TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Method: (M1) further involves intermittently agitating X2 for 30 seconds-10 minutes every hour The method further involves collecting the methane generated in step (b)-(d). Methane is also generated in step (a) and collected. X1 has a pH of 4.5-6.5, and X2 has a pH of 6.5-7.5. The hydrolysis unit comprises a bacterial culture which is the member chosen from Aerobacter, Aeromonas, Alcaligenes, Bacillus, Bacteroides, Clostridium, Escherichia, Klebsiella, Leptospira, Micrococcus, Neisseria, Paracolobacterium, Proteus, Pseudomonas, Rhodopseudomonas, Sarcina, Serratia, Streptococcus, Streptomyces, Methanobacterium omelianskii, Mb.formicium, Mb.sohngenii, Methanosarcina barkerii, Ms.methanica and Mc.mazei and their mixtures. The methanogenesis unit also comprises a bacterial culture as described above.(M1) further involves pretreating the solid organic material prior to the first period of incubation, where the pretreatment is a chemical pretreatment, mechanical pretreatment, heat pretreatment or their combinations. The mechanical pretreatment is cutting and/or grinding. The chemical pretreatment is chosen from bicarbonate treatment, radiation treatment, alkaline peroxide treatment, ammonia treatment or their combinations. The heat pretreatment is carried out at 50 degrees Centigrade-120 degrees Centigrade, preferably 60 degrees Centigrade-90 degrees Centigrade. In preferred conditions, the solid organic material is rice straw, where the pretreatment comprises grinding the rice straw to produce rice straw sized from about 5-50 mm, heating the rice straw at 50-150 degrees Centigrade, preferably 60-90 degrees Centigrade, and treating the rice straw with ammonia, whose amount is equal to about 0.5-10% (preferably 1-5%) of total weight of rice straw. Preferably, a first incubation period and/or fourth incubation period occur in a hydrolysis reaction which is not the first hydrolysis reactor. Preferred System: (II) further comprises 1-15 additional hydrolysis reactors, where the hydrolysis reactors and the methanogenesis reactor are linked by parallel linking and/or series linking. The perforated support unit is chosen from grids, filters, grates, sieves, slats, strainers or their combinations. (II) further comprises a pump operably connected to the first hydrolysis reactor. EXAMPLE - Rice straw was collected in bales from a county in northern California and transported to the laboratory. The anaerobic phased solids digester (APS-digester) systems had one hydrolysis reactor and one biogasification reactor. The rice straw was processed in batches, i.e. the system was operated in a batch mode with a retention time of 24 days. During the digestion, liquid was circulated intermittently between the hydrolysis reactor and the biogasification reactor to transport the soluble substances from the hydrolysis reactor and the biogasification reactor to transport the soluble substances from the hydrolysis reactor to the biogasification reactor. After each batch of treatment, the residual solids were removed from the hydrolysis reactor and a new batch of rice straw was loaded. All the reactors were heated to a constant temperature of 35 degrees Centigrade.    The hydrolysis reactor was not mixed while the biogasification reactor was mixed intermittently (1 min every 2 hr). Each reactor was connected to a gas collection bag and a wet-tip gas meter which was used to record the daily biogas production volume. Gas samples were taken twice a week from the sampling port on the gas collection line of each reactor and analyzed for the contents of methane and carbon dioxide using a gas chromatograph (GC). The liquid samples were taken from each reactor and measured for pH. For each batch digestion, samples of straw before and after digestion and samples of the reactor contents before and after digestion were taken and analyzed vent for total solids (TS), volatile solids (VS) and pH. The reductions of TS and VS, daily biogas production, and total biogas production during the 24 day period were used to evaluate the performance of the digester system under different operational conditions.    A total of 17 runs were conducted including 3 repetitions. All the digestion runs were at a temperature of 35 degrees Centigrade and a retention time of 24 days. Effects of different pretreatment methods, including physical (mechanical), thermal, and chemical (ammonia) treatment, on the digestion of rice straw were investigated. The physical pretreatment included grinding the straw into two lengths (10 mm and 25 mm) and chopping the straw into one length (25 mm). Thermal treatment was conducted by heating the straw in a pressure cooker for two hours at three different temperatures (60, 90 and 110 degrees Centigrade). Chemical treatment was carried out with 58% ammonia hydroxide solution. The amount of ammonia added to the straw for each digestion run was 2% based on the dry weight of the straw digested. This level was determined based on the adjustment of C/N ratio of the treated straw to around 25. A higher temperature resulted in higher conversion rates of solids and higher biogas production. As compared with non-pretreatment, the TS and VS reductions were increased by 3.4-22.4% and 3.6-22.6%, respectively, and the biogas yield increased by 2.5-17.5% when pretreatment temperature varied from 60-110 degrees Centigrade. The temperature effect was not linear.    The increase of solids reduction (15.6%) and biogas production was more significant when the temperature increased from 60-90 degrees Centigrade. A higher pretreatment temperature resulted in higher daily biogas production rate during the first six days of digestion. The digestion process slowed down after two weeks when the hydrolysis of straw and release of soluble sugars became the limiting step. About 75-80% of the biogas was produced in the first two weeks. Effects of physical pretreatment were as follows. The smaller the straw particles, were, the better the digestion as i.e. the more solids reduction was and the higher the biogas yield was. Grinding yielded the best digestion results. More soluble sugars were available in the reactors during the initial 9 days, yielding a higher biogas production rate, if the straw was processed into smaller particles. The biogas yield of ground, 10 mm, thermally pretreated straw was 0.47 l/g VS fed, which was 17.5% higher than the yield of thermally pretreated whole straw (0.40 l/g versus fed). The chopped straw was very close to the whole straw for the digestion, showing only 2.5% increase in the biogas yield. Improvement of digester performance by mechanical processing (milling and chopping) was very small if the straw was not thermally pretreated.