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A Primer on Anaerobic Reactors | Fundamentals & Applications |
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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. SRT = total active biomass concentration / net growth of active biomass ABATTOIRS 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. HAFs are not good candidates for treating dairy wastes, Because of the high FOG and high biomass yields, the media tends to plug and float. UASBs are much better if the FOG is less than about 100 mg/L. Low rate or completely mixed reactors usually are best overall. 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
flow rates. LANDFILL LEACHATE 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.
PHARMACEUTICAL WASTE 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 treatment. SUGAR INDUSTRIES 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. BREWERIES 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.
TANNERIES 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 kg/m3/day.
ANAEROBIC TREATMENT AND HYDROGEN SULFIDE
CONTROL
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