Combining Chemical Flocculation and Disc Filtration with Managed Aquifer Recharge

water

Article Combining Chemical Flocculation and Disc Filtration with Managed Aquifer Recharge

Kristofer Hägg 1,2,* , Michael Cimbritz 3 and Kenneth M. Persson 1

1 Division of Water Resources Engineering, Faculty of Engineering LTH, Lund University, John Ericssons Väg 1, V-Hus, 221 00 Lund, Sweden; [email protected] 2 Sweden Water Research AB, Ideon Science Park, Scheelevägen 15, 223 70 Lund, Sweden 3 Department of Chemical Engineering, Faculty of Engineering LTH, Lund University, Naturvetarvägen 14, Box 124, 221 00 Lund, Sweden; [email protected] * Correspondence: [email protected]; Tel.: +46-706-222-391

Received: 9 November 2018; Accepted: 12 December 2018; Published: 14 December 2018

Abstract: Natural organic matter (NOM) is a growing concern for artificial recharge plants. In the future, it is predicted that warmer climates and more precipitation will cause higher NOM production in lakes and more NOM transport to lakes. This, coupled with increasing drinking water demand due to the population increase, is pushing operators of water treatment plants (WTPs) to find new ways to treat water. In this study, the possibility of reducing the organic load in infiltration basins through a compact pre-treatment technique utilizing microsieves, or disc filters, instead of bulky sedimentation basins and rapid sand filters after chemical flocculation to separate flocs, was investigated. The experiments were conducted using a laboratory-scale flocculator, bench-scale disc filters (10 µm and 40 µm), FeCl3, an anionic synthetic polymer, and water from Lake Vomb, a lake in southern Sweden. Raw water was flocculated using FeCl3 and the polymer, and the filtrated samples were analyzed by measuring UV–VIS absorbance, total organic carbon (TOC), and permeate volume. The results when using 10-µm and 40-µm disc filters demonstrate that it is possible to reduce NOM (by approximately 50%) and separate flocs from raw water. The experiments also highlight the importance of sufficient flocculation times and the use of appropriate polymer dosage to achieve higher permeate volumes and avoid residual polymers in the effluent. In this paper, the possibility of using this technique as a standalone treatment step or as a pre-treatment step in order to manage the aquifer recharge is demonstrated.

Keywords: coagulation; ﬂocculation; disc ﬁltration; pre-treatment

1. Introduction One-quarter of all drinking water in Sweden is produced by artificial groundwater recharge plants [1]. The techniques most commonly used to recharge the groundwater are induced infiltration and infiltration through infiltration basins [2]. The latter is more common in Sweden among water treatment plants (WTPs) with high production. The technique utilizes services provided by nature to reduce natural organic matter (NOM), bacteria, and viruses [3]. Artificial recharge plants can, depending on the study, remove 50–85% of NOM from surface waters [3–6]. However, problems arise when WTPs are pushed to the limit due to increasing drinking water demands. This problem is exacerbated with the growing concerns regarding global warming. It is expected that the increasing temperature and precipitation [7] will cause lakes and rivers, which are used for drinking water, to deteriorate due to organic carbon transport [8,9] to surface waters. This, combined with the increased demand for drinking water due to the increasing population, has put pressure on water utilities to plan ahead to secure water production for future generations. At Vomb Water Works, an artificial

Water 2018, 10, 1854; doi:10.3390/w10121854 www.mdpi.com/journal/water Water 2018, 10, 1854 2 of 10 recharge plant in southern Sweden, an additional raw water source, i.e., Lake Bolmen, will be used in an effort to increase drinking water production. However, the limitation of the infiltration fields’ capacity to treat large volumes of lake water from the NOM-rich waters of Lake Bolmen has prompted the water utility to investigate different pre-treatment methods. One solution common among utilities, which is effective to reduce NOM, is to pre-treat the raw water by chemical flocculation before infiltration [5]. This technique is used in different ways in Sweden, with coagulation and flocculation steps using Fe(III)- or Al(III)-coagulants [10]. Depending on the production volume needed, different separation methods are commonly used. In Sweden, smaller WTPs with low production (<20,000 m3/year) tend to remove flocs by contact filtration, while larger WTPs with higher production (>20,000 m3/year) tend to use sedimentation [4]. Microscreens were originally used for pre-treatment for slow and rapid sand filters at potable water treatment works in order to reduce coagulant doses and solids loading [11], but nowadays, there are several alternative applications available for both water and wastewater treatment. Disc filtration, one type of microscreen based on gravity flow, is widely used for tertiary filtration in wastewater treatment, mainly because of the resulting low footprint. Difficulties when adding coagulants to improve tertiary treatment were, however, noticed quite early [12,13]. This was due to flocs not being strong enough to withstand shear forces in the filter. A polymer addition was suggested as a solution, and recent development has shown the possibility of combining coagulation/flocculation with disc filtration in order to achieve very low levels of effluent phosphorus [14]. The objective of this study was to investigate the possibility of combining chemical flocculation and disc filtration in drinking water production, either as a standalone treatment step or as a pre-treatment step before aquifer recharge.

2. Materials and Methods During this study, jar tests [15] were performed in combination with laboratory-scale disc ﬁltration. The jar tests and subsequent ﬁltration were conducted as described in [14] with some alterations. The procedure is described in the following sections.

2.1. Preparation for Jar Tests

To ensure successful coagulation, FeCl3-assisted ﬂocculation needs to occur around pH 5.1 [16]. This was achieved by adding FeCl3 and HCl solutions to 1 liter of raw water and measuring the pH before the jar tests were conducted. The pH was measured using a WTW pH-197 (Sigma-Aldrich, Saint Louis, MO, USA) pH meter after FeCl3 was added. An HCl solution was then added until the targeted pH was reached. The volume of HCl needed was later added before the coagulants during each jar test.

2.2. Jar Tests and Disc Filters Jar tests, as described in [15], were conducted in the water treatment plant in Vomb, Sweden, using a program-controlled flocculator (Flocculator 2000, Kemira, Helsingborg, Sweden). In the setup, a 1-L glass beaker was used in combination with a laboratory-scale disc filter (Nordic Water Products AB, Gothenburg, Sweden), as shown in Figure1 below. The horizontally set disc filter ( ϕ = 9 cm) was attached to a 1.2-L cylinder through which water flowed by gravity, and the flocs were retained on the filter material. The test procedure was as follows:

1. The ﬂocculator was programmed to 30 s rapid mixing (400 rpm) and 20 min slow mixing (75 rpm). The beaker was ﬁlled with 1 L of raw water. 2. Before the program was started, the pre-calculated amount of HCl was added to the beaker while mixing.

3. The program was started, and when 10 s of the rapid mixing remained, FeCl3 was added. 4. When the polymer was used, it was added during slow mixing. Water 2018, 10, 1854 3 of 10

5. Once the flocculation program had finished, the raw water was transferred to the disc filter. The permeate was collected and measured after 60 s of filtration. Water 2018, 10, x FOR PEER REVIEW 3 of 10

Figure 1.1. Photo of the Kemira Flocculator 2000 (Helsingborg,(Helsingborg, Sweden)Sweden) and the laboratory-scalelaboratory-scale disc filteﬁltersrs from Nordic Water.Water.

To ensureensure replicability,replicability, the the water water sample sample was was transferred transferred from from the the beaker beaker over over to to the the disc disc filter filter in thein the same same manner, manner, by carefully by carefully pouring pouring the water the water into the into disc the filter disc while filter angling while theangling filter tothe minimize filter to flocminimize breakage. floc breakage. Once the Once sample the was sample transferred, was transferred, the disc filterthe disc was filter set was upon set a 1-Lupon beaker a 1-L beaker to collect to thecollect permeate. the permeate. In preparation for the trials using disc filters,filters, the experiments were conducted to findfind the appropriate FeClFeCl33 dosages.dosages. TheThe experimentsexperiments followed followed the the procedure procedure mentioned mentioned above above with with a couplea couple of exceptions;of exceptions; no no polymer polymer was was added, added, and and instead instead of of transferring transferring the the raw raw water water to to the the disc disc filter filter (step (step 5) a5) 30-mina 30-min sedimentation sedimentation step step was was added. added.

2.3. Coagulants and Additives All the coagulants used in the study were of food food-grade-grade quality and were used directly without further preparation apart from the synthetic polymer (a(a co-polymerco-polymer of acrylamide and sodium acrylate), MLT 30 (BASF, Ludwigshafen, Germany). A H SO (97%) and HCl solution was used acrylate), MLT 30 (BASF, Ludwigshafen, Germany). A H2SO2 4 (97%)4 and HCl solution was used to tocontrol control the the resulting resulting pH pH due due to the to thehigh high alkalinity alkalinity of the of lake the lake water. water. The Theprimary primary coagulant coagulant used used was wasPIX-311, PIX-311, a 40% a by 40% weight by weight FeCl3 solution FeCl3 solution produced produced by Kemira by ( KemiraHelsinpgborg (Helsinpgborg,, Sweden) and Sweden) provided and providedby Sydvatten by Sydvatten AB, Sweden AB, (municipal Sweden (municipal drinking water drinking producer water, producer,Malmö, Sweden Malmö,). The Sweden). 0.5% synthetic, The 0.5% synthetic,anionic, polymer anionic, was polymer diluted was the diluted day of the the day experiment of the experiment in distilled in distilled water resulting water resulting in a 0.05% in a 0.05%polymer polymer solution. solution. 2.4. Water Sample Analysis 2.4. Water Sample Analysis Raw water and the treated water samples were tested for total organic carbon (TOC) and UV–VIS Raw water and the treated water samples were tested for total organic carbon (TOC) and UV– absorbance using a TOC analyzer (TOC-L, Shimadzu, Kyoto, Japan) and spectrophotometer (DR 5000, VIS absorbance using a TOC analyzer (TOC-L, Shimadzu, Kyoto, Japan) and spectrophotometer (DR Hach Lange, Loveland, CO, USA), respectively. The absorption was measured at λ = 254 nm and 5000, Hach Lange, Loveland, CO, USA), respectively. The absorption was measured at λ = 254 nm λ = 436 nm using a 5-cm cuvette. These wavelengths were used by the water treatment plant for daily and λ = 436 nm using a 5-cm cuvette. These wavelengths were used by the water treatment plant for process control. daily process control.

2.5. Vomb Lake and Drinking Water Quality The raw water samples used in this study were taken from Lake Vomb in southern Sweden. Lake Vomb is a hypertrophic lake with seasonal algae blooms, mostly caused by effluents from the surrounding agriculture [17], and has been used by Vomb Water Works as a raw water source since

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2.5. Vomb Lake and Drinking Water Quality Water 2018The, 10 raw, x FOR water PEER samples REVIEW used in this study were taken from Lake Vomb in southern Sweden.4 of 10 Lake Vomb is a hypertrophic lake with seasonal algae blooms, mostly caused by efﬂuents from the 1948.surrounding The water agriculture quality varies [17], anddaily has and been seasonally. used by VombThe experiments Water Works in asthis a rawstudy water were source conducted since during1948. The the watersummer qualitys of 2016 varies and daily 2017, and and seasonally. during this The time experiments, the water quality in this was study aswere shown conducted in Table 1.during the summers of 2016 and 2017, and during this time, the water quality was as shown in Table1.

Table 1. Average rawraw waterwater qualityquality ofof Lake Lake Vomb Vomb from from 2016 2016 and and 2017. 2017. Compared Compared with with other other lakes lakes in insouthern southern Sweden, Sweden, the the total total organic organic carbon carbon (TOC) (TOC concentration) concentration and and color color in Lakein Lake Vomb Vomb are are within within the thelower lower to middle to middle range range [18 –[1820].–20]. Sampling TOC Color Alkalinity Turbidity U VISVI Sampling pH TOC Color Alkalinity Turbidity U VISVI Point pH [mg/L] [mg Pt/L] [mg HCO3−/L] − [FNU] [254 nm/m] [436 nm/m] Point [mg/L] [mg Pt/L] [mg HCO3 /L] [FNU] [254 nm/m] [436 nm/m] Lake 8.3 ± 0.07 1 9.1 ± 0.39 1 17.7 ± 0.52 1 1.54 ± 0.23 1 8.3 ± 0.07 1 9.1 ± 0.39 1 23.8 2 152.9 2 2.9 2 17.7 ± 0.52 1 1.54 ± 0.23 1 LakeVomb Vomb (n = 7) (n = 4) 23.8 2 152.9 2 2.9 2 (n = 8) (n = 8) Limit 3 >9.0(n, <7.5= 7) 4( n = 4) 15 n/a 0.5 n/a (n = 8) n/a(n = 8) Limit 3 >9.0, <7.5 4 15 n/a 0.5 n/a n/a Note: 1 Measurements conducted during the experiments from summer 2017 from June to August; 2 1 2 Note: Measurements conducted during the experiments from summer 2017 from June to August; Data3 reﬂects Dataaverage reflects annual average values taken annual from values Sydvatten taken AB’s from annual Sydvatten rapport [ 20AB’s]; 3 Swedish annual regulatory rapport limit[20]; for Swedish drinking regulatorywater [21]. limit for drinking water [21].

3.3. Results Results and and Discussion Discussion

3.13.1.. Fe Fe3+3+: :Coagulant Coagulant Dosage Dosage

ToTo ensure successfulsuccessful coagulationcoagulation and and ﬂocculation, flocculation, varying varying FeCl FeCl3 dosages3 dosage weres were tested tested within within a pH a pHrange range of 4.9–5.3.of 4.9–5.3. In In Figure Figure2, the2, the residual residual UV–VIS UV–VIS absorbance absorbance after after ﬂocculation flocculation and and sedimentationsedimentation areare presented presented..

Figure 2. The residual UV–VIS absorption after chemical ﬂocculation and sedimentation using FeCl3. FigureThe average 2. The raw residual water UV UV–VIS–VIS absorption absorption after was chemical (n = 12) 19.1 flocculation± 0.6 (254 and nm) sedimentation and 2.1 ± 0.5 using (436 nm).FeCl3. The average raw water UV–VIS absorption was (n = 12) 19.1 ± 0.6 (254 nm) and 2.1 ± 0.5 (436 nm). The greatest reduction in the UV–VIS absorption seemed to occur between 148 and 173 mmol/L Fe3+,The followed greatest by reduction a marginal in improvement the UV–VIS absorption up to 297 mmol seemed Fe3+ to. occur For the between remaining 148 experiments and 173 mmol with/L 3+ 3+ 3+ Fedisc, ﬁlters,followed 173 by mmol a marginal Fe (70 improvement mg FeCl3 solution/L) up to 297 wasmmol chosen. Fe . For the remaining experiments with disc filters, 173 mmol Fe3+ (70 mg FeCl3 solution/L) was chosen.

3.2. Effects of Flocculation Time, Polymer Addition, and Disc Filter Filtration

Water 2018, 10, 1854 5 of 10 Water 2018, 10, x FOR PEER REVIEW 5 of 10 3.2. Effects of Flocculation Time, Polymer Addition, and Disc Filter Filtration In order to produce strong flocs using FeCl3 and the synthetic polymer, different flocculation times Inwere order tested to. produce Figure 3 strongshows flocsthe resulting using FeCl permeate3 andthe volume synthetic and residual polymer, UV different absorbance flocculation (UVA) aftertimes floc were separation tested. Figure using3 disc shows filters. the resulting permeate volume and residual UV absorbance (UVA) after floc separation using disc filters.

Figure 3. The effects of the ﬂocculation times on the UV reduction and permeate volume when using the Figure10-µm 3 disc. The ﬁlter, effects with of thethe maximumflocculation theoretical times on permeatethe UV reduction volume beingand permeate 1000 mL. volume The polymer when dosageusing

thewas 10 0.5-µm mg/L disc andfilter was, with added the maximum after 720 s theoretical of flocculation permeate time after volume the FeClbeing3 addition. 1000 mL.Each The p samplingolymer dosagepoint is was the result0.5 mg/ ofL three and iterationswas added and, after in total,720 s 45 of tests. flocculation The average time rawafter water the FeCl UV absorbance3 addition. (UVA)Each samplingwas (n = 4) point 17.3 is± the0.13 result (254 nm). of three iterations and, in total, 45 tests. The average raw water UV absorbance (UVA) was (n = 4) 17.3 ± 0.13 (254 nm). As the flocculation time increased, both the residual UVA and the permeate volume were reduced. AfterAs 120 the s, flocculationthe residual timeUVA increasewas stilld decreasing,, both the residual while the UVA permeate and the volume permeate started volume to gradually were redincrease.uced. After During 120 thes, the first residual 120 s of UVA flocculation, was still thedecreasing variation, while of the the results permeate was volume most likely start dueed to to graduallyinsufficient increase. flocculation During time, the and first once 120 the s of flocculation flocculation, time the reached variation about of 240the s,results the UV was absorption most likely was duereduced to insufficient to a lower valueflocculation than that time, of the and raw once water. the After flocculation 240 s, the time permeate reache volumed about slowly 240 s, increased, the UV absorptionand at the samewas reduced time, the to residual a lower UVAvalue decreased. than that of Once the theraw polymer water. After was added,240 s, the the permeate permeate volume volume slowlyincreased increase considerably,d, and at and the thesame residual time, UVAthe residual decreased UVA more decrease than ind. the Once samples the polymer without was the polymeradded, theaddition. permeate The volume permeate increase volumed increasedconsiderably with, and the flocculationthe residual time. UVA This decrease couldd bemore a result than of in larger the samplesand stronger without flocs the over polymer time, addition allowing. T morehe permeate water to volume pass through increase thed discwith filter.the flocculation The UVA stayedtime. Thisconstant, could however,be a result which of larger meant and that stronger increasing flocs theover polymer time, allowing flocculation more times wate byr to more pass thanthrough 60 s the had disclittle filter. or no The effect UVA on stay the reductioned constant of, thehowever, UVA. Thewhich difference meant that in the increasing flocs when the usingpolymer the flocculation polymer can timesbe seen by inmore Figure than4. 60 s had little or no effect on the reduction of the UVA. The difference in the flocs when Basedusing onthe the polymer results can seen be in seen Figure in 3Figure, the flocculation 4. times for the FeCl 3 and the polymer addition were set to 720 and 240 s, respectively, in the remaining experiments. This way, the experiments could be conducted during shorter periods of time, reducing the uncertainties associated with the raw water quality variations.

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(a) (b)

Figure 4. Photo of Kemira Flocculator 2000. (a) Flocs formed without the polymer; and (b) flocs formed with the polymer. The difference in the floc size can clearly be seen, where the flocs formed after the polymer addition (b) were larger than those formed with only the FeCl3 (a).

(a) (b) Based on the results seen in Figure 3, the flocculation times for the FeCl3 and the polymer additionFigure were 4. PhotoPhoto set of to of Kemira 720Kemira and Flocculator Flocculator 240 s, 2000. respectively, 2000. (a) Flocs(a) Flocs formed in formed the without remaining without the polymer; the experiments polymer; and (b )and.flocs This (b formed) wayflocs , the experimentsformedwith the with could polymer. the be polymer. Theconducted difference The duringdifference in the shorter floc in sizethe period floc can size clearlys ofcan time, be clearly seen, reducing be where seen thethe, where flocsuncertainties the formed flocsafter formed associated the with afterpolymerthe rawthe polymer additionwater quality addition (b) were variations. ( largerb) were than larger those than formed those with formed only with the FeClonly3 the(a). FeCl3 (a).

3.3. Polymer Dosage, 10-µm and 40-µm Disc Filters 3.3. PolymerBased onDosage the , results 10-µm and seen 40 in-µ m Figure Disc Filters3, the flocculation times for the FeCl3 and the polymer addition were set to 720 and 240 s, respectively, in the remaining experiments. This way, the In the following experiments, the the polymer polymer dosage dosage was was varied varied,, and and the the response responsess in in the the UVA, UVA, experiments could be conducted during shorter periods of time, reducing the uncertainties associated TOC,TOC, andand permeate permeate volume volume were were studied studied using using 10- 10and- and 40-µ 40m- discµm filters.disc filters. The flocculation The flocculation time before time with the raw water quality variations. beforethe polymer the polymer additions additions was 720 was s, 720 and s, it and was it 240wass 240 after s after the polymerthe polymer additions. additions. Each Each datapoint datapoint in inFigure Figur5 eis 5 the is the average average result result ( n =(n3) = after3) after flocculation flocculation and and filtration filtration using using 10 10µm µm disc disc filter. filter. 3.3. Polymer Dosage, 10-µm and 40-µm Disc Filters In the following experiments, the polymer dosage was varied, and the responses in the UVA, TOC, and permeate volume were studied using 10- and 40-µm disc filters. The flocculation time before the polymer additions was 720 s, and it was 240 s after the polymer additions. Each datapoint in Figure 5 is the average result (n = 3) after flocculation and filtration using 10 µm disc filter.

Figure 5. The effects of the polymer dosage on the UVA, TOC, and permeate volume using a 10-µm disc Figure 5. The effects of the polymer dosage on the UVA, TOC, and permeate volume using a 10-µm ﬁlter. The average raw water UVA (n = 4) and TOC (n = 2) were 17.3 ± 0.4 and 8.8 mg/L, respectively. disc filter. The average raw water UVA (n = 4) and TOC (n = 2) were 17.3 ± 0.4 and 8.8 mg/L, The average permeate volume when ﬁltering only the raw water was 1167 ± 58 mL after 1 min.

Figure 5. The effects of the polymer dosage on the UVA, TOC, and permeate volume using a 10-µm disc filter. The average raw water UVA (n = 4) and TOC (n = 2) were 17.3 ± 0.4 and 8.8 mg/L,

Water 2018, 10, x FOR PEER REVIEW 7 of 10

respectively. The average permeate volume when filtering only the raw water was 1167 ± 58 mL after Water 2018, 10, 1854 7 of 10 1 min.

AsAs seen seen in in the the results, results, the the polymer polymer additions additions had hada large a large impact impact on the on permeate the permeate volume, volume, and a moderateand a moderate effect effect on the on residual the residual UVA. UVA. The The permeate permeate volum volumee gradually gradually increase, increase, similar similar to to the the flocculationflocculation times times (Figure (Figure3 ),3 until), until the the polymer polymer dose dose reached reache 0.6d mg/L. 0.6 mg/ AfterL. that,After the that, permeate the permeate volume volumedecreased decrease with thed with increasing the increasing amount amount of thepolymer. of the polymer. This could This becould the be result the result of an overdoseof an overdose of the ofpolymer the polymer occurring occurring above 0.6above mg/L, 0.6 causing mg/L, causing the clogging the clogging of the disc of filters. the disc Moreover, filters. Moreover, the overdosing the overdosingof the polymer of the might polymer cause might re-stabilization cause re-stabili fromzation the chargefrom the reversal charge [reversal22]. The [22] UVA. The was UVA reduced was reducedto about to the about same the point same where point thewhere permeate the permeate volume volume reached reache its maximum,d its maximum, after whichafter which the UVA the UVAbecame bec constant.ame constant. The residual The residual TOC stayedTOC stay essentiallyed essentially at the sameat the level, same until level, the until polymer the polymer dose reached dose reache0.8 mg/L,d 0.8 whenmg/L, it when started it start to increaseed to increase at the same at the rate same as rate the polymeras the polymer dose. The dose. only The organic only organic matter matteradded added to the samplesto the samples was the was polymer, the polymer, and because and because the polymer the polymer did not d significantlyid not significantly absorb absorb the UV thelight, UV it light, is reasonable it is reasonable to assume to assume that the that TOC the increase TOC increase was caused was bycaused the increase by the increase of the polymer. of the polymerTo illustrate. To illustrate this, experiments this, experiments were done were where done the where polymer the polymer was gradually was gradually added to added Milli-Q to Milli water- Q(see water Figure (see6 Figurebelow). 6 below). In Figure In6 F,igure it is possible 6, it is possible to see an to increasesee an increase in the UVAin the when UVA thewhen polymer the polymer dose doseapproaches approaches 2 mg/L. 2 mg/L. Note Note that thisthat increasethis increase is only is 0.02only m0.02−1. m Following−1. Following the Swedishthe Swedish regulatory regulatory limit limitfor polyacrylamide for polyacrylamide in drinking in drinking water water [21], [21] using, using 0.5 mg 0.5 polymer/L mg polymer/ rawL raw water water resulted result ined a in UVA a UVA and andTOC T reductionOC reduct ofion 74% of 74 and% and 49%, 49 respectively.%, respectively.

Figure 6. The effects of the polymer dosage on the UVA and TOC added to the Milli-Q water. Each Figuredata point 6. The is theeffects result of ofthe three polymer iterations. dosage The on first the dataUVA point and TOC is for added the pure to Milli-Qthe Milli water.-Q water The. labelsEach datafor each point data is the point result show of three the added iteration polymers. The dosagefirst data measured point is for in mg. the pure Milli-Q water. The labels for each data point show the added polymer dosage measured in mg. Figure7 shows that similar trends were observed when a 40- µm disc filter was used. The UVA decreased until the polymer dose reached about 0.5 mg/L, while the residual TOC remained constant with a small increase when the polymer dosage was 0.2 mg/L. The variation of the results at lower polymer dosages was most likely due to insufficient polymer dosages. This could be an explanation for why a temporary increase in the TOC and permeate volume could be observed. As mentioned before, if 0.5 mg/L of polymer was used, a UVA and TOC reduction of 70% and 50%, respectively, could be obtained. This result is similar to what was observed when using the 10-µm disc filters but this time with a higher permeate volume. However, reaching a sufficient polymer dosage appeared to be more important.

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Figure 7. The effects of the polymer dosage on the UVA, TOC, and permeate volume using a 40-µm disc Figure 7. The effects of the polymer dosage on the UVA, TOC, and permeate volume using a 40-µm filter. The average raw water UVA (n = 4) and TOC (n = 2) was 18.2 ± 0.15 and 9.3 mg/L, respectively. disc filter. The average raw water UVA (n = 4) and TOC (n = 2) was 18.2 ± 0.15 and 9.3 mg/L, Each data point in the figure is the average of three iterations (n = 3). respectively. Each data point in the figure is the average of three iterations (n = 3). Water with a TOC content above 4 mg/L is unsatisfactory for drinking water but is suitable as Figure 7 shows that similar trends were observed when a 40-µm disc filter was used. The UVA pre-treated water used for artificial groundwater recharge. Lowering the content of organic matter to decreased until the polymer dose reached about 0.5 mg/L, while the residual TOC remained constant 4 mg/L would reduce the organic matter load in the infiltration basins and likely ensure high-quality with a small increase when the polymer dosage was 0.2 mg/L. The variation of the results at lower groundwater [3,5,6]. polymer dosages was most likely due to insufficient polymer dosages. This could be an explanation There are some alternatives remaining that could be investigated in future studies. Other for why a temporary increase in the TOC and permeate volume could be observed. As mentioned coagulants and different polymers could be used, in particular, aluminum-based coagulants and before, if 0.5 mg/L of polymer was used, a UVA and TOC reduction of 70% and 50%, respectively, so-called green polymers, which might be more suitable for drinking water due to the trace amounts could be obtained. This result is similar to what was observed when using the 10-µm disc filters but of acrylamide (monomer) found in synthetic polymers. The concern might be that these options this time with a higher permeate volume. However, reaching a sufficient polymer dosage appeared might not be as effective in reducing NOM. One important uncertainty that needs to be investigated to be more important. is the residual polymer, which can cause clogging in the infiltration basins and other subsequent Water with a TOC content above 4 mg/L is unsatisfactory for drinking water but is suitable as treatment steps. Lastly, how these results, in particular, permeate volume, will carry over to pilot-scale pre-treated water used for artificial groundwater recharge. Lowering the content of organic matter to implementation remains to be seen and would make for an interesting study. It is notable that disc 4 mg/L would reduce the organic matter load in the infiltration basins and likely ensure high-quality filters have high fluxes and, therefore, can be used for very compact waterworks designs. groundwater [3,5,6]. 4. ConclusionsThere are some alternatives remaining that could be investigated in future studies. Other coagulants and different polymers could be used, in particular, aluminum-based coagulants and so- In this study, the possibility of combining chemical flocculation and disc filtration for drinking called green polymers, which might be more suitable for drinking water due to the trace amounts of water treatment was demonstrated. The most important results can be summarized as follows: acrylamide (monomer) found in synthetic polymers. The concern might be that these options might not• beWhen as effective combining in reduc jar testsing withNOM. 10- One and important 40-µm disc uncertainty filters, it was that possible needs to to be reduce investigated the UVA is andthe residualTOC polymer, by 74% andwhich 49% can and cause 70% clogging and 50%, in respectively. the infiltration basins and other subsequent treatment steps.• The Lastly, optimum how polymer these results, dosage in for particular the TOC, permeate and UVA volume, reduction will and carry permeate over to volume pilot-scale was implementation0.6 mg/L when remains using to 10- beµ seenm disc and filters. would Good make results for an were interesting also achieved study. following It is notabl thee that Swedish disc filtersregulatory have high limits fluxes for and drinking, therefore water,, can 0.5 be mg/L. used for The very polymer compact dosage waterworks exceeding designs. 0.6 mg/L showed signs of clogging and residual polymer in the effluent. 4•. ConclusionsFor 40-µm disc filters, the results varied more at the lower polymer dosage and reached stable resultsIn this study, at 0.5mg/L. the possibility Increasing of combin the polymering chemical dosages flocculation above 0.5 mg/L and disc did filtration not improve for drinking the TOC waterand treatment UVA reduction, was demonstrated. although permeate The most volume important increased. results can be summarized as follows: • Switching the 10-µm disc filters for the 40-µm disc filters using 0.5 mg polymer/L, the permeate  When combining jar tests with 10- and 40-µm disc filters, it was possible to reduce the UVA and volume was increased by approximately 30%. TOC by 74% and 49% and 70% and 50%, respectively.  The optimum jar tests showed polymer the dosage importance for the ofTOC sufficient and UVA polymer reduction dosage and andpermeate flocculation volume times was for0.6 UVAmg/L reduction when and using the 10 maximization-µm disc filters. of permeateGood results volume. were Atalso least achieved 180 s wasfollowing needed the to Swedish achieve successfulregulatory flocculation limits for and drinking UVA reduction water, 0.5 when mg/L. using The p Feolymer3+, with dosage an observed exceeding improvement 0.6 mg/L showed up to signs of clogging and residual polymer in the effluent.

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1200 s. Moreover, a 60-s flocculation time for the polymer, seemed to be sufficient for UVA reduction. However, the permeate volume increased with extended flocculation times.

Author Contributions: Data curation, K.H.; Investigation, K.H.; Supervision, K.M.P.; Original draft preparation, K.H. and M.C.; and Review and editing, M.C. and K.M.P. Funding: This research received no external funding. Acknowledgments: The authors acknowledge Nordic Water for supplying laboratory scale disc filters and BASF for providing anionic polymers. We would also like to acknowledge Tobias Persson and Britt-Marie Pott for their advice, and Marianne Franke, Dan Bogren, and Agneta Jönsson for their technical support. Conflicts of Interest: The authors declare no conflicts of interest.

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