Synechococcus PCC 7002 growth in anaerobic digestion effluent and off-gas

Beyl, Talíta (2018-12)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Anaerobic digestion (AD) effluent is a low-cost and nutrient-rich medium produced at significant volumes, on which algae can be grown. The biogas that is produced by AD is commonly burnt, for heating or electricity generation. The off-gas from this process can be used to enrich the gas fed to an algal cultivation system, to enhance inorganic carbon supply to the growing photoautotrophic species. Therefore, a significant opportunity exists for the integration of the wastes from AD (the nutrient rich effluent, and the carbon dioxide (CO₂) rich off-gas) with the production of microalgae. The potential of using photosynthetic microalgae to produce valuable products is of interest currently. Not only does increasing dependence on non-renewable energy sources jeopardize sustainable supply but increasing CO2 emissions also promote climate change. In addition, rising global energy demands of developing nations threaten the availability of sustainable energy for future generations. Since genetically modified strains on microalgae are available, integrating their growth with AD, which will produce nutrients and inorganic carbon, could support the cause for commercial sustainable chemical production. In this study, AD effluent was treated with a cyanobacterial species, Synechococcus PCC 7002, to reduce turbidity and high concentrations of nutrients present. It was illustrated that by lowering the pH, increasing the total dissolved salts and adding a specific macro-nutrient necessary for growth to the AD effluent, the addition of valuable resources (such as water for effluent dilution and CO2 enriched gas for enhanced growth) can be avoided. This ultimately increases the feasibility of AD-microalgae integrated systems. The study organism was grown in 150 mL batch reactors in modified AD effluent at moderate light conditions to biomass concentrations of up to 1550 mg/L within 310 hours – nearly 3 times faster than in synthetic growing media. By enriching the headspace gas with carbon dioxide to simulate conditions of burnt biogas generated from anaerobic digestion, this biomass concentration was increased to 2230 mg/L within 282 hours. The experiment was repeated, comparing the growth in synthetic media and the optimised AD media in an eight-litre air-lift reactor. Keeping conditions conservative by performing the experiments in low light conditions with air bubbling instead of carbon dioxide enriched air (at a gas hold up of 10% of the total volume of the reactor), the experiment illustrated what growth could be expected at large scale production with minimal costly enhancements. Although both experiments were nutrient limited, higher biomass yields were observed for cyanobacteria grown in AD media (μmax = 0.018 hr-1). For batch experiments in 85% AD effluent enriched with inorganic carbon, total nitrogen was reduced by 90% and in the airlift reactor with atmospheric air bubbling, by 60% within 10 days.

AFRIKAANSE OPSOMMING: Anaërobies verteerde afvalwater kan teen lae kostes in groot volumes vervaardig word en is ryk in voedingstowwe wat noodsaaklik is vir die groei van alge. Die biogas was geproduseer word tydens anaërobiese vertering (AV) kan gebruik word om die gas wat in alge-kweek-sisteme ingevoer word te verryk met koolstofdioksied. Gevolglik is die absorbsie daarvan deur foto-outotrofiese spesies verhoog. Daar bestaan dus ‘n merkwardige geleentheid om the afval van AV (‘n voedingstof-ryke vloeistof en ‘n koolstof-ryke gas) te integreer met groot skaalse produksie van mikroalge. Die potentiële voordele om so ‘n sisteem in plek te stel vir die produksie van waardevolle chemikalieë wat elders die vervaardig sou word is van kardinale belang. Toenemende afhanklikheid op nie-hernubare energiebronne plaas nie net druk op nationale ekonomieë nie, maar die toenemende koolstof-uitlaatgasse bevorder wêreldwye klimaatsveranderinge. Die stygende aanvraag vir energie in ontwikkelende lande plaas ook druk op vermoëns vir die volhoubare produksie van energie vir toekomstige generasies. Siende dat ‘n geneties gemodifiseerde spesies mikroalge ontwikkel is kan die groei daarvan met AV gekombineer word om die kommersiële produksie van biobrandstowwe te bevorder. In hierdie studie was AV avfalwater behandel met ‘n sianobakteriële spesie genaamd Synechococcus PCC 7002 om wolkerigheid en hoë konsentrasies van kontaminante te verlaag. Die byvoeging van waardevolle hulpbronne (soos water vir verwatering en koolstofdioksied-verrykte gas vir groei bevordering) kan tot ‘n mate vermy word deur eksperiment bedryfde pH te verlaag, opeloste sout konsentrasies te verhoog en sekere makro-voedingstowwe noodsaaklik vir mikoralge groei by te voeg. Dit bevorder die lewensvatbaarheid van AV-mikroalg-integreerde sisteme. Die organisme was in 150 mL bondel reaktors in gemodifiseerde AV afvalwater teen middelmatige lig-blootstelling gegroei tot en met biomassa konsentrasies van 1550 mg/L in 310 ure. Die groeispoed is byne 3 keer vinniger as wat waargeneem was vir dieselfde eksperiment wat in sintetiese media uitgevoer is. Deur die hoofruimte-gas te verryk met koolstofdioksied (om verbrande biogas te simuleer wat deur AV geproduseer kan word), was die biomassa verhoog tot 2230 mg/L in 282 ure. Die eksperiment was herhaal in ‘n 8 L lugbrug fotobioreaktor. Kondisies vir groei was in lae lig met atmosferiese lug-borreling in plaas van koolstof-verrykte gas om die konserwatiwiteit van resultate te bevorder. Dit is belangrik aangesien dit die geloofwaardigheid van grootskaalse produksie beter verteenwoordig, waar groeikondisies uitgelewer is aan natuurlike elemente en dikwels sub-optimaal is. Ten speite van die feit dat albei eksperimente se groei beperk was, is hoër biomassa opbrengste waargeneem vir sianobakterië wat in AV afvalwater gegroei is (μmax = 0.018 hr-1). Vir bondel eksperimente in 85% AV afvalwater wat met 10 % (v/v) koolstof-verrykte gas gegroei is was totale stikstof met 90% verlaag en in die lugbrug fotobioreaktor met atmosferiese lug borreling tot 60%, beide binne 10 dae.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105112
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