Original Report
Water Purification and Clogging Phenomena
Observed in Infiltration-Percolation Processes
in Experimental Sand and Soil Tanks
KuniyoshiTAKEUCHI YasushiSAKAMOTO MakotoKUNO （Received August 31，1981） Abstract In五ltration−percolation processes in sands and soils were examined quantitatively and qualitatively feeding with tap water and synthetic wastewater． The experimental ap・ paratus was designed so as to produce capillary upward flow to be followed by gravita− tional downward flow． Temporal changes in flow rate， water quality and permeability， and their interrelatiolls were analyzed． The flow rate did not decrease monotonously over time under continuous feeding， but occasiollal catastrophic recoveries occurred seemingly as a result of status changes of micro・ bial or nonliving organics from丘xed to suspended form． In saturated sands， the fiow rgte depended primarily on the permeability at a thin boundary layer between sands a？dgravels， and water quality changed primarily within the upward flow zone． In unsaturated soils， particle size was found to be a controlling factor for water quality distribution as well as permeability distribution in the system．
Introduction
Land application of wastewaters or treated ef− fluents has been regarded as one of the promis・ ing methods of advanced treatment because of its practical simplicity，10w energy consumption， and little impact to the environment． Some field level experiments have already been reported （Niimi and Arimizu，1977）． At the same time， techniques of infiltrating treated efHuents or rain water into the ground are now of particular in． terest for groundwater recharge and for urban flood contro1（lshizaki and Kitagawa，1979）． From these points of view， many studies have beeエ1 performed on water and wastewater infil・ tratiOn SyStemS． ＊Former graduate student of the Department of Environmental Engineering， currently a伍1iated with the Water Resourges Development Corpo− ration．Matsumoto and Ookubo（1980）reported the
results of experiments using synthetic wastewater charged into 19cm−diameter cylinders packed ．with sands． They attempted to simulate an in一 丘ltration pond and observed the relation between the infiltration rate and the total ammount of influent． Ishizaki and KitagaWa（1980）investi− gated in丘1tration processes of clean water charged into underground sand layers through pipes へ with many holes． However， the relationship between water quality and san（玉properties was』not examined． Acommon problem observed in
these studies was the gradual decrease in infiltra− tion rates as a result of clogging formed ill the sands． This paper therefore focuses on the clogging phenomena introduced by microbiological activi． ties・ The experimental apParatus used are sand and soil tanks， in which both gravitational infil． tration and capillary upward flow were simulated．一102一
Table l Constituents of diluted synthetic wastewater Major constituent Concentration （mg／1）
CH3COONa
NH4Cl 20．0＊ 38．2＊ ”・”、J．，’ t1 Other nutrients ・．L K2HPO4， KH2PO4， MgSO4， NaC1， CaC12． FeSO4， H3BO4， MnCl， ZnC1， CuSO4，（NH4）6 Mo7024． ．＋一．．・ Ch；’ ）》・・ o ＊The concentrated wastewater has five times more of these． つ o Mariotte bottle Water or Wastewater Influent water level This system is a model for infiltrating wastewater from buried pipes， where Percolation can be considered as a ・』狽翌潤￨dimensional flow through unsatu− rated soil．Q
v Active wall height ’T’ 、 ぶ Sampling holes K麺
1、 1 、メ
董
。8 ．、 ＼こ・Methods
Fig．1 』＼ ・＼へ・ o ＼oo
oo
ooo
Materials丘11ed in the tanks，2．5cm thickness， 34cm width and 43cm height， made of 5mm poly・・ vinyl chloride plates， were standard sands br∩ught from Toyoura Bay， Yamaguchi Prefecture， and commercial soils for gardening， termed Akadama− do． The sallds’diameter ranged from O．105 to O．250 mm， specific gravity was 2．65， porosity 38．4％， and permeability coe伍cient O．045cm／sec at 25°C． The soil particles had diameters ranging O・42 to O．84mm for丘ne particles and O．84 to 2．00mm for coarse ones， specific gravity was 2．70， poros− ity 80．0％， and bulk density O．549／cm3・ The liquids fed were tap water and synthetic waste− water containing nutrients listed in Table 1． The experimental apParatus is shown in Fig・1・ Apartition wall was placed in the tank at a distance 9．5cm from the inlet so that water must cross over it by means of capillary attraction to flow down toward the outlet． The water perco− 1ated to the direction． of the arrows． Thickness of 2．5cm was selected to grasp the phenomena two−dimensionally． There were about 30 holes of lcm diameter on a side wall with rubber stoP− pers． For the case of sand tanks， the water o o 」＿一叫トー 9．5cm ＼ o Sand o Soil o o o 0 ＼ o o Gravel−
0 0 E o cり 寸 Outlet＿＿
24．5cm 6ｨμ
Piezometer or Tensiometer Experimental apParatus・ quality was determined by total organic carbon （TOC）measurement of lml samples taken from these holes． For the case of soil tanks， piezom・ eters were placed at these holes for the determi・ nation of distribution of piezometric head． In this case， most part was unsaturated under neg・ atlve pressure and thus the samples for quality analyses could not be taken out・ in the piezometer was connected with the in the soils through a porous medium， paper and cotton filter， which could keep under such negative pressure as in these ments． Depth of 43cm was consideredwater
Water
water
folded air of〔 expe「1’ to be adequate for evaluation of water quality changes within the unsaturated soil where aerobic condi− tions can be maintained． The inlet and outlet were covered with small gravel to prevent sand and soil from leaking． ＼The sands and soils were flooded with deaer・ ated w暇ter to start experiments from saturated conditions． Furthermore in the cases of soils tap water had been fed for a week before start− ing wastewater feeding in order to know the differences of the flow rates at the early stage among tanks． The waters were stored in Mari− otte bottles and fed at cbnstant pressure． The synthetic wastewater was sterilized in an auto・December 1981 Report of the Faculity of Engineering， Yamanashi University No．32 Table 2 Description of experimetal conditions Run No．
Medium丘11ed
Active wall height （cm） Initial fiOW rate （m／day） Organic carbon conCentratiOn （mg／1） 「・〆 1 2 3 4 5 6 7 8 Standard sand Standard sand Standard sand Standard sand Fine soil Coarse soil Fine soil Coarse soi1 10 5 10 5 3 3 3 3 1．50 1．90 1．57 1．99 4．28 4．31 3．64 4．09 29．25 29．25 5．85 5．85 ＊ ＊ 29．25 29．25 Sand（diameters O．15 to O．25mm）． Coarse soi1（0．84 to 2．00mm）． clave at 120°C for 15min prior to feeding to sands． Table 21ists the experimental condi− tions of each run． Runs of sands were different in water level or water quality， or both． Runs of soils were different in water quality or particle size， or both． The experiments were carried out at room temperature（23．3 to 29．5°C）． TOC concentrations were measured with a Shimadzu infrared carbon analyzer． E田uent samples with suspended solids were homogenized with an ultrasonic homogenizer． Samples for TOC analyses were obtained through a lml sy− ringe． For filtration， the syringe was equipped with a O．45μm membrane filter in a MilliporeSwinex丘1ter holder． NHt−N was determined
by the nesslerization method with filtrated sam一A
ら 勺 ＼H ） o る ≧ ￡ 』 1．0 0．1 0．01 0 （a） Sands ObO 漁 ＆％。 Conc． waste （Run 2） ●Dil． waste （Run 4） AConC． waste （Low water level） （Run 1） 翁．1．0 唱 這 §1
ξo．1 Fihe soi1（0．42 to O．84mm）． ＊Tap water was fed． ples． For the measurement of ignition loss（IL）， sand and gravel samples were dried at 110°C for four hours， then ignited at 6000C for one hour・ ReSultS and diScUSSiO皿S 1．Flow rate （i） In sand tanks Before starting the experiments for wastewater， apreexamination was performed to ascertain that continuous feed of tap water alone does not cause significant changes in flow rate over time． There・ fore， its decrease shown below can be inter・ preted as an effect of constituents of wastewater・ Fig．2・a shows the changes of flow rates． The rates are represented in m／day by dividing the total outlet discharge per day by whole surface （b）Fine s・i1・． oWater（Run 5） ●Wastewater（Run 6） ・° U 鴎 k i8 00 “ ♂。。。％ ●eo ●● oOo 、● （，）’ b。arse s・il・ o Water（Run 7） ：° ●Wastewater（Run 8） o●●1●● 。x 90・ lOOo● s°8・ 禽o o● 窪 ・● °°e。Po° ジ 100 0 ． 100 0 100 Time （days） Time（days） Time（days） Fi9．2 Flow rate v．s． time：The dotted lines in（b）and（c）indicate the end of preliminary tap water feeding and the stqrt of wastewater input．一104一
t”area of the tank，85cm2． At a glance， it is no− ticeable that the flow rates fluctuate widely with− out reachillg stable state to the end． During the first several days， the rates of all runs de・ creased by more than 80％of that of the begining， then they recovered suddenly to some extent about 30 days later． After that， they gradually decreased with some fluctuations up to 2％or less of the initial values． Differences between runs were not necessarily explainable regarding wide ranges of fluctuations and occasional recovery phenomena． With waste− water of high concentrations（Run l and 2）， a mass of organics was observed around the outlet gravels， which might have controlled the flow rates． The differences of water levels at the．inlet seemed not so much in且uential to the flow rates コaS COnCentrat10nS． （ii） In soil tanks In the case of soil， even when tap water was fed the flow rate continued to change for a long time（Fig．2−b and c）． This was probably due to the changes in physical formation of soil struc− ture caused by displacement and consolidation of small particles， because soils had aggregated structure， high porosity， and varied distribution of part．icle sizes． The flow rates did not show monotollous decrease but catastrophic recoveries were observed at times similarly in the sand cases． Therefore， it seems improper to make a model for temporal changes of flow rates only with continuous functions． A theory for catas− trophic phenomena is required． Infiltration−per− colation through sands and soils is inevitably an intermittent process due to biological and chem− ical cloggings and fluctuation of feed rates， hence perfectly steady state conditions can not be ac・ complished in practice・ 、lt was around the 30th day when difference between tap water and wastewater became con・ spicuous both in the fine and coarse particle soils． In the cases of fine particles， the flow rates reco・ vered to a considerable extent approximately on the 80th day， while in the cases of coarse parti・ cles such recovery did not occur． This phenom一 enon would be interpreted as follows：In the 丘ne particle soils， water paths once formed hardly disappear even under considerably low pressure because of high capillarity forces． The flow could thus recover anytime when the hydraulic gradient increased． On the other hand， in the coarse particle soils， water paths might easily be lost once experienced some critical low（or nega− tive）pressure or low flow rates unable to hold suf五cient soil moisture to keep water paths． Those paths lost would not be recovered when the hy・ draulic gradient increased． In other words， in the coarse soils， the area available for water to fiow is dif五cult to recover once lost， hence acts as virtually an irreversible controlling factor． 2． Perlneability （i） In sand tanks The decrease in the fiow rates is due to the decrease in permeability or in percolating area， or both． At丘rst， the area where percolation occurred was determined by tracing stream lines with inlection of rhodamine． The distribution of stream lines on the 38th day， however， was similar to the one at the start of feeding． This fact leads to the conclusion that the flow rate decrease occurred in the case of sands was caused by the decrease in permeability either in some particular control section or in the whole area uniformly． In order to clarify this point， the permeability oΦ cう ＼日
3
u
お 8 栖 ．出 f−P
Φ E Φ 10−1 5−8 × 3−5 10−2 2−3 10−3 Run 1 1−2 10−4 1 2 3 4 5 6 7 8 9 Time（days） Fig．3 Permeability coethcients between the points on the in Fig．9． stream line indicatedDecember 1981 Report of the Faculity of Engineering， Yamanashi University No．32 coeMcients between various points on the stream line shown in Fig． g were calculated． The results of calculation are exhibited in F嬉．3． The permeability coe伍cient between Points l and 2 decreased at the early stage of experiment， and that between Points 2 and 3 gradually decreased later． This indicates that the resistance against flow was predominant at the boundary between sands and gravels． This resistance would have been caused by biological clo99ings． Subtle and rapid fluctuation of the rates suggests that clog− gings were produced in easily breakable丘lm form． ・ （ii） In soil tanks In the case of soils， permeability was determin・ ed by measurement of piezometric head， whose distribution was shown in Fig．4． The piezo− metric head observed through a tensiometer is considered as the total head because the velocity head is negligible in percolation process， and accordillgly， the measured value of tensiometer itself is regarded as the potential at the point． （1）Fine s・ils （a） Initial state （On the lst day） Since potential difference in the horizontal direc・ tion was small， only difference in the vertical direction was examined． Fig．4clearly indicates that in the case of fine particles the major resistance against flow was present at the boundary between gravels and soils the same as the case of sands． After the 80th day， the resistance there was reduced， and the flow rates recovered． The controlling factor of the flow could therefore be attributed to the permeability at the boundary． Fj遮．5depicts the apParent lfiow velocity against the potential gradient in the upward flow zone on ogarithmic paper． In the case of丘ne particles， l Fig．5−a shows that the slope is nearly unity following the Darcy’s law． With wastewater， the permeability of this zone varied more widely than with tap water． In the case of coarse particles， the relation was not clear． It might be because the potential gradient calculated f rom the difference between potentials at only one pair・of points in the zone was not precise （b）Tap water （On the 50th day） （c）Wastewater ．（On the 50th day） 一“24 ．．一．．．2 （II）Coarse s・ils （a）Initial State レ （O・th・1・td・y）
19
一∼7
16 iy多三1・
二＿ζ子
’92 （b）Tap water （On the 50th day） 8 2 （c）Wastewater （On the 50th day）．27
22
Fig．4 Distribution of potential heads observed through tensiometers．一106一
（a）Fine soi▲s with tap water （R皿5） 1．0 ．三 日 ＼庄 ぷ 0．1 む ’5 ヱ 巴 甘 2 8 画 く 0．01 （b）Fine soils with wastewater （Run 6） （c）Coarse s・ils with tap water （Run 7） o o o oo o o o o o oo o oo潤@o
@o
O o Oo o o oo oo o o、 o ● o o o oo潤@Oo o o ● @ o @ ● o @ o @■ @ o o E 8° 怐@● o o 08 ・ ● ● 怐@ ● ●● @ ●●● o@ o
D●● 怐@ ● Oo°°08 Ooo ● ● o 盾盾 oo Oo ● △ ム ム ム o o ● △ △ ● ●8● △ ●● ■ ● ● ● ● 0．01 0．05 0．01 0．05 0．01 0．05 Potential gradient （cm／cm） Fig．5 ApParent velocity v．s． potential gradient in the upward flow zone； o −Before the first recovery of the flow rate； ●−After the first recovery； △−After the second recovery； The points outside the rectangulars in（a） and（b）correspond to the preliminary results with tap water． The periods when the outside points were observed in（a）and（b）coincided． 日 o ＼ 日 、ぷ G ．巴R
秘． 百 ＝ 5 ぢ 1．5 1．0 （a）Fine s・ilS Wastewater（Run 7） Upper zone 0．5 Water（Run 5） 日 o ＼日 巴 岩 ．巴 勺 ●D 駕 ’」口 5 ぢ 1．5 1．0 （b）Coarse soils 1．5 1．0． Water（Run 6） Wastewater（Run 8）0 100 0 100
Time （days） Time（days） Fig．6 Potential gradients in the downward flow zone． enough to be related to the flow rates ill this case． To evaluate the permeability in the downward flow zone， potential gradient was calculated along the proximity of the partition wall， because the vertical component of fiow velocity was consid・ ered predominant there as compared with the horizolltal component． Potential gradients in the upper zone（20 to 30cm from the bottom）and in the lower zone（10 to 20cm from the bottom） changed over time as shown in Fig．6．・With tap water， the potential gradients were about unity with little variatioll except in the lower』zone in the case of丘ne particles， where water might not be flowing but rather held as capillary pore water between particlesドWith wastewater， the varia− tion of potential gradient was relatively irregular and ranged widely． This might result from perme・ ability changes caused・by丘xation and irelaxation of micro−organisms， a subtle and discontinuousDecember 1981 Report of the Faculity of Engineering， Yamanashi University No．32 30 20 10 （a） Sands ミミ 10 葦 § § 毒 ・60「’T，Ah oRun 1 ●Run 2 ，A 60 （
N
＼bρ 日 ） 40 ．9 甘 お 毒 り 5 200
（b） Soils 0 A◆ ／」件㎏治
Time（days） 100’°1 。蕊
＼e6 10 20 30 40 ・
Time（days） Fig．7Quality of effluent． The dotted line is the same as indicated in Fig．2． process． Increase of potentia19radient was con． sidered dependent on saturation ratio of water， which would control oxygen supply from the atmosphere and affect biological growth rates． 3． Quality of efflue皿t （i） From sand tanks Ωuality of efHuent was determined as F匂．7・a；’ Total organic carbon（TOC）concentration fluc− tuated widely ranging from 20 to 60タ∠for the first 20 days and decreased thereafter to about 1／80f the influent concentr皐tion． The fluctua． tions well coincided with those of the flow−rates shown in Fig．2． They could therefore be attributed to、 the丑uctuations of retention time of water in the tanks which were caused by the permeability changes depending upon microbial growth and degradation・ During the first 20 days when． TOC concentrati皿 was relatively high， turbidity was also high． This implies that TOC of this period was in the form of organic sus． pended solids produced by biological activities． The presence of suspended solids must−have re・ duced’ 狽??@permeabilites、and affected the flow rates． NHオーN concentration’did not change so widely as the case of soils（Fig．7−b）． （ii） From soil tanks Changes of TOC and NH；−N concentrations of eMuent from the soil tanks are presented in Fig・7−b・ TOC and NH亡N of inHuellt were first affected by biodegradation in the gravels at the inlet， and then in the soils depending on the retention time・ However， e田uent quality was more or less constant（5mg−TOC／1 for fine parti− cles and 7mg−TOC／1 for coarse partilces）after the 30th day regardless of flow rate or retention time． This may suggest that organic carbon could not be removed at the same rate everywhere but that the region effective to improvement of water quality was limited to particular areas． NHオーN removal was generally greater in soils than in sands， especially in the case of fine particles． The difference of NHオーN removal between the fine and coarse particles was even greater than that of TOC． These facts seem to suggest that the NHオーN removal was influenced primarily by particle size， which would afEect the oxygen supPly for nitri丘cation and the adsorption area． 4・ Distributio皿of orga皿iCS The measurement of TOC in efHuent did not clearly show the difference of water puri丘cation一108一
Water Purification and Clogging Phenomena Observed Processes in Experimental Sand and Soil Tanks in In丘ltration−Percolation （a） ・026 ’025 Run 2
028 025
028
029
、、、40
・26 運
（）28 036 0311’ （d） 018 ・11j
028
”ご’n
017
／／／014
014
Day 30013 013
（b） ’022021
027
021
018
23、’024llO
013x（）15
（e）016’
A．．．；．：：；．．；．：．：．‘ 040・゜19a22
09蕊。1、
Day 40 60 し／ （c）019
も ■ ：：∵・・：・・∵ エ ． コ ．∴：（）20・ コ コ ． コ ． ： ・・．． ・ ．・・ ≒｝’∴∵予・：・：・ ● ・ ． ・ ， ・ ． ・ ’ ● ■ 令 ’ ● ・ ’ ，． ロ ひ ロ ． コ ゲ ・…@027・
26 覆，》蓼4
”21’1
019
（f） （Filtrated）ol5
016
）016
160
010
／05
・．． @■ ・●・ ●・●・・ ．． … φ．・．●● の ． ’ ■ ， ． タ コ ． ほも ロ 〔）27・． ， り． 、 ， ■ ・ ’ ● ● コ ■ … ㌔’ご’5㍉．・ ：・，三lcス’：与 ロ ロ ロ ：．’：．二・’・‘・．亨 ゜”謔O29ζ Day 20（86
Ol6−016
12
ｽ7
14
09
3 08
＼＼
Day 40 4060
uo
Fig．8 TOc distribution profile in the sand tank．（mg／の capability between saturated sands and unsatu・ rated soils． For more detailed compariSon， it is necessary to know where and how water quality changed in sands and soils． For unsaturated soils， however， samples for water quality analyses could not be obtained． The distribution of or・ ganics was therefore examined only in the sand tanks． TOC determined from lml samples of about 20 points in the sand tank for Run 2 is illustrated in F㎏．8． T・he d三stribution profile and its changes shoW where the removal of organic carbon occur・ red． Between the l st and the 10th day， the TOC 、values higher than that of influent concentration appeared at the bottom and the center of the tank． High concentrations in these portions might be re丑ecting the suspended solid orgapics initially existed in sands． TOC was measured り without filtration， but the contribution、 of．sus三 pended solids was found not negligible as discussed later． ．・ On the 20th day， the concentratiop was high near the partition wall and、 decreased along the14
Ol1
09
1 ．ピ5
Run 2 ノ㌻Q4i；
・ll ： ・ hl ’ ： ● ・ 1 ● …．年、 3 ・ ． F∴い、 O：：㌧∵；・、 ‘ ’ ● ・ e ● ・ @ ’ ●． @・ ● ● P■ ， Pバ∴6． ・・’㌦‘：吟 2①
．・‘．’D、千 ．． 、@ ’⑦ 8
， Fig． g Dark zone under anaerobiC conditions and sampling points on the s仕eam ！ine（①一⑧）for Figs．3and 10． stream lines． On the 30th day， the distribution was similar to that of the 20th day， but the values of qoncentration showed elimination． This similarity probably、indicates that the System was relatively steady 4μring this period． tt The reduo・ tion of valUes might・・result from ipcrease in the retention．．tjme d鴫to decrease．in the flow rate， On the 40th day， some loca日concentrations were over．i20mg／1． Between the 30th・and the 40th day， dlark zone u算der．anaerobic conditionsDecember 1981 Report of the Faculity of Engineering， Yamanashi University No．32 （一 ＼㎏ E ） ．9 言
5
奉 茎 8u
o
巨 60 50 40 30 20 10’ 0 0 釆 ’IL、＼ 10ODay
△Day 20 口Day 30 ●Day 40 、±＼藍．
20 30 40 協一一一一，tN こ工’．’二仏・ 50 6070cm
12
34 5
6 7．8 Distance from the inlet（cm）and point no． ・nthe stream line in Fig．9（①一⑧）． Fig．・10 TOC variation along the stream line shown in Fig．9・ appeared as shown in F㎏．9． They must have ・caused such high concentratiops． The concen− tration profile of filtrated samples on the 40th day is given in Fig・8−f・ Comparing Figs・8−e with 8−f，it is evident that the high values of TOC concentration on the 40th day were due to the presence of organic carbon in the suspended solid form． These suspended solids were produced probably by the release of fixed micro−organisms under anaerobic conditions． Fig．10 shows TOC at several points along the stream line shown in Fig．9． Of these points， Point l was situated in gravels，2was in sands −about 5mm above the gravel−sand boundary， and 4was at the same height with the water level of the in且uent． At Point 2， TOC values increased with time， while at Point 4 they decreased． This difference implies that TOC materials were accumulated near the boundary and their remov・ al occurred within the upward flow zone． On the other hand， within the downward flow zone the removal rate little increased． In order to examine the influence of suspended solids， TOC of丘ltrated sam「ple from each point was also mea・ sured， and the difference between the filtrated and unfiltrated samples was found less than 2 mg／l except for 10mg／l at Point 2． From this， it was supposed that organics’were accumulated close to the bo皿dary in the suspended solid form as well as in the fixed form． For evaluation of weights of fixed micro−orga・ nisms， ignition loss（IL）was determined for the samples taken from about 20 points on the 40th day． The values of IL of the sand samples ranged from 2．7 to 3．4mg／g except Point 2 being 4．3mg／g． For the gravel samples， the value at the三nlet was 10．8mg／g and that at the outlet was 12．9mg／g． The average value for standard sand itself was 2．9mg／9． It illdicates that most 丘xed micro−organisms were present in the gravels and in the sands near the boundary to the grav・ els at the inlet． The distribution of organics were determined only ill the case of sand tanks but some results would also apply to the case of soil tanks． For instance， the behavior of organics in suspended solid form plays a significant role ill water qual． ity distribution， and only limited areas serve for active biodegradation of organics．Conclusions
Infiltration−percolation processes in sands and soils were examined quantitatively and qualita・ tively feeding with tap water and synthetic waste− water， and the following conclusions were ob− tained． 1．The flow rates did not show the monoto・ nous decrease with continuous feeding， but oc− casional catastrophic recoveries occurred as．the results of changes of solid organics from fixed form to suspended form． 2．Major cloggings were observed within thin boundaries betweell sands and gravels or betweell soils and gravels． 3． In saturated sands， clo99ings were induced only by microbial and nonliving organics and inorganics produced from Ilutrients in Wastewater， whereas ill unsaturated soils， the physical displace− ment of soil particles also contributed to the t gene「ation of cloggings． 4． In saturated sands， the flow rate depended primarily on the permeability at a thin boundary layer between sands and gravels；and water qua1・ ity improved primarily within the upward flow 一1↓0一Processes in Experimental Sand and Soil Tanks zone． 5． In unsaturated soils， particle size was found tσbe a controlling factor for the flow rate and water quality improvement． The mechanism of this phenomenon， however， is yet unknown． 6． In unsaturated soils， the flow rates were influenced not only by permeability of soils but also by the area acting for water paths which seemed irreversible once lost by insuf五cient water supply・ 7．Sampling dif五culty prevented the phenom・ enological analyses of water quality improvement performed in the soil tanks． Water puri丘cation and clo99ing by microbio・ logical activities in sands and soils are dif五cult phenomena． Many problems are left to be solved including the relationship between permeability and moisture distribution in unsaturated soils， the relationship between degree of saturation and biodegradation rates， the adsorption and diffusion rates in aggregated soil structure， the physical and chemical properties of biological cloggings， the end products of biodegradation in sands and soils， and．so forth・ Acknowledgement Authers are greatly indebted to Mr． Tetsur6 K6no， Lecturer， Department of Environmental Engineering， Yamanashi University for his inval， uable suggestions and technical assistance in design of experiment．s・The sterilization procedure was introduced upon his suggestions． Dr． Ooki Nakayama， Professor of the same department is gratefully acknowledged for his encouragement and technical offer of the ingenious craftsman． ship in glass works・ Gratitude is also extended to the former students of the department， Mr． Yukihisa Katayama， the City of Shizuoka and Mr． Hikaru Nozawa， Fuji Electronics Co． for their devotion to carring out the experimental study． 1） 2） 3） 4） Refere皿ces Ishizaki and Kitagawa （1979） Groundwater recharge with injection wells． Suirik5enkai Ronbunsh五，23． Ishizaki and Kitagawa（1980）Principles and effectiveness of buried pipe infiltration sys・ tems．］Kensetsush5 Doboku・kexlky両o Shi・ Iyo． No．1590． Matsumoto and Ookubo（1980）The effects of infiltration rate on the biological clo99ing of infiltration ponds． Eiseik5gaku Kenky五・ t5ronkai Ronbunsh五，16． Niimi and Arimizu（1977）Study on waste・ water purificatio皿i皿soils， M；kan−」；ka Kenkyukai． ’
