Evaluating energy balance and environmental footprint of sludge management in BRICS countries

Abstract

Climate change is driving global endeavours to achieve carbon neutrality and renewable energy expansion. Sludge, a nutrient-rich waste, holds energy potential yet poses environmental challenges that need proper management. We conducted a comprehensive life cycle assessment to evaluate the energy balance and environmental footprint of the most commonly used sludge management scenarios in BRICS countries, namely Brazil, Russia, India, China, and South Africa. Technologies such as incineration and anaerobic digestion with energy recovery units (i.e., cogeneration unit) maximize energy balance and minimize the environmental footprint, with incineration showing a superior performance. Shifting sludge management scenarios from the worst to the best can boost energy production by 1.4-98.4 times and cut the environmental footprint by 1.5-21.4 times. In 2050, these improvements could lead to a 98-fold boost in energy generation and a 25-fold drop in carbon emissions, according to the Announced Pledges Scenarios. Optimizing parameters such as volatile solids and anaerobic digestion efficiency further boosts energy output and minimizes the environmental footprint. This study offers robust evidence to support sustainable sludge management and thus promote energy recovery and carbon neutrality goals, guide technological transitions, and inform policymaking for sustainable development.

Keywords: BRICS countries; Carbon neutrality; Energy balance; Environmental impacts; Life cycle assessment; Sludge management.

Graphical abstract

Fig. 1

Population size and annual biosolids (dry sludge) production of the BRICS countries (Brazil, Russia, India, China, and South Africa, sourced from http://infobrics.org/). The world share indicates each BRICS country's proportion of the global population.

Fig. 2

Mainstream sludge treatment and disposal processes used in the BRICS countries. For each country, three mainstream sludge management scenarios were considered and labelled accordingly. For instance, BS1, BS2, and BS3 represent three different sludge management scenarios that are commonly used in Brazil (sourced from a governmental report, see Methods). For each sludge management scenario, the dots are connected sequentially from top to bottom, representing the sequence of steps in the process. UASB represents the up-flow anaerobic sludge blanket.

Fig. 3

Net energy balance and total environmental impact of sludge treatment and disposal scenarios in BRICS countries. The total environmental impact of the sludge management scenarios was calculated using the sum of the normalized impact categories (see Methods). Negative total environmental impact values denote environmental benefits, while positive values represent environmental burdens. For the net energy balance, positive values indicate energy production, whereas negative values represent energy consumption. BS1–3, RS1–3, IS1–3, CS1–3, and SS1–3 denote the selected sludge management scenarios in Brazil, Russia, India, China, and South Africa, respectively. The specific processes and mass flow of each sludge management scenario are outlined in Figs. 2, S1–S5. The total environmental impact and net energy balance were calculated based on the functional unit of 1 ton of dry sludge solids.

Fig. 4

Comparison of the environmental impacts in various categories between sludge management scenarios used in BRICS countries. The results for environmental impact categories of the sludge management scenarios used in Brazil (a), Russia (b), India (c), China (d), and South Africa (e). BS1–3, RS1–3, IS1–3, CS1–3, and SS1–3 denote the sludge management scenarios adopted in the BRICS countries; the detailed processes are outlined in Fig. 2.

Fig. 5

Energy balance and carbon emission projections for the best and worst sludge management scenarios in each BRICS country for the years 2023 and 2050 (according to the Announced Pledges Scenarios: see Methods). The green and purple values represent the differences between the best and worst sludge management scenarios in terms of the net energy balance and carbon emission, respectively, across the BRICS countries in the indicated years (for detailed calculations, see Methods). Positive net energy balance values represent energy production in the sludge management scenarios, whereas negative values represent energy consumption. Negative carbon emission values represent environmental benefits, whereas positive values represent environmental burdens. Notably, the projections of carbon emissions for 2050 are based on the predicted electricity generation sources in BRICS countries as derived from the International Energy Agency.

Fig. 6

Sensitivity analysis of the optimal sludge management scenarios used in BRICS countries. (a) India, IS1 (Thickening–Anaerobic digestion–Cogeneration unit–Dewatering–Land application). (b) South Africa, SS2 (Thickening–Dewatering–Thermal drying–Incineration–Landfill). GWP and NEB represent the global warming potential (kg CO₂ eq) and net energy balance (MJ), respectively. Regarding India, sensitivity analysis was used to explore parameters such as the volatile solids content (range: 50 %–80 %), anaerobic digestion efficiency (range: 40 %–70 %), and electricity sources (coal, hydropower, wind, and solar). Regarding South Africa, sensitivity analysis was used to explore parameters such as the volatile solids content (50 %–80 %) and electricity source (coal, wind, solar, and nuclear). The global warming potential is displayed in the upper left (Fig. 6a; India) and upper right (Fig. 6b; South Africa) halves of the circles. The lower left (Fig. 6a; India) and lower right (Fig. 6b; South Africa) halves of the circles illustrate the results of sensitivity analysis of the net energy balance.