High rate algal ponds (HRAPs) are an advanced pond that provide efficient and cost-effective wastewater treatment, as well as the ability to recover nutrients in the form of microalgal biomass. Microalgal photosynthesis, nutrient uptake and subsequent growth, coupled with aerobic bacteria degradation of organic compounds, are fundamental to the process of wastewater treatment in HRAPs, yet are often limited in these ponds and, in particular, microalgal photosynthesis is well below the reported theoretical maximum. Understanding how the physico-chemical environment affects microalgal performance is therefore critical to improved wastewater treatment and nutrient recovery, yet has been the subject to few studies to date. This research focused on the enhancement of microalgal photo-physiology, growth and nutrient removal efficiency (NRE) through modification to the physical and chemical environment in wastewater HRAPs. In this study, I first examined the seasonal dynamics of microalgal performance in full-scale wastewater HRAPs. While both retention-time corrected chlorophyll biomass and photosynthetic potential increased from winter to summer, the summer-time performance was considered to be constrained, as indicated by the decreased light absorption, light conversion efficiency and NRE. The physico-chemical environment in the full-scale HRAPs were characterised by high day-time pH, high light attenuation and long, straight channels with low turbulence. This led to questions regarding 1) effects of nutrient supply, in particular carbon and 2) the role of the HRAP light climate on microalgal performance. I addressed these questions using a series of experiments that involved either changing the nutrient concentration and its supply or by modifying the light environment, through changes in pond operational parameters including CO2 addition, influent dilution, pond depth, hydraulic retention time (HRT), mixing speed and frequency. The overall results from these experiments showed that carbon was the primary and light the secondary limiting factors of microalgal performance. These limitations negatively affected light absorption, photosynthesis, productivity and NRE. While each operational parameter tested impacted on microalgal performance, to some degree, CO2 addition had the greatest influence on light absorption, photosynthetic efficiency and productivity, while continuous mixing had the greatest effect on NRE. Adding CO2 increased light absorption by 110% and 128%, maximum rate of photosynthesis by 185% and 218% and microalgal biovolume by between 150 – 256% and 260 – 660% (species specific), when cultures were maintained at pH 8 and 6.5, respectively. Providing sufficient mixing to achieve continuous turbulence enhanced NRE by between 300 – 425% (species specific), increased biomass concentrations between 150% and 4000% (species specific) compared to intermittent and no mixing, respectively, and increased harvest-ability of colonial species. However, at present, both CO2 addition and mechanical mixing attract high capital and operational costs. Modification to these technologies would be required to meet the objectives of cost-effective wastewater treatment and biofuel production. A more immediate and cost-effective solution demonstrated in this study was the altering pond depth, influent concentration and HRT. Doubling pond depth from 200 to 400 mm increased both microalgal nutrient removal and photosynthetic efficiencies which led to areal productivity increasing by up to 200%. When increased pond depth was coupled with decreased HRT, light absorption and photosynthetic performance further increased due to decreased internal self-shading and improved pond light climate. For nutrients, high influent loads increased productivity, while moderate loads increased effluent water quality. Overall, this work demonstrated that optimising the chemical and physical environment of wastewater treatment HRAPs (CO2 addition to maintain pH at 6.5 – 7, 400 mm pond depth, continuous mixing with vertical speed of 200 mm s-1, moderate nutrient load (15- 30 g m-3) and moderate HRT (4 / 6 days summer / autumn) can enhance microalgal biomass productivity, nutrient recovery as well as improve effluent water quality, particularly during summer when growth can be constrained.

Made available in DSpace on 2015-03-26T13:28:11Z (GMT). No. of bitstreams: 1 texto completo .pdf: 1416633 bytes, checksum: 22095939bdd40881c123d814a5962c73 (MD5) Previous issue date: 2012-02-29 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The integration between wastewater treatment in high rate algal ponds (HRAPs) and microalgae culture, aiming biomass production, has been largely discussed, in order to reduce costs associated with both process. Nevertheless, for that association being feasible, it is still necessary reaching in the field the observed productivity in the laboratory scales. In many cases, this lack is due to the fact that when microalgae are exposed to high sunlight rates, sunlight saturation and photoinibition effects may influence their growth. On the other hand, solar radiation represents an important factor in the pollutants removal on HRAPs. The aim of this work was to evaluate the influence of different levels of solar radiation on the wastewater treatment efficiency of the HRAP, specifically on the removal of nutrients and indicator microorganisms, trough the use of different covers in high rate ponds fed with UASB effluent. Reductions of 22, 30 and 60% of the natural Photosynthetically Active Radiation (PAR) (1.920 μE.m-2.s-1) were assessed. A 30% reduction on the PAR resulted on greatest values of dissolved oxigen, whereas a 60% reduction resulted on smallest values of pH and temperature. Concerning nitrogen removal, the 20% reduction showed to be more efficient for total kjeldahl nitrogen and ammonium removal. The variation on PAR did not influence Phosphorus or indicator microorganisms removal. The observed productivity, from 5,3 to 7,4g.m-2.d-1, and pollutants removal efficiency (45 to 61% for TKN, 58 to 78% for NH3, 9 to 16% for total Phosphorus and from 1.5 to 2.0 removed logs for total coliforms and E.coli) support the viability of the association between algal biomass production and wastewater treatment. / A integração dos processos de tratamento de esgoto em lagoas de alta taxa (LATs) e cultivo de microalgas com vistas à produção de biomassa tem sido amplamente discutida, como forma de reduzir os custos associados a ambos. No entanto, para que tal associação seja de fato viável, ainda é necessário alcançar em campo as produtividades observadas em escala de laboratório. Muitas vezes, essa lacuna se deve ao fato de que, quando as microalgas estão expostas a altas intensidades luminosas, os efeitos de saturação luminosa e fotoinibição podem interferir no seu crescimento. Por outro lado, a radiação solar representa um importante fator na remoção de poluentes nas LATs. O objetivo deste trabalho foi avaliar a influência de diferentes níveis de radiação solar na eficiência de pós-tratamento de efluente de reator UASB das LATs, especificamente na remoção de nutrientes e microrganismos indicadores de contaminação. Através da utilização de diferentes formas de cobertura, avaliou-se reduções da ordem de 22, 30 e 60% na Radiação Fotossinteticamente Ativa (RFA) média natural, de 1.920 μE.m-2.s-1. A redução da RFA em 30% propiciou os maiores valores de oxigênio dissolvido, e a redução de 60% resultou em menores valores de pH e temperatura. No que diz respeito à remoção de nitrogênio, a redução de 20% apresentou-se mais eficiente para remoção de NTK e NH3. Os diferentes níveis de radiação avaliados apresentaram resultados semelhantes para a remoção de fósforo e microrganismos indicadores. A produtividade observada, de 5,3 a 7,4g.m-2.d-1, e as eficiências de remoção de poluentes (45 a 61% para NTK, 58 a 78% para NH3, 9 a 16% para Ptotal e de 1,5 a 2,0 logs removidos para coliformes totais e E.coli) sugerem a viabilidade da associação entre sistemas de produção de biomassa algal e tratamento de esgoto.