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Particle Separation

Particles in water play an important role in all kinds of water quality and treatment issues. Since the early beginnings of centralised water production and treatment, the main goal of water purification was primarily the removal of water turbidity in order to produce clear water free from visible particles. Although the connection between hazardous organisms in water and diseases caused by water consumption was only proven in the second half of the 19th century (John Snow and others, see figure I), experience learned human civilisation to treat water especially from surface water sources by removing the turbidity. Sedimentation in ponds and large pots, filtration through porous materials and textiles and other means of water treatment were applied in smaller communities and single households. With population growth and settlements of larger cities, water treatment and disease control was often neglected. Especially in medieval civilizations the idea that diseases would originate from (drinking) water was lost to a great extent.

Only the discovery and recognition of pathogenic micro-organisms in water and their distinct effects on humans and animals during several water-born epidemics in (the first mega) cities in the World, created by explosive growth due to industrialisation around 1850, brought the idea of hygiene in daily life back to the population. Pushed by medical and pharmaceutical specialists, water filtration in central treatment plants, water boiling in households and after 1900 also centralised water disinfection were introduced to produce safe drinking water. The necessity of controlled discharge of particle-rich human wastewater and treatment of it was recognised as well. Of course, particle removal and the transfer of soluble material into biological particles play an important role also in wastewater treatment. Around 1940, so called primary treatment was applied consisting of simple sedimentation facilities to remove about 50% of the particles contained in raw sewage. Later, biological treatment was introduced where biological flocculation and the removal of the activated sludge is an important solids separation step for small colloidal particle fractions.

Figure I: John Snow’s submission to human health (from Van Nieuwenhuijzen, 2002)

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Content Table

Handbook on Particle Separation Processes

Advances in particle removal in water and wastewater treatment are still important developments in understanding and optimising treatment processes and concepts. In 2008 a major summer course on this topic was organized by Delft University and UNESCO-IHE Delft, more than 30 years after the first International Summer Course on Particle Separation, in 1977 in Cambridge. The Academic Summer School on Advances in Particle Separation in Water and Wastewater Treatment is a selective symposium and workshop for invited lecturers and participants only, organised under supervision of the IWA Specialist Group Particle Separation. The purpose of the Summer School was to exchange knowledge and expertise from a selected group of honourable experts with a long track record in the field of particles and particle separation in water and wastewater treatment to emerging new and young specialist in this area. Since the older generation of experts is more and more retiring, information exchange is essential to provide future experts with available expertise.

The IWA Handbook on Particle Separation Process (Van Nieuwenhuijzen and Van der Graaf, 2010) provides an overview of the latest developments on particle separation in water and wastewater treatment. This book has been edited from the presentations and workshops held at the Academic Summer School Particle Separation in Water and Wastewater Treatment.  The purpose of the Handbook is to provide knowledge and expertise from a selected group of international experts with a wealth of experience in the field of particles and particle separation in water and wastewater treatment. The book contains material ranging from Methods and Instrumentation for Particle Analysis and Characterisation (by Prof. Markus Boller), over Natural Organic Matter: Particles, Colloids and Macromolecules (by prof. Gary Amy), Nom Removal Technologies (by Prof. Hallvard Ødegaard), Several Physical Chemical Treatment Technologies (Prof. Yoshima Watanabe), Filtration Characteristics of Wastewater Treatment by Prof. Jaap van der Graaf and Flotation for Particles Removal by Prof. Mooyoung Han. Other topics like Modelling Particle Removal (Prof. Desmond Lawler), Particle Separation in Drinking Water Treatment (Prof. Rolf Gimbel) and Nanoparticles in Water and Wastewater (Prof. Mark Wiesner) are presented in other gremials and publications.

Focus on particles

It becomes obvious that the removal of particles from water and wastewater is crucial for safe potable water production and efficient wastewater treatment. If particles are present in a water source, it is the primary issue of all purification techniques to eliminate or inactivate the particles and with them also eventual hygienic hazards.

For several reasons particles represent undesired pollutants in most product waters. Apart from the mass of suspended matter as often used bulk parameter, many other quality indicators are strongly associated with particles such as hygienic contaminants and adsorbed chemicals. On one hand, particles may negatively interfere in various treatment processes and supply systems, on the other particulate matter in the form of biomass is a necessary prerequisite in many water treatment schemes. The removal of particulate matter was and will be one of the most crucial steps in water and wastewater treatment. In order to understand the behaviour of particles in water and to develop and design efficient treatment facilities, the characteristics of particles has to be known on the basis of individual solids and of whole particle populations. In water treatment, particles are of extremely heterogeneous nature with respect to size, density, shape, chemical composition, shear strength, surface charge, etc. which represent information that is in most cases not available. The conference aims at gathering knowledge on particle characterisation by presenting research studies including results with innovative new instruments and methods.

In order to understand the role of particles in water quality evaluation and water purification and wastewater treatment processes, the particles and their behaviour in aqueous systems have to be known and characterized.

Occurrence of Particles in Water

Solids in water are of very different origin and appear in a large variety of sizes, shapes, chemical composition, etc. An incomplete list of particles in water is shown below and illustrates the complex nature of aqueous solids.

Domestic Wastewater

Coarse solids    gravel, sand, faeces, paper, hair, cotton, wood pieces, plastic pieces (e.g. ear sticks), cigarette-ends, etc.

Fine solids faeces, road dust, atmospheric solids deposits, aggregates of micro-organisms, single micro-organisms, worm eggs, viruses, biological debris, clay minerals, chemical precipitates, nanoparticles

Industrial wastewater

great variety according to production and treatment processes cellulose fibres, asbestos fibres, incineration ashes, precipitation products, pigments, emulsified oil droplets, polymers, coal dust, silicates, metals, blood, milk, etc.

River water

Gravel, sand, silt, clay, algae, decay products of plants, protozoa, bacteria, viruses, etc.

Lake water

Phytoplankton, zooplankton, detritus, precipitation products, bacteria, viruses, protozoa, etc.

Groundwater/Spring water

Precipitation products (Fe, Mn, Ca), soil colloids, bacteria, viruses, protozoa, etc.

Potable water

nano-size particles, bacteria, viruses, corrosion products, calcite particles, particles of natural organic matter (NOM).

There are many analytical parameters to express the solids content in water. Among the most frequently used ones are:

TSS                  

Total suspended solids: expressing the dry weight of the filtered solids mass (0.45 µm membrane filtration) in gTSS/m3

POC                 

Particulate organic matter as difference between TOC – DOC (0.45 µm filtered) in gC/m3

COD                 

Particulate organic matter as difference between CODtotal – CODfiltered (0.45 µm filtered) in gO2/m3, (COD = chemical oxygen demand)

VSS                  

Volatile suspended solids: expressing the organic combustible part of TSS

TS                    

Total solids: total solids mass after drying the water sample (including mass of dissolved matter) in gTS/m3

Turbidity          

Light absorption or light scattering through a water sample; compared to standard turbidity of defined particles expressed in NTU (nephelometri turbidity), FTU (formazine turbidity) and others.

Particle no.     

 Number of particles per volume (number per ml or L), usually with laser methods; size range of the analytical method has to be indicated, e.g. 0.5 to 200 µm or 1 to 1,000 µm for conventional particle counters.

While most of the above parameters do not give much information on particle characteristics, detailed analysis of particle numbers include the evaluation of the particle size. A coarse indication of the size range of some well known particle classes is given in Fig II. It becomes clear that the standard procedure for solids analysis by filtering water samples with 0.45 µm membrane filters is not an accurate procedure to define solid and dissolved matter. There is still suspended matter in the size range below 0.45 µm which would have to be clearly classified as particulate.

Figure II Size categories of aqueous particles (Van Nieuwenhuijzen, 2002)

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Figure III Example of particulate matter in municipal wastewater (Van Nieuwenhuijzen, 2002)

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Particle Separation Processes

In most treatment trains for water treatment, particle separation is the first treatment step. There are different alternatives for solids separation and their beneficial application depends strongly on the quality of the water or more precise on the characteristics of the particle suspension to be treated. The processes most widely applied in water treatment are:

-           sedimentation

-           flocculation

-           flotation

-           granular media filtration

-           contact filtration

-           screening, straining

-           membrane filtration

-           NOM-removal technologies.

Among these processes flocculation is not really a solids separation process but it helps to improve solids separation considerably by particle agglomeration. A primary factor for the decision which processes are suited under which conditions, is the solids concentration in terms of mass quantity, volume or number concentrations. On the other hand, the particle size is an important parameter which determines the removal mechanisms which are best be promoted in order to achieve optimal solids separation performance. Based on the relationships depicted in IV, different areas can be identified in which certain solids separation processes are best be applied.

Figure IV. Appropriate action fields of solids separation processes (Boller et al., 2010)

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Figure V. Particle size fractionation (Van Nieuwenhuijzen, 2002)

Particle size fractionation of waters can be of interest for selection of an appropriate separation technology. Figure V presents a standardised method for particle size characterisation developed by the author.

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Removal of particles > 30 µm

Particles > 30 µm can be removed by screening processes such as microstrainers. The smaller range of mesh sizes is in the order of 8 µm. Also a granular media filter may remove particles > 20 µm completely. The combination rapid filter/slow sand filter can reach a particle removal efficiency of 100% for particles >10 µm. Screens and granular filters are subject to clogging and need to be cleaned regularly. At higher concentrations, sedimentation for particles with a higher density and flotation for particles with a low density are more suited. Filters and screens may be used below concentrations in the order of 50 mgTSS/l. At higher concentrations sedimentation or flotation are necessary.

Removal of particles between 0.5 µm and 30 µm

Many important particles in surface waters are in the size range between 0.5 and 30 µm such as bacteria, blue and green algae, diatoms, and partly also inorganic particles. In this size range orthokinetic flocculation (particle transport by shear) has to be applied in order to agglomerate the particles to larger flocs and thus enable removal by sedimentation, flotation or filtration. A combination with sedimentation or flotation is suited at concentrations above 50 mgTSS/l or filtration or contact filtration at concentrations below this value. Also membrane filters such as micro- and ultra filters may cope with this particle class. In full scale, membrane filters are usually protected against too large particles by screening filters with openings in the order of 20 µm.

Removal of particles < 0.5 µm

The smaller particles in the nano-size range, the more difficult it is to describe their behavior in water. Viruses and some types of inorganic precipitates such as calcite and iron (hydr)-oxides typically belong to this size range. From particle analysis in this size range, it becomes clear that the number of particles is increasing with decreasing size. Particle numbers of nano-sized colloids in natural waters may easily reach 108 to 109/ml. In addition to the mentioned colloids, decay products of suspended organic material are present in large amounts. In future also an increasing number of synthetic nano-particles in wastewater, natural water and drinking water have to be expected.

The aggregation of submicron particles is relatively fast if their surface chemistry is suited and if their concentration is high enough (> 108/ml). The transport of submicron particles for aggregation is brought about by Brownian motion, also know as perikinetic flocculation. Often the agglomerates are still small and cannot be removed by sedimentation or filtration, further agglomeration with the help of orthokinetic flocculation, contact filtration or sludge blanket clarifiers is necessary.

The particle class < 0.5 μm can efficiently be removed by ultra filtration. Again pre-filters with openings of 10 to 20 µm are applied. In view of the small pore size of ultra filters in the order of 0.01 µm, viruses and other colloids may be removed completely by ultra filter membranes. Ultra filters may cope with particle concentrations up to more than 1000 mgTSS/l. However, membrane fouling caused by accompanying organic material such as polysaccharides, proteins and humic substances can limit the application of ultra filters to much lower concentrations in the order of 100 mgTSS/l and lower.

Flocculation

This chapter summarized the previous researches on the flocculation conducted by Tambo and Watanabe, and on the inclusion of flocculation in the monolith ceramic membrane filtration conducted by the authors. The floc density function describes the quantitative relationship between the size and effective (buoyant) density of flocs. The exponent Kρ in the function is related to the fractal dimension (D) for the aggregates formed in Cluster-Cluster Aggregation (CCA) as D = 3 – Kρ.  The Kρ is a function of the ALT ratio and has the numerical value of 1.00 and 1.25 for the ALT ratio of around 1/100 and 1/20, respectively. These values coincide with the D value determined for the reaction and diffusion limited case (2.05 and 1.75), respectively, determined in the field of the Fractal Physics. Based on the dimensionless flocculation kinetics derived by Tambo and Watanabe, the GC0T value has been proposed as the design criterion of flocculator and its usefulness was clearly demonstrated in this paper.

The authors have also clarified the characteristics unique to monolith ceramic membrane with pre-coagulation by referring to the behaviour of micro-particles. The region exists in the monolith channel with the optimum G and GC0T value for good flocculation. The flocculation of micro-particles reduces the membrane fouling. If pre-coagulation and chemically enhanced backwashing are included in the monolith ceramic membrane filtration system, extremely high filtration flux is possible for a long operation period.

Flotation

DAF plants have several advantages compared to conventional sedimentation plants. DAF is more efficient than sedimentation in removing particles (turbidity). Lower particle loading to the filters yields longer filter runs and high filtered water production. Integrating this into the design of new plants allows for filters to be designed at higher rates.

DAF plants have a smaller footprint compared to conventional sedimentation and high rate plate and tube sedimentation plants. Shorter flocculation times reduce the size of the flocculation tanks. Another advantage is that DAF has been shown to be more effective than sedimentation in removing Giardia and Cryptosporidium (Edzwald et al 2000, 2003). Finally, the sludge solids from DAF have a higher percentage of solids than sedimentation, thereby reducing sludge treatment and disposal costs.

Below, we provide a list of some research needs. This is not meant to be comprehensive, but rather point out some important areas for additional research. The first has to do with making positively-charged bubbles. Coagulants are used in drinking water treatment to produce flocs of little or no electrical charge to enhance bubble attachment. For water supplies low in natural organic matter, the primary objective of coagulation is altering the negative particle charge. Eliminating coagulant addition to the raw water flow may be possible by adding chemicals to the recycle flow to produce positively-charged bubbles (Han et al, 2006).

A second need has to do with optimising and controlling bubble size. DAF produces bubbles over a wide range of sizes from 10 to 100pm, depending on the saturator pressure and nozzle type and design. Optimising the bubble size could improve treatment efficiency and also reduce energy consumption.

DAF is known to strip out some taste and odour compounds. A third need is to quantify what compounds are removed and identify how the process can be optimised. A fourth need has to do with integrating DAF as a pre-treatment for membrane processes, especially reverse osmosis and nanofiltration. To prevent fouling of membranes and to make the membrane plants more cost effective, pre-treatment is often necessary. DAF could be used to reduce particle fouling and fouling from algal polysaccharides. Finally, research is needed to reduce the energy consumption from producing air bubbles or to utilize the physical characteristics that might occur when bubbles break for the removal of microorganisms or micro pollutants.

Particle removal with advanced flotation

In this study, the water quality of first-flush runoff and stored rainwater, as well as the efficiency of removal and behaviour of particles in a rainwater storage tank, was investigated using three rainfall events that had different histories of previous dry days and rainfall volume. Several considerations to be born in mind while designing a rainwater storage tank were suggested.

Turbidity of first-flush runoff was over 100 NTU in the case of long antecedent dry days (13 days) and slight rainfall (6 mm), but, within 1 hour, runoff turbidity decreased to under 30 NTU. Right after runoff stopped, the initial turbidity of stored rainwater in the tank was 2.1~11.2NTU for the three rainfall events. As antecedent dry days increased and the amount of rainfall decreased, so turbidity increased. The pH was high due to contact with the marble terrace. The average particle size in all cases was 8~10㎛.

The main tank, which consists of 2 rooms of 125 m3, was operated at a fixed water level, without inflow and outflow, after stopping the access of runoff to examine the removal efficiency and behaviour of particles during sedimentation. The removal rate increased regularly according to retention time for stored rainwater of around 2 NTU. However, the removal rate increased rapidly early on, but then increased gently, for stored rainwater of 6~11 NTU. The particle number and peak of PSD in stored rainwater decreased according to the retention time.

The removal efficiency was increased by having a considerable distance between inlet and outlet, even when there were long antecedent dry days and little rainfall. If possible, it is recommended to design the effective water depth to be over 3 m, and to supply rainwater near the water surface by using a floating suction.

Jet-Mixed Separator

The JMS is an economical and effective solid-liquid separator with the phenomenon of simultaneous flocculation and sedimentation. It has porous plates inserted vertically in the channel perpendicular to the flow direction. The water passes through holes in the plates, thus creating jets which gently mix the water on itself resulting in the promotion of the flocculation of suspended particles. According to the local velocity measurement, it was demonstrated that large-scale eddies in the vertical plane are almost absent in the JMS. Therefore, larger flocs can settle in the JMS. The JMS incorporated with inclined tube settlers was applied to the rapid sand filtration system instead of the combination of mechanical flocculator and sedimentation basin. In a hydraulic detention time of less than 1 hour, the effluent turbidity from the JMS was below 1 TU. The JMS without inclined tube settlers was applied to the municipal wastewater treatment where it was used as pre-treatment process to the RBC. The JMS produced an effluent with low concentration of suspended solids and TOC at a hydraulic detention time of less than 1 hour.

Membrane filtration (effluent particle characterisation)

The Specific Ultra filtration Resistance (SUR) is proposed as a new parameter for measuring the filtration characteristics of effluent in dead-end ultra filtration. The SUR is calculated from the filtration data measured with a lab-scale device. The SUR is calculated from the ratio of filtration time and filtrate volume (t/V) as a function of the total filtrate volume and is the product of the specific cake resistance and the solids concentration. The process conditions during measurement affect the SUR of the effluent, therefore the SUR is defined at a constant TMP of 0.5 bar and an effluent temperature of 20°C. The SUR has an accuracy of more than 95% and is measured within 30 minutes of filtration.

The experiments on the SUR revealed also additional information about the filtration characteristics of WWTP-effluent. The MWCO of a PES/PVP membrane did not influence the SUR. This indicates that effluent constituents larger than the pore sizes determine filtration characteristics. Experiments with varying Trans Membrane Pressure showed that the occurring fouling layer is highly compressible (s = 0.6-0.75).

This implies that the fluxes should not increase too much, as the accompanying TMP increases more than linear. Pre-treatment induced an increase in filterability of 20% to 30%. Both pre-filtration and coagulation influenced mainly particles larger that 5-10 μm. This relatively small increase in filterability by pre-treatment indicates therefore that the filterability is only partly determined by particles larger than 5-10 μm.

The SUR was measured for effluent of various WWTP’s in the Netherlands and showed great variations for the different WWTP’s. The SUR was found to range from 5•1012 to 30•1012 m-2. Although the effect of pre-treatment on the filterability (measured as SUR) was relatively small, the change in filterability can be measured accurately with the SUR. In these tests coagulation as well as multi-media filtration showed a decrease of the SUR (of approximately 20% to 30%), greatly depending on the local conditions.

Membrane Bioreactor (sludge particle characterisation)

An modification of the SUR for membrane bioreactors (MBR’s) is the Delft Filtration Characterisation methode (DFCm).  For research into fouling in MBR Delft University of Technology has developed the Delft Filtration Characterisation method (DFCm), described in detail by Evenblij et al. (2005). With the DFCm different activated sludge samples can be filtrated with the same membrane and under identical hydraulic circumstances. In this way differences in filterability can be related exclusively to differences in sludge characteristics. The DFCm consists of a small scale filtration unit and an accompanying measuring protocol.

The heart of the filtration unit is a single tubular side stream X-Flow UF membrane with a length of 95 cm, a diameter of 8 mm and a nominal pore size 0.03 µm. A peristaltic pump circulates an activated sludge sample through the system; the cross-flow velocity in the membrane tube is fixed at 1 m/s. Another peristaltic pump is used for permeate extraction; permeate flow rate can be adjusted by tuning the pump speed. The permeate production is measured in time with a mass balance, so the flux J (l/m2•h) can be calculated. Using three pressure gauges (feed, concentrate and permeate) the transmembrane pressure TMP (bar) during an experiment is monitored. The viscosity η (Pa•s) of permeate can be assumed equal to pure water. From these three parameters the filtration resistance R (m-1) can be calculated according to Darcy’s law: R = TMP / (η•J).

Fouling is a complex process that can be analysed from different points of view. The DFCm can not cover all aspects of fouling that will occur in a full-scale installation. Therefore it is important to identify the possibilities and the limitations of DFCm.

Table I Typical ranges of different fouling rates (based on Kraume 2007)

Fouling “form”Fouling rate [mbar/min]Time intervalCleaning
Cake fouling0.1 – 110 minutesMechanical
Residual fouling0.01 – 0.11 – 2 weeksMaintenance
Irreversible fouling0.001 – 0.016 – 12 monthsRecovery
Long-term irreversible fouling0.0001 – 0.001Several yearsNot applicable

The shortest-term fouling mechanism is cake formation, which is only combated by mechanical measures. Apart from creating turbulent flow conditions near the membrane surface through coarse bubble aeration also relaxation and back flushing can be used to prevent or remove cake layer fouling. Periodical chemical cleaning of the membranes is indispensible to maintain sufficient permeability on a longer term. Depending on the system configuration low-intensive maintenance cleanings and high-intensive recovery cleanings can be carried out. Fouling that can not be removed by any physical or chemical cleaning measure is referred to as irrecoverable or long-term irreversible fouling. Ultimately the irrecoverable fouling determines the lifetime of a membrane (apart from other forms of damage).

The DFCm is not capable of covering long-term fouling phenomena; it only provides insight about cake fouling. Though on first sight DFCm thus only seems to cover a small part of the total fouling spectrum, it is however very important to have insight in the cake fouling potential of MBR activated sludge. In the first place low cake fouling offers room for optimisation of the membrane operation; when the filterability is good, energy and thus costs can be saved concerning relaxation time, back flushing and/or coarse bubble aeration rate. Besides this it is not improbable that there is a relation between short-term and long-term fouling rates. This can be investigated by comparing short-term DFCm results with long-term permeability data from investigated plants.

Direct membranbe filtration of wastewater

Water sources are depleting, both in quality and quantity, leading to stricter discharge policy and increasing number of wastewater reuse projects all around the world (Bixio et al., 2004).

Because of its specific characteristics, membrane separation plays a significant role in water reuse: membranes provide both rejection of harmful pathogens and clear filtrate. As reported also by Wintgens et al. (2005), currently membranes are applied to the treatment of municipal wastewater mainly in MBR’s and in MF/UF filtration of effluent, eventually followed by reverse osmosis (RO). In both cases, a biological treatment precedes the application of membranes, aiming to remove COD (chemical oxygen demand), BOD (biological oxygen demand), and nutrients (Nitrogen and Phosphorous).

However, membrane separation is a purely physical process that may be considered as a stand-alone process as well. The direct application of a membrane to wastewater would have the following characteristics:

1)      The typical limitations of biological processes (influence of temperature, feed stability and toxicity, start up period etc.) would be avoided;

2)      Wastewater constituents would be “separated” rather than “removed”.

And in facts,  this process has also been referred to as Direct Membrane Separation (DMS) (Ahn et al.,2001).

In the frame of the worldwide interest for the development of novel applications for membrane processes, the concept of direct ultrafiltration of wastewater with crossflow tubular membrane has been explored.

The application has great potential with regard to water recycling and sanitation, especially because being a purely physical process, it can produce water on demand starting from common wastewater.  The main applications could be irrigation and advanced pre-treatments, especially for the production of high quality water with dense membranes and nutrients recovery.

The research at TU-Delft has shown that the process is technically feasible, with sustainable fluxes in the order 70-80 l/m2h. This makes the application virtually economically reasonable. The cost of water extraction is indeed estimated below 0.30 eurocents/m3.

The application should now be evaluated at pilot scale for continuous operation. Meanwhile, a deeper analyses of the permeate characteristics should be carried on, to evaluate in details treatment and reuse path.

It must be remarked that the application of simple pre-treatments, such as sedimentation and flocculation, does not affect significantly neither fouling formation nor permeate quality, and therefore appear useless.

Perspectives and recommendations on NOM-removal

Natural organic matter (NOM - with its main constituent humic substances), has several negative influences in water that is to be used for water supply and needs therefore to be removed. A better understanding of NOM character and its removal by various treatment methods is essential to improve treated water quality meeting increasingly stringent standards. In order to have a better insight into the types of organic compounds present before and after different water treatment processes several characterization techniques have been developed worldwide. These have provided considerable knowledge in understanding the impact of NOM on treatment processes.  The characterization techniques differ considerably in terms of analytical approach, NOM fractionations or components analyzed, time and skills required, costs, and the form of the output or results (whether it can be interpreted easily and used by the treatment plant operators).

Comparative analysis of different methods of characterization of NOM has clearly shown that there is no single method which can fully reveal its characteristics that are important for water treatment practice. It is obvious that use of combinations of different methods would be required for proper analysis of the fate of different fractions of NOM during different treatment processes. However, the methods of characterization to be applied under given conditions depend on the source of NOM and treatment methods applied.

In the absence of high skills and costly instruments, tracking DOC and SUVA changes along the treatment process train could be a basic approach in understanding the removal of NOM.  High pressure liquid chromatography using gels have proved useful in combination with UV/vis, fluorescence, light scattering and sensitive dissolved organic carbon detection techniques, yielding information on molecular absorbance, size distribution, molar mass and reactivity. Information on biodegradability of NOM can be deduced from experimental measurement of bacterial growth under defined conditions. The nature and amount of biologically assimilable organic carbon (AOC) in combination with the bacterial cell number and growth rate constants can provide a meaningful characterization of microbial stability in aquatic systems. The characteristics of humic substances (MW, charge, hydrophobic and, aromatic nature etc) give the opportunity of several removal methods:

(1)        Because of the large molecular size NOM may be removed by molecular sieving, i.e. filtration through NF membranes. According to the Norwegian experiences, predominantly with cellulose acetate membranes (typically 3 nm effective pore size), the plants should be designed for a moderate flux (< 20 LMH) and recovery (< 70 %) and operated with daily light cleaning for fouling control. NF is suitable when the NOM concentration and color is high.

(2)        Because of the charge and colloidal nature, NOM can be removed by coagulation and floc separation. Coagulant dose and pH of coagulation are the two most important factors for achieving optimal treatment result. In most cases, the maximum residual metal concentration level (0.15 mg Me/L) determines the required coagulant dose level. Contact filtration are often used for raw water color levels up to about 50 mg Pt/L and turbidity levels less than 1-2 NTU. Above this a pre-separation step (settling/flotation) is recommended.

(3)        The color of NOM may effectively be removed by ozonation (or another strong oxidative method). Oxidation has to be proceeded by biofiltration in order to take the growth potential out of the water. Typical O3-dosages are 0.15-0.20 mg O3/mg Pt or 1-1.5 mg O3/mg TOC. Necessary biofilter EBCT is around 20-30 min. Ozonation/ biofiltration is only adviced for relatively low color levels, typically below 35 mg Pt/l. Otherwise the biogrowth potential created by the ozonation may be too high for the biofilter to handle.

(4)        Sorption processes are less used. GAC adsorption as the only process is unsuitable because of pore blocking resulting in low capacity and short filter runs. GAC may, however, be suitable in combination with pre-ozonation. Ion exchange (based on macroporous anion exchangers) is used in small plants, but is only recommended at relatively low raw water color levels, typically below 30 mg Pt/L.

References

Ahn, K.-G. Song, I.T.-Yeom and K.Y.Park (2001) Performance comparison of direct membrane separation and membrane bioreactor for domestic wastewater treatment and reuse Water Science and technology: Water Supply 1, no 5-6 pp.315-323

Amy, G., S.K. Sharma, S.G. Salinas Rodriguez, S. A. Baghoth and S.K. Maeng (2010), Characterization Profiling of NOM - as a Basis for Treatment Process Selection and Performance Monitoring, Handbook on Particle Separation Processes, IWAPublishing Editor(s): Arjen Van Nieuwenhuijzen and Jaap Van der Graaf, ISBN: 9781843392774

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Resources

This article was authored by:

Dr.ir. Arjen F. van Nieuwenhuijzen, PhD

Witteveen+Bos Consulting Engineers

PO Box 233 - NL 7400 AE Deventer

a.vnieuwenhuijzen@witteveenbos.nl

www.witteveenbos.com

The issues in the article are covered in more detail in his up and coming title, 

Handbook on Particle Separation Processes, edited by Arjen Van Nieuwenhuijzen and Jaap Van der Graaf.

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