Evaluation of Wastewater Treatment Plant Efficiency and Suitability of Treated Wastewater for Irrigation Purposes at Sirte city, Libya

INTRODUCTION
In many arid and semi-arid areas water is becoming an increasingly scarce
resource and the planners are forced to consider any source of water which
might be used economically and effectively to promote further agricultural
development (Widaa and Saeed, 2008). Recycling of wastewater has emerged
as a realistic option to overcome problems related to water shortages and its
adverse environmental impacts. However, water-scarce countries will have to
rely more on the use of wastewater resources to partly alleviate water scarcity
(Qadir et al., 2007). Wastewater reuse could free huge amounts of fresh water
currently used for irrigation and make this resource available to meet the growing
needs for fresh water for domestic uses.
Municipal wastewater is marginal quality water because of the associated
health hazards. It is mainly comprised of 99.9% water together with relatively
small concentrations of suspended and dissolved organic and inorganic solids
(Pescod, 1992). Wastewater is needed suitable pre-use treatment and
appropriate soil–water–crop management strategies when used for irrigation
(Qadir et al., 2007).
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Water quality refers to the characteristics of water supply that will influence
its suitability for specific use. Quality is defined by certain physical, chemical and
biological characteristics as stated by Ayers and Westcot (1985). Scherer et al,
(1996) reported that water quality for irrigation purposes is determined by its salt
content. They also stated that the analysis of water for irrigation should include
the cations: calcium, magnesium, and sodium, and anions; bicarbonate,
carbonate, sulfate, and chloride. Water high in N can cause quality problems in
crops such as barley and sugar beets and excessive vegetative growth in some
vegetables (Bauder et al., 2011).
In Sirte City substantial quantities of wastewater are usually discharged to
Mediterranean Sea. This amount of water increases day by day due to the
increase of population and industrial sector in such towns, so planners and
authorities should thinking seriously to find safe disposal of this water instead of
to be dumped in the sea. Reuse of this water in Agriculture represents one of
safe disposal solution. This study specifically aims to assess the efficiency of the
wastewater treatment plant at Sirte city and to investigate the quality of treated
wastewater for irrigation purposes.
MATERIAL AND METHODS
The study was conducted at wastewater treatment plant of Sirte city which
located at 31.21˚ E, and 16.59˚ N, Libya. The area is characterized by 14.1 C˚ as
minimum temperature and 28.2 C˚ as maximum, while the annual average wind
speed is 5 knot and the annual average rainfall is 20 mm.
Composite waste water samples were taken from the effluent before
entering the treatment plant and at the final treatment point during November
2012. Samples were taken every two hours starting at 8.00 am and ending at 4
pm during the day. The bottles used for sample collection were cleaned with hot
water and suitable detergent, which rinsed with hot water to remove all traces of
detergent, and finally were sterilized in an autoclave. The collected samples were
brought to the laboratory, filtered and the analysis was done for the determination
of bicarbonate and carbonate (HCO3
- and CO3
=) ions, sulfate (SO4
=), chloride
(Cl-), nitrate (NO-
3-N), phosphors (P), other major and minor nutrients and heavy
metals according to the methods described by Gupta (2007). Also the analysis
included pH, electrical conductivity (EC) and (B). SAR and adjSAR were
calculated using the following equations according to Ayers and Westcot (1985):
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Where Na, Ca and Mg are in meq/l and pHc was measured using the
following equation as stated by Ayers and Westcot (1985):
pHc= (pK2 –pKc) + p(Ca+Mg) +p(A1K)
The treatment efficiency in wastewater treatment plant of Sirte city was
evaluated using the average values of influent parameters before treatment as
(input), and the average values after treatment as (output) using the following
equation according to Duangporn et al. (2009):
The success of the process, determined by measuring the size of which
indicate the quantity of treated wastewater at the exit, as well as water quality,
which leads to the entrance facility for treatment. The relationship between these
parameters, at the entrance and exit process in order to obtain data on the same
level of success of implementation, is called the degree of efficiency of waste
water.
RESULTS AND DISCUSSION
The parameters EC, SAR, N, Mg, SO4, HCO3 and P of wastewater effluent
before and after treatment were detected and used to evaluate the treatment
plant efficiency (Table 1). The result showed a great variation in purification of
the wastewater constituents. The highest efficiency (33.45%) was recorded for
bicarbonate (HCO3) ions and the lowest one for phosphorus (3.16%). On the
other hand, pH and Cl values revealed increment after treatment, (Figures1, and
3). This results may be attributed to absent or ineffective monitoring system,
unskilled labors, weather conditions, weeds and sedimentation (Ball and Widaa,
2013). The increased percentage of Cl after treatment may be attributed to the
added chloride material before the final stage as a part of treatment processes.
In the same time no noticed variation in the K values (Figure 2).
As presented in Table(2), the quality of Sirte treated wastewater in
comparison with FAO standards for irrigation water, showed that the
concentrations of EC, NO3–N, and Cl and SO4 ions were exceeded the
restriction sever level of the FAO standards. Also, the values of Co, Cr, Mn and
Mo ions exceeded the levels of the FAO standards (Table 3). The values of pH
(figure 1), SAR, adj.SAR (Figure 4), Boron (Table 2), calcium, magnesium and
other trace elements (Table 3) were found to conform to FAO standards for
irrigation water (Ayers and Westcot, 1985) . The increasing levels of previous
mentioned parameters may be due to the variations in the sources of
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Table 1: The monitored parameters and purification efficiency of Sirte
treatment plant
Parameter Unit Before
Treatment
After
Treatment Efficiency %
EC dS/m 4.25 3.654 0.014
SAR Indicator 6.73 5.83 13.37
N mg/l 488 403 17.42
Ca+2 mg/l 200 180 10.00
Mg+2 mg/l 122 109 10.66
SO4
= mg/l 787 576 26.81
HCO3
- mg/l 293 195 33.45
P mg/l 1.012 0.98 3.16
Figure 1: pH values before and after wastewater treatment
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Figure 2: Concentration of Potassium before and after wastewater
treatment
Figure 3: Concentration of chloride before and after wastewater treatment
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wastewater, as well as the addition of chloride after treatment to control
microorganism in wastewater. This result supported by Commission on
Geosciences Environment and Resources (1996).
Table 2: The parameters of Sirte treated wastewater in comparison with
FAO standards for irrigation water
Parameter Unit
Treated
wastewater FAO standards
None Slight to
Moderate
Severe
EC dS/m 3.65 < 0.7 0.7- 3.0 > 3.0
pH Indicator 7.93 Normal Range 6.5-8.5
SAR Indicator 5.83 <3 3-9 >9
NO-
3 -N mg/l 403 <5 5-30 >30
B mg/l 0.70 <0. 7 0. 7 – 3. 0 >3. 0
Cl- mg/l 704 <142 142-355 >355
HCO-
3 mg/l 195 <92 92-519 >519
SO=
4 mg/l 576 Maximum recommended concentration <250
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Table 3: The concentration of different element in Sirte treated wastewater
in comparison with FAO standards for irrigation water
Element Symbol Unit Sirte FAO
standards
Exceeding
Amount
Mercury Hg mg/l 0.02 0.05
Calcium Ca meq/l 9.00 10.0
Magnesium Mg meq/l 10.0 5.00
Cobalt Co mg/l 0.06 0. 05 +0.01
Chromium Cr mg/l 0.40 0. 10 +0.30
Copper Cu mg/l 0.03 0. 20
Iron Fe mg/l 0.60 5. 00
Silver Ag mg/l 0.06 2. 50
phosphorus P mg/l 0.98 3.70
Manganese Mn mg/l 6.00 0. 20 +5.80
Molybdenum Mo mg/l 0.05 0. 01 +0.04
Nickel Ni mg/l 0.04 0. 20
Lead Pd mg/l 0.03 5.00
Selenium Se mg/l 0.02 0. 02
Zinc Zn mg/l 0.01 2. 00
Fig. 4: Comparison of SAR and adj. SAR of treated wastewater with
permissible levels of the FAO irrigation water standards
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CONCLUSION
From standpoint of that irrigation water quality refers to its suitability for use,
the Sirte treated wastewater according to the report of Ayers and Westcot (1985);
it can be classified as saline water. These because the electrical conductivity
(EC) is exceeding the permissible level as recommended by FAO for irrigation
water and at the same time lowers values of bicarbonate and calcium were
recorded. The toxicity problem is expected when water used to irrigate crops
sensitive to Chloride, Cobalt, Chromium, and Manganese. Also the excessive
quantities of nitrate- N is present, which upset the production or delay the crop
maturity (Bauder et al., 2011). Therefore any plan of using Sirte treated
wastewater for irrigation purposes need appropriates soil–water–crop
management strategies.
REFERENCES
Ayers, R. S. and D. W. Westcot (1985). Irrig. And Drainage paper29 rev.1