Membrane and Desalination Technologies - TCE Moodle Website

488 P. Kajitvichyanukul et al.
4. COMBINATION OF BIOTECHNOLOGY AND FILTRATION
TECHNOLOGY
4.1. Biofiltration
4.1.1. General Description
Biotechnology is normally important in industrial and commercial wastewater renovation.
However, specific problems in potable water treatment such as NO3 – and NOM have opened
the door for application of biotechnology to this field. Biofiltration is one of biological
technology applications in potable water treatment. In general, biofilters in drinking water
treatment are typically operated for the dual purpose of particle removal and removal of
dissolved BOM (47). Biofiltration is most often used after flocculation and clarification
(sedimentation or flotation), either in the filtration unit itself or as a separate subsequent
unit. Other uses of biological technology in drinking water are:
(a) conversion of ammonia to NO 2 – and then to NO3 – and finally to nitrogen gas
(b) BOD removal
(c) oxygen addition
(d) carbon dioxide removal
(e) removal of excess nitrogen and other inert gasses
(f) turbidity removal and water clarification
(g) removal of various organic contaminants
Biofiltration is distinguished from other biological waste treatments by the fact that there is
a separation between the microorganisms and the treated waste. In biofiltration, the microbial
biomass is static, immobilized to the bedding material, while the treated water is mobile and it
flows through the filter (48).
A biofilter consists of one or more beds of solid biologically active materials, such as peat,
compost, soil, leaves, or woodbark (49). The packing may be mixed with inert support
materials such as porous clay, polystyrene spheres, GAC, diatomaceous earth, perlite, or
vermiculite to increase its reactive surface and durability, to reduce back pressure, and to
prevent compaction. In biofilters, bacteria are attached not only to the filter media, but also
form a biofilm on the surface of the filter grains. A large available surface area for biofilm
growth is one of the most essential properties of media used in this process. The available
surface area is dependent on the grain size and density (for porous materials) of the media.
The feed water with biodegradable organics, suspended cells and other particulates,
dissolved oxygen (DO), and other dissolved nutrients are pumped into the top of the filter,
and flow down through the filter bed. On the filter media, biomass accumulates and grows as
feed water flows through the filter. Biodegradation of substrates and growth of cellular mass
also occurs in the bulk liquid. Biomass on the filter media is removed by decay, fluid shear,
and backwashing. The major mechanism of pollutant removal in biofiltration is by biological
degradation of the contaminants. The contaminants are incorporated into the microbial
biomass or used as energy sources (electron donors or electron acceptors) (48). In addition,
the pollutants may also be removed from the fluid by adsorbing to the microbial film or to the
bedding material.