Integrating ponds and activated sludge process in the PETRO concept

PII: S0043-1354(98)00389-3

Wat. Res. Vol. 33, No. 8, pp. 1767±1774, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter

INTEGRATING PONDS AND ACTIVATED SLUDGE PROCESS IN THE PETRO CONCEPT M O. V. SHIPIN1*, P. G. J. MEIRING1* , R. PHASWANA2 and H. KLUEVER2

Meiring, Turner and Ho€mann, P.O. Box 36693, Menlo Park, 0102 Pretoria, South Africa and Rietgat Puri®cation Works, Northern Pretoria Metropolitan Substructure, Soshanguve (Tswaing), South Africa 1

2

(First received April 1998; accepted in revised form September 1998) AbstractÐThe activated sludge process (ASP) can be successfully integrated into the ponding system in the framework of the PETRO concept (ASP-variant). The role of microalgae in the ASP was elucidated and biological mechanisms of algae removal are suggested. Microalgae contribute to ¯occulation by the production of exopolysaccharides (EPS) under stress conditions. Superior ¯occulation appears to result in a greater nitri®cation rate contributing to N-removal in the ASP reactor. Consequences and advantages of the secondary position of the polishing ASP (downstream of the ponds) are discussed. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐmicroalgae, exopolysaccharides, nitri®cation, nitri®ers, oxidation ponds, lagoons, activated sludge, sewage

INTRODUCTION

The PETRO system was designed to combine oxidation ponding as a low tech primary stage and a polishing facility as a secondary stage (Meiring, 1993; Shipin et al., 1998). The full scale systems has two variants in which the secondary facility can be either a trickling ®lter (TF) or an activated sludge process (ASP). A series of oxidation ponds treat the bulk of organic load (up to 70%), which substantially decreases the size of the relatively high tech secondary facility hence the acronym PETRO (Pond Enhanced Treatment and Operation). The rationale behind the concept should not be considered just in terms of the unit process mechanisms Ð ponds (Mara and Pearson, 1986), TF and ASP (Hawkes, 1983a,b), but rather in terms of the synergism characterizing the PETRO process. Compelling evidence elucidating mechanisms underlying e€ective microalgae removal have been obtained by label studies on the TF variant of the system (Shipin et al., 1996; Meiring et al., 1997; Shipin et al., 1999). It was demonstrated that microalgae, acting as heterotrophs in the dark, contribute to the mucilage production in the TF acting as heterotrophs in the dark. Bio®lm slime, predominantly exopolysaccharide (EPS) in nature, was shown to be produced by the microalgae experiencing stress transfer from the mixotrophic (growth on organics in the light) conditions in the ponds to the heterotrophic conditions in the TF (metabolism in the dark). Microalgae are thought to heterotro*Author to whom all correspondence should be addressed.

phically utilize low molecular weight organics such as amino acids, monosaccharides, VFA, etc. (Neilson and Lewin, 1974; Abeliovich and Weisman, 1978; Pearson et al., 1987). Microalgae appear to play a prominent part in the PETRO TF and more importantly, help produce the superior quality of its ®nal e‚uent. This paper reports the results of an investigation into the role of microalgae entering an ASP reactor of the ASP variant plant, as well as the consequences for the ASP performance and the treatment process as a whole.

MATERIALS AND METHODS

Description of the full scale PETRO ASP-variant plant The plant is situated in Soshanguve (Tswaing), north of Pretoria, South Africa. The area is characterized by a moderate highveld climate with hot summers (December± February, average Twater=198C) and mild winters (June± August, average Twater=148C). The plant receives 7 MLD (70000 p.e.) of municipal sewage (640 COD mg/l, Nammonia 28 mg/l) at an organic loading rate 4500 kg COD/d. The principal ¯ow diagram of the process is shown in Fig. 1. The primary treatment stage comprises a series of oxidation ponds consisting of a primary facultative pond (volume 5360 m3, depth 4 m), 2 secondary facultative ponds (total area 15,000 m2, depth 1.5 m) in a loopwise arrangement supplying algae-rich recirculated water to the surface of a primary pond, another small primary pond is used to contain sludge carry-over. An activated sludge reactor of the BNR type (volume 2403 m3, organic loading rate 0.75 kg COD/m3/d, six 22.4 kW vertical shaft aerators) is fed from the primary pond and followed by two clari®ers and an aerobic digester to stabilize waste sludge before drying on sludge drying beds.

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Fig. 1. Flow diagram of the Soshanguve ASP variant plant. (A) Primary facultative pond, broken line square- fermentation pit, (B±D) secondary oxidation ponds, (E) BNR reactor, (F) clari®ers, (G) chlorination facility, (a) stream containing VFA for P-removal in the BNR reactor, (b) primary pond e‚uent, (c) oxidation pond recycle; recycle rate = c:(a + e) = 1.5, (d) algae-rich recycle, (e) BNR aerobic zone feed, (g) return activated sludge, (h) waste activated sludge to the aerobic sludge digester, (j) e‚uent to river, (k) to the sludge drying beds. The system operation parameters (COD, ammonia, nitrate, MLSS, SVI, SSV, ash, protein and chlorophyll a were determined according to the standard methods (APHA, 1989). Algal concentration was expressed as cells per l or as chlorophyll a (mg/l). Direct measurement of chlorophyll in the mixed liquor containing microalgae proved impossible due to methodological problems. Pigment extraction with a solvent (acetone, ethanol) yielded a coloring agent from the sludge of which light absorbance at the required wavelengths was several times higher than that of chlorophyll. Therefore the algal biomass in the MLSS had to be evaluated as cells per l (Table 1). For the lab experiments an authentic sludge from a full scale ASP was used in the two lab reactors (50 l) to simulate conditions in the full scale ASP reactor. The reactor conditions were maintained at: MLSS = 55002200 mg/l; SSV = 5802 20 ml; SVI = 106 ml/mg SS; pH 7.1 20.2; DO = 4.020.5 mg/l (mixing was ensured by two pumps); sludge age = 15 days. Feed parameters: CODtotal=550 mg/l; CODalgal=200 or 500 mg/l (depending on the experiment); NH+ 4 =45 mg/l; NOÿ 3 =1 mg/l; organic loading rate 750 mg/l/day COD. Algal biomass, typical of the ponds was pregrown in 0.5 l ¯asks on PHM-1 mineral medium (Borowitzka and Borowitzka, 1988). It was concentrated by centrifugation and added to the ASP feed at the concentration of 200 mg/l as COD (on average 2100 mg/l chlorophyll a as a ®nal concentration). Dead algal biomass was produced by drying in an oven at 608C for 1 h, then washing in a phos-

phate bu€er (pH 7.0) to remove lysed organic material. The supernatant was discarded while the algal slurry added to the ASP feed. For viable cell enumeration purposes (total heterotrophs or ammonium oxidizers) sludge ¯ocs were homogenized in an Ultra-TURRAX homogenizer (Janke and Kunkel) for 2±3 s at high speed. Viable heterotrophic bacteria were counted on NWRI agar (APHA, 1989) with an extract from PETRO ASP sludge used as a liquid medium. Sludge extract was obtained by autoclaving the sludge for 3 h at 1218C. Direct microscopic enumeration of microalgae, protozoa and rotifers was carried out according to the standard methods (APHA, 1989). Concentration was achieved by centrifugation. Nitrifying bacteria (ammonium oxidizers) were counted by the most probable number technique in liquid medium (Schmidt and Belser, 1982) with bromothymol blue as indicator (indicator turns yellow due to activity of ammonium oxidizers). Tubes were incubated at 208C for 9 weeks. EPS concentration in sludge and solution (after sludge ¯ocs were separated by centrifugation at 5000 rpm for 15 min) was measured by the calibrated ruthenium red adsorption method at wavelength 532 nm (Figueroa and Silverstein, 1989). The nitri®cation rate was determined as mg NH3-N oxidized per g of mixed liquor suspended solids per day according to a method for the short-term nitrifying activity determination (Schmidt and Belser, 1982).

Table 1. E€ect of microalgae in the feed on microbiological parameters of the full scale PETRO ASP reactora (Soshanguve). Standard deviations are in parentheses 6 Aug 1996 Microalgae in the in¯ow, cells/l Microalgae in the mixed liquor,cells/l Protozoab, per l Rotifers, per l Heterotrophic bacteria, CFU/l Ammonium oxidizers, per l Nitri®cation rate, mg NH3N oxidized per g MLSS per day

6

2 Sept 1996 5

6

4 Dec 1996 5

7

25 Feb 1997 6

7

17 June 1997 6

4.4x10 (21.7x10 )

3.1x10 (21.2x10 )

2.3x10 (21.0x10 )

9.9x10 (23.9x10 )

3.7x106 (21.2x105)

7.9x106 (25.1x105)

8.0x106 (26.3x105)

1.1x107 (21.0x106)

8.9x107 (22.9x106)

n.d.c

0 5.8x104 (21.4x103) 10 5.7x10 (21.5x109)

3.5x103 (21.7x102) 3.9x102 (21.0x102) 4.8x1010 (22.0x109)

2.0x102 (20.1x102) 1.3x103 (21.5x102) 9.1x1010 (23.9x109)

0 4.0x103 (22.2x102) 10 4.9x10 (24.0x109)

0 4.1x103 (21.6x102) 10 8.1x10 (22.3x109)

8.2x109 (23.0x108) 63 (23.6)

7.0x109 (22.1x108) 60 (22.5)

9.8x109 (24.3x108) 71 (23.1)

9.9x109 (25.3x108) 75 (23.3)

4.6x109 (21.8x108) 57 (22.0)

a MLSS in the system ¯uctuated to a certain extent from date to date (ranging from 5193 to 6580) the values for parameters measured per l were recalculated to MLSS 5500 mg/l for comparison.bOnly protozoan species capable of feeding on microalgae were counted (Opercularia sp.).cn.d. means not determined.

Ponds and ASP in the PETRO concept RESULTS AND DISCUSSION

Full scale plant study Certain microbiological parameters (with an emphasis on sludge capacity to nitrify and produce exopolysaccharide, EPS) have been monitored over the period of 10 months which spanned di€erent seasons characterized by ¯uctuating concentration of algal biomass in the ponds (Table 1). All microalgae entering the polishing ASP reactor originate from the secondary oxidation ponds. The pond e‚uent rich in algae is supplied to the aerobic section of the ASP reactor. Another portion of this algal pond e‚uent is recirculated to the surface of the primary pond from where microalgae eventually end up in the anaerobic chamber of the ASP. The chamber was speci®cally designed for high rate Premoval (the work on optimization of P-removal is currently in progress). Finally the microalgae already enmeshed in sludge ¯ocs enter the aerobic zone through the anoxic zone designed for P- and N-removal. Microalgae in the oxidation ponds (ASP reactor feed) were represented by dominant Chlamydomonas sp., Phacus sp. and Euglena sp. with lower numbers of Chlorella sp., Micractinium sp. and occasionally Scenedesmus quadricauda. Speciation of dominant algae varied seasonally with a tendency for Chlamydomonas sp. as the most resilient alga to become the only species in winter when

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lower light intensity and oxidation rate put higher stress on other species. It was observed that seasonal algal blooms in the oxidation ponds preceding the ASP characteristically resulted in an enhanced EPS content of sludge ¯ocs (Fig. 2). Apparently EPS content directly ¯uctuated with algal concentration increasing from 110 to 846 mg/l (as chlorophyll a). EPS concentrations were as high as 2.8%, though non¯occulated polysaccharide did not exceed 0.01%. Meanwhile ash and protein content of the ¯ocs remained constant ranging from 30 to 35% and from 2.3 to 2.8 wt%, respectively. No correlation with microalgal concentration or any other parameter has been observed. Although the protein content of the sludge ¯ocs was equal to or sometimes greater than that of the EPS, the relative importance of these polymers in terms of ¯occulating capacity was di€erent. Rheological properties of EPS are known to substantially exceed those of proteins (Sutherland, 1990). The number of nitri®ers (ammonium oxidizers) in the ¯ocs appeared to grow with an increase of the microalgal population in the ponds (Table 1). At the same time the number of heterotrophic bacteria did not correlate with these variations and remained relatively constant. Nitri®cation rate in the ¯ocs closely followed both the microalgal concentration and EPS content ¯uctuations. Apparently better EPS

Fig. 2. Correlation of microalgal biomass concentration (measured as chlorophyll a, mg/l) entering full scale ASP reactor and exopolysaccharide (EPS) concentration in sludge ¯ocs. (w) EPS in ¯ocs, wt%, max standard deviation (SD) 0.30; (Q) chlorophyll a in in¯uent (max SD 29); (R) chlorophyll a in clari®er over¯ow (max SD 10).

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production and ¯occulation e€ected higher retention of sensitive nitri®ers. This suggests that the presence of algae leads to higher rates of ammonia oxidation which in its turn enhances N-removal in the system. Though a variety of protozoa species were found in the reactor (Thecamoeba sp., ciliates: Epistylis sp., Aspidisca sp.) only the sessile colonial ciliate Opercularia sp. was observed to contain microalgae. Its concentration varied from 0 to 9.6  103 per l, but no correlation has been found with algal load on the reactor (data not shown). This indicated that in contrast to the PETRO TF, protozoan predation plays a relatively minor role in algae removal in the ASP. Rotifer population and algae removal by rotifers is con®ned to the ASP reactor and downstream of the clari®ers, where the ammonia concentration is low (<17 mg/l ammonia-N). The ASP reactor is characterized by the following NH3-N concentrations: aerobic zone <1, anoxic zone <5, anaerobic zone 13 mg/l. Sensitivity of rotifers to high ammonia concentrations, characteristic of the oxidation ponds (18±30 mg/l), precludes them from playing a signi®cant role in algae removal in the ponds (Lincoln et al., 1983). Rotifers were represented by two species: Philodina sp. (usually about 80%) and Trichocerca sp. No correlation has been found between their ¯uctuating numbers (1.3  103±5.8  104 per l) and algal load. Nevertheless under the conditions of the PETRO polishing ASP, usually characterized by the presence of high number of unicellular microalgae rotifers, may be important agents in eliminating non¯occulated material, such as single algae. Rotifers have an enormous potential for microalgae removal (Seaman et al., 1986).

Nematodes (200±350 mm) present in the reactor were not observed to feed on microalgae. Laboratory study Lab PETRO ASP pond reactors were set up to investigate the e€ect of the algal loading rate on critical parameters of the activated sludge process (Figs 3 and 4) and further elucidate the algae removal mechanism in the full scale ASP. The reactors ®lled with an activated sludge, developed in the full scale reactor (Soshanguve) were supplied with algal loads exceeding those of the full scale facilities: CODalgal 200 mg/l (2100 mg/l by chlorophyll a). It was demonstrated, that under these conditions sludge EPS content increased signi®cantly due to an arti®cial algal ``overdose'' (Fig. 3). Floccular EPS concentration increases from 1.3 to 3.5% over 6 weeks of feeding live microalgae, a tremendous achievement considering the colloidal properties of EPS when small weight increments lead to a logarithmic volumetric and rheological increase. Specially prepared dead algal biomass representing the same COD load, 200 mg/l (in addition to a nonalgal load of 350 mg/l) failed to induce a similar e€ect in the parallel reactor. This suggests that only metabolically active algae capable of EPS production enhance ¯occulation in the ASP. Rotifer (Philodina sp., Trichocerca sp.) and Opercularia sp. populations were consistently more stable in the ®rst reactor (varying from 5.5  104 to 9.5  104 and from 1.6  104 to 6.3  107, respectively). At the same time these populations in the 2nd reactor varied from 4.1  102 to 5.5  104 and from 7.6  102 to 4.8  105, respectively. The algal ``overdose'' signi®cantly boosted the nitrifying capacity of the activated sludge in the reactor over the period of 6 weeks. The number of

Fig. 3. Correlation of EPS in ¯ocs and nitri®cation rate in the lab PETRO ASP reactor fed either live or dead algal biomass. Feed: live algae, (w) EPS in ¯ocs, wt%, max SD 0.24; (Q) nitri®cation rate, mg NH3-N oxidized per g MLSS per day (max SD 5.1). Feed: dead algae, () EPS in ¯ocs, wt% (max SD 0.20); (R) nitri®cation rate, mg NH3-N oxidized per g MLSS per day (max SD 4.3).

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Fig. 4. EPS in ¯ocs versus nitri®cation rate in the lab PETRO ASP reactor operating under conditions of nonalgal organic underload. (w) EPS in ¯ocs, wt%, max SD 0.17; (Q) nitri®cation rate, mg NH3-N oxidized per g MLSS per day (max SD 3.9).

the nitrifying bacteria (ammonium oxidizers) increased from 4.7  109 to 7.9  109, while the total heterotrophs population remained constant. Concomitant enhancement in nitri®cation rate was observed (Fig. 3). In contrast, dead algal biomass did not bring about such improvement, indicating that a metabolically active biomass is required to enhance nitri®cation in the PETRO ASP. One possible mechanism is the better retention of nitrifying bacteria due to a higher sludge EPS content. In an experiment (Fig. 4) under conditions simulating underload in CODnonalgal (50 mg/l) and an overload in CODalgal (200 mg/l), sludge EPS content decreased signi®cantly over 9 weeks despite the presence of algae. This suggests that a balanced supply of the algal and nonalgal organic loads is a prerequisite of the ecient operation of the polishing ASP reactor. Oxidation ponds (particularly the primary oxidation ponds) tackle substantial COD loads (up to 60%) through the action of the algo-bacterial consortium, thus decreasing the load on the polishing ASP reactor. As a result, apparently large amounts of microalgal biomass are generated by the combined action of sunlight and bacterial CO2 production. Nevertheless an increase in the algal portion of the ASP load (and algal suspended solids) does not represent a proportional increase in COD load on the ASP sludge. Every 100 mg/l in chlorophyll a values were empirically determined to represent approximately 10 mg/l(3 mg/l) in COD (BOD) values and 20 mg/l SS. Furthermore, the recalcitrant nature of microalgal biomass under the secondary ASP conditions (light availability, low organic load) results in even lower algal organic load being imposed on the ASP. The highest algal

loading was registered in February 1997: 846 mg/l was found to have a COD value of only 85 mg/l, which in terms of biodegradability under aerobic conditions in the ASP, may be regarded as insigni®cant. The algal biomass appears to be fairly recalcitrant under aerobic conditions. Microscopic observations of the activated sludge support these calculations. In contrast to the TF (Shipin et al., 1996), microalgal cells in the ASP appear to ¯occulate and get embedded in activated sludge ¯ocs without signs of substantial lysis and degradation. Microalgal biomass in the waste activated sludge dries well without causing any odor problems. The dried sludge is quite stable and does not reabsorb water. When absorbed by the activated sludge, microalgae do not lose viability, even during the extended process of aerobic digestion (20 days), by which time the wasted sludge has stabilized. Mechanisms of algae removal in TF and ASP It is thought that several adaptive and metabolic strategies are exhibited by microalgae to overcome the light limitations. One of importance for the current investigation is a phenomenon of an active turbidity reduction in the surrounding waters by microalgae. As a rule only a fraction of surface light (often as low as 1%) penetrates the water column and is thus available to benthic algae (Fenchel and Staarup, 1971). Light absorption is even more signi®cant in natural and arti®cially turbid waters containing large amounts of suspended solids e.g. clay (not dissimilar to the mixed liquor of the ASP). To secure minimal light requirements for survival and growth microalgae are known to excrete ¯occulating substances of polysaccharide nature,

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usually acidic EPS (Zur, 1979; Fattom and Shilo, 1984). Precipitation of clay particles and clari®cation of the Huleh swamp in Israel caused by production of a soluble algal ¯occulant has been reported (Avnimelech, 1975). Avnimelech et al. (1982) have shown that mutual aggregation of microalgae and clay particles is a rather widespread phenomenon. Of the four algal species tested, representing di€erent morphological, physiological and taxonomic groups, three, blue± green ®lamentous Anabaena sp., nonmotile green unicellular Chlorella sp. and motile green Chlamydomonas sp., had a de®nite and well-pronounced tendency to form clusters with clay. Severe light limitations in the turbid waters appear to trigger microalgal production of extracellular polysaccharide which causes ¯occulation. In this way a microalgal population loses some cells through the ¯occulation of nonalgal solids, but ensures higher light availability for the remaining non¯occulated cells. All studied microalgae were reported to produce acid exopolysaccharides (Percival and Foyle, 1979; Barclay and Lewin, 1985; Borowitzka and Borowitzka, 1988). Bacteria also produce mostly acid EPS (Sutherland, 1990). It is not feasible to di€erentiate between bacterial and algal EPS in the ASP sludge ¯ocs. Nevertheless it is possible to correlate an increase of EPS in the activated sludge ¯ocs with an increase of the microalgal biomass content in the ASP feed (pond e‚uent). This correlation was observed at full scale ASP (Fig. 2) and corroborated by simulation in a lab reactor (Fig. 3). Transfer experiments carried out on the dominant pond algal species (Shipin et al., 1996) were repeated for Chlamydomonas sp., a dominant algal species in the ASP variant plant (data not shown). Chlamydomonas sp. was shown to grow heterotrophically in the dark on a variety of organic compounds (glucose, amino acids, glycerol, ethanol, acetate, formate). The results obtained con®rmed ®ndings that some algae produce EPS after the stationary phase transfer from mixotrophic to heterotrophic mode of growth. The data suggest that stress-induced ¯occulation is at least one of the major mechanisms responsible for the algae removal in the PETRO ASP. It appears that the relative importance of the di€erent mechanisms involved in the removal of microalgae in the TF and ASP varies. Removal by rotifers and protozoa plays a greater role in the TF. The removal in the TF is characterized by both ¯occulation (due to algal and bacterial EPS production) and degradation (through bacterial activity) of algae. In the ASP, algae removal is achieved primarily through the stress-induced EPS production (both algal and bacterial) and subsequent ¯occulation (embedding of the algal biomass in the sludge ¯ocs).

Consequences of the secondary position of the PETRO ASP The ASP reactor (used as a secondary facility after the ponding and therefore a low rate process in terms of organic loading) compares very favorably with a conventional ASP employed as a much more commonly used primary (medium or high rate) facility. Its economically important features (e.g. lower energy and operational costs) are complemented by a lower susceptibility to problems of microbiological nature (e.g. bulking was never observed in Soshanguve). In this arrangement the ASP microbial consortium operates under conditions when most of COD load has already been removed in the upstream ponds (primary and secondary oxidation pond). Microbial biomass production is lower, and less excess sludge is produced, since endogenous respiration (and secondary metabolite production e.g. EPS) is greater than in a high/medium rate ASP. Forced endogenous respiration results in a production of biomass with a high mineral level and a more stable sludge is produced. It has long been known that to maintain an e€ective population of nitrifying bacteria in the ASP the proportionate increase in their population must at least equal that of the heterotrophic population. The heterotrophs under nonlimiting nutrient conditions are known to have a much higher growth rate than the nitrifying autotrophs. Hence the need to limit nutrients available to heterotrophs by controlling the food/mass ratio to a low ®gure. This ensures e€ective nitri®cation (Hawkes, 1983b). At high loading rates the sludge is more prone to poor settleability, and high supernatant turbidity. At such times changes in the chemical composition of the extracted biopolymer (¯occulant) were noted and there was a considerable loss of ¯occulating capacity (Ki€, 1975). The above-mentioned requirements are best achieved in the PETRO concept by placing the ASP facility downstream of the low tech ponding system removing the bulk of organic load. It is suggested that using the PETRO ASP as a secondary treatment produces better sludge ¯occulation. Reciprocally, better ¯occulation would lead to higher retention of slow-growing nitri®ers enhancing the nitri®cation potential of the ASP. The higher nitri®cation rate due to lower heterotrophic conditions is particularly important at lower temperatures (below 158C). At the lower substrate concentrations a nitrifying population is built up at shorter sludge ages in PETRO ASP than in a conventional ASP.

CONCLUSIONS

Exopolysaccharides are considered to be produced only by bacteria and fungi in conventional

Ponds and ASP in the PETRO concept

ASP systems (Hawkes, 1983a). Our data suggest that algal contribution to the EPS production in the PETRO ASP reactor is substantial. This EPS production appears to be triggered by recognized stress phenomenon. To secure minimal light requirements for survival and growth under conditions of high particulate turbidity, microalgae excrete a ¯occulating substance of polysaccharide nature reducing turbidity by ¯occulation of the particles (Avnimelech et al., 1982). Compelling evidence has been obtained with radioactive label studies for the algal heterotrophic activity and concomitant algal EPS production in a PETRO trickling ®lter (Shipin et al., 1996; Shipin et al., 1999). Data obtained for the polishing ASP reactor suggest an analogy. Though microalgae are ecient EPS producers in the reactor, they do not constitute a reactive COD load on the consortium oxidizing waste organics. The microalgae rather become a component of the microbial ¯oc consortium. As they are resilient, they retain viability throughout the ASP cycle, and reemerge metabolically intact in the waste sludge. By analogy with the PETRO TF, their activity in the reactor is thought to involve conversion of the low molecular weight dissolved organics (monosaccharides, amino acids, VFA, etc.) into e€ective ¯occulating agents (EPS). Algae imbedded in the ¯oc, convert one form of COD component into another. Mucilage of EPS nature albeit voluminous in appearance is insignificant in terms of COD load. The nature of the microbial EPS in general and algal EPS in particular is such, that a small increment in weight results in a log increase in viscosity and ¯occulation capacity. Superior ¯occulation leads to a complete single cell removal, high ¯oc settleability and superior clarity of the clari®er e‚uent. Secondary position of the ASP reactor leads to an increased ¯occulation and nitri®cation potential of the activated sludge ¯ocs. Apart from enhancing ¯occulating characteristic of the activated sludge, microalgae appear to promote N-removal through a higher nitri®cation rate. This possibly occurs due to the secondary position of the low rate ASP, resulting in the greater limitation in dissolved organic matter and better conditions for the autotrophic nitri®ers. A greater retention capacity of the ¯oc due to its higher EPS content may also be of signi®cance. The mechanism of the algae removal in the ASP, the same in principle, appears to di€er from that of the TF in several respects. Removal by rotifers and protozoa, though of signi®cance in both cases, plays a greater role in the PETRO TF. Algae removal in the TF is thought to be characterized by both ¯occulation (due to algal and bacterial EPS production) and degradation (through bacterial activity) of algae. In the ASP this removal is achieved primarily through the stress-induced EPS pro-

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duction (both algal and bacterial) and subsequent ¯occulation (embedding of the algal biomass in the sludge ¯ocs). AcknowledgementsÐThe authors gratefully acknowledge the support of the South African Water Research Commission which funds the PETRO project, as well as the Town engineer and sta€ of the town of Akasia. REFERENCES

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