State of the Art and New Opportunities
for Membranes in municipal Water Treatment
L. Durand-Bourlier*, K. Glucina*, I. Baudin* and P. Aptel**
*Lyonnaise des Eaux-CIRSEE, 38 rue du Président Wilson, 78230 Le Pecq, France. E-mail: , ,
**Laboratoire de Génie Chimique - UMR 5503, Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse cedex 4, FRANCE Toulouse. E-mail:
ABSTRACT
With more stringent regulations to maximize removal of particles, disinfectant tolerant microorganisms, natural organic matter which generates desinfection by-products and synthetic organic compounds, membranes as physical barriers are more and more applied for water treatment. The efforts of manufacturers and researchers to decrease the investment and operating costs have allowed membrane processes to compete with conventional processes even for very large capacity facilities (more than 100,000 m3/day). Membrane is a very flexible and reliable water treatment process, which makes it very attractive for engineers and operators. They can be used as a single process or in combination with conventional treatments such as coagulation, adsorption, oxidation or biological purification to meet additional treatment objectives. They are mainly used for drinking water treatment and to a lesser extent for waste water and reuse applications. Many research needs are already identified to still optimize the technologies.
KEYWORDS
Membrane, microfiltration, nanofiltration, ultrafiltration, reverse osmosis, water treatment.
INTRODUCTION
The application of membrane processes has greatly expanded in the past decades. First, reverse osmosis (RO) membranes, and in a lesser extent ion-exchange membranes for electrodialysis, were applied for the water desalination. The trend was the increasing part of membrane processes compared to thermal processes such as multi-stage flash (MSF) distillation. Then low-pressure membranes, microfiltration (MF) and ultrafiltration (UF) appeared on the market and with lower and lower costs and more and more stringent regulations in terms of produced water quality, their growth is assured. The nanofiltration (NF) membranes have a cut-off between UF and RO membranes (Figure 1) and became also very viable process for water treatment, especially for water softening.
The objectives of this paper is, after reminding some history, to describe the state of the art for each type of membrane process, the reason for their success and the new opportunities especially in terms of research and development trends for municipal water treatment applications.
Figure 4. Membranes cut-off threshold and corresponding separation processes
HISTORY
Reverse Osmosis (RO)
By the seventies a competing technology to the conventional distillation process began to be commercialized. The desalination of water is accomplished by developing semi-permeable membranes, which under pressure separate the inorganic salts from the water. This process is called reverse osmosis.
Numerous factors have contributed to the rapid growth of RO membrane application to treat water:
ü Development of new material (asymmetric membranes, then thin film composite (TFC) membrane)
ü Lower operating pressure so lower energy consumption (at least by a factor 4)
ü Improved specific flux, so enhancement of product flow
ü High salt rejection
ü Increased surface area (now up to 40 m2) and compacity (spiral wound modules)
ü Improved flow distributors
ü Optimization of pretreatment process (intermittent chlorination or chloramination, multi-media filtration, MF and UF)
ü Decreasing cost: spiral wound modules have decreased by almost 70% during the last twenty years (Furukawa, 1999).
Thus for single purpose facilities (where a thermal energy is available on the site), RO has lower capital and operating costs (Moch & May, 1997). The following graph illustrates the chronological evolution of RO cumulative capacity, which has caught up the MSF capacity. Today more than 10 million cubic meter are produced per day by RO especially for seawater and brackish resource water, which represents about 0.5% of the drinking water worldwide production (Aptel, 2000).
Figure 5. RO growth compared to MSF in terms of cumulative capacity (Furukawa, 1999)
Low pressure membranes (MF and UF)
MF and UF processes are also based on differential pressure, which is the driving force to separate some solutes. With a larger cut-off than RO membranes, MF and UF membranes stop all particles: mineral, organic such as algae and biological such as bacteria and Giardia or Cryptosporidium, but no dissolved inorganic compound. MF with larger pores than UF does not remove viruses as UF does. Virus removal by UF depends on the commercial product: some presents total removal (experimentally >7 log) whereas some others remove 5log or less.
Their clarification and disinfecting properties contribute to their larger and larger application for drinking water treatment and in a lesser extent for wastewater treatment. Further, as the technology has been optimized, it has become economically competitive with conventional processes up to a production capacity of more than 100,000 m3/day.
Since the first UF membrane plant using Aquasource membranes started in 1988 in France, UF has boomed: there are now more than 80 recorded Aquasource drinking water treatment plants worldwide (Figure 3). It should also be noted that now out of the low pressure membrane full-scale drinking water treatment plants identified worldwide, UF
Figure 6. Aquasource evolution for municipal drinking water treatment
applications represent more than 60 percent of the total installed capacity. The worldwide capacity is estimated at more than 2 million cubic meters per day for potable use. For all water treatment (including wastewater but above all industrial use) it is estimated at more than 3 million cubic meters per day.
Nanofiltration (NF)
NF membranes with an intermediate cut-off between UF and RO membranes are mainly used to soften waters. In fact, NF membranes remove divalent ions (Ca2+, Mg2+, SO42-...); they typically provide 80 to 95 % rejection of hardness. The advantage of softening with NF rather than RO membrane is mainly due to the lower pressure applied for NF process compared to RO process. NF membranes also remove dissolved organics (humic and fulvic acids) which are generators of disinfection by-products (DBPs) and some synthetic organic compounds (SOCs) such as some pesticides which cannot be removed by MF or UF used as single process without other treatment step.
Reasons of Membrane success
Water treatment objectives
Membrane as a single process. Membranes as physical barrier with different molecular weight cut-off (MWCO) can remove many contaminants and so they reach various treatment objectives as shown in Tableau 1.
Tableau 1. Summary of removal abilities for each type of membrane
Parameters / MF / UF / NF / ROTurbidity / X / X / X / X
Bacteria and cysts (Giardia, Cryptosporidium…) / X / X / X / X
Viruses / X / X / X
Color / X / X
Natural Organic Matter (NOM), DBPs / X* / X / X
Micropollutants (Pesticides, taste & odor causing compounds) / X* / X / X
Hardness / X / X
Sulfate / X / X
Specific monovalent ion removal (Fluoride, Nitrate, Arsenic…) / X
* Removal by CristalÒ process (Aquasource UF membrane coupled with powdered activated carbon (PAC)).
It should be noted that removal efficiency of microorganisms is not only linked to the MWCO of the membrane but also to the module configuration. For spiral-wound element, which is the most common configuration for NF and RO due its high compacity, the desinfection is not reliable because of seal use. Seals are also commonly used for immersed membrane (MF or UF) systems. The most reliable configuration is the hollow fibers module where the fibers are maintained by a resin layer (e.g. epoxy). This configuration is illustrated by the Figure 4.
Figure 7. Hollow-fiber membrane modules of Aquasource (surface area: 55 & 64 m2)
Membrane processes can be used on their own but can also be coupled with other type of treatment
Low-pressure membrane combined with other treatment processes. To meet additional water quality criteria, low-pressure membrane can be coupled with conventional treatment process such as coagulation, activated carbon, oxidation or biological activity.
The CRISTALÒ process (Combination of Reactors, Including membrane Separation Treatment and Adsorption in Liquid) couples adsorption onto powdered activated carbon (PAC) with Aquasource UF through a hollow fiber membrane (Baudin & Anselme, 1995). To remove organic compounds, this process combination is a viable alternative to more conventional methods like granular activated carbon (GAC) filtration or ozonation. The membrane provides a physical barrier preventing the passage of the PAC, so retaining the organic compounds adsorbed on the PAC, which otherwise would not all be trapped by the membrane. These compounds include organic matter, pesticides, compounds responsible for taste and odor, precursors of DBPs, etc. CRISTALÒ combines the advantages of UF and of PAC adsorption. This process can be applied directly to raw water, or as a polishing treatment. More than 15 full-scale plants using this process are in operation with capacities up to 65,000 m3/day.
An other innovative treatment combination has been proposed for the water sources which present high variations and high level of organic matter: coagulation including a sludge blanket clarifier step prior to the UF process. The settling pretreatment smoothes the variations of the surface water quality, which could cause some fouling of membranes. Thus for the San Antonio (Texas, US) drinking water treatment plant of 34,000 m3/day capacity, the selected process was the combination of a SuperpulsatorÒ (clarifier of Degrémont) and Aquasource UF membrane among 3 membrane competitors (Durand-Bourlier et al., 2001). It should be noted that PAC can be added upstream to the SuperpulsatorÒ and/or in the hollow fiber (CristalÒ process) to polish the treatment (micropollutants removal such as pesticides or taste and odor causing compounds). Another advantage of this treatment chain is the reduction of water loss to less than 1% because the backwash water of the UF process is recycled into the clarifier.
For biological purification, membrane bioreactor (MBR) has also been developed especially for municipal or industrial wastewater treatment but also in a lesser extent for drinking water with the Biocristal-DNÒ process. This process is a combination of a biological reactor with Aquasource UF (to separate off solids and disinfect) hollow-fibers in which PAC is added to adsorb also micropollutants such as pesticides. The example of the industrial scale drinking water plant at Douchy shows how useful and advantageous this denitrification process is (Urbain et al., 1996). The UF Douchy plant, in operation since 1989, was easy to modify by adding a bioreactor to make up for increased nitrate concentrations in the karstic feed water. This way it is possible to treat the nitrates while keeping the advantages of UF to reduce turbidity and disinfect the water. MBR is especially attractive for wastewater treatment where space and efficiency requirements are at a premium.
High-pressure membrane combined with other treatment processes. NF and RO operation often requires extensive pretreatment, especially for the spiral-wound configuration and for surface water treatment, in order to avoid membrane fouling and scaling, and to control the membrane life. The pretreatment often included a first step of pH adjustment, chlorination, coagulant addition, sedimentation, clarification, dechlorination, possibly adsorption onto activated carbon, often complexing agent addition, another pH adjustment and final polishing. This pretreatment train is a rather complex process, so the feed stream of membrane can vary significantly which make difficult the facility operation. Moreover, due to the membrane material, which is often very sensitive to chlorine, the disinfectant must be neutralized very carefully.
A recent and attractive treatment combination has been developed to solve these difficulties: MF or UF membrane before NF or RO process. Low-pressure membrane guarantees a reliable feed-water without particles and so ensure smooth operation of high-pressure membrane.
Flexibility and reliability
Flexibility. As seen before, membranes can achieve various treatment objectives and many others when coupled with other type of treatment (e.g. coagulation, adsorption...).
Thus they can be easily integrated in existing plants to upgrade the facilities. For example, CristalÒ process has been successfully applied at the Vigneux plant in France, 55,000 m3/day, as a polishing treatment to upgrade the facility, which treat the Seine River. TOC has been reduced to less than 1 mg/L at the outlet of the facility, thus reducing the production of DBPs in the distribution network (trihalomethanes, have decreased from an average of 50 mg/l to less than 10 µg/L). This reduction in organic content has also allowed to reduce the chlorine consumption, but also means that a higher chlorine residual is maintained throughout the distribution network even for the furthest distributed point (Baudin et al., 2000). Pesticides and their ozonated by-products are removed by the PAC in the hollow fibers. Reduction of taste and odor causing compounds has also been observed.
As membrane processes are modular equipment, they are easily adaptable to variations of flow rate and water quality. As they are compact, they can be easily applied where the available building area is limited. It is also important to note that the equipment is automated which makes easier and more reliable the operation.
Reliability. As physical barrier, membranes provide good and constant quality. Despite of the high variability of certain surface water quality like storm events which can causes high turbidity up to 500 or 1000 ntu, MF or UF membrane will still insure a permeate turbidity below 0.1 ntu. The treated quality in terms of particle and microbial removal is independent of the raw feedwater quality. In comparison, conventional clarification and disinfection processes have limited efficiency and reliability which depends on the resource quality variations and on the plant operating conditions, such as reagent concentration, pH, temperature, contact time, and hydraulic flow pattern in the reactors.
However membrane efficiency must be regularly checked. In case of fiber breakage or default, the membrane barrier is not still intact, consequently monitoring is needed. There are various methods to control the integrity of modules. Some are indirect methods such as turbidity or particle counting for low-pressure membrane or conductivity analysis for high-pressure membrane. Others are direct methods such as bubble point measurement, air pressure tests with various procedures of measurement and sonic or acoustic sensors. Most of these methods can be automated on the installation. The monitoring is continuous or activated at the wanted frequency depending on the integrity test. Obviously sensitiveness of these various tests can be different.