초록
낙동강 원수에 대한 정수처리과정의 주요 공정인 응집과 여과에 미치는 조류의 장애를 조사하기 위하여 낙동강 하류지역인 칠서정수장 취수원에서 1995년 1월부터 1998년12월까지 발생되는 계절별, 종별 조류 천이과정과 조류가 응집과 여과에 미치는 영향을 실제 정수장에서 적용 사례를 중심으로 조사한 결과는 다음과 같다. 취수원인 낙동강의 Chlorophyll-a 농도는 최저 $20.7{\mu}g/l$ 에서 최고 $180.9{\mu}g/l$ 이었고,총질소와 총인의 농도는 각각 3.4mg/l 와 0.1mg/l 이상으로 년중 과영양상태로 평가되었다. 조류 우점종은 $Stepanodiscus\;sp.\;{\to}\;Asterionella\;sp.\;{\to}\;Melosira\;sp.\;{\to}\;Microcystis\;sp.\;{\to}\;Synedra\;sp.\;{\to}\;Stepanodiscus\;sp.$로 천이되었다. 정수장애는 여과시 폐쇄 및 응집, 침전불량 이었고, 장애 발생빈도는 Stepanodiscus sp.와 Synedra sp.가 각각 42%와 38%이었다. 조류 개체수가 Stepanodiscus sp.와 Synedra sp.는 각각 1,820cells/ml와 800cells/ml일 때 정수장애가 시작되었다. Chlorophyll-a 농도가 $18.9{\mu}g/l$ 일 때 Alum과 PACS의 최적응집제 주입량은 각각 40mg/l 과 16mg/l 에서 처리수 탁도가 가장 양호하였으며, Chlorophyll-a 농도가 $154.1{\mu}g/l$ 로 증가될 때 Alum과 PACS의 최적응집제 주입량은 각각 75mg/l 과 35mg/l 에서 처리수 탁도가 양호하여 Chlorophyll-a 농도가 증가할수록 응집제 주입량이 증가하였다. 응집제를 최적으로 주입하였을때의 조류 제거율은 Stepanodiscus sp. 경우 Alum과 PACS 주입량이 각각 60mg/l 과 30mg/l 일 때 각각 85%와 83%이었으며, Synedra sp. 경우 Alum과 PACS 주입량이 각각 50mg/l 과 25mg/l 일 때 각각 79%와 81%이었다. 여과지에 유입된 Synedra sp.의 개체수가 1,500cells/ml일 때 여과지 폐쇄를 유발하였고, 이 때 여과지속시간은 8hr이었다. 여과지 폐쇄를 일으키는 대부분의 조류는 안트라사이트 표층으로부터 10cm 깊이에서 억류되었으며, Synedra sp.는 표층 5cm 깊이에서 97% 억류되었다.
Seasonal variations of dominant algae species and the effects of these algae on coagulation and filteration of water treatment were investigated at Chilseo water treatment plant in downstream of Nakdong river from January in 1995 to Desember of 1998. The water quality of Nakdong river was found to be a hyper eutrophic state during the investigation periods. In the measurement, Chlorophyll-a contents ranged $20.7{\sim}180.9{\mu}g/l$ and total nitrogen contents(T-N) and total phosphorus contents(T-P) exceeded more than 3.4mg/l and 0.1mg/l, respectively. The changes in dominant algae species was in the order of Stepanodiscus sp., Asterionella sp., Melosira sp., Microcystis sp. and Synedra sp. from spring to winter. Microcystis sp. especially, was blooming during summer and Synedra sp. and Stepanodiscus sp. during winter. Although most diatomous algae appeared in the water treatment process caused filter clogging and reduced efficiency of coagulation and sedimentation, Synedra sp. and Stepanodiscus sp were revealed as the main trouble algae. Malfunction of water treatment process caused by Synedra sp. and Stepanodiscus sp. started at the algae concentrations of 800cells/ml and 1,820cells/ml, respectively. When chlorophyll-a content was $18.9{\mu}g/l$, the optimum amounts of coagulant were found to be 40mg/l of Alum and 16mg/l of PACS. Under condition of chlorophyll-a content of $154.1{\mu}g/l$, addition of Alum at the level of 75mg/l and PACS at the level of 35mg/l showed the lowest turibidity. The result indicates that increased amounts of the coagulants should be added for a better water treatment as chlorophyll-a contents increased. Addition of Alum at the amount of 60mg/l and 30mg/l of PACS removed Stepanodiscus sp. algae at the rate of 85% and 83%, respectively. In case of Synedra sp., 50mg/l of Alum and 25mg/l of PACS showed removal rates of 79% and 81%, respectively. Synedra sp. algae at the standing crops of 1,500cells/ml started filter clogging and a filtering process was completely inhibited after 8 hours. At this situation the filter clogging by Synedra sp. algae occurred at the depth of 5cm from the top anthracite layer. On the other, other algae did filter clogging at the depth of 10cm.
(Dept. of Agricultural Chemistry, Sunchon National University)
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