A Primer on UASB Reactors | Fundamentals & Applications
GENERAL GUIDELINE: A MOST USEFUL COMPASS
To a certain extent, perhaps the following graph may be one of the most helpful charts ever to help assess the various anaerobic technologies/configurations available for targeting a specific wastewater.
Abattoirs are well suited targets to low-rate, anaerobic process because of the usually low COD and high O&G levels. It may be also possible to design as competitive, higher rate systems. As with most every anaerobic approach, process temperature is key. One will want to avoid seeing an anaerobic treatment plant for wastewaters that contain grease, such as meat processing, milk, cheese, etc., operate at less than 32C. If client or engineers insist, they have to deal with the consequences of designing and operating at lower temperatures. First , a larger reactors. Second, most of the grease will float to the top and form a scum layer that in some cases with meat slaughtering operations has approached six ft (2 m) in depth. This scum layer is very difficult to breakup especially if the reactor is covered with a membrane type material. These problems may occur at 32C, but to a much lesser extent. Therefore stick to and maintain the higher temperature range. Below 20°C removals are simply settling, hardly any anaerobic biological process contribution.
Abattoir wastewater from fowl is particularly difficult to deal with in
that it contains a lot of protein and gut waste which is high in nitrogen.
The protein is slow to degrade with strong odor potential, as rotting
protein tends to have. Cadaverine for example is one of the organic
chemicals produced by decaying flesh that gives it the ugly smell. In warm
climates, picking things like open, trickling filters or aerobic biotowers
where biomass and flesh could get trapped on the media and wherein the
decay odors could get passed directly to air leaving the tower, would be
definitely unadvisable. Many of abattoirs in the U.S. are using oversized
and relatively inexpensive oxidation ditches to process these wastewaters.
CITRUS AND WET MILLING WASTE
Anaerobic treatment is the best option for both citrus and wet milling
wastewaters. To a certain extent high rate configurations such as
UASBs and EGSBs can be considered as well as low rate reactors. Low
rate rate systems will feature design loading rates probably somewhere in
the 0.5 to 1.0 kg/m3/day range, so as with high rate designs one can
roughly work out the approximate volumes. Low rate systems are good
for wastewaters such as thin stillage.
Corn wet milling wastewaters are ideal candidates for anaerobic
treatment. Whole stillage is best treated using completely mixed reactors;
thin stillage -- after solids removal -- can be treated using high rate
processes. One can give preliminary reactor sizing if the general
wastewater characteristics are known -- COD, TSS, VSS, TKN, etc. -- and
Landfill leachates are difficult to treat anaerobically. While HAFs have been used, their history is not good. The high inorganic dissolved solids usually include substantial amounts of calcium which precipitates as calcium carbonate in the reactor. This can eventually plug the media.
Landfill leachates contain significant amounts of nonbiodegradable or very slowly biodegradable organics and often contain substantial amounts of color. Heavy metals usually are a minor problem since they will be absorbed by the biomass. Nitrogen levels usually are high so that ammonia released or there will be a demand for oxygen to satisfy nitrification. High O&G figures probably point to other hexane extractable materials --- probably organic acids. It is highly recommended highly that treatability tests be conducted so that the designer will know exactly what efficiencies can be achieved.
A possible approach could be a low rate mixed reactor or a contact process
that includes a mixed digester and a clarifier for solids removal. Since
the wastes usually are not amenable to granulation and because of the high
dissolved salt content, landfill leachates usually are not treated using
UASB or EGSB reactors. A low rate type reactor would be designed for
operation at up to 1 kg/m3/d and a contact process would be designed to
operate at around 4 kg/m3/day. Still be careful with this one. The
problems can be difficult to manage.
In general, one needs to know more about this wastewater before making
a decision about the best way to treat it. At first thought it may not
seem a good candidate say for TF treatment because of the high strength
and the consequent need for high recycle rates, in addition to the concern
for odors. Anaerobic would be much better, but pharmaceutical wastewaters
often contain antibiotics and sulfates that can make anaerobic treatment
difficult. Because of the nature of the waste, a treatability test
should be commissioned to review the actual characteristics and determine
if there are any constituents that would cause problems with anaerobic
Biomass from sugar wastewaters form granules readily, so there is
little risk to using UASB/EGSB reactors. For this wastewater type UASB/EGSB
processes are much better than say attached growth, anaerobic processes
such as HAFs. Sugar wastewaters produce a lot of biomass that
can accumulate in the media and cause floatation and damage to the
reactor. Unfortunately one has seen this happen too many times, in some
cases within the first year. There are some ways to avoid these
problems, such as installing gas purge systems to blow the excess solids
out, but not every project/design makes provisions for this
HAFs are much better for acetic acid and protein wastewaters that contain
little suspended solids.
UASB or EGSB reactors are best suited for treating bottling wastewaters
but low rate installations do exist.. One advantage of the low rate
reactor is that it is much more forgiving and requires less EQ volume up
front. One would expect the cost is not greater than an UASB or IC
reactor; otherwise low rate alternates would not be promoted.
The greatest challenge in breweries is control of toxic agents used as
lubricants and biocides in brewing operations, and diatomaceous earth can
cause significant abrasion of granules.
Feedlot wastes are treatable anaerobically, but require a process configuration that allows the owner to process high solids content.
Tannery wastewaters are very difficult to treat anaerobically because
of the salts, acids and chemicals used for processing the hides. One
further needs to know the type of tanning process -- vegetable,
chrome, etc. -- and the general characteristics -- COD, VSS etc. If the
wastewater is treatable, one would expect one will need to use DAF to
remove the solids before anaerobic treatment. A low rate type reactor
could be OK, but probably design COD loading rate should be close to 1.0
ANAEROBIC TREATMENT AND HYDROGEN SULFIDE
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