Avaliação do impacto do ciclo de vida e avaliação do custo do ciclo de vida para estações de tratamento de águas residuais centralizadas e descentralizadas na Tailândia

Scientific Reports volume 12, Número do artigo: 14540 (2022) Citar este artigo

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Esta pesquisa investiga a relação custo-benefício de quatro cenários de tratamento de lodo para estações de tratamento de águas residuais (ETAR) centralizadas (C) e descentralizadas (D) usando avaliação de custo do ciclo de vida (LCCA). Os impactos e custos ambientais são quantificados pelo Stepwise2006. A opção de construção de WWTP ambientalmente e financeiramente mais viável para Bangkok, Tailândia (2022–2031) é determinada em termos de LCCA e valor presente líquido (VPL). Os custos ambientais dos cenários de tratamento D são menores do que os dos cenários de tratamento C. Os custos ambientais totais dos cenários de fertilizantes C e D são menores do que os dos cenários de desidratação C e D. O fluxo de caixa líquido por unidade funcional das ETAR-C é superior ao das ETAR-D. O cenário do fertilizante C é o cenário de tratamento ambientalmente e economicamente mais viável devido ao menor déficit de LCCA (-5,58 THB2020 por m3 de efluente tratado). A compostagem deve, portanto, ser adotada para tratar o lodo. A opção de construção de WWTP ambientalmente e financeiramente mais viável é a opção I (construir quatro C-WWTPs em 10 anos) devido ao menor déficit LCCA (-19925 milhões de THB2020) e menor perda financeira (NPV = -6309,96 milhões de THB2020). Essencialmente, a administração local da capital deve adotar a opção I como diretriz na formulação da política de gestão de tratamento de águas residuais de 2022–2031.

O rápido crescimento populacional e a urbanização contribuem para aumentar a demanda por coleta e tratamento de águas residuais. Em áreas urbanizadas, as águas residuais domésticas são coletadas e tratadas em uma estação de tratamento de águas residuais (ETAR) centralizada (C) ou descentralizada (D). A gestão de águas residuais C normalmente envolve extensas redes de esgoto, sistema de coleta de águas residuais complexo e eficiente, tecnologia de tratamento padrão e alta eficiência de tratamento. Enquanto isso, na gestão de águas residuais D, as águas residuais domésticas são coletadas e tratadas próximo à fonte usando subsistemas modulares, tornando desnecessária a construção de redes de esgoto complexas, o que, por sua vez, aumenta a flexibilidade do sistema1.

Vários fatores influenciam a decisão de investimento entre os sistemas de gestão de águas residuais C e D, por exemplo, abastecimento de rede de esgoto, oportunidade de uso da terra, disponibilidade de pessoal qualificado e capacidade financeira e técnica2. Como resultado, em muitos países em desenvolvimento, dada a restrição financeira, a gestão de águas residuais D é considerada uma alternativa economicamente viável à gestão de águas residuais C.

Os custos de construção e operação dos sistemas de tratamento de águas residuais D variam muito, dependendo do número e do layout dos subsistemas modulares. Além disso, o custo total do sistema de tratamento D equipado com grandes subsistemas modulares é geralmente menor do que o do sistema de tratamento de esgoto C, devido às menores necessidades de operação e manutenção do sistema de tratamento D. Além disso, os subsistemas modulares D bem projetados possuem uma vantagem de custo sobre o gerenciamento de águas residuais C3.

Life cycle thinking focuses on the environmental and socio-economic impacts of a product or service through the entire lifecycle (2022)." href="/articles/s41598-022-18852-y#ref-CR4" id="ref-link-section-d75446236e369"> 4. A avaliação do ciclo de vida (LCA) normalmente se concentra nos impactos ambientais, por exemplo, toxicidade humana, ecotoxicidade, aquecimento global, eutrofização e esgotamento de recursos, consistindo em quatro etapas: (1) definição do limite do sistema, unidade funcional e suposições, (2) vida inventário de ciclo (LCI), (3) avaliação de impacto do ciclo de vida (LCIA) e (4) interpretação5,6. Para o impacto econômico, o custo do ciclo de vida (LCC) leva em consideração o fluxo de caixa líquido, ou seja, fontes de receitas e despesas, enquanto a avaliação do custo do ciclo de vida (LCCA) leva em consideração o LCC e os custos ambientais7.

Os estudos de LCA existentes que incorporam o conceito de custo do ciclo de vida (LCC) estão listados na Tabela 1. Essencialmente, os estudos existentes se concentram principalmente nos sistemas centralizados de tratamento de águas residuais, por exemplo, Awad et al.8, Tabesh et al.9, Polruang et al .10, Bertanza et al.11. Enquanto isso, Lorenzo-Toja et al.12, Lorenzo-Toja et al.13 investigaram os sistemas de tratamento de águas residuais C e D em termos de LCA e LCC.

The 2016–2017 average inventory data of the centralized (i.e., C-dewatering and C-fertilizer) and decentralized sludge treatment scenarios (D-dewatering and D-fertilizer) are also respectively provided in Fig. 2 and Table SI-1 of Supplementary Information (SI) (2018)." href="/articles/s41598-022-18852-y#ref-CR24" id="ref-link-section-d75446236e1529"24. In the analysis, this study focuses on the existing eight centralized WWTPs and seven (out of 12) decentralized WWTPs due to either the temporary closure of the remaining decentralized WWTPs for renovation or a lack of data. The average capacity of the centralized and decentralized WWTPs are 139,000 and 2357 m3 per day, respectively. The useful life of the WWTP and sewer network systems are assumed to 30 years./p>

In the decomposition, 70% sludge and 30% organic matter are composted by the windrow method to improve the quality of compost (2018)." href="/articles/s41598-022-18852-y#ref-CR24" id="ref-link-section-d75446236e1562"24. According to Seleiman et al28, sludge contains 25.77, 12.98, and 3.40 g of nitrogen, phosphorus, and potassium per kg dry matter./p>

The environmental costs are determined by Stepwise monetary weighting factors that detail in Table SI-3 of SI29,30 and converted into the year 2020 Thai currency (THB2020) (2020)." href="/articles/s41598-022-18852-y#ref-CR31" id="ref-link-section-d75446236e1594"31 using purchasing power parity (PPP) (i.e., PPPUS$2002 and PPPTHB2002) and Thailand's gross domestic product (GDP) deflator index of 2002 and 2020. The details of currency conversion are provided in Table SI-4 of SI./p>

In the LCCA, the source of revenue (or cash inflow) is the sale of decomposed sludge fertilizer which is priced at 2 THB/kg. For the expenditures (or cash outflow), the construction costs, including the costs of collection system, treatment plant, and dewatering system, are gleaned from publicly available data and prior publications34,35,36. The operation and maintenance (O&M) costs include the costs of electricity, water supply, chemical reagents, sludge treatment, and administrative overheads, e.g., wage, management fee (Department of Drainage and Sewerage (2018)." href="/articles/s41598-022-18852-y#ref-CR24" id="ref-link-section-d75446236e1638"24./p>

The construction and O&M costs are converted into the 2020 Thai baht (THB2020) based on the purchasing power parity (PPP) and gross domestic product (GDP) deflator index (2020)." href="/articles/s41598-022-18852-y#ref-CR31" id="ref-link-section-d75446236e1647"31. The PPP and GDP deflator index are used to reconcile differences between the three currencies (US$, EUR and Thai Baht) and multiple time periods./p>

The current total capacity of the centralized and decentralized WWTPs in the capital Bangkok is 1,112,000 and 25,000 m3 per day, respectively. The new centralized WWTP in Minburi district is currently under construction and expected to be complete in 2022, with the maximum wastewater treatment capacity of 10,000 m3 per day. In 2021, all the existing WWTPs combined are capable of treating only 68.33% of Bangkok's municipal wastewater, given the per-capita daily wastewater generation of 0.2 m3 (2017)." href="/articles/s41598-022-18852-y#ref-CR37" id="ref-link-section-d75446236e1670"37 and the population of 8.39 million38./p>

By 2027, the population of Thailand's capital Bangkok is projected to be 8.48 million, with the wastewater generation of around 1.70 million m3 per day. According to Department of Drainage and Sewerage (2018)." href="/articles/s41598-022-18852-y#ref-CR24" id="ref-link-section-d75446236e1683">24,Japan International Cooperation Agency34, it takes two years to construct a centralized WWTP at the cost of 3358.27 million THB2020; and one year for a decentralized WWTP at the cost of 118.95 million THB2020. An annual budget of around 4500 million THB2020 is set aside for the construction of new WWTPs (Department of Drainage and Sewerage (2018)." href="/articles/s41598-022-18852-y#ref-CR24" id="ref-link-section-d75446236e1698"24./p>

In finance, net present value (NPV) is used in capital budgeting and investment planning to determine the profitability of an investment project. Mathematically, NPV is the present value of the future cash flows, discounted at the required rate of return, minus the initial investment. In this research, the discount rate or required rate of return is 10%, given that the discount rate of public infrastructure projects in developing countries is around 10%39. For the planned WWTPs to be constructed in the capital Bangkok, the sources of revenue are fee from wastewater treatment and sale of decomposed sludge fertilizer, while the expenditures include the O&M and environmental costs, excluding the construction cost since the WWTPs are public infrastructure projects funded from state coffers. The wastewater treatment fee is 2 THB2020 per m3 wastewater (2020)." href="/articles/s41598-022-18852-y#ref-CR40" id="ref-link-section-d75446236e1716"40. This study also assumes that the BMA could collect 80% of the treated wastewater fee./p>

Table 2 shows the contribution analysis results in terms of the environmental impacts of the four sludge treatment scenarios (C-dewatering, C-fertilizer, D-dewatering, D-fertilizer). Under all treatment scenarios, electricity consumption contributes negatively to almost all environmental impact categories, except for human toxicity (non-carcinogens), aquatic ecotoxicity, and aquatic eutrophication. Human toxicity (non-carcinogens) and aquatic ecotoxicity are inversely correlated to heavy metals in sludge, while aquatic eutrophication is inversely correlated to effluent quality. Electricity consumption of C-dewatering and C-fertilizer is the main contributor of mineral extraction, while the main contributor of mineral extraction of D-dewatering and D-fertilizer is tap water consumption. The mechanical aeration is responsible for the lion's share of the electricity cost in wastewater treatment10,16,32. The electricity consumption of the centralized treatment scenarios (0.873 kWh/m3 treated wastewater) is greater than the decentralized treatment scenarios (0.363 kWh/m3 treated wastewater). The average electricity consumption of 22 WWTPs in Spain (0.36 kWh/m3 treated wastewater)12 is lower that both centralized and decentralized treatment scenarios of this study. In comparison with Arashiro et al.21, the electricity consumption and sludge of the decentralized treatment in this study is lower. All of the environmental impacts, excluding aquatic eutrophication, of the centralized treatment scenarios are higher than the decentralized treatment scenarios. The aquatic eutrophication of the centralized treatment scenarios is lower than the decentralized treatment scenarios. This is attributable to lower total phosphorus in the effluent of the centralized treatment scenarios (0.73 g total P per m3 treated wastewater), compared to that of the decentralized treatment scenarios (1.52 g total P per m3 treated wastewater). In comparison with dewatering, sludge decomposition (i.e., for fertilizer) generates lower environmental impacts. According to Seleiman et al.28,Kominko et al.41, sludge is rich in nutrients that are beneficial for crop growth without contaminating groundwater and agriculture produce. However, in this current research, the heavy metals in sludge fertilizer, including copper, cadmium and mercury, exceed the regulatory limits on organic fertilizer standards42. To minimize food-related toxicity in human, the authorities thus stipulate that sludge fertilizers should be used in ornamental plants (Department of Drainage and Sewerage (2018)." href="/articles/s41598-022-18852-y#ref-CR24" id="ref-link-section-d75446236e1795"24./p>

The construction costs of the existing decentralized treatment scenarios are higher than the centralized treatment scenarios since most of the existing decentralized WWTPs in Thailand were constructed more than three decades and have treated wastewater using energy-inefficient technology, e.g., mechanical aerations46. The decentralized treatment scenarios are classified by the demand for electricity as the small general service and the centralized treatment scenarios as the large general service (2018)." href="/articles/s41598-022-18852-y#ref-CR24" id="ref-link-section-d75446236e2686">24. The electricity cost (THB per kWh) of the small general service (or the decentralized treatment scenarios) of 1.21 THB2020 per m3 treated effluent was higher than that of the large general service (or the centralized treatment scenarios) of 0.70 THB2020 per m3 treated effluent (2021)." href="/articles/s41598-022-18852-y#ref-CR47" id="ref-link-section-d75446236e2699"47. The administrative overheads, e.g., wage, management fee, of the decentralized treatment scenarios (6.33 THB2020 per m3 treated effluent) are higher than the centralized treatment scenarios (1.46 THB2020 per m3 treated effluent)./p> (2022)./p>

(2018)./p> (2020)./p> (2017)./p> (2020)./p> (2021)./p>