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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

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 20C 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.

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.

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.

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.

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
One possible alternative to deal with hydrogen sulfide  is to add ferric or ferrous chloride to the influent to the anaerobic reactor to tie up the sulfide as ferrous sulfide. This alternative works well but will increase the density of the sludge so may not be best for UASB or EGSB reactors. It takes about 1.1 kg of Fe per kg of sulfide if no competing reactions are present.  Another alternative is to strip the sulfide from the gas stream using a caustic scrubber at pH > 9.  This method would require a spray tower, a caustic (NaOH or KOH) feeder, and a sump for recycling the scrubber water. This method works well but can require substantial amounts of caustic because some carbon dioxide also will be scrubbed as sodium or potassium bicarbonate.  Biofilters work well for vent gases but would be very large and expensive for biogas streams. The Hiperion and similar processes provided by a number of vendors convert the hydrogen sulfide to elemental sulfur, but can be quite expensive. Activated carbon works well but probably will be very expensive.

ANAEROBIC TREATMENT AND HYDROGEN SULFIDE CONTROL
One possible alternative to deal with hydrogen sulfide  is to add ferric or ferrous chloride to the influent to the anaerobic reactor to tie up the sulfide as ferrous sulfide. This alternative works well but will increase the density of the sludge so may not be best for UASB or EGSB reactors. It takes about 1.1 kg of Fe per kg of sulfide if no competing reactions are present.  Another alternative is to strip the sulfide from the gas stream using a caustic scrubber at pH > 9.  This method would require a spray tower, a caustic (NaOH or KOH) feeder, and a sump for recycling the scrubber water. This method works well but can require substantial amounts of caustic because some carbon dioxide also will be scrubbed as sodium or potassium bicarbonate.  Biofilters work well for vent gases but would be very large and expensive for biogas streams. The Hiperion and similar processes provided by a number of vendors convert the hydrogen sulfide to elemental sulfur, but can be quite expensive. Activated carbon works well but probably will be very expensive.

 

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Thomas Irwin, M.S. Environmental Scientist/Rutgers
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