Removal of Cyanobacterial Toxins in Water Treatment Processes: Laboratory and Pilot-Scale

Experiments

A.M. KEIJOLA, University of Helsinki, Department of Microbiology, SF-00710 Helsinki, Finland; K. HIMBERG, Technical Research Cen- tre of Finland, Food Research Laboratory, Biologinkuja 1 , SF-02150 Espoo, Finland; A.L. ESALA, K. SIVONEN, University of Helsinki, Department of Microbiology, SF-Helsinki, Finland; and L. HIIS- VIRTA, National Board of Health, P.O. Box 220, SF 00531, Helsinki, Finland

Abstract The effectiveness of various drinking water treatment processes has been tested in laboratory and pilot scale experiments using neurotoxic and hepatotoxic cyanobacterial blooms and laboratory-grown algal cultures as test material. The species involved be- longed to the genera Microcystis, Oscillatoria, and Annbaenn. Microcystis and Oscillato- ria hepatotoxins were analyzed by high performance liquid chromatography. The Ann- baena neurotoxin, anatoxin-a, was de t edned using gas chromatographylmass spectrometry techniques. Toxicity waa tested and verified by mouse bioassay. The labo- ratory study, made with freeze-dried cyanobacteria, showed that toxins were not re- moved by conventional flocculation treatment procedures. The simultaneous addition of a small amount of powdered activated carbon with the flocculation chemical did not improve the removal of hepatotoxins significantly. Substantial reduction of toxins was achieved by granulated activated carbon filtration and by ozonization. Pilot plant stud- ies supported the laboratory findings. Results of the slow sand filtration experiments suggested some removal of toxins.

INTRODUCTION The increasing eutrophication of water supplies from urban and agri- cultural sources has increased the occurrence and intensity of cyano- bacterial blooms. The bloom-forming cyanobacteria may produce tox- ins: hepatotoxic peptides and neurotoxic alkaloids. These have caused animal poisonings all over the world. Toxic blooms have been reported from at least 25 countries. It has been suggested that toxic cyanobacte- ria may also be a health risk for humans (Carmichael, 1988; Codd and Poon, 1988)

Toxicity Aseessment: An International Journal Vol. 3, 643-656 (1988) 0 1988 John Wiley & Sons, Inc. CCC 0884-8181/881050643-14$04.00

644/KEIJOLA ET AL.

Acute poisonings of humans by freshwater cyanobacteria have not been reported, but some evidence of the susceptibility of humans to these toxins exists (Gorham and Carmichael, 1988). The hepatoen- teritis outbreak at Palm Island in Australia in 1979 affected 148 peo- ple, mostly children, and it was later suspected to be due to a dense growth of hepatotoxic Cylindrospermopsis rasiborskii in the drinking water supply of the island (Byth, 1980; Hawkins et al., 1985). Epidemi- ological investigations made in a hospital in Armidale, Australia, showed increased levels of hepatic enzymes in plasma from patients whose drinking water was supplied from the reservoir contaminated with toxic Microcystis aeruginosa bloom (Falconer et al., 1983a). In both cases the bloom had been treated with copper sulphate.

Only some research has been done about toxin removal in water treatment. Hoffman (1976) found in laboratory scale investigations that toxins produced by Microcystis aeruginosa were not removed by the normal coagulation-filtration-disinfection treatment process. The removal was achieved only by adding large amounts of activated car- bon powder to the process or by activated carbon filtration. Falconer et al. (1983b) found in a pilot plant study that a 7-8 cm layer of granular activated carbon on the top of a sand filter removed microcystin.

Toxic cyanobacteria also occur in Finnish fresh- and coastal wa- ters. In a recent survey, 44% (83/188) of the bloom samples were found to be toxic. Both hepatotoxic and neurotoxic blooms were also found in raw water sources (Sivonen et al., 1988).

Half of the raw water intake in Finland is surface water and the majority (70%) of surface water treatment plants have the conven- tional coagulation treatment process (National Board of Health, 1986). Several raw water sources are eutrophicated. For this reason it was essential to study remedial methods, in case of toxic cyanobacterial bloom in raw water sources.

The aim of our experiments was to study the behavior of the hepa- totoxins of Microcystis and Oscillatoria agardhii and the neurotoxin of Anabaena flos-aquae under different water treatment processes. This paper presents a summary of the results of the laboratory and pilot- scale experiments made at the Technical Research Centre of Finland, University of Helsinki, and the water treatment plant of Pitkakoski, Helsinki. More detailed results of the laboratory scale experiments are published elsewhere (Himberg et al., 1988). The processes studied were the conventional coagulation treatment process as such and also com- bined with simultaneous activated carbon treatment or preozon- ization. The effect of slow sand filtration on toxin removal was also tested.

REMOVAL OF CYANOBACTERIAL TOXINS1645

MATERIALS AND METHODS

Cyanobacteria Materials

Natural blooms of cyanobacteria were collected by plankton net (mesh size 25 pm) from lakes in southern Finland. In the laboratory the material was concentrated by filtering through nylon mesh (25 pm), and then frozen and freeze-dried. The experiments with 0. ugardhii were made with a laboratory-grown culture (NIVA CYA 126). The cyanobacterial materials used in experiments are presented in Table I.

Analysis of Toxins

Acute toxicity of the materials and isolated toxin fractions were tested by mouse bioassay by intraperitioneal injection: female mice (NMRI) weighing 20-25 g were used. The LD60 of the materials was calculated using probit analysis (Finnley, 1963) .

The hepatotoxins were analyzed by the HPLC (high performance liquid chromatography) technique using toxin standards that had been isolated from corresponding hepatotoxic cyanobacterial materials (Ta- ble I).

TABLE I Fresh and freeze-dried natural bloom and laboratory-grown culture materials

used in experiments

Toxicity Material Species Origin LDSO mg/kg Experiment

Freeze-dried M. wesenbergii Lake Pitka- MLDloo 250' Laboratory bloom (dominating) jarvi, Espoo a

M . viridis Freeze-dried Oscillatorin Lake LIngsjbn 190' Laboratory,

laboratory agardhii CYA h a n d slow sand culture 126 filter

Freeze-dried Anabaem flos- Lake Sayhteen- 90b Laboratory, bloom aquae (domi- jarvi slow sand

nating) + M . filter aeruginosa

Freeze-dried M . aeruginosa Lake Mallus- 60" Pilot, slow sand

Fresh bloom M . aeruginosa Lake Mallus- 75" Pilot bloom jiirvi filter.

j w

a Hepatotoxic. Neurotoxic. The amount of the bloom material was inadequate for the determination of LDw,

MLDIw = approximated minimal lethal dose 100%.

646/KELJOLA ET AL.

Hepatotoxin standards were extracted from freeze-dried cyanobac- teria by a method used for hepatotoxins of cyanobacteria (Siegelman et al., 1984; Krishnamurthy et al., 1986; Berg et al., 1987). It involved extraction of the freeze-dried cells with 5% butanol-20% methanol in water, separation of the crude toxin from cell pigrnents to Sep-Pak C18 (Waters) or to Bond-Elut C18 cartridge (Analytichem lnc.), and finally, purification of the toxin on a semipreparative reverse-phase C18 HPLC column.

Two toxin fractions were found in each hepatotoxic material. The first toxic fraction isolated from M. aeruginosa bloom material of Lake Mallusjarvi had such a small peak in HPLC chromatogram that it was not considered when toxins were analyzed from water samples. Codes M1 and M2 are used for the toxin fractions isolated from Microcystis bloom material of Lake Pitkajarvi, and 01 and 0 2 for toxin fractions of 0. agardhii. The isolated toxins were considered to be pure enough for comparative analysis of toxins.

The principle for the determination of hepatotoxins in water sam- ples was the same both in laboratory and pilot-scale experiments. Dif- ferences occurred mainly in instrumentation and sample volumes.

In laboratory investigations, 1.0 1 volume of the water sample was vacuum filtered through six Sep-Pak C18 cartridges, which were then eluted with 3 mL of methanol. For samples taken before treatment operations, the determination was made from filtered (Whatman 41) 1.5 L water volume samples and the number of cartridges used was eight. Combined methanol fractions were evaporated to dryness under nitrogen flow and the residue was dissolved in 1 mL of 80% HPLC solvent.

In pilot-scale investigations 0.5-1.5 of the 0.5-9 L water volume samples was vacuum filtered through a Bond Elut C18 cartridge, which was eluted with 5 mL of methanol. Combined methanol frac- tions were evaporated (40°C) to dryness and the residue was dissolved in 2 mL of HPLC solvent. Samples taken before treatment processes or after preozonization were first filtered through a Whatman GF/C filter. To check the analytical method, the HPLC solvent used in analysis was collected and its toxicity was tested by mouse bioassay.

The HPLC systems used for determinations of hepatotoxins in water samples are presented in Table 11.

The anatoxin-a analysis was performed with the GC/MS (gas chro- matographylmass spectrometry) technique without a standard be- cause of the instability of the isolated toxin. Anatoxin-a was analyzed from 100 mL water volume samples that were first made alkaline (pH > 11) and extracted with chloroform. The chloroform phase was sepa- rated and made acidic with hydrochloric acid. After adding 5 mL of

REMOVAL OF CYANOBACTERIAL TOXINS1647

TABLE II The HPLC systems used in hepatotoxin analysis of water samples

Laboratory Pilot scale investigations investigations

Column NOVA-PAK Cl8 ,5 pm 2.6 x pBONDAF'AK C18,5 p m 3.9 x 300 mm (waters) 150 mm (waters)

inserts C18 inserts

ammonium acetate ammonium acetate

Precolumn Guard-PAK with 10-pm C18 Guard-PAK with 10-pm

Solvent 26% acetonitrile in 10 mM

Solvent flow 0.9 mL1min 1.0 mL1min Monitored at 240 nm 238 nm Injection 20 pL 50 pL

26% acetonitrile in 10 mM

Retention Microcystis 3.5 and 4.2 M ~ ~ K J Q W ~ S 15-16 timeslmin Oscillntoria 5.2 and 5.6 Oscillntoria 11 and 15

methanol to the solution, it was evaporated to dryness. The residue was dissolved in chloroform. Acetylic derivate was made with acetic acid and acetic anhydride. The sample was evaporated, after which the residue was dissolved in 0.1 mL of chloroform for GUMS analysis. The analytical method will be published separately elsewhere (Himberg et nl., unpublished).

Laboratory Investigations, Experimental Design Based on surface-water treatment processes applied in Finland, the following treatment processes were selected for laboratory investiga- tions:

-flocculation with aluminium sulphate + sand filtration + chlo-

-flocculation with ferric chloride + sand filtration + chlorination -addition of activated carbon powder + flocculation with alumin-

-flocculation with aluminium sulphate + sand filtration + filtra-

-0zonization + flocculation with aluminium sulphate + sand fil-

The presence of a cyanobacterial bloom was simulated by adding 50 mg of freeze-dried hepatotoxic cyanobacteria to 1.5 L volume, and 5 and 50 mg of freeze-dried neurotoxic cyanobacteria to 1.0 L volume of un- treated raw water, and thereafter stirring the water for 30 min in order to liberate the toxins into the water.

rination

ium sulphate + sand filtration + chlorination

tion through granulated activated carbon + chlorination

tration + chlorination

648fKEIJOLA ET AL.

The experiments were performed in triplicate and separately with hepatotoxic M . wesenbergii (+ M. viridis) and neurotoxic Anabaena flos-aquae ( + M . aeruginosa) blooms and hepatotoxic Oscillatoria agardhii culture. The toxin concentrations were determined before and after treatment procedures. The general treatment effectiveness was monitored by determinations of KMn04 value, turbidity, and residual concentrations of flocculation chemicals.

The dosage of flocculation chemicals was based on the KMn04 value of the raw water containing cyanobacteria: for aluminium sul- phate IX and for ferric chloride 0 . 8 ~ KMn04 value. The flocculation pH was adjusted with lime to 6.3 (aluminium sulphate) or to 6.5 (ferric chloride). After rapid stirring, the water was agitated for 30 min at 15 rpm and then clarified for 2 h.

The sand filter consisted of 300 cm3 of washed quartz (particle size 0.5-1.5 mm) with an average filtration rate of 5 m3/m2/h. The acti- vated carbon filter consisted of 85 cm3 granulated activated carbon with an average filtration rate of 1 m3/m2/h.

Dilute sodium hypochlorite was used for chlorination, with the dosage about 0.5 mg CldL. Ozone dosage was 1 mg/L. The effect of a smaller ozone dosage were also tested with some individual tests. Acti- vated carbon powder dosing was 5 mg/L.

Pilot-Scale Investigations, Experimental Design

To study the feasibility of the results from the laboratory investiga- tions for predicting the behavior of cyanobacterial toxins in real water treatment practice, pilot-scale investigations were conducted in the pilot scale water treatment plant of Helsinki City Waterworks. In order to find out whether there is a difference between fresh or freeze- dried cyanobacteria material, the experiments were conducted with both fresh and freeze-dried M . aeruginosa bloom from Lake Mallus- jarvi. Based on the results of the laboratory scale investigations, the following processes were chosen for pilot-scale investigations:

-normal process = aluminium sulphate flocculation + sedimen-

-0zonization + normal process -activated carbon powder 20 mg/L + normal process -activated carbon powder 100 mg/L + normal process -activated carbon powder 200 mg/L + normal process

tation + filtration

The effect of the normal treatment process on the removal of Microcp - tis toxin was tested only with freeze-dried material. Chlorination was not used in the experiments because it has not been shown to remove the hepatotoxins in previous experiments.

REMOVAL OF CYANOBACTERIAL TOXINS/649

The pilot plant received the same raw water as Helsinki City Waterworks, from Lake Paijanne. It has two parallel treatment lines, both having a capacity of 100 L/h.

The basic process without activated carbon and cyanobacteria ad- dition was the following: Raw water was first treated in a carbon diox- ide tower (feed 9.5-13.5 mg COZ/L) and then transferred to an ozoniza- tion tower. Ozone dosage, when used, was 1.0-1.5 mg/L. For chemical coagulation, aluminium sulphate was added to the water and the pH was adjusted to 6.3 with lime. After mixing the chemicals, the water was flocculated in a three-stage flocculator. The water was then clari- fied in a sedimentation tank and filtered through a filter that consisted of anthracite (45-cm layer) and sand (50-cm layer).

The control of plant operation was partly regulated by a computer system that controlled flow rates, chemical dosages, and turbidity of the treated water.

The water containing cyanobacteria was added to the process as a side flow of 5 L/h straight after raw water pumps (95 L/h). The fresh cyanobacteria content in the water to be treated was adjusted to corre- spond to the the dissolved solids concentration of 35 mg/L. Freeze- dried cyanobacteria feed was 24 mg/L. Aluminium sulphate dosage was about 1.3 x KMn04 value of the water containing cyanobacteria (62-75 g/m3). The activated carbon powder (Merck, analytical quality) slurred with tap water was fed as a side flow of 5 L/h to the flocculation basin. Cyanobacteria feed lasted 7.5-8 h. Samples were taken at 2-h intervals at the following sampling points: after cyanobacteria addi- tion, after filtration, and if ozonization was used, also after preozoniza- tion. The pilot process is presented in Fig. 1.

Slow Sand Filtration Experiment

Slow sand filtration was selected for investigation because it is known to remove many organic compounds that cause bad taste or odor, or might otherwise be harmful. Furthermore, one application of slow sand filtration, artificial groundwater intake, is widely used in Fin- land. This process also has problems with algal growth.

Two identical filters A and B with a diameter of 14 cm were built from transparent material and placed indoors. To start the biological functions, the sand used in the filters was taken from a full-scale slow sand filter operating in the water treatment plant of the town of Vaasa. The sand beds were 24-cm deep and had 15-17-cm layers of water on top of them.

Thirty to forty liter volumes of raw water were recycled in each filter and changed twice a week. The filtration was first started with

650/KEIJOLA ET AL.

17 17

Prechemicals Flocculation Mixing Sedimentation Filtration

Al Act.C CaO

2x6 3x75

Raw water MI I

C yanobacterial Sample t

Sample

Fig. 1. Pilot water treatment plant process used in experiments.

the raw water from Lake Paijanne. The filtration rate was 0.085 m/h. To ensure the biological functioning of filter B, treated wastewater was passed through it after three weeks of operation.

The slow sand filtration experiments were made with raw water that had first been mixed with freeze-dried cyanobacteria in order tc liberate the toxins into the water and then filtered with Whatman GF/C filters. Neurotoxic A. flos-aquae and hepatotoxic M. ueruginosu and 0. ugurdhii were used in these experiments.

The hepatotoxins were analyzed from a water volume of 1.0-2.0 L with the method used in the pilot plant investigations.

RESULTS Laboratory Investigations

The reductions of toxin concentrations in laboratory-scale treatment processes are presented in Table 111. The conventional coagulation pro- cesses did not remove the toxins. Effective removal was achieved with

REMOVAL OF CYANOBACTERIAL TOXINS/6!51

TABLE I11 Toxin reductions in laboratory scale (1.5 L) water treatment experiments with

different cyanobacterial materialsa

Treatment process Toxin Toxin

fractionb reduction/%

1. Al-flocculation + sand filtration + chlorination

2. Fe-flocculation + sand filtration + chlorination

3. Activated carbon powder 5 mg/L + Al-flocculation + sand filtration + chlorination

4. Al-flocculation + sand filtration + granulated activated carbon + chlorination

5. Ozonization 1 mg/L + Al-floccualation + sand filtration + chlorination

M1 M2 0 1 0 2 A M1 M2 0 1 0 2 A M1 M2 0 1 0 2 A M1 M2 01 0 2 A M1 M2 0 1 0 2 A

11 11 29 32 14 (0) 0

16 0

<O 49 (0) 20 34 19 13 58 (82)

100 100 100 100

100 100 100 100

94 (97)

96 (100)

a The figure given in parentheses is the toxin reduction achieved from the experi- ments with a small amount (5 mg/L) of Anabaena flos-aquae bloom.

The average concentrations of hepatotoxins in water before treatment processes after 30 min of mixing: M1 = microcystin, first toxic peak = 58 2 7 pg/L, M2 = microcystin, second toxic peak = 38 f 8 pg/L, 0 1 = oscillatoria toxin, first toxic peak = 30 2 1 pglL, 0 2 = oscillatoria toxin, second toxic peak = 44 f 8 pglL. The A (= anatoxin-a) content of the freeze-dried bloom material used in the experiments was 4.4 mg/g.

simultaneous activated carbon filtrations and with preozonization when the dose was 1.0 mg Os/L.

In other respects the water treatment process proved to function normally: The reduction percentage of the KMn04 value was at the level of 80%, and when activated carbon filter was used, almost 100%. The highly turbid water turned clear and opalescent during treatment processes. Residual aluminium and iron concentrations always stayed below 0.6 mg/L.

652/KEIJOLA ET AL.

Pilot Plant Investigations

When freeze-dried M. aeruginosa bloom was used as test material, toxin was not removed by the normal treatment process. There was no reduction of toxin concentration from the most toxic sample.

In experiments made with fresh M. aeruginosa bloom, the toxin concentration of the water before treatment was at the level of 50 pgl L. At the preozonization stage the toxin concentration was reduced by more than 90%. Already the activated carbon dosage of 20 mg/L was sufficient to remove more than 99% of the toxin.

Experiments using freeze-dried cyanobacterial material gave slightly lower reductions of toxins than experiments with fresh mate- rial. In these experiments the toxin concentration before treatment was at the level of 15 pg/L. Preozonization removed about 5 0 % of the toxin and in samples taken after the whole treatment process the toxin concentration was only about 10% of the original. The activated carbon dosage of 20 mg/L removed 90% of the toxin. Dosages of 100 and 200 mg/L removed the toxin completely.

A new peak at a retention time of 11 min appeared in the HPLC chromatogram when the analysis was made from the samples taken after preozonization. The toxicity of this peak was not, however, inves- tigated separately.

Examples of HPLC chromatograms of toxin analysis are presented in Fig. 2.

Both cyanobacterial material and activated carbon powder caused some disturbance in the function of the process. This fact appeared, e.g., from the high turbidity of the treated water in all experiments, except when ozone was used. Ozonization caused flotation of cyanobac- teria in flocculation basins. Residual aluminium concentrations in the treated water were high. The total organic carbon (TOC) and the KMn04 value, however, were at normal levels.

Results are not shown in detail because of the inaccurate nature of the pilot plant experiments. The expected dosage rates were achieved only with an accuracy of + 2 0 % because of the instability of the raw water, activated carbon, and cyanobacteria flows.

Slow Sand Filtration Experiments

Substantial removal of hepatotoxin from M. aeruginosa and neuro- toxin of A. flos-aquae was achieved in slow sand filtration, while in experiments with OsciZZatoria only some reduction of the first toxin fraction was achieved (Table IV). Because of the lack of properly puri- fied standards, the toxin results are given as reduction percentages

5 10 15 20 Time (mid

0 5 10 15 20 Time (mid

1 I I I 1 0

(d)

\

5 10 15 20 Time (min)

I I 1 I I

0 5 10 15 20 Time (mid

Fig. 2. HPLC chromatograms of M. aeruginosa toxin in pilot plant investigations: (a) bloom material, 6) toxic raw water (sample volume 500 mL), (c) sample taken from the ozonization tower (sample volume 3L), (d) sample taken from the activated carbon feed (20 mg/L) containing process (sample volume 8.5 L), (M) measured toxic peak, (N) not measured toxic peak.

TABLE lV Toxin removal in two identically built slow sand filters"

- Reduction of toxin(s)/%

Toxic species Filter A Filter B

Anabaena flos-aquae bloom 68 74 Microcystis aeruginosa bloom 86 82 Oscillatoria agardhii culture

1. Toxic fraction 0 0 2. Toxic fraction 29 65

During the starting period filter A used lake water and filter B treated wastewater.

calculated from differences in chomatogram peak heights. Some of the reduction could be caused by the dilution effect of nontoxic raw water used in filtering before the toxin experiments.

Although the treatment results obtained at the starting stage of the slow sand filtration did not reveal any biological activity in the filter, the development of a biofilm was, however, observed by micro- scopic examination. Especially filter B, where treated wastewater had been used at the starting stage, had problems with algae and it had to be scraped twice during the experiments.

General treatment results achieved from the cyanobacteria exper- iments also showed that the function was biological: the KMn04 value of the toxic raw water was 40-46 mg/L. After filtering, the amount was 32-36 mg/L. The color of the toxic raw water was blue or bluish green. After filtration the water was colorless. The turbidity reduc- tions were 30-70%. During the last experiments (with 0. agardhii) the filters were already overloaded and signs of anaerobiosis could be no- ticed.

DISCUSSION Coagulation-sedimentation-rapid sand filtration-chlorination pro- cesses did not remove the hepatotoxins of Microcystis and Oscillatoriu, or neurotoxin anatoxin-a of A. flos-aquae, from water in laboratory- scale experiments. These results were to be expected. The pilot-scale experiments with freeze-dried M. aeruginosa confirmed this result.

Activated carbon has ben shown to remove cyanobacterial toxins from water (Hoffman, 1976; Falconer et al., 1983b; this study). In labo- ratory-scale experiments, simultaneous addition of activated carbon powder to the coagulation process in a dose of 5 mg/L removed some of the hepatotoxins and more than half of the anatoxin-a. Good reduc-

REMOVAL OF CYANOBACTERIAL TOXINS/655

tions of Microcystis toxin (more than 90%) were achieved in pilot plant experiments when the activated carbon powder dosage was 20 mg/L. The simultaneous addition of high amounts of activated carbon powder makes it difficult, however, to run a water treatment process.

The laboratory-scale granulated actived carbon filter removed Mi- crocystis and Oscillatoriu toxins completely. Anatoxin-a removal was over 90%. The filters were new and the amount of carbon used was relatively high. In long runs the filters might have become saturated.

The effect of ozonization on the removal of cyanobacterial toxins has not been studied before. In laboratory scale experiments, an ozone dosage of 1 mg/L was sufficient to remove the hepatotoxins and most of the anatoxin-a. Individual experiments with lower ozone concentra- tions gave smaller reductions. In pilot scale experiments, 1-1.5 mg/L ozone dosage removed most of the Microcystis toxin when the concen- tration was determined from samples taken after the whole treatment process. Ozone caused flotation of the cyanobacterial material in the flocculation basins, which might explain the improvement of the treat- ment result after preozonization in the normal flocculation process. Postozonization could be a better alternative in toxin removal because more of the oxidation capacity could be used on toxins instead of sec- ondary reactions with other organic material.

Cells disintegrate during the freezing stage, which releases the toxins from cells. The experiments with freeze-dried cyanobacteria cor- respond to the situation where the bloom is treated with copper sul- phate or the cells are dying of old age and the toxins are therefore released into the water phase. An effective water treatment process removes cyanobacteria cells and the toxins within them, which might explain the slightly better toxin reductions with fresh rather than with freeze-dried cyanobacterial material.

The results from the slow sand filtration experiments showed it is possible to achieve substantial removal of cyanobacterial toxins during the biological water treatment process. This method is, however, un- certain because different filters have different biofilms and their abil- ity to remove toxins may thus vary. Further investigations with large- scale filters and longer filtration times are needed before final conclusions can be drawn about the effectiveness of biological filtration for the removal of cyanobacterial toxins.

This study was supported by the National Board of Health, Helsinki Univer- sity, and the Academy of Finland. The authors thank Helsinki City Water and Sewage Authority for the use of their facilities and water treatment pilot plant, the many technical assistants for their help, and Professor S.I. Niemala for the critical reading of the manuscript.

656/KEIJOLA ET AL.

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