(760a) Ecosystems As Unit Operations:  Designing Integrated Networks of Technological and Ecological Systems

One of the requirements to meet the goal of sustainable design is to ensure that impacts of engineering systems do not exceed the regenerative capacity of ecological systems [1], in order to prevent unintended harm. Industries striving towards sustainable development also aim to become carbon and emissions neutral, with zero water withdrawal. Many companies aim to go even beyond such goals by having "net positive" manufacturing systems. While the conventional approach to meeting this goal has been to increase the use of renewable fuels, optimize the system efficiency to reduce emissions and rely on end-of-pipe pollution control methods, such solutions tend to neglect the ecological overshoot resulting in some perverse outcomes.

In this work, we present an approach to designing coupled techno-ecological systems in order to minimize ecological overshoot within an â??industrial-serviceshedâ?, by including ecological systems in a manner analogous to unit operations within the engineering design framework. As a result, simultaneous decisions are made regarding the design of engineering systems to minimize the ecosystem service demand and the design of supporting ecological systems to supply the necessary ecosystems services demanded by industrial sites. Sustainability indices for such systems are quantified using the TES (Techno-Ecological Synergy) framework [1], as a function of the design variables and the objective is to identify optimal coupling between Technological and Ecological systems, to minimize the overshoot.

Traditional engineering unit-operation models capture the demand for ecosystem service, while the supply is quantified by spatially explicit unit-operation models of ecosystems. Similar to process level constraints regarding the plant operating conditions, feedstock availability etc. ecological constraints like land availability, type of ecological systems and other geo-spatial constraints based on remote-sensing and aerial survey are identified and included within the design problem. The objective of TES system design is to maximize the net ecosystem service supply, quantified by the ecosystem service supply less the demand as opposed to the conventional approach of minimizing only the ecosystem service demand.

The TES design framework is applied to the design of a biofuel production system. Ecological systems included within the design framework include a forest ecosystem captured using the Urban Forest Effects model [2] and a horizontal subsurface flow wetland systems for treating the wastewater, designed using steady-state first order equations of an ideal plug flow reactor [3]. The problem is formulated as a multi-objective nonlinear program (Mo-NLP) and tradeoffs are identified between maximizing the net present value of the system and the net ecosystem service demand for multiple ecosystem services. The dynamic behavior of ecological systems and changes in the design and operation of systems are captured using a multi-period approach. Seasonal variations due to weather fluctuations are also bound to cause a change in ecological processes and thus robust designs are identified at each time-period accounting for these external disturbances. The problem formulation also accounts for the uncertainty associated with the critical design and operating variables.

Preliminary results from the case study demonstrate that systems designed by accounting for ecosystem service supply results in novel designs that have a higher economic value, and higher sustainability index, resulting in â??win-winâ? designs. Thus, coupled TES systems are expected to have economic and environmental advantages over conventional systems design, mainly due to the reduction in external inputs by moving towards a closed loop production system.

References

[1] Bakshi, Bhavik R., Guy Ziv, and Michael D. Lepech. "Techno-ecological synergy: A framework for sustainable engineering." Environmental science & technology 49.3 (2015): 1752-1760.

[2] Nowak, David J., and Daniel E. Crane. "The Urban Forest Effects (UFORE) Model: quantifying urban forest structure and functions." (2000).

[3] Rousseau, Diederik PL, Peter A. Vanrolleghem, and Niels De Pauw. "Model-based design of horizontal subsurface flow constructed treatment wetlands: a review." Water research 38.6 (2004): 1484-1493.