INTRODUCTION
Main sources of heavy metals in wastewater especially the industrial wastewater
are plastic water pipes, fertilizers, electroplating, paint pigments, catalytic
processes, metal ceramics and alloys industries (Ajay et
al., 2005; Brezonik, 1974; Patterson
and Passino, 1987; Emsley, 1992). The heavy metals
affect human health and cause serious diseases such as kidney failure, anemia,
destroyed liver, cancer and brain damage when present above the permissible
concentration levels (Ajay et al., 2005; Emsley,
1992; Abdulkarim and Abu Al-Rub, 2004).
In general, wastewater treatment processes such as chemical precipitation,
extraction, reverse osmosis and adsorption methods are normally used in removing
the heavy metals from wastewater. It has been observed that the adsorption method
is an economically feasible and viable process especially using the natural
material such as clay as an adsorbent. Many investigators used clay for the
removal of cobalt, lead, cadmium, zinc and chromium ions from wastewater (Orumwense,
1996; Yadava et al., 1991; Pradas
et al., 1994; Singh et al., 1992;
Ceylan et al., 2005; Al-Jlil
and Alsewailem, 2009).
The Wastewater Treatment Plant (WTP) at King Abdulaziz City for Science and Technology (KACST) was established to treat wastewater from KACST for landscape irrigation in and around KACST compound.
An extensive review on the subject showed that a very little research and investigation have been carried on the removal of the heavy metals from sewage wastewater in Riyadh City. Therefore, the aim of this study was to remove a mixture of heavy metals from the sewage waste water in Riyadh City by using the available wastewater treatment plant at King Abdulaziz City for Science and Technology (KACST). Firstly, treatment of wastewater by using Reverse Osmosis (RO) unit and secondly, by replacing the RO-Unit in the sewage plant by adsorption unit using Saudi bentonite clay as an adsorbent. Then, the results will be compared for determining the heavy metals removal efficiency of RO-Unit and the adsorption method for economical evaluation.
MATERIALS AND METHODS
Materials
The Saudi bentonite clay from Jeddah city was used as adsorbent in the experiment.
The chemical analysis of the Saudi bentonite clay was done by XRD, XRF and surface
area analyzer. Mean chemical analysis is shown in Table 1 and
2. The XRD results verified the presence of montmorillonite as the major
component with small amount of Kaolinite, quartz and illite minerals.
The adsorbates were a mixture of multi-components namely: copper, cobalt, zinc,
lead, arsenic, cadmium and chromium ions in the wastewater of Riyadh City (Table
3).
Methodology
The experiment was carried at the King Abdulaziz City for Science and Technology
(KACST) during 2008-2009. The wastewater was passed through a series of treatment
processes to obtain clean water for landscape development around KACST compound
by reducing COD and BOD and the removal of heavy metals. The Wastewater Treatment
Plant (WTP) at KACST consists of RO-Unit for advanced treatment of product water
from biological treatment process where aeration, biofilters and activated sludge
were used for the reduction of COD and BOD from domestic wastewater. The RO-Unit
at the WTP facility was used for the removal of heavy metals from product water
obtained from biological treatment process of WTP.
| Table 1: |
Chemical analysis of the bentonite clay by XRF |
 |
| Table 2: |
Surface area and pore characteristics of bentonite clay |
 |
| Table 3: |
Heavy metals concentrations in wastewater in Riyadh City |
 |
 |
| Fig. 1: |
Wastewater treatment plant with Reverse Osmosis Unit |
 |
| Fig. 2: |
Sewage wastewater treatment plant with Adsorption Unit |
Then, the RO-Unit was replaced by the Adsorption Unit using Saudi bentonite
clay as an adsorbent for the removal of heavy metals. The results, from both
the treatment techniques were compared for heavy metals removal efficiency and
its economical feasibility.
The treatment process using RO-Unit consists of the following steps:
| • |
Primary treatment stage i.e., screening, aeration and sedimentation
in clarifiers |
| • |
Biological treatment stage includes two processes: |
| |
• |
Activated sludge tank treatment process |
| |
• |
Bio-filters tank |
| • |
Reverse Osmosis (RO) Unit for removing heavy metals |
| • |
Chlorination (disinfection) |
The whole experiment was repeated three times to obtain reliable data.
The schematic diagram of Wastewater Treatment Plant (WTP) is presented in
Fig. 1.
The second treatment process was the Adsorption Unit for removing heavy metals
instead of RO-Unit. This treatment process is identical to the first process
except that the RO-Unit was replaced by the Adsorption Unit as shown in Fig.
2.
Determination of Equilibrium Time for Metal Adsorption
Equilibrium time for Saudi bentonite clay was determined by using a constant mass of adsorbent (3 g) in 50 mL of wastewater in 250 mL capacity glass bottles and placed on a constant agitating shaker. The particle size of the adsorbent was 0.25 mm and the temperature was maintained at 25°C.
The adsorbed metal ions and the adsorbent system reached to equilibrium after 1 h which was based on the run experiments at different time intervals. For this reason, the equilibrium process was left for 3 h to ensure that the adsorption process has reached the state of equilibrium.
Determination of the Concentration of the Collected Cobalt Ions Solution Samples
The sample from adsorption unit were collected after 3 h and filtered. After
this, the absorbance of samples from adsorption unit and RO-Unit was measured
by atomic absorption spectroscopy. Then the absorbance of samples was converted
to concentrations directly using calibration curve for each metal ion. The removal
efficiency of the metal ions adsorption on the bentonite clay and after reveres
osmosis was obtained from the following equation:
where, Co is the initial concentration of metal ions (mg L-1) and Cf is final concentration of the metal ions (mg L-1) from Adsorption Unit and from RO-Unit.
RESULTS AND DISCUSSION
The concentration of heavy metals (Co, As, Cd and Cr) in the sewage wastewater
of Riyadh city was above the permissible limits (Table 3)
as compared to the recommended permissible limits for these metal ions for crop
production (Table 5). For example, the concentration of Cd
and As 6.36 and 0.972 mg L-1, respectively which was above the permissible
limits as shown in Table 5. It is well known that Cd and As,
being toxic metals, can cause human health and environmental hazards. Therefore,
wastewater treatments such as adsorption process and reverse osmosis were applied
to remove the heavy metals from the sewage wastewater. In the Adsorption Unit,
Saudi bentonite clay was used as a low cost adsorbent, whereas in the RO-Unit
polyamide membrane was used for water treatment.
The concentration of heavy metals were less in the treated water by the two
techniques than the recommended permissible limits for crop production (Table
4).
The concentration of heavy metals decreased significantly in the treated water
when compared to the untreated wastewater. The results, showed that the Saudi
bentonite clay and RO-Unit were very effective for the removal of heavy metals
from the wastewater. Mean minimum removal efficiency of heavy metal ions was
88.89% by Saudi bentonite clay and 87.92% by RO-Unit as shown in Table
4. In conclusion, the Saudi bentonite clay and RO-Unit showed an excellent
potential for the removal of some heavy metals from the sewage wastewater.
| Table 4: |
Effect of treatments on heavy metals concentrations in waste
water in Riyadh City |
 |
| Table 5: |
Comparison of heavy metals concentration in the treated wastewater
for crop production |
 |
| *Source: Pescod (1992) |
| Table 6: |
Cost of Saudi bentonite clay as compared to reverse osmosis
module |
 |
| *Price according to the National Water Company, Saudi Arabia,
**Selling price by Dirah Drilling and Heavy Equipment Company, Saudi Arabia |
The study findings agree with the results of Al-Jlil (2010),
who found that the removal of heavy metals was 97% by using bentonite clay and
Saudi roasted pits. Similar results were reported by Erdem
et al. (2004) who studied the adsorption behavior of natural (clinoptilolite)
zeolites with respect to Co2+, Cu2+, Zn2+ and
Mn2+ in order to consider its application to purity metal finishing
wastewater. Their results showed that natural zeolites hold great potential
to remove cationic heavy metal species from industrial wastewater.
Adsorption Mechanism of Metal ions on Saudi Bentonite Clay
The metal ions adsorption on Saudi bentonite clay is due to the electrostatic
attraction between the negative sites of clay and the positive sites of heavy
metal ions. The number of negative sites formed on the bentonite surface is
mainly due to the presence of silica (Elliott and Huang,
1981). Similar results were reported by many investigators who used clay
for the removal heavy metals such as cobalt, lead, cadmium, zinc and chromium
ions from wastewater (Orumwense, 1996; Yadava
et al., 1991; Pradas et al., 1994;
Singh et al., 1992; Ceylan
et al., 2005).
Brief Economic Study
A rough economic study regarding the removal of heavy metals using RO-Unit in
the WTP, KACST and Adsorption Unit using Saudi bentonite clay as an adsorbent
was estimated. The cost is estimated by dollar kg-1 of bentonite
clay and per membrane module (Table 6).
The data in Table 6 showed that Saudi bentonite clay is the
least expensive adsorbent as compared to the reverse osmosis module which in
very expensive. In addition to above, since the Saudi bentonite clay is not
expensive, its regeneration may be not required after the adsorption process
as it would be for the other expensive adsorbents. The reason is that the Saudi
clay do not need regeneration because it is natural, locally available and cheep
adsorbent suitable for heavy metals removal. On the other hand, the RO-Unit
needs frequent cleaning due to fouling on the membrane pores. This will increase
the cost of using RO-Unit for removing the heavy metals from wastewater.
CONCLUSIONS
The concentration of heavy metals in the sewage wastewater in Riyadh city were above the permissible limits for crop production. The heavy metals concentration in the treated water were within the permissible limits for crop production. The mean minimum removal efficiency for the adsorption of metal ions on Saudi bentonite clay and reverse osmosis method were 88.89 and 87.92%, respectively.
The economical evaluation of the study showed that Saudi bentonite clay is the least expensive adsorbent as compared to RO-Unit. Also, Saudi bentonite clay do not regeneration after the adsorption process. Whereas in the case of RO Unit, the membrane module needs frequent cleaning due to fouling and is very expensive.
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