New and Emerging Drinking Water Treatment Technologies
The development and implementation of water treatment technologies have been mostly driven by three primary factors: the discovery of new rarer contaminants, the promulgation of new water quality standards, and cost. For the first 75 years of this century, chemical clarification, granular media filtration, and chlorination were virtually the only treatment processes used in municipal water treatment. However, the past 20 years have seen a dramatic change in the water industry’s approach to water treatment in which water utilities have started to seriously consider alternative treatment technologies to the traditional filtration/chlorination treatment approach. This paper identifies and discusses some of these “emerging” technologies.
Technology Development and Implementation Process
For a new technology to be considered it must have advantages over traditional treatment processes. These can include lower capital and operations and maintenance costs, higher efficiency, easier operation, better effluent water quality, and lower waste production. Nevertheless, for a water treatment technology to be accepted and implemented at a large municipal scale, it must be demonstrated in stages. Understanding this process is necessary in order to properly plan and introduce new technology to municipal water treatment. A typical sequence of these stages might be summarized as follows:
Stage 1: Successful demonstration in another field.
Stage 2: Testing and development at bench- and pilot-scale levels (1 to 50 GPM).
Stage 3: Verification at demonstration-scale level (>100 GPM).
Stage 4: Multiple successful installations and operations at small full-scale level (0.5 to 5 MGD).
Stage 5: Implementation at a large-scale municipal water treatment plant.
Two important milestones must be achieved in parallel with the above stages: obtaining regulatory approval and reducing costs to competitive levels. Commonly, regulatory approval is necessary by the end of the demonstration-Page 221Suggested Citation: “11 New and Emerging Drinking Water Treatment Technologies.” National Research Council. 1999. Identifying Future Drinking Water Contaminants. Washington, DC: The National Academies Press. DOI: 10.17226/9595.
scale Verification stage (stage 3) and prior to installation at small full-scale plants (stage 4). However, for new technology to reach full acceptance (stage 5), its cost must be competitive with that of other more conventional processes that achieve the same objective.
The time duration for each of the above stages can vary greatly depending on the technology being considered, how urgent it is to have it implemented, how long it takes for its cost to reach competitive levels, and the significance of its role in the overall water treatment train. The last factor is different from the others in that it recognizes the difference between a technology that is proposed as an alternative to filtration, for example, which is an essential component of water treatment, versus a technology that is proposed to replace a less important component such as a pump, automation, chemical feed, taste-and-odor control, or peroxidation.
A wide range of water treatment technologies have been developed or are currently in development. This paper focuses on technologies that can be applied in municipal water treatment plants. Such technology should meet the following criteria:
- The technology can be scaled to large applications (i.e., > 5 MGD).
- The technology can be cost competitive with existing technologies at large scale.
- The technology can produce water that meets regulatory requirements.
- The technology has a high degree of reliability.
In this paper, the following technologies are screened and evaluated: membrane filtration (low pressure and high pressure), ultraviolet irradiation, advanced oxidation, ion exchange, and biological filtration. Many of these technologies are certainly not new to the water industry. However, either their application has been limited or they have introduced to the water industry so recently that many questions remain unanswered about their large-scale application.
Membrane Filtration Technology
There are two classes of membrane treatment systems that should be discussed: low-pressure membrane systems (such as microfiltration and ultrafiltration) and high-pressure membrane systems (such as nanofiltration and reverse osmosis). Low-pressure membranes, including microfiltration (MF) and ultrafiltration (UF), are operated at pressures ranging from 10 to 30 psi, whereas high-pressure membranes, including nanofiltration (NF) and reverse osmosis page 222 Suggested Citation: “11 New and Emerging Drinking Water Treatment Technologies.” National Research Council. 1999. Identifying Future Drinking Water Contaminants. Washington, DC: The National Academies Press. DOI: 10.17226/9595.
(RO), are operated at pressures ranging from 75 to 250 psi. Figure 11-1 shows a schematic of the pore size of each membrane system as compared to the size of common water contaminants.
If there is a “Cinderella” story of a water treatment technology it is that of the application of low-pressure membranes for surface water treatment. The idea of using low-pressure membrane filtration for surface water treatment began developing in the early 1980s. At the time, low-pressure membranes had long been used in the food-processing industry as nonchemical disinfectants. During the latter half of the 1980s, several research projects were initiated by west coast water utilities (East Bay Municipal Utilities District and Contra Costa Water District), the American Water Works Association (AWWA) Research Foundation, and other organizations to evaluate MF and UF for municipal surface water treatment. The studies clearly showed that both MF membranes (with a nominal pore size of 0.2 mm and UF membranes (with a nominal pore size of 0.01 mm are highly capable of removing particulate matter (turbidity) and microorganisms. In fact, the research results showed that, when it came to these contaminants, membrane-treated water was of much better quality than that produced by the best conventional filtration plants. Figure 11-2 shows an example plot of turbidity removal by an MF membrane. The majority of treated-water samples had a turbidity level near the limit of the online turbidimeter (less than 0.05 Nephelometric Turbidity Units (NTU)). In addition, membrane filtration (both MF and UF) was proven to be an “absolute barrier” to Giardia cysts and Cryptosporidium oocysts when the membrane fibers and fittings were intact. Finally, the particular UF membranes tested by Jacangelo et al. (1995) were also proven to act as absolute barriers to viruses because of their nominal pore size of 0.01 mm.
As a surface water treatment technology, low-pressure membrane filtration has several advantages over conventional filtration and chlorination. These include smaller waste streams, lower chemical usage, smaller footprint, greater pathogen reduction, no disinfection byproduct formation, and more automation. For a while, it was also believed that low-pressure membrane filtration is highly susceptible to excursions in raw water turbidity. However, pilot- and full-scale operational data have demonstrated that low-pressure membranes can treat turbidity excursions as high as several hundred NTUs with manageable impacts on process operation and efficiency (Yoo et al., 1995). All of the above advantages greatly favor membrane filtration over conventional filtration with chlorine.
On the other hand, because of their porous structure, low-pressure membranes are ineffective for the removal of dissolved organic matter. Therefore, color-causing organic matter, taste-and-odor-causing compounds such as Geosmin and methylisoborneol, and anthropogenic chemicals can pass through the membranes into the treated water. This limits the applicability of low-pressure membrane filtration to surface water sources where the removal of organic matter is not required. One UF membrane system has overcome this page 223Suggested Citation: “11 New and Emerging Drinking Water Treatment Technologies.” National Research Council. 1999. Identifying Future Drinking Water Contaminants. Washington, DC: The National Academies Press. DOI: 10.17226/9595.
Pore size ranges of various membranes.
Example plot of turbidity reduction by MF membranes.Page 224Suggested Citation: “11 New and Emerging Drinking Water Treatment Technologies.” National Research Council. 1999. Identifying Future Drinking Water Contaminants. Washington, DC: The National Academies Press. DOI: 10.17226/9595.
limitation by introducing powdered activated carbon (PAC) as part of the system. PAC injected into the influent water to the membrane is retained on the concentrate side of the membrane and disposed of with the waste stream. This approach is certain to expand the domain of low-pressure membrane applications in surface water treatment, especially at sites where organic removal is only occasionally required.
With all of these positive aspects, there were several obstacles that low-pressure membrane filtration had to overcome. First, for several years the cost of membrane filtration systems at “municipal” scale (i.e., greater than 1 MGD) was prohibitively high. Second, membrane filtration did not have regulatory acceptance and required extensive evaluation on a case-by-case basis. Third, information on its reliability in large-scale municipal applications was not available.
However, since the early 1990s, the cost of low-pressure membranes has decreased dramatically, which has made it more attractive to water utilities for full-scale implementation. In addition, a number of water utilities realized all the benefits that low-pressure membrane systems provided and decided to undergo the regulatory approval process to install these systems at relatively small and cost-effective scales. This has opened the door for the installation of increasingly larger low-pressure membrane plants. Until 1994, all MF or UF plants in the United States and around the world had capacities of less than 3 MGD. In 1994, the first large-scale MF plant (5 MGD) went online in San Jose, California, after undergoing significant testing to obtain California Department of Health Services approval. Since then the application of low-pressure membrane filtration has been on the rise. Figure 11-3 shows the recent profile of low-pressure membrane installation in North America in cumulative plant capacities. Today, membrane filtration is rapidly becoming accepted as a reliable water treatment technology. The California Department of Health Services has certified one MF membrane system for water treatment in the state and has granted it 3-log Giardia removal credit and 0.5-log virus removal credit. It has also certified one UF membrane system and granted it 3-log Giardia removal credit and 4-log virus removal credit. Others are either being considered for certification or are actively undergoing the required testing. Membrane system construction costs are believed to be comparable to conventional plant construction costs up to a capacity of 20 MGD. However, this upper ceiling is rapidly rising. In fact, there are membrane plants being considered in the United States with capacities ranging from 30 to as high as 60 MGD.
As noted earlier, included in this category are nanofiltration (NF) and reverse osmosis (RO) membranes. NF membranes are actually thin-film composite Re membranes that were developed specifically to cover the pore size between Re membranes (<1 nm) and UF membranes (>2 nm) (Matsuura, 1993)–hence the name nanofiltration. Thin-film composite (TFC) membranes are discussed later in this paper. The result was a type of membrane that operatesPage 225Suggested Citation:“11 New and Emerging Drinking Water Treatment Technologies.” National Research Council. 1999. Identifying Future Drinking Water Contaminants. Washington, DC: The National Academies Press. doi: 10.17226/9595.×SaveCancel
Profile of low-pressure membrane installations in North America.
at higher flux and lower pressure than traditional cellulose acetate (CA) RO membranes. In fact, NF membranes are sometimes referred to as “loose” RO membranes and are typically used when high sodium rejection, which is achieved by RO membranes, is not required, but divalent ions (such as calcium and magnesium) are to be removed (Scott, 1995). Nevertheless, NF membranes are viewed by the water industry as a separate class of membranes than RO membranes and are discussed in this paper as such. NF membranes are commonly operated at pressures ranging from 75 to 150 psi (Lozier et al., 1997). NF membranes have been used successfully for groundwater softening since they achieve greater than 90 percent rejection of divalent ions such as calcium and magnesium. Several NF membrane-softening plants are currently in operation in the United States, with the first plant installed in Florida in 1977 (Conlon and McClellan, 1989). By 1996 the combined total capacity of NF plants in the United States was greater than 60 MGD, all in Florida (Bergman, 1996). It is estimated that approximately 150 NF membrane plants existed around the world by 1995, with a combined total capacity of approximately 160 MGD (Scott, 1995). Because most commercially available NF membranes have molecular weight cutoff values ranging from 200 to 500 daltons (Bergman, 1992; Scott, 1995), they are also capable of removing greater than 90 percent of natural page 226Suggested Citation: “11 New and Emerging Drinking Water Treatment Technologies.” National Research Council. 1999. Identifying Future Drinking Water Contaminants. Washington, DC: The National Academies Press. DOI: 10.17226/9595.
organic matter present in the water. Therefore, they are also excellent candidates for the removal of color and, more importantly, disinfection byproduct (DBP) precursor material (Taylor et el., 1987; Tan and Amy, 1989; Bleu et el., 1992; Chellam et al., 1997).
Currently, NF membranes are being considered as total organic carbon (TOC) removal technology in surface water treatment. The idea is to install NF membranes downstream of media filtration in order to maintain a very low solids-loading rate on the membranes. Although NF membranes have been designated by the U. S. Environmental Protection Agency (EPA) as one of the two best available technologies (BATs) for meeting stage 2 of the Disinfectants/Disinfection Byproducts Rule, they have not been applied for surface water treatment at full scale. To date, pilot studies have been conducted to evaluate the applicability of NF membrane filtration downstream of media filtration during surface water treatment with mixed results (Reiss and Taylor, 1991; Tooker and Robinson, 1996; Chellam et al., 1997). The study reported by Chellam et al. (1997) clearly demonstrated that the fouling rate of NF membranes downstream of conventional filtration was two times higher than that of NF membranes downstream of MF or UF membranes. This was supported by the study of Reiss and Taylor (1991), which showed that conventional filtration pretreatment did not reduce the fouling rate of NF membranes to acceptable levels. Nevertheless, the Information Collection Rule includes data gathering on the applicability of NF membrane filtration for TOC removal from surface water sources. The majority of the data will be from bench-scale testing, which does not include information on long-term operational design and reliability, but some data will be obtained from pilot-testing programs. These data will provide additional input into the viability of NF membranes for surface water treatment.
RO membranes have long been used for the desalination of seawater around the world. These membranes can consistently remove about 99 percent of the total dissolved solids (TDSs) present in the water, including monovalent ions such as chloride, bromide, and sodium. However, for a long time, these membranes were predominantly made from CA and required operating pressures at or greater than 250 psi. Recent innovations in Re membrane manufacturing have developed a new class of Re membranes, called TFC membranes that can achieve higher rejection of inorganic and organic contaminants than CA Re membranes while operating at substantially lower pressures (100 to 150 psi). In addition, CA Re membranes commonly require acid addition to lower the pH of the water to a range of 5.5 to 6.0 to avoid hydrolysis of the membrane material. TFC RO membranes do not hydrolyze at neutral or high pH and therefore do not require pH depression with acid addition. It should be noted that the need for pH depression for preventing the precipitation of salts on the membrane surface (such as CaCO3) may still be necessary in some cases depending on the quality of the water being treated and the availability of suitable antiscalants.
TFC RO membranes are currently being evaluated for water reclamation. Results from ongoing pilot studies have shown that TFC RO membranes can achieve greater than 90 to 95 percent rejection of nitrate and nitrite, compared to 50 to 70 percent removal with CA Re membranes. The same pilot studies also show that the TOC concentration in the effluent of TFC Re membranes can be as low as 25 to 50 g/L.Page 227Suggested Citation: “11 New and Emerging Drinking Water Treatment Technologies.” National Research Council. 1999. Identifying Future Drinking Water Contaminants. Washington, DC: The National Academies Press. DOI: 10.17226/9595.×SaveCancel
Because of their existing applications for water softening and seawater desalination, high-pressure membrane treatment is currently accepted by the regulatory community and the water industry as a reliable technology. The main obstacle to the increased application of high-pressure membranes in municipal water treatment is their high cost. By nature of the current modular design of membrane systems, economies of scale are not recognized for large treatment plants. However, several membrane manufacturers are currently modifying their membrane system designs to make them economically attractive a large scale.
Two-Stage Membrane Filtration
From the above discussion, it is apparent that low-pressure membranes are highly effective for particulate removal, while high-pressure membranes are effective for dissolved matter removal (both organic and inorganic). Conceptually, a combination of the two membrane systems in series (MF or UF followed by NF or RO) would provide a comprehensive treatment process train that is capable of removing the vast majority of dissolved and suspended material present in water. Such a treatment train is commonly termed “two-stage membrane filtration.” Other names include “integrated membrane systems” or “dual-stage membrane filtration.” The only material that is believed to pass through such a treatment train includes low-molecular-weight organic chemicals. However, compared to existing treatment, a two-stage membrane filtration process (possibly coupled with PAC addition) would produce far superior water quality. The main concern about such highly treated water is that it may be more corrosive. Special corrosion inhibition measures for low-TDS waters of this kind require further development.
Several studies have been conducted to evaluate two-stage membrane systems for surface water treatment (Wiesner et al., 1994; Chellam et al., 1997; Kruithof et al., 1997; Vickers et al., 1997). The results of these studies have clearly shown that MF or UF membranes are excellent pretreatment processes to NF or RO membranes and that the combined particulate removal and organic removal capabilities of this treatment scheme produce excellent water quality that complies with existing and forthcoming regulatory requirements.
The primary obstacle that a two-stage membrane treatment system needs to overcome is its cost. Lozier et al. (1997) estimated the capital cost of a 40-GPM, the two-membrane system at $4/gpd. The capital unit cost of a large-scale, two-stage membrane system may range from $2 to $3/gpd of capacity. This is still substantially higher than the cost of conventional treatment, which is estimated at $1 to $1.5/gpd.