Chemical exergy as a unified and objective indicator in the assessment and optimization of land treatment systems

The thermodynamic concept of chemical exergy is introduced in land treatment systems (LTS) in order to develop unified and objective indicators for the assessment of plant species, in terms of biomass production, treatment efficiency, and potential impacts to the environment arising from specific pollutants in the soil. For the purposes of this study, data from a three-year-field trial of four different LTS (planted with different plant species: Acacia cyanophylla syn: A. saligna, Eucalyptus camaldulensis, Populus nigra, and Arundo donax) were collected and analysed. All species received partially treated domestic wastewater based on their water requirements. Results showed that chemical exergy could be used as a common platform in the assessment of LTS. With this concept, plant biomass, applied effluent, and changes in soil properties due to effluent application, were assessed using a single exergetic indicator. It was found that chemical exergy approach presented in this study provides a unified and objective assessment of the effluent quality and the changes in the properties of soil, since the calculations were based on the concentration and the chemical exergy of all parameters considered, in accordance to the laws of thermodynamics and without the need of using subjective weighted factors for each parameter. This is an important advantage, compared to the other holistic approaches, which allows consistent and overall comparisons among different plant species as well as comparisons between the results obtained and the existing water quality standards. Furthermore, insights into the efficiency with which plant species used the applied wastewater were also gained through the chemical exergy use efficiency (CEUE), the calculation of which was based on the chemical exergy additions due to effluent application and the chemical exergy stored in plant biomass. Overall, the results obtained in this study suggested that the chemical exergy concept would help in the development of unified and objective indicators for the successful selection of plant species in LTS. It appears that chemical exergy and exergy as a whole can offer more than just a simple description of LTS, in fact, variations in the exergy flow within these systems may provide a much better understanding of LTS in the scope of a broader theoretical framework. However, at the present time, the exergy concept is still not applicable in the assessment and optimization of LTS. To increase its acceptance further research is needed, especially in the area of environmental impacts.