Overview on the Upflow Anaerobic Sludge Blanket (UASB)
The
Upflow Anaerobic Sludge Blanket
or UASB is one among many different types of
Methane Digesters. Prof Chan terms the UASB design the "biggest breakthrough in the design of digesters." This is based on the realization that UASB has implications in making renewable energy production feasible in marginalized regions where there is currently a surplus of
organic liquid animal waste?.
From
UASB.org (History of UASB):
The UASB process was developed by Dr. Gatze Lettinga and colleagues in the late 1970's at the Wageningen University (The Netherlands). Inspired by publications of Dr, Perry McCarty (from Stanford, USA), Lettinga's team was experimenting with an anaerobic filter concept. The anaerobic filter (AF) is a high rate anaerobic reactor in which biomass is immobilized on an inert porous support material. During experiments with the AF, Lettinga had observed that in addition to biomass attached on the support material, a large proportion of the biomass developed into free granular aggregates. The UASB concept crystallized during a trip Gatze Lettinga made to South Africa, where he observed at an anaerobic plant treating wine vinasse, that sludge was developing into compact granules. The reactor design of the plant he was visiting was a "clarigestor", which can be viewed as an ancestor to the UASB. The upper part of the "clarigestor" reactor design has a clarifier but no gas cap.
Prof Chan notes that "Methane Digestion has been practiced in China for decades now, not as a result of research until the Cultural Revolution
got the academics to spend a year or two in their villages, but because the
peasant knew that air should not get into the digester, even if they did not
know about the necessary anaerobic conditions to get useful METHANE instead of useless AMMONIA."
WikiPedia Excerpt on the
Upflow Anaerobic Sludge Blanket:
The UASB reactor is a methanogenic (methane-producing) digester that evolved from the anaerobic clarigester. A similar but variant technology to UASB is the expanded granular sludge bed (EGSB) digester. A diagramatic comparison of different anaerobic digesters can be found here.
UASB uses an anaerobic process whilst forming a blanket of granular sludge and suspended in the tank. Wastewater flows upwards through the blanket and is processed by the anaerobic microorganisms. The upward flow combined with the settling action of gravity suspends the blanket with the aid of flocculants. The blanket begins to reach maturity at around 3 months. Small sludge granules begin to form whose surface area is covered in aggregations of bacteria. In the absence of any support matrix, the flow conditions creates a selective environment in which only those microorganisms, capable of attaching to each other, survive and proliferate. Eventually the aggregates form into dense compact biofilms referred to as "granules". A picture of anaerobic sludge granules can be found here.
How does the UASB Work?
Fine
granular sludge blanket 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 center 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:
- 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;
- Acidogenesis or acetogenesis - The result from stage one utilized by a second group of organisms to form organic acids;
- 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, we are 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 (polyculture) and agriculture and the innovative aquaponic that can be utilized to rapidly grow plant mass from the mineralized water. The health related aspects associated with re-use of the effluent for aquaculture and agriculture are very important, and need more or further research and testing. It is important to note that the IF&WMS is giving 100% treatment to the original wastes, so we are NOT really using actual wastes.
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). Digester 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.
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