Examining the capacity for cultivating marine macroalgae using process liquids from biogas digestate as nutrient source and cultivation medium

Highlights

• Nutrient supply and culture medium of biogas digestate for production of marine macroalgae.

• Processed biogas digestate resulted in algal biomass increase similar to f/2-standard medium.

• Pretreated biogas digestate provide suitable nutrients for marine macroalgae cultivation.

Abstract

The land-based and seawater-independent production of macroalgae as a source of renewable biomass with a multitude of potential applications increasingly attracts the attention of science and industry. However, the availability of a feasible nutrient source acceptable for supplying a large-scale cultivation facility on land remains a persisting obstacle that has not yet been overcome. Hence, this study aimed to test an uncomplicated technological approach to sufficiently reduce the nutrient load of the biogas digestate and to identify an optimal ratio of pretreated digestate to volume of cultivation medium. Based on an initial solid-liquid separation of biogas digestate, the liquid phase was treated with flocculation agent and fine separation (liquid A), followed by filtration through an organic sorption filter (liquid B) and finally through a high-performance soil filter (liquid C). The results showed notable differences regarding the nutritional value of the different process liquids. The mixtures of process liquids A to C with artificial seawater (ASW) optimal for biomass increase were 1:50, 1:10 and 1:1, respectively. Maximum biomass increase was achieved by using a mixture of process liquid A and ASW with the ratio 1:50, which resulted in a growth rate of 7.02% FW d⁻¹ in comparison to a growth rate of 5.71% FW d⁻¹ of the untreated control group that used f/2-standard nutrient supply. Regarding the attempt of using a maximum amount of digestate and of reducing the necessity of diluting the process liquid with ASW, process liquid C with a mixing ratio of 1:1 appeared promising for a large-scale land-based cultivation of marine macroalgae. However, the transfer of the data to other agricultural biogas plants needs to be adapted to the respective case as changing feeding strategies of the biogas fermenters with different biogas substrates might lead to a large variation of nutrients in the biogas digestate resulting in changing quality and quantity of nutrients in the produced process liquids.

Introduction

Marine macroalgae are a highly interesting source of renewable biomass with a wide range of possible applications. In addition to use as food, as an additive for animal feed or as raw material for pharmaceutical, biotechnological and agricultural applications, the ability of some marine macroalgae to purify wastewater has been successfully demonstrated [[1], [2], [3], [4], [5]]. In this context, it was shown that marine macroalgae, similar to microalgae, can be used to fix valuable and/or harmful ingredients from wastewater [6]. However, not only the potential use of marine macroalgae as raw material but also their use for energy production has been proven many times [7,8]. The energy potential available by converting marine biomass into energy through anaerobic digestion has been demonstrated for various macroalgae [9]. Algal biomass for energy production is a sensible strategy in particular for countries with direct access to the sea where seaweed is constantly washed up on the beaches or where nutrient-rich coastal waters cause massive seasonal algal blooms. Without access to the sea, the use of marine algal biomass for energy production on a regular basis appears hardly realistic as the costs of transport, i.e. shipping of the biomass is quite high compared to conventional agricultural resources used as biogas substrate like corn silage or cattle manure [[10], [11], [12]]. Utilization of microalgae biomass for biogas production, although theoretically possible, seems even less promising as dry mass content of e.g. mass produced Chlorella or Spirulina are low (dry mass below 1%) and current prices of microalgae as food supplements are quite high [[13], [14], [15], [16]]. Thus, the possibility of a land-based and seawater-independent production of macroalgal biomass with various potential applications increasingly attracts the attention of science and industry [[17], [18], [19]]. Different technological approaches for the land-based production of marine macroalgae have been tested and have proven the feasibility of that approach [[20], [21], [22]]. In particular, a combination of a land-based macroalgae cultivation with an agricultural biogas plant offers various additional effects, which can facilitate the production of algae. Firstly, the costs of energy for agitation and temperature control of the cultivation medium or for the supply of artificial illumination can be reduced by using energy converted from the biogas of the combined heat and power unit as long as the fluctuating market prices for electricity do not render it economically more viable to sell the energy [[23], [24], [25]]. Secondly, produced biogas contains high concentrations of carbon dioxide, which can be filtered by the algal CO₂-sequestration ability. In this regard, it was shown that an increase of CO₂ feed to the cultivation medium resulted in a stimulation of macroalgal productivity [22,26,27]. However, the problem of a feasible nutrient source acceptable for supplying a large-scale on-land and seawater-independent cultivation has not been solved. Artificial nutrient media like Provasoli Enrichment, f/2-medium or Von-Stosch medium are highly expensive and, thus, only used in hatcheries under lab-scale conditions [28,29]. Nonetheless, it was shown that nutrient-rich waste products like pig manure, biogas digestate or municipal wastewater might also provide sufficient nutrients for the growing of algae [30]. In regard to the use of biogas digestate as nutrient source, the issue of a possible influence of different biogas substrates or of different process conditions on the quality and nutrient composition of the residual digestate remains critical. Additionally, biogas digestate is normally very rich in nutrients and still contains a high amount of solid particles, which would cause turbidity of the medium. Moreover, the particles might hamper the circulation of the algae and might also lead to an highly unpredictable nutrient load. Furthermore, algal cultivation requires only small amounts of certain nutrients, whereas a simple dilution of the biogas digestate may lead to a disproportionately high and diverse spectrum of nutrients in the algal medium. Moreover, depending on the amount and ratio of input substrates as well as on the retention time of the substrates in the biogas fermenters, nutrient concentration of solid-liquid separated digestate might significantly differ. Hence, these still unclear uncertainties regarding the usage of biogas digestate as nutrient source demand more complex and elaborate separation technologies of the digestate. Thus, these still unsolved uncertainties might hamper the implementation of a land-based macroalgal cultivation. Our study aims to test an uncomplicated technological approach to sufficiently reduce the nutrient load of the digestate and to identify an optimal ratio of pretreated biogas digestate to volume of cultivation medium. Thus, the usability of pretreated digestate of an agricultural biogas plant as nutrient source for a land-based macroalgae cultivation was evaluated.