ANAEROBIC FILTERS (FIXED-FILM) PROCESS FUNDAMENTALS
Anaerobic fixed-film reactors were developed in 1968 and have grown to represent an advanced technology that has been used effectively for treating a variety of industrial wastes. A number of variations have been developed in the intervening years including the fully packed upflow anaerobic filter (AF) , the fully packed downflow anaerobic filter and the upflow hybrid anaerobic filter (HAF).
Fixed-film reactors are basically contact processes in which wastes pass over or through a mass of biological solids contained within the reactor, which is attached to the surfaces of a media matrix as a thin biofilm, entrapped within the media matrix, or held as granulated or flocculated biomass within the reactor by the action of the media or a gas-solids separation device. Soluble organic compounds passing in close proximity to this biomass diffuse into the surfaces of the attached or granulated solids, where they are converted to intermediates and to end products, specifically methane and carbon dioxide.
UPFLOW ANAEROBIC FILTERS
Anaerobic filters consist of cylindrical or rectangular tanks having an enclosed fixed media within the reactor. Volumes of full-scale reactors have ranged from about 100 to 2,000 m3. Media heights have ranged from essentially full-depth (i.e. fully packed anaerobic filter) to placement only in the upper 50 percent to 70 percent of the reactor height (hybrid anaerobic filter).
Almost all upflow fixed-media reactors constructed since 1985 have been designed using the hybrid configuration.
The biomass in upflow hybrid anaerobic filters is retained as a granular sludge layer or flocculant sludge blanket in the lower levels of the reactor and attached to the media in the upper levels. The distribution of biomass varies with the specific design, the type of media, and the method of operation.
DOWNFLOW ANAEROBIC FILTERS
Downflow anaerobic filters have been used in full-scale systems since 1983 to treat industrial wastewaters having COD concentrations of 15,000 to 85,000 mg/L. Reactor configurations have included cylindrical tanks ranging in volume from 600 to 4,000 m3. The media generally fills the reactor with the exception of shallow distribution and collection zones. The active biomass in downflow reactors is retained almost exclusively as a biofilm attached to the media. High recycle ratios are typically used in downflow reactors to maximize contact between waste constituents and the attached biomass.
A notable feature of the downflow anaerobic filter is its ability to accept wastes having solids concentrations as high as 4 percent with concern for excess solids accumulation. However, most of those that have been in operation [say at Bacardi and BP/Amoco plants] did not work well because of plugging and most have been abandoned. As of now, we’re not aware of any vendor who provides full design and follow up service on this type.
HYBRID ANAEROBIC REACTORS
The hybrid reactor design combines an lower section functionally identical to an UASB and an upflow AF on top, the idea being to combine the strengths of each approach in a single tank. Thus, the lowermost 30 to 50 percent UASB-like portion of the reactor volume is responsible for flocculant and/or granular sludge formation. The upper 50 to 70 percent of the reactor is filled with crossflow plastic media and behaves as an fixed-film anaerobic filter.
ANAEROBIC FILTER MEDIA
The media in fixed-film anaerobic reactors helps to provide uniform flow through the reactor, improves contact between the waste constituents and the biomass contained within the reactor, and causes accumulation of the large amounts of biomass needed to produce the long solids retention times that are required to treat complex industrial wastes. Commercial media available for use in anaerobic filters include Pall rings (or similarly designed loose-fill media) and modular, block-type media formed from corrugated plastic sheets. The channels in the media modules may be tubular so that no lateral flow occurs through the height of the block, or counter-stacked so that a crossflow redistribution effect occurs at the contact points within the unit’s height.
The amount of media to use in upflow hybrid anaerobic filters is quite subjective. Since the growth on the media surfaces provides substantial COD removal, and since the media aids in flocculating the biological solids, there is a limit to how little media can be used. The media-to-height ratio seems to be the critical factor, and reactors having 70 percent or less of the volume occupied by media generally have experienced increased biomass loss and reduced efficiency.
Media used in full-scale upflow anaerobic filters averages 30 sq.ft./cu.ft. (100 m2/m3) specific surface arearegardless of the type of media. Research indicates that less than 5 percent improvement in COD removal efficiency is gained by more than doubling said specific surface area. Therefore, it seems likely that the additional cost of high-density media cannot be justified routinely. While pilot- and full-scale data shows lower efficiencies for loose-fill and tubular media than for crossflow media, site-specific considerations, economics, and operating factors should ultimately be the determining factors.
Clogging of media has been a concern of a number of designers and potential users of upflow anaerobic filters. While clogging was a problem with early fully packed designs using rock and loose-fill media methods to scour and flush excess solids from the media periodically have been included in recent designs with loose-fill media. No instances of plugging have been reported for crossflow or tubular modular-media having specific surface areas of about 30 sq.ft./cu.ft. (100 m2/m3)
Since waste conversion in downflow reactors is associated almost exclusively with attached biomass, these reactors must be essentially filled with media to realize maximum use of the contained volume. While a number of types, configurations, and sizes of media have been evaluated in laboratory and pilot-scale downflow anaerobic filters, only tubular media having 30 sq.ft./cu.ft.specific surface area have been used in full-scale downflow reactors.
INFLUENT DISTRIBUTION SYSTEM
A major factor in the design of full-scale fixed-film reactors is the proper distribution of the influent wastewater and the associated effluent recycle. Placement of the distribution orifices should take into consideration the hydraulic limits of piping and orifice sizing. Generally, distribution orifices for upflow anaerobic filters are placed no farther apart than 1 m. This spacing requires orifice diameters of about 18 mm (3/4″) to maintain an outlet velocity of 1 m/s. Smaller orifices would be subject to increased plugging potential, and larger orifices would require greater spacing, which could result in poor distribution.
The difference between a full-packed or hybrid configuration is the placement of media relative to the distributor piping. In a full-packed design, the media rests on top of the distribution piping; in a hybrid configuration, the media is placed on a support grid located about one-third to one-half depth above the distribution piping. A rock-filled unit must be fully-packed since it is not possible to build a support to hold the weight of the rock above the distribution piping.
Anaerobic filters almost always experience plugging and a need for cleaning because of the entrapped solids, and usually this means replacing the rock at one to three year intervals. There are no other options with rock. Plastic media is easier to clean either by removing the media blocks or by using a water jet from the top if the media depth is less than four feet. The structure that holds the media must be strong enough to hold the media with solids attached so that it does not fail during the rinsing process. A hold-down grid must be place on top of the media to prevent is flotation when the biomass in the media traps gas bubbles.
Some installations use two fixed-film reactors in series. Not only is the COD removal efficiency improved over that of single-stage reactors operating at the same hydraulic retention time, but operation in the cyclic mode – that is, by periodically reversing the lead and follow reactors – produces significant reduction in excess solids production. A number of plants of this type have been placed in operation since 1987. One kind of two-stage option uses the first stage as a hydrolysis reactor, while the second is designed to optimize methane production.
TOXIC IMPACT HINT
Occurrence of high pH in conjunction with high VFAs indicates strongly a toxic impact, possibly due to high ammonia concentrations or chemicals used in the industrial processing plant.
SULFURIC ACID AND ANAEROBIC TREATMENT
It is not good to add sulfuric acid to anaerobic treatment processes. The associated production of hydrogen sulfide can lead to odors and toxic conditions. Hydrogen sulfide can react with trace minerals, specifically iron, to produce black finely dispersed solids.