Anaerobic digestion of wetland biomass from conservation management for biogas production

Highlights

• Biogas yield of wetland plants is lower compared to conventional energy crops.

• Biomass of reed rushes can provide a substantial source of energy.

• Wetland plants could be an attractive substrate in agricultural biogas plants.

• Economy of the process depends on EU subsidies and agro-environmental payments.

Abstract

In this study, we examined the biogas and methane production of wetland plants species harvested in the Narew River Valley protected as national park, NE Poland. In the experiment, we used the aboveground biomass of five species: reed canary grass (Phalaris arundinacea), common reed (Phragmites australis), reed sweet-grass (Glyceria maxima), tufted sedge (Carex elata) and woollyfruit sedge (Carex lasiocarpa). The plants were harvested in three seasons of the year 2015: mid-summer, late summer, and early fall.

The chemical composition of wetland plants was not beneficial for the anaerobic digestion, because of the sub-optimal C:N ratio. Consequently, the wetland species exhibited rather average methane production (102–221 NL kg⁻¹ VS). These values were similar to that observed for grasses collected in extensive grasslands and wetlands by other researchers. They were, however, much lower than those obtained for species harvested in cultivated grasslands. Methane and gross energy yield differed significantly (p < 0.05) depending on the season of the biomass cut. The optimal period for the harvest of wetland biomass for biogas generation was the middle of summer, July–August.

Phragmites australis appeared to be the most promising energy species. Despite the low methane yield per kg VS, common reed, due to its high aboveground biomass, achieved quite high methane production per area (1,16·10⁶ NL CH₄ ha⁻¹). Our findings suggest that biomass of reed rushes can provide a substantial source of renewable energy used for biogas generation, which will make possible economic exploitation of wetlands combined with the conservation of their natural functions and services.

Introduction

Biomass is now the largest global contributor of renewable energy supplying 10% of global annual primary energy consumption, and it can still expand in the production of heat, electricity, and fuels for transportation [1]. In European biogas plants, agricultural biomass is a primary substrate. This is particularly evident in Germany, the leader of biogas energy production in Europe [2].

Although the exploitation of biomass for the energy generation may reduce the GHG emissions, the positive impact on other components of the environment and economic benefits of the biomass use are often less evident [3,4]. Numerous studies demonstrated that large-scale mass production or harvest of biomass could result in contamination and depletion of soil [5] and water resources [6], a substantial decrease of biodiversity [7] and loss of traditional rural landscape [[8], [9], [10]].

All these negatives caused numerous attempts to promote a more environment-friendly alternative to energy crops, which are agricultural and municipal wastes and forest residues. Recently biomass from the management of tree-less ecosystems: wetlands, wet meadows and buffer strips has emerged as a promising option for biogas generation [11]. According to Phillips et al. [12] in the United Kingdom a vegetation management in nature reserves, conservation areas and other areas, such as raised bog and estuary reed beds, can bring as much as 0.5 million Mg y⁻¹ of biomass, which can be used for energy purposes. In Poland, permanent grasslands can provide ca. 2.3 million Mg y⁻¹ of biomass, without detriment to the supply of forage, which can result in the generation of 1.1–1.7 billion m³ y⁻¹ of biogas [13]. Particular attention should be paid to biomass of fluviogenous wetlands, mainly consisting of Phragmites australis, Phalaris arundinacea, and Carex sp., which is mowed annually in large quantities as a part of conservation programs [14] and has been suggested to be an interesting alternative to energy crops for agricultural biogas plants [15].

The aim of the study was to examine the biogas and methane production of five wetland plants species: reed canary grass, common reed, reed sweet-grass, tufted sedge and woollyfruit sedge, harvested in three different dates in the protected area of the Narew National Park, northeast Poland (Natura 2000 sites: Special Area of Conservation PLH200002 and Special Protection Area PLB200001).