The following characteristics are important when selecting marine microalgal species which may have use in wastewater treatment: high and consistent nutrient removal capability; tolerance to a wide range of nutrient concentrations and salinities; the ability to grow heterotrophically; long term dominance in culture; high specific growth rate; resistance to predation and bacterial contamination; tolerance to low oxygen levels; ease of harvest; production of usable biomass or extracts (Marelie et al, 1982; Abeliovich, 1986; Borowitzka, 1988c; Oswald, 1988a, b & c). Microalgal species with some of the characteristics outlined above are shown in Table 1.8.
Table 1.8 Selection of marine microalgal species for potential use in wastewater treatment
Species Division Characteristic Reference
Chaetoceros
calcitrans Bacillariophyceae Dominant in massculture Aquaculture
Wohlgeschaffen et ah
(1992)
Nelson et al (1992);
Nitzschia
longissitm Bacillariophyceae Growth onwastewaters Palmer (1969)
Nitzschia ovalis Bacillariophyceae Growth on
wastewaters Palmer (1969)
Phaeodactyliim
tricornutum Bacillariophyceae Growth anddominant on wastewaters Dominant in mass culture
Aquaculture
Ryther et al (1972); Goldman & Stanley (1974); Goldman et al
(1974a); Goldman & Ryther (1976a) Wohlgeschaffen et al
(1992)
Herrero et al (1991)
Skeletonema
costatum Bacillariophyceae Growth onwastewaters Dominant in spring bloom Loch Striven Aquaculture
Goldman & Stanley (1974)
Marshall & Orr (1927) Regan (1988)
Thalassiosira
weissflogii Bacillariophyceae Dominant in massculture Aquaculture
Wohlgeschaffen et al
(1992) Regan (1988)
Botryococciis
braunii Chlorophyceae Hydrocarbon fuel Birch & Bachofen (1988) Chlamydomonas
reginae Chlorophyceae Aquaculture Regan (1988)
Chlorella salim Chlorophyceae Aquaculture Spectorova et al (1982)
Chlorella
stigtnatophora Chlorophyceae Aquaculture Fabregas & Herrero(1987a)
Dunaliella salina Chlorophyceae 6-Carotene, glycerol Borowitzka & Borowitzka (1988b)
Dunaliella
tertiolecta Chlorophyceae Dominant in massculture Aquaculture 6-Carotene, glycerol Wohlgeschaffen et al (1992) (Spectorova et al (1982); Herrero et al (1991) Borowitzka & Borowitzka (1988b) Nannochloropsis
Table 1.8 Selection of marine microalgal species for potential use in wastewater treatment (continued).
Species Division Characteristic Reference
Stichococcus
bacillaris Chlorophyceae Aquaculture Regan (1988) Rkodomonas baltica Chryptophyceae Aquaculture Regan (1988)
Rhodomonas
marina Chryptophyceae Aquaculture Regan (1988) Rhodomonas sp. Chryptophyceae Aquaculture Regan (1988)
Oscillatoria
animalis Cyanophyceae wastewatersGrowth on Aquaculture
Palmer (1969) Pantastico (1987)
Spirulina platensis Cyanophyceae Aquaculture
Harvestibility Benemann (1992)
Amphidinium
carterae Dinophyceae Aquaculture Witt et al (1981) Oxyrrhis marina Dinophyceae Aquaculture Regan (1988)
Micromonas pusilla Prasinophyceae Aquaculture Regan (1988)
Tetraselmis rubens Prasinophyceae Aquaculture W itt et al (1981)
Tetraselmis sp. Prasinophyceae Growth on wastewaters Aquaculture
Goldman & Stanley (1974)
Spectorova et al (1982); Regan (1988)
Tetraselmis suecica Prasinophyceae Dominant in mass culture Aquaculture Wohlgeschaffen et al (1992) Fabregas et al (1984, 1985b & 1987b); Herrero etal (1991) Tetraselmis
tetrathele Prasinophyceae Dominance in massculture Materassi et al (1984)
Tetraselmis
verrucosa Prasinophyceae Aquaculture Regan (1988) Chrysochromulina
chiton Prymnesiophyceae Aquaculture Regan (1988) Coccolithophora sp. Prymnesiophyceae Aquaculture Regan (1988)
Coccolithus sp. Prymnesiophyceae Aquaculture Regan (1988)
Isochrysis galbana Prymnesiophyceae Dominant in mass culture Aquaculture Wohlgeschaffen et al (1992) Spectorova et al (1982); Fabregas et al (1985a); Herrero et al (1991);
Pavlova lutheri Prymnesiophyceae Growth on wastewaters Dominant in mass culture
Goldman & Stanley (1974)
Wohlgeschaffen et al
(1992)
Phaeocystis
poucheti Prymnesiophyceae Aquaculture Regan (1988) Prymnesium
One of the main problems with mass cultivation of microalgae is contamination by other microalgal species (Goldman, 1979; Shelef & Boeder, 1980; M aterassi et ah, 1984; De Pauw et ah, 1984; Richmond, 1986c). Maintenance of a unialgal culture in open air systems usually requires strict management and manipulation of the environment including temperature, nutrient composition and dilution rate (Borowitzka, 1988c). Less than 50 of the 30,000 species of microalgae identified have been studied in some detail, with respect to their metabolism and chemical composition (Boeder, 1981; Borowitzka, 1988a; Richmond, 1986b). Furthermore, details of physiology, biochemistry, and potential for mass culture are known for only a few of those studied (Borowitzka, 1988c; Benemann, 1989; de la Noüe et ah, 1992). Therefore the physiology of microalgal species capable of treating wastewaters needs to be studied.
Although unialgal cultures of microalgal species can be obtained from culture collections, Borowitzka (1988c) stressed the importance of selecting microalgal strains that are suited to the climatic conditions under which the microalgal treatment will occur. Local endemic species should be most suited to the prevailing climate (Witt et al, 1981; Oswald, 1988c) and they may be introduced to treatment ponds through the dilution of the sewage with seawater. Any comprehensive screening of the potential of marine microalgae to treat wastewaters ideally should include endemic species.
The traditional use of the sea for disposal of wastewaters and the relatively recent adverse effects of this practise on the environment has meant that little research into the use of wastewater treatment processes to prevent eutrophication has been made in the UK. Although the climate of the UK is not the most favourable for the use of a photosynthetic wastewater treatment process, outdoor semi-continuous mass cultures of
marine species including Phaeodactylum tricornutum have already been successfully grown using artificial nutrients in the UK (Ansell et at, 1963a & b; 1964). Year-round treatment of wastewaters by freshwater microalgal cultures has also been demonstrated in the northern cold climate of Quebec, Canada, where Scenedesmus obliquus was cultivated on secondary effluent in greenhouses to maintain temperatures during the winter (Pouliot & de la Noüe, 1985). The productivity of freshwater mass algal culture systems, even in the temperate latitude of the UK has been shown to be comparable to that of traditional agricultural crops (Fallowfield & Garrett, 1985a & b).
Apart from the limitations to algal growth imposed by environmental conditions, there are several other problems associated with microalgal wastewater treatment systems. These include the large areas of land required for ponds, the lack of an efficient and economic method of harvesting the algal biomass and the difficulty of maintaining the algae in suspension (de la Noüe & De Pauw, 1988; Oswald, 1988a & c; Raven, 1988; Benemann, 1989). Therefore a screening of marine microalgal species for use in a wastewater treatment system should also address these drawbacks.
1.8 Aims
The preceding sections of this chapter have highlighted: the problems for marine disposal of wastewaters; the EC guidelines for the nutrient content of coastal discharge which have to be met by 1998; the shortfall of conventional treatment methods in meeting these guidelines; and how a marine microalgal wastewater treatment process may offer an efficient and economic alternative to conventional methods. Although extensive research has been made into microalgal treatment systems using freshwater species, the use of marine species has been little explored. Even less is known of the
potential of microalgal species from temperate climates for wastewater treatment. A comprehensive screening of microalgal species of every division has not been made, and few authors have included endemic species in their studies.
The specific aims of this study were as follows:
a) To screen a wide range of marine microalgal species for their potential to remove nutrients from wastewater and remain dominant in culture. In order to select marine microalgal species appropriate for a detailed study of wastewater treatment.
b) To determine the range of tolerance to environmental conditions of the best-treating species from the screening experiments and to evaluate their ability to remove nutrients under continuous culture.
c) To test the best-treating species selected from the continuous culture experiments for treatment under ambient temperate conditions using larger-scale apparatus modelled on a high-rate pond.