Trickling filters (also called biofilters) have been used to remove organic matter from wastewater for nearly 100 years. The TF is an aerobic treatment system that utilizes microorganisms attached to a medium to remove organic matter from wastewater. The colonies of microorganisms attached to solid surfaces are called biofilms due to their thin layers of biological structures. This type of system is common to a number of biological wastewater technologies, such as rotating contactors and packed bed reactors (also called bio-towers). TFs are mainly composed of four major components: a filter medium such as stones, plastic shapes, or wooden slats; an enclosure to hold the liquid; a distribution system; and an under-drain system. The filter medium provides the surface on which the microorganisms grow. The enclosure holds both wastewater and filter medium, while a distribution system ensures a uniform hydraulic load over the entire TF and the under-drain system provides drainage, holds filter medium, and supplies oxygen to the bottom section.
A rotary or stationary distribution system distributes wastewater from the top of the filter, percolating it through the interstices of the medium (see Fig. 4.6). As the wastewater flows over the medium, the organic matters in the wastewater are adsorbed by a population of microorganisms (aerobic, anaerobic, and facultative bacteria; fungi; algae; and protozoa) attached to the medium as a biological film or slime layer (approximately 0.1 to 0.2 mm thick). As the wastewater flows over the medium, microorganisms already in the water gradually attach themselves to the rock, crushed granite, or plastic structure surface and form a film. The aerobic microorganisms in the outer part of the slime layer (biofilm) then decompose the organic material. As the layer thickens through microbial growth, oxygen cannot penetrate the medium surface, and anaerobic organisms prosper. As the biological film continues to grow, the microorganisms near the surface lose their ability to cling to the medium, and a portion of the slime layer falls off the filter. This process is known as sloughing. The sloughed solids (sludge) are collected by the under-drain system and transported to a clarifier for removal from the wastewater.
There are advantages and disadvantages of TFs associated with biological treatment of food and agricultural wastewater; their importance depends on the needs of the end user and characteristics of wastewater.
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• Simple, reliable, biological process
° Suitable in areas where large tracts of land are not available for landintensive treatment systems
0 May qualify for equivalent secondary discharge standards
• Effective in treating high concentrations of organic matters, depending on the type of medium used
• Rapidly reduces soluble BOD5 in wastewater streams
• Efficient nitrification units o Low power requirements o Moderate level of skill and technical expertise needed to manage and operate the system
130 Food and Agricultural Wastewater Utilization and Treatment • Disadvantages
° Additional treatment may be needed to meet more stringent discharge standards
° Possible accumulation of excess biomass that cannot retain an aerobic condition and can impair TF performance ° Requires regular operator attention ° Incidence of clogging is relatively high ° Requires low loadings, depending on the medium ° Flexibility and control are limited in comparison with activated-
sludge processes ° Odor problems ° Snail problems
A TF consists of permeable medium made of a bed of rock, slag, or plastic over which wastewater is distributed to trickle through, as shown in Fig. 4.6. Rock or slag beds can be up to 60.96 m (200 ft) in diameter and 0.9-2.4 m (3 to 8 ft) deep, with rock size varying from 2.5-10.2 cm (1 to 4 in). Most rock media provide approximately 149 m2/m3 (15 sq ft/cu ft) of surface area and less than 40% void space. Packed plastic filters (biotowers), on the other hand, are smaller in diameter (6 to 12 meters (20 to 40 ft)) and range in depth from 4.3 to 12.2 m (14 to 40 ft). These filters look more like towers, with the media in various configurations (e.g., vertical flow, cross flow, or various random packings). Research has shown that cross-flow media may offer better flow distribution than other media, especially at low organic loads. When comparing vertical media with the 60° cross-flow media, the vertical media provide a nearly equal distribution of wastewater minimizing potential plugging at higher organic loads better than cross flow media. The plastic medium also required additional provisions, including ultraviolet protective additives on the top layer of the plastic medium filter and increased plastic wall thickness for medium packs that are installed in the lower section of the filter where loads increase. The design of a TF system for wastewater also includes a distribution system. Rotary hydraulic distribution is usually standard for this process, but fixed nozzle distributors are also being used in square or rectangular reactors. Overall, fixed nozzle distributors are being limited to small facilities and package plants. Recently some distributors have been equipped with motorized units to control their speed.
Distributors can be set up to be mechanically driven at all times or during stalled conditions.
In addition, a TF has an under-drain system that collects the filtrate and solids, and also serves as a source of air for the microorganisms on the filter. The treated wastewater and solids are piped to a final settling tank where biosolids are separated from the water. Table 4.1 lists common trickling filters used in biological wastewater treatment.
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