Urban, industrial and agricultural wastewaters contain up to three magnitudes higher concentrations of total nitrogen and phosphorous, compared with natural water bodies.1 Normal primary and secondary treatment of these wastewaters eliminates the easily settled materials and oxidizes 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 treatment of the effluent is therefore required, and both chemical and physical methods which are used are very expensive. Oswald2 estimates that the relative cost of tertiary treatment to remove PO 4 3− , NH 4 + and NO 3 − is about 4 times the cost of primary treatment. Higher orders of treatment, such as quaternary treatment required 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.3,4 The possibility exists that the algae produced in these systems can be used as animal feed supplements,5,6 or be composted. The use of waste-grown algae may ultimately also have application in closed cycle life-support systems,7,8 or may be used in conjunction with power stations, not only to treat wastewaters, but also to act as a CO2 sink for the amelioration of the impact of greenhouse gases.9–13

Limits to Growth

High rate outdoor production units of microalgae can undergo temperature fluctuations. Seasonal temperature variations as well as more rapid daily fluctuations are liable to modify the growth conditions of microalgae and hence affect production efficiency. The effect of elevated temperatures, above optimal growth temperatures, on growth is seldom reported in literature, but often described as more deleterious than low temperatures. Depending on the species, different strategies are deployed to counteract the effect of above optimal temperatures such as energy re-balancing and cell shrinking. Moreover, long term adaptation of certain species over generation cycles has also been proven efficient to increase optimal temperatures. Physical models coupled to biological kinetics are able to predict the evolution of temperature in the growth media and its effect on the growth rate, highlighting the downstream drastic economic and environmental impacts. Regarding the relative elasticity of microalgae towards temperature issues, cell mortality can depend on species or adapted species and in certain cases can be attenuated. These elements can complement existing models and help visualize the effective impacts of temperature on outdoor cultures.

/pdf/temperature-effect-on-microalgae-a-crucial-factor-for-45ho9ewt6a.pdf

Temperature effect on microalgae: a crucial factor for outdoor production

Modeling the effects of light and temperature on algae growth: State of the art and critical assessment for productivity prediction during outdoor cultivation

http://www.nt.ntnu.no/users/skoge/prost/proceedings/dycops-2010/Papers_DYCOPS2010/WePlenaryT5-01.pdf

Hurdles and challenges for modelling and control of microalgae for CO2 mitigation and biofuel production

Several microalgae, such as species ofChlorella, Spirulina andDunaliella, are grown commercially and algal products such as β-carotene and phycocyanin are available. The main focus of algal biotechnology continues to be on high value fine chemicals and on algae for use as aquaculture feeds. This paper provides the outline for a rational approach in evaluating which algae and which algal products are the most likely to be commercially viable. This approach involves some simple market analysis followed by economic modelling of the whole production process. It also permits an evaluation of which steps in the production process have the greatest effect on the final production cost of the alga or algal product, thus providing a guide as to what area the research and development effort should be directed to. An example of this approach is presented and compared with other models. The base model used here gives a production cost of microalgal biomass at about AS 14 to 15 kg−1, excluding the costs of further processing, packaging and marketing. The model also shows that some of the key factors in microalgal production are productivity, labor costs and harvesting costs. Given the existing technology, high value products such as carotenoids and algal biomass for aquaculture feeds have the greatest commercial potential in the short term.

Algal biotechnology products and processes — matching science and economics

Modeling algal productivity in large outdoor cultures and waste treatment systems

A method is provided for optimally supplying autotrophic organisms with CO2 nutrient, the requirement for the nutrient fluctuating in dependence on growth and thus on the incident light, temperature and the density of the organisms. The pH in a culture channel containing the organisms is continuously measured and the introduction of CO2 into the culture channel is regulated in response to the measured pH. An arrangement is provided for practicing the method and comprises a channel in which a suspension of the organisms can be moved in one direction, at least one discharge means in the channel for introducing the CO2 gas into suspension, at least one control member connected to the discharge means for controlling the operation of the discharge means, at least one pH measuring means in the channel for immersion into the suspension, and a circuit connecting the pH measuring means and the control member to actuate the control member for introducing the CO2 in response to the pH measured by the pH measuring means.

Eberhard Dr Stengel

Papers

Modeling algal productivity in large outdoor cultures and waste treatment systems

Method and arrangement for optimally supplying autotrophic organisms with CO2 nutrient