Waste water from the biodigesters in the cassava factory, and from the rubber factory, cannot be discharged to the river due to COD levels being higher than permitted by the Vietnamese Government standards.
There is no immediate use for this by-product which accounts for about 5% by weight of the cassava root that is processed in the factory. Thus with daily processing of 500 tonnes of cassava roots, the rind represents 25 tonnes.
The production of gas in one factory is below expectations apparently due to the pH value being too low because of too much starch entering the biodigester. It may be possible to increase the methane yield (and to reduce the COD Of the effluent) in all the biodigesters by adding critical nutrients to the influent.
The most appropriate way to deal with the waste water is to use it to irrigate the rubber tree plantations that are now displacing other crops on the region. Rubber trees are traditionally fertilized with a combination of organic and inorganic fertilizers. It should be possible to replace completely these sources of plant nutrients by applying the waste water directly to the soil in the rubber plantations.
This by-product has a high content of fiber which should make it suitable as a feedstock in a down-draft gasifier. It is estimated that the moisture content of the root waste is about 30%. To be suitable as feedstock in the gasifier the moisture should be reduced to less than 20%.
The expected yield of electricity from the syngas after processing 22 tonnes of root rind of 20% moisture in a downdraft gasifier is 14 MWh daily. Assuming a price for electricity of VND 1000/KWh, this is a daily production equivalent to 20 million VND (USD 1000).
The pH in the biodigester can be raised by adding urea to the cassava waste entering the biodigester. Research in Lao PDR with an experimental biodigester charged with orange peel (Inthapanya et al 2013: http://www.lrrd.org/lrrd25/11/sang25198.htm) showed that the methane production was increased by 32% by the addition of urea. A similar response can be expected by adding urea to the biodigesters in the cassava factory.
Yield of latex will be increased when rubber trees are fertilized with the waste water from the biodigesters charged with cassava waste. The increase in latex yield will be higher than from the traditional application of fertilizer because of the added benefits of the water as well as the plant nutrients.
Measurements will be made of pH, nitrogen, phosphorus and potassium, COD and suspended solids.
A production function design will be used with 5 levels of waste waster (from 0 to 150% of the level of nitrogen traditionally applied to rubber trees): (i) Rubber trees in the second year of growth; and (ii) mature rubber trees. There will be two replications of each of the levels of waste water giving a total of 10 plots.
Individual plot sizes will be equivalent to the areas that hold 10 trees. Application of waste waster should be continued over a full 12 month cycle
For the young trees in option (i); height and circumference of trunk (1 m above ground level) will be measured at monthly intervals.
For mature trees in option (ii) measurements are made of latex yield and quality.
For options (i) and (ii) soil quality will be monitored by application of the biotest ( http://www.lrrd.org/lrrd21/7/rodr21110.htm) together with determinations of organic matter and pH.
Gasification of the cassava root rind will produce process heat (or electricity) and biochar. Biochar when applied to soil will improve nutrient and water use and sequester carbon.
The 150 KVA gasifier, imported from Cambodia in MEKARN I, will be installed in the WUSONS (DAWU GROUP) Cassava factory to evaluate the potential of using the cassava root rind as feedstock. It is expected that partial drying of the rind waste (to<20% moisture) will be necessary before it can be gasified.
The inputs and outputs of the gasifier will be measured to determine: (i) the yield and quality of the gas (hydrogen and carbon monoxide) and the conversion rate (kg cassava rind per unit KWh) when the gas is used to power a gas engine to produce electricity; and (ii) the yield and composition of the biochar; and (iii) the effect of the biochar as soil amendment for growth and production of rubber trees.
The addition of urea to the input stream will raise the pH and increase the yield of methane by up to 32%; in addition the COD concentration in the waste stream emerging from the biodigester will be reduced and the fertilizer value increased.
Five experimental biodigesters (Kounnavongsa Bounthavone and Preston 2009 http://www.lrrd.org/lrrd21/2/boun21029.htm) each with 250 liters of liquid volume will be established in the WUSONS cassava factory to determine the optimum level of urea to be added to the cassava waste water at the point of entry to the biodigesters.
Urea will be added daily at levels of 0, 0.5, 1.0, 1.5 and 2.0 % of the solids entering the biodigester.
Gas production and methane in the gas will be monitored daily over a production cycle of 30 days. The COD and N, P, K in the effluent will be measured and the fertilizer value assessed using the biotest.
If the hypotheses underlying the project are proved there will be multiple benefits to the Cassava and Rubber industry and to the environment. Waste products presently having a negative value will be transformed into (renewable) energy. Pollution caused by discharge of waste waters from biodigesters and rubber processing will be transformed into positive end uses as source of renewable energy, as fertilizer for rubber trees, as a soil amender (biochar) which at the same time will sequester carbon in the soil.
Initial experiments have been made with the waste resulting from removal of the rind from the cassava roots. After drying to <20% moisture and mixing with 50% rice husks the waste was successfully gasified in an updraft gasifier stove. This material probably needs to be reduced to a homogeneous size with particles less than 10mm, in order to use it as the sole feedstock for gasification.
The gasifier imported from Cambodia is presently being installed in the cassava factory. A technician from Cambodia will assist in commissioning this unit. Initial tests will be with rice husks but the final objective is the use of the root waste as the main feedstock.
Initial experiments will investigate the effect of adding urea to the waste streams used as influent to the biodigester. The study will be done with small scale plug-flow tubular polyethylene biodigesters of 400 liters liquid volume. This research will be started in late May 2014