WASTE TREATMENT BY ALGAL CULTIVATION

Edited by

Michael A. Borowitzka and Kuruvilla Mathew

Proceedings of a Seminar held at Murdoch University, Western Australia

29th November, 1991

THE ALGAE: THEIR BIOLOGY AND LIMITS TO GROWfH

ABSTRACT

Michael A Borowitzka

Algal Biotechnology Laboratory School of Biological & Environmental Sciences

Murdoch University Murdoch, W.A, 6150, Australia

The growth of algae in high rate oxidation ponds is limited by the availability of light. The effect of light on productivity is influenced by temperature. Furthermore, for optimal productivity the population density of the culture h~s to be controlled and mixing has to be optimised as well. Algae in HROPs are generally not carbon limited, and probably use both inorganic carbon and organic carbon. In order to maintain stable cultures the amount of total ammonium need to be controlled, and this is affected, in part by the total organic loading. Furthermore, the pond system has to be managed so that the effects of predators and pathogens is minimised.

KEYWORDS

productivity, light, temperature, heavy metals, predators, pathogens, mixing, heterotrophy, mixotrophy, pond management.

INTRODUCTION

Urban, industrial and agricultural wastewaters contain up to three magnitudes higher concentrations of total nitrogen and phosphorous, than natural water bodies (de Ia Noiie et al., 1992). Normal primary and secondary treatment of these wastewaters eliminates the easily settled materials and oxidises the organic material present, but does not remove the nutrients which will cause eutrophication of the rivers or lakes into which these wastewaters may be discharged. Tertiary

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treatment of the effluent is therefore required, and both chemical and physical methods which are used are very expensive. Oswald (1988a) estimates that the relative cost of tertiary treatment to remove POl-, NH4 + and N03- is about 4 times the cost of primary treatment. Higher orders of treatment, such as quaternary treatment to remove refractory organics and organic and inorganic toxicants, and quinary treatment to remove inorganic salts and heavy metals are 8 to 16 times as expensive as primary treatment. Algae can be used as a biological alternative tertiary treatment and also for the removal of heavy metals and possibly other toxic substances (Gerhardt et al., 1991; Wu and Kosaric, 1991). The algae produced in these systems can possibly be used as animal feed

Figure 1. Some of the algal species found in HROPs. (a) Chiarella vulgaris; (b) Scenedesmus obliquus; (c) Oocystis; (d) Chlamydomonas; (e) Euglena; (f) Micractinium. (not to scale).

supplements (Lipstein and Hurwitz, 1980; Sandbank and Hepher, 1980), or be composted. The use of waste-grown algae may ultimately also have application in closed cycle life-support systems (Wharton et al., 1988; Oguchi et al., 1989; Shimada et al., 1990), or may be used in conjunction with power stations, not only to treat wastewaters, but also to act as a C02 sink (Laws and Berning, 1991 ), a feature which is desirable for the amelioration of the impact of greenhouse gases. Recently, Negoro and coworkers (1991) have screened a number of microalgae for their ability to grow at high C02 concentrationa and for their sensitivity to SOx and NOv in order to evaluate their potential as C02 scrubbers.

The bio-treatment of wastewaters with algae to remove nutrients such as nitrogen and phosphorous, and to provide oxygen for aerobic bacteria was proposed over 30 years ago by Oswald and Gotaas (1957). Since then there have been numerous laboratory and pilot studies of this process and several sewage treatment plants using various versions of this system have been constructed (Shelef et al., 1980; Oswald, 1988a, b). Microalgal systems for the treatment of other wastewaters such as piggery effluent (De Pauw et al., 1980; Martin,C et al., 1985a, b; Pouliot et al., 1986), the effluent from food processing factories (Rodrigues and Oliviera, 1987) and other agricultural wastes (Phang and Ong, 1988) have been studied. More recently, algae based systems for the removal of toxic minerals such as Pb, Cd, Hg, Se, Sn, As and Br are also being developed (e.g. Soeder et al., 1978; Kaplan et al., 1988; Gerhardt et al., 1991) and at least one small commercial plant is already in operation in the USA

In high rate oxidation ponds the algae species of particular interest are: (a) for freshwater systems -the green algae Chlorella, Chlamydomonas, Scenedesmus, Micractinium and Oocystis, the euglenophyte Euglena, and the blue-green algae (cyanobacteria) Oscillatoria and Phormidium (Caldwell Connell Engineers, 1976; Azov et al., 1980a; Balloni et al., 1980; Oswald, 1988a). (b) for marine systems - the diatoms Phaeodactylum, Nitzschia and Skeletonema and the green alga Chlorella (De Pauw et al., 1980). Some of these algae are illustrated in Figure 1.

Over the last 20 years there have been major advances in the large-scale culture of microalgae with the primary purpose being to produce algal biomass for food or feed, and to use the algae as a source of valuable biochemicals. Several commercial operations exist in Australia, the USA, Israel, Thailand, Taiwan and Mexico (Borowitzka,MA and Borowitzka, 1988b; Borowitzka,U and Borowitzka, 1989a, b). The development of these facilities, in particular, has led to significant advances in our understanding of the biology and ecology of large-scale algal culture, as well as in the engineering of large-scale culture systems and algal harvesting methods all of which are important to the design and operation of high rate algal oxidation ponds. This paper will attempt to summarise the present state-of-the art, with particular emphasis on high rate wastewater oxidation ponds (HROPs).

Algal growth in HROPs is influenced by a number of physical and biotic factors and these are summarised in Table 1. Furthermore, effective operation of an HROP requires a stable algal community, and the first prerequisite for this is to establish an equilibrium between algal growth rate and the rate of algal dilution by the wastewater input stream. If this is achieved, good algal growth rates can be maintained for a long time. The maintenance of good algal growth rates, and therefore reduced residence time, requires an optimisation of light and nutrient utilisation, the avoidance of toxic levels of certain compounds in the feed stream, a control of potential pests and algal pathogens and the maintenance of an optimal algal population size by regular harvesting of the algal biomass. These are considered in more detail below.

LIMITING FACTORS TO ALGAL GROWTH

High algal productivity is essential for successful economical operation of high-rate oxidation ponds. The problems of optimising growth rate and photosynthetic efficiency in HROPs are the same as those which affect the microalgal industry in general. These problems are the maximisation of photosynthetic efficiency, nutrient supply and assimilation efficiency, especially the supply of C02 ,

and the control of potential predators and competitors. HROPs have additional problems resulting

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from the variable nutrient load and the possible presence of inhibitor& and/or toxins ( cf. Abeliovich, 1980; Shelef et al., 1980; Lewis, 1990).

Table 1. Factors influencing algal growth

Abiotic factors - Light (quality, quantity) - Temperature - Nutrient concentration ( esp. N, P and organic C) -02 , C02 -pH - Salinity -Toxic chemicals

Biotic factors - Pathogens (bacteria, fungi, viruses) -Predation by zooplankton - Competition

Operational Factors -Mixing - Dilution rate -Depth - Addition of bicarbonate or C02 - Harvest frequency

Light and Temperature

Although some algal species can grow heterotrophically in the dark (Neilson and Lewin, 1974), most algae are phototrophs and may grow either photoautotrophically or photoheterotrophically (mixotrophically). Light is therefore of fundamental importance and in algal cultures light rapidly becomes limiting due to the absorption of the light by the algal cells in the water column (Richmond, 1986). In HROPs this is exacerbated by the high content of particulate matter and coloured substances in the sewage. For this reason, almost all algal culture systems are shallow with depths ranging from about 15 to 30 em (Fontes et al., 1987) and depth is a very important operational variable. The effect of depth on biomass productivity is illustrated in Figure 2.

In outdoor cultures, algal yields are often closely correlated with incident radiation (Tamiya et al., 1953; Ayala et al. 1988). Figure 3 shows the productivity of a 110 m2 pilot plant at Antofagasta, Chile over a 12 month period. The average irradiance in summer was 315 W.m-2 and in winter 152 W.m2, and the average summer temperatre was 38°C and the winter temperature 25°C. (Ayala et al., 1988). More detailed studies have shown, however, that the relationship between growth, yield and light is more complex (Vonshak et al., 1982; Fontes et al., 1987; Vonshak and Richmond, 1988). Figure 4 shows the general relationship between cell yield (productivity) and specific growth

>-..... > ·--0 :I

"0 0 "­a..

10 20 30 40 50 60

Depth (em)

Figure 2. The effects of culture depth on productivity (g.m2.day-1) of an outdoor algal culture in a raceway-type pond.

rate over a range of algal population densities for outdoor cultures of Spirulina under conditions where nutrients and temperature are not growth-limiting. This graph shows that the maximum yield outdoors does not coincide with the highest specific growth rate; ithe highest specific growth rate is achieved at the lowest cell density when mutual shading and light limitation are minimal, and, with increasing population density the specific growth rate decreases as the culture becomes more light­limited. The highest yield, however, is achieved where the specific growth rate is not at maximum, but where the net photosynthetic efficiency is at its highest; at low cell densities the photosynthetic efficiency is actually reduced due to photoinhibition (Dauta et al., 1990; Grande et al., 1991 ); the optimum irradiance is not normally the maximum irradiance. In denser cultures, not all the cells receive the maximum amount of light all the time, rather the cells are exposed to a variable and intermittent light regime if appropriately mixed, and this alternating high-light/low-light regime appears to enhance growth. This effect of an alternating light regime has been shown practically by Laws et al. (1983) who placed airplane-type wings or foils at regular intervals in the flow of his cultures (Figure 5). These 'wings' increased turbulence. By appropriate spacing of the 'wings', he could optimise the high-light/low-light regime and thus enhance the productivity (Laws et al. 1986).

In practice there are several ways to ensure that the algae receive an optimum amount of light. These include: (1) working with shallow ponds, and (2) mixing the culture. Of course, it is also important to determine the optimum cell density for the culture and to maintain the culture at this density, but the rate of mixing must also be optimised. If mixing is too slow, then many of the algal cells do not receive the optimum amount of light; increased mixing will therefore increase the cell

75

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24

20

>- 16 -·-> -0 :::J 12

"'0 0 !o..

a.. 8

4

0 M J J A s 0 N D J F M A

Month

Figure 3. Productivity (g.m-2.day-1) of Spirulina mass culture at a pilot plant at Antofagasta, Chile. (from Ayala et al., 1988).

yield. The relationship between yield, mixing rate and population density is shown diagrammatically in Figure 6 .

. Temperature, together with irradiance, is the major factor affecting the algal yield in cultures. The growth rate of algae increases with increasing temperature until the optimum temperature is reached. Further increases in temperature, beyond the optimum, usually lead to a rapid decline in the growth rate (Jitts et al., 1964; Dauta et al., 1990; Fiala and Oriol, 1990). At higher temperatures, closer to the optimum for growth, the algae are also generally better able to tolerate much higher irradiances before photoinhibition sets in (Tamiya et al., 1953; Dauta et al., 1990; Talbot et al., 1991 ). Figure 7 illustrates these effects of irradiance and temperature on two species of freshwater algae.

This interaction between light and temperature also has been observed in outdoor cultures where it may not only affect productivity, but also lead to shifts in the species present in the pond (De Pauw et al., 1980). For example, Vonshak et al. (1982) found that the specific growth rate of Spirulina declined from summer to winter. This decline was most pronounced at low algal densities compared with higher densities (Figure 8). Similar results have also been obtained by Castillo et al. (1980) in Peru with Scenedesmus. Not only are there seasonal changes in temperature and total irradiance, but there are also diurnal variations, and maximum pond productivity will only occur when both pond temperature and irradiance are near their optimum, something which rarely occurs. Ponds cool down at night and in the morning the pond temperature is often sub-optimal and the algae therefore cannot utilise all the available light; i.e. they are temperature limited: Depending on how

>--> -()

:::1 "'0 0 I.. a..

\ ·~ ~

\ ~.

.... \ ~ ~

..........

·. .. ·, '• .. .. .. .. .. .. . ,,...,

............ , .......... , .. ~·~ .. , ..

Population Density

Q) -0 0:::

.s: -~ 0 I..

(.!)

0 -0 Q)

c.. Vl

Figure 4. The effect of population density (mg dry wt.P) on the specific growth rate (day-1)

[dotted line] and the productivity (g.m-2.d-1) [solid line] for Spimlina. (based on Richmond (1988)).

long the pond takes to warm up, the optimum productivity will only be reached later in the day. The rate of pond warming will depend, of course, mainly on the air temperature and pond depth; shallower ponds warm faster but, the also cool faster at night. Therefore, some consideration might be given to altering the pond depth between night and day. Furthermore, in cold climates algal ponds will have to be heated for adequate algal growth is to occur (De Pauw et al., 1980; Richmond et al., 1990). Temperature limitation in colder climates can be overcome, in part, by using algae with lower temperature optima (Talbot et al., 1991) ..

The relationship between irradiance and temperature has been modelled by several workers, and such models can be used to determine the optimum conditions for biomass production or the estimate the potential yield of an algal plant (Martin,NJ and Fallowfield, 1989). The following describes the model developed by Grobbelaar et al. (1990).

The irradiance (I) at any depth (z) can be calculated from the equation

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Flow

High pressure under

Pressure over

Vortex

Crossover

Figure 5. Design of a single foil showing mechanism of vortex production by the foil in an algal raceway.

>­-> -0 ::J

"'0 0 '­

a..

Population Density

Figure 6. Diagrammatic representation of the interaction of algal population density and productivity in a raceway pond at two mixing speeds (based on Richmond (1989)).

(1)

where 10 is the irradiance at the surface in mol photons.m-2.h-1 and kd is the extinction coefficient at depth z in meters. For 'clean' autotrophic cultures kd is a function of the biomass concentration only. For wastewater systems kd can be partitioned into the various light absorbing components, i.e.

(a) (b)

1.5

Q) -0 1.0 a:: .s: -~ 0 1-

(!)

0.5

0.0 i 0 200 400 600 800 0 200 400 600 800

lrradiance lrradiance

Figure 7. The effect of increasing temperature and irradiance (1J.E.m-2.sec-1) on growth rate (doublings.day-1) for (a) Ankistrodesmus falcatus; (b) Phormidium bohneri. The temperatures are 5, 15, 25, 35°C in order from the bottom to the top of the graph.

kd = kw + k8

+ k, + kp (2)

where kw = the attenuation of light due to water, k, is due to dissolved yellow and brown compounds (gelbstoff), k1, is due to tripton (non-living particulate matter), and k, is due to the phytoplankton. A basic equation for productivity can thus be written as:

PROD(mg dry wt.m-2.h-1) = PRD -RES - INB (J)

where PROD = 'productivity', PRD = 'gross productivity', RES = 'respiration', and INB = 'photoinhibition'.

PRD can be calculated from inputs of biomass concentration present in the culture, culture temperature, and light impinging on the surface of the culture:

PRD = A1X1(A~T l/s(A3)T lz + ls(AJT

(4)

where A1 • A3 are constants, XI is the biomass concentration in mg dry wt.I-1, I, is the light half saturation constant in mol photons.m-2.h-I, and T is a temperature factor. The constant AI can be interpreted as the efficiency of light utilisation (photosynthetic efficiency), A2 as the Q10 of photosynthesis, A3 as the Q10 for light half saturation, and the factor llA3jT as the temperature

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Q) -0 0::

..c -3: 0 1...

(.!)

0 -0 Q)

a. (/)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Population Density

Figure 8. The effect of Spirulina population density (absorbance at 540 nm) on the specific growth rate (J.L) in summer (top), autumn (middle) and winter (bottom). (from Vonshak et al. 1982).

dependence of Is (light half saturation constant) in the photosynthesis versus irradiance curve of photosynthesis (Harris, 1978).

The component RES is a total loss factor i.e. losses due to respiration and exudation of organic compounds from the cells. RES increases with an increase in temperature (Grobbelaar and Soeder, 1985) and, on the basis of experience with outdoor cultures (Soeder, 1980), can be approximated by

RES = X 1.5r- 0.54 1 100

(5)

Photoinhibition is also temperature dependent (Dauta et al., 1990), and an approximation based on experience with outdoor cultures is given by

INB + PRD 2·5 T I 7.5 z

when 12 > 1 mol photon.m-2.h-1 (Grobbelaar et al., 1990).

(6)

The above model gives good agreement with the data collected over a 16-month period for cultures of Spirulina and Coelastrum grown in several different raceway ponds at Dortmund in Germany.

However, the model could not fully account for the afternoon reduction in photosynthesis, which occurred sometimes (Berner et al., 1986). Recently, Kroon et al. (1989) have refined this model for microalgal productivity in HROPs, and Guterman et al. (1990) have developed a macromodel for outdoor algal mass production.

Nutrient load - Carbon and Nitrogen

The algae in HROPs utilise both inorganic carbon (C02 and HC03-) and organic carbon. Unlike 'clean' algal culture systems, HROPs have a high organic load and this organic carbon may be utilised by the algae either photoheterotrophically near the surface, or heterotrophically in the deeper parts of the pond. The organic load is probably responsible for the usual pattern of species dominance observed in these ponds; i.e. at high loads mainly Euglena and Chlamydomonas, at intermediate load mainly Chlorella, Scenedesmus and Micractinium and, at low loads other algae (Palmer, 1969; Abeliovich, 1980, 1986).

Inorganic carbon in HROPs is derived from both atmospheric C02 and C02 produced by bacterial respiration. Data presented by Abeliovich (1980) and Abeliovich and Weismann (1978) indicate that the bacterially-generated C02 must be quite small, and that about 25 to 50% of the algal carbon is derived from heterotrophic utilisation of organic carbon. Although little work has been done to study the heterotrophic potential of microalgae found in HROPs, it is likely that these algae are very efficient at organic carbon utilisation (Burrell et al., 1984; Martinez et al., 1987). For example Martinez et al. (1991) have isolated a strain of Chlorella vulgaris from the wastewater ponds of a sugar refinery which requires glucose to reach maximal growth rate even under saturating light conditions. In fact, the growth of this alga with glucose in light was equal to or greater than the sum of photoautotrophic (light without glucose) and heterotrophic (dark with glucose) growth. This has also been found by Ogawa and Aiba (1981) with Chlorella and Scenedesmus, and also appears to be the case with Haematococcus pluvialis (Borowitzka,MA et al., 1991 ). It is therefore very likely that all the dominant algae in HROPs are efficient (photo )heterotrophs.

The bacteria in HROPs are not only C02-generators, but there is also evidence that heterotrophic bacteria may reduce algal growth (Dar and Svi, 1980), whereas the algae produce substances inhibitory to bacterial growth (Kobbia and Zaki, 1976; Dar, 1980) including Escherichia coli (Sebastian and Nair, 1984). The main role of bacteria in HROPs is probably to break down complex organic molecules to make them available to the algae, however the interaction between algae and bacteria and the exact role of the bacteria in these systems still requires more detailed study.

Algae in HROPs are generally not nutrient limited, however the ammonia concentration in the pond is an important factor in the successful management of the HROP. 2.0 mM total ammonia (NH4 + + NH3) at pH 8.1 has been shown to inhibit photosynthetic 0 2 evolution by about 50% in many algal species (Abeliovich and Azov, 1976; Admiraal, 1977; Azov and Goldman, 1982), and high concentrations of ammonia can lead to death of the algae (Thomas et al., 1980; Borowitzka,MA and Borowitzka, 1988a). This effect is intensified at higher temperatures since a higher proportion of the total ammonia occurs as free ammonia. Therefore, one of the main concerns in managing HROPs is to determine the dilution time of water in HROPs so as to avoid this critical concentration of total ammonia since it would reduce the algal growth rate causing washout of the algae; total ammonia concentration generally should not exceed 1.5 mM.

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Photosynthetic COz-uptake will raise the pH in the pond and, if there is a high concentration of total ammonia, i.e. > 2.0 mM, inhibition of photosynthesis will occur when the pH reaches about pH 8.1. The only effective method of lowering the pH is by respiration and, therefore, a balance must be maintained between photosynthesis which occurs mainly near the surface of the pond, and respiration which occurs throughout the water column. Thus, when ammonia concentration is high, a careful balance should be maintained between its concentration and BOD, to enable the continued lowering of pH via respiration; i.e. if ammonia concentration is high then the BOD should also be high (Abeliovich, 1983). Furthermore, if algal ponds are fed by diluted wastewater or by secondary effluents, carbon limitation may develop (de la Noiie et al., 1984; Abeliovich, 1986ii) and this will, in tum, limit the rate of nutrient uptake, especially nitrogen uptake.

Inhibitory Substances

Clearly, algal treatment is not suitable for all wastewaters. If the wastewater contains too high concentrations of ammonia then this will affect the performance of the plant (see above) and the wastewater must be either diluted or the feed rate reduced. Extreme concentrations of heavy metals, herbicides or pesticides may also prove toxic, and in these cases alternative treatment strategies must be used. If the wastewater is mainly domestic sewage, then it may also contain surfactants from detergents and household cleaning and personal care products etc., and these may be toxic to the algae. The toxicity of these surfactants varies with algal species and type of surfactant and reported toxic levels vary from 0.003 to 17,784 mg.I-1• It has been shown that laboratory populations are more susceptible to surfactants than ''wild" algal populations under natural conditions (Ukeles, 1965; Yamane et al., 1984; Wangberg and Blanck, 1988; Lewis, 1990). Furthermore, the cationic detergents have been shown to be more toxic than nonionic or anionic detergents. Unfortunately, the amount of information on the effects of these substances on the algae in HROPs is extremely limited and their impact on HROP performance cannot be estimated at this time.

Predators, Parasites and Pathogens

In order to maintain a stable steady state in the HROP one must avoid the development of complex food chains. The development of rotifers, copepods, mosquito larvae, fly larvae, and other invertebrates will result in an unstable algal population at the mercy of the population dynamics of herbivorous grazing zooplankters and their predators (Groeneweg et al., 1980). This can lead to sudden collapses of the algal population. The development of these animal populations can be prevented by establishing a pond regime which leads to diurnal anaerobic conditions for a short period.

Another potential problem is infection of the algae by parasitic fungi, mainly of the genus Chytridium (Abeliovich and Dickbuck, 1977; Payer et al., 1978). These fungi also cannot complete their life cycle if anaerobic conditions prevail for several hours. Algal viruses may also be a problem although, as yet, these have not been reported in HROPs.

REMOVAL OF HEAVY METALS AND OTHER TOXIC COMPOUNDS

Microalgae are efficient 'absorbers' of heavy metals. For example, Soeder et al. (1978) showed that Coelastrum proboscideum absorbs 100% of the lead from a 1.0 ppm solution within 20h at 23°C and about 90% after only 1.5 h at 30°C. Cadmium was absorbed a little less efficiently, with about 60% of the cadmium being absorbed from a 40 ppb solution after 24 h. The process of 'absorption' appears to by" largely passive, probably through ion exchange on the cell wall polysaccharides (Soeder et al., 1978; Ting et al., 1989), although a slower, metabolically mediated uptake step also occurs (Khummongol et al., 1982; Nakajima et al., 1982). The efficiency of absorption step depends on the nature and charge of the cell wall polysaccharides. The studies of Kaplan et al. (1988) with Chiarella stigmaphora and C. salina, showed that the highly charged native polysaccharides of C. stigmaphora. which contain high amounts of uronic acids had a high copper-complexing capacity compared to the less charged C. salina polysaccharides. Similar results have also been obtained by Mangi & Schumacher (Mangi and Schumacher, 1979) with Mesotaenium and Xue and Sigg (1990) with Chlamydomonas reinhardii. Maeda et al. (1990a, b) and Costa and Leite (1990) have also shown that Cd is accumulated mainly by adsorption whereas Zn is actively taken up by the cells of Chiarella vulgaris and C. homosphaera and bound by metallothionein-like proteins (Robinson, 1989).

For heavy metal removal, however, algal immobilised in an alginate or carrageenan matrix in a packed-bed reactor can be used, and such systems have been proposed for the removal of a range of heavy metals (Nakajima et al., 1982; Darnall et al., 1986; Kuyucak and Volesky, 1988, 1989; Wilkinson et al., 1990).

Although this property of heavy metal absorption has application in wastewater treatment it may, however, render the algal biomass unsuitable for use as an animal feed component (eg. Payer et al., 1975; Yannai et al., 1980) which is one of the most effective uses for the algal biomass. Experiments on feeding algal biomass grown on sewage to various animals such as Daphnia (de Ia Noiie and Proulx, 1986), fish (Sandbank and Hepher, 1978, 1980) and poultry (Yannai et al., 1978; Upstein and Hurwitz, 1980) however, have shown that this algal biomass is acceptable. It is unlikely that sewage-grown algae can be used for human nutrition (Jassby, 1988), however algae grown on agroindustrial wastes could find application in this, often higher value, area.

Algae are also good accumulators of compounds such as organochlorides and tributyl tin (Payer and Runkel, 1978; Lal et al., 1987; Wright and Weber, 1991). They have also been reported to break down some of these compounds (Lee et al., 1989; Wu and Kosaric, 1991 ). Recently Baeza­Squiban et al. (Baeza-Squiban et al., 1990; Schmidt, 1991) have shown that the green alga Dunaliella bioculata produced an extracellular esterase which degrades the pyrethroid insecticide Deltamethrin.

Algae have also been shown to degrade a range of hydrocarbons, such as those found in oily wastes (Cerniglia et al., 1980; Carpenter et al., 1989).

OPERATION OF HIGH RATE OXIDATION PONDS

Algal growth can be described by the first order equation:

...IIIII

84

dX =!-LX

dt

(7)

where X is the algal biomass (g.m3) and J.l is the specific growth rate (day-1). Equation 7 can be expressed in terms of the pond productivity:

(8)

where Pis the productivity (g.m-2.day-1) and dis the pond depth (m). Accumulation of algae in the pond operating as a completely mixed reactor can be described as:

dX - = !-L(X- D) dt

(9)

where Dis the dilution rate (sewage inflow rate per unit volume per day) at steady state dX!dt = 0 and J.l = D. Thus, at steady state, Equation 8 becomes:

P = DXd (10)

Thus, the two controllable variables which determine pond productivity are dilution rate (or its reciprocal, retention time) and depth. Because of seasonal variations in temperature and light, retention time and depth may need to be varied over the various seasons if the maximum productivity is to be maintained. There are four alternative strategies for doing this (Azov et al., 1980b):

(1) Constant retention time, area and depth

This is the simplest alternative, however it means that in winter the algal concentration may become too low to supply all the oxygen required for waste biodegradation through photosynthesis. This means that supplemental oxygen has to be supplied.

(2) Variable retention time

A variable retention time takes into account the effects of temperature and light on algal growth ( cf. Figure 3). For example, at the experimental HROP operated at Haifa in Israel (Shelef et al., 1980) a 2 day retention time was required in summer, and a 6 day retention time in winter. The variable retention time can be achieved by either (a) varying both area and depth, (b) varying area at constant depth, or (c) varying depth at constant area.

(a) Varying both area and depth

This method was first suggested by Oswald (1969). For Californian conditions he suggested an optimum depth of 20 em in winter and 40 em in summer for maximal areal yields, and 75 em in winter and 120 em in summer for optimal sewage treatment. The problem with this method is that a pond system operating at a constant sewage inflow at 20 em depth and 6 days retention time in winter and 40 em depth and 2 days retention time in summer would require six times as much area in winter as in summer. The cost of constructing the extra pond area would probably make the whole operation uneconomical (Borowitzka,MA, 1992).

(b) Varying area at constant depth

This method also requires extra pond area, and has been trialled at Haifa (Azov et al., 1980b; Shelef et al., 1980). The pond area not required during the warmer months was used to grow fish. Although this sort of polyculture may be economical under some circumstances, it complicates operations and requires a workforce skilled both in algal culture and fish culture.

(c) Varying depth at constant area

This option appears to be the most effective, both in terms of the capital cost of pond construction and in the ease of operation.

The above strategies assume that the sewage stream composition remains fairly constant over the year. Where this is not the case, a strategy must be developed to vary the retention time and/or pond depth in response to the changed nutrient inputs.

Effective and reliable operation of any large-scale algal culture, including HROPs, requires effective monitoring of the culture and clearly defined strategies for pond management. As previously mentioned, the concentration of ammonium is a critical, and easily monitored, factor for maintaining an effective culture. Other easily monitored variables are pH and 0 2 concentration. pH in HROPs is usually between pH 7.5 and 10. High pH (>8.0), coupled with high concentrations of ammonia and low BOD load mean long detention times, whereas low pH ( < 7.5) and low ammonia concentrations mean short detention times. pH values less than about pH 6.0 usually lead to algal death and should be avoided. If such low pH values do occur, then the pH can be adjusted by the addition of lime (Carberry and Brunner, 1991). With optimal photosynthesis, HROP pH values may reach > pH 10 at the pond surface without having any adverse effects on the stability of the algal community or on biomass production. High oxygen concentrations (> 30 mg.I-1) should also be avoided since these may lead to photooxidative damage to that algal cells. This can be avoided with proper mixing which will increase the transfer of the supersaturated oxygen from the water to the atmosphere and will also transfer oxygen for algal and bacterial respiration below the photic zone (Abeliovich, 1986). Table 2 gives a general outline of suggested operation times for HROPs on domestic sewage.

It may, however, also be possible to treat acid wastes without first adjusting the pH by using algae such as Euglena gracilis which has been used to treat swine manure digest at pH 4.0 with or without the addition of whey (Waygood et al., 1980).

CONCLUSIONS

The application of algae for wastewater treatment needs careful consideration of the type of wastewater to be treated and the prevailing climatic conditions. Sufficient information is available to allow the design of an effective and reliable plant for the treatment of wastewaters with algae. In the last two decades major advances have been made in our understanding of algal physiology and ecology and in the area of algal mass culture to allow the rational assessment of the suitability of an algal treatment process compared to alternative processes for a given waste stream and site and further evidence has accumulated showing that HROPs and other algal treatment systems can be both effective and economical.

/

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Table 2. Parameters for a stable operation of a high rate oxidation pond (domestic sewage only) based on data compiled by Abeliovich (1986).

Raw Sewage Detention time (days) Emuent

BOD(% Ammonia removed;

Ammonia Total BOD Algal cone. (% of less (mg.J-1) (mg.J-1) 8- 14°C 22- 25°C (g dry wt initial removed

J-1) cone.) algal biomass)

80-90 400- 500 10- 12 0.25- 0.5 70-80 90-99

80-90 400- 500 4 0.5- 1.0 80-90 90-99

40-50 300- 400 6-7 0.1 - 0.2 40-50 90-95

40-50 300- 400 2-3 0.2 - 0.3 40-50 90-95

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