ANAEROBIC FILTERS: MOST COMMON APPLICATIONS
GENERAL ANAEROBIC PROCESS SELECTION HOW TO GUIDELINES: 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
The first task is to get reasonably accurate measures of waste flows and COD/solids loads and other critical parameters such as TKN and sulfates. Then one can work toward process selection and reactor sizing. Other treatment objectives such as nitrogen removal need to be factored into the process selection and sizing. Without treatability testing, it usually is not beneficial to use complex modeling so a simple, educated spreadsheet approach to reactor sizing and estimating of biogas production is OK for starters.
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 WASTEWATER
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.
DAIRY PROCESSING AND RELATED INDUSTRY 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.
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.
WASTEWATERS FROM PHARMACEUTICAL INDUSTRIES
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 aerobic trickling filter 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.
WASTEWATERS FROM SUGAR RICH 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.
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.
CONFINED ANIMAL OPERATIONS & FEEDLOTS
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 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.
CALCULATION OF SRT (SOLIDS RETENTION TIME)
For the purpose of rough calculations we can, whatever the units used, and using say COD as constituent, combine these two formulas:
Yn = Yo * ( 1 + 0.2 * Kd * SRT) / ( 1 + 1.2 * Kd * SRT)
Yn = bugs in tank * 100 / ( SRT * kgCOD/day * COD removal points)
Thus we have
bugs in tank * 100 / ( SRT * kgCOD/day * COD removal points) = Yo * ( 1 + 0.2 * Kd * SRT) / ( 1 + 1.2 * Kd * SRT)
Solving for one of the SRT we get the coveted iteration formula:
SRT = bugs in tank * 100 * ( 1 + 1.2 * Kd * SRT) / ( kgCOD/day * COD removal points * Yo * ( 1 + 0.2 * Kd * SRT) )
From now on it’s just a matter of allowing Excel to perform circular reference iterations (you may have to enable this setting in your Excel version).
For those unfamiliar with iterative methods (a numerical analysis classic) one way to find a root of an equation is to solve for the variable and give iterative methods a try.
Were we to find a root for say
a * x^2 + b * x = (a * x + b) * x = 0
One possible iteration formula to try could be solving for x as follows:
x = 1 / (a * x + b)
While it may seem laughable to do it for this case the truth is that it works great for the not so immediate cases. With minor tweaks it can be used for UASBs, HAFs, BVFs, BNR TFs of all sorts.