Anaerobic Digestion of Laminaria japonica Waste from Industrial Production Residues in Laboratory- and Pilot-Scale

Abstract

The cultivation of macroalgae to supply the biofuel, pharmaceutical or food industries generates a considerable amount of organic residue, which represents a potential substrate for biomethanation. Its use optimizes the total resource exploitation by the simultaneous disposal of waste biomaterials. In this study, we explored the biochemical methane potential (BMP) and biomethane recovery of industrial Laminaria japonica waste (LJW) in batch, continuous laboratory and pilot-scale trials. Thermo-acidic pretreatment with industry-grade HCl or industrial flue gas condensate (FGC), as well as a co-digestion approach with maize silage (MS) did not improve the biomethane recovery. BMPs between 172 mL and 214 mL g(-1) volatile solids (VS) were recorded. We proved the feasibility of long-term continuous anaerobic digestion with LJW as sole feedstock showing a steady biomethane production rate of 173 mL g(-1) VS. The quality of fermentation residue was sufficient to serve as biofertilizer, with enriched amounts of potassium, sulfur and iron. We further demonstrated the upscaling feasibility of the process in a pilot-scale system where a CH₄ recovery of 189 L kg(-1) VS was achieved and a biogas composition of 55% CH₄ and 38% CO₂ was recorded.

Keywords: Laminaria japonica; biogas; biomethane; flue gas condensate; industrial residuals; macroalgae; maize co-digestion; thermo-acidic pretreatment; waste management.

Figure 1

Thermo-acidic pretreatment: Net accumulated methane production of pre-treated (PT) and untreated (U) Laminaria japonica waste. Pre-treatment was applied for 2 h at 20 °C (PT-20), 50 °C (PT-50) and 80 °C (PT-80) in 0.05 M HCl (a), 0.1 M HCl (b) and 0.5 M HCl (c). (d) shows the histogram of the final net methane yield in mL·g⁻¹ VS algae biomass. The values of the final methane yield are reported in Table 3 and were taken at Day 22.

Figure 1

Thermo-acidic pretreatment: Net accumulated methane production of pre-treated (PT) and untreated (U) Laminaria japonica waste. Pre-treatment was applied for 2 h at 20 °C (PT-20), 50 °C (PT-50) and 80 °C (PT-80) in 0.05 M HCl (a), 0.1 M HCl (b) and 0.5 M HCl (c). (d) shows the histogram of the final net methane yield in mL·g⁻¹ VS algae biomass. The values of the final methane yield are reported in Table 3 and were taken at Day 22.

Figure 2

Inoculum adaptation to substrate: Net accumulated methane production of untreated (U) Laminaria japonica waste (a) and final net methane yield in mL per g of VS algae biomass (b). Results are from different succeeding trials (U-1, U-2, U-3). The values of the final methane yield are reported in Table 3 and were taken at Day 22.

Figure 3

Pretreatment with flue gas condensate: Net accumulated methane production of pre-treated and untreated (U) Laminaria japonica waste. Pre-treatment was applied for 2 h at 80 °C in 0.2 M HCl (HCl-80) and FGC (FGC-80) and at 100 °C in HCl (HCl-100) (a). (b) shows the histogram of the final net methane yield in mL g⁻¹ VS algae biomass. The values of the final methane yield are reported in Table 3 and were taken at Day 22.

Figure 4

Co-digestion with maize silage: Net accumulated methane production of digestion from LJW (LJ-U), maize silage (MS-U) and their respective blends, 50% LJ/50% MS (50/50) and 25% LJ/75% MS (25/75) (a); (b) shows the histogram of the real final net methane yield in mL·g⁻¹ VS algae biomass, along with the theoretical BMPs (T). The values of the final methane yield are reported in Table 3 and were taken at Day 22.

Figure 4

Co-digestion with maize silage: Net accumulated methane production of digestion from LJW (LJ-U), maize silage (MS-U) and their respective blends, 50% LJ/50% MS (50/50) and 25% LJ/75% MS (25/75) (a); (b) shows the histogram of the real final net methane yield in mL·g⁻¹ VS algae biomass, along with the theoretical BMPs (T). The values of the final methane yield are reported in Table 3 and were taken at Day 22.

Figure 5

Continuous anaerobic digestion of Laminaria japonica waste in laboratory-scale: Laboratory-scale continuous AD of untreated LJW showing the process parameters; cumulative CH₄ volume (L), specific volumetric CH₄ production (calculated) per VS feed load (L·g⁻¹·day⁻¹) (a); pH value and redox potential in mV (b); conductivity EC in mS·cm⁻¹, VS and TS concentration (% from total weight) (c). VS and TS represent average values from three individual measurements and error bars are defined as standard deviation (±SD).

Figure 6

Dynamics of VFA concentration: The graph shows the distribution of VFA concentration (g·L⁻¹) for formic acid, acetic acid, propionic acid (a); butyric acid, iso-butyric acid, iso-valeric acid (b); valeric acid, caproic acid and heptanoic acid (c); The last subfigure shows the total sum concentration of VFAs (d).

Figure 7

Continuous anaerobic digestion of Laminaria japonica waste in pilot-scale: Pilot-scale continuous AD of untreated LJW over time showing the process parameters; cumulative biogas volume (m³), volumetric biogas flow rate (L·h⁻¹) (a) volumetric CH₄ and CO₂ concentration (%) (b); pH value and redox potential in mV (c); VS, TS concentration (% from total weight) and conductivity (mS·cm⁻¹) (d). VS and TS represent average values from three individual measurements and error bars are defined by standard deviation (±SD).

Figure 8

Laminaria japonica waste substrate: Dry and native Laminaria japonica waste material as obtained from Qingdao CoDo International Limited, Qingdao, China (a) containing plastic braids (c). For laboratory use, the biomaterial was shredded and sieved to a particle size of <0.5 cm (b).