"which the palm oil is emulsified by glycoprotein Gum Arabic (GA) as the chemical emulsifyingagent.GA has been tested that it has no influence on the bacteria growth and could not be used as anutrient source by the bacteria.Sometype of bacteria could synthesize surfactants which couldlower the interfacial tension and increase the bioavailability of hydrophobic carbon sources(Rosenberg & Ron., 1999).The palm oil is known to produce high yield of PHB by its high carbon source pergram.(Charles et al.,2011)had cultured the Ralstoniaeutropha bacteria in emulsified palm oil toproduce PHB at a high yield level as 79% of cell dry weight consisted of PHB.In thefermentation of R.eutropha,the polymer was formed at a yield of 0.61g PHB/g palm oil and0.84g PHB/g palm oil if only the PHB production phase of the culture is considered (Charles etal., 2011).PHB FROM GLYCEROLGlycerol is a secondary product of the biodiesel industry. A recent increase in biodieselproduction has generated a surplus of crude glycerol and subsequently driven down the price.This allows that glycerol can be used as a cheap raw material for the production of PHB (John A.et al, 2011). According to John (2011), the bacteria strain called RalstoniaeutrophaJMP 134 canproduce PHB through fermentation process using crude glycerol. Since the crude glycerolcontains low concentration of glycerol and other impurities, a purification process is necessarybefore producing PHB. The feedstock is evaporated first and 90% methanol is recovered and thisstep decreases the purification costs up to 37.5%. Afterwards, an acid solution is added toneutralize the feedstock stream. Then centrifugation, evaporation and distillation processes arecarried out in order to obtain 98 wt% of glycerol.Crude Methanol Neutralization CentrifugationglycerolRecoveryDistillation Water DecantationProduct streamevaporation Figure2. The purification process of crude glycerolThe fermentation process is performed into two stages for mass cell growth and PHBaccumulation. The fed-batch culture is the most widespread method to obtain the needed nutrientlimitation and to reach higher cell density. Raw materials with the concentration of 249g/L werepumped into the bioreactor in 25 atm outlet pressure. This fermentation process started withsterilization operation. Subsequently, for the purpose of achieving 15 to 20 DOC% saturation,pure oxygen and air were fed in the first tank continuously. Consequently, the cell massconcentration reached 91.5g/L while the concentration of PHB was 57.1g/L. Cell massFeedstock SterilizationPumping HeatgrowthexchangerPHBaccumulationFigure3. Process for PHB fermentationA downstream process that is more efficient in PHB extraction is used after the fermentationprocess.Fermentation CentrifugationH2O2-water Heatbrothwashing exchangerSprayWaterPHBdryingevaporationFigure4. Downstream processIt is evident that using crude glycerol as raw materials can lower production costsbecause the cost of carbon source accounts for nearly 45% of total cost (Sindhu et al, 2013).Nevertheless, the yield of PHB is very lower compared to some other carbon sources. Therefore,there are still many challenges existing for the production. PHB FROM CASSAVA WASTE WASTERCassava manufacturing industries are found on a large scale in sub-tropical regions, for examplein Africa. Research has been carried out before on using cassava starch hydrolysate as a suitablesource for producing PHB. Cassava waste water (CSW) consists of carbohydrates, phosphorus,nitrogen and various fatty acids therefore it has potential to be substrate for producing poly-3- hydroxybutyrate (Marcia N &Glaucia M., 2003). A research from (Sangyoka et. al., 2012) indicated that CSW can be used as a substrate tosynthesise PHB through RalstoniaeutrophaKKU38 using a 250 ml Erlenmeyer flask. CSWrequires some pretreatment processes so that higher yields of PHB can be generated. Firstly,simple gravity settling was applied to remove solids suspended and then filtrated to obtain a clearphase. Secondly, acidogenic fermentation process of the clear phase of CSW performed into 5Llaboratory bottle which contains anaerobic sludge in order to achieve higher concentrations ofvolatile fatty acids (VFAs). According to (Yu, 2001), VFAs are the effective elements forsynthesis of PHB and comparison between chemical characteristics of CSW and fermented CSWdemonstrated that the concentrations of VFAs dramatically increased after acidogenesis. In terms of microorganism RalstoniaeutrophaKKU38, a bacteria strain isolated from CSW was cultured innutrient broth at temperature 30 ? and shaking at 150rpm. As for the concentration of culturemedium and substrate, soluble Chemical Oxygen Demand (sCOD): nitrogen: phosphorus ratioshas an extremely influence on yield of PHB. Therefore, the original pH of culture medium was 7;KH PO and NH Cl were added into culture medium to adjust the cultivated condition to best.2 4 4 (Sangyoka, 2012) pointed out that sCOD: nitrogen: phosphorus at ratios of 100:0.5:11 was themost suitable conditions. At this ratios the bacteria could efficiently synthesize PHB up to themaximum PHB content of 85.53% and yield of 0.31 g PHB/ g COD consumed.The main advantages of CSW as a carbon source are the positive environmental effect ofremoving the SCOD from the CSW and the high by-product yield of starch. The yield of PHB islower than the other processes listed but is still comparable. See figure 5 for a simple block flowdiagram of the process. Sludge acidogenic PHBWaste waterAcidogenic effluentfermentation productionFig 5.Process conversion of cassava starch manufacturing waste water into PHBDISCUSSION - CHOICE OF RAW MATERIALAs introduced above, PHB can be produced by different carbon source and synthesized invarious bacteria. A comparison of various raw materials in terms of carbon source and bacteriaand PHB production in the sides of yield and biomass is illustrated in table 1 below. Table 1. Comparison of PHB yield and biomass with different carbon source and bacteriaPHB YIELD C-SOURCE BACTERIA (g/gram of PHB BIOMASSsubstrate) (%)Cassava starch Ralstoniaeutropha 0.31 85.53 %wastewater KKU38Glycerol RalstoniaeutrophaTISTR 0.54 54.01%1096Sugarcane Unspecified 0.34 72.10%RalstoniaeutrophaPalm oil Unspecified 0.61 79 %RalstoniaeutrophaAs shown in Table 1, PHB can be synthesized in different yields from various carbonsources and bacteria. Since the carbon source has been identified as one of the major cost- absorbing factors in PHB production, cheap and renewable substrates are currently beinginvestigated as substitutes for existing sugar-based feedstock(Kumar et al.,2011).Since sugarcontained in sugarcane can be used directly by the bacteria to produce PHB; the sugarcane is amajor carbon source for PHB. However, the cost of sugarcane as feedstock has no advantageswhen compared with the cassava starch wastewater.Compared with sugar, the palm oil could has been investigated as an economical andefficient feedstock in both laboratory and commercial scale since it offers competitive price andthe higher yield of PHB per gram of carbon substrate. At the same time, there are still somepotential problems need to be considered if the palm oil is chosen to used in the PHB production.The major problem is that the demand for palm oil will continue to rise due to increasing humanconsumption, which may cause an increase in the price of palm oil(Kumar et al., 2011).Thesecond, since the palm oil creates a heterogeneous two-phase medium with the water due to itsheterogeneity, it is necessary to emulsify the palm oil before used as a carbon feedstock (Charleset al., 2011).In the aspect of glycerol, it can be used as an inexpensive raw material for theproduction of PHB since the price of crude glycerol stay at a low level due to the increasing "