Wastewater treatment stations take advantage of the biogas produced from sludge in anaerobic digesters to generate electricity (reciprocating gas engines) and heat (cooling water and engine exhaust gases). A fraction of this electricity is used to operate the plant while the remaining is sold to the grid. Heat is almost entirely used to support the endothermic anaerobic digestion and a minimum fraction of it is rejected to the environment at a set of fan coolers. This generic description is applicable to on-design conditions. Nevertheless, the operating conditions of the plant present a large seasonal variation so it is commonly found that the fraction of heat rejected to the atmosphere increases significantly at certain times of the year. Moreover, the heat available in the exhaust gases of the reciprocating engine is at a very high temperature (around 650 ºC), which is far from the temperature at which heat is needed for the digestion of sludge (around 40 ºC in the digesters). This temperature difference offers an opportunity to introduce an intermediate system between the engines and the digesters that makes use of a fraction of the available heat to convert it into electricity. An Organic Rankine Cycle (ORC) with an appropriate working fluid is an adequate candidate for these hot/cold temperature sources. In this paper, the techno-economic effect of adding an Organic Rankine Cycle as the intermediate system of an existing wastewater treatment station is analysed. On this purpose, different working fluids and system layouts have been studied for a reference wastewater treatment station giving rise to optimal systems configurations. The proposed systems yield very promising results with regard to global efficiency and electricity production (thermodynamically and economically).

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We employ an ARDL bounds testing approach to cointegration and Unrestricted Error Correction Models to estimate the relationship between income and CO2 emissions per capita in 21 Latin American Countries (LACs) over 1960–2017. Using time series, we estimate six different specifications of the model to take into account the independent effect on CO2 emissions per capita of different factors considered as drivers of different dynamics of CO2 emissions along the development path. This approach allows to address two concerns. First, the estimation of the model controlling for different variables serves to assess if the EKC hypothesis is supported by evidence in any of the LACs considered and to evaluate if this evidence is robust to different model specifications. Second, the inclusion of control variables accounting for the effect on CO2 emissions is directed at increasing our understanding of CO2 emissions drivers in different countries. The EKC hypothesis effectively describes the long-term income-emissions relationship only in a minority of LACs and, in many cases, the effect on CO2 emissions of different factors depends on the individual country experience and on the type and quantity of environmental policies adopted. Overall, these results call for increased environmental action in the region.

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