-76 c"q pd F

2%= Membrane Processes in Water Reuse

James C. Lozier, P.E. Senior Membrane Process Engineer

and

Robert A. Bergman, P.E. Membrane Treatment Technical Director

CH2M HILL PO Box 147009

Gainesville, FL 32614-7009

Introduction

One of the largest potential sources of water is reuse of municipal wastewater effluent, particularly in arid regions of the western United States. Today, between 60 and 90 percent of municipal water delivered to city residents is discharged into wastewater collection systems. In southern California, over 2 million acre-feet per year of municipal and industrial wastewater is discharged to the ocean (1).

Currently, for water reuse for such activities as agricultural and parks irrigation, industrial reuse, and groundwater recharge through surface infiltration (when salinity reduction is not necessary), most wastewater (following conventional secondary treatment) is typically further treated by simple or direct filtration followed by disinfection. These additional processes further reduce the level of suspended solids and to minimize health risks from microbial pathogens. Where a greater degree of contaminant removal is necessary to meet reuse goals or requirements. many water agencies are using or evaluating reverse osmosis (RO) or nanofiltration (NF) membrane technology following high lime clarification or microfiltration (MF) as part of the reuse process.

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This paper describes several pilot- and demonstration-scale reuse projects currently underway or recently completed that use membrane processes. Treatment applications rapge from nonpotable reuse to indirlzct and (potentially) direct potable reuse. In each case, membrane treatment plays an integral goal in meeting water quality objectives.

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Types of Water Reuse

Nonpotable

Nonpotable reuse includes using reclaimed water from wastewater for all applications except drinking and food preparation. The mO!jt common types of nonpotable reuse include landscape and agricultural irrigation, recreational or environmental enhancement, and industrial cooling water. Examples of indirect reuse using membrane treatment in this paper are limited to the first two categories.

Recreational and Environmental Enhancement at RCID

In 1989, the Reedy Creek Improvement District (RCID) in central Florida initiated an advanced water reclamation program (AWRP) to investigate whether effluent from their tertiary wastewater treatment plant (WWTP) could be treated to a quality suitable for direct discharge to two recreational water bodies on their property and eventually to a highly regulated surface water, Reedy Creek. After negotiations with the Florida Department of Environmental Protection (FDEP) and downstream Reedy Creek users, the following treatment goals were established for AWRP effluent quality:

Meet EPA-established health risk criteria for direct discharge to recreational water bodies or a maximum health risk of one chance of infection in 1,000 exposures

Meet the "unaffected background water quality" of Reedy Creek for total nitrogen (TN) and total phosphorus (TP) levels not to

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exceed 1.46 milligrams per liter ( m a ) and 0.04 mgL, respectively, on an average annual basis

New process improvements to the existing RCID WWTP were designed to reduce nitrogen and phosphorus levels in plant influent to 3 and I m a , respectively, but would not be able to meet the more stringent nutrient discharge requirements. With CH2M HILL’S help, RCID identified an advanced treatment sequence to meet these goals and conducted bench and pilot testing for 2 years to demonstrate the ability of the selected processes to meet the objectives and to develop criteria for the design of a 5 million gallon per day (mgd) AWRP facility.

The process train selected consisted of membrane MF, ultraviolet irradiation (UVI), and RO. A coagulant addition system was also included to evaluate the impact of alum and ferric addition on MF filtrate quality and the effectiveness of coagulation to improve phosphorus removal by MF. A process schematic is shown in Figure 1. Before a process was selected, a desktop comparison of MF, ultrafiltration, and lime clarification was performed. MF was cost competitive with lime treatment and eliminated the need to handle and dispose of large quantities of chemicals and sludge. This was the f i s t known pilot- scale application of MF for the treatment of municipal wastewater in the U.S.

The pilot study was conducted over 6 months in three phases. Initially, two MF units featuring tubular ceramic (US Filters Membralox) and polymeric hollow fiber membrane (Memtec America-Memcor) configurations were operated and comparlxl as a potential pretreatment for RO and f i s t barrier for pathogen and phosphorus reduction. Phase I results led to the selection of the hollow fiber (Memcor) MF technology for use in later phases. In Phase 11, seven types of RO membranes were screened on the MF/UVI pretreated water. In Phase lII, which clomprised the bulk of testing, two different RO membranes (selected for their performance in the previous phase) were operated on MF filtrate for over 2,0001 hours at full-scale conditions. UVI was discontinued during the final phase and replaced by free chlorination because it proved to be more effective for controlling bacterial activity.

Testing results showed that the AWRP process could more than meet all treatment goals. As shown in Table 1 , phosphorus levels in the WWTP

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effluent were reduced, on average, to less than detectable (0.008 mg/L) and nitrogen levels were reduced to between 0.45 and 0.78 m a , depending on the RO membrane type. The combination of alum coagulation and MF proved to be an effective means of controlling phosphorus. MF filtrate phosphorus levels averaged less the 0.04 m a target. All observed nitrogen removal was provided by RO. Cellulose acetate (CA) RO membranes performed more effectively than the thin film composite (TFC) membranes evaluated in this study.

Using seeded virus studies on each AWRP process and applying the rotavirus model, the potential health risk from bodily contact with water from the AWRP process was estimated to be one in lo7 after MF, one in 10"' after MF and UVI, and one in 1014 after MF, UVI, and RO. These figure!; equate to a negligible health risk and one that is orders of magnitude less than the Environmental Protection Agency's (EPA's) criteria.

From testing results, a full-scale process was developed for ]potential future implementation at RCID. By modifying the deep bed filters. recently installed at the WWTP for biological denitrification, the need for RO as an integral treatment step can probably be eliminated and thereby significantly reduce capital and operating costs. A process schematic for the proposed 5-mgd AWRP facility is shown in Figure 2. The required degree of nitrogen reduction would be achieved through denitrification; phosphorus removal would be attained by alum coagulation and MF. MF and UVI would provide more than the requisite degree of pathogen control and provide multiple treatment barriers. The total unit cost for construction and operation of a 5-mgd facility is estimated to be approximately $0.80/1,000 gallons.

An important pilot outcome was the demonstration that, in comparison with lime treatment, RO operations could be enhanced by pretreating the WWTP effluent with MF. The CA RO system was operated for over 2,500 hours with no need for membrane cleaning. RO systems operated on lime-treated water at other locations required cleaning every 4 weeks or less (2).

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Landscape Irrigation

Membrane processes are demonstrating their suitability for water reuse projects where municipal wastewater effluent is reclaimed for irrigation. Two such projects are the Horticultural Reuse project sponsored by the City and County of San Francisco and the Moody Gardens Project in Galveston, Texas. In the San Francisco pilot-scale project, secondary effluent is being treated by MF and RO to provide suitable water quality for landscape irrigation in the Golden Gate Park. Treatment goals include meeting California Title 22 requirements for turbidity and total coliform and a maximum chloride concentration of 150

m a . As with the RCID AWRP, using MF to pretreat the secondary effluent appeared to provide stable RO performance with a limited need for membrane cleaning (3) . The Ntemcor MF and CA RO elements were used in this study.

The City and County of San Francisco also evaluated the combination of MF and UVI in conjunction with the planning of a 14 mgd water reclamation plant to provide water for a number of nonpotable uses, including landscape irrigation and industrial water supply. The feasibility of this process combination will be compared with direct filtration and chlorination to meet California Title 22 requirements for virus, turbidity and suspended solids reduction (4).

With Moody Gardens, wastewater from the City’s airport WWTP is treated by sand filters, activated carbon and electrodialysis reversal (EDR) and is used to irrigate rare and exotic tropical plants and flowers (5). Filtration reduces suspended solids levels and granular activated carbon (GAC) reduces organic carbon. EDR removes about 85 percent of the salts, thus reducing the total dissolved solids (TDS) of the wastewater from 2,600 to less than 450 m a . The current capacity of the treatment system is 350,000 gallons per day (gpd). Estimated treatment (cost is reported to be $0.50/1,000 gallons.

These and selected other projects that use membrane processes for nonpotable reuse are presented in Table 2.

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Potable

Use of reclaimed water from wastewater treated for potable consumption can be direct or indirect. Direct potable reuse is the piped connection of reclaimed water to a potable water supply distribution system or a water treatment plant (WTP). Indirect reuse occurs when reclaimed water is disclharged to a water source (either ground or surface) to reuse the water for potable purposes rather than as a means of disposal (unplanned reuse). Most potable reuse efforts fall into this category. Currently, there are few studies attempting to demonstrate direct potable reuse because of the extreme difficulty in garnering the necessary degree of regulatory and public acceptance.

Indirect Potable Reuse

Potable Aquifer Recharge at Water Factory 21. Water Factory 21, operated by the Orange County Water District (OCWD) of southern California, pioneered the use of RO for wastewater reclamation. The 15-mgd facility has operated since 1976 and treats secondary effluent with high pH lime clarification, recarbonation, filtration, and chlorination (referred to as conventional treatment), and GAC and RO (see Figure 3). After filtration, the flow is split and 5 mgd receives RO treatment while the remainder is treated by GAC. Effluent from the facility is currently blended with deep well water and injected into a potable aquifer to prevent seawater intrusion. The effluent meets all California drinking water standards. Although both GAC and RO provide organics removal, RO produces a much lower TOC and is also the primary removal step for inorganics, including TN, chloride, sulfate, and TDS. The high degree of treatment provided by RO is illustrated in Table 3.

Since the installation of RO at WF-21 in 1977, CA RO membranes have been used exclusively for injection water production. OCWD staff have conducted numerous studies with a variety of TFC-type RO elements to identify alternative membranes that might reduce RO operating costs. These studies have shown that the CA membranes are less prone to fouling and can be more effectively cleaned than the TFC-type membranes.

After their long and successful operating history, OCWD has recently been issued a permit to inject 100 percent reclaimed water from WF-21 into aquifers

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used for potable supply. Regulations associated with the permit require that the injection water meet the following quality requirements (6):

Be free from viruses (Slog reduction through coagulation, filtration and disinfection)

Have a TOC of 2 mgL or less

Contain 10 mg/L or less of TN

Meet all drinking water standards

Be a designated research and demonstration facility (which WF- 21 is)

To meet most of these requirements, 0 0 will continue to use RO, despite the fact that it has the highest associated operating costs of any of the reclamation processes. Total costs for treatment at WF-21 are estimated to be approximately $1.85/1,000 gallons of which $.90/1,OoO gallons are attributed to RO. (Costs are derived from OCWD, 1979, and have been updated.)

Although they are exploring other treatment alternatives for removal of organic and inorganic contanunants, RO is currently considered to be the most critical treatment process for continued use and future expansion of WF-21. With the permitting of 100 peircent reclaimed water injection, OCWD is planning to expand the RO capacity of WF-21 from 5 mgd to 25 mgd (1).

OCWD is currently piloting MF as an,alternative to lime clarification, recarbonation, and filltration for RO pretreatment. Future plans include the construction and operation of a 500-gallons per minute MF demonstration plant, which will include MF, UVl, and RO. This facility will be used to demonstrate the relialbility of full-scale MF technology, to evaluate its cost effectiveness for meeting Title 22 regulations when coupled with UVI, and to further prove its value for RO pretreatment.

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Surface Supply Augmentation

Tampa Water Resource Recovery Project. The City of Tampa, Florida, in conjunction with the West Coast Regional Water Supply Authority and CH2M HILL, conducted a water resourace recovery program to determine the feasibility of treating municipal wastewater to a level that is acceptable for indirect potable reuse via surface water augmentation. The pilot-scale program, operated over a 7-year period, compared the effectiveness of the following processes after treatment of secondary effluent by pre-aeration, high pH lime clarification, two-stage recarbonation, and gravity filtration followed by four parallel process schemes:

1. Disinfection only 2. GAC and disinfection 3. RO and disinfection 4. Nanofiltration (NF) and disinfection

Disinfection included either chlorination or ozonation. A flow schematic for the process sequence is shown in Figure 4.

The following conclusions were drawn from an extensive ainalytical monitoring and toxicological testing program (7):

Product water from each process train complied with all primary and secondary drinking water standards for inorganic and general parameters either existing and currently proposed (at the time of the study, 1986 to 1992). Because RO and NF can remove dissolved solids from water2 they provide a product water significantly lower in TDS than GAC treatment.

Ozonated membrane product waters were lower in TOC content than the ozonated GAC product water; however the GAC product water exhibited no mutagenic response when it was screened with the Ames Test employing Salmonella strains. The membrane-treated waters show varying levels of mutagenic response (see Table 4).

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GAC was shown to have higher operational reliability and lower cost than either RO or NF.

From these results and the fact that it was anticipated that there might not be a need to demineralize the reclaimed water to meet regulatory requirements, GAC and ozonation were selected as the most suitable process combination for long-term toxicological studies. In these studies, the ozone-disinfected GAC effluent produced a water quality as good as or better than other sources of raw water, including the ]present Hillsborough River supply. Total unit cost for a 20-mgd reclamation facility using GAC and ozone following lime clarification, recarbonation, and filtration was estimated at $1.50/1,000 gallons for year- round operation (8).

San Diego Total Resource Recovery Program. As part of a Total Resource Recovery Program, the City of San Diego operated a 1-mgd wastewater treatment system, referred to as AQUA 11, which was designed to upgrade secondary effluent equivalent in quality to imported water currently used for potable supply. The treatment train consisted of water hyacinths to reduce biological oxygen demand (BOD) and total suspended solids (TSS) followed by coagulation, filtration, RO, air stripping, GAC, and disinfection. RO was used primarily for inorganic ion removal, although it did provide substantial TOC removal and served as an additional barrier to viruses and other pathogens. Water from this process sequence was deemed to have equal or lower health risk than the existing raw water supply (9).

Direct Potable Reuse

Denver Potable Water Demonstration Plant. Beginning in the early 1980s, the feasibility of employing multiple types of treatment processes, both conventional and advanced, to treat municipal wastewater to a quality suitable for direct use as potable water were investigated. Usually in these studies, multiple barriers of contaminant removal were used to ensure a degree of treatment reliability unprecedented in conventional WTPs. One study that employed membrane processes as an integral treatment step was the Denver Potable Water Demonstration Study.

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In 1983, the City of Denver, Colorado, and CH2M HILL began a 2-year study to demonstrate the technical feasibility of producing potable water with a quality comparable to the City’s existing drinking water supply from secondary-treated municipal wastewater. At the end of the study, the following treatment sequence was selected to provide water for a succeeding 2-year comprehensive health effects testing program: high pH lime clarification, recarbonation, filtration, UVI, activated carbon adsorption, IRO, air shipping, ozonation, and chloramination (10) (see Figure 5).

RO’s original purpose was to reduce sodium and chloride levels in the reclaimed water. Testing results indicated that RO also removed about 85 percent of the ammonia in the s e c o n d e effluent so that selective ion exchange, which was evaluated early in the study, could be eliminated. RO was also found to be an effective barrier to pathogens. Near the end of the study, a small-capacity NF system was evaluated to determine its suitability as a substitute for RO.

The results of the health effects testing, which included whole animal testing, indicated that the health risks associated with the reclaimed water were no greater than, and possibly less than, those from the City’s current drinking water supply.

Preliminary total costs for a full-scale treatment facility were estimated between $2.00/1,000 gallons and $2.60/1,000 gallons (10).

Membrane-related potable reuse projects described here and selected other projects are presented in Table 5.

Multiple Purpose Reuse

West Basin Municipal Water District. The West Basin Municipal Water District is currently constructing a 20-mgd water reclamation plant in El Segundo, California. The facility designed by CH2M HILL is designed to reuse secondary effluent from the Los Angeles Hyperion WWTP for multiple purposes. Fifteen mgd will be treated by direct filtration and chlorination to Title 22 standards for landscape higation. An additional 5 mgd will receive decarbonation, high pH lime clarification, recarbonation, gravity fitration, air

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* stripping and chlorination to provide injection water for a saltwater intrusion barrier. A process schematic for the facility is shown in Figure 6. RO treatment is primarily to reduce TDS and TOC, although it will also serve as one of multiple barriers for virus and pathogen control. Like WF-21, the RO product water will be blended with other potable quality water before deep well injection. Future plans include expanding the lime treatment facilities to provide water of a quality suitable for industrial cooling-tower makeup and to possibly use MF as RO pretreatment for barrier water capacity expansion. The facility’s ultimate capacity is 70 mgd of which 10 mgd will use RO. Estimated construction cost for the initial phase is $1.50/gpd (CH2M HILL study for West Basin).

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City of Scottsdale. The City of Scottsdale, Arizona, is currently piloting Memcor MF followed by either NF or RO to reclaim wastewater from their WWTP. Both CA and TFC membranes are being evaluated. The City plans to use the reclaimed water for golf course irrigation during the summer months (1 1) and aquifer recharge during the winter months. Current plans are to construct 8 mgd of capacity for irrigation and 6 mgd for recharge.

Summary and Future Trends

Since its first major application at WF-21 in the late 1970s, RO has played an integral role in water reuse. Besides its traditional role of demineralization, RO has proven to be an effective way to control other major contaminants in wastewater, including nitrogen species and dissolved organic substances, and serves as one of several barriers for pathogens. More recently, other membrane processes are seeing greater application in the wastewater reclamation arena. MF is being evaluated as an alternative to lime clarification, recarbonation, and filtration for RO or NF pretreatment with the potential to reduce costs and eliminate large chemcal and sludge disposal requirements associated with lime use. Applying MF alhead of RO may allow the cost-effective use of low- pressure TFC membranes and permit membrane operation at greater fluxes, thereby reducing RO capital and operating costs.

NF is also being evaluated as a substitute for RO where lesser rates of demineralization are sufficient to meet water quality goals. NF membranes

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have lower energy requirements than RO membranes and their use could reduce costs associated with demineralization and TOC removal.

Where TDS reduction is needed but control of TOC and a barrier to pathogens is not required, EDR might be an appropriate process in recllamation applications such as irrigation.

References

1.

2.

3.

4.

5.

6.

U.S. Department of the Interior, Bureau of Reclamation (USBR). National Desalting and Water Treatment Needs Survey. Water Treatment Technology Program Report No. 2. Denver, Colorado. May 1993.

Lozier, J. Evaluating Reverse Osmosis Membrane F’erformance on Secondary Effluent Pretreated by Membrane Microfiltration. AWWA Proceedings. San Antonio, Texas. June 1993.

Lozier, J. Personal Communication with William Dunivin 11, Superintendent of Water Production, Orange County Water District. February 1992.

Lozier, J. Personal Communication with Rochelle Company, Project Engineer, City and County of San Francisco, Department of Public Works. February 1993.

Journal of the AWWA. Reuse Water Nourishes Moody Gardens. Vol. 85. Issue 9. September 1993. Pp. 118-121.

Mills Jr, William R., Martin G. Rigby, Michael P. Wehner, and Orange County Water District. Direct Injection of Treated Wastewater into Potable Water Aquifer+Uegulatory Issues. American Water Works Association Proceedings. 1993 Annual Conference. San Antonio, Texas. June 6-10, 1993.

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7. Jones, Gregory, et al. Recovery of Municipal Wastewater Using Advanced Treatment Technology. Tampa, Florida, Proceedings of the AWWA Annual Conference. June 1989.

8. CH2M HILI,. Tampa Water Resource Recovery Project Summary Report. 1993.

9. Thompson, IKen, Robert C. Cooper, Adam W. Olivieri, Don Eisenberg, Lon A. Pettegrew, and Richard E. Danielson. City of Sun Diego potable reuse of reclaimed water: Final results. National Water Supply Improvement Association Proceedings. Newport Beach, California. .4ugust 23-27, 1992.

10. Lauer, William C., Stephen E. Rogers, Anthony M. LaChange, Myron K. Nealey. Denver’s Potable Water Reuse Demonstration Project Process Selection for Potable Reuse Health Effects Studies. Denver Water Department. March 1989.

11. Lozier, J. Clonversation with Marty Craig, Water Resources Engineer, City of Scottsdale. February 1994.

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Average Concentration

TP ~ 0 ~ 1 ~ 0 ~ TKN TN (mg/L) (mg/L as N) (mg/L as N) (mg/L)

WWTP ERluent 0.34 2.79 0.83 3.62 MF Filtrate 0.023 3.09 0.77 3.86 RO CA Permeate cO.008 0.65 0.09 0.78 RO TFC Permeate c0.008 0.37 0.07 0.45 Target Level 0.04 1.46

Notes: CA = Cellulose Acetate TFC = Thin Film Composite MF = Membrane Microfiltration

Table 1. Nutrient Reduction through Advanced Water Reclamation Process.

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U 9

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Table 2. Summary of Selected Nonpotable Membrane-Based Reuse Projects.

TDS and Chloriie Reduction > 90% Nitrogen Removal > 80% TOC Removal z 90% Pathogen Barrier

Treatment Costs': Conventional + GAC $0.95 RO $0.90

Total $1.85 -

Updated from 1979 Published Costs -

Table 3. WF-21 (Oran e County Water District) Benefits and 8ost of RO Use.

Filter GAC RO NF

7.7 2.2 0.2 0.6 I Water

TOC (mg/L) Ames Tests' (liter-equivalent)

Test 1 110 5.0 1 .o Test 2 >10 6.0 1.5 Test 3 >lo 1.5 3.0 Test 4 >10 3.6 4 . 4

Table 4. Tampa Water Resource Recovery Project Comparative Reduction of TOC and Toxicity by Candidate Processes.

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-.... __

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Table 5. Summary of Selected Potable Membrane-Based Reuse Projects.

s c 3.

Methanol

A Unchlorinated J

Secondary Effluent

Deep-Bed Denitrifying

Filters

SO2

Cl2

Continuous Ultraviolet Microfiltration Disinfection

Contact Microstrainer Basin

To Recreational

Waters

Figure 2. Process Flow Schematic of Proposed RClD Advanced Reclamation Process.

Hig,hPH . Unchbrinated SecondaryEffluent - Lime C rkation

Groundwater

- Recabnation Filtration

Figure 3. Water Factory 21 (Orange County Water District) Process Schematic

To Barrier Aquifer 4- RO

Denitrified Unchlorinated Effluent -

Figure 4. Tam a Water Resource Recovery Project Sclematic of Process Trains.

High Lime Two-Stage Treatment - Recarbonation - Filtration Aeration -

4 c~2103

4 ~12103 - GAC

4 ~12103 RO

4 ~ c12/03 NF

.4 0 W

High Lime - Recarbonation - Filtration - uv - GAC Unchlorinated

Effluent secondary - Clarification

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Figure 5. Denver Reuse Demonstration Project Process Schematic.

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4 Air - CI2 03 Stripping RO or

NF

H erion

Effluent &P

To Barrier Water System

(5 mgd initial, 10 mgd ultimate)

Industrial Use

(20 mgd ultimate) + (future)

Disinfection - Dechlor (CIA .

Hi hpHLime Reca* - Filtration -

Figure 6. WBMWD Water Recycling Program Process Schematic.

- ~eca*on - Qreatment - ~~~~h~ - Disinfection - RO -