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[JapaneseJournal of Water Treatment Biology Vol.33 No.4.179-186 1997]

Inconsiderable Amount of Phytoplankton Leakage by Algal Mat Peeling from a Slow Sand Filter

NOBUTADA NAKAMOTO, NORIYASU IWASE1, KENTARO NOZAKI2, and MASASHI SAKAI3

Department of Applied Biology,Faculty of TextileScience and Technology,Shinshu University, /Ueda, Nagano 386,Japan 1 Nippon JogesuidoSekkei Co. Ltd., 2Center forEcological Res., Kyoto Univ., 3NiigataEnvironment . Hygene Res.Inst., Niigata

Abstract Heavy algal mat of a filamentous diatom is sometimes observed in an open slow sand filter. Amount of particleleakage breaking through a slow sand filterwas measured as an indicatorof chlorophylla concentration in filtratewater during the filterrun. Chlorophyll a concentration decreased rapidly since the beginning of filteroperation. Chlorophyll a concentration after ll days of filterrun reached to inconsiderableamount of 0.005 Chl. a mg m-3. This means the particleconcentration breaking through the slow sand filterwas negligiblysmall amount. Algal mat peeling during a continuous culture condition of filamentous diatom does not cause the par ticleleak through a slow sand filter.

Key words: slow sand filter, algal mat, leakage, particle, chlorophyll

varians. Development of algal mats of INTRODUCTION filamentousdiatom in slow sand filtershas a Algal mat of filamentousdiatom Melosira beneficialeffect of the prevention of filter varians is observed in slow sand filters clogging, the promotion of heterotrophic during the warm period which received activityby the oxygen production and the nutrient rich water.1,2)The suspended removal of nutrientsfrom the raw water.2,3,6) matter from a river is trapped within this Therefore the filamentous diatom in this algalmat. Development of algalmat reduces system acts as an automatic bio-eliminator the particulateload to the sand filterand and purifier.2,7)However it is said that reduces the filterresistance at Someya water pullingsome of the sand and filterskin with works in Ueda city,Nagano.2,3) Large pieces floatingalgal mat reducesthe effidencyof filter.8-12) of algal mat floatto the surface liftedby It is said that the suspended matter enters oxygen bubbles generated by photo- into the sand layer from the bare spot and synthesis. They flows out through the is easy to break through into the filtrate. overflow outlets.Algae reproduces again on Phytoplankton leakage from the rapid sand the spots leftbare on the sand bed during filterto a filtratewater was sometimes the warm period. The filterbecomes a reported and it gave a problem.13) continuous culture system of Melosira Chlorophyll measurement in the filtrate 180 Japanese J.Wat. Tret. Biol. Vo1.33 No.4

water is one of the most sensitive and the chlorophyll a concentration as an convenient techniques to evaluate the indicator of suspended matter. Chlorophyll a relativelysmall amount of phytoplankton for concentration was determined by the the water purificationplant of the surface ordinary UNESCO method using a glass fiber water of a lake and a stream.14) filter and acetone extraction. A special This study was done to confirm the designed technique in Figure l was adopted inconsiderable amount of particle leakage to collect suspended matter in a filtrate broken through the filter during filter water. The filtratewater passing through the operation to evaluate the significance of a slow sand filter was taken from the continuous culture system of algae in a slow regulating well. The filtratewater from the sand filter. regulating well was directly led to an in-line filterholder for 47mm filter(Gelman Science FIELD AND METHOD Co.) at the lower water level using vinyl tube The study was carried out at slow sand (inner diameter 6 mm). The water was gently filtersof Someya waterworks in Ueda city, filtered by a Whatman GF/C glass fiber Nagano Prefecture. The water source is the filter(retention particle size 1.2micro meter) surface water of a stream which runs using a natural gravity. The amount of through an active agricultural area. The filterablewater by one disk of a glass fiber water taken passes through a simple filterof 47mm GF/C was from 50 to 150 sedimentation basin before entering the slow itersl which was measured the amount stored sand filter.The depth of supernatant water in a plastic container (200 lters). The in a filterpond is about one meter and its quantitative sampling of algal mat on the area of each filterbed is 780 m2. Each filter filterbed was done using an algalmat sampler.15) pond has 1 or 2 overflow outletsto keep the The chlorophyll a was used for an indicator water level and for the evacuation of floating of algal mat development. scum. When the filter is clogged, filtering RESULT process is stopped and the supernatant water is drained off.And one or two centimeters Raw Water and SupernatantWater Turbidity of the surface mud is scraped and is removed of the raw water and the inflow water to the to recover the filterresistance. This surface filterpond during the experiment is shown mud scraping is done about once a month in Figure 2 and chlorophyll a concentrations from May to November and about twice in of the both water are shown in Figure 3. winter and early spring when the filamentous The turbidity was reduced to about half diatom does not well developed3). value after the sedimentation basin. The The experiment was performed at 3 filter turbid water flew into the works from beds during August g to September 21, September 8 to September 16. The coagulant 1993.The turbidity data was provided from of poly aluminum chloride was added into the Ueda city waterworks. The turbidity unit raw water when the turbidity exceeded 10 was shown in ppm which was measured by ppm in this waterworks. During this period a turbidity meter. The raw water from the the turbidity of the inflow water to the filter surface water of a stream, the inflow water was effectivelyreduced by this treatment. to the filter pond which was same as a The chlorophyll a concentration increased supernatant water of filter pond and the with the inflow of the turbid water. The filtratewater were collecteddaily to measure inflow water after the sedimentation basin Inconsiderable Amount of Phytoplankton Leakage by Algal Mat Peeling from a Slow Sand Filter 181

Figure 1 Schematic diagram of sampling technique for algal mat on the filterbed and suspended matter in filtratewater by an in-line filterholder using the natural gravity.

Figure 2 Changes of turbidityin the raw water (solidline) from a riverand in the inflow water (broken line)to the filterpond during experimental period(from August 9 to September 21). 182 Japanese J.Wat. Tret. Biol. Vol.33 No.4

Figure 3 Changes of chlorophyll a concentrations in the rawwater (solid line) from a river and in the inflow water (broken line) to the filterpond during experimental period.

decreased by an addition of coagulant. The mat on the sand bed ranged 100 to 500 chl. average concentration of the inflow water to a mg m-2. The filterpond thus became a the filter pond was 1.41±0.95 chl. a mg continuous culture system of filamentous m-3 (mean±std, n=24) during the algae.Nakamoto and Sakai 1994 reported experimental period. average amount of floating algal mat under

Development of Algal Mat Developmental this condition in this water works was 44.4 patterns of algal mat as changes of chl. a mg m-2 during 11 days by a harvest chlorophyll a per area against filter run are experiment. In this system, 32.4% of the shown in Figure 4. The lines expresses the algal mass on the bed lifted off every day data of three filter beds during the and flew out through the overflow outlet.6) observation period. The initial amount of An area of 32.4% of the sand bed appeared chlorophyll a after scraping was about 10 chl. as the bare spot every day. a mg m-2 and it increased exponentially until Chlorophyll a concentration as anindicator

10 days. The amount of algal mat reached of breaking through suspended matter in the about 150 chl. a mg m-2 after the initial filtratewater during the experiment period exponential growth. After a submerged was shown in Figure 5. The concentration on growth, formation of oxygen bubbles the day just after the scraping was 0.009 generated by the photosynthesis of the algal and 0.024 chl. a mg m-3 when the filtering mat caused parts of algal mat to be released rate was extremely slow of less than 1m from the bottom and to come floating to the d-1 on the first day of filterrun. Then the surface to the pond. And it flew out through filteringrate speeded up to about 1 m d-1 the overflow outlets. Filamentous algae then on the 2nd day. The filteringrate speeded reproduced again on the bare spots of the up gradually and it reached 3 to 5 m d-1 surface of the sand bed. The amount of algal until several days of filterrun and it kept Inconsiderable Amount of Phytoplankton Leakage by Algal Mat Peeling from a Slow Sand Filter 183

Figure 4 Development of algal mat as an indicatorof chlorophylla amount against days of filterrun on the slow sand filterduring the filteroperation. The data express the changes at three differentfilter beds during experimental period.

Figure 5 Changes of chlorophylla concentrationin the filtrateagainst days of filterrun during the filteroperation. The data express the changes at three differentfilter beds during experimental period.

almost same flow rate until the end of filter algal mat was matured after 11 days, the operation. The maximum concentration of concentration in the filtrate water was

0.25 chl.a mg m-3 was measured on the 2nd 0.005±0.005 chl. a mg m-3(mean±std, n= day of filteroperation. However it decreased 43). rapidly within initialseveral days. When the 184 Japanese J.Wat. Tret. Biol. Vol.33 No.4

the water must pass before reaching the DISCUSSION filter medium itself.The Schmutzdecke Clogging of slow sand filteris caused by consistsof threadlikealgae and numerous the accumulation of schmutzdecke on the other froms of life,including plankton, sand surface.It is said that the suspended diatoms,protozoa, rotifers, and bacteria. matter is effectively trapped by this The small chlorophylla concentrationin schmutzdecke. Then the strainer effect the filtrateon the firstday just after the increases with the accumulation of dirty scraping may be due to the extremely slow material on the bed.8-12,16)When the filteringrate of less than 1m d-1. The development of a filamentousdiatom mat is particulate suspended matter may be observed and the filterbecomes under a precipitatedby the settlingaction onto the continuous culture of algae, however, the grains.11) filters are not easily clogged.2-6)This The initialleakage of relativelyhigh experiment shows the amount of particle concentrationof this experiments may be broken through the filteris inconsiderable caused by absence of suspensionfeeder such amount during the continuous culture as protozoa,rotifer and other small animals condition.However, the suspended matter in the schmutzdecke and in the upper sand may enter the sand layer from the bare spots layer which organisms are removed by the of the surfaceof the sand bed. It means there scraping. Lloyd20) and Wotton et al.21) is a trap mechanism in the upper sand layer. emphasize the role of suspensionfeeder as a It is well known that almost all the remover of particlein the filterbed. These suspended matter penetrated into the sand organisms livesin a biologicallyactive zone are easily trapped and are decomposed within the upper layer of the filterbed. It within the surface sand layer.16-19)This trap takes a time to mature the biologicalactive may cause by the physical,chemical and zone within the sand layer. biologicalmechanism. Our result shows it This low concentrationof chlorophylla of took a time for maturity to give fullplay of 0.005 chl. a mg m-3 in the filtrateis suspended matter trapping in the sand layer. extremely low level in comparison with Generally speaking,8-12)peeling the algal natural lakes.22)The concentrationof Lake mat and making bare spots on the filterbed Biwa is 2 to 7 chl.a mg m-3 at the center reduce the efficiencyof filter.And the of the northern basin which secchi suspended matter enters into the sand layer transparency is 5 to 8 meters. The from the bare spots and breaks through into chlorophyll a concentration of the most the filtrate.On the contrary,our resultshows clearestnatural lake of Lake Mashu in Japan that the peeling gives no damage to leak is 0.22 to 0.35 chl.a mg m-3 which secchi through the sand filterafter 11 days of filter transparency is 22 to 25 meters. The run. However some amount of suspended chlorophylla concentrationof 0.005 chl.a mg matter leaks through at the beginning of m-3 in the filtrateis almost equivalentto the filterrun. concentrationin a clearwater at 100m depth Huisman and Wood 1974 described the in the open ocean.23)We conclude that the mechanisms of biologicalfiltration.11) On the significantharmful leakage of suspended surface of the sand there is a thin slimy matter in the filtratewater does not occur matting of material,laragely organic in origin, during the continuos culture condition.The known as the schmutzdecke, through which peelingof algalmat on the sand filterbed is Inconsiderable Amount of Phytoplankton Leakage by Algal Mat Peeling from a Slow Sand Filter 185 not harmful under the continuous culture The Hague The Netherlands, (1985). condition. 9) Kojima, S.: Biology of drinking water. 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