(728c) A Techno-Economic Analysis of Greenhouse Waste and Water Recycling Using Anaerobic Digestion: A Case Study in Food-Energy-Water Nexus

The US DOE estimates that about 111 million tonnes of agricultural crop residues are produced every year (Turhollow, Perlack et al. 2014). The total world production is estimated at nearly 1 billion tons/year (Cherubin, Oliveira et al. 2018). Sustainable agriculture would require us to recover and recycle as much of the materials and energy contained in these wastes. In this paper, we will examine the techno-economics of crop residue recycling using anaerobic digestion for greenhouses in Almería, Spain, which supplies nearly 20% of produce for Spain (about 3 million tonnes/year) (Sayadi-Gmada, Rodriguez-Pleguezuelo et al. 2019). Much of the cultivation is done on a year around basis. Large amount of waste material is produced after every season, which is typically hauled away to landfills (with a high cost for farmers for the transportation) where it decays producing unwanted greenhouse gases polluting the atmosphere. The total waste is estimated to contain around 1.35 million tonnes of crop residues and 0.5 million tonnes of rejected produce (Egea, Torrente et al. 2018).

Anaerobic digestion (AD) of the waste represents a potential route to create an integrated and sustainable agricultural model (Kalra and Panwar 1986, Corneli, Dragoni et al. 2016, Sukhesh and Rao 2018). As shown in attached figure, AD produces mainly 4 product streams all of which have value associated with it. The biogas produced can be used to generate heat and power both of which are needed commodities for greenhouse operation. The waste water produced is rich in nutrients and therefore can supply much needed irrigation water thus reducing the stress on local aquifers and desalination plants, which is a critical issue in the South-East of Spain. The solid digestate produced can be converted to compost manure after treatment and thus forms a source of organic fertilizer for the next round of crops. The main issue facing the application of this technology is one of economics. Implementation of such a waste recycling system can provide a sustainable solution from the point of view of Food-Water-Energy Nexus as applied to greenhouse agriculture (Haltas, Suckling et al. 2017, Hoolohan, Soutar et al. 2019, Falconer, Haltas et al. 2020).

For the case study, we selected a 100 hectare greenhouse cooperative and carried out an assessment of the economics of constructing and operating an AD system from the crop residues produced. Based on prior studies, the total crop residues was estimated at 4900 tonnes/year (tpy) containing about 3400 tpy of Volatile Solids (VS) and about 500 tpy of water. Additional water needed for dilution in the AD was estimated at 20,000 tpy. The biogas production from the digester was estimated at 1400 tpy containing 660 KW of energy which is roughly 1/3 of the energy content in the waste biomass. The biogas can be used to produce nearly 200 KW of electricity and 460 KW of heat energy both of which are needed in the greenhouse operation and therefore directly recyclable. The wet digestate is sent to a filtration system where much of the water enriched with nutrients is recovered. Therefore ,after some pretreatment, this water can be recycled back for greenhouse irrigation. Likewise the solid digestate is rich in minerals and after treatment, becomes valuable as a compost fertilizer which is also a needed commodity for the greenhouse cultivation.

In this paper, we will present the results of a preliminary economic analysis of the process. We will take a look at the economic feasibility of the process as well as the sensitivity to variables such as waste disposal tipping fee, capital expenditures required and the cost of energy recovered from the AD system.

References

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