Anaerobic Co-digestion of steam pretreated wheat straw and sewage sludge

Abstract

Anaerobic digestion (AD) of wastes and residues is a widely applied and explored process. However, when the process is not optimized, inadequate concentrations of nutrients and inhibitory compounds are common and can lead to low maximum organic loading rates and increased risk of process problems, low organic dry solids degradation rate and reduced methane yields. To overcome process instability and low performance, the simultaneous digestion of two or more substrates, named co-digestion, has been implemented in the last 30 years. Nowadays, the expansion of co-digestion to the use of anaerobically slowly converted agro-waste, via pretreatments, is of major interest due to their availability and energy densities. Among the sustainable feedstocks, wheat straw is a good candidate for co-digestion with sewage sludge, since it has a high concentration of organic compounds, low water content and low concentration of many nutrient elements, which can be provided in higher amount by sewage sludge, being instead characterized by low organic content, high water content and high concentration of several nutrient elements in relation to carbon.

This study investigates co-digestion of sewage sludge (combined primary sludge and waste activated sludge) and steam pretreated wheat straw in continuous stirred-tank reactors. Wheat straw was pretreated through acetic acid catalyzed steam explosion at 190°C for 10 minutes, leading to 89% recovery of the total solids. A constant proportion of the two substrates was maintained in the inlet, where 35% of the feedstock volatile solids was provided by wheat straw, giving a C/N ratio of 12.6, and TS content of 10.7%. Digestions of the two single feedstocks were used as controls to compare mono- and co-digestion performance. Micro- and macronutrients were added in the wheat straw controls to reach the same concentrations in the feed as for co-digestion. Similar organic loading rate were applied 2.1 ± 0.1gVS/Ld in all reactors, and the hydraulic retention time was maintained at 22 days.

All processes were stable most of the operational time, with methanogenesis inhibition occurring only at the beginning when wheat straw was introduced in the substrate, accompanied volatile fatty acids accumulation. Hence, the microbial population likely adapted to the new substrate. After 26 days the concentration of total volatile fatty acids was maximum only 0.30 g/L within 7 hours after feeding and less than 0.15 g/L at 7 hours after feeding, suggesting that higher organic loads might be tolerated by the system.
The average methane yield from co-digestion was 0.27 LCH4 / gVS. This was significantly different only from the methane yield obtained from wheat straw mono-digestion, and a synergistic effect was not demonstrated. Similar degradation degree of volatile solids were obtained from co-digestion and sludge mono-digestion, meaning a good accessibility to degradable compounds in wheat straw was gained from its pretreatment. More experiments need to be performed to assess the digestate quality and its impact on the economy and applicability of the process.

Finally, in a full-scale plant digesting sludge only, an increase in annual methane production of 43% was estimated through co-digestion of 35% wheat straw. Due to the high energy density (low water content) of wheat straw, the plant could operate at the same hydraulic retention time as today, with an increase in reactor volume of only 11%.

Popular Abstract

The increasing demand of renewable energy is boosting research and optimization of well-established processes, such as anaerobic digestion of wastes and residues. In the process, the reduction of the organic content of sludge and other wastes is performed by microorganisms under anaerobic condition, giving biogas (mainly methane and carbon dioxide) and sludge digestate. Biogas provides power and heat, and can be upgraded to biomethane (by removal of most carbon dioxide), used as vehicle fuel and in the gas grid. The digestate, when in sufficient quality, can be used as fertilizer.
The simultaneous digestion of two or more substrates, named co-digestion, has been implemented widely in the last 30 years, improving the economic viability of anaerobic digestion plants. Nowadays, the expansion of co-digestion into the use of pre-treated agro-waste, is of major interest due to the availability of agro-waste and the potential of increased biogas production rate after pre-treatment. In this study, steam pre-treated wheat straw is evaluated as co-substrate for sewage sludge digestion, in continuous experiments.
Wheat straw is characterized by a high content of organic compounds, low water content and low concentration of many nutrients, as well as recalcitrancy that makes its pre-treatment necessary to increase structure accessibility and degradability. On the other hand, sewage sludge has a low organic content, high water content and a good availability of many nutrients. Their combination is promising, since it leads to an overall good content of micro- and macronutrients, dilution of inhibitory compounds, and balance between moisture and organic content. So far, the influence of wheat straw pretreatment on the methane yield as well as co-digestion effects have been investigated mainly in batch reactors. In this study, the use of the two substrates in continuous experiments instead, gives the possibility to evaluate co-digestion effects without influence of the composition of inoculum as in batch test.
Wheat straw was milled and pre-treated through acetic acid catalysed steam explosion at 190°C for 10 minutes. It was combined in a constant portion with sewage sludge in the combined feedstock (0.35:0.65 volatile solid based), increasing the energy density of it. In sewage sludge, 12 nutrients were found to be below the concentrations suggested in literature for optimal digestion. However, sewage sludge provided N, Fe, Cu and Zn in the co-digestion reactors close to the optimal concentrations.
Digestions of the two single feedstocks, were used as controls to compare mono- and co-digestion performance. Stable operation was observed for co-digestion, after a short adaptation period. The influence of wheat straw high energy density (low water content) is emphasized in a full-scale plant case study. Hence, in a full-scale plant digesting sludge only, an increase in annual methane production of 43% was estimated through co-digestion of 35% wheat straw, with an increase in reactor volume of only 11%. More experiments need to be performed to assess the digestate quality and its impact on the economy and applicability of the process.

• author: Tosi, Virginia
• supervisor: Emma Kreuger; Åsa Davidsson
• organization: Biotechnology (MSc); Biotechnology (M.Sc.Eng.)
• course: KBTM01 20211
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Anaerobic co-digestion, wheat straw pretreatment, nutrients availability, OLR, VFA acumulation, Biotechnology
• language: English
• id: 9073619
• date added to LUP: 2023-09-14 10:33:15
• date last changed: 2023-09-14 10:33:15