Su-re.co
Su.re.co installs small-scale biodigesters with local Indonesian farmers, turning their cow & pig manure into organic fertilizers and biogas, providing a sustainable alternative to firewood, LPG gas, and chemical fertilizers.
Carbon offsets from biogas are a result of the carbon saved from LPG and firewood usage, the carbon saved from chemical fertilizer usage, and the carbon saved from manure degradation, subtracted by carbon emitted by burning biogas and carbon emitted by the bioslurry (organic fertilizer) (see Formula Below). Further sources can be seen in the appendix next to each parameter.
CARBON OFFSET = OCS +CF + CMO - BG - BF
OCS: Carbon saved from old cooking source (LPG/firewood)
CF: Carbon saved from chemical fertilizer
CMO: Carbon prevented from setting free when cow manure decomposes (cow manure offset)
BF: Carbon emitted by bio-fertilizer
BG: Carbon emitted by cooking with biogas
Following this scheme, the carbon offset per biodigester is approximately 1 tCO2e/year (precisely: 0.96 tCO2/year).
This value of 0.96 tCO2 is the theoretical result for a digester running under ideal circumstances.
The gasmeter formula reduces the assumption of “Flow rate * average gas production” of 90 minutes per day.
Therefore, precise gas consumption includes non-ideal parameters, such as a lower feeding rate, shorter cooking times, colder temperatures, etc. In the calculation.
The formula, which includes the gasmeter data, is the following:
Carbon Offset [tCO2] = 0.004437758 * Gas Consumption [m^3]
Key Assumptions (Baseline Scenario)
It is important to understand that the calculations are estimates based on existing data and assumptions. Therefore, the following assumptions contribute largely to the variability of carbon offset estimate results.
- Ideal Feedrate of 15L/day of manure (total with water is 26 L/day)
- Usage factor of 50 % firewood and 50 % LPG
- Average feedstock: 50 % cow manure and 50 % pig manure
- Slurry rate: 15 L/day
- 90 minutes cooking time per day
- Ideal retention time of 25 days (how much time it takes for all the slurry to be replaced)
The Gasmeter formula also follows the previous assumptions, plus several new ones. Since the gasmeter data replaces average cooking time, it only influences BG (biogas emissions) and OCS (cook stove). The gasmeter formula thus contains the slope 0.004437758.
Areas for Improvement
This calculation is a rough estimate by Su-re.co and is not yet validated by external experts. As knowledge and data are updated by us and others, the following areas can help us refine our calculations.
- Parameters used in the cow manure management calculation need to be updated to current standards. Furthermore, some data still uses agricultural data from the US, which need to be changed to data for Indonesia.
- We need to include emissions of biogas production subtracted by emissions of LPG and Chemical fertilizer production. This would require a proper LCA (Lifecycle assessment) analysis.
- Su-re.co can also reduce the assumptions by collecting more data about the manure type usage and type of the replaced cooking source. By starting a survey, the assumption will be replaced by specific data. Besides that, examining the consistency of our biogas and slurry would help get even more detailed results.
These measurements, however, will be costly and require extensive monitoring.
In short, regardless of the calculations' details, the results remain an estimate. Using the final estimates needs careful communication and transparency in the process.
Appendix
Carbon saved from old cooking sources, LPG/firewood (OCS):
The carbon saved by replacing the old cooking sources LPG and firewood comprises two parts.
The replacement of LPG multiplied by the usage factor of LPG (FLPG), and the replacement of firewood multiplied by the usage factor of firewood (FFW).
This calculation is based on the assumption of the following usage factors: LPG 80% and Firewood 20%.
The saved carbon comprises the mass of the old fuel that was burned multiplied by the emission factor of that source (EF). The energy needed for cooking is based on the assumption that it is possible to cook 90 minutes per day with the biogas. (not always true, e.g., on colder days).
The energy is, therefore, a product of the cooking time (tcooking), the supplying flow rate, the density of biogas (rhoBG) (the amount of gas used), and the heating value of biogas (HHVBG).
Carbon saved from chemical fertilizer usage (CF):
To calculate the carbon offset of the chemical fertilizer, we first need to know how many hectares can now be supplied by the bioslurry fertilizer. For this, we first calculate the amount of bioslurry produced per day (SO), using the level of bioslurry in the digester (2/3) divided by the retention time (ideally 25 days). The produced bioslurry multiplied by the density (rhoBS) will result in the mass of bioslurry fertilizer that can be used. By dividing this by the fertilizer needed per hectare (UBS), the number of hectares that can be theoretically supplied with the bioslurry per day is calculated.
In reality, this bioslurry (x25) can only be used every 25 days, not per day. Once we know the supplied hectares, we can calculate the chemical fertilizer usage for this area, which is prevented by multiplying it with the chemical fertilizer usage (UU). The calculation is based on the assumption that urea was used as fertilizer before, but this could be more specified if wanted. Lastly, multiplying this with the emission factor of kgCO2/kgurea will lead to the prevented chemical fertilizer offset.
Carbon emitted by cooking with biogas (BG):
The carbon emitted when cooking with biogas can be easily calculated by multiplying the amount of biogas needed for the daily cooking time of 90 minutes (energy needed for cooking as calculated before divided by the heating value of biogas and biogas stove efficiency) with the emission factor for biogas (EFBG).
Cow Manure Offset (CMO):
Cow/Pig Manure, when left on the field will release N2O and CH4. Therefore, by reusing cow/pig manure for the bioslurry it is not released into the atmosphere and can be included in our carbon offset calculation.
This calculation is again divided into two parts, one regarding pig manure and one regarding cow manure.
The factors FC and FP include each farmer's usage factor of the two manure types.
The theoretical calculation per digester is based on the assumption that pig and cow manure are used 50:50; for the COC, since gasmeters are calculated, the factors are calculated based on actual usage.
In the formula, the amount of emission of CH4 in cow manure (ECCH4) and in pig manure (EPCH4), as well as the emission of N2O in cow and pig manure (ECN2O/EPN2O), are multiplied by the related global warming potential (GWP).
To calculate ECCH4/EPCH4 and ECN2O/EPN2O, Su-re.co used the following formulas from GHG Protocol: Global Protocol for Community-Scale Greenhouse Gas Emission Inventories. The results are all based on the ideal feedrate of feedstock per day, calculated by dividing the slurry level in the digester by the retention time.
Carbon emitted by bio-fertilizer (BF):
The calculation for carbon emitted by bioslurry fertilizer is similar to the calculation for chemical fertilizer. The daily slurry outcome (SO) is multiplied by the emission factors for CO2/m3 slurry used and CH4/kg slurry. As the emission factor for CH4 is given in a different unit (per ha on which bioslurry was applied), Su-re.co included the density of bioslurry (rhoBS) and usage factor of bioslurry (kg/ha) to calculate the hectares that can be fertilized by the given bioslurry per day. Additionally, the global warming potential is included again to receive only kgCO2.
Digester Dimensions
Feedstock Pig Manure
Feedstock Cow Manure
Chemical Data CH4
Chemical Data CO2
Chemical Data H2O
Chemical Data N2O
Chemical Data N2O
LPG/Firewood Usage
Biogas Stove
LPG/Firewood Stove
Bio-Fertilizer Usage
Chemical Fertilizer Usage (Urea)
Co-benefits Research
Livelihood benefits
The livelihood benefits are the benefits that farmers get from the reduction of cost and time spent on getting LPG and / or firewood. The livelihood benefits scope of calculation follows the one stated in this Gold Standard document.
There are some assumptions made in this calculation that enable us to make our co-benefits tangible:
1. The usage factor of biogas replaces a prior use consisting of 50% firewood and 50% LPG
2. Farmers are cooking using biogas for 90 minutes / day
3. All the farmers used urea as fertiliser before we installed a biodigester
4. The output of organic fertiliser (Bioslurry) is 15L / day, assumed to reduce the amount of chemical fertiliser
5. Time spent collecting firewood interferes with working hours (farming).
Livelihood benefit = LPG + FW + CF + TS
LPG : cost reduction of LPG
FW : cost reduction of Firewood
CF : cost reduction of Chemical Fertilizer
TS: cost of time saved from searching firewood
We acknowledge other important impacts (gender, education, access to energy) but not taken into quantitative manner.