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Produção de biogás a partir de biomassa microalgal residual usada em tratamento de águas residuais domésticas
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Authors
Abstract(s)
A utilização de microalgas no tratamento de águas residuais representa uma alternativa
sustentável aos processos convencionais, permitindo cumprir os limites legais de descarga e
produzir energia a partir da biomassa produzida, contribuindo para a autossuficiência
energética das ETARs e para os objetivos de neutralidade energética até 2045, definidos pela
Diretiva Europeia 2024/3019.Neste contexto, o presente trabalho avaliou o potencial da
aplicação de microalgas autóctones no tratamento de águas residuais domésticas e na
valorização energética da biomassa via produção de biogás.
O estudo foi desenvolvido em parceria com a ETAR de Gaia Litoral, recorrendo a uma mistura
50:50 (v/v) de efluente após tratamento primário e secundário, em cultivos escalados até 30 L.
Durante o tratamento biológico observou-se uma elevada eficiência na remoção de nutrientes,
como de nitrogénio (N) e fósforo (PO₄³⁻). Verificaram-se elevadas eficiências de remoção para
ambos os nutrientes, sendo o melhor resultado para o nitrogénio. Assim, obteve-se uma
redução de 34,43 mg/L para 5,63 mg/L, resultando numa redução de 83%, enquanto para o
fósforo se registou uma diminuição de 5,66 mg/L para 1,01 mg/L, correspondente a cerca de
82% de remoção de fósforo. Entre as espécies de microalgas observadas, a que melhor se
adaptou e desenvolveu foi a Scenedesmus sp., reconhecida pela sua resiliência e elevada
capacidade de assimilação de nutrientes, confirmando o seu potencial como organismo-chave
para processos de biorremediação e produção de biocombustíveis. Nos primeiros ensaios de
codigestão anaeróbia da biomassa microalgal com a lamas do digestor verificaram-se
inicialmente produções de biogás reduzidas, entre os 250 e 400 NmL, este baixo rendimento
pode ser explicado pela baixa razão C/N (≈4/5) e AGV/ALC (<0,2), muito abaixo do valor
recomendo pela bibliografia. Para superar esta limitação, procedeu-se à adição de lamas mistas
(primárias e secundárias), aumentando assim a fração de matéria orgânica biodegradável e
consequentemente a razão de C/N aumentou para 8/9 e uma razão AGV/ALC estabilizou em
0,2-0,4, o que permitiu obter um aumento na produção de biogás. Esta estratégia resultou num
claro aumento do rendimento em biogás, produzindo valores superiores a 2000NmL de volume
total de biogás. Em particular, a codigestão de microalgas pré-tratadas com a mistura das lamas
originou incremento de cerca de 10% na produção total de biogás, atingindo 2320 NmL face aos
1990 NmL sem biomassa microalgal, confirmando sinergias entre substratos e validando esta
solução como uma via eficaz de valorização energética em ETAR.
The use of microalgae in wastewater treatment represents a sustainable alternative to conventional processes, allowing compliance with legal discharge limits while producing energy from the generated biomass. This contributes to the energy self-sufficiency of wastewater treatment plants (WWTPs) and supports the goals of energy neutrality by 2045, as defined by the European Directive 2024/3019. In this context, the present work evaluated the potential application of native microalgae for the treatment of domestic wastewater and for the energetic valorization of the resulting biomass through biogas production. The study was conducted in partnership with the Gaia Litoral WWTP, using a 50:50 (v/v) mixture of effluent after primary and secondary treatment, in scaled-up cultures up to 30 L. During the biological treatment, high nutrient removal efficiencies were observed, particularly for nitrogen (N) and phosphorus (PO₄³⁻). The best results were achieved for nitrogen, with concentrations decreasing from 34.43 mg/L to 5.63 mg/L, corresponding to an 83% reduction. Phosphorus concentrations dropped from 5.66 mg/L to 1.01 mg/L, representing approximately 82% removal efficiency. Among the observed microalgae species, Scenedesmus sp. showed the best adaptation and growth, known for its resilience and high nutrient assimilation capacity, confirming its potential as a key organism for bioremediation processes and biofuel production. In the initial anaerobic co-digestion tests of microalgal biomass with digester sludge, low biogas yields were recorded (between 250 and 400 NmL). This limited performance can be explained by the low C/N ratio (≈4/5) and volatile fatty acids/alkalinity ratio (AGV/ALC <0.2), which are far below the values recommended in the literature. To overcome this limitation, mixed sludge (primary and secondary) was added, thereby increasing the biodegradable organic matter fraction. Consequently, the C/N ratio rose to 8/9 and the VFA/ALK ratio stabilized between 0.2 and 0.4, leading to an improvement in biogas production. This strategy resulted in a clear increase in biogas yield, producing more than 2000 NmL of total biogas. In particular, the codigestion of pre-treated microalgae with mixed sludge led to an approximately 10% increase in total biogas production, reaching 2320 NmL compared to 1990 NmL without microalgal biomass. These results confirm the synergistic effect between substrates and validate this approach as an effective pathway for energy recovery in wastewater treatment plants.
The use of microalgae in wastewater treatment represents a sustainable alternative to conventional processes, allowing compliance with legal discharge limits while producing energy from the generated biomass. This contributes to the energy self-sufficiency of wastewater treatment plants (WWTPs) and supports the goals of energy neutrality by 2045, as defined by the European Directive 2024/3019. In this context, the present work evaluated the potential application of native microalgae for the treatment of domestic wastewater and for the energetic valorization of the resulting biomass through biogas production. The study was conducted in partnership with the Gaia Litoral WWTP, using a 50:50 (v/v) mixture of effluent after primary and secondary treatment, in scaled-up cultures up to 30 L. During the biological treatment, high nutrient removal efficiencies were observed, particularly for nitrogen (N) and phosphorus (PO₄³⁻). The best results were achieved for nitrogen, with concentrations decreasing from 34.43 mg/L to 5.63 mg/L, corresponding to an 83% reduction. Phosphorus concentrations dropped from 5.66 mg/L to 1.01 mg/L, representing approximately 82% removal efficiency. Among the observed microalgae species, Scenedesmus sp. showed the best adaptation and growth, known for its resilience and high nutrient assimilation capacity, confirming its potential as a key organism for bioremediation processes and biofuel production. In the initial anaerobic co-digestion tests of microalgal biomass with digester sludge, low biogas yields were recorded (between 250 and 400 NmL). This limited performance can be explained by the low C/N ratio (≈4/5) and volatile fatty acids/alkalinity ratio (AGV/ALC <0.2), which are far below the values recommended in the literature. To overcome this limitation, mixed sludge (primary and secondary) was added, thereby increasing the biodegradable organic matter fraction. Consequently, the C/N ratio rose to 8/9 and the VFA/ALK ratio stabilized between 0.2 and 0.4, leading to an improvement in biogas production. This strategy resulted in a clear increase in biogas yield, producing more than 2000 NmL of total biogas. In particular, the codigestion of pre-treated microalgae with mixed sludge led to an approximately 10% increase in total biogas production, reaching 2320 NmL compared to 1990 NmL without microalgal biomass. These results confirm the synergistic effect between substrates and validate this approach as an effective pathway for energy recovery in wastewater treatment plants.
Description
Keywords
Microalga Waste Water Treatment Energy Biofuels Biogas Microalgas Tratamento de Água Residual Energia Biocombustíveis Biogás