Biofuel co-products as livestock feed - Opportunities and challenges

428Biofuel co-products as livestock feed – Opportunities and challengesin open outdoor shallow ponds for simple maintenance andrelatively low input costs.Open cultivation systemsMost algae for commercial use are grown in the open air.The two most common open cultivation systems are circularand raceway ponds, in use for more than five decades.These systems can be developed using natural water bodiessuch as lagoons and ponds or artificial ponds such asraceways. Raceway ponds are generally oval shaped, closedloop channels of 0.2–0.5 m in depth where mixing is generallyprovided using paddle wheels. Raceway ponds are usuallyconstructed from cement, or sometimes compacted soilchannels with plastic liner (Brennan and Owende, 2010).Their shallow nature and continuous mixing meets thelight requirements of the culture and prevents sedimentation.The open cultivation pond is cheap and they do notcompete for agricultural land as they can be built on nonproductive(marginal) land, or coastal regions for marinealgal cultivation. The open system requires minimal investmentin terms of light source and operations (Borowitzka,1997; Chisti, 2008; Brennan and Owende, 2010; Mutandaet al., 2011).Open systems such as coastal shallow brackish-waterponds are extensively used for feed production in aquacultureand for other industrial applications. Dunaliella spp.are widely grown in these systems. These natural ponds areeconomical in terms of their operations, but only a limitednumber of species can be grown. Other physico-chemicalparameters that affect productivity in open systems areevaporation losses, temperature fluctuations, inefficientmixing and light limitation. The evaporation losses increasethe ionic concentration in the media causing severe osmolaritychanges (Becker, 2004; Pulz, 2001; Lee, 2001). CO 2requirements are more than can be met from the naturalenvironment and thus constrain productivity. Hence carbonatesand bicarbonates are used as carbon sources. Fluegases and CO 2 can be directly used as inorganic carbon forgrowth of cells in autotrophic mode. Due to the abovementionedlimitations, the productivity of open ponds is low,and hence developing closed bioreactor systems for biomassproduction is preferred. One of the possible solutionsto prevent contamination and severe evaporation losses areto cover the ponds with a greenhouse, which limits pondsize considerably but gives a quasi-controlled environment.Pond managementSince open ponds are highly susceptible to environmentalfluctuations and contamination, certain measures areneeded to keep the cultures healthy and productive.Contamination by other algae is very common in openponds; this can be effectively managed by maintaining acritical cell concentration, preventing competing speciesgrowth. Contamination by rotifers can be controlled byreducing the culture pH, since they are sensitive to lowpH, and later re-adjusting cultures to optimum pH. Mixingis essential as accumulation of biomass in one place leadsto anaerobic decomposition and accumulation of bacteria.The most efficient way of maintaining cultures is througha batch system, with fresh unialgal inoculum as startingmaterial for every batch. An initial optimal optical densityof the culture is a key factor for health of culture andmaintenance. Since the emphasis is on the production ofbiomass with the minimum of energy inputs, it is desirableto use windmills for agitation of cultures and also usemarine forms to avoiding or minimize competition fromcontaminating organisms (Borowitzka, 2005).Closed cultivation systemsMaintenance of uni-algal culture in open ponds is verydifficult, but can be achieved in closed bioreactor systems.In closed configurations, various culture parameters canbe controlled and environment-sensitive strains growingin near-neutral pH conditions can be grown with higherproductivities in closed photobioreactors. The closed reactorsystems have high biomass productivity compared withopen ponds since culture parameters such as illumination,turbulence and air exchange can be carefully regulated(Grobbelaar, 2009, 2010).Six parameters or subsystems for photo bioreactors are,light source, optical transmission system, reaction area, gasexchange system, filtration system (removal of biomass)and sensing system. Light source is an important designconsideration, which includes variables like type of lightsource, intensity of light source, effect of light source on celldevelopment in the algal culture, and dark period requirementof the algae (Anderson, Anil and Schipull, 2002).Several of these parameters interact, such as the opticaltransmission system and gas exchange system via the mixingthat takes place in the reaction area.Three types of closed reactors are commonly employedfor mass cultivation: tubular; cylindrical or columnar; andflat plate (Lehr and Posten, 2009).Tubular bioreactors consist of an array of glass or plastictransparent tubes connected by U bends to capture moresunlight (Tredici and Materassi, 1992). They can be alignedin a flat plane or as a coil around a vertical cylindrical supportframework (Borowitzka, 1999). The tubes are generally5–10 cm in diameter. The algal cultures are circulatedin these narrow tubes using mechanical pumps or airliftsystems (Brennan and Owende, 2010). Tubular bioreactorshave high surface to volume ratio, hence light capture ishigher and gives high productivities. Spirulina platensisand Chlorella spp. have been successfully grown in thesesystems. Combined airlift-tubular systems have been usedin production of Porphyridium cruentum, Phaeodactylum