Hazards associated with the use of chlorinated oxidation pond effluents for irrigation

Water Research Pergamon Press 1973. Vol. 7, pp. 853-862. Printed in Great Britain

HAZARDS ASSOCIATED WITH THE USE OF CHLORINATED OXIDATION POND EFFLUENTS FOR IRRIGATION YEHUDA KOTT Environmental Engineering Laboratories, Technion, Israel Institute of Technology, Haifa, Israel (Recei~'ed 10 Attgust 1972)

Abstract--Eight milligrams per litre chlorine applied to oxidation pond effluents caused no algal kill within the first 2 h of contact. The available chlorine attacks bacteria causing coliform count to drop from l0 s 100 ml -t to a few tens. Enterovirus counts dropped from about 80 100 ml -t before chlorination to 37 100 ml -~ (after chlorination). Vibrio cholerae (EI-Tor) were killed under these adverse conditions, and MPN dropped from 10a 100 ml-t in the influent wastes to 2 100 ml- z in the effluents. A 5 mg l- a dose of chlorine at 1 h contact time killed these sensitive bacteria decreasing MPN to less than 2 100 ml- z. Differences between the efficiency of chlorination experiments under laboratory and field conditions would necessitate the application of 15 mg i-~ chlorine for 2 h of contact. INTRODUCTION OXIDATION ponds are commonly used where climatic conditions are favourable and land is inexpensive. M a n y studies have been published to show that unstable organic matters found in wastewater entering such ponds will undergo disintegration by proteolytic enzymes in bacteria (BROCKETT, 1971) to organic compounds of lower degree, which are in turn utilized by other bacteria and algae. Various metabolic activities were investigated by some researchers (MCIONNEY, 1962; KOT'r and INGERMAN, 1966). In these studies it was proven that although oxidation pond effluents are "stabilized organic material", they contain organic matters, with BOD values ranging from 15 to 80 mg 1-1. The decrease in coliform counts in those ponds is from one or two orders of magnitude at most. The advantage of this method ofwastewater treatment is its being the least expensive one presently used worldwide. It is for this last reason that the World Health Organization (WHO) released special design criteria for waste stabilization ponds (GLOYNA, 1971). In many countries where water shortage problems are non-existent, effluents of such ponds are diverted to rivers without any further treatment. In a few places, where it is feasible, wastes are diverted to a series of ponds, where final effluent quality would show very low BOD values. Only in a few countries are such effluents used--mainly for agricultural purposes. In Israel more than 200 oxidation ponds are in use, most of them serving rural communities. It is therefore natural to assume, that in a country where a b o u t 90 per cent ofthe water resources are utilized (GuR~vICI and VILENCIUK, 1970), most of the oxidation pond effluents will be used for irrigation purposes (HERSHKOVITZ and FEXNMESSER, 1967; FEIrCMESSER, 1971). The Israel Ministry of Health forbids, by law, direct irrigation of raw-eaten crops by oxidation pond effluents. Such effluents are considered safe only for industrial crops like cotton and potatoes (STATE OF ISRAEL, 1965). 853

854

"~'EH~t~,a Korr

In the )ear of 1970 some sporadic cases of Vibrio cholerae were isolated and these bacteria were found in wastewater (CoHEy et al., 1971 ; SWARrZ, 1971). As a result, the irrigation of potatoes was forbidden as well. Currently oxidation ponds are used in summer for restricted irrigation which interfere with agricultural activities, and in winter time an overflow system permits the effluents of such ponds to flow freely, which may cause groundwater pollution. The purpose of this study was to investigate the possibility of overcoming the health hazards, which may arise due to irrigation with oxidation pond effluents, by chlorination. The ultimate goal is to find a way for using such effluents for unrestricted irrigation. MATERIALS AND METHODS 1. lndoor activities Oxidation pond effluents were taken from the Environmental Engineering Laboratories experimental ponds, and from three different districts in Israel, brought to the laboratories and examined. MPN for coliforms at 35°C and for fecal coliforms at 44°C using EC medium were carried out according to APHA Standard Methods (APHA, 1971), using the Multiple Tube Fermentation Technique. A 5 day, 20°C BOD test of pond effluents was performed according to A P H A Standard Methods (APHA, 1971). Algal counts were carried out using haemacytometer cells (KIn'T, 1970). Vibrio cholerae counts were performed as follows. Vibrio cholerae (EI-Tor strain) was added and mixed daily to 14 1. of domestic wastewater in quantities to make a final concentration of 103 100 ml-1. The mixture was introduced into a 70 1. capacity model oxidation pond. The effluent, containing the suspected vibrio bacteria was inoculated to obtain a most probable number of Vibrio cholerae using the technique recommended for M P N for coliform bacteria (APHA, 1965), using test tubes containing peptone broth p H 8.6. After 5 h growth in these tubes at 37°C a loopfull from each tube was inoculated to another set of peptone broth p H 8.6 tubes, followed by 5 h incubation at 35°C. A loopfull from each of these tubes was inoculated to a T.C.B.S. cholera medium*, to receive isolated colonies. After an overnight incubation at 37°C, isolated colonies were transferred to agar slants, and incubated for 18 h at 37°C. The agar slants were checked serologically with polyvalent antisera and results were considered positive when a clear agglutination was received. Samples showing autoagglutination were discarded; results were recorded as Vibrio cholerae bacteria found in 100 ml of oxidation pond effluents. Enteroviruses were isolated and counted using the ultrafilter method proposed by Gartner (GARrr~ER, 1966), the concentrated viruses were inoculated on primary M K cells as mentioned in previous work in detail (Kor:r, 1971). Results were interpreted as P F U in I00 ml of original sample. Two-litre lots of pond effluents were placed in a thermostatic bath, sodium hypochlorite solution was added resulting in various concentrations of chlorine ranging from 6 to 40 mg I-1. A magnetic stirrer was used to ensure good contact. Samples were dechlorinated after 1 or 2 h contact, and the above mentioned tests were performed. * T.C.B.S. cholera medium, Oxoid code No. CM333.

Hazards of Chlorinated Oxidation Pond

Effluents

855

2. Outdoor experiments

(a) A 350-1. capacity aquarium was used for continuous chlorination experiments performed outdoors. Domestic wastewater from a residential area was pumped to the pilot plant area of the Environmental Engineering Laboratories. The wastes were diluted with tap water in a ratio of 1:1, to obtain approximately 200 mg 1-1 BOD in the influent. The pond was operated for 10 h daily, with a detention time of 5 days. To this pond, a rectangular vessel, divided by four baffle plates, was connected. The effluents of the experimental pond passed through this vessel. Sodium hypochlorite was added in concentrations resulting in 5-20 mg 1-1 chlorine. Contact time in chlorination vessel varied from 30 rain to 2 h, and samples were taken from the chlorinated effluents. Samples were tested for residual chlorine and dechlorinated by the addition of sodium thiosulphate. M P N (confirmed) test for fecal coliform bacteria was carried out by multiple tube fermentation method, fecal coliforms were tested on EC medium at 44°C. (b) An oxidation pond at Regba, containing 350 m 3 of liquids, was chosen for further large-scale experiments. A 350-1. capacity vessel divided by four baffle plates, was installed near the pond. The pond liquids were pumped from 10 and 50 cm below the surface area to this chlorination vessel. Chlorination was carried out by pumping sodium hypochlorite solution in concentrations resulting in 10-40 mg 1-1 chlorine in the chlorination vessel. Contact times used were 1 and 2 h, the precision of time being maintained within 5 per cent. Chlorine residual examinations and multiple tube fermentation tests for the M P N of coliforms were carried out on the spot. Incubated test tubes were brought to the laboratories. Fecal coliforms were counted on EC medium at 44°C. RESULTS Some years ago it was regarded as useless to chlorinate oxidation pond effluents because of the possibly high chlorine demand of living algal cells found in such TABLE 1. CHLORINATIONOF ARTIFICIALAND OXIDATIONPONDEFFLUENTS

MPN I00 ml-t after chlorine dose of Source of sample*

Time (min)

8 mg 1-L

10 m g 1 - l

15 m g I - t

1

0

7.9 × 107

2

60 0 60

9.2 × 104 1"3 × l0 s 2"0 × 10a

7.9 x 107 4'9 X I0 a 1"4 × 10a 4'8

1-0 × 107 13 1'3 x 10a 2

0

1"3 x 106

--

--

60

9.3

--

--

4"9 x 105

--

--

5"4

~

--

3 4

0 60

x

102

* Source of samples: 1--Nutrient broth + E. coli B + 3000 Chlorella sorokiniana/mma; 2--Triclding filter effluent + 3000 Chlorella sorokiniana/mma; 3 - - E x p e r i m e n t a l oxidation p o n d effluent (70 1); 4 - Effluents o f a full-scale oxidation p o n d (Regba).

w.R.7/6--o

1"3 X 10 "~ 23 <2 40

33

130

1"1 x 107 2 <2

I O0

34

2-8 x 107 <2 <2

230

38

2"4 X 106 <2 <2

320

43

3"3 x 107 <2 <2

1850

45

1"6 x l0 s <2 <2

3000

19

3"5 x 107 <2 <2

5000

M P N (confirmed) per 100 ml when n u m b e r of algae mill -3 was

10

1.3 × l0 s <2 --

8880

Experimental conditions: Chlorine dose applied, 8 nag 1- ~; temperature, 30~C; source of eMuents, experimental oxidation pond (70 I.).

0 30 60 Initial B O D (mg 1- ~)

Contact time (min)

TABLE 2. INFLUENCE OF CONSTANT CHLORINE DOSE VS. CIIANGING ALGAL NUMBERS ON COLIFORM KILL

15

7"9 ?(11) 4 <2 ,1-5

10,()(X~

O

Z~

,<,

G~

<2 4.9 X 10" 5"5 x IO s

4-9 x 104 1"1 X 106

2-4 x IO T

2-3 x 104

<2 2.1 x 102

1-8 X 102 5-4 x 104

<2

3-5 x 104

<2 15

20°C

2-4 x IO s

<2 1"3 x 10'

1.0 x 103 2"2 x IO*

<2

2-4 x IO s

<2 17

30°C

3-3 x 102

<2 17

1.0 × 102 2.3 × 104

<2

5"4 x IO s

<2 1"0 x 102

IO°C

1"3 × 102

<2 33

3.5 x 104

5

<2

>2"4 × IO s

<2 6.1

20°C

2 h contact at

2.4 × IO s

<2 3"3 × 102

15 1.6 × 102

<2

2-3 x 104

<2 <2

30°C

79

<2 <2

<2 33

<2

49

<2 <2

1h

<2

<2 <2

<2 13

<2

4

<2 <2

2 h

M P N fecal coliformt in 100 ml

* Based on a total of over 50 samples examined, taken from 14 different ponds. 1" N u m b e r of fecal coliforms is nearly identical to that established by M P N coliform (confirmed) test. Chlorination carried out at 20°C.

Minimum 50 perc. Maximum

1.3 x 102 3"5 x I04

5"0 x 106 2.4 × I0 s

Emek Hefer

<2

4"3 × 10"~

Minimum

Maximum

2-3 x IO s

1-3 x lO 7

Yizrael

<2 1-2 × 102

7-0 x 10" 2-3 x 106

Minimum 50 perc. Maximum

50 perc.

IO°C

Control

Emek

Western Galilee

District

1 h contact at

M P N coliform (confirmed)/100 ml

TABLE 3. COLIFORM KILLING EFFECT OF 8 mg 1- t CHLORINE IN OXIDATION P O N D EFFLUENTS*

,.,,4

EB

o ta. m

5"

~."

O

~"

O

;:3"

o

m

858

YEHUDA. KOTT

effluents. However, previous publications (KoTT et al., 1966: BETZERand KOI"T, 1969) had shown that the algal cell wall is resistant to chlorine penetration for about 2 h in tap water. TABLE 1 represents results of experiments where various doses of chlorine were applied to artificial and oxidation pond effluents. It can be very clearly seen, that in 1 h of contact (with nutrient broth medium to which Chlorella sorokiniana cells were added together with E. coli B strain--representing coliform bacteria) a decrease of four orders of magnitude in the number of E. coli B has been achieved using 8 mg lchlorine. Applying 15 mg 1-1 chlorine, only 13 100 ml-~ of E. coli B survived. Better results were achieved when trickling filter treatment plant effluent was used. The same results were recorded when using effluents of an experimental oxidation pond. It should be noted that in these series of experiments a coliform reduction of only three orders of magnitude was achieved using large-scale oxidation pond effluent. The results in general indicated that good chlorination would be achieved and could be applied to algae in oxidation ponds. In order to prove this, chlorination of oxidation pond effluents was performed with increasing number of algae, keeping all other parameters constant. TABLE2 summarizes results showing clearly that the number of coliform bacteria decreased very rapidly--no matter if high or low number of algal cells were present in pond effluents chlorinated. As it was found that 8 mg 1-1 applied chlorine caused a very pronounced decrease in coliform counts, it was decided to investigate whether these results apply generally to most oxidation ponds. TABLE 3 summarizes results of experiments carried out on effluents from oxidation ponds from three different districts in Israel. From the data appearing in TARL~-3 it can be seen that the number of coliforms in all three districts was of the same order of magnitude, and in no case was less than 10'~ (100 ml)-* effluent. The 50 percentile value was found to be 106 (I00 ml)-* TABLE 4. CONCENTRATION OF RESIDUAL CHLORINE AFTER ADDITION OF 8 mg I- ~ TO OXIDATION POND EFFLUENTS* J

Residual chlorine (rag 1- ~) after chlorination for

District

BOD (mg 1-:) (range)

Western Galilee

42-85

Emek Yizrael

Emek Hefer

1 h at

2 h at

10'C

20°C

30°C

10°C

20°C

30°C

Minimum 50 perc, Maximum

0.48 0.61 2-65

0 0.31 1.25

0 0 1.14

0-14 0.30 2.3

0 0-20 1.75

0 0 1.08

41-150

Minimum 50 perc. Maximum

0-49 1"42 1-82

0 0"44 0.88

0 0 0.28

0 1-06 1.12

0 0 0.42

0 0 0.18

46--142

Minimum 50 perc. Maximum

1.45 2"42 3.40

0 0.56 1.35

0 0 1"2

0-36 1.70 2.40

0 0"55 1"16

0 0 0.80

* Based on a total of over 50 samples examined, taken from 14 different ponds.

Hazards of Chlorinated Oxidation Pond Effluents

859

Chlorination of effluents using 8 mg 1-1 chlorine decreased coliform counts to values of 102 (I00 ml)- ~ (50 percentile value) without any correlation to the temperature at which the experiment was carried out (10 ~, 20 ~ or 30°C). It is interesting to mention that at 2 h all minimum values showed less than 2 coliforms (100 ml) -~, 50 percentile values varying between less than 2 in I00 ml (low level) and several hundreds (100 ml) - t (high level). In all cases the number of fecal coliforms was low. The highest result after 1 h contact was found to be 79, but after 2 h results did not exceed 13, most of the results showing less than 2 (I00 m l ) - L It should be noted that all the ponds in the three districts tested showed more or less the same effluent quality, a fact which was reflected in the experimental results. As already mentioned, only 8 mg 1- ~ chlorine was used in these experiments. Chlorine residuals are recorded in TABLE 4, where it can be seen that many times there was no chlorine residual at fifty percentile values; however, residuals measured other times were high enough to indicate further chlorine activity. It is interesting to note that at 30~C in most experiments no residual chlorine was recorded--these results were often in good correlation when higher coliform counts were recorded than those at 20°C. TABLE 5 contains results from various experiments carried out in order to follow up Vibrio cholerae bacteria survival in oxidation ponds. It can be very clearly seen that this Vibrio cannot stand adverse conditions and the die-off rate is very fast. TABLE 5. Vibrio cholera (EL-TOR) SURVIVAL IN EXPERIMEN'rALOXIDATION POND

Bacterial count

I00

ml-1

Influent

Effluent Vibrio cholerae

Coliforms 3"3 2.2 7.9 2.5 3"3 3"3

× x × x x x

IO s IO s IO s I0 9 IO s lO 6

Vibrio cholerae*

1.3 × 10 3 to 8"0 × I0 3

Coliforms 1-3 7"9 3"5 2"8 2.4 1"3

x x × x x x

I0 7 I0 "L 10 7 I0' I0 6 I0 6

Before chlorination

After chlorination

2.2 2.2 0 2"2 2-0 2"2

0 0 0

* Tests were carried out on the liquids of the 701. experimental pond, non-simultaneously, during the research period. Most probable numbers in influent wastewater were 103 (100 ml)-1 and dropped to between less than 2 to 2 (100 ml)-1 bacteria. However, after chlorination at a contact time of 1 h using chlorine concentrations as low as 5 mg I- 1 no Vibrio cholerae bacteria were isolated. TABLE 6 summarizes results obtained from oxidation pond effluents examined for presence of enteroviruses. It can be seen that the number of viral particles was never lower than 30 (100 ml) -~ and did not exceed 290 (100 m l ) - L The ratio of coliform bacteria to viral particles was (at 50 percentile values) 104: 1. After application of 8 mg 1-1 chlorine, coliform count dropped by five orders of magnitude, while the decrease in the number of viral particles was only 37 per cent. It should be emphasized

860

YEHUDA K o r r

TABLE 6.

SURVIVAL OF COLIFORMS AND EN"TEROVIRUSES 1N CHLORINATED OXIDATION POND EFFLUENTS

N u m b e r o f e n t e r o p a t h o g e n s (100 m l ) - t

I n effluents Minimum 50 p e r c e n t i l e Maximum

Coliforms (confirmed)

Fecal coliforms

Enteroviruses

1.3 :< 10 '~ 7.0 × 1 0 ' 1.1 N 107

4"9 :< 10 "~ 4.6 -< 1 0 ' 1-3 x 106

30 85 290

< 2 4.5 2.3 x I 0 '~

< 2 < 2 49

40 57 100

I n chlorinated effluents Minimum 50 p e r c e n t i l e Maximum

Figures based o n m o r e t h a n 20 d i f f e r e n t e x p e r i m e n t s . E x p e r i m e n t a l conditions: 20°C, 1 h contact time, 8 m g I- l chlorine.

that all experiments mentioned above were performed in the laboratory, under optimal stirring and contact time. TABLE 7 summarizes results recorded when 8 mg l- 1 chlorine was added to a small size, continuous flow system, and to a pilot plant installation assembled in a field experiment. It can be clearly seen that although oxidation pond effluent qualities were similar in the experiments, best results were achieved under laboratory conditions. Relatively good results were recorded in chlorination experiments performed using baffle plates in the chlorination vessel, while in Regba oxidation pond effluent results seemed to be somewhat higher. TABLE 7. EFFECT OF CONTINUOUSCHLORINATION ON COLIFORM SURV[VALIN OXIDATION POND EFFLUENTS

MPN (100 ml) -z Before chlorination Origin of sample 1 2 3 4 5*

Mode of chlorinationt Batch Continuous Continuous Continuous Continuous

t BOD (°C) ( m g l - t) pH 10 9 II 16 32

35 45 52 52 40

8"6 8"5 8"0 8"3 9"4

Coliform (confirmed) 1'7 3"1 3"3 1.9 1"3

x x x × x

t05 l0 T 10~ 102 l0 o

Fecal coliform 6"8 1"3 1.3 2.4 7'8

x × x x x

l0 "~ l0 T 107 l0 s l0 ~

After chlorination Coliform (confirmed) <2 7.9 x 4.9 x 1.3 x 2.1 x

l0 s 10z 102 102

Fecal coliform <2 3-3 x 105 4-9 × 10 4"5 × l0 78

Chlorine

residual

(rag I- 0.?-" 6.6 4.4 5.3 7.5 3.5

Origin of samples: I. Effluents o f a 350-1. experimental oxidation pond, chlorinated in beakers. 2. Effluents o f a 350-1. experimental oxidation pond, chlorination vessel without baffle plates. 3. Effluents o f a 350-1. experimental oxidation pond, chlorination vessel with 2 baffle plates. 4. Effluents o f a 350-1. experimental oxidation pond, chlorination vessel with 4 baffle plates. 5. Effluents o f Regba oxidation pond, continuously chlorinated in a 350-I. vessel. * 15 m g - t chlorine added t 8 m g - l chlorine added 2 h contact time

DISCUSSION

In many countries where water shortage does not exist, treated wastewaters are diverted to a nearby river or to the sea, with or without prior chlorination. In other countries, such wastes are reused, mainly for agricultural purposes. In Israel water shortage induced farmers to reuse wastewaters. Most of the agricultural settlements

Hazards of Chlorinated Oxidation Pond Effluents

861

look on oxidation ponds as equivalents of secondary wastewater treatment, and the pond effluents are used for restricted farming purposes. The farmers would like to use the water without limitations and therefore it seemed that by removing restrictions new possibilities for agricultural use of such water can commence. In this study it was found that for about 2 h algal cells do not show chlorine demand. This fact enhanced chlorination of oxidation pond effluents, which was formerly thought impractical. It was observed that during the first 2 h of contact, the number of algal cells did not decrease, no matter what was their initial concentration, and the disinfection rate of chlorine remained constant. Comparing results obtained from three different districts in Israel it was shown that within a certain range of BOD (25-80 mg 1-1) all the ponds yielded the same results, showing lower counts of coliform after 2 h of contact time, reaching as low results as a few tens of coliform bacteria in 100 ml of sample. Survival of various pathogens in oxidation ponds is a controversial theme. However, previous study showed that although Salmonella could be detected in influents of oxidation ponds, it was not found in the effluent samples (KOrT, 1961). In 1970, when Vibrio cholerae bacteria were found in Israel, the Ministry of Health tightened restrictions for the use of wastewater for agricultural irrigation purposes. It was therefore interesting to follow up Vibrio comma survival in oxidation ponds. Results (TABLE 5) indicated very clearly that these bacteria are very sensitive to adverse conditions, despite high pH which is favourable. It might be that some of the bacteria drifted to pond effluent causing positive results of M P N 2 (100 ml)-1. However, a low concentration of chlorine caused a total kill of Vibrio. It is much more important and worrying to note, that any oxidation pond effluent contain about 80 100 m l - l enterovirus particles. It is not known yet, how many of these are contributed by vaccination of children and how many are of wild types. It is worthwhile noticing the preliminary results showing that a concentration of about 40 mg 1-1 chlorine is necessary to destroy virus particles in oxidation pond effluents. It is thought that as a big difference was observed between results of bench and pilot plant experiments, a concentration of 15 mg 1-1 chlorine would be necessary for a contact time of 2 h, resulting in effluents containing a few tens of coliform bacteria. From the results reported in this paper it is concluded that further study is needed before it is safe to use oxidation pond effluents for unrestricted irrigation unless strict regulations are issued for their treatment.

Acknowledgements--The following persons were engaged at various stages in the performance of this study: BEN ARI HANNA, BETZER NACHUM, KENDE ARIE, MARKOVITZYOAV and STAHL BARUCH. Virus enumeration was carried out by SPERBERSHOSHANA.Their work and efforts are very much appreciated. The study was sponsored by the Israel Ministry of Agriculture and by the National Council for Research and Development. Partial sponsorship was received from the Environmental Protection Agency, United States Department of Interior--Project 16030 DQN. REFERENCES AMERICANPUBLICHEALTHASSOCIATION,AMERICANWATERWORKSASSOCIATIONand WATERPOLLUTION CONTROLFEDERATION(1971) Standard Methods for the Examination of Water and Wastewater, 13th edn, A.P.H.A. Washington. BETZERN. and KOTTY. (1969) Effect of halogens on algae--II. Cladophora sp. Water Research 3, 257-264.

$62

YEHUDA KOTT

BROCKETT D. O. (1971) Some aspects of the microbiological activity of the Mangere oxidation ponds. Ph.D. Thesis, University of Auckland. New Zealand. COHEN J., EVER HaDAN~ SH., SHaLLO.'4 Y. and EDEN T. (1971) The choIera outbreak in Jerusalem 1970 as part of the intestinal infections problem in Israel. Public Health 14, 3-12. FEINXlESSER A. (1971) Survey of sewage utilization for agricultural purposes in Israel. In: Adcances in Water Pollution Research (Edited by JENKINS S. H.) pp. 1-33 1-I-33:7 Pergamon Press, Oxford. GARTNER H. (1966) Detection and recovery of pol[oviruses on a soluble uttrafilter. In: Transmission of Viruses by' the Water Route (Edited by BERG G.) pp. 121. Interscience, New York. GLOYNA E. F. (197l) Waste Stabilization Po.'lds. W.H.O.. Geneva. GUREVICI R. and VILENCtUK I. (1967) Econon,ia apei in Israel, pp. 1. Simpozion de IrrigatiiIsraelRomania, Tel Aviv, Israel. HERSHKovrrz S. L. and FEtNMESSERA. (i967) Utilization of sewage for agricuRural Purposes. Water Sew. Wks. 114, 181-184. MCKINNE¥ R. E. (1962) Microbiology for Sanitary Engineers, pp. 239-245. McGraw-Hill, New York. KOTT Y. (1961) Possible public health hazard through use of sewage oxidation ponds effluents for irrigation. Bull. Res. Counc. Israel 9E. K o r r Y., HERSHKOVtTZ G., SHE~,ITOBA. and SLESSJ. B. (t966) Algicidal effect of bromine and chlorine on Chlorella Pyrenoidosa. Appl. 3,Iicrobiol. 14, 8-1 I. K o r r Y. and INGERMAN R. (1966) The biochemical dynamics of waste stabilization ponds. Int. jr. Air War. Pollut. 10, 603-609. K o r r Y. (1970) Chlorination dynamics in wastewater effluents. In: Proc. National Specialty Conf. on Disinfection, pp. 585. ASCE, New York. KOTT Y. (1971) Coliphages as virus indicators in water and wastewater. Second Annual Report, FWQA Research Grant 16030 DQN, Technion Research and Development Foundation Ltd., Haifa, Israel. STATE OF ISRAEL, MINISTRY OF HEALTH (1965) Restrictions for irrigation with Wastewaterfor Agricttltural Purposes. SWARTZ T. A. (1971) The Jerusalem cholera outbreak--The course of the epidemiologic investigation. Public Health 14, 13-18.