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Produção de estruvite a partir das escorrências da desidratação de lamas de digestão anaeróbia
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Authors
Abstract(s)
A crescente escassez de recursos como o fósforo e o azoto exige soluções sustentáveis e
circulares no tratamento de águas residuais. Este trabalho teve como objetivo a recuperação
de estruvite (MgNH₄PO₄·6H₂O) a partir das escorrências geradas na desidratação de lamas da
digestão anaeróbia, com foco na ETAR de Gaia Litoral, operada pela SIMDOURO. Foi feita a
verificação das condições de formação de estruvite em escala laboratorial, com base num
estudo anterior, seguindo-se o aumento para uma escala semi-piloto. Finalmente, fez-se o
dimensionamento do equipamento necessário à produção de estruvite e uma análise
económica do projeto.
Inicialmente, caracterizou-se o efluente em termos de teor de ortofosfatos, azoto amoniacal e
pH. Posteriormente, foram realizados ensaios à temperatura de 20 °C e pH 9, variando a razão
molar Mg:P (1:1, 1,5:1, 2:1), com a adição de MgCl₂·6H₂O. Após a validação na escala de 0,5 L,
o processo foi testado para volumes crescentes de 2, 5, 8, 14 e 27,5 L. Foram ainda testados
dois tratamentos prévios, filtração e decantação. Finalmente, foi avaliada a pureza da estruvite
obtida em cada ensaio. Os resultados demonstraram eficiências de remoção de ortofosfato
superiores a 93%, com ligeira redução de eficiência nos ensaios realizados com volumes maiores,
podendo dever-se a problemas de agitação. Verificou-se um aumento proporcional na massa
de estruvite obtida com o aumento do volume de teste. Adicionalmente, realizaram-se ensaios
comparativos entre amostras de escorrências com pré-filtração e pré-decantação. A préfiltração
promoveu um aumento da pureza da estruvite (95%) apesar de uma diminuição da
quantidade total de estruvite recuperada.
Para aplicação industrial, dimensionou-se um sistema de reatores para operação em batch, com
dois tanques de 200 m³, capazes de processar 360 m³/dia de escorrências, e produzindo 24
kg/dia de estruvite. Foram também dimensionados um sistema de filtração prévio aos reatores,
um secador para secar o sólido obtido e todos os restantes equipamentos como tanques para
reagentes, bombas e válvulas.
A análise económica, contudo, indicou a inviabilidade financeira: custos operacionais anuais de
entre 91257 e 98040 € em reagentes e energia, que superaram as receitas com a
comercialização da estruvite, 3662 € por ano. No entanto, benefícios como a prevenção de
incrustações anuais, redução de riscos de eutrofização ambiental e alinhamento com diretivas
europeias, reforçam o valor estratégico deste processo.
Conclui-se que a recuperação de estruvite é tecnicamente viável e ambientalmente relevante,
mas a sua viabilidade económica exige reduções nos custos com os reagentes, a utilização de
fontes alternativas de magnésio, integração de energias renováveis e políticas de incentivo.
The growing scarcity of resources such as phosphorus and nitrogen requires sustainable and circular solutions in wastewater treatment. This work aimed to recover struvite (MgNH₄PO₄·6H₂O) from the runoff generated in the dewatering of anaerobic digestion sludge, focusing on the Gaia Litoral WWTP, operated by SIMDOURO. The conditions for struvite formation were verified on a laboratory scale, based on a previous study, followed by an increase to a semi-pilot scale. Finally, the equipment necessary for struvite production was dimensioned and an economic analysis of the project was carried out. Initially, the effluent was characterised in terms of orthophosphate content, ammoniacal nitrogen and pH. Subsequently, tests were carried out at a temperature of 20 °C and pH 9, varying the Mg:P molar ratio (1:1, 1.5:1, 2:1), with the addition of MgCl₂·6H₂O. After validation on a 0.5 L scale, the process was tested for increasing volumes of 2, 5, 8, 14 and 27.5 L. Two preliminary treatments, filtration and decantation, were also tested. Finally, the purity of the struvite obtained in each test was evaluated. The results showed orthophosphate removal efficiencies greater than 93%, with a slight reduction in efficiency in the tests performed with larger volumes, which may be due to agitation problems. A proportional increase in the mass of struvite obtained was observed with the increase in test volume. In addition, comparative tests were carried out between runoff samples with pre-filtration and pre-decantation. Prefiltration promoted an increase in struvite purity (95%) despite a decrease in the total amount of struvite recovered. For industrial application, a batch reactor system was designed with two 200 m³ tanks, capable of processing 360 m³/day of leachate and producing 24 kg/day of struvite. A filtration system prior to the reactors, a dryer to dry the solid obtained, and all other equipment such as reagent tanks, pumps, and valves were also designed. However, the economic analysis indicated financial unfeasibility: annual operating costs of between 91257 € and 98040 € in reagents and energy, exceeded the revenue from the sale of struvite, 3662 € per year. Nevertheless, benefits such as the prevention of pipe scaling, reduction of environmental eutrophication risks and alignment with European directives reinforce the strategic value of this process. It is concluded that struvite recovery is technically feasible and environmentally relevant, but its economic viability requires reductions in reagent costs, the use of alternative sources of magnesium, integration of renewable energies and incentive policies.
The growing scarcity of resources such as phosphorus and nitrogen requires sustainable and circular solutions in wastewater treatment. This work aimed to recover struvite (MgNH₄PO₄·6H₂O) from the runoff generated in the dewatering of anaerobic digestion sludge, focusing on the Gaia Litoral WWTP, operated by SIMDOURO. The conditions for struvite formation were verified on a laboratory scale, based on a previous study, followed by an increase to a semi-pilot scale. Finally, the equipment necessary for struvite production was dimensioned and an economic analysis of the project was carried out. Initially, the effluent was characterised in terms of orthophosphate content, ammoniacal nitrogen and pH. Subsequently, tests were carried out at a temperature of 20 °C and pH 9, varying the Mg:P molar ratio (1:1, 1.5:1, 2:1), with the addition of MgCl₂·6H₂O. After validation on a 0.5 L scale, the process was tested for increasing volumes of 2, 5, 8, 14 and 27.5 L. Two preliminary treatments, filtration and decantation, were also tested. Finally, the purity of the struvite obtained in each test was evaluated. The results showed orthophosphate removal efficiencies greater than 93%, with a slight reduction in efficiency in the tests performed with larger volumes, which may be due to agitation problems. A proportional increase in the mass of struvite obtained was observed with the increase in test volume. In addition, comparative tests were carried out between runoff samples with pre-filtration and pre-decantation. Prefiltration promoted an increase in struvite purity (95%) despite a decrease in the total amount of struvite recovered. For industrial application, a batch reactor system was designed with two 200 m³ tanks, capable of processing 360 m³/day of leachate and producing 24 kg/day of struvite. A filtration system prior to the reactors, a dryer to dry the solid obtained, and all other equipment such as reagent tanks, pumps, and valves were also designed. However, the economic analysis indicated financial unfeasibility: annual operating costs of between 91257 € and 98040 € in reagents and energy, exceeded the revenue from the sale of struvite, 3662 € per year. Nevertheless, benefits such as the prevention of pipe scaling, reduction of environmental eutrophication risks and alignment with European directives reinforce the strategic value of this process. It is concluded that struvite recovery is technically feasible and environmentally relevant, but its economic viability requires reductions in reagent costs, the use of alternative sources of magnesium, integration of renewable energies and incentive policies.
Description
Keywords
Phosphorus recovery precipitation semi-pilot scale struvite wastewater Águas residuais Escala semi-piloto Estruvite ETAR Precipitação Recuperação de fósforo