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In this article we will discuss about the municipal wastewater desalination by Reverse Osmosis (RO).
M. Abdel-Jawad, F. Al-Atram and S. Al-Shammari
Water Resources Division, Water Desalination Department Kuwait Institute for Scientific Research P.O. Box 24885, Safat 13109, Kuwait
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Abstract:
Potable water quality can be produced at a reasonable cost, if reverse osmosis (RO) technology is applied to renovate secondary/tertiary wastewater effluent. Such implementation will yield many advantages for Kuwait, namely, satisfying the increasing agricultural, industrial and domestic demands for good quality water that is free from viruses and bacteria; preserving the natural strategic water resources; reducing the environmental pollution resulting from the direct discharge of secondary/tertiary municipal effluents to the sea; and meeting unexpected emergency cases of shortages in freshwater produced from the sea for certain domestic applications.
This paper describes preparatory work at the Ardiya site for the operation of the RO unit, experimental results, problems and proposed solutions. It also includes some preliminary results obtained from operation of the RO plant.
The results indicate that wastewater can be treated with very low pressure, and excellent permeate quality can be obtained. The permeate produced is almost devoid of salts, bacteria and pollutants. However, virological tests are still required.
Introduction:
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Kuwait, as an arid country, has very limited resources of freshwater. Available resources are desalinated seawater, brackish groundwater and treated wastewater. Almost all of Kuwait’s freshwater needs are supplied by seawater distillation. The brackish water resource is limited and nearly un-replenishable. Urban wastewater is collected, treated to a tertiary level and returned to the sea. Limited quantities are utilized for landscaping purposes.
In Kuwait, wastewater effluent is treated to a secondary or tertiary level (i.e., a secondary effluent is exposed to chlorination, sand filtration and rechlorination to yield a tertiary effluent). The relatively low salinity of the treated wastewater (1000-1500 mg/l) compared with that of brackish water (4000-5000 mg/l) makes the treated wastewater a potentially excellent source of good quality water.
Over 100 MIGD of treated wastewater has a great potential to supplement /replace the brackish water supplies, and might be the solution needed to redress the balance and complement the need for additional water supplies in Kuwait.
The brackish water network of Kuwait has a large pumping capacity of over 160 MIGD. This capacity is underutilized in the transmission of brackish water, but it will satisfy the needed transmission of polished treated wastewater effluents for over three decades to come.
A problem remains in the dissolved organics and other contaminants present in the tertiary-level treated effluents. Different methods can be employed to renovate this effluent and to utilize it in agricultural, industrial and certain domestic applications.
Direct human consumption of the treated effluent cannot be accepted due to ethical and physiological reasons. Reverse osmosis (RO) is a successful desalination method applied to seawater, brackish water, and industrial and urban wastewater. It relies on a membrane separation technique that requires pressure to force clean water through a membrane, which thus rejects dissolved salts and harmful contaminants, including bacteria, viruses and chemicals, with the reject stream.
The state of the art of RO is such that the site-specific design specifications of a new plant are still far from routine. This is particularly true in the case of seawater RO. Some of the parameters, such as the quality of feed-water, the recovery of freshwater and problems associated with membrane sensitivity to scaling, fouling, etc., are highly site- dependent. Nevertheless, great advances have been made in RO technology during the last decade, and they have resulted in rapid use of RO.
In spite of the recent advances in the application of RO technology to wastewater reclamation, issues related to membrane performance, pre-treatment, membrane cleaning and brine disposal must be addressed and solved. Technical and economic assessment must be carried out to obtain an actual unit cost for wastewater renovated by the RO process.
Therefore, the aims of this project are to assess the technical viability and the economic feasibility of implementing RO membrane technology to renovate Kuwait’s tertiary-level treated wastewater and to find solutions for the problems encountered in wastewater renovation by the RO process.
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This paper covers the operation of large sand filters, the operation of the RO plant and the preliminary results.
Process Description and Results:
Based on the characteristics of the tertiary-level treated wastewater from the Ardiya plant, and the results of the bench-scale and pilot plant tests, the pre-treatment facilities were constructed. Figure 1 shows the layout of the constructed pre-treatment and RO facilities, and Figure 2 shows the complete schematic diagram of the pre-treatment and the RO plant. The technical details of the pre-treatment system and the RO plant are shown in Tables 1 and 2, respectively.
Further optimization of the pre-treatment system was achieved using the following chemical dosing rates:
1. Fe III dosage (5.0 mg/l
2. Cationic polyelectrolyte (0.5 to 1 mg/l)
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3. Sanitizing agent (peroxyacetic acid with 3.0 mg/l peroxygen residuals)
4. Dechlorination agent (1 to 2 mg/l)
Average physical and chemical analyses of the feed-water and treated water after filtration are shown in Table 3.
Through the constructed system shown in Fig 1 and Table 1, SD1 values of 2 were normally obtained from the sand filters of the pilot pre-treatment plant. However, using the sand pressure filters that are supplied with the RO unit (Fig. 1 and Table 1) could not give an acceptable SDI value. This was due to the fact that these sand filters are designed for very clean groundwater feed and not water that is heavily loaded with pollutants such as wastewater.
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The flocculated suspended solids that can be removed from the treated wastewater feed were found to be more than 20 g/l. This quantity is almost double the maximum permissible content of suspended solids in the feed-water entering a filter bed similar to the bed supplied with the RO unit described in Table 1 and operating at a rate of 10 m/h (4 gpm /ft2).
Knowing that the available filter bed is designed to be operated at 80 m3/h (31.5 m/h) (three times the normal rate of flow) and the flocculated suspended solids are double the normal load for such a filter which is designed to be washed every eight hours, it is evident that the filters have to be backwashed every one hour and twenty minutes.
Therefore, the SDI of the treated water could not be measured in such a short time. Lowering the feed flow rate of the filters to 20 m3/h (8 m/h), an excellent SDI value of less than 1 per cent was obtained. Although, the backwash interval was extended to four hours, it was still a very short interval and the feed flow rate was not sufficient to operate the 250,000- USGPD RO unit.
Operation of the Reverse Osmosis (RO) Plant:
The RO membranes were preserved for almost one year, before the plant began operation in May 1996. A compromise flow rate of 50 m3/h (20 m/h) was adopted for operation of the RO unit. This adjustment resulted in a two-hour backwash interval and 50 per cent recovery. Table 4 shows the typical operating conditions of the RO unit. Table 5 shows the chemical and bacteriological analyses of the permeate product water.
The individual conductivities of the permeate from the eight RO pressure vessels are shown in Table 6. During July 1996, chemical cleaning of the membrane was performed using citric acid and EDTA. Chemical analysis of the cleaning solution revealed that iron was the major membrane pollutant (55 mg/l). The RO plant is operated daily for a period of one hour. The filters are also operated, backwashed and rinsed daily.
Discussion:
The RO unit was originally designed to treat brackish feed-water with an excellent SDI. Treated wastewater contains a much higher load of flocculated suspended solids. Analysis of the characteristics of the sand filters and the feed wastewater quality revealed that these filters cannot cope with more than one hour and twenty minutes of normal acceptable operation of the RO unit.
Lowering the feed flow to the RO unit can extend the normal operation to two hours. This mode of operation does not satisfy a main objective of the study, i.e., technical and economic evaluation of this application in Kuwait. Therefore, a small RO unit, and a proportional pre-treatment and filtration system were transferred to Ardiya and modified to allow 24-h operation.
The results obtained show that the pre-treatment technique adopted is very efficient in producing quality acceptable for RO feed. However, the filters ought to be much larger than the ones available. Preliminary results, as presented in Tables 4 and 5, show that > 50 per cent recovery and 97.4 per cent salt rejection can be achieved at a very low feed pressure, i.e., around 10 bar.
Physical, chemical and bacteriological analyses of the permeate water show that an excellent quality of water can be produced by RO. However, virological analysis of the product water is still required.
