In the Integrated Farming System (IFS), the digester is considered the heart of the system. Volatile organic solids such as plant, agro-industrial and animal waste are collected mixed with water to create the desired slurry mix and then are fed into the Digester. The diagram above shows how the IFS system works.

Many different digester designs are used. The Upflow Anaerobic Sludge Blanket (UASB) is preferred by Prof Chan the developer of the Integated Farming and Waste Management System (IF&WMS) which is considered one derivative approach of IFS. He considers it to be the biggest breakthroughs in the digester design, because it requires no energy to operate but still provides a high level of anaerobic productivity. The practice of using digesters in China has gone on for decades now.

How does the UASB Work?
Fine granular sludge blanket that acts as a filter to prevent the solids in the incoming wastes to flow through as the liquid part does. So if the hydraulic retention time (HRT) does not change, which is limited to 1-3 days (the bigger the digester, the shorter time it is, because the size costs money), the solid retention time (SRT) can be 10-30 days or more for more effective digestion, depending on the shape of the digestion chamber. It means that the digester becomes much more efficient without having to increase the size, which costs money. Wageningen University in the Netherlands has started to do R & D along these lines.

Standing and hanging baffles are used, with a conic separation with a small outlet at the centre will be much more effective to keep the anaerobic sludge blanket in the lower part of the digester. This will act as a very good filter to retard the flow of solids in the wastes and prolong the solid retention time for more bacterial action. However, the digester will be more economic if the loading can be increased for a specific size of digester with the conic separation. COD reduction of 58% now obtained is adequate, and no attempt should be made to increase the bacterial action at such high costs. It is better to use much cheaper open tanks and basins for more effectiveness and efficiency, as in the IF&WMS.

Bio-Chemical Activities in USAB Digesters
Bacterial actions are in 3 phases in the digester and they occur IN SEQUENCE:

1. Hydrolysis or solubilization - The first phase takes 10-15 days, and until the complex organics are solubilized, they cannot be absorbed into the cells of the bacteria where they are degraded by the endoenzymes;
2. Acidogenesis or acetogenesis - The result from stage one utilized by a second group of organisms to form organic acids;
3. Methanogenesis - The methane-producing (methanogenic) anaerobic bacteria then use the product of (2) to complete the decomposition process.

Sludge Stabilization
2 low-cost sumps retain most of the floating scum and solids. They can be removed at leisure, because it is too much to ask a farmer or operator to do this tedious work DAILY. Biogas can be used to heat the liquid in both sumps to 70-80 degrees Celsius, to make sure no pathogens survive. Scum and solids are instant compost, good for healthy vegetables or pretty flowers. Desludging is NOT necessary, as near the outlet of the digester, after more settling on the way which is collected in sumps, some stabilized sludge will come out with the effluent daily. It is already good compost for mushroom and earthworm cultures as food or feed respectively.

USAB’s Key Role in the Integrated Farming System
In the IF&WMS/IFS, George Chan and ZERI have developed an innovative form of PolyCulture. This involves not only converting all organics into more nutrients in cheaper tanks and basins, as already stated above, but are using all of them in both aquaculture,  and agriculture and innovative aquaponics that can be utilized to rapidly grow plant mass from the mineralized water.

Digester Variations
A digester can be made of a simple plastic bag of 10 m3 for the isolated family or a reinforced concrete or steel tank of up to 8,000m3 (so far) for farms, factories, distilleries, etc. In all cases, the pay-off time is 2-3 years if the digester operates with an Upflow Anaerobic Sludge Blanket (UASB). The marvelous part is that it does not require much space or costly equipment like other sources of renewable energy.

China is already building UASB digesters of 2,000 m3 as standard equipment to deal with human, livestock and most industrial wastes and make their factories and farms self-sufficient in energy and fertilizers, and is now developing 8,000 m3 reactors. For more read Dr Mae Won Ho's Article Biogas China.

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