Tertiary treatment is generally practiced when the discharge requirements have stringent limitations or when treated effluent quality has to be inline with recycle standards. Filtration systems often are also used for pre treatment of water prior to membrane systems. Some examples of tertiary filtration system offered by Sigma include:

o Multigrade Filtrations (MGF)
o Activated Carbon Filtration (ACF)
o Slow / Rapid Sand Filtration
o Engineered Media Filtration
o Fluidized Bed Oxidation Reactor (FBOR)
The following sections describe typical tertiary filtration offerings from Sigma.


A stacked media bed or two layers (dual media) is one answer to providing coarse to fine filtration in a down flow pattern. The two material selected have different grain sizes and different specific gravities. Normally, ground anthracite is used in conjunction with silica sand. The anthracite grains with specific gravity of 1.6 and a grain size of 1 mm settle slower than sand with a specific gravity of 2.65 and a grain size of 0.5 mm, so the coarse anthracite rest on top of the fine sand after back washing. The coarse anthracite allows deeper bed penetration and provides longer filter runs at higher filter media. The filter sand polishes the effluent.

Just as coarse-to-fine dual media is more effective than a single medium filter, further improvement can be gained by introducing a third, smaller, heavier media under sand. Garnet with a specific gravity of 4.5 and a very fine grain size settling faster than the silica sand can be used as the bottom layer. This filter operates at higher flow rates and provides deeper penetration and longer filter runs than a single or dual media filter.

Flow rate through a filter is critical, since it limits the throughput and dictates the number of filter required. Generally, as flow rate increases, penetration into the filter increases. The flow rate is limited by the head available and the media size. As the media starts to load with solids, the net velocity at a given flow rate increase until shear forces tear the solids apart and they escape into the effluent. The filters are designed to be back-washed before this breakthrough occurs at a point determined by head loss.


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Activated carbon filters are one of the popular means of removing toxic, including chlorine, and most organic chemical contaminant.

Activated carbon is prepared by slowly heating coal, walnut shells, or hardwood in the absence of air, then washing and chemically treating the resulting carbon to remove impurities. There are roughly 500 - 1000 square meters of surface area for one gram of activated carbon. Almost all the surface area is within a carbon granule.

With the exception of chlorine, activated carbon works by “adsorbing” chemical contaminants onto the surface of the carbon granule. The term adsorption refers to the ability or one substance (in this case, carbon) to attract and hold molecules of another substance (contaminants).


The sigma Fluidized Bed Oxidation Reactor (FBOR) is an aerobic biological process that utilizes a catalyst base to adsorb and degrade dissolved organics. The FBOR is an integrated system of biological and chemical oxidation carried out in a single hybrid reactor packed with activated carbon based redox resin. The process involves wet air oxidation using carbon which contains Chemo Autotrophic bacteria in immobilized state. Oxygen required for the oxidation of organics is facilitated by supplying air from the bottom of the reactor. Since only the stoichiometric quantity of oxygen is supplied for oxidation, utilization of electrical energy is minimized, leading to a saving of electrical power consumption to an extent of 75%, as compared to the conventional aerobic system.

The Sigma FBOR displays elevated removals for suspended solids, BOD, COD, sulfide, and odor with high degree of performance consistency.

How the process works:

o Based on the flow requirements, the FBOR can be designed in multiple modules operating in parallel.
o Wastewater is distributed through a special distribution system in the FBOR reactor.
o The organics and contaminants contained in the wastewater have the ability to flow upwards/downwards through the reactor.
o Immobilized microorganisms perform biological degradation of the organics, with the media assisting as a catalyst, performing catalytic oxidation at the same time.
o Once acclimated, the microbes are highly resistant to shock loadings and offer a consistent performance.
o The catalyst media is kept fluidized by the air supply as well as a recycle stream entering the FBOR, allowing complete mixing of the media, providing a continuous cleaning mode. Since biological growth on the catalyst surface is relied upon for organic removals, the system is self regenerating and the site is repeated available for oxidation.
o Treated wastewater is collected and discharged in an outlet manifold. Solids/sludge generation is minimal and does not require post clarification.
o Operating costs for FBOR are low. The system provides very efficient oxygen transfer to the water, minimizing the airflow required. The inherent simplicity of the design minimizes maintenance requirements.
o The FBOR can be tailored to suit anaerobic applications and can be adapted for nitrogen and phosphorous removal.

The Sigma FBOR Advantage:

o Units are modular in design and require less installation time
o Pre-assembled units can be supplied for rapid installation
o Units are easily transported
o Compact and efficient design, requires less land area
o Minimal regular maintenance
o Simple to maintain
o Minimal to sludge free operation
o Excellent resilience to refractory organics removal
o Robust and economical tertiary treatment
o Quiet operation with low odor and low visual impact – ideal for sensitive area
o High quality effluent, consistent performance