -7.1
Results and conclusion:


Phosphorus concentration increased with HAc (acetic acid) consumption under
anaerobic conditions, and phosphorus was biologically removed under anoxic
conditions (Fig. 2). PHB concentrations increased in the anaerobic phase, and
decreased in the anoxic phase. The variation of glycogen concentration was in
reverse.
It is clear that during the nitrate addition the rate of phosphorus release is
decreases( Fig.3, a b). The HAc consumption rate increased due to nitrate addition
because of denitrification. Despite the presence of nitrate the DPB released
phosphorus as long as HAc was present and PHB was produced from HAc (Fig.
4.)
DPB have mechanism according to Mino model, and nitrate doesn’t block
phosphorus release in the DPB, but HAc uptake increase and the P/C ration
decreases. A reduction of phosphorus release by nitrate is due to HAc utilization
for denitrification.
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Simultaneous Presence of HAc and NO3-
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