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Water hyacinth (Eichhornia crassipes) causes ecological and economic problems because it grows very fast and quickly consumes nutrients and oxygen in water bodies, affecting both the flora and fauna; besides, it can form blockages in the waterways, hindering fishing and boat use. However, this plant contains bioactive compounds that can be used to produce biofuels. This study investigated the effect of various substrates as feedstock for biogas production. A 125-l plug-flow anaerobic digester was utilized and the hydraulic retention time was 14 days; cow dung was inoculated into water hyacinth at a 2:1 mass ratio over 7 days. The maximum biogas yield, achieved using a mixture of natural water hyacinth and water (NWH-W), was 0.398 l/g volatile solids (VS). The cow dung/water (CD-W), hydrothermally pretreated water hyacinth/digestate, and hydrothermally pretreated water hyacinth/water (TWH-W) mixtures reached biogas yields of 0.239, 0.2198, and 0.115 l/g VS, respectively. The NWH-W composition was 70.57% CH4, 12.26% CO2, 1.32% H2S, and 0.65% NH3. The modified Gompertz kinetic model provided data satisfactorily compatible with the experimental one to determine the biogas production from various substrates. TWH-W and NWH-W achieved, respectively, the shortest and (6.561 days) and the longest (7.281 days) lag phase, the lowest (0.133 (l/g VS)/day) and the highest (0.446 (l/g VS)/day) biogas production rate, and the maximum and (15.719 l/g VS) and minimum (4.454 l/g VS) biogas yield potential.

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The COVID-19 pandemic has caused most waste recycling activities to be terminated due to several factors, such as concerns about the spread of coronavirus through the collected solid waste. This study investigates the socio-economic impact of the situation of the closed-loop system of solid waste recycling. Several recommendations for tackling this problem are presented in this research. Primary data collection for the waste bank and informal recycling sector was carried out in the eastern part of Surabaya during large-scale social restrictions. In-depth interviews were conducted with waste bank customers, waste bank unit representatives and the informal recycling sector to understand the pandemic’s socio-economic impact on the closed-loop system. Results show that this pandemic has significant impacts on individuals and stakeholders engaged in waste recycling activities. Customers of waste banks, who mostly belong to low-income communities, mentioned that the waste bank closure gave rise to social and economic problems, such as increasing unmanaged solid waste and decreasing income. This result also applied to the informal recycling sector. The government can use the recommendations in this study to generate related policies, such as enforcing the health protocol within solid waste management to keep the recycling system in place and the business alive.