The treatment of wastes by algal culture

Journal of Applied Bacteriology Symposium Supplement 1985, 187s-205s

The treatment of wastes by algal culture

H . J . FALLOWFIELD & M.K. GARRETT Department of Microbiology, The West of Scotland Agricultural College, Auchincruive, Ayr K A 6 5H W , U K and Department of Agriculture for Northern lreland and The Queen’s University of Belfast, Newforge Lane, Belfast BT9 5 P X , U K

1. Introduction, 187s 2. Biology and chemistry of algal treatment, 188s 3. Treatment systems, 188s

3.1 Waste stabilization ponds, 189s 3.1.1 Anaerobic ponds, 189s 3.1.2 Facultative ponds, 190s 3.1.3 Maturation ponds, 190s

3.2 High rate algal ponds, 190s 3.3 Comparison of high rate algal ponds and waste

stabilization ponds, 191s 4. Pilot plant experience with high rate pond systems

in Northern Ireland, 194s 4.1 Nutrient stripping and BOD reduction, 194s 4.2 Biomass production, 195s 4.3 Stability and species composition, 196s

5. Harvesting technology, 196s 6. Product composition, 197s 7. Energy balance of the treatment process, 199s 8. Rate limiting factors, 200s 9. Conclusions, 201s

10. Acknowledgements, 201s 11. References, 201s

1. Introduction

Most designs for wastewater treatment systems have been developed for temperate climatic conditions but many have been adopted subsequently for ellluent treatment in tropical and subtropical regions of the world. These systems have been designed primarily to reduce the organic load (biochemical oxygen demand, BOD) placed upon the receiving water body and utilize technologies such as activated sludge, intensive aeration and aerated lagoons, oxidation ditches and trickling filters etc. Frequently mechanical aeration is used to increase the dissolved oxygen content of the wastewater. Such systems are usually energy intensive and require technical skill for operation and maintenance, skills which may be in short supply within many developing countries.

Liquid organic wastes with low carbon to nitrogen ratios are inherently suited to the growth of photosynthetic organisms. The treatment of both domestic and agricultural wastewaters by algae is attractive since the technology is relatively simple and the energy requirements are low, whereas the standard of ellluent treatment can be high. During the growth of algae in wastewater oxygen derived from algal photosynthesis is available for bacterial respiration which subsequently reduces the BOD of the efluent. In addition nitrogen and phosphorus are assimilated by the algae and this may have important implications for the trophic status of the receiving water body. Whilst effective ellluent treatment may be achieved by sub-optimal algal growth, as in waste stabilization ponds (WSP), the optimization of algal growth in high rate algal pond (HRAP) systems is attractive since it combines the effective treatment of a noxious waste and the production of potentially valuable protein rich algal biomass.

188s H . J . Fallow$eld and M . K . Garrett 2. Biology and chemistry of algal treatment

The principal features of the ‘symbiotic’ algalibacterial interaction within an algal treatment system are shown in Fig. 1 . The major external factors affecting algal growth within such systems are light and temperature (Goldman 1979b). In mass cultures of algae in defined inorganic media and in the absence of bacteria. growth is frequently carbon limited (Soeder 1980), the carbon being supplied by diKusion from the atmosphere or by addition as gaseous CO, (Heussler et a / . 1978). In wastewater

ALGAE BACTERIA

Trace elements PO4

Fig. 1. Processes occurring during the treatment of wastes by algal culture.

treatment systems the C 0 2 for algal growth originates both from the atmosphere and from bacterial respiration but there is controversy regarding CO, availability in relation to pH (Moss 19’73). Azov (1982) has suggested that free CO, may be limiting in dense algal cultures with high pH values although Hill & Lincoln (1981) noted that bacterial respiration produced sufficient free CO, from organic matter in pig slurry to retard the depletion of the HCO; + CO, reservoir thus minimizing the pH rise. The situation is complicated by the existence of algal/bacterial flocs which could alleviate a growth limitation which could otherwise be ascribed to free CO, level (Schiefer & Caldwell 1982).

Successful operation of algal treatment systems depends upon establishing a dynamic equilibrium between algal 0, production and bacterial 0, consumption. Marked diurnal variation in these parameters has been recorded (Goldman 1979b; Hill & Lincoln 1981; Buhr & Miller 1983). Various estimates have been made for the optimum ratio of bacteria to algae in these systems (Oron et al. 1979; Hill & Lincoln 1981). These range from 1 : 3 to 2 : 3 on a biomass basis and on a numerical basis a ratio of 100 : 1 has been suggested. The algal/bacterial interaction is normally considered to be symbiotic in nature but antagonistic mechanisms can also operate (Humenik & Hanna 1971; Allen & Garrett 197’7; Toerien et al. 1984).

Algal growth is also accompanied by a reduction of nitrogen and phosphorus concentration. This nutrient stripping property of the system, especially the luxury uptake of phosphorus by the algal cells (Kuhl 19’74), means that the final effluent is less likely to cause eutrophication in the receiving water body. Moreover, the harvested biomass from the treatment process normally contains approximately 50% protein (Soeder & Binsack 1978) and represents a potentially valuable by-product which may offset treatment costs.

3. Treatment systems

Many ponding systems may correctly be described as having an algal component. These include waste stabilization ponds (WSP) which usually consist of facultative and maturation ponds in series,

Treatment of wastes 189s both of which rely to differing degrees upon algae for successful operation. In addition, a non-algal anaerobic pretreatment pond may be included in the WSP system depending mainly upon the temperature of the location. In contrast to this the high rate algal pond system may be used as the basis of a complete treatment system relying entirely upon the association between bacteria and algae for effective treatment. A combined approach has recently been suggested by Soeder (1983) in which facultative or anaerobic ponds would be used as pretreatments for the HRAP system.

3 . 1 W A S T E S T A B I L I Z A T I O N PONDS

A typical WSP design is shown in Fig. 2. The pond requirement will depend in practice upon the anticipated loadings and individual component ponds may be operated in parallel.

WSP System

HRAP System

Anaerobic ponds (2.5-4.0 m deep)

Facultative pond (1.2- 1.8 rn deep)

Maturation ponds ( I . O - 1 . 5 r n deep) 7

0 40rn LLu_J scale

I rn (0-2 -0.6 m deep)

Fig. 2. Comparative plans of typical WSP and HRAP systems.

3.1.1 Anaerobic ponds

These are the first ponds in the WSP series. They are typically 2.5-4.0 m deep and operate at BOD, loadings of between 4000 and 16000 kg BOD,/ha/d and effect a 45-70% reduction in BOD, at a retention time of 2 d (Arthur 1983). At such short retention times BOD, reduction is mainly attribut- able to settling of suspended solids. Some reduction is effected by biological action, however, since the efficiency is temperature dependent. Biological action is the result of the enzymic hydrolysis of complex organic polymers to simple monomers from which volatile fatty acids, CO, , NH, and H, are produced. These compounds are subsequently converted to methane under alkaline conditions (Lettinga 1981). It has been suggested that anaerobic ponds are a warm climate phenomenon (Ellis 1983) and that they should not be used in areas where mean monthly ambient temperatures are below 12°C for two months of the year (Arthur 1983). Where anaerobic ponds are employed odour control may be achieved by ensuring loadings are optimum or by mixing back oxygen rich waters from HRAP or maturation ponds, removing the need for surface aerators (Arthur 1983).

i 90s H . J . FalIowjeld and M . K . Garrett

3.1.2 Facultatitie ponds

In the absence of an aerobic pond the facultative pond can be the first pond within a WSP series. In either case they are the principal site of waste stabilization. Pond depth is usually 1.2-1.8 m as shallower ponds allow vegetation to develop on the pond bottom and leave insufficient volume for sludge accumulation. The facultative pond relies for its effectiveness upon both the anaerobic processes occurring within the sludge blanket and the more eficient aerobic processes occurring in the surface waters. Oxygen for bacterial respiration and BOD reduction is derived from algal photosynthesis and by diffusion from the atmosphere. The calculation of permissible BOD, loadings is an area of some confusion and has recently been reviewed by Ellis ( 1983). There is disagreement regarding ‘the relationship providing the best agreement with all the available data’ Arthur (1983). Thus:

i., = 20 T-60 Arthur (1983) A, = 20 T-I20 Ellis (1983)

where, i., = Areal loading rate (kg BOD,/ha/d).

Estimated permissible loading rates at ambient temperatures of 12 and 25°C respectively are within the range 180440 kg BOD,/ha/d. Reduction in BOD, is similarly temperature dependent (as are retention times) and ranges from 75-84% as retention time varies from 7-15 d (Arthur 1983).

T = Minimum mean monthly ambient temperature (“C).

3.1.3 Maturation ponds

Maturation ponds are shallow (1.2-1.5 m) and fully aerobic. These ponds have lower BOD, loading rates (15-50 kg BODJhajd) and are primarily concerned with ‘polishing’ the final emuent by reducing the bacterial count, lowering the suspended solids of the wastewater and removing nitrogen and phosphorus. The role of maturation ponds within the WSP system has also been reviewed by Ellis (1983).

i t should be noted that no mechanical mixing is employed in any of the ponds within a WSP system although many of the design equations assume a completely mixed system. Mixing is effected only by wastewater inflow and wind action. Thermal stratification of facultative and maturation ponds has been noted by several workers (Stahl & May 1967; Marais 1970) and results in a decrease in mixing and oxygen transfer to the deeper layers of the pond. The depth of the aerobic zone within the facultative pond is then severely reduced and the efficiency of waste stabilization is decreased. Furthermore as a result of thermal stratification the warmer wastewater entering the pond may flow within the warmer upper layers without dilution in the total pond volume (Drews 1966), so that retention times may be considerably lower than design values. Mara (1975, 1976) has further considered the design of waste stabilization ponds for operation in warmer climates.

3.2 H I G H R A T E A L G A L P O N D S

The technology for mass algal culture and the concept of the HRAP system for wastewater treatment was researched and developed initially by Oswald et a/. 1955, 1957 and Oswald, 1963, 1970, 1976. HRAP technology for the treatment of domestic wastewaters in Israel was refined and developed by Shelef er a!. (1976, 1978, 1980) and Goldman & Ryther (1976) extended the concept to include wastewater-seawater mixtures. A selection of wastes in which algae have been cultured is shown in Table 1.

H R A P systems are usually of shallow ( 0 . 2 4 6 m) meandering channel design (Fig. 2). Shallow ponding results in a greater proportion of the algal culture remaining within the photic zone and the meandering channel design permits greater economy of mixing. Mechanical mixing is incorporated, not as 3 method of aeration, but to prevent thermal stratification and ensure that the algal cells receive maximum exposure to solar radiation. Mixing has been effected by air-lift pumps (Persoone et ctl 1980). propeller pumps (Shillinglaw & Pieterse 1980; Oswald 1981) and by paddlewheels (Groenweg et a!. 1980; Lincoln & Hill 1980; Oswald 1981; Fallowfield & Garrett 1984). Typical

Treatment of wastes I91 S Table 1. Wastes in which algae have been cultured

Waste Location Algae Reference

Domestic Settled sewage

Raw sewage

Sewage effluent, high salinity

Urban wastes Rural wastes

Settled sewage

Industrial Canning factory

effluent Soya bean wastes,

Sewage sludge Palm-oil mill

sludge Nitrogen industry

wastewaters

Poultry waste Pig manure

Swine manure Swine manure Pig waste Pig manure

Pig manure

Manure and sewage sludge

Pig slurry

Agricultural

California, USA

Israel

Hong Kong

Manitoba, Canada Mysore, India

Melbourne. Australia

Bloemfontein, South Africa

Hong Kong

Penang, Malaysia

Warsaw, Poland

California, USA Galway, Ireland

Oregon, USA Belgium Singapore Florida, USA

Dortmund, Germany

Hong Kong

Northern Ireland, UK

Chlorella, Scenedesmus, Chlamydomonas, Micractinium Scenedesmus, Micractinimum, Phytoconis, Oocyotis Chlorella

Euglena Scenedesmus, Spirulina

Chlorella, Scenedesmus, Anacystis, Microcystis, Oscillatoria

Scenedesmus

Chlorella

Chlorella

Chlorella

Unspecified Chlorella

Scenedesmus, Chlorella Chlorella, Scenedesmus Unspecified Chlorella, Euglena, Chlamydomonas Scenedesmus

Chlorella

Chlorella

Oswald (1963)

Shelef et al. (1976)

Chan et a/ . (1979)

Waygood et al. (1980) Venkaturaman et a!.

Ip et al. (1982) (1982)

Gaigher et al. (1980)

Wong & Lay (1980)

Sivalingham (1980)

Przytocka-Jusiak et al. (1984)

Dugan et al. (1972) Wilson & Houghton

Boersma et al. (1975) De Pauw et al. (1978) Lee & Dodd (1980) Lincoln & Hill

Groenweg et a/ .

Cheung & Wong (1981)

Garrett & Fallowfield

(1974)

(1980)

(1980)

(1981)

mean surface velocities are 0 .0542 m/s. There is some evidence that the mode of mixing and the surface velocity may affect the harvestability of the algal biomass (Oswald 1981). Factors affecting the design of HRAP systems were considered further by Oswald (1976).

HRAP systems may be used as primary ponds receiving effluent directly (Oswald 1976; Shelef et al. 1980; Lee & Dodd 1980; Groenweg et a/. 1980) or as secondary ponds receiving effluent from facultative or anaerobic ponds (Oswald 1976; Hill & Lincoln 1981; Oswald 1981). Effluents from anaerobic digesters have been shown to be suitable though not ideal media for algal growth (Bennemann et a/. 1977). BOD, loading rates are usually 15&600 kg BOD,/ha/d and a 93-96% reduction in BOD, may be achieved at retention times of 2-8 d (Azov & Shelef 1982).

3 .3 COMPARISON OF HIGH RATE ALGAL P O N D S A N D WASTE STABILIZATION P O N D S

The essential features of HRAP and WSP systems are shown in Table 2. The operation of both systems depends upon numerous complex environmental interactions of which those concerning light and temperature are probably the most important (Goldman 1979b). The recommended design equations for WSP, however, are based predominantly upon considerations of mean ambient air

192s H. J . Fallow$eld and M . K . Garrett

Table 2. ComDarison of various features of HRAP and WSP svstems

HRAP WSP

Areal loading rates high. 100-600 kg BOD,!ha,'d.

Retention times short, 2-5 d

Ponds shallow. 0 . 2 4 6 m and require continuous or intermittent mixing.

Sludge does not accumulate.

Ponds are operated to maximise algal growth and photosynthesis. Algal yields are high, 50-90 t /ha/ y .

Loading rates vary: facultative ponds, 2W600 kg BOD,/ha/d maturation ponds, 15-50 kg BOD,/ha/d

Retention times long, 10-48 d depending on number and type of ponds.

Ponds deep, 1.0-1+3 m, mixed by inflow and wind action, thermal stratification common with photic and euphotic zones. Hydraulic short circuiting a problem.

Sludge accumulates in both anaerobic and facultative ponds.

Photosynthesis and algal growth not optimized. Productivity low but use of algal biomass not intended.

temperature. Calculated permissible areal BOD, loadings for anaerobic and facultative ponds at three mean ambient air temperature are shown in Table 3. Although suggested permissible areal loadings for anerobic ponds are between 4ooO-16 OOO kg/ha/d, design calculations (Ar!hur 1983) imply a constant loading of 7ooO-9OOO kg/ha/d for ambient temperatures between 12 and 2 5 T , BOD, reduction, however is strongly temperature dependent. Loadings and BOD, reductions within a facultative pond are both temperature dependent.

Various HRAP operational strategies were researched and discussed by Azov & Shelef (1982). These included: (i) constant pond area with seasonal depth and retention time variations, and (ii) constant depth with seasonally variable pond area, the latter strategy being experimentally equivalent to changing retention times within a constant depth pond. Calculated areal BOD, loadings for these two operational strategies. together with experimentally determined percentage BOD, reductions (Azov & Shelef 1982) at mean winter, spring and summer air temperatures in Israel (Azov et al. 1980) are shown in Table 3. When the constant area: variable depth strategy is employed constant areal BOD, loadings of about 400 kg/ha/d may be used at all ambient temperatures considered. Loadings may vary from 167-660 kg/ha/d when using a variable area: constant depth strategy. BOD, reductions were high at all temperatures regardless of the operational strategy.

Experimentally derived and predicted area and retention times necessary to attain the above BOD, loadings and reductions for various configurations of WSPs and operational strategies for H R A P s at three mean ambient air temperatures are shown in Table 4. The values for the WSP

Table 3. Calculated areal BOD, loadings (kg/ha/d) and percentage BOD, reduction for WSP and HRAP systems ~~

BOD, load Percentage (kg/ha/d) BOD, reduction

Mean ambient temperature ('C) 12 20 25 12 20 25

WSP Anaerobic' 8000 9Ooo 7000 45 62 70 Facul tativet 180 340 440 15 80 84

- 86 92 94 Anaerobic + facultative + maturation - - - 86 90 93 Facultative + maturation - -

H RAP: Constant area, variable depth 358 403 39 1 94 94 94 Variable area, constant depth 167 331 660 96 94 93

* Volumetric loadings (kg BOD,/m3/d): 0.2 (12"C), 0.25 (20°C) and 0.192 (25°C); mid pond depth is 4 m;

t Permissible BOD, loadings (kg BOD,/ha/d) calculated from = 20T - 60, where T = "C (Arthur 1983). $ Calculated from Azov er al. (1980) and Azov & Shelef(1982).

Arthur (1983)

Treatment of wastes 193s Table 4. Comparison of area requirements and retention times for WSP and HRAP systems to treat emuent from

a town of 50000 inhabitants

Retention time (d) Total pond area (ha)

Mean ambient temperature "C 12 20 25 12 20 25

Anaerobic + facultative + maturation 29.7 18.8 13.0 10.7 7.7 5.1 Facultative + maturation 48.9 25.4 17.6 18.5 9.4 6.8

WSP*

HRAPt Constant area; variable depth (depth, m) 4 7 3.7 2.8 4.33 4.33 4.33

(0.52) (0.40) (0.28) Variable area; constant depth (0.45 m) 8.7 4.4 2.2 10-1 5.1 2.5

* From Arthur (1983), pond depths of 4 m, anaerobic; 1.8 m, facultative; 1.5 m maturation. Influent BOD, =

t Calculated from Azov et al. (1980) and Azov & Shelef (1982). Influent BOD, = 323 mgjl; flow rate normal- 384 mgjl.

ized to 5 200 m3/d.

system are for the treatment of effluent from a town of 50000 inhabitants with a flow rate of 5 200 m3/d and an influent BOD, of 384 mg/l and were predicted by Arthur (1983). The retention times suggested were sufficient to produce an effluent BOD, of 25 mg/l. Retention times for the two configurations of HRAP's were determined experimentally in a 0.1 ha HRAP by Azov & Shelef (1982) and produce an effluent BOD, of 20 mg/l. Pond area requirements were calculated from the data of Azov & Shelef (1982) for a town of 50000 inhabitants. The flow rate given was 15000 m3/d whilst maintaining an influent BOD, of 323 mg/l. The difference in the two flow rates for similar populations merely reflects differences in the volume of effluent discharged from towns within the developed and the developing world.

The advantage in terms of both area and particularly retention time of incorporating an anaerobic pond within a WSP series is apparent from the data shown in Table 4. However, the reduced area requirements and shorter retention times for HRAP's operated under similar conditions are even more striking. The retention time at 25°C for a 4.33 ha, 0.28 m deep HRAP was 2.8 d compared with 13 d for a 5.1 ha, 1.8 m deep, three cell WSP system incorporating anaerobic, facultative and maturation ponds. Similarly at 12°C retention times were 4.7 and 29.7 for HRAP and WSP systems respectively.

In considering HRAP operational strategies Azov & Shelef (1982) concluded that the constant area: variable depth strategy was the preferable method for optimal year round operation. The strategy was relatively simple to operate, accommodated seasonal climatic changes and had the highest biomass production whilst maintaining a high degree of efluent treatment. The variable area: constant depth mode was ruled out for most locations due to increased land requirements and complexity of operation.

The pond area requirements and retention times for HRAP systems discussed above are primarily concerned with BOD, reduction. Within a three cell WSP system, with three maturation ponds each with a 5 d retention time, there would also be a 99.99999% reduction in faecal coliforms i.e. from lo6 to 10/100 ml (Arthur 1983). Design equations for maturation ponds assume that faecal coliform (FC) removal is a first order kinetic process (Marais 1974), defined by the following equation:

Bi 1 + KB(T) t

Be =

where, Be = FC 100/ml of effluent. B, = FC 100/ml of influent.

KB(T) = first order FC removal rate constant at T"C/d. t = retention time.

K,(T) = 2.6 (1.19)'T-20).

194s H . J . FallowJield and M . K . Garrett Values for the removal rate constant K,(T) vary from 0-65/d at 12°C to 62/d at 25°C (Arthur 1983). Available data for FC removal in HRAP systems is poor, merely suggesting that a removal of 99% may be achieved (Oswald 1980) but this equates only to a tenfold reduction in FC numbers. Calcu- lations from the data of Shelef ef al. (1976, 1978, 1980) imply that K,(T) values are within the range 6.05-426id (20°C) for HRAP systems compared with 3.09/d (20°C) for maturation ponds (Arthur 1983).

Exposure to U . V . radiation has been shown to affect the rate of FC removal (Drews 1966; Gameson & Saxon 1967; Moeller & Calkins 1980). The implied higher values of K,(T) within HRAP systems may be due to mixing which exposes a larger pond volume to effective doses of U.V. radiation. The larger algal biomass present in HRAP's compared with WSP systems may also affect the FC removal rate since algal endotoxins (Pratt 1942; Spoehr et a/. 1949; Jorgensen 1962; Chrost 1975) and high pH brought about by intensive algal photosynthesis (Parhad & Rao 1974) have both been suggested to be effective in FC removal.

For reasons of public health the re-use of treated wastewater for the irrigation of many agricultural crops is dependent upon FC content. Sewage influent typically contains lo7 FC/100ml and reductions to 106'100 ml are currently achieved in HRAP systems. Pond eflluents with counts at this level could only be used for irrigating crops such as cotton, sugar beet, cereals and olives.

4. Pilot plant experience with high rate algal pond systems in Northern Ireland

In 1981 pilot plant studies of algal culture in the liquid phase of pig slurry were initiated at the Agricultural Research Institute of Northern Ireland (latitude 50" 2 6 north, longitude 6" 06' west). This followed approximately 10 y of laboratory work to evaluate the microbiology and biochemistry of the process, the nutritional quality of the biomass and the performance of the system under simulated local climatic conditions (Garrett & Allen 1976; Garrett et al. 1976; Allen & Garrett 1977; Garrett ef al. 1978). The pilot plant was operated outdoors for a 2 y period using both pig slurry and domestic sewage effluent. The performance of the system on pig slurry will be considered here since a more comprehensive set of data are available for this effluent.

The pilot plant consisted of a rotary press screen slurry separator, 2 culture tanks of raceway design each having a surface area of 11.1 mz and containing 2200 1 each at a culture depth of 0.2 m, together with ancillary equipment. The cultures were mixed by paddle wheels and could be operated in either batch or continuous modes (Garrett & Fallowfield 1981). Continuous cultures were operated from September 14 to November 26 1981 and from May 10 to August 6 1982 with continuous mixing at a surface velocity of 0.21 m/s. A reduced mixing strategy and heating of the cultures was also evaluated.

The suspended solids (SS) content of the input liquid presented a major problem with this system. Liquid from the separator had a SS content of 5.32% which had to be further reduced by addition of a cationic flocculent and subsequent dilution 1:9 with water to give an input liquid containing 2200 mg SS,t This contrasted with a 1 :1 dilution necessary for laboratory scale work in which slurry liquid phase was prepared by centrifugation. This dilution was necessary to reduce solids concentration per SL' rather than to reduce effluent strength in order to permit algal growth. The influent slurry liquid following dilution 1 :9 with water contained 19 mg/l total P, 137 g/1 total N and had a BOD, of 1500 mg/L

4 1 N U T R I E N T S T R I P P I N G A N D B O D R E D U C T I O N

Various aspects of the performance of the system are summarized in Table 5. In the period September to Kobernber 1981 phosphorus removal averaged 58% of input whereas that for the May to Septem- ber 1982 period averaged 64%, the final effluent containing 3.29 mg total P/l. This represented a mean removal rate of 0.45 g total P/mz/d. Approximately 50% of the inflowing nitrogen was removed from the slurry in the September to November period producing an eftluent containing 1-5 mg N 1. In the May to September period a mean percentage removal of 63% was achieved giving an effluent containing < 100 mg N/I. The mean removal rate was 6.48 g N/m2/d. For the entire

Treatment of wastes 195s

Table 5. Selected loading and performance data for a pilot-plant HRAP system treating the diluted liquid phase of pig slurry (Mean monthly figures)

1981 1982

September October November May June July August

Retention time (d) 5.1 8.2 12.8 6.7 4.4 4.4 4.4 Slurry solids: loading (kg/ha/d) 91 75 19 47 106 173 134 Phosphorus : loading (kg/ha/d) 7.1 4.8 2.9 - 3.2 1.3 1.6

removal (%) 16.7 55.6 42.0 - 45.3 67.7 89.7 Nitrogen: loading (kg/ha/d) 91 64 43 72 105 98 122

removal (%) 96.3 92.9 98.3 13.2 62.3 63.8 54.8 BOD,: loading (kg/ha/d) 562 314 207

removal (%) 96.3 93.7 98.3

culture period the mean removal rate was 77.5% and the maximum rate (98.3%) was recorded when algal productivity was at a minimum in November. Substantial nitrogen losses also occur through volatilization.

BOD removal ranged from 92.6 to 98.6%. Estimation of oxygen production using an oxygen yield factor of 1.6 g O,/g of algal dry matter (Oswald 1963) validated by Hill & Lincoln (1981) indicates that only 2G40% of the reduction in BOD can be attributed to photosynthetically produced oxygen.

4.2 BIOMASS PRODUCTION

High rate pond systems are normally operated to maximize BOD reduction and nutrient stripping through optimizing algal growth. A consequence of this is that production of biomass is also maxi- mized and this provides a by-product of potential value as an energy source, for animal feeding or as a source of certain biochemicals. Figures quoted for biomass productivity in HRAP systems are normally uncorrected for the solids input to the system and are thus overestimates of the true productivity of these systems. In the pilot plant study considered here, biomass producitivity was calculated by subtraction of the input solids content from the dry matter output data and algal productivity was also assessed from cell counts and measurement of chlorophyll a.

Cell counts and chlorophyll a contents remained almost constant indicating steady state culture conditions in the period September to November. Biomass production during this period however fluctuated between 16.60 g/m2/d in September and 1.58 g/m2/d in November, the mean value being 6.69 g/m2/d. Significantly higher rates were recorded in the summer period May to September. Mean production for this period was 18.29 g/mz/d with a range from 7.65 to 32.95 g/m/d. These values give a projected yield in tonnes per hectare per year ranging from 14.67 to 81.58 depending on the mean monthly figure used for the calculation and with an annual mean of 49.41 t/ha/y. Total dry matter (DM) output, uncorrected for solids input to the system, would range from 43.47 to 130.31 t/ha/y with a mean of 83.39 t/ha/y. A more feasible expectation based upon a 153 d growing season (May to September) would be 26.25 t/ha corrected for solids input, or 41.51 t/ha uncorrected. Mass algal culture systems are more productive in terms of dry matter yield than conventional agricultural systems using land plants. A comparison of attainable dry matter yield for various crops and algae both in the UK and in subtropical climates is presented in Table 6.

These results compare favourably with other reported yields in Europe, especially in view of the relatively high solids and nitrogen loading used here. Soeder (1976) reported algal productivities of between 18 and 28 g DM/m2/d for algae grown outdoors in a complete inorganic medium in Germany. Groenweg et al. (1980) culturing Scenedesmus spp. and Coelastrum sp. in more dilute pig slurry than that used here obtained algal productivities of between 2.5 and 14.0 g DM/m2/d in November and July respectively. Similarly De Pauw et al. (1978) reported mean productivities of approximately 12 g DM/mZ/d for Scenedesmus acutus grown in continuous culture in filtered diluted pig slurry, although productivities of 30 g DM/mZ/d were also recorded. Higher irradiances and culture temperatures also increase algal productivity in studies in the USA to 22 g DM/m2/d in

1965 H . J . Fallow$eld and M . K . Garrett

Table 6. Comparison of some attainable dry matter yields for various agricultural crops and mass algal culture systems

Dry matter yield (tlhalv)

United Kingdom Wheat 5.0 Barley 7.0 Maize 17.0

Potato 11.0 Algal Biomass* 26.3

Ryegrass 23.0

Subtropical Wheat (Mexico) 18.0 Maize (California) 26.0 Bermuda grass (Southern USA) 21.0 Rice (California) 22.0 Potato (California) 22.0 Chlorella/Scenedesmus spp. (California) 50.0 Synechocysris spp. (Florida) 52.0

* For a 153 d growing season in Northern Ireland. Based on data from Slesser & Lewis (1979).

Oregon (Boersma et al. 1975) and 30 g DM/m2/d in Florida (Lincoln & Hill 1980). Similar high values, 25 g DM/m2/d, for algal culture in pig slurry have been obtained in Singapore by Lee & Dodd (1980). The 153 d growing season suggested for Northern Ireland on the basis of the pilot plant study is similar to that reported by Ciferri (1980) for enclosed outdoor culture of Spirulina in Central Italy.

4.3 S T A B I L I T Y A N D SPECIES COMPOSITION

Dry matter productivity figures require supplementation with qualitative data since species composition can be relevant to harvesting techniques, end product use etc. In the September to November period cultures were essentially homogeneous consisting of the inoculated alga, Chlorella vulgaris. In the summer period May to September, however, the population evolved from one dominated by C. nulgaris through a period in which Chlamydomonas spp. dominated to a mixed culture dominated by the inoculated alga but including also Ulothrix sp., Ankistrodesmus sp. and a pennate diatom Nitzs- chia sp. Dry matter production was unrelated to species composition although transitions in domin- ant forms were accompanied by transient depressions in dry matter production.

The understanding of environmental factors affecting species composition may aid in the selection of algal species capable of producing commercially valuable metabolites. A wide range of environmental factors have been implicated including light (Mur et a!. 19771, pH (Goldman et al. 1982a & b), interactions concerning pH and NH; - N and CO, equilibria (Goldman 1979b; Groen- weg et a/. 1980; Azov & Goldman 1982) and temperature (Goldman & Ryther 1976; De Pauw et a/. 1980). Other factors affecting species composition include retention time Bennemann et al. 1980; Shillinglaw & Pieterse 1980; Fallowfield & Garrett 1984) and grazing by zooplankton (Lee & Dodd 1980; Lincoln & Hill 1980; Oswald 1980). It has been suggested that rotifers could be exploited for harvesting algae in HRAP systems (Groenweg & Schluter 1981).

5. Harvesting technology

Algae are removed from the discharges of WSP and HRAP systems to improve efiluent quality in terms of both suspended solids and BOD. Whilst the mechanism of algal removal should generally be eficient in capital and operating costs and in its energy requirements, relatively sophisticated removal

Treatment of wastes 197s techniques can be justified where the biomass productivity is high (as in HRAP systems) and where the harvested material has economic value. In WSP systems algal removal is performed primarily to meet discharge standards. Methods employed include slow sand filtration, rock filters, intermittent discharges, variable level discharges and zero discharge and these methods have been reviewed by Parker (1976) and also by Ellis (1983). Floating plants such as water hyacinth (Dinges 1978) and duckweed (Wolverton & McDonald 1979) have also been used for algal ‘removal’ but more correctly act by limiting algal growth.

Golueke & Oswald (1965) pioneered work on algal harvesting techniques including centrifugation, flocculation and sedimentation. Natural sedimentation rates for algae in effluents are low; 0.02-0.06 m/d (Stutz-McDonald & Williamson 1979). Flocculation of algal cells using alum (40-80 mg/l at pH 6.0-7.0) and lime (10&200 mg/l at pH 10.5-11.2) has been shown to be effective (Golueke & Oswald 1965). The flocs produced by these treatments are generally loose and difficult to dewater but if gas bubbles are allowed to nucleate the flocculated algal material floats and may be skimmed from the liquid surface. Alum is most commonly used as a flocculating agent. Small gas bubbles have been generated by pressurization (Van Vuuren et al. 1965) by electrolysis (Sandbank et al. 1974) and by carbon dioxide injection (Conway et al. 1981). Bare et al. (1975) have used a pressurized system of dissolved air flotation to achieve 90% removal of algal biomass from an effluent containing 125 mg suspended solids/l using 175 mg alum/]. Methods are available for the recovery of alum from flocculated material (Golueke & Oswald 1965; Shelef et al. 1978) to improve the economics of the process and the possible nutritive value of the biomass produced.

Photosynthetically derived dissolved oxygen (DO), frequently at supersaturated levels within surface waters of HRAP, may also be used as a source of minute gas bubbles (Van Vuuren & Van Duuren 1965) and Koopman & Lincoln (1983) have used this method to harvest algae from HRAP effluents using both alum and a cationic polymer as flocculants, at rates of 1.42 and 0.35 g/g algal SS respectively. Harvesting was not attempted when the DO fell to below 16 mg/l. Viviers & Briers (1982) reported unsuccessful attempts at dissolved oxygen flotation with alum additions of up to 200 mg/l at a DO of 20 mg/l, although polymer additions of 3.8 to 7.4mg/l did achieve 56 to 85% algal recovery. Significantly ‘bleeding’ air into the feed pump (4 I/min at a liquid feed of 100 l/min) permitted harvesting to continue 8 h after sunset when the DO had declined to 8 mg/l. Encouraging the natural autoflocculation produced by high pH and alkalinity (Arad et a\. 1980) may in the future obviate the need for the addition of flocculants.

Further options for upgrading effluents from HRAP and WSP systems include land spreading, aquaculture and anaerobic digestion. Land spreading utilizes the algal biomass as an organic manure and may be attractive as green algae in HRAP’s may produce 5.8 t N/ha/y this being 20 to 30 times the productivity of soya beans (Oswald 1980). Alternatively thickened algal sludge may be used as feed for an anaerobic digester (Golueke et al. 1957) or for aquaculture (see following section).

More capital and energy intensive harvesting methods were recently reviewed by Mohn (1978, 1980). Microscreening has received much attention (Korminak & Cravens 1978; Harretson & Cravens 1982) but successful operation has been found to be largely species dependent. Bennemann et al. (1980) have pointed out that there may be a trade-off between pond operation for maximum biomass production and that for successful harvesting by microscreening and this may have important implications for pond operation if it extends to other harvesting methods.

6. Product composition

The biomass produced in a HRAP system consists of cells of the cultivated alga(e) together with a minor contribution from the indigenous microbial population (Oron et al. 1979). In the pilot plant studies in Northern Ireland a striking feature has been the similarity of chemical composition of the biomass despite differing culture conditions, although the pilot plant products differed significantly from material produced previously in laboratory chemostats (see Table 7). Interestingly the fatty acids C18:2 and 18:3 comprised a third of the total fatty acid content.

Algal biomass is most frequently evaluated as a source of protein, but Ackman (1981) recently

198s H . J . FallowJield and M . K . Garrett Table 7. Composition and nutritional qualities of harvested products of algal culture

Laboratory* product Batcht Continuoust Total nitrogen 8.2 9-22 8.71

True protein 42.0 46.34 46.18 Carbohydrate 23.5 8.83 7.13 Starch 17.0 1.36 1.28 Phosphorus 4.1 1.74 2.42 Lipid 16.2 11.01 11.74 Chlorophyll LI 3.8 0.71 0.14

h 2.7 0.27 0.3 1 it h 1.4 2.85 2.39

Copper - 0.045 0.071 Ash 6.4 18.82 16.85 Nucleic acid 9- 1 12.18 14.5

Crude protein 51.2 57.65 54.35

Chemical score 32-36 29-34 3 540 Modified essential amino acid index 6 S 7 1 74 68-81 FAO!WHO protein score 56-60 50-80 50-79

* Garrett er al. (1976). t Pilot-plant studies by Failowfield & Garrett.

considered algae as sources of edible lipids and many other cellular constituents may be potentially valuable sources of tine chemicals. Cklorococcum oleofaciens and Neochloris oleoabundans have been shown to accumulate 42-88% of their dry weight as lipid with lipid productivities of 1.5 g/l (Metzger et nl. 1982) and manipulation of cultural conditions can have significant effects upon the lipid content and composition of green algae (Materassi et al. 1980). Algal biomass may also be a future source of hydrocarbons and Bofryococcus brnunii can accumulate 1 4 3 6 % of dry matter in this form (Largeau 1981). Algae show a greater structural diversity in their pigments than higher plants (Becker 1981a). Dunaliella hnrdnwil contains not only 2% /?-carotene but also produces 40% glycerol, a potentially valuable chemical feedstock, at a dry matter productivity of 20 g/m2/d (Ben-Amotz & Avron 1980). A detailed analysis of the composition of Spirulina spp. including pigments, lipid and vitamin content was presented recently by Santillan (1982). Algal proteins may also have structural properties such as foam stability which could find application in the food industry (Lee & Picard 1982). The production and potential uses of algal biomass were recently reviewed by Becker (1981a & b).

A major limitation to the utilization of green algae as novel sources of protein for non ruminants is the low digestibility of most species studied (Waslien 1975). Hasdai & Ben-Gheldalia (1981) have suggested that a highly digestible algal biomass may be produced by a harvesting technology which produces a young biomass with a low mineral and aluminium content. Various pretreatments have been shown to have little eff'ect upon the digestibility (Subbulakshmi et al. 1976; Lee et al. 1982) whilst others have been shown to increase digestibility significantly (Waslein 1975). Garrett et al. (1976) have related low digestibility to the widely occurring but not ubiquitous wall component sporopollenin and have shown that algae such as Chlorella vulgaris strain number 21 1-le, lacking this wall component have relatively high digestibilities. Thus, rat feeding experiments gave a digestibility of 81.7, a net protein utilization of 48.2 and a biological value of 59.0 (Strain et al. 1984). The biomass from HRAP systems treating pig slurry has a nutritional value similar to casein or soya flour (Table 7 ) .

Many uses have been suggested for a harvested product of such high nutritional value. These include the use oF'clean cultures' for human food (Jaleel & Soeder 1973), as chicken feed (Mokady et tr l . 1980), pig feed (Lincoln & Hill 1980), for aquaculture (Ryther et al. 1972, 1975; Kawasaki et al. 1982; Tarifeno-Silva rt nl. 1982a & b: Gordon et al. 1982) and for pisciculture (Sandbank & Hepher 1980). Further uses. possible markets and potential costs of production of algal biomass are considered further by Soeder & Binsack (1978), Shelef & Soeder (1980), Soeder (1980) and by Lesley et al. (1981 )

Treatment of wastes 199s 7. Energy balance of the treatment process

If algal culture is operated as an autotrophic process amplification of energy can occur through photosynthesis. The pilot plant study in Northern Ireland included a detailed energy analysis in which batch and continuous processes were compared and strategies for reducing supplementary energy inputs were assessed. The major proportion of the supplementary energy input was for continuous mixing (688 kJ/mZ/d). The efficiency of the system could be improved, however, and it was shown that an 8 h mixing strategy required less energy whilst algal productivity was maintained. Calculations suggest an energy requirement of 94.2 MJ/ha/d (26.2 kWh/ha/d) for larger pond systems on an 8 h mixing regime. Further calculations using data from Table 4 suggest an energy input of 215 MJ (60 kWh) to treat a 1000 m3/d of wastewater. This compares favourably with activated sludge (724 kWh), aerated lagoon (605 kWh) and rotating disc systems (86 kWh) (calculated from data of Gloyna & Tischler 1979). HRAP systems are also more energy efficient in oxygenation, providing 20 kg O,/kWh compared with 2.5-3.5 kg O,/kWh in aerated systems (Anon. 1983). Con- tinuous cultures operated on an 8 h (day light) mixing regime in Northern Ireland had a positive energy balance throughout most of the culture period with surpluses ranging from 650 kJ/m2/d in July and August to 50 kJ/m*/d in October and November (Fig. 3).

0) I I I I I I 0 0 C Sept Oct N ov May Jun Jul Aug

1981 I982 - O n

+ 200 I I I

Fig. 3. Energy balance for a HRAP system treating the diluted liquid phase of pig slurry in Northern Ireland. (a) Continuous mixing, (b) 8 h (day light) mixing.

Data such as these can be used to calculate a theoretical energy budget for larger HRAP systems. Depending upon whether total solids or slurry corrected algal solids are used in the calculations of biomass productivity, energy surpluses of between 3.3 GJ/ha/d (916.9 kWh/ha/d) and 5.64 GJ/ha/d (1565 kWh/ha/d) could be achieved. Sedimentation of the biomass followed by anaerobic digestion offers a possible strategy for conversion of the energy harvested by photosythesis in a HRAP system. Assuming 60% of the solar energy fixed by the algal cells might be recovered as methane via anerobic digestion and that electrical energy may be generated at an efficiency of 25% from methane (Oswald & Eisenberg 1981), a net energy suplus of between 495 kJ/ha/d (137.5 kWh/ha/d) and 845 kJ/ha/d (234.7 kWh/ha/d) might be achieved. Similar claculations by Oswald & Eisenberg (1981) for the sewage HRAP system suggested a net exportable electrical energy output of 67 kWh/ha/d. The difference between this result and those calculated above arise from differences in the length of the growing season used to estimate potential productivity so that on a 365 d/basis there would be little difference in the theoretical energy production values for HRAP systems in California and Northern Ireland. Thus, mass algal culture for energy harvesting is normally considered appropriate to areas

200s H . J . Fallowfield and M . K . Garrett

having relatively long growing seasons and has been suggested, for example, for coastal desert regions (Balloni et a/. 1982). This leads to a consideration of rate limiting factors in the HRAP system.

8. Rate limiting factors

Light and temperature are generally considered to be the key environmental variables regulating algal growth at the relatively high nutrient levels which prevail in waste treatment systems using sewage or farm waste. There is considerable self-shading of algal cells in HRAP systems so that the light level below 10 cm depth may frequently be below light compensation point for the algal cells. In mixed systems individual cells will circulate through various levels of irradiance at different depths and this complicates analysis of light limitation in the system. Recently many models have been developed to describe algal culture in waste waters (Goldman 1979a; Azov & Shelef 1982; Buhr & Miller 1983) and in pig slurry (Hill & Lincoln 1981). One approach has been to assume that the algal cells effectively integrate the available light levels at different tank depths so that the average irradiance within the tank volume adequately describes the overall light environment. A second approach has been to use the Beer Lambert Law (Azov & Shelef 1982) and relate algal growth at a particular depth to the light intensity at that depth. The Beer Lambert Law, however, is applicable to dilute solutions illuminated by monochromatic light and possessing only a small scattering component and therefore has limited application to HRAP systems. Moreover, most models are based upon incident irradiation rather than that absorbed by the algal cells and Fallowfield & Garrett 1984) have suggested that a model combining the concepts of fractional absorption (absorptance)

and the Kubelka-Munk colorant layer analysis (Judd & Wyszecki 1975) which considers both scat- tered and absorbed light within a dense solution is more applicable. These mathematical treatments of light penetration with depth indicate that the concentration of particulate matter in the treatment system has a more mzirked effect than surface irradiance upon light attenuation with depth. Since irradiances world wide are higher than I, (the incident light value which saturates the rate of photosynthesis). algal concentration within the culture rather than incident irradiance would appear to be the factor most affecting light limitation. This is consistent with the observation that large increases in surface irradiance produce only relatively small increases in algal biomass yields (Goldman 1979b) and this may explain why yeld values in the range 15 to 25 g/m2/d are most commonly reported for HRAP systems for waste water treatment.

These considerations indicate the importance of culture depth and retention time as control parameters for optimizing biomass productivity in HRAP systems although there is controversy regarding how this can be achieved. It has been suggested that systems should be operated with greater depths in summer than in winter when surface irradiance is lower (Goldman 1979b; Oron & Shelef 1982; Buhr & Miller 1983). Recently Azov & Shelef (1982) noted that algal concentration at optimum retention times was not generally the same in winter as in summer. It was suggested that systems should be operated at greater depths in the winter when the algal concentration is lower, and therefore light penetration greater and at shallower depths in the summer when both algal concentration and light attenuation are greater.

The effects of temperature upon biomass production rates have been widely reported (Goldman 1980: Azov & Shelef 1982; Buhr & Miller 1983). Tamiya et al. (1953) showed that growth rate was dependent upon incident irradiance only at low irradiance values and at high incident irradiances growth rate was dependent upon temperature. Azov & Shelef (1982), however, concluded that light and temperature have combined effects upon maximum algal production and also upon both optimum and wash-out retention times. In a well mixed HRAP system algal cells will encounter high irradiances when growth rate become more temperature dependent and low irradiances when growth rate is more dependent upon photosynthetic rate. In modelling studies consideration should be given to the relative merits of using photosynthetic rates (P) or growth rates (p) versus incident irradiance ( I ) relationships since photosynthetic rate may be uncoupled from growth rate under certain culture conditions (Cohen & Parnas 1976). A more detailed consideration of limiting factors in HRAP systems is given in Fallowfield & Garrett (1984).


9. Conclusions

The prospects for large scale production of micro-algae were optimistically reviewed in a report published by the Carnegie Institution of Washington in 1953 (Burlew 1953). Since that time research and development work has been stimulated by growing interest in biomass production and pollution control but it has been the combination of these objectives which has provided the greatest impetus to development of HRAP technology to a practical effluent treatment process. Algal treatment systems now operate with high efficiency in various parts of the world and future developments are likely to centre on end-product utilization, refinement of harvesting technology and the further development of mixing strategies for optimum process performance.

10. Acknowledgements

Pilot plant studies in Northern Ireland were supported in part by a grant from the Energy Tech- nology Support Unit of The United Kingdom Department of Energy.

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The treatment of wastes by algal culture