Study on the Ratio between Total Nitrogen and Phosphorus with
Inorganic Nitrogen and Phosphates in Seawater and Sewage
Instructor：Professor Luoping Zhang, +86-592-2185855(o), lpzhang@xmu.edu.cn
Research Assistant: Mr JunBi Wang, +86-135-15967514, jbwang@xmu.edu.cn
Research students: Erik Karlsson*, Linglong Yu, Bai Yu and Shanshan Lin
*Author: ee05ek6@student.lth.se
Abstract
Eutrophication of coastal waters is now widely recognized as a major environmental problem.
This problem stems from an excess use of the nutrients phosphorus (P) and nitrogen (N). These
nutrients reach the oceans through river and sewage estuaries, rainfall and through dry deposition.
Nitrogen can also be fixated by certain species of bacteria in the water. The effect of high
concentrations of these nutrients is a massive growth of algae, or so-called red tides. This
phenomena depletes the ocean of oxygen, releases toxic substances, leading to mass-deaths of
sea-living organisms and has been occurring with an increasing frequency over the past decades.
In general, P is considered to be limiting the growth of these algae in freshwater and in marine
systems, N is limiting. For these reasons, a project at the Xiamen university is aiming to determine
the environmental capacity of the Xiamen bay area, i.e. what load of nutrients these waters can
accept and digest without any adverse environmental effects. As part of this project the aim for
this report is to determine the concentration of total, inorganic and organic N and P in the Xiamen
bay, and the ratio between the different species of these nutrients. In order to do so, samples were
collected from five different sites in the Xiamen bay as well as from the Xiamen no 1 sewage
treatment plant at Hubinnan Road. The samples were then analyzed in the laboratory using
standard methods. Unfortunately some of the experiments failed and therefore no major
conclusions could be drawn. For P, the ratio between organic and inorganic phosphorus was 0.927
in average. For N, the results of the experiments were insufficient and the ratio could not be
determined.
Introduction
In the last few decades eutrophication in natural waters worldwide has become the subject of
intense debate. There is now little doubt that eutrophication mainly is caused by anthropogenic
activities. This is because so many of human activities release nutrients in various forms into the
environment. In eutrophication processes two nutrients, nitrogen (N) and phosphorus (P) are of
major concern (Anderson et al. 2002). In general, nutrients reach coastal waters through river
estuaries, sewage discharge, runoff and through rainfall. The input of P into oceans has increased
times three compared to pre-industrial levels and the amount of N has increased even more
dramatically (Anderson et al. 2002). The effects of eutrophication are harmful algal blooms that
lead to the deaths of many organisms. In order to combat these severe environmental effects,
information about the lifecycles of N and P is crucial.
Aim
The aim of this project is to determine the concentrations of the different species of N and P. In
specific, the project will strive to determine the ratio between organic and inorganic N and P.
Acknowledgements
The study has been conducted as a group effort. Experiments for the different species were
divided among the students in order to save time and then the results were put together. The study
has been conducted under the supervision of Professor Zhang and Mr Wang.
Background
The department of Environmental Science and Engineering at Xiamen university is currently
running a long-term project which aims to determine the environmental capacity of the Xiamen
bay area. In this project the total pollutant input will be estimated and used in digestion and
underwater-current simulations. In order for these simulations to be carried out correctly the
nitrogen and phosphorus content must be determined. In China, however, only the inorganic
nutrient concentrations have been monitored so far. This is because the toxic algae that are the
product of eutrophication can only utilize inorganic nutrients, which has led scientists to
concentrate on these species. However, it is now evident that microorganisms rapidly can convert
organic nutrients into inorganic, making them accessible to algae.
Nutrients in sea water
What is often discussed nowadays is which nutrient is to be considered limiting in a specific
natural water system. Indeed, it can vary significantly with factors such as salinity, water and
surface temperature, depth and nutrient availability (Yang & Hodgkiss 2004). In freshwaters P is
the less abundant nutrient and the availability of this nutrient is therefore what limits the growth.
Moreover, P can limit or co-limit the growth in estuarine or marine systems that sustain heavy
loads of N from e.g.. agriculture. In many temperate and polar marine waters N is the limiting
factor that limits the primary production of photosynthetic organisms. Furthermore, if the input of
P into a N-limited environment is reduced (e.g. through better cleansing) a possible outcome is
that this area will become P-limited instead. Moreover, other factors such as temperature and light
may also be limiting (Anderson et al. 2002). In the waters surrounding Xiamen phosphorus is
considered to be limiting (Hong et al 1999). What is clear is that an increasing body of evidence
suggests that the nutrient input in many coastal waters constantly is on the rise (Hong et al 1999:
Economist 2008). Therefore, efforts must be focused on reducing the release of nutrients into the
environment, thus limiting eutrophication. A first step to do so is to determine what amount of
nutrients can be handled by the natural environment without causing adverse effects, which is why
this project as described above is so important.
Phosphorus
There are various pathways that lead to inflow of phosphorus to coastal waters. Up until 1990s
atmospheric deposition of phosphorus into sea water was considered unimportant, because this
exchange mechanism was poorly understood. As the nature of this mechanism is becoming more
well-known, it is clear that this assumption was correct. The atmospheric deposition can be
divided into two categories: first, dry deposition which can be defined as the process by which
chemicals are transferred in gaseous or solid phases to the sea, and second, wet deposition through
rainfall (Hong et al. 1999).
Another possible input pf phosphorus into coastal regions is through benthic release, i.e. the
release of phophorus compounds from sediments. Depending on various factors, sediments can
either act as a sink or a source of phosphorus. In the Xiamen sea it was estimated that there is a
significant release of phosphorus from sediments (Hong et al. 1999).
Freshwater runoff from the Jiulong River as a considerable discharge which is greatest in May and
June. Moreover, there are a number of sewage drainage outlets into the Xiamen sea which
contributes with a considerable amount of phosphorus (Hong et al. 1999).
Nitrogen
The atmosphere consists of about 78% Nitrogen in its inert form of nitrogen gas. However, there
are far more reactive forms of nitrogen that form in different chemical reactions. When burning
fossil fuels, for example, nitrogen in these compounds reacts with the oxygen in the air and form
nitrogen oxides. These gases are released into the atmosphere and then returns to the surface of the
earth as acid rain. Further, large amounts of atmospheric nitrogen are fixed in the process that is
used to produce fertilizers in the form of ammonia. Because fertilizers often are used excessively,
the largest share of this ammonia is washed away from fields by rain and transported to the sea
(Economist 2008).
The same processes that transport phosphorus into the Xiamen waters will also transport nitrogen
into these waters. However, besides these mechanisms nitrogen has another unique input
mechanism. A significant contribution of nitrogen into seas is through nitrogen fixation by
blue-green algae (cyanobacteria) (Fogg 1982).
Effects
As previously mentioned, an excess input of nutrients in coastal waters will lead to eutrophicaiton.
Eutrophication, in turn, can have severe effects on ecosystems (Economist 2008). In the South
China Sea the perhaps most well-known effect of eutrophication is the occurrence of red tides, i.e.
algal blooming. These red tides have been known to occur with increasing frequency in the
Xiamen area (Hong et al. 1999). Algal growth rapidly depletes the water of oxygen (Economist
2008). The algae, themselves, can also produce toxic substances. Either way, the end result is the
same: mass-deaths of many species including fish, clams, shrimps, crabs and other
bottom-dwelling species (Anderson et al 2002). The Hong Kong red tide in 1998 is known to have
killed over 2500 tonnes of fish at an estimated value of US$ 32 million (Yang & Hodgkiss 2003).
By consuming seafood from waters affected by red tides, humans may also become ill or even die
(Anderson et al. 2002). Thus, the possible effects of nutrient release into costal regions are severe
from an environmental perspective as well as a financial.
Method
In order to efficiently analyze the different species of N and P in the samples, different
experiments aiming to analyze different species were carried out simultaneously in the laboratory.
In Table 1, a summary of what method was used to analyze which species is presented.
Table 1 Analyzed species and method used
Species
Analyzing method
Nitrogen
Total nitrogen (N)
Nitrite-N (NO2-)
Nitrate-N (NO3-)
Ammonium-N (NH4+)
Phosphorus
Alkaline potassium persulfate digestion
Naphtalene ethylenediamine spectrophotometry
Zinc-cadmium reduction method
Hypobromite oxidation method
For all the above methods the last step was analysis in a UV spectrophotometer in order to attain
the concentrations of the different species in the samples. However, before samples can be
analyzed correctly, calibration curves for the various species will have to be constructed. The
standard calibration curves will then be used in order to determine the concentration from the
absorbance pf the samples.
For all the methods, a significant issue is that of contamination from water and air. In specific,
contamination may ruin the results from the analysis of ammonium very easily. Therefore washing
of the equipment is of utmost importance. The glass equipment should first be washed with XX
and then rinsed with tap water and minicule water. Thereafter, the equipment should be soaked in
hydrochloric acid and then rinsed as before again.
Sampling
Sampling Time: Morning of July 3, 2008, estimated high tide at 12:00.
Sampling Site: five sea water site (WS, WN, TX, TD, TI); two samples from Xiamen No.1
Sewage Treatment Plant, Hubinnan Road (influent and effluent).
TD
TX
WN
TI
WS
Fig.1 Xiamen bay
Fig. 1 above is a map of the sampling area with Xiamen island in the middle surrounded by the
bay. WN and WS are th two different sampling that are par of what is referred to as the Xiamen
western sea. The three northern sampling sites are TX, TD and TI in what is called Tongan bay.
These geographical areas are divided by a solid bridge (red circle) that allows very little water to
be exchanged between the two areas. Just south-west of the Xiamen western sea (XS), is the
estuary of the river XX. South of the southernmost tip of Xiamen island is the Taiwan strait.
Sampling Boat: motorboat (Fig.2).
Sampling Staff: Luoping Zhang, Junbi Wang, Jiangrui Cui, Canming Pan, Zimiao Zhao, Erik
Karlsson, Linlong Yu (Fig.3).
Sampling Method: A ladle was used to pour the seawater into the sampling bottle (Fig.4); A
sampling barrel was used to pour water sample from Sewage Treatment Plant into the sampling
bottles (Fig. 5). Fig.6 shows the sewage samples.
Sampling Bottle: 1.5 L Robust pure drinking water bottles (Fig. 6).
Sample Quantity: 1.5 L per bottle.
The sampling staff got on boat in Xiamen Harbor No. 1 Pier at 8:50, and started to take the first
sample at 9:00. Table 1 shows the sampling protocol.
Table.2 Sampling Protocol (July 3, 2008)
ID
Site
Time
Site
Number
1
WS
Longitude
Water State
Remark
and Latitude
0900
Huoshao
24.29.277N
light green,
Island ES
118.03.946E
relatively
clear
2
3
WN
TX
0910
0922
Baozhu Island
24.31.655N
SSE
118.04.436E
Airport W；
24.33.899N
green, clear
2 bottles
WN1/WN2
green, clear
Jimei Bridge S 118.06.846E
1Km
4
TD
0935
Eyu Island W
24.35.614N
light green,
118.09.855E
relatively
clear
5
6
TI
Sewage
0945
1102
Wuyuan
24.33.733N
Bridge N
118.10.655E
water inlet
green, clear
relatively
Treatment
feculent,
Plant
no obvious
smell
7
Sewage
1105
water outlet
transparent,
2 bottles,
Treatment
clear, no
1 for
Plant
smell
standby
PS: taking 2 samples from WN for QA/QC
Fig.2 Sampling Boat
Fig.3 Sampling Staff
Fig.4 taking sample at sea
Fig.5 taking sample in water outlet
Fig.6 Sewage samples (influent on the left, effluent on the right and in the middle)
Fig.7 Operation in the lab (pumping filtration)
The samples were used to determine total nitrogen, total phosphorus, nitrate, nitrite, ammonium,
and phosphate in sea water and sewage, in order to confirm the ratio between inorganic and
organic species in sea water and sewage.
Experimenters: Luoping Zhang, Junbi Wang, Jiangrui Cui, Canming Pan, Zimiao Zhao, Erik
Karlsson, Linlong Yu, Zhenshun Tu, Yu Bai, Shanshan Lin.
Lab: Room 308 室, Haiyang Building A (Fig. 7).
Results
After analysis in the laboratory and calculation according to standard methods the following
results for the different species were attained:
Nitrogen species
In the analysis of the different nitrogen species, the experiment designed to determine the total
nitrogen content in the samples failed. Thus, the ratio between inorganic and organic nitrogen in
the samples could not be determined. Below follows a presentation of the other results.
Table 3 Concentration of nitrate and nitrite at different sampling sites
NO3-
NO2-
Sampling site
concentration
concentration (mg/L)
(mg/L)
WS
0.97289
0.06039
WN1
1.03429
0.06526
WN2
0.99023
0.06509
TX
1.12643
0.07519
TD
0.80856
0.07666
TI
1.13727
0.07566
Sewage plant influent
1.15083
0.00816
Sewage plant effluent
4.52611
0.57814
The results of the analysis of nitrate and nitrite can be seen in Table 3 above. For these two species,
no general trend or significant difference between samples from the western sea and the Tongan
bay could be distinguished.
Table 4 Concentration of ammonium at the different sampling sites
NH4+
Sampling site
concentration (mg/L)
WS
0.09095
WN1
0.09933
WN2
0.08057
TX
0.16591
TD
0.05839
TI
0.05901
Sewage plant influent
27.82096
Sewage plant effluent
20.30127
For ammonium the results from different sampling sites were too random for a trend to be
distinguished. What can be seen is that the concentration of ammonium is much higher in the
samples from the sewage plant unlike the concentrations of nitrate and nitrite. This could is
probably due to the high levels of ammonium in urine, generally found in wastewater.
In general for nitrogen, what can be seen from the results of the experiments is that nitrate, NO3-,
is the major component with a concentration of approximately 1 mg/L. This is much higher than
the Chinese average for sea water at about 0.3 mg/L. The most probable reason for this is that the
river estuary is a major input of nitrogen from nearby agricultural use of fertilizers. Another reason
for the high proportion of the nitrate species in the samples is that other species are easily oxidized
into nitrate.
Phosphorus
The experiments for phosphorus went well and the results are presented in Table 5 below.
Table 5 Concentration of total phosphorus and phosphate at different sampling sites
TP
PO4
Sampling site
Concentration
concentration (mg/L)
(mg/L)
WS
0.03048
0.02955
WN1
0.03315
0.03184
WN2
0.03670
0.02926
TX
0.06156
0.06253
TD
0.04292
0.04618
TI
0.04741
0.04647
Sewage plant influent
0.16423
0.04188
Sewage plant effluent
0.11268
0.03342
From the results in Table 5 it is evident that the concentration of phosphorus was lower in the
western sea compared to the Tongan bay. No difference in the concentrations of phosphorus
species in sewage water and sea water could be identified. Further, the proportion of phosphate in
the samples was so high that concentration of PO4 easily can be used to estimate TP in samples.
Moreover, given the concentration of total phosphorus as well as that for phosphate, the ratio
between these could be calculated.
Table 6 Ratio PO4/TP at different sampling sites
Sampling site
Ratio PO4/TP
WS
0.96943
WN1
0.96068
WN2
0.79731
TX
1.01579
TD
1.07598
TI
0.98030
Sewage plant influent
0.25503
Sewage plant effluent
0.29660
Average ratio sea water:
0.927
As can be seen in Table 6, something went amiss in the experiments for TX and TD as the results
indicate that the concentration of PO4 is higher than that of TP. The average ratio in the sea water
samples of 0.93 between PO4 and TP seems reasonable (the samples from TX and TD has been
left out of these calculations as these are clearly erroneous). This gives a ratio between organic and
inorganic phosphorus of approximately 1:9.
For the water from the sewage plants the ratio PO4/TP was much lower than that of sea water.
This can be explained by the fact that wastewater is generally high in organic matter, thus
lowering the ratio inorganic/organic content. Also, the wastewater probably contains a lot of
detergents, i.e. other forms of phosphorus
Conclusion
The experiments did not produce clear results and therefore no major conclusions concerning the
ratio between inorganic and organic nitrogen and phosphorus could be drawn. It was found that
nitrate was the major component of the nitrogen species and phosphate dominated TP content. The
ratio between PO4/TP in the sea water samples was found to be 0.93 and thus the ratio between
organic/inorganic P was found to be 1:9. In the wastewater the portion of phosphate was much
lower which is probably due to high levels of organic matter and detergents.
References
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Eutrophication:Nutrient Sources, Composition and Consequences. Estuaries 25. 704-726
The Economist 387. 2008. Science and Technology: Dead water; The Environment. 102
Fogg, G.E. 1982. Nitrogen cycling in sea waters. Phil. Trans. R. Soc. 296. 511-520
Hong, H., Shang, S., Huang, B. 1999. An Estimate on External Fluxes of Phosphorus and its
Environmental Significance in Xiamen Western Sea. Marine Pollution Bulletin 39. 200-204
Yang, Z.B., Hodgkiss, I.J. 2003. Hong Kong’s worst “red tide”—causative factors reflected in
a phytoplankton study at Port Shelter station in 1998. Harmful Algae 3. 149–161