High-rate algal pond performances in faecal coliforms and helminth egg removals

War. Res. Vol. 28, No. 1, pp. 171-174, 1994 Printed in Great Britain. All rights reserved

0043-1354/94 $6.00+0.00 Copyright © 1993 Pergamon Press Ltd

HIGH-RATE ALGAL POND PERFORMANCES IN FAECAL COLIFORMS AND HELMINTH EGG REMOVALS B. EL HAMOURI~), K. KHALLAYOUNE,K. BOUZOUBAA,N. RHALLABI a n d M. CHALABI Groupe de Recherches sur rEpuration des Eaux Us6es et leur R6utilisation (GREEUR) Institut Agronomique et V6t6rinaire Hassan II, BP 6202 Rabat-Instituts, Rabat, Morocco

(First received May 1992; accepted in revised form April 1993) Abstract--Experiments on wastewater treatment in high-rate algal ponds (HRAPs) were conducted at Rabat (Morocco). The results indicate a BOD removal of 88%, while N-NH + and P-PO 3- removals reached 69 and 52%, respectively. Investigations on faecal coliforms (FC), pathogens (Salmonella) and helminth egg (nematodes) removals by the treatment system showed that (i) the design parameters of the HRAP play an important role in (FC) die-off. It is demonstrated that high FC die-off was obtained throughout the year when the area/volume ratio of the HRAP reached 3.3 m-~ while seasonal variations were recorded for lower figures of this ratio; (ii) a complete removal of Salmonella sp.; and (iii) a complete removal of the nematode eggs initially found in the raw wastcwater particularly those of the Ascaris and Trichuris genera.

Key words--high-rate algal pond, BOD, design parameters, faecal bacteria, die-off, Salmonella, nematode eggs

INTRODUCTION

MATERIALS AND METHODS

Wastewater and experimental HRAPs

The high-rate algal p o n d ( H R A P ) described by Oswald a n d Golueke (1968) a n d M c G a r r y a n d T o n g k a s a m e (1971) for wastewater t r e a t m e n t a n d n u t r i e n t recycling is based o n a symbiotic interaction between the h e t e r o t r o p h i c bacteria a n d the algal cells living in the p o n d (El H a m o u r i et al., 1992). While extensive i n f o r m a t i o n o n the H R A P capabilities for B O D a n d n u t r i e n t removals in m a n y areas in the world is available, very few studies have been

Wastewater of the city of Rabat was pumped into a reservoir at the end of the main sewer serving a habitation zone called Takaddoum and immediately despatched (15-20 rain) to the treatment pilot-plant. Wastewater was settled for 24 h before its treatment in the HRAP (3 days retention time in the hot season and 6 days in the cold season). Three HRAP-pilots were used in the experiments. Their dimensions were as follows:

performed to assess the H R A P performances in pathogen removal. Almost all the data available on this subject originate from waste stabilization ponds (WSP). Thus, the effect of physico-chemical and environmental factors on F C die-off have been studied: dissolved oxygen, ultra-violetlight (Moeller and Calkins, 1980; Middlebrooks et al., 1982), predation (Ellis, 1983) and p H (Parhad and Rao, 1974; Pearson et al., 1987). On the other hand, helminth egg removal performances were reported for W S P (Schwartzbrod et al., 1987; Silva et al., 1987) and for conventional wastewater treatment plants (Sticn and Schwartzbrod, 1988). N o work has yet bccn done to assess those performances for HRAPs. In this paper, B O D and nutrient removals are reported to check the H R A P performances under the Moroccan climate. Most of the results reported here deal with the performances of the H R A P in F C and helminth egg removal. F C die-off is discussed in relation to the H R A P design parameters and the physico-chemical factors prevailing in the pond. 171

Area

Depth

Area/volume

Pond

(m 2)

(m)

(m- l)

1

12 24 4.7

0.6 0.5 0.3

1.6 2 3.3

2 3

The pond content was continuously mixed by paddle wheels operated by electrical motors (0.5 kW). The speed of water at the surface of the pond was maintained at 0.15 m/s. The experiments described in this paper took place from 1986 to 1989.

Analytical procedures BOD 5, COD, N-NH~, N-NO~-, P-PO43- and suspended solids (SS) were analysed according to Standard Methods (APHA, 1985).

Parasitological analysis Helminth eggs were concentrated from 2-1. composite samples by centrifugation at 1000g for 15 min and isolated by flotation following the method of Janeckso and Urbanyi (1931). The eggs were enumerated in a MacMaster cell under a standard microscope.

Bacteriological analysis Bacterial counts were performed following the fermentation tubes method (APHA, 1985) or on a solid medium

B, EL HAMOURIet al.

172

Table 1. Some of the physico-cbemicalcharacteristics of Rabat's wastewater before and after treatment in a HRAP: 24 h settling period followed by a 3-day retention time in the HRAP (or 6 days in the cold season) Wastewater

Raw Settled Treated pH 7.4 73 8. I EC 1.56 1.49 0.96 SS (rag/l) 220 160 390 COD (mg/l) 695 439 163 BOD5 (rag/I) 417 275 50 N-NH~- (mg/I) 39 31.2 11.9 N-NO3 (mg/I) 2.12 1.97 1.01 P-PO3- (mg/l) 12.56 11.02 6.07 SS = suspended solids; COD = chemical oxygen demand; BOD5= biochemicaloxygen demand in 5 days at 20~'C; EC = electrical conductivity at + 25°C (dS/m).

following the method described by Rodier (1985) for faecal bacteria and the poured plate method (APHA, 1985) for total coliforms. Salmonella counts were performed on 100 ml samples following the 3-steps method: (i) pre-enrichment, (ii) enrichment (Muller-Kauffman tetrathionate broth), (iii) isolation (Harvey and Price, 1979) followed by a biochemical screening and a serological identification (Cox and Williams, 1976). Physico-chemical analysis • Dissolved oxygen was measured using a Clark electrode CG 867 (Schott Gerate). • pH was recorded using a combined electrode and a pH-meter (Metrohm Hersian E 603). • Electrical conductivity at 25°C was measured using a Radiometer CDMZF. • Light intensity was measured using a quantum-meter (Li-Cor E 603). RESULTS AND DISCUSSION BOD, N-NH~

and P-PO34 - removals

Table 1 shows the physico-chemicat characteristics of Rabat's wastewater before and after each step of the treatment. BOD5 removal reached 88% (34% being eliminated during the 24 h settling period) while the concentrations of N - N H ~ and P-PO43- decreased by 69 and 52%, respectively. The algal concentration in the effluent reached a maximum of 230 mg/1 in summer conditions. These performances confirm those already reported for H R A P s in arid and sub-arid areas (Shelef and Azov, 1987). Faecal coliforms removal Faecal coliforms (FC) and faecal streptococcus (FS) counts performed on Rabat's wastewater

throughout the year and their removal rates are shown in Table 2, while the removal rate expressed in log units for the four seasons is shown in Table 3. These tables show that FC removal by the treatment under the Moroccan climate reaches more than 3.5 log units in summer conditions. This interesting result was not expected regarding the relatively short retention time adopted by comparison with WSP systems where retention times are generally around 20 days for slightly better results (Oragui et al., 1987; Silva et al., 1987). On the other hand, Table 3 shows that to keep the effluent quality equivalent to that obtained under summer conditions, it is necessary to extend the retention time in the cold season over the 6-day period which would lead, in turn, to an area extension of the H R A P . Such an alternative is not always possible, this is why we investigated other ways to improve the faecal bacteria removal during wastewater treatment in the H R A P . Figure 1 shows that design parameters of the H R A P are indeed of great importance in this case. It is shown that when the area/volume (A/V) ratio reaches 3.3 m ~, the effect of the season on FC dieoff becomes negligible, while the dependence on the seasons increases for lower values of this ratio. Figure 1 also shows that for the 3.3 m ~ value of A/V, a slightly higher FC die-off may be obtained in spring than in summer as was the case for lower values of this ratio. This might be explained by the inhibition of photosynthesis (photoinhibition), which is likely to occur in H R A P s under high light intensities and high temperatures (El Hamouri et al., 1989), leading to the reduction of the lethal effects of pH and dissolved oxygen on faecal bacteria. Several physico-chemical factors may affect the FC die-off during wastewater treatment in systems based on algae-bacteria interactions. Those frequently reported are: ultra-violet light penetration (Moeller and Calkins, 1980), dissolved oxygen concentration (Pearson et al., 1987) and pH (Parhad and Rao, 1974). Pearson et al. (1987) showed that pH approaching or above 9 increased FC die-off. Such pH values are reached daily in the H R A P (Fig. 2). Figure 2 also shows that the dissolved oxygen concentration reached 2 to 3 times the concentration of saturation during the second half of the day. Conjugated to high light intensities, 1500/~E m -~ s -t in summer and 550 in winter, this oxygen-rich environment may result in the formation of singlet oxygen and/or superoxide as a result of excessive light

Table 2. Rabat's wastewater counts* of faecal bacteria throughout the year and their removal rate in a HRAP (treatment conditions as in Table l) Raw Treated Rate of removal Total coliforms/100ml 2.4 10~t 2.0 107 91,66 Faecal coliforms/100ml 2.1 107 4.6 103 99.98 Faecal strcptococcus/100ml 2.2 l07 2.3 104 99.89 *Geometric means of faecal bacteria and total coliforms counts performed throughout the year; treatment conditions: 3-h day retention time in the hot season and 6 days in the cold season; tnumber of analyses = 50.

Wastewater treatment in high-rate algal ponds

173

Table 3. Effect of the seasons on faecal coliformsand faecal streptococcus removalrates* (in log units)duringwastewatertreatmentin a HRAP (treatment conditions as in Table I) Autumn-wintert

Spring

Summer

2.27 + 0.36 2.22 +_0,32

2.74 + 0.56 2.67 + 0.26

3.19 +_0.43 3.16 + 0.29

Faecal coliforms Faecal streptococcus

*Geometric means of the bacterial counts before and after treatment for the 3 HRAPs at each of the 4 seasons were used for calculation; ?autumn and winter figures were similar.

capture by the algal chlorophyll. These two molecules are very reactive forms of oxygen. They were reported to provoke irreversible damage to the photosynthetic apparatus (Hipkins, 1984) and to microorganism D N A (Decuyper et al., 1984; Cadenas, 1989). Both high pH and high dissolved oxygen concentrations depend on seasonal variations because of light intensity and temperature changes. This explains why the retention time must be extended from 3, in the hot season, to 6 days in the cold season. Such an operating mode has been adopted for optimal waste oxidation by Azov and Shelef (1982) and is confirmed here for FC removal performances as well. On the other hand, Table 2 shows that the faecal coliforms content of the effluent is 5 times higher than the 1000 FC/100 ml recommended by the WHO for unrestricted irrigation (WHO, 1989). Unless a 2 or 3-day retention time, in maturation ponds, is added to the treatment scheme to reach the guidelines allowing unrestricted reuse, the effluent must only be used for industrial crops, cereals or crops to be cooked for maximum public health preservation (WHO, 1989). Salmonella isolation in wastewater before and after the treatment in HRAP showed that only 14 strains among the 262 isolated during the 3 years were suspected as Salmonella sp. and serologically confirmed. Ten of these strains (71%) were isolated from raw and four from settled wastewater (29%).

Among the serotypes isolated, S. typhimurium, S. enteritidis, S. edimburg and S. paratiphi were predominant either in raw or settled wastewater. None of these strains was isolated from the HRAP effluent whatever the operating conditions and pond design parameters demonstrating, thus, the high efficiency of the treatment based on the HRAP in removing this particular pathogen. Helminth egg removal The helminth egg counts in the raw and treated wastewater are shown in Table 4. Among the parasites of great concern for wastewater reuse in irrigation (WHO, 1989), Ascaris sp. and Trichuris sp. were found with 17 and 7 eggs/l, respectively. None of these parasites were found in treated water. In a previous study, Ascaris sp. and Trichuris sp. were found in treated water at an average of 6 eggs/! (Rhallabi et al., 1990). This is not in accordance with the figures shown in Table 4. The discrepancy is actually due to the eggs isolation method used by Rhallabi et al. (1990). We used the sedimentation method of Ballanger et al. (1977) which does not seem appropriate for wastewater examination since numerous debris made identification and quantification difficult (Barbier et al., 1989). Complete removal of the nematode eggs, except for Toxocara sp. and Enterobius sp., is not surprising since the contribution of the treatment step included for egg removal (24 h settling period) reached 100% for the genera of greatest concern in wastewater reuse

4..

9,4

i

30

9,2

t~

A/V~ 1.6

/

lO

J

8,6 i

Ant. - W i n t e r

Spring

Sy4

Summa-

Fig. !. Faecal coliforms (FC) removal for the three HRAPs with different area/volume ratios in the four seasons. Seasonal geometric means of FC counts for each pond, before and after treatment, were used for the calculations. W R 25/I--L

8,g

•

6

!

8

-

!

10

•

|

12

•

|

14

-

!

16

•

!

18

•

0

20

l~y.time

Fig. 2. Typical day time-course of pH and dissolved oxygen concentration in a HRAP wastewater treatment system in Rabat, Morocco.

B. EL HAMOURI et al.

174

Table 4. Helminth egg counts (eggs/I) in Rabat's wastewater before and after treatment (treatment conditions as in Table I) Raw Settled Treated Nematodes Ascaris sp Toxascaris sp Trichuris sp Capillaria sp Toxocara sp Enterobius sp Cestodes Hymenolepis sp Moniezia sp Trematodes Fasciola hepatica Total

17 6 7 2 25 21

0 0 0 0 3 I

0 0 0 0 0.4 0.4

26 5

0 0

0 0

4 113

0 4

0 0.8

in agriculture ( W H O , 1989). F o r Toxocara sp. a n d Enterobius sp., the H R A P t r e a t m e n t improved egg removal by a n o t h e r 10 a n d 3%, respectively. W e do n o t have any indications yet as to whether this additional h e l m i n t h egg disappearance in the H R A P was due to a simple settling effect occurring, even t h o u g h the p o n d c o n t e n t was continuously mixed, or to a n actual egg die-off. Acknowledgements--The authors wish to thank the following organizations for their financial and logistical support: DER, Minist~re de l'Agriculture et de la Rfforme Agraire, Rabat; the International Foundation for Science (IFS), Stockholm and le Commissariat G~n~ral aux Relations Internationales de ia Communaut6 Franq, aise de Belgique (CGRI), Brussels. The authors also thank Pr. Ettalibi for reading and discussing the manuscript. REFERENCES

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