**1. Introduction**

Environmental performance indicators are considered to assist decision-making processes in managing important environmental, social and financial aspects and perspectives and improving the assessment of the impact caused by business activities [1–5]. Improving environmental performance requires effective control of the activities, products and processes of the business that may trigger a significant burden [6–8]. Improving the business performance can be achieved through a wide spectrum of modifications in corporate activities, products or processes and can range from small fragmentary changes to integrated environmental management.

Changes in this direction include, among others, inter-alias environmental education of workers and stakeholders, informing and/or sensitizing customers and suppliers, investing in environmental protection technologies, adopting optimally available techniques to minimize gaseous pollutants, etc. A significant process is the so called "Design for Environment" (DfE), i.e., the ecological design of products and services and the creation of environmental reports (corporate environmental reporting), as well as the installation of environmental managemen<sup>t</sup> systems [9–12].

Environmental performance appraisal is an internal process of business and is essentially a tool designed to provide managemen<sup>t</sup> with reliable and verifiable information on an ongoing basis to determine whether an enterprise's environmental performance meets the criteria set by the organization's managemen<sup>t</sup> [13–15]. The literature presents several available environmental indicators that are used at different scales of business activities, namely international, national or local. As evaluation methods vary, environmental performance indicators, as well as the concept of sustainability, also differ and include diverse groups of indicators [16,17]. In particular, indicators related to specific environmental consequences (e.g., climate change) have been developed, using resources (e.g., water footprint) with ecological efficiency measures [18].

There are different approaches to measuring environmental performance, namely production, control, ecological, accounting, economics and quality [19]. These approaches have different guidelines, focus and measurements. It is obvious that performance measurement activities vary in different countries and also among different industries, due to the variety of environmental issues, organizational variables (e.g., the size of the organization or the way it is structured), national conditions and individual corporate strategies [20].

According to the definition provided by the Organization for Economic Co-operation and Development (OECD), an environmental indicator is characterized as "a parameter that describes the state of a phenomenon/environment/area, with significance that extends beyond that directly related to a parameter value" [21]. Therefore, an indicator needs to provide important information about the parameter to be described. If a parameter is complex, such as sustainability, more than one indicator may be required. On the other hand, a group of various indicators may be combined to produce a single indicator, i.e., a Composite Sustainable Development Index (CSDI), if desired. The Environmental Sustainability Index (ESI), the Dow Jones Sustainability Index (DJSI) and the Global Reporting Initiative (GRI) are examples of CSDI. Furthermore, there are various environmental performance indicators, but also specific indicators focused mainly on environmental impacts, such as climate change [22], air pollutants [23] and ad-hoc application of systems of indicators as decision-support tools towards sustainable urban development [24].

The indicators that best describe environmental performance can be divided into the following groups [25]: (a) lagging indicators, which are measures at the end of a process, such as the amount of emitted pollutants; (b) leading indicators, which are performance measurement measures, i.e., they measure the implementation of practices or measures that are expected to lead to improved environmental performance, such as the percentage of facilities that carry out self-monitoring; (c) Environmental Condition Indicators (ECIs) measure the direct impact of an activity on the environment, such as air, water, groundwater and soil concentrations, changes in the size of a population of a particular species in a given area. Each group of indicators has discrete strengths and weaknesses, aiming a different target audience, and this is the reason why many companies use a mix of indicators to measure their environmental performance [25].

In addition to the use of indicators, progress on environmental issues can be assessed by comparative evaluation between companies or by the average performance of the industry to which a company belongs. The International Organization for Standardization (ISO) has compiled a list of conditions that a marker must meet to be useful and relevant to the measurement of environmental performance. According to this list, an indicator must be: (i) in accordance with the environmental policy and the important environmental aspects; (ii) suitable for management, business or environmental activities; (iii) useful and representative of the environmental performance criteria; (iv) understandable to internal and external stakeholders; (v) easily accessible, measurable and informative; (vi) adequate in relation to the quality and quantity of data; (vii) able to respond to changes in environmental performance [26].

This paper focuses on the development of a holistic framework for the assessment of industrial investments' sustainability with the use of indicators. The key research question that is examined in the present work is the identification of the optimal bundle of indicators which could be used for the assessment of alternative industrial investments. It is evident that in most cases, available funds are not adequate to cover all needs, thus decision-makers (either industrial or public) require a concrete methodology for the assessment of alternative investments and the identification of the optimal one, based on sustainability criteria. To date, the evaluation and selection of an investment over competition in most cases is solely realized based on their economic performance and indicators (Return on Investment, Net Present Value, etc.). However, the selection of the optimal criteria for the assessment of investments is, without any doubt, a highly multi-dimensional problem. The proposed framework is the outcome of a research e ffort that incorporates collection of data with the use of a structured questionnaire. For the determination of the optimal set of indicators that best capture the environmental performance of a given investment, the opinions of a number of di fferent stakeholders, with mutually contradicting priorities (e.g., environmental organizations, companies, universities, public bodies, environmentalists and economists), are considered in the present work. The proposed environmental (non-monetary) indicators are proposed to be used in parallel to the currently widely used economic ones, in order to more e fficiently and holistically assess alternative industrial investments.
