The invention relates to water treatment methods and systems, particularly for the treatment of surface water and/or water from natural sources, most particularly water with a high natural organic matter (NOM) content. Such waters can be found in many places through the world, for example, Scandinavia, Scotland,

Mediterranean countries, Australia, North America and Siberia. Surface water is a common drinking water source in many parts of the world. As an example, approximately 90% of the population in Norway consumes drinking water which originates from surface water (80% lakes, 10% rivers). Due to the natural conditions (e.g. peat deposits), such waters often contain a high content of natural organic matter. This results in high colour, very low turbidity, low alkalinity and low hardness as well as a low buffer capacity. Such waters typically have a pH of around 6-7 and are poorly buffered (have low mineral content). Another common source of drinking water is ground water. Similar characteristics can also be found in some ground waters. Although the main part of NOM is not harmful, some fractions can cause colour, taste and odour problems or can even be toxic (e.g. algae toxin microcystin). Also, certain fractions of NOM in drinking water can lead to bacterial regrowth in the distribution system. NOM is a precursor for disinfection by-product formations that are known to be carcinogenic (e.g. during chlorination or ozonation) and should preferably therefore be removed during drinking water treatment. NOM is also known to bind microorganic pollutants (e.g. pesticides, herbicides, pharmaceuticals etc.) and heavy metals. The removal of NOM in drinking water production is therefore necessary to eliminate these problems. Furthermore, high NOM concentrations may impact subsequent treatment steps, e.g. by increasing chemical demand, sludge production etc. The removal of NOM is therefore an important consideration and requires advanced drinking water treatment.

The amount of NOM in surface waters is increasing in places due to environmental changes and impacts of climate change. For example increased precipitation has caused the flow paths of water ways to change, leading to an increase in leaching of organic compounds from the upper forest floor to lakes. Increase in production of NOM and changes in characteristics are also predicted due to increasing temperatures and other climate change issues. NOM is a complex material and comprises a multitude of different organic substances. Classification and fractionation of these is a difficult task. However, some main properties are commonly used to describe the NOM characteristics. Key fractions are humic and fulvic substances, which are typically very hydrophobic compounds. These substances typically have a high specific UV absorption due to a high content of aromatic carbon. In conventional treatment such waters are recognized as being well treated by coagulation.

Existing treatment processes typically use coagulation with coarse media filtration. The process primarily consists of a rapid depth filtration (e.g. sand-based, dual media etc.) preceded by a coagulation / flocculation treatment stage in which particulate matter is aggregated into floes large enough to be filtered out by the rapid filtration stage. Aluminium and iron based metal salts are commonly used coagulants. Coagulation is defined as the process of destabilization of a given suspension or solution. Flocculation is the process whereby destabilized particles or particles created by destabilization, are promoted to come together and make contact, in order to form larger aggregates. These are then readily removed in subsequent processes.

Rapid filtration has the advantage of allowing high throughput, but also has disadvantages including the fact that it does not itself filter out bacteria or protozoan parasites thus necessitating further (usually chemical) disinfection steps. The sludge produced in the rapid filtration process also requires extensive processing.

In general, there are many factors that can be varied in a given water treatment process, each factor typically affecting other factors. Thus a particular process is optimized by using a specific set of operating conditions. These operating conditions will not typically be optimal for other water treatment processes. In drinking water treatment, rapid filtration processes are typically used on water with a pH range of around 6-9 and which are well buffered (i.e. insensitive to pH changes). The addition of coagulant can reduce the pH to some extent, however in well buffered water this will be negligible. The optimal pH range for NOM and colour removal is lower than these conditions. Nevertheless, the NOM and colour removal is adequate at such pH values and further pH adjustment is not normally performed. If pH adjustment is required, acid or base is added to the water accordingly. The final treated water must not be corrosive, normally necessitating a final pH adjustment to acceptable levels to meet drinking water regulations.

Drinking water regulations vary from country to country, but typically require the water to have a pH in the neutral range or a little higher. For example, the

Norwegian regulations currently stipulate a pH range of 6.5-9.5 for treated water, under the condition that the treated water does not cause corrosion problems in the distribution net. That condition is normally fulfilled in a pH range from 7.8 to 8.5

One particular optimization of the rapid filtration process has been used for the treatment of Nordic surface waters with a low pH and low buffering. In this process, the raw water is subjected to a coagulation and flocculation pre-treatment stage, followed by a rapid sand filtration stage followed by a marble filtration stage. The addition of coagulant in this process lowers pH to around 3.5 to 4.5. This range is mostly below the optimum pH range for coagulation of NOM and colour removal by ferric based coagulants. Often no further pH adjustment is required. However, the metal coagulant has a high solubility at this pH range. This results in a high residual metal content after the rapid filtration stage. The marble filter serves to increase the pH into a range where the metal solubility is low and the metal species will thus precipitate in the marble filter, reducing the residual metal content of the treated water. This process, where coagulation is coupled with rapid filtration, is in many ways an adequate solution. It is ideal for soft and coloured waters. It performs coagulation at low pH (3.5-4.5). Often no pH adjustment is needed and furthermore residual metal is captured in the marble filtration.

As an alternative water treatment process, recent studies have looked at the possibility of using membrane filtration instead of a conventional rapid depth filtration. Membrane filters have a number of advantages for water treatment, including an improved hygienic barrier and a smaller physical footprint, but one major disadvantage is the potential for membrane fouling, i.e. blocking or obstructing the pores of the membrane and thus reducing the membrane's efficiency. By contrast, the rapid filtration process is not particularly sensitive to clogging / fouling. Membrane fouling can be classified into two categories:

reversible and irreversible. Reversible fouling is fouling that can be cleaned from the membrane by regular backwashing techniques without using chemicals.

Irreversible fouling cannot be removed by regular backwashing and can only be removed by chemical cleaning. Commonly alkaline or acidic cleaning solutions are used, sometimes oxidizing agents such as chlorine). Fouling results in a gradual degradation of performance over time, typically causing a pressure build up across the membrane. Membrane fouling is a complex phenomenon and can be difficult to predict. Coagulation / flocculation has been used as a technique for membrane fouling mitigation. However, the choice of coagulant and the operating conditions are very important. The wrong choice of coagulant and/or operating conditions can have drastic effects causing severe fouling. The optimal dose and coagulation condition is required to avoid these effects. For example, if an insufficient amount of coagulant is dosed, membrane fouling can increase drastically, with residual metal concentration remaining too high and with low NOM removal. Also low pH increases metal solubility and consequently the increased residual metal. It is also known that changes in pH level can alter fouling rates. Furthermore, the membrane material must also be taken into consideration as many membrane materials are not suitable for low pH operation and general pH adjustment

(acid/base addition) is often needed in the coagulation based process. For these reasons, membrane treatment processes coupled with coagulation pre-treatment have typically been carried out at higher pH levels.

The inventors have previously investigated the use of a coagulation pre-treatment step coupled with a membrane filtration step. It was found that if the coagulant type and concentration were carefully controlled, membrane fouling could be kept within acceptable levels. These previous studies also found that the optimum pH for the overall process (i.e. for the particular experimental circumstances) was around 6 to 7, which is higher than the optimal range for NOM and colour removal. A higher pH was required in order to avoid excess residual metal content in the effluent water. This was the limiting factor in the process. Optimal conditions for NOM and colour removal at lower pHs (i.e. below 6) requires a post filtration pH adjustment both to reduce the residual metal concentrations and to meet the drinking water standards of a non-corrosive water. Such chemical additions are ideally avoided or minimized. Specifically, Meyn, T., (201 1 ). ["NOM Removal in Drinking Water Treatment Using Dead-end Ceramic Microfiltration: Assessment of Coagulation/Flocculation

Pretreatment", Department of Hydraulic and Environmental Engineering. Doctoral Thesis, Norwegian University of Science and Technology (NTNU), Trondheim, Norway] showed that in spite of high NOM content in the raw water (DOC 6.8 mg C/L, colour 55 mg Pt/L), stable operation was achievable at high membrane fluxes of up to 250 L/(m ² h), achieving irreversible membrane fouling below 1 mbar/h, a dissolved organic matter (DOC) removal of 70% and colour removal of around 90%, at a coagulant dosage of 0.65 mg Al per mg DOC (using Polyaluminium Chloride (PACL) at pH 6, with 60s of inline flocculation). This study also found that the optimization of coagulation pre-treatment is crucial. If, for example, an insufficient amount of coagulant is dosed, membrane fouling increases drastically, residual metal concentration is high and NOM removal is minimal.

"NOM removal technologies - Norwegian experiences" by H. 0degaard et al, Drink. Water Eng. Sci. Discuss., 2, 161-187, 2009 provides a summary of various NOM removal systems and technologies which have been used in Norway. These include direct nanofiltration techniques in which a calcium carbonate filter is used. The purpose of this filter is largely for replacement of divalent ions which are removed by the very small pores of the nanofiltration device. There is no coagulation treatment in this process. The document also describes

coagulation/filtration systems. Two such systems are described. In the first system, a non-membrane filtration system is combined with an alkaline filter for corrosion control. In the second system, membrane filtration is used, but with no alkaline filter. This second system is earlier work by one of the inventors of the present invention and was designed for different operating conditions, in particular different applied pH values.

US 6,416,668 B1 describes a filtration system which predominantly relates to the use of "dense" membranes such as spiral wound nanofiltration (NF) and reverse osmosis (RO) membranes. These membranes have very small pore sizes, and essentially remove everything from the influent. NF membranes are still considered porous, meaning they actually have pores, with sizes of roughly 0.001 μηη to 0.01 μηη. Such membranes can retain organic molecules and divalent ions. RO membranes are non-porous and the separation mechanism is based on the ability of a solute to diffuse through the membrane. Theoretical pore sizes of such membranes are between 0.0001 and 0.001 μηη. They can retain monovalent ions, e.g. Na+ and CI-. In order to make such NF and RO membranes work efficiently, all matter with a significantly larger size than the pore size needs to be removed upstream of the membranes. US 6,416,668 therefore provides a chain of filtration stages before the NF / RO treatment step. If this pretreatment is not done properly, the NF / RO membrane modules will be fouled (clogged) immediately.

According to one aspect of the present invention, there is provided a method of treating water comprising: a coagulation step in which a coagulant is added to an input water to induce coagulation of natural organic matter particles within the water; a membrane filtration step in which the water output from the coagulation step is passed through a semi-permeable membrane; and an alkaline filtration step in which the water output from the membrane filtration step is passed through an alkaline filter to raise the pH of the water and induce precipitation of residual metals from the water.

The combination of the three process stages makes it possible to use membrane filtration as a viable industrial water filtration method. In particular, it is possible to treat soft / unbuffered waters or low pH waters efficiently.

The pH may be lowered by addition of chemicals prior to the flocculation stage. However, in preferred embodiments of the invention the addition of coagulant reduces the pH of the water. This effect is particularly pronounced in the case of soft waters (i.e. with low mineral content) as the water is not well buffered. This means that the addition of acid or alkaline material produces rapid changes on the overall pH of the water. By contrast, in the case of hard waters (i.e. well buffered waters), the high mineral content can react with the additives, thus slowing down the overall pH change of the water. It has been found that in the case of certain soft waters, the addition of an optimal quantity of coagulant will reduce the pH of the raw water input to the optimal pH level for NOM and colour removal. Ferric based coagulants are particularly suitable for this. In these cases there is therefore no need for additional chemical pH adjustment in order for NOM and colour removal to be optimized. Preferably therefore, the amount of coagulant added is sufficient to reduce the pH of the water to the level which is optimal for NOM removal. In preferred embodiments, the optimum pH for NOM removal may be in the range of 2 to 8, preferably 3 to 6, 4 to 6 or 5 to 6, alternatively 3 to 5, preferably 3.5 to 4.8, or more preferably still in the range of 4.3 to 4.8. Preferably the pH of the water is adjusted to within this range prior to or during the coagulation stage. More preferably still, the amount of coagulant is sufficient to reduce the pH to within the range of 2 to 8, preferably 3 to 6, 4 to 6 or 5 to 6, alternatively 3 to 5, preferably 3.5 to 4.8, more preferably still in the range of 4.3 to 4.8. In some particularly preferred embodiments, the coagulant is Iron based and the pH is in the range of 3.5 to 4.8, preferably 4.3 to 4.8. In other particularly preferred embodiments, the coagulant is Aluminium based and the pH is in the range of 5 to 6. Preferably, coagulant is added to reach a concentration which is optimal for NOM removal. This will depend on the initial concentration of NOM and its characteristics / properties. Moreover, it has been found that in these circumstances the fouling of the membrane (both reversible and irreversible) is still within acceptable limits. A consequence of the low pH of the filtered water is that metal solubility is high and thus residual metal content is high in water after the membrane filtration step. The alkaline filtration stage brings the pH of the filtered water back up to a level which is acceptable for drinking water (e.g. around 7.8 - 8.5). At the same time, the alkaline filtration step induces precipitation of residual metal from the treated water, and increases the mineral content of the water, thus providing buffering and corrosion control. The result is a high quality treated water. Preferably the residual metal concentration is reduced to levels below the legislated maximum allowed concentration, thus producing a treated water well within the drinking water regulations.

The use of membrane filtration provides a particular benefit in that, even if the coagulation step should fail for some reason, the effluent water from the water treatment plant still benefits from a certain level of filtration. For example the membrane filter can exclude bacteria and parasites regardless of the coagulation efficacy. This provides a certain minimum hygienic barrier effect. This is not the case with a conventional coarse media rapid filtration system where a failure of coagulation will result in a failure of the system as a whole, i.e. neither natural organic matter nor parasites would be removed. The present invention is thus able to provide a higher default water quality even in the event of failure.

The process of the invention is particularly applicable to treatment of water with low pH, low buffer capacity and high natural organic matter content. The preferred embodiments of the invention are used for treatment of natural fresh water, surface water or ground water, e.g. river bank infiltration. Particularly preferred examples of source water which can be efficiently treated according to the invention are Nordic (The term Nordic refers to the countries Norway, Sweden, Finland, Denmark and Iceland and their associated territories, the Faroe Islands and Greenland) surface and ground waters and Scottish highland waters as well as surface / ground waters in Tasmania, North America, Northern Russia and Siberia / Northern Asia. The invention is particularly applicable to surface waters as these tend to be less buffered compared with ground waters which typically have a higher mineral content. Nevertheless, the invention will apply to some ground waters. The invention also applies to surface waters treated by bank filtration (which may sometimes be referred to as ground water).

The membrane filtration step may use a variety of different membranes. For example, organic polymer membranes can be manufactured with a large variety of different properties and can be tailored to a particular process in order to achieve optimum efficiency. However, although it is possible to design polymer membranes which can operate efficiently with low pH fluids, these are very specialized and can be costly. Preferred embodiments of the present invention use a ceramic membrane in the membrane filtration step. Ceramic membranes are more costly to produce and therefore require a higher initial investment. However they are hydrophilic and thus can be operated at higher throughputs. Therefore for a given size they can treat larger quantities of water. From an alternative perspective, for a given water treatment rate, plant size can be reduced with the use of ceramic membranes. Additionally, ceramic membranes are more resilient to chemical cleaning agents and it is therefore easier to clean a fouled ceramic membrane. Due to their high mechanical strength, ceramic membranes are expected to have a longer life time compared to polymeric membranes. For these reasons, cost savings can be made in the long run. Preferred pore sizes of the membrane are in the ultrafiltration/microfiltration

(UF/MF) range. The larger pore size of such membranes (compared with nanofiltration and reverse osmosis processes) results in lower applied pressures and, as a consequence ultrafiltration/microfiltration is more efficient (less energy intensive than nanofiltration or reverse osmosis).

Like rapid filtration plants, membranes require regular backwashing to clear the inevitable fouling. Backwashing results in the generation of sludge which must normally be subjected to further processing. However the sludge generated by membrane filters is quite different in constitution from that generated by rapid filtration. Indeed the sludge generated by different membranes can vary

significantly. A possible benefit arising from the constitution of membrane filtration sludge, particularly from tubular membranes with inside-out filtration channels (such membranes being preferred since they are efficiently backwashable), is that it tends to have a more pellet-like form. This lends itself to easier collection and processing, thus reducing the operating costs of the water treatment plant as a whole.

Coagulation can take the form of traditional coagulation / flocculation setups. In these, one or more tanks are provided in which the water and coagulant are mixed so as to encourage flocculation. Once floes have achieved a sufficient size they settle out. Conventional arrangements often include successive arranged tanks, starting with a higher mixing rate to ensure good mixing of the coagulant in the water, and successive tanks with a lower mixing rate to allow for better flocculation. However, a preferred arrangement is to use inline coagulation. In such

arrangements, the coagulant is added into the pipeline ahead of the membrane filtration stage. The residence time and flow rate of the fluid are designed to allow sufficient mixing and flocculation to occur. Such arrangements may involve a wider section of pipe so as to reduce the speed of fluid flow and thus encourage flocculation. Structure such as an obstruction (e.g. static mixers or baffles, constrictions, nozzles, etc.) may be used to introduce turbulence for improved mixing. Typical residence times in the pipeline between the addition of coagulant and the membrane filter are preferably greater than 30 seconds, more preferably 60 seconds or more. It has been found that inline coagulation can provide sufficiently good NOM and colour removal and has the advantage of simplifying the apparatus and reducing the size of the apparatus. Inline coagulation setups are particularly suitable for use with membrane filtration as less flocculation is required for filtration to be effective.

The optimum type of coagulant should be selected according to the exact nature (i.e. mineral content, NOM content, colour, turbidity etc.) of the water to be treated, but in preferred cases the coagulant is a metal salt. Preferred metal salts are aluminium salts and iron salts and in the most preferred embodiments the coagulant is an iron salt. In preferred embodiments, the optimum pH range for NOM removal by iron salts is achievable by simple addition of the appropriate amount of coagulant, without any further chemical pH adjustment required.

Aluminium salts may also be used, but the addition of Aluminium salts may cause a drop in pH below the optimal range for NOM and colour removal for such coagulants, (which is in the range of 5.5 to 6.0, i.e. higher than for ferric based coagulants). Therefore, additional chemical pH adjustment may be required in order to bring the pH back to the optimal level. An additional issue with Aluminium is that it may have an increased probability of leaking from the alkaline filter, resulting in a lower quality effluent.

The alkaline filtration step may use any suitably chosen alkaline material as the filtration medium. An appropriate material will depend on the composition of the water to be treated and the expected pH and residual metal content. Some non- limiting examples of suitable alkaline filter materials are: marble, limestone, dolomite, calcite, water glass. Preferred embodiments of the invention use carbonate mineral filters, such as a marble filter. An alkaline filter provides good buffering at the same time as raising pH due to the water absorbing minerals from the filter medium as it passes through the filter.

It will be appreciated that the preferred features described above may be applied in various combinations as will be readily understood by a person of ordinary skill in the art.

According to another aspect, the invention provides a water treatment system, comprising: a coagulation stage comprising a coagulation stage water input, a coagulant input and a coagulation stage water output; a membrane filtration stage comprising a membrane filtration stage water input connected downstream of the coagulation stage water output and which is situated on one side of a semipermeable membrane, and a membrane filtration stage water output on the other side of the semi-permeable membrane; and an alkaline filtration stage comprising an alkaline filtration stage water input connected downstream of the membrane filtration stage water output and which is situated on one side of an alkaline filtration medium, and an alkaline filtration stage water output on the other side of the alkaline filtration medium.

The preferred features described above also apply to the apparatus. Accordingly, the semi-permeable membrane is preferably a ceramic membrane. The

coagulation stage may comprise one or more sequentially arranged tanks (e.g. with different mixing rates), but is preferably an inline coagulation stage. The alkaline filtration medium is preferably a marble filter. It will be appreciated that, while in some embodiments additional treatment steps may be provided, in many preferred embodiments no further treatment steps are necessary between the coagulation stage and the membrane treatment stage or between the membrane treatment stage and the alkaline filtration stage. In some embodiments there may be pH adjustments made in between these stages, but preferably no further filtration stages are provided (either membrane filtration or granular filtration).

The invention also extends to the use of the above-described systems for treatment of natural water and/or treatment of surface water or ground water.

Preferred embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

Fig. 1 shows a schematic of the proposed treatment system;

Fig. 2 shows a graph of DOC removal in dependence on pH value;

Fig. 3 shows a graph of Zeta potential (particle surface charge) in dependence on coagulation pH;

Fig. 4 shows residual iron concentration in dependence on the coagulation pH; Fig. 5 shows irreversible fouling in dependence on pH; and

Fig. 6 shows reversible fouling in dependence on pH. An embodiment of the invention and some experimental results are presented here. This system provides a tailor made treatment process for surface waters such as Nordic, surface waters (or other waters of similar characteristics as described above) based on membrane filtration technology. A schematic of the proposed treatment concept is shown in Figure 1.

Fig. 1 shows a water treatment system 100. A raw water inlet 105 provides the input for raw water (taken from natural, surface water sources such as rivers or lakes, or possibly from ground water sources) into the system. A volume of raw water is held in buffer tank 1 10 to ensure steady flow conditions within the system 100.

An optional pH adjustment may take place by chemical addition at 1 15 if conditions require it, but this step is ideally omitted as the optimal pH adjustment can be achieved via the coagulant addition. Therefore in water treatment plants designed for soft, low buffered waters, the system will preferably not have a chemical pH adjustment. A pump 120 feeds water to the membrane 150. Coagulant is added into the system at 125. The coagulant is typically a metal salt, most often either Iron or Aluminium salts. Coagulation and flocculation takes place at 130. The setup shown in Fig. 1 is an inline coagulation arrangement comprising a structure 135 for generating turbulence and mixing and a section of expanded pipe 140 in which the flow rate is reduced and flocculation is encouraged. The residence time in this pipe section

140 is around 60 seconds. It should be noted that in large scale plants, there may be no need for such extra structure to encourage flocculation as the piping which connects the coagulation stage 135 with the membrane may be sufficient on its own.

After the coagulation / flocculation treatment stage 130, the water passes into the membrane 150. The membrane may be an organic polymer membrane or it may be a ceramic membrane. Other membranes may also be used, although ceramic membranes are preferred for their ability to operate at high fluxes and for their robustness to low pH operation and to chemical cleaning processes. If polymer membranes are used, these will have to be designed carefully to withstand the operating conditions. The membrane module 150 is a low pressure UF/MF membrane module. Filtered water is output from the membrane module 150 at 155 and sludge is output at 160. The sludge is taken away for further treatment and disposal. For example, the sludge may be concentrated in a hydro cyclone, the concentrated part disposed or discharged and the diluted stream fed back to the raw water. The membrane module 150 filters out flocculated NOM and particles as well as bacteria and parasites, thus producing a high quality permeate at its output 155.

The permeate at this point in the process is low in pH and thus the solubility of metals is high meaning that the permeate is likely to have a high residual metal content.

An optional C0 ₂ input 157 may be provided. This is used to adjust the amount of marble which dissolves in the marble filter 165. This directly influences alkalinity and pH of the treated water.

The permeate is then passed through the marble filter 165 before exiting the system 100 at 170. The marble filter 165 increases the pH of the water, bringing it back to a normal drinking water level (typically around 7.8-8.5). At the same time, the permeate absorbs minerals from the marble and thus the buffering capability of the water is increased, thus providing corrosion control in the same step as the pH adjustment. Due to the increase in pH, dissolved residual metal is precipitated as hydroxide, e.g. iron hydroxide. Since this is a coagulation process, still remaining organic matter may be either incorporated into or adsorbed onto the hydroxide floes and thus separated by the alkaline filter.

The treated water output at 170 may be subjected to further disinfection steps if necessary before being distributed as drinking water.

The use of membranes in the above system and process improves the hygienic barrier of the treatment, since depending on the type of membrane, at least parasites and most of the bacteria are retained even if the coagulation pre- treatment fails. Any turbidity present in the raw water is also reliably removed.

The application of ceramic membranes allows high permeate fluxes, which eventually leads to a reduced plant size, and guarantees that the membranes are stable also at low pH values.

In addition, when the process is applied to soft, low pH, low buffered raw water, no extra pH adjustment is required since the coagulation pH will be in the optimal range for removal of NOM and colour just after dosing of the coagulant, as described above.

Finally, the proposed treatment scheme already includes corrosion control and the removal of dissolved residual coagulant/metal in one treatment step.

Experimental results

A number of experiments have been carried out using the setup of Fig. 1 , investigating the performance of coagulation pre-treatment in combination with ceramic microfiltration. Low coagulation pH values were applied together with an iron based coagulant. A summary of the operating conditions is shown in Table 1.

Process parameter Value / Range

Raw water DOC of 6.8 mg/L and colour of 55 mg

Pt/L

Coagulation pH • 4.3

• 4.7

• 5.4

• 6.0

Coagulant / dosage JKL (Iron chloride sulphate), 8.5 mg

Fe/L

Coagulation / Flocculation • Classic configuration (rapid and slow mixing with HRT of 20 min); and • Inline configuration (static mixing followed by pipe flocculation with HRT of 60s and G of 31 s-1 )

Filtration parameters 250 L/(m2 h) flux, backwash every 60 minutes

Experiments Duplicates, operation for 48h

Where:

HRT is the Hydraulic Retention Time;

G is the hydraulic gradient; and

Duplicates indicates the standard practice of using multiple samples and analyses.

The results show that the DOC removal is highest (75 to 78%) in the pH range from 4.3 to 4.8 (see Figure 2). However, it was greater than 70% in an extended pH range from 3.5 to 5.4. This illustrates clearly the advantages of coagulation with iron based coagulants under such conditions. The same trends were found for colour removal.

The optimal DOC and colour removal efficiency of iron coagulants at pH values below 5 can be explained by an improved destabilisation of these components under such conditions. This can be directly derived from surface charge

measurements, as shown in Figure 3. With decreasing pH the zeta potential is approaching zero, indicating that the charge of the coagulated particles is decreasing further and further, which leads to improved coagulation and flocculation since electrostatic repulsion is significantly reduced. Positively charged iron species dominate at pH values below 5, which can efficiently neutralise the negatively charged NOM molecules.

The coagulation pH range from 4.3 to 4.8 may be beneficial for the removal of organic matter. However, while this advantage is gained, there is a trade-off of higher dissolved metal concentrations under such conditions (see Figure 4). Iron concentrations of several hundred micrograms were measured after the membrane filtration step. This is significantly higher than the typical limiting value in drinking water guidelines (e.g. 150μg Fe/L in Norway). The results further show that low membrane fouling can be achieved at conditions optimal for coagulation of NOM with iron based coagulants. In the pH range where NOM removal is best, i.e. around 4.7 (e.g. 4.6-4.8), low irreversible membrane fouling is also observed (Figure 5). In addition, low membrane fouling was observed for both applied pre-treatment configurations, inline and classic coagulation.

Coagulation reduced irreversible membrane fouling, compared to conditions with no pre-treatment.

With regard to reversible membrane fouling, a clear advantage was found for the classic coagulation setup (Figure 6). Without wishing to be bound by theory, it is currently believed that this is due to a difference in the size of floes resulting from the two processes, with smaller inline floes causing greater clogging of membrane pores. Again, without wishing to be bound by theory, another reason for the observed differences may be found in the floe stability. While at first large floes are created with the classic coagulation setup, which then are broken down into smaller fractions while passing the membrane feeding pump, the resulting fragments may be mechanically stronger, and thus form a fouling layer which is less compressible. In contrast, floes created by inline coagulation have most likely a more fluffy consistence, thus forming a fouling layer which is more compressible and as a consequence, has an increased hydraulic resistance.

The advantages of the 3-step water purification process described here are that the system can be made simple and compact. The system and method may be customized for several local areas such as, for example, Nordic conditions. The process effectively removes NOM and colour due to the low coagulation pH.

Additionally there is a low residual metal concentration due to the alkaline filter and additional pH adjustments are minimized or are not required at all. The process has low chemical costs and extra corrosion control is not normally necessary. It provides high operation fluxes with ceramic membranes and importantly provides an increased barrier against bacteria and parasites. The result is a constant permeate quality.