Sustainable production of biogas from waste rice straw (SubProM project)

Project summary

The project proposes a simple and low-cost biotechnological process (RSTM process; see 9.2) for the conversion of waste rice straw into biogas. It is proposed that microbial fermentation of waste rice straw could become a significant supplementary energy source in rice producing regions in Asia. Rice straw is a major agricultural waste product in Asia with an estimated production of approx 700 million tons/year. Currently most of this waste biomass resource is not utilized or recycled in a sustainable manner.
Due to its simplicity, the RSTM process is especially suited to enhance energy production in rural communities where small manure based biogas plants are already in operation.
If experimental proof-of-concept is obtained this could lead to a substantial increase in construction of small biogas digesters in rice producing regions thus greatly reducing the demand for fossil fuels and creating possibilities for new jobs and small enterprises. Consequently this will allow for a substantial increase in energy production and standard of living in rural areas – although in a climate friendly way.
The project will also investigate socio-economic aspects of implementing green energy production (biogas) in a rural rice producing community exemplified by the Mekong Delta. Capacity building in biotechnology  will be provided in Vietnam through research collaboration and PhD/Postdoc projects in Vietnam and Denmark under supervision of senior scientists in both countries.

Outputs

Project completion report:

Extensive laboratory experiments using batch and semi-continuous reactors were conducted in 2013 and 2014. A large part of these results were published in BSc and MSc project theses, Mr. Nam’s PhD thesis and subsequently in Vietnamese scientific journals and in national and internal conference proceedings (Appendix 3c). The laboratory experiments unequivocally showed that rice straw (RS) and water hyacinth (WH) are excellent substrates for biogas production when applied in co-digestion with pig manure (PM).

From 2014-2015 a total of 15 on-site farm-scale test were performed using different reactor designs and different combinations of PM, RS and WH. These tests were all very useful in establishing the proof of concept and leading to the development of the improved HDPE farm-scale digester (see below).

Farm-scale experiments showed that a normal farm household (with only 2-3 pigs) could´obtain a stable production of biogas for their daily cooking needs when RS and WH were used as co-substrates. In addition biogas yields were significantly enhanced (usually by more than 50%) when also using biomass. See also Appendix 3b.

Parallel experiments (lab-scale and farm-scale) on biomass pre-treatment carried out from 2013 to 2016 showed that simple procedures of pretreatment such as pre-soaking of biomass in tap water, river water or digester effluent are sufficient for good biogas yields. These pretreatment methods are simple, easy to perform and very time-saving. For that reason on-site production of carbohydrases by microbial monocultures in VN - as originally contemplated - was abandoned.

At a meeting at CTU in November of 2016 consensus for “best practice” concerning biogas production from RS and WH in simple farm-scale biogas reactors was obtained. The best reactor design to be promoted in the future by SupProM/CTU and by the SDP is the so-called Improved (= sloping) High Density PolyEthylene tube reactor (IHDPE). The standard 8 m3 IHDPE tube digester is made of tough (0.75 mm) HDPE with a total price of approx. 400 USD (including installation and gas tank).

This reactor design has proved very versatile when using high proportions of different biomass substrates since biomass flotation within the reactor is very unlikely to occur when using presoaked biomass (Appendix 3b).

Timeline - outcomes
# Proof - of- concept obtained in lab-scale digesters (2013 – 2014).
# Proof - of - concept obtained in farm-scale digesters (2014 – 2016)
# Decision Point:
Best practice (concerning reactor design; pretreatment of biomass and operational parameters/conditions) established ultimo 2016.
# Sponsored Digester Program (SDP) approved by Danida on August 17, 2017 (Total available funding for SDP: 70 kDKK)
# Installation of SDP Reactors begins at the end of 2016.

Brief popularized abstract:

The main mission of the SubProM project has been to investigate whether waste rice straw (RS) and water hyacinth (WH) could be used as sole substrates or co-substrates for biogas production at small farm households in the Mekong Delta (MD) when the pig stock is too small (temporarily or constantly) for biogas production. A constant pig stock of 5 adult pigs is a necessity for producing enough biogas for the daily cooking needs of a farm household in a biogas digester. Many small farms in the MD only hold 1-2 pigs and therefore cannot use biogas digesters for production of cheap and sustainable biogas. Research by the SubProM team has unequivocally showed that biogas yields are enhanced significantly (often more than 50%) by co-digestion of even high proportions of RS and WH which are available in almost unlimited quantities in many parts of Vietnam (and Asia). These results provide very high prospects for a significant and economic increase in sustainable biogas production in Vietnam and other parts of Asia.

The SubProM team at CTU also developed a new type of tube biogas reactor (total price of 400 USD) especially suited for co-digestion of high fraction of RS and WH biomass with pig manure.
# Ultimo 2017 a total of 27 improved HDPE digesters have been installed in provinces close to CanTho (cf. Appendix A).
# Extensive questionnaire including biogas users and non-biogas users in the Mekong Delta have been completed and the results published (Ultimo 2017) (Appendix 3c).