ADVANCED SEPARATION TECHNOLOGY APPLICATION FOR NOM ...

ADVANCED SEPARATION TECHNOLOGY APPLICATION FOR NOM REMOVAL FROM A FRESHWATER SUPPLY ANDREA G. CAPODAGLIO1, ARIANNA CALLEGARI1, PHILIPPE SAUVIGNET2 1 University of Pavia, Via Ferrata 1, 27100 Pavia Italy 2Veolia Eau, Paris, France Abstract. Natural Organic Matter (NOM) in drinking water supplies is not known to have any direct effects on human health; however, it impacts significantly on supply water quality and ts treatment needs due to its reactivity with many dissolved and particulate species. Several technologies are nowadays used to remove NOM from supply water, including molecular sieving through nanofiltration membranes, coagulation with subsequent floc separation, oxidation followed by biofiltration and sorption processes including chemisorption (ion exchange), and physical adsorption (activated carbon). The evolution of water-related directives and progressively more restrictive standards for drinking water, however, call for the investigation of advanced, more efficient and cost-effective water treatment processes. The paper contains a brief overview on the state-of-the-art methods for NOM removal from supply waters, and describes a new technology, developed and patented by the Research Center of Veolia Environment, tested and validated at field scale on the supply water source of a town in Brittany (France). Keywords: Natural Organic Matter (NOM), coagulation, nanofiltration, adsorption, oxidation, flocculation

1. Introduction Natural Organic Matter (NOM), deriving from various sources, is commonly present in the water of surface and ground bodies: it can be microbially derived (autochthonous), result from leaching and extracellular release processes of algae and bacteria, originate from decomposition and leaching of plant and soil organic matter (allochthonous). NOM is therefore a complex, heterogeneous mixture of organic compounds, consisting of aromatic, aliphatic, phenolic, and quinonic structures with varying molecular sizes and properties. Due to this complexity and heterogeneity, the structural and functional characterization of aquatic NOM is extremely difficult. Commonly, it could be characterized into a hydrophilic and a hydrophobic fraction. The hydrophilic fraction includes, carboxylic acids, carbohydrates and proteins, the hydrophobic fraction includes humic substances (HS) (Crouè et al. 2000), a term referring to a broad class of interrelated compounds, including humic and fulvic acids. The compositions of HS varies from source to source with respect to solubility and reactivity (Aiken et al. 1985; McCreary and Snoeyink 1980). NOM classification cab be represented as in Figure 1 (Leenheer and Crouè 2003). Althought NOM is not known to have any direct effects on human health, it should be removed from drinking water for a number of reasons, it may in fact: affect properties of water such as color, taste and odor, that may render its consumption unpleasant; react with disinfectants used in water treatment, reducing their disinfection power and producing undesirable disinfection by-products (DBPs) and increasing disinfectants demand; affect process design, operation and maintenance; affect stability and removal of inorganic particles; modify coagulation conditions and coagulation process performance, and increase coagulants demand; affect corrosion processes; affect biostability and promote unwanted biological re-growth in distribution systems; form complexes with, or increase mobility of, chemical substances found in nature; foul membranes; reduce adsorption capacity of granular or powdered 2011 IWA Specialty Conference on Natural Organic Matter, Costa Mesa, CA, USA, July 27-29, 2011, www.nwri-usa.org\nom2011.htm

activated carbon (GAC/PAC) by pore blocking; compete with taste and odor compounds for adsorption sites in GAC/PAC (Eikebrokk et al. 2006).

Figure 1. NOM classification.

2. Drinking water nom removal: a current state-of-the-art summary In drinking water treatment applications, NOM can be removed through mechanisms including: GAC adsorption, coagulation, membrane filtration, and biological degradation. It can also be partially transformed through oxidation by advanced oxidation processes. Individual methods will be briefly analyzed in the following sections. 2.1. ACTIVATED CARBON FILTRATION The adsorption behaviour of NOM is difficult to study due to its heterogeneous nature (Newcombe 1999); some studies showed that this process is controlled predominantly by the relationship between NOM’s molecular size distribution and the pore size distribution of the carbon (Newcombe et al. 2002). Direct adsorption on AC is generally not recommended, since substrate capacity is quickly reduced by the pore blocking caused by large humic substance molecules. Coagulation prior to GAC filtration has sometimes proven to be capable to remove particles that might otherwise clog the filter. Precoagulation also removes a fraction of NOM, reducing therefore the load on GAC filters (Jacangelo et al. 1995). The lifetime of GAC filters can be extended by thermal reactivation, however, this can lead to the enlargement of the medium macropores caused by burn-off effects, and as a consequence increase the removal of the high molecular weight NOM and decrease that of low molecular weight (Boere 1992).

GAC adsorption can be preceded by water oxidation by ClO2: it has been shown that small doses of this compound can increase the molar masses of some NOM molecules and thus increase GAC adsorption of high M.W. NOM (Swietlik et al. 2002); however, it was also observed (Swietlik et al. 2004) that higher dosages of ClO2 may break-up some of the larger NOM molecules. Also, this may lead to the undesired formation of organic by-products due to interactions between GAC, NOM and ClO2. 2.2. COAGULATION Coagulation is probably the most commonly used method for NOM removal. The conventional process incorporates several physicochemical processes including rapid mixing, slow mixing (flocculation), sedimentation, filtration, and disinfection. Coagulation reactions take place almost instantaneously in the rapid mix stage of the water treatment process and continue until the water is filtered. The effectiveness of coagulation affects the efficiency of the subsequent sedimentation and filtration processes. Effective coagulation is achieved through addition of charged (or other destabilizing) species into the water source. This process may be accomplished using coagulants, the two most commonly used in practice being: hydrolyzing metal ions Al3+ and Fe3+, typically supplied as aluminum sulfate (Al2 (SO4)3·14H2O), and ferric chloride (FeCl3·6H2O). Aluminum sulfate, commonly referred to as alum, is the most widely used coagulant in water treatment processes. NOM removal in this case is due to several mechanisms which include double layer compression, charge neutralization, sweep coagulation, and inter particle bridging (Check 2005). Several factors influence the efficiency and effectiveness of coagulation by metal salts. These factors include, but are not and limited to, coagulant dose, pH, alkalinity, temperature, and ions present in solution. Ambient pH is critical in maximizing NOM removal effectiveness. Although maximum adsorption of both humic and fulvic acids occurs under acidic conditions, studies have shown that adsorption is the key mechanism involved in their removal over the entire pH range (Dempsey 1984). NOM removal by mineral adsorption occurs primarily due to Van der Walls forces or polarization arising from the rearrangement of macromolecules. Through this mechanism, polar moments in two adjacent molecules will cause a net attractive force. Hydrophobic humic molecules, the most easily removed by coagulation, are strongly influenced by physical absorption (Matilainen et al. 2002). There is general evidence that higher molecular weight NOM compounds are more easily removed than their low molecular weight counterparts. 2.3. MEMBRANE FILTRATION Nanofiltration (NF) technology has proved to be a successful alternative process for drinking water treatment, due to its superior removal of disinfection by-product precursors, minimal use of chemicals, reduction in sludge production, and potential for use in compact systems. The cost of this process, however, is at the moment higher than those of coagulation and GAC adsorption (Table 1); on the other hand, the rate of decrease in costs of this particular technology over the past years has been greater than that associated to other treatments, so that it might become economically competitive in the near future. NOM can be effectively removed by NF, and to a less extent, by tight-UF membranes through a combination of diffusion, convection, and electrostatic repulsion mechanisms. Dominant transport mechanisms of NOM through NF depend on the operating conditions as well as the size of solutes and pores. Typical pore size of these membranes is 1–5 nm, operated at the pressure of 4–8 bar (Ødegaard et al. 2006).

TABLE 1. Qualitative summary of selected aspects of some technologies used for NOM removal (Jacangelo et al. 1995). Treatment Process Coagulation GAC adsorption Nanofiltration

NOM removal efficiency Fair good Very good Excellent

Process complexity Low-medium Medium-high Medium

Process cost Low-medium Medium Medium-high

A typical flow diagram of a membrane filtration plant is shown in Figure 2: raw water passes through a pre-treatment unit, normally a micro-sieve with openings of typically 50 μm. After this, pressure is raised up to the operating pressure of the membrane unit. Cross flow filtration takes place in the membrane unit resulting in a clean water stream (permeate) that has passed through the membrane and a dirty water stream (concentrate) that flows back to atmospheric pressure through a reduction valve. Since the reduction of calcium and bicarbonate concentrations through the membrane is about 15– 30%, an alkaline filter (calcium carbonate) can be included in order to increase these levels. The NF process is often adopted when NOM content/color is high (>30 mg/l) and turbidity low (