SostRice Project – Searching for a sustainable model of rice straw management

Usually, the blog is focused on the facilities and process associated with the biorefining and green chemistry sector. On this occasion, I will address the topic of this post from a wider perspective, taking into account important aspects related to the feedstock. This is possible because I had the opportunity to attend a conference on the sustainable management of the rice straw where representatives of the stakeholders (technician, farmers, administration and ecologists) gave its opinion on the challenges and solutions in this matter. Listening to the experts of the agricultural field, that is more unfamiliar to me, I took most of the notes included in this post.

The conference took place within the frame of the SostRice (CO2 Emission Reduction of the Rice Cultivation through Energy Valorisation of the Rice Straw) Project in the facilities of AINIA (Valencia, Spain). Such project is funded by the LIFE Programme of the European Commission. The partnership is formed by IAT (leader), AINIA, CITAGRO, CTAER and Ludan Renewable Energy. It is a demonstration initiative that aims to reduce the energy, the fertilizers and the water consumption in the rice cultivation through the development of a sustainable system of straw management.

Introduction of the rice straw management issue

The activities of the SostRice Project are focused on two of the most important rice-growing zones in Spain: “las Marismas del Guadalquivir” (Andalusia, 35.000 ha) and “la Albufera” (Valencia, 15.000 ha). Both zones are characterized by being very close to protected areas, “el Parque Natural de Doñana” in the first case and “el Parque Natural de la Albufera” in the second one. Also, both are suffering serious environmental problems related to rice straw management. To give a sense of the amounts involved, it is estimated that 4,5 tonnes of straw are generated by hectare in “las Marismas”.

In a traditional way, several operations have been used to manage this waste:

1. “Fangueado”

It is the total incorporation of the straw into the soil. It allows to seal the terrain and reduce the necessity of water pumping.

It presents problems resulting from the uncontrolled fermentation of the waste: methane and sulfide emissions; appearance of physiopathies in the crops; fish mortality.

2. Burning of the stubble

It is an inexpensive procedure for the farmers to eliminate the pests.

It presents problems resulting from the combustion: release of large quantities of CO₂; negative effects for the human health; potential danger for areas of high nature value. Moreover, a partial loss of nutrients occurs (some of them remain in the ashes).

3. Packing to animal feed

It is a not usual option due to the rice straw is not to the liking of the livestock and has a high silica content.

The collection of the rice straw

In light of the above considerations, it seems clear that a more sustainable alternative for the straw management calls for its valorization. However, before talking about this process, it is very important to take into account some operations highlighted by the experts as essential for all the management strategy: collection and transport.

Figure 1. Flooded rice paddy fields (extracted from SostRice web page)

Due to the special features of some rice-growing areas, the straw logistics is not simple. For instance, the ground of “la Albufura” low zones (roughly 40% of the total area) gets muddy during the harvesting and if the straw falls down, its collection and transport is very complicated. Therefore, it would be necessary to develop the appropriate technology in order to a further valorization. The economic profit of any ulterior solution can vary depending on the progress in those operations.

Another important point is the seasonality of the resource. Any valorization model pretending to work in a continuous way (like the prototypes to be mentioned below), it should consider the relevance of the storage of the straw generated in a short period.

Valorization strategies and management model in the SostRice Project

The SostRice Project takes into account two solutions to value the rice straw: combustion and anaerobic digestion. Two demonstration prototypes have been designed, built and started-up to study them. In both cases, the necessary data to make an integral assessment of the sustentability (technical, economic and social) through tools based on the Life Cicle Analysis technique are being taken. The data will be compared to those obtained studying traditional models.

During the conference, the attention was drawn over the anaerobic digestion pilot plant that Ludan has designed and erected (watch video about the prototype below, only in Spanish). Before starting the process, the straw is pretreated by means of mechanical media. Once inside, it is subjected to two stages of digestion to obtain biogas and a stable digestate. It should be pointed out that the rice straw is not a usual feedstock for anaerobic digestion processes and, therefore, it is necessary to carry out a technical development to successfully treat this lignocellulosic waste. In the pilot plant, previous advances achieved in the labs of AINIA are being tested. According to the first data collected, the quantity of biogas could reach 200 m³ per ton of straw. The nature of the digestate vary depending on the co-digested substrate. If it is co-digest with water, the result is poor in nutrients, whereas it is co-digested with manure, a richer product is obtained.

Video about the anaerobic digestion pilot plant (only in Spanish)

The global model raised by the project tries not only to reduce the greenhouse gases emissions and the energy consumption of the straw management but also to close the cycle of nutrients through the use of the digestate as mineral biofertilizer. The rice paddy fields have a very high organic matter load so they do not need to be supplemented with fertilizers to increase it. However, mineral fertilizers (nitrogen, specially) are used and this is the point where the project claims to impact on. Although is out of the scope of the project, the biofertilizers could also be removed from the cycle to supplement other crops which really need organic matter.

Likewise, it should be highlighted that the SostRice model considers that the plants must be close to the zone where the waste is generated. In this way, logistics is optimized and transportation expenses are minimized.

Other valorization strategies

Other projects and strategies to manage the straw were mentioned during the conference:

1. Small-scale uses (“Banco de Paja” Project): pads in organic gardens, bioconstruction, organic blankets to stabilize slopes.

2. Manufacturing of blankets or rolls of straw to prevent the erosion (LIFE Eco-Rice Project).

3. Composting of the rice straw with sewage sludge and further production of organic fertilizer (LIFE Biocompost Project).

Finally, I would like to underscore another initiative that was not mentioned in the conference. It is the LIFE WALEVA Project (From Whatever Residue into Levulinic Acid – an innovative way to turn waste into resource) aimed to valorize the rice straw through its conversion into levulinic acid. It is a very versatile platform molecule with numerous applications in different sectors. The project is supported by the farmers association of “Don Benito” region (Extremadura), another of the main rice-growing zones in Spain. The partnership is formed by Técnicas Reunidas, CICYTEX and feiQue. A future fusion of the results of both initiatives could result in an integrated biorefinery with rice straw as feedstock.

NOTE: I would like to express my thanks to Andrés Pascual (Jefe del departamento de Medio Ambiente, Bioenergía e Higiene Industrial de AINIA) for the invitation to the conference.