Assessment of Effluent Quality of Tertiary Wastewater Treatment Plant at Buraidah City and Its Reuse in Irrigation
Ahmad I. Al-Turki
The present study aimed to examine effluent quality of the Tertiary Wastewater Treatment Plant at Buraidah City (TWTPB) and to demonstrate its compliance with regulations of the Ministry of Water and Electricity (MWE) for tertiary treated effluent. Wastewater samples were monthly obtained form the influent and effluent in TWTPB and analyzed for Total Suspended Solid (TSS), Electrical conductivity, pH, turbidity, NH4+ concentration, some heavy metals, Na concentration, Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD) and counts of fecal and total coliforms over a period of three consecutive years. The maximum values obtained of TSS, NH4+, BOD and COD in the effluent were 5.85, 0.90, 5.75, 24.4 mg L-1, respectively. Removal efficiency for TSS, BOD and COD exceeded 97% in the effluent indicating a high performance of TWTPB. Counts of fecal and total coliforms in the effluent were always less than 2 cfu 100 mL-1 in the effluent showing a high level of sanitation. Sodicity hazard indexes such as Sodium Adsorption Ratio (SAR) and Exchangeable Sodium Percentage (ESP), revealed that effluent salinity was of low to moderate potential problems when reused for irrigation. Residual Sodium Carbonate (RSC) exhibited a negative value implying that no residual carbonate or bicarbonate to react with Na to exacerbate the sodium hazard in soils was found. The effluent of TWTPB was found consistently to satisfy the stringent standards required by MWE for unrestricted irrigation in respect to all parameters measured in this study. It is, therefore, recommended to reuse TWTPB effluent for unrestricted irrigation, taking into consideration soil properties and leaching requirements.
Received: March 29, 2010;
Accepted: May 13, 2010;
Published: June 26, 2010
Water resources are becoming increasingly scarce in many arid and semi-arid
areas such as Saud Arabia due to agricultural development and increased demand
(Hussain and Al-Saati, 1999). In Saudi Arabia, more than
80% of the water is met from non-renewable ground sources (MAW,
1996). While water demands continue to increase, the limited amount of groundwater
will always impose great challenges to water resources management in the region.
In the last two decades the reuse of municipal wastewater has emerged as an
important available alternative water source and a viable mean to meet water
requirement for the agricultural sector in many of countries (Hussain
and Al-Saati, 1999; Al-Turki, 2003; Bashaar,
2007) . In many instances, the reuse of municipal wastewater is promoted
as a mean of limiting wastewater discharges to the environment (Huertasa
et al., 2008).
Wastewater reuse for agricultural practices depends mainly on the quality of
wastewater effluent which has to be sufficient to protect environment and human
health and also be suitable for soil and plants (Huertasa
et al., 2008). Therefore, many wastewater treatment plants in Saudi
Arabia have implemented tertiary filtration of secondary-treated effluent to
improve effluent quality and meet the requirements for its reuse in crop irrigation
(Abu-Rizaiza, 1999; Abdel-Magid, 2001).
In Buraidah City, the capital of Qassim Region in central of Saudi Arabia,
there was a secondary wastewater treatment plant which was stopped in 2006 because
of its limited capacity and poor quality of its effluent. Abdel
Magid and Al-Oud (2000) studied the effluent quality of this plant and concluded
that the secondary treated effluent was of unacceptable quality and it might
cause problems for environment, health and aesthetic when disposed to wadis
in the vicinity of urban areas. A new tertiary wastewater treatment plant was
recently constructed in Buraidah City with design capacity of 70,000 m3
d1. The influent wastewater in this plant is preliminary treated
by semi-rotary mechanical bar screens and grit removal. This wastewater is then
secondary treated by activated sludge process with a non-conventional continuous
channel hydraulic design using carrousel system. Settlement tanks receive the
secondary treated water to settle heavy activated sludge in the bottom of the
tank while the clear treated water flows over a weir in to outlet pipe. The
settled activated sludge coming from settlement tanks is then lift to an elevation
for gravity return to the aeration tanks and the excess activated sludge disposed
off to sludge thickeners. Clear water flows to the sand filtration unit for
further removal of suspended solids, turbidity, organic matter and microbial
populations. Filters are composed of 100 cm sand layer each, overlaying a 80
cm gravel layer acting as support media. Filters are designed and operated to
provide an average filtrate rate of 5.7 m3 m2 h-1.
Backwashing of the filters is accomplished for 20 min daily using compressed
air and pressurized water. Wastewater is then subject to chlorination after
sand filtration. Three chlorinators are available with capacity of 25 kg h-1
each. Chlorine dosage rate is controlled by chlorine analyzer, which measures
free residual chlorine and gives signal proportionally to the chlorine control
valve provided on each chlorinator. Twelve contact tanks receive wastewater
in the final stage of the plant in order to provide enough detention time to
complete the chlorination reaction and destroy pathogenic bacteria present in
the effluent water. No coagulants are used in any stage of this plant. The effluent
water is continuously pumped out through a pipe to a part of Wadi Al-Rumma.
Although huge amount of treated wastewater is disposed to Wadi Al-Romma, no
clear strategy was developed for reusing the effluent wastewater for irrigation
Several parameters are commonly employed to evaluate the quality of treated
wastewater including Total Suspended Solid (TSS), turbidity, salinity, Biological
Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) (Alberta
Environment, 2000; Huertasa et al., 2008).
Heavy metals such as cadmium (Cd), lead (Pb), Zink (Zn) and Nickel (Ni) are
measured in wastewater as an indicator for effluent quality (Mensah
et al., 2009). Microbiological quality of wastewater is usually indicated
by quantification of fecal and total coliforms (Fattouh and
Al-Kahtani, 2002; Wand et al., 2007; Srinivasan
and Reddy, 2009). Other paramount wastewater criteria affecting plant growth
and soil properties depend on specific ion concentrations in particular sodium,
calcium magnesium, chloride, carbonate and bicarbonate. Wastewater contents
of these ions can be used to calculate sodicity hazard indices such as Sodium
Adsorption Ratio (SAR), Residual Sodium Carbonate (RSC), Exchangeable Sodium
Percentage (ESP) and Soluble Sodium Percentage (SSP) (Hussain
and Al-Saati, 1999; Harussi et al., 2001;
Al-Shammiri et al., 2005).
For safe reuse, standards have been established by different institutions to
control the quality of the irrigation water. Current legislations in Saudi Arabia
have been recently set and issued by Ministry of Water and Electricity (MWE,
2006). Regulations identify two types of irrigations: restricted and unrestricted
irrigations which depend mainly on kind of crops. Stringent regulations have
been set to meet unrestricted irrigation which is intended for any crop and
any type of soil without limitations (Alberta Environment,
2000). Such tight standards cannot be met unless tertiary treatment is applied
(Abu-Rizaiza, 1999). More flexible standards, however
are proposed for restricted irrigation depending on the type of soil, the proximity
of the irrigated area to a potable aquifer, irrigation method, crop harvesting
technique and fertilizer application rate (Abdel Magid, 2001; Huertasa
et al., 2008). Regulations of restricted irrigation can be achieved
normally by applying secondary wastewater treatment.
The objective of this study was to evaluate the performance of tertiary treatment plant in Buraidah City (TWTPB) with respect to physiochemical and biological characteristics of effluent and to examine its suitability for unrestricting irrigation.
MATERIALS AND METHODS
This study was conducted in Buraidah City, the capital of Qassim Region in central of Saudi Arabia to evaluate TWTPB over a period of three successive years, starting from January, 2006 until December, 2008.
Sampling: Grab samples of influent and effluent of TWTPB were collected on monthly basis, in jars of one liter each and preserved at 4°C during transporting to the laboratory. For bacteriological analyses, jars were sterilized prior to sampling. All analyses were performed immediately after arrival to laboratory.
Analyses: All samples were analyzed for the followings:
||Physiochemical parameters including total suspended solid
(TSS), turbidity, pH, Electrical Conductivity (EC)
||Biochemical parameters including BOD and COD
||Heavy metal contents including Zn, Mn, Cu, Pb, Cd, Ni ( only in the effluent)
||Concentration of Na+, Ca+2, Mg+2, CO3-2,
HCO3¯ (only in the effluent)
||Microbiological analyses by the quantification of fecal coliforms and
total coliforms population
All above measurements were conducted following the Standard Methods for the
Examination of Wastewater
and Waters (APHA, 1998). Some analyses were carried out
by the staff of the Central Laboratory of TWTPB and results were rendered available
upon the author's request while other analyses were carried out in Water Laboratory,
College of Agriculture and Veterinary Medicine.
Removal efficiency: The effectiveness of removal of TSS, BOD, COD, NH4+ and fecal coliform was calculated using the following formula:
where, P is the measured parameter, inf stands for influent and eff stands
for the effluent.
Sodicity hazard calculation: Concentrations of Na+, Ca+2,
Mg+2, CO3-2 and HCO3¯ were
transferred to millequivalent concentrations and employed to evaluate sodicity
hazard by calculating SAR, SAR adj, SSP, ESP and RSC according to Eaton
(1950), Richards (1954) and Suarez
(1981) using the following equations:
where, PHc = [PK + PK calcite] + p[Ca + Mg] + p[HCO3]
PK = log(second dissociation constant of the carbonic acid).
PK calcite = log (solubility equilibrium constant of calcite).
Statistical analysis: Means, maximum values, minimum values and standard
deviations of parameters measured in this study during three successive years
were calculated using Microsoft Excel Program 2007.
RESULTS AND DISCUSSION
Volume of disposed effluent: The average volume of monthly effluent
of Buraidah Tertiary Treatment Plant (TWTPB) during the three years ranged from
1.5x106 m3 in 2006 to 1.9x106 m3
in 2008 (Table 1). Annual effluent volume reached 2.28x107
m3 in 2008. Currently MWE is expanding the TWTPB to increase its
capacity to 140,000 m3 day-1 in 2014. This means that
TWTPB will discharge about 4.56x107 m3 year-1.
Reuse of such large quantities of treated wastewater for irrigation can potentially
participate in saving groundwater, the main source of irrigation in Saudi Arabia.
This application is of great importance since the total cropped area in Buraidah
City has dramatically increased during the last two decades resulting in exploitation
of the Saq aquifer which sets the warning for an imminent water crises (Badr,
1984; Al-Saati, 1995). Currently, wastewater effluent
is totally disposed to Wadi Al-Romma forming a large swamp, which may result
in microbial regrowth and considered as a source of mosquitoes breeding and
a potential source of hazardous drinking water for animals and undesirable odors.
Physiochemical characteristics: Table 2 summarizes
the physiochemical characteristics of raw and treated wastewater during the
period of study. Means of TSS concentrations of raw influent were 163.40, 165.47
and 225.61 mg L-1 in 2006, 2007 and 2008, respectively, while means
of TSS of tertiary treated effluent were found to sharply decline to 5.14, 5.96
and 4.66 mg L-1 in 2006, 2007 and 2008, respectively. Although TSS
concentration of raw influent had a great variations (between 167.01 and 263.90
mg L-1), a small variation was observed in TSS concentrations of
tertiary treated effluent (between 4.54 and 5.58 mg L-1) during the
three years (Table 2) which implied that the performance of
TWTPB is independent on the influent characteristics. Hamoda
et al. (2004) and Pollice et al. (2004)
reported similar results when sand filter was used in the tertiary wastewater
treatment in Kuwait and Italy, respectively. In contrast, Colmenarejoa
et al. (2006) carried out a survey of eight wastewater treatment
plant with different technology in Spain and found that TSS range from 40 to
139 mg L-1 indicating low quality of the effluent in respect to TSS.
As shown in Fig. 1, TWTPB achieved high removal efficiency
of TSS exceeding 96.8% of raw influent TSS.
||Monthly wastewater influent and effluent volume (m3)
||Some physiochemical properties of row influent and tertiary
effluent in TWTPB
|*Each value represents a mean of 12 replicates taken monthly
during the year. **Standard deviation; ***Not determined
||Removal efficiency of selected tested parameters of effluent
in TWTPB. Each value represents a mean of 12 replicates taken monthly during
Healy et al. (2006) and Al-Jlil
(2009) found that sand filter was able to remove 97% of wastewater TSS.
However, in another study, removal efficiency of TSS using sand filter was low
and ranged between 58 to 71% (Hamoda et al., 2004).
The maximum turbidity of tertiary treated effluent recorded during the period
of study was 2.90 NTU (in 2007), while the highest mean of turbidity was 2.47
NTU in 2006 (Table 2). This low level of turbidity is a good
indicator of the high performance of the plant in the removal of organic and
inorganic suspended materials (Alberta Environment, 2000;
MWE, 2006). TSS and turbidity are indicators of the aesthetic
aspects of water and are increasingly accepted as physiochemical parameters
for monitoring performance of wastewater treatment plant and quality in water
reuse. They are of low cost, easy to analyze and they are informative (Hamoda
et al., 2004; Arevalo et al., 2009).
In the present study, the concentration of NH4+ decreased
dramatically in tertiary treated effluent compared to that in the influent .
The highest value of NH4+ concentration was 41.10 mg L-1,
while the highest value in the tertiary effluent was found to be 0.91 mg L-1
(Table 2) which may indicate a high rate of ammonium oxidation.
Removal efficiency of NH4+ was 97.98, 98.54 and 97.60%
in 2006, 2007 and 2008, respectively (Fig. 1). Free chlorine,
used for disinfection, is routinely measured in the effluent to ensure the presence
of certain concentrations capable of disinfection. Results of this work revealed
that the concentration of free chlorine ranged between 0.81 and 0.31 mg L-1
in the tertiary effluent (Table 2). The minimum accepted level
of active chlorine residue in wastewater effluent to inhibit any microbial re-growth
is 0.5 mg L-1 (MWE, 2006; Wand
et al., 2007). Accordingly, active chlorine level should be maintained
at this level and continuous microbial assessment of the disposed wastewater
is imperative in order to ensure water sanitation prior to reuse. Generally,
pH values of the influent and the effluent were slightly alkaline ranging from
7.32 to 7.82 and from 7.38 to 7.91, respectively during the period of the study
(Table 2). The increase in effluent pH compared to influent
pH is attributed to the decrease in dissolved CO2 concentration through
a reduction in the concentration of organic matter due to oxidation during the
treatment (Colmenarejoa et al., 2006).
Biochemical and microbial characteristics: Biological Oxygen Demand
(BOD) and Chemical Oxygen Demand (COD) are two of the most important biochemical
parameters commonly used to examine wastewater quality since they reflect the
organic load in wastewater (Uz et al., 2004; Huertasa
et al., 2008).
||Some biological properties of row influent and tertiary effluent
|*Each value represents a mean of 12 replicates taken monthly
during the year. **Standard deviation
As indicated in Table 3, TWTPB was able to reduce BOD concentrations
from 156.96, 159.62 and 208.51 to 4.80, 4.98, 4.32 mg L-1 and to
reduce COD concentrations from 290.50, 350.98 and 420.87 to 17.38, 18.59 and
16.27 mg L-1 in 2006, 2007 and 2008, respectively. Efficiency of
BOD removal was 96.93, 96.8 and 97.95%, while efficiency of COD removal was
94.11, 94.66, 96.12% in 2006, 2007, 2008, respectively (Fig. 1).
The large reduction in BOD and COD was due to effectiveness of sand filter.
Results obtained in the current study are in agreement with those of Hamoda
et al. (2004) and Al-Jlil, (2009) who reported
BOB and COD removal of up to 97% when sand filter was used. However, Colmenarejoa
et al. (2006) observed low removal efficiency of BOD and COD in final
effluents in six plants using different technologies other than sand filtration
, which ranged between 39 and 84 and between 37 and 70.4 mg L-1,
The BOD/COD ratio has been proposed as indicator for biodegradation capacity
(Metcalf and Eddy Inc., 1985). If BOD/COD ratio is more
than 0.5, biodegradation will readily take place, if between 0.2 and 0.4 biodegradation
will occur only in favorable thermal situation and if the ratio is below 0.2
biodegradation will not proceed (Contreras et al.,
2003). It was found that domestic wastewater has typically a BOD/COD ratio
between 0.4 and 0.8 (Metcalf and Eddy Inc., 1985) and
as reference, a BOD/COD ratio of 0.4 is generally considered the cut-off point
between biodegradable and not biodegradable waste (Uz et
al., 2004). In the present study, BOD/COD ratio in row influent was
around 0.5, which indicates the presence of considerable amount of organic materials
vulnerable to biodegradability. This ratio decreased during the different stages
of treatment to reach 0.25 which reveals a high stability of the effluent and
no further biodegradation is expected to occur. As shown in Fig.
1, the average removal efficiency of BOD was above 97% and of COD was above
96% during the period of study indicating a very high performance of TWTPB,
which lies within the highest values of BOD and COD removal efficiency reported
in the literature for wastewater tertiary treatment plant in different countries
(Hamoda et al., 2004; Colmenarejoa
et al., 2006; Tyagi et al., 2009).
||Monthly variations in BOD of tertiary-treated effluents in
It is clear from Fig. 2 that the monthly variations in tertiary
treated effluent for BOD concentrations remain always below 6 mg L-1
in spite of high variations in BOD concentrations of the raw influent. Monthly
variations for COD hade similar pattern observed for BOD (data not shown). These
findings clearly revealed consistency and stability in TWTPB performance in
respect to BOD and COD removal.
The influent counts of Total Coliforms (TC) were found to vary during the period
of study from 4.1x107 FCU 100 mL-1 (in 2008) to 5.1x107
FCU 100 mL-1 (2006), while counts of Fecal Coliforms (FC) were found
to vary from 4.8x106 FCU 100 mL-1 (in 2008) to 6.9x106
CFU 100 mL-1 (in 2006) (Table 3). TC and FC counts
in chlorinated effluent were substantially reduced to negligible counts that
ranged from 0 to 2 FCU 100 mL-1 (Table 3). This
result indicates that disinfection process is highly effective and can completely
inactivate both TC and FC in the tertiary treated effluent. Removal efficiency
for TC and FC was almost 100% during the three years of the experiment. Such
high level of microbial elimination from the effluent is rarely reported in
the literature, where counts of FC in tertiary treated effluents of many plants
range from 100 to 1000 FCU100 mL-1 (Fattouh and
Al-Kahtani, 2002; Bhattacharjee et al., 2003;
Wand et al., 2007; Srinivasan
and Reddy, 2009). It worth mentioning that use of FC as indicator of wastewater
sanitation was subjected to criticism since different pathogens such as protozoa
and viruses are more resistant to disinfection compared to FC (Wand
et al., 2007; Fattouh and Al-Kahtani, 2002).
Therefore, it is advisable to examine the presence of these pathogens in the
effluent, at least every two months, to assure microbial quality of the effluent.
Evaluation of effluent quality for reuse in irrigation: Several parameters related to human health, soil properties and plant types were considered to evaluate compliance of TWTPB effluent with MWE standards for unrestricted irrigation. The most important criteria affecting human health are presence of pathogens, BOD and COD concentration and heavy metal contents, whereas the most important criteria affecting soil properties and plant growth are salinity and concentration of sodium, calcium, magnesium, carbonate and bicarbonate and microorganism population. Results were compared to MWE standards in order to assess effluent quality and its suitability for irrigation.
The mean value of BOD concentration in the effluent was 4.70 mg L-1 which is 53% less than the highest permissible limit set by MWE for BOD concentrations in tertiary-treated effluent (10 mg-1) for unrestricted irrigation (Table 4). It should be noted that BOD
concentration in the effluent did not exceed the value of 5.54 mg L-1
during the period of study (Table 2), indicating the stability
of BOD reduction in the effluent and its suitability for unrestricted irrigation.
Mean value of COD concentration of the effluent was 17.04 mg L-1.
MWE did not specify a limit for COD for wastewater effluent, but other water
associations such as those in Kuwait and Italy have set 75 mg L-1
as a maximum limit (Ayers and West cot, 1985; Environment,
2000; Hamoda et al., 2004) and accordingly the
COD of effluent met this standard. Similarly, a high stability was observed
in COD reduction in the effluent where the maximum COD concentration was 21.4
mg L-1 during the three years of study.
As shown in Table 4, the average values of TSS, pH, turbidity
and NH4+ of the effluent were 5.25 mg L-1,
7.73, 2.4 NTU and 0.64 mg L-1, respectively which clearly satisfied
the maximum accepted limits required by MWE Standards for unrestricted irrigation.
Concentrations of heavy metals detected in the effluent were below the permissible
limit except that of Pb which was 50% above the limit set by MWE (Table
4), thus Pb may accumulate with time in soil and inhibits plant cell growth
at high concentrations (Bhattacharjee et al., 2003;
Mensah et al., 2009). However, solubility and
mobility of Pb is pH dependent and its adsorption to soil constituents increases
as soil solution pH increases and reaches a maximum at pH 6 (Bhattacharjee
et al., 2003). Therefore adsorption process will be presumably predominant
for effluent Pb when reused for irrigation since pH of effluent and soils in
Buraidah City is always more than 7, which may reduce its toxicity to environments
For hygiene purposes, MWE regulations required that the maximum count of FC
in tertiary treated effluent should be 2.2 CFU100 mL-1 to be accepted
for unrestricted irrigation. TWTPB effluent strongly met this regulation and
always contained no more than 1 cfu 100 mL-1 (Table
5) indicating a very high degree of sanitation.
||Compression of characteristics of final tertiary effluent
in TWTPB to Saudi guidelines for agriculture irrigation proposed by MWE
|*Each value represents a mean of 36 replicates taken during
three successive years (2006, 2007 and 2008)
||Salinity properties of tertiary treated effluent in TWTPB
|*Each value represents a mean of 36 replicates taken during
three successive years (2006, 2007 and 2008)
Therefore, no microbial contamination hazard is expected from irrigation reuse
of the effluent.
Water salinity, expressed as Electrical Conductivity (EC), is perhaps one of
most important parameters affecting soil characteristics and plant growth. The
upper limit proposed by MWE for reuse of wastewater effluent is 3.5 dS m-1.
The mean of salinity level of the effluent was 3.34 dS m-1 (Table
4) indicating its suitability for irrigation. According to Ayers
and Westcot (1985) guidelines for interpretations of water quality for irrigation,
no serious problems are expected from using the effluent with this salinity
level for irrigation. Saline water with EC value of 5.3 dS m-1 was
used in a similar condition in Qatar Country to grow fodder for milking cows
with no adverse effect (Arab Water World, 1991). Gratten
and Oster (1993) indicated that the threshold irrigation water salinity
for a 100% yield potential for crops varied from 1 dS m-1 for salinity
sensitive crops to 2.7 dS m-1 for salinity tolerant crops. However,
several classifications were suggested for irrigation waters for salinity hazard
ranging from an EC value of 0.75 dS m-1 (no salinity problems) to
an EC value of 7 dS m-1 (sever salinity problems) (US Salinity Laboratory,
1969). Therefore, soil properties, crop types and irrigation technique should
be taken into consideration when evaluating water salinity for irrigation.
Sodium is the most dangerous of the major constituents of irrigation water
causing what so called Sodicity hazard. Although it is a limiting factor for
wastewater effluent reuse, MWE Standards did not specify regulations for sodicity
parameters. The upper limit of Na in water irrigation in Kuwait was suggested
by some regulatory agent to be 185 mg L-1 (Al-Shammiri
et al., 2005). Excessive concentration of Na can reduce water uptake
by plant roots and effectively disperse soil colloids, resulting in a loss of
soil structure. Concentration of Na in tertiary treated effluent under investigation
was 16.5 meq L-1 (379 mg L-1) (Table 5),
which exceeded the upper limit of Na by more than 200%. It is, seemingly, evident
that when tertiary treated effluent is used as the sole source of water for
irrigation, the soil will accumulate injurious amounts of exchangeable Na with
time. However, Na concentration per se is insufficient indicator for
evaluating such water suitability for irrigation. The adverse impact of sodium
on soil and plant growth depends on replacing sodium ions for both calcium and
magnesium ions (Leal et al., 2009). Therefore,
different sodicity hazard indices including SAR, SAR adj, SSP and ESP have been
suggested to describe the extent of this substitution and hence effect of sodium
hazard (Eaton,1950; Richards, 1954;
SAR value is the ratio of sodium to calcium and magnesium concentrations. U.S.
Salinity Laboratory, (1969) considers SAR less than 4 to be safe, form 4-9
to be possibly safe, more than 9 to be hazardous. The average SAR of the effluent
was 5.5 (Table 5) which is in the category of low risk to
soil and plant. The SAR adj is an SAR value corrected to account the removal
of Ca+2 and Mg+2 by their precipitation with HCO3¯
and CO3-2 ions in the water added (Eaton,
1950). To avoid salinity toxicity, the adjusted SAR of irrigation water
should be less than 10 (Rowe and Abdel-Magid, 1995), hence
sodium ions are not expected to cause any problems when used for irrigation
purposes since its SAR adj was 9.35 (Table 5).
The SSP indicator is the ratio of Na to other cations including Ca, Mg, K and
Na. When it exceeds a value of 60%, the treated water is considered harmful
and not suitable for irrigation (Hoffman et al., 1980).
As indicated in Table 5, the SSP of the effluent under study
was 46.78% which can be considered of low risk for irrigation purpose. ESP is
an important indicator showing the effect of sodium on soils physical properties.
High value of ESP means high sodium concentration in the effluent water which
causes dispersing soils by replacing the Ca+2 and Mg+2
ions from soil exchange complex (Leal et al., 2009).
The desired value of ESP is less than 5%, while values between 6 and 9% mean
the possibility of increasing problems with soil infiltration and permeability
and ESP value above 15% mean sever problems for soil (Rowe
and Abdel-Magid, 1995; Al-Shammiri et al., 2005).
The ESP calculated for the effluent was 6.41% (Table 5) which
is of moderate risk in respect to soil physical properties.
Another index for examining quality of irrigation water is the Residual Sodium
Carbonate (RSC) (Eaton, 1950). The RSC is an accurate
indicator for the assessment of sodicity hazards because it considers the hazard
of carbonate and bicarbonate water irrigation (Barbiero
et al., 2001). High level of HCO3- and CO3-2
will result in precipitation of calcium and magnesium leading to alkali condition.
Water is not suitable for irrigation when RSC value is greater than 2.5 and
is of critical side when RSC is between 1.25 and 2.5 and is safe when RSC is
less than 1.25 (Eaton, 1950). The effluent under investigation
exhibited negative value for RSC (Table 5), implying that
the cumulative concentration of HCO3¯ and CO3-2
is lower than the combined Ca+2 and Mg+2 concentration
and hence there is no residual carbonate or bicarbonate to react with Na and
increase sodicity hazard. It worth mentioning that all sodicity hazard indices
other than RSC are expected to underestimate sodicity hazard since they do not
take into consideration carbonate and bicarbonate concentration. It is, therefore,
evident that the reuse of TWTPB effluent for irrigation will not impose any
sodicity problems to soil properties or plant growth provided that suitable
irrigation techniques and leaching requirements are taken into consideration.
The present study indicated that TWTPB is capable of producing a high quality effluent with respect to both physiochemical and biological parameters. Final effluent consistently meets the stringent regulations proposed by MWE for unrestricted irrigation in particular those set for TSS, EC, turbidity, BOD and total fecal coliforms. Sodicity hazard parameters of the effluent were in the category of low risk. The RSC exhibited always a negative value which implies that reuse of the effluent for irrigation will not cause soil dispersion or infiltration problems. However, soil properties, plant type and irrigation techniques should be taken into consideration to avoid salts accumulation with time upon reuse. Since concentration of Pb is higher than the permissible limit, there is a need to monitor its concentration in the effluent, irrigated soil and planted crops.
The Author wishes to acknowledge the cooperation and assistance of Water and Sewage Authority of Qassim Region, Saudi Arabia during the course of this work. Thanks are due to Dr. Essam AbdolMonem for his valuable help with laboratory analyses of heavy metals and some ions.
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