Estimation of methane production through the anaerobic digestion of greenhouse horticultural waste: A real case study for the Almeria region

Highlights

• Anaerobic digestion of the GWH produces biogas with acceptable quality.

• To increase the methane production in the biogas, the GWH should be pre-treated or co-digested.

• The application of the empirical models gives a methane production value of GWH close to the experimental test.

• Anaerobic digestion can be a potential solution to manage the GHW produced in Almeria (Spain).

Abstract

The methane production of greenhouse horticultural waste (GHW) from Almeria (Spain), from where fruits and vegetables are exported to all parts of Europe, was calculated in this work through a combination of experimental and theoretical methods. To this end, eight samples of GHW were collected and characterized in a waste treatment plant. The collection of samples was fairly distributed throughout the year to ensure a representative characterization. The amount of methane produced in a hypothetical anaerobic digestion process was predicted through empirical models fed by experimental data. The experimental characterization revealed that GHW contained an adequate content of volatile matter (65.72% TS), but a high value for total dry matter (53.46%) and lignin content (9.36%), as well as a low moisture content (46.54%) and C/N ratio (17.46). Inhibiting compounds were also observed in the characterization, such a S (0.43%) and Cl (1.41%). The methane production predicted was 0.229 Nm³ CH₄/kg volatile matter, which may seem low in comparison to other waste potentially usable for anaerobic digestion. Nonetheless, the co-digestion of GHW with other waste could be an interesting alternative to enhance methane production and solve seasonality issues. Suitable pre-treatment can be also explored to increase the usability of GHW in anaerobic digestion. All in all, this work establishes a theoretical basis for potential solutions to manage the GHW produced in Almeria.

Introduction

Almería is one of the most economically efficient areas in Spain. Despite being the driest region in Europe, it is the major producer of horticultural products in Spain and Europe, due to the highest extension of greenhouses settled in this region, the use of groundwater and desalinated water, and high rate of sunny days (Sayadi-Gmada et al., 2019). Almería may produce more than 3,200,000 tons of horticultural products per year, of which 80% are generated in the west of this province. Here, more than 1,300,000 tons of horticultural waste per year are produced throughout a greenhouse land surface which boasts 31,614 ha (Duque-Acevedo et al., 2020). It not only means an important landscape visual impact, but also a relevant environmental problem.

Currently, the majority of the organic fraction waste produced is managed in vegetable waste treatment plants, where it is transformed into vegetable compost; although another fraction is reused as animal feed or soil addition. However, the high volume of waste produced per year in this region exceeds the processing capacity of the treatment plants, making its management poor and posing serious problems for the province of Almería (Duque-Acevedo et al., 2020). Under the European framework for progressing in Circular Economy Action Plans, Andalucía has recently published a list of performances to manage vegetable waste produced from horticultural crops (Rural, 2016). The main strategic actions are aimed at reducing the volume of horticultural waste sent to the treatment plant, in the form of on-site self-management of waste and its use as animal feed. The promotion of research on the diversification of horticultural waste recovery as part of the circular economy is also addressed here.

For many years, several studies have been carried out to analyze the properties of waste from the main crops of the greenhouses of Almeria, with the aim of recycling this waste and recovering energy through thermal treatment (García-Raya et al., 2019; Garzón et al., 2017; Callejón-Ferre et al., 2014; Pinna-Hernández et al., 2019). Nevertheless, most of the literature shows a lack of knowledge about the development of energy recovery studies based on the application of biological conversion techniques to produce heat or electricity from Almeria's greenhouse horticultural waste.

Anaerobic digestion (AD) is considered a suitable way to handle wet and easily biodegradable organic material. It is a phased process where the organic residues are transformed into two different products, a biogas and a digestate, through a series of metabolic reactions: hydrolysis, acidogenesis, acetogenesis and methanogenesis. The quantity and the quality of the biogas produced depends on the characteristic of the substrates fed into the reactor, such as dry matter content (DM), organic dry matter content (ODM), pH and C/N ratio (Vitez et al., 2020). These parameters affect the living conditions of anaerobes and determine the success or failure of the digestion process. AD not only brings an efficient approach to energy production, but it also solves the waste management dilemma. In this sense, this technology is used with a wide range of waste, such as organic fraction municipal waste (OFMSW), food waste, manure and meat waste, olive mill residues, textile industry residues, agro-industrial waste and vegetable residues (Vitez et al., 2020; Van et al., 2020; Shahbaz et al., 2019; Handous et al., 2019; González-Arias, 2019; Ren et al., 2018). AD is applied to process vegetables such as rotten tomatoes, onion skin, cauliflower leaves, cauliflower stems, carrot leaves, beet leaves and others (Ji et al., 2017). However, there is no evidence of the digestion of waste with similar characteristics to the ones evaluated in this work.

Methane content in the biogas produced from the AD of organic substrates mainly depends on their contents of substances that can be degraded to CH₄ and CO₂. Thus, the composition and degradability, namely the lignin content, of the mixture of horticultural waste analyzed in this study are key factors for the energy or methane yield, expressed as mL CH₄·g VS⁻¹. Fig. 1 shows the concentration of CH₄ and CO₂ in the biogas produced from the AD of different well-known residues. The energy yield is listed as high. In some cases, these values reach between 50 and 67% in volume base of CH₄, around 26 and 50% of volume of CO₂, and between 89 and 600 mL of CH₄ per gram of volatile solids (Martinez, 2014; Cebrián, 2013; Moller et al., 2004; Ghanem et al., 1992; Mayer et al., 2013; Palatsi et al., 2010).

The production of methane is also affected by the low pH value, whose ideal value should be above 7–7.75, and the concentration of short-chain fatty acids (SCFAs), such as acetic acid, propionic acid and butyric acid, which is generated during the hydrolytic process and also affects the pH value (Garcia-Peña et al., 2011). Several studies have shown the potential associated with the digestion of common vegetables to produce energy through biogas. However, some properties of this type of waste, such as total solid contents (TS = 10%), volatile solid contents (VS = 6%–18%), Carbon and Nitrogen ratio (C/N = 20) and moisture content (W > 80%), as well as the contents of the organic components (fructose and hemicellulose - 75%, cellulose - 5% and lignin - 1%), can suppose challenges to the implementation an effective AD, due to the ranges in values inhibiting the production of methane through the acidification of the fermentation broth (Ji et al., 2017).

The estimation of the potential to produce methane through the AD of waste is essential to predict the viability of the process to be used, for example, as a suitable management tool. Existing models show the influence of the pH value, volatile fatty acid (VFA) content or ammonia content on the kinetics of AD (Batstone et al., 2002; Batstone et al., 2000), but they were not developed to predict the methane yield of energy crops or agricultural waste. Buswell (1936) and Boyle (1970) developed a model to estimate the biogas composition (CH₄, CO₂, H₂S and NH₃) from the chemical composition of organic substrates, such as C, H, N and S (Buswell and Hatfield, 1936; Boyle, 1977). However, this model did not serve to calculate the methane yield, because it did not integrate the effect of degradability. In 1980, Chandler found associations between substrate degradability and substrate composition, resulting in an estimation of this parameter through lignin content (Chandler and Jewell, 1980). Thus, from the model developed in the study by Santaloria Capdevilla et al. (Santolaria Capdevilla et al., 2014a) is was possible to estimate the energy or methane yield by considering, on one hand, the VS content, the TS content and the degradability of the organic substrate and on the other hand, the content of CH₄ in the biogas produced.

The purpose of this study is to assess the methane yield associated with the AD of a mixture of greenhouse horticultural waste (GHW) collected from a waste treatment plant (WT plant) located in the west of Almería. Therefore, this study entails the following:

• Characterizing the mixture of greenhouse horticultural waste through the analysis of different samples collected from the WT plant during 2019–2020.

• Studying the application of empirical models based on previous ones in order to predict methane production from the digestion of GHW.

• Discussing the feasibility of the digestion to be applied as a sustainable management tool.

This work aims to contribute to the exploration of a new and complementary technique for the management of waste generated in the province of Almería based on the recovery of its energy rather than its reuse and recycling. In this way, the problem of saturation of the management techniques currently used could be solved and the accumulation of these residues in the environment could be reduced.