BAF has reached majority after 21 years, says Black & Veatch's Frank Rogalla
The largest facilities today have unitary filter sizes close to 200m2 and serve communities of up to 8M PE. Table 1 gives an overview of the main technologies and the largest plants built to date. The principle of biofiltration is rather old and had been applied in some drinking water filters. If disinfection of the media is avoided and oxygen saturation favoured, ammonia concentrations of up to 2mgN/l can be eliminated by nitrification.
The main idea of BAF is to use the filter grain both for the retention of solids and the attachment of a biofilm. The growth of bacteria has to be favoured by providing the right nutrients, like nitrate and a carbon source (often methanol) for denitrification or oxygen through aeration - the latter created the acronym biological aerated filter. Like classical filters, regular backwash is applied to release the accumulated solids and remove the excess biomass grown by the degradation of pollutants during the filtration cycle. In BAF, all treatment can take place in one compact reactor, with no need for additional solids separation. Because biomass is attached to the media and does not need to be concentrated in a downstream clarifier, total biomass concentrations of three to five times the typical activated sludge mixed liquor values of 3-5g/l can be achieved.
In addition to high biomass concentration, specific bacteria with slow growth rates, such as nitrifiers, can be retained. The fact that only one compact bioreactor is needed, even to reach high-quality effluents, made BAF technology a favourite for tight sites. These compact reactors are often covered and provided with odour removal for better integration into urban neighbourhoods, such as along the French Riviera, in Milford Haven or in Aberdeen. Alternatively, the reactors are wedged into existing sites for upgrades or extensions and to meet new effluent requirements, such as in northern England - Davyhulme, Halifax and Mitchell Laithes. As far as technological options, initially the advantages of classical downflow filtration prevailed:
- the counter-current backwash, where the part of the filter most heavily loaded with solids, in the upper part, is also most vigorously backwashed,
- the accumulated solids are washed out in the shortest way possible, the reverse path they came from,
- in addition, the filtration proceeds in the sense of media compression,
- backwash nozzles are only in contact with already filtered water, reducing the risk of clogging.
The first generation of BAF technologies, Biocarbone, Biodrof and Denite, were derived from classical downflow filtration. The only real difference was the media type and grain size:
- denite used silica sand in the 2-3mm range,
- biodrof and Biocarbone used expanded shale in the 3-6 mm range.
While Denite was used for nitrate removal with methanol addition, Biodrof used downflow co-current aeration, trying to pull the air through the media in a similar fashion as classical trickling filters. On the contrary, Biocarbone had an aeration grid placed in the bottom of the media and used counterflow aeration. But it was soon realised that the presence of a gas, in addition to the filter grain, water and solids, complicates the hydraulics of the reactor. The Biodrof system, modelled after traditional trickling filters, was soon abandoned because on a larger scale it proved too difficult to achieve even distribution of co-current downflow air into the fine granular media. In downflow denitrification special bumping procedures, regular counter-current flushes, have to be introduced to release the gas bubbles accumulated in the media.
While the counter-current air and water system of Biocarbone was reasonably efficient, filtration cycles could be shortened dramatically at high flows, when in the upper, more clogged part of the filter bed, upflowing air bubbles would be trapped by the high downflow water velocity.
The main advantage of BAF processes is compact nitrification, since in classical activated sludge, to reach a sufficient sludge age to accommodate the slow growing nitrifiers, aeration basins need to be increased three or four fold. In addition, long sludge ages tend to lead to less settleable solids, increasing the necessary clarifier surfaces. Therefore, the nitrogen removal requirements of the European Wastewater Directive increased the use of biofiltration in the early 1990s. But if denitrification needs to be achieved in BAF and additional carbon source avoided, the nitrified effluent has to be recirculated into an upstream anoxic filter.PREDICTABLE PATTERNS
The denitrification efficiency will be proportional to the recycle rate but with diminishing return. The recycle causes high filtration velocities, often prohibitive for economic operation of downflow systems. Increased needs for nitrogen removal therefore also favoured the application of upflow BAFs, such as Biofor, which can reach a more even and predictable headloss pattern at high flows. Alternatively, open media was applied, such as plastic packings in the biopur reactor or larger grain blast furnace slag in the SAF to avoid trapping bubbles and decrease headloss at high velocities. The open media also reduces backwash needs but requires an additional solids separation step, often a downstream granular filter, with or without aeration. Regarding technological choices, the classical upflow filter, with the influent being fed through a distribution device in the bottom and expanding the media, needs fine screens upstream to protect the nozzles.
Upflow operation is also prone to release solids if the air and water velocities are high enough to expand the media. Additionally, co-current backwash is rather difficult because the solids have to be pushed through the whole filter bed, increasing the risk of a longer filter ripening period after start-up.
To avoid these difficulties, floating filters were introduced, using lightweight media retained by a grid or floor in the upper part of the filter. But the most interesting feature was the possibility to lodge the aeration grid in the middle of the filter bed, separating a non-aerated zone in the bottom of the media for prefiltration or denitrification if the treated, nitrified effluent is recycled to the influent. This was first applied in the Biostyr process, which uses expanded polystyrene beads as media.
The Biostyr process for total nitrogen removal has been applied in some sites where total nitrogen requirements are very stringent, such as in Frederikshave and Nyborg (Denmark) with the annual average of less than 8mgTN/l or in Herford (Germany) where the maximum spot sample concentration allowed is 13mgTN/l. In addition, backwash could be simplified to downflow gravity flush, with the clean water stored on top of the filter, eliminating backwash pumps.different types of media
Depending on media density, lower velocities of air and water can be applied to reach the expansion necessary for solids release, as practised with the biobead system, which uses recycled plastics (such as polypropylene and polyethylene) as media. Plastic media sizes are more regular and can be more easily adapted to various needs as mineral media, with the grain size varying from 2-3 mm, 3-4 mm, up to 5-6 mm.
Depending on the raw influent characteristics and effluent objectives, the best media size can be chosen, either favouring high-specific surface with finer media or higher solids storage and less hydraulic resistance with larger grains. In the Colombes WwTW, west of Paris, designed for a dry weather flow of 2.8m3/s, short-term storm peaks of up to four times higher are tolerated. To achieve nutrient residuals of 10mgTN/l and 1mgTP/l for 1M PE, the plant consists of chemically enhanced primary treatment and three BAF steps for COD removal, nitrification and tertiary denitrification.
As the rain peaks, it also dilutes the influent concentration, all flows can be biologically treated while still coming close to the dry weather effluent quality. This is achieved by switching all BAFs in parallel and converting the anoxic filter to aerobic COD removal. Up until recently, to minimise clogging potential, most biofilters relied on efficient preliminary treatment, often a chemically enhanced primary treatment or a first biological step.
A further development to reduce sensitivity to influent characteristics and place the BAF system directly on the screened sewage without primary settling was the multi-media upflow filter. This reactor is known as B2A for the anaerobic and aerobic conditions it allows, with an aeration grid in the middle of the multi-layer media build-up.
It allows the recycling of effluent in order to achieve denitrification in the bottom of the filter but a dual backwash system has to be applied to flush both the bottom part and backwash the upper layers and settling of the backwash solids has to be provided separately since no primary basin is included upstream.
Like anybody growing up, BAF technology has accumulated a lot of experiences in the last two decades, and has been learning from its mistakes. It is now a mature process that can adapt to various situations and react flexibly to new challenges especially when high effluent quality on a small footprint is required