Evaluating adaptability of filtration technology to high-turbidity water purification

Transcription

1 Evaluating adaptability of filtration tecnology to ig-turbidity water purification Hiroyuki Takino*, Yuici Izutsu*, Mami Nakamaci*, Daiji Nagasio* *Hansin Water Supply Autority, Kobe City, Japan, Abstract Hansin Water Supply Autority takes surface water from te Yodo River as te source water for water supply. In 212 te turbidity of te Yodo River markedly increased because of torrential rain. Te settled water turbidity reaced 4 degrees, and te filtered water turbidity exceeded.1 degree. In tis study, we examined te effects of various factors on filtered water turbidity at a water treatment plant upon te introduction of ig-turbidity water. In te course of experiment te filtered water turbidity was observed to rise wen ig-turbidity water was introduced immediately after backwasing of te filter medium. But te increase was suppressed in a sorter duration of time wen te filtration speed and recoagulant rate were iger. In-advance recoagulant injection is concluded to be capable of suppressing filtered water turbidity after te introduction of ig-turbidity water. Te filtration using antracite was able to maintain filtered water turbidity under.1 degree for over 3 at te filtration speed of 17m/d and te recoagulant injection rate of 2mg/L, even wen turbidity of te inflowing settled water was as ig as 4 degrees. Keywords Filtration; Hig-turbidity water; Direct filtration; Drinking water; Sand; Antracite INTRODUCTION Recently, several Asian countries ave been experiencing unusual fluctuations in te amount and pattern of precipitation caused by climate cange. In Japan, irregular and localized torrential rain events are increasing, and a concern is tat water purification will be adversely affected by sudden increases in source water turbidity caused by torrential rain. Drinking water turbidity in Japan must be kept under.1 degree as te measure against Cryptosporidium. However, te turbidity of filtered water is at risk of reacing te upper limit of.1 degree because of ig source water turbidity caused by localized torrential rain. Hansin Water Supply Autority (HWSA) takes surface water from te Yodo River as te source water for water supply. In 212 te turbidity of te source water from te Yodo River markedly increased and exceeded 1, degrees because of torrential rain, wic was te first incident in 4 years. Te delay in increasing precoagulant injection togeter wit oter factors caused te settled water turbidity to reac 4 degrees, and resulted in te filtered water turbidity exceeding.1 degree for 2 ours. Tis incident motivated us to examine te filtration,

2 wic is te final process of drinking water purification. In tis study, we examined te filter medium, filtration speed, and recoagulant injection rate under ig-turbidity water conditions to determine teir effect on te filtration efficiency and to devise a means of adapting te direct filtration metod. OUTLINE OF WATER TREATMENT PROCESSES OF HWSA AND SPECIFICATIONS OF THE FILTERS An outline of te water treatment processes of HWSA is sown in Fig. 1. Te specifications of te filter media are sown in Table 1. Te treatment processes of HWSA start wit coagulation, followed by settling, ozonation, granular activated carbon adsorption (up-flow fluidized bed), intermediate clorination, recoagulation and filtration. We apply a coagulation/filtration metod in wic we add a small additional amount of recoagulant to te water prior to filtration, and te water is directly filtered to remove microorganisms and fine particles of activated carbon. Aluminum sulfate is used as te coagulant for bot precoagulation and recoagulation. As te filter medium, sand is used in Inagawa Water Treatment Plant (WTP), and antracite in Amagasaki WTP, bot in monolayer beds. Te filtration speed was designed to be 15m/d for Inagawa WTP and 24m/d for Amagasaki WTP. Presently, te average filtration speed is 11m/d for Inagawa WTP and 17m/d for Amagasaki WTP. Fig.1 Water treatment flow of HWSA Table 1 Specification of filter media Inagawa WTP Amagasaki WTP Designed Actual Designed Actual Filter medium Sand Antracite Layer tickness (mm) ,5 1,39 Effective grain size (mm) Uniformity coefficient Harmonic mean diameter(mm) L/D* 699 1,418 *Te ratio between te filter bed dept and te effective grain size of te filter medium

3 MATERIALS AND METHODS Equipment used in experiment Two sets of experimental filtration equipment (columns: φ2mm) were provided at a WTP, namely Column A and Column B. Column A was filled wit sand (layer tickness: 6mm, effective grain size:.59mm, uniformity coefficient: 1.53), and Column B was filled wit antracite (layer tickness: 1,5mm, effective grain size:.78mm, uniformity coefficient: 1.41) bot in monolayer beds. Te specifications of tese filter media matced tose used at actual plant facilities. An outline of te process flow in experimental setup is sown in Fig. 2. Filter specifications and backwasing details are sown in Table 2. In te experiment, we removed intermediate-clorine-treated water from te actual treatment facility (average turbidity ca..3 degree), added recoagulant (3-5mg/L), and ten mixed tem in a mixing tank (G value: 18sec -1, retention: ca. 1min). Aluminum sulfate was used as te recoagulant in te experiment. Te ig-turbidity water used in te experiment was created by adding residual matter, sieved (sieve size: 63μm) from te bottom mud in te sand settling basin of te intake station, to te intermediate-clorine-treated water and calibrating te water turbidity until it reaced ca. 4 degrees. Fig.2 Process flow in experimental setup

4 Table 2 Filter specifications and backwasing details Air was Air and water was Water was Column A Column B Total duration of filtration 48 6 Settling time Duration Speed Duration Air-was speed 3min.8m/min 1min.8m/min Water-was speed.2m/min.8m/min Duration Speed 1min 1min.8m/min Metods We canged te filtering parameters to identify teir effect on te filtered water turbidity from tree viewpoints: Filtration speed and recoagulant injection rate Te filtration speed was canged between 5m/d and 15m/d for Column A and between 6m/d and 24m/d for Column B. Te recoagulant injection rate was canged between 5 and 2 mg/l for bot columns. Hig-turbidity water was filtered under different combinations of filtration speed and recoagulant injection rate, and te filtered water turbidity was measured. In eac experiment, ig-turbidity water was introduced into te columns immediately after backwasing of te filter medium. Timing of increase in recoagulant injection After continuous filtration of intermediate-clorine-treated water wit an aluminum sulfate injection rate of 5mg/L for 17 at filtration speed of 11m/d for Column A and 17m/d for Column B, ig-turbidity water was introduced into te experimental columns. Te recoagulant injection rate was increased to 2mg/L at tree different timings; before, simultaneously wit, and after te introduction of ig-turbidity water. Prolonged filtering of ig-turbidity water Te filtration speed was set at 11m/d for Column A and 17m/d for Column B. Te recoagulant injection rate was 2mg/L. Te ig-turbidity water was filtered for a prolonged period of time over 3 and te transition of te filtered water turbidity was investigated. RESULTS AND DISCUSSION Effect of filtration speed and recoagulant injection rate Te acquired data are sown in Table 3 for Column A and in Table 4 for Column B. At all

5 filtration speeds and recoagulant injection rates, suspended matter was detected in te filtered water immediately after te introduction of ig-turbidity water, and te filtered water turbidity increased. Te maximum turbidity of te filtered water was 5.6 degrees in Column A and 3.8 degrees in Column B. Te turbidity decreased wit te progress of filtration. Te turbidity decreased more rapidly wen te filtration speed was iger. Tis penomenon is considered to be attributed to te porosity of te filter media, wic was temporarily increased at te beginning of te experiment because of backwasing and caused te breaktroug of turbidity. And te iger te filtration speed, te more rapidly te filter layer returned to its normal porosity and stability, recovering its ig filtering capacity. Te effect of recoagulant injection rate was also examined. Te iger te injection rate was, te more rapidly te turbidity decreased in tis experiment. At te recoagulant injection rate of 2mg/L, te filtered water turbidity decreased to below.1 degree in 6min in Column A (filtration speed 11m/d) and in 3 min in Column B (filtration speed 17m/d). On te oter and, wen te recoagulant injection rate was not increased and fixed at te usual value of ca. 5mg/L, none of te cases except for Column B wit a filtration speed 24m/d, acieved filtered water turbidity of under.1 degree witin 18min. Tis result indicates tat te recoagulant injection rate must be increased for te purification of ig-turbidity water. Table 3 Filtered water turbidity for different filtration speeds and recoagulant injection rates (Column A) Filtration speed (m/d) Recoagulant injection rate (mg/l) Maximum turbidity (degree) Time required to reac turbidity of below.1 degree (min) : Not acieved witin 18 min Table 4 Filtered water turbidity for different filtration speeds and recoagulant injection Filtration speed (m/d) rates (Column B) Recoagulant injection rate (mg/l) Maximum turbidity (degree) Time required to reac turbidity of below.1 degree (min) : Not acieved witin 18 min Effect of timing of increase in recoagulant injection Fig. 3 and 4 sow te results wen recoagulant injection was increased before te introduction of ig-turbidity water. Te turbidity and particle count of te filtered water increased for bot

6 Columns A and B but did not usually exceed.1 degree. Column B sowed an immediate increase in te filtered water turbidity and particle count after te introduction of ig-turbidity water. Tis is considered to ave been caused by te failure of coagulation triggered by te excessive recoagulant injection, wic was not appropriate for te amount of intermediate-clorine-treated water used in te experiment. Fig. 5 and 6 sow te results wen recoagulant injection was increased simultaneously wit te introduction of ig-turbidity water. Te turbidity and particle counts of te filtered water increased for Column A but did not exceed.1 degree. Column B sowed muc a smaller increase in te particle count tan Column A, and te turbidity remained very low trougout te period wit ig-turbidity water. Fig. 7 and 8 sow te results wen recoagulant injection was increased after te introduction of ig-turbidity water. Te turbidity and particle count increased immediately after te introduction of ig-turbidity water, and Column A ad a maximum turbidity of.15 degree for te filtered water. From te above results, te recoagulant is identified capable of suppressing filtered water turbidity wen its injection is made before te contact of ig-turbidity water.

7 (degree) (number of particle >2μm/mL) (degree) Fig.3 and turbidity wen recoagulant injection was increased BEFORE te introduction of ig-turbidity water (column A) 2 Increase in recoagurant Introdtion of ig-turbidity water Fig.4 and turbidity wen recoagulant injection was increased BEFORE te introduction of ig-turbidity water (column B) 2 Introdution of ig-turbidity water and increase in recoagurant Fig.5 and turbidity wen recoagulant injection was increased SIMULTANEOUSLY WITH on te introduction of ig-turbidity water (column A)

8 (degree) (degree) (degree) 2.2 Introdution of ig-turbidity water and increase in recoagurant Fig.6 and turbidity wen recoagulant injection was increased SIMULTANEOUSLY WITH on te introduction of ig-turbidity water (column B) 2 Increase in recoagulant.2 Introdution of ig-turbidity water Fig.7 and turbidity wen recoagulant injection was increased AFTER te introduction of ig-turbidity water (column A) 2 Increase in recoagulant.2 Introdution of ig-turbidity water Fig.8 and turbidity wen recoagulant injection was increased AFTER te introduction of ig-turbidity water (column B) Effects of prolonged filtration of ig-turbidity water Fig. 9 and 1 sow te turbidity and particle count during te prolonged filtration of ig-turbidity water. Te particle count in te filtered water started to increase 18 after te introduction of ig-turbidity water in Column A, and te filtered water turbidity exceeded.1 degree 29 after te start of filtration. In contrast, Column B exibited no increase in te particle count in te filtered water 3 after te introduction of ig-turbidity water, and te turbidity of te filtered water remained below.1 degree trougout. Tis is considered to be due to te larger L/D ratio of te Column B filter medium. In oter words, te filtration using antracite as a larger capacity for depositing suspended matter tan filtration using sand.

9 (degree) (degree) Fig.9 and particle count during filtration (column A) Fig.1 and particle count during filtration (column B) Conclusion In tis study, we examined te effects of various factors on filtered water turbidity at a WTP upon te introduction of ig-turbidity water. Wen ig-turbidity water was introduced immediately after backwasing of te filter medium, te filtered water turbidity was observed to rise. Wen te filtration speed was iger, te increase persisted for a sorter time, and wen te recoagulant rate was iger, te filtered water turbidity was lower. In-advance recoagulant injection is concluded to be capable of suppressing filtered water turbidity after te introduction of ig-turbidity water. Te filtration using antracite was also identified as aving excellent filtering capacity under all filtering conditions, it was able to maintain filtered water turbidity under.1 degree for over 3 at te filtration speed of 17m/d and a recoagulant injection rate of 2mg/L, even wen turbidity of te inflowing settled water was as ig as 4 degrees. Tis study was based on sort-term experiments wit experimental columns, but by optimizing te recoagulant injection rate and filtration speed, we could suppress te filtered water turbidity to below.1 degree for many ours using bot sand and antracite monolayer filters. On te basis of tese results, it is suggested tat recoagulation wit direct filtration is a igly effective and feasible water purification metod for water treatment facilities subject to te intake of igly turbid water as te source water. ACKNOWLEDGEMENTS Tis work was carried out in collaboration wit Swing Corporation as a partner of te study.