3.6 Common Biological Method to Treat Dairy Wastewater
3.6.1 Activated sludge
Activated sludge is a process that uses air and microorganism to treat wastewater.
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Activated sludge consists of different groups of microorganisms entrapped within a
polymeric network. The floc of activated sludge is negatively charged and the multivalent
cations are important for the stability of the structure. Among all cations, Ca+2 plays a key
role for bridging polymeric networks and bacteria together. Filamentous microorganisms
are important for the structure of activated sludge. Poor quantity of filamentous
microorganisms contributes to low settle ability, poor dewater ability and compaction. And
over quantity of that lead to slowly settling, poor compaction and bulking sludge [30].
Activated sludge process converts organic pollutant to carbon dioxide, water and
new cells. The excess cells, called sludge, will be separated from treated water at settling
tank, where 1 kg BOD removed will produce 0.5 kg dry excess sludge. The removal of
excess sludge is efficient and low cost because this treatment takes 25% to 65% of total
treatment cost. There are several methods have been used to reduce sludge—
alkaline-thermal treatment, ozonation, chlorination, metabolic uncoupler, and dissolved
oxygen [31].
Biological treatment, generally the activated sludge process, has become the main
wastewater treatment method to treat municipal and industry wastewater all over the
world[32]. Activated sludge is one of the common suspended growth reactors and most of
them are continuous stirred tank reactor (CSTRs). The suspension of suspended growth is
typically provided by an air pump which provides constant stirring of wastewater and
microorganism. The mixing is sufficient and all constituents are uniform throughout the
CSTR. This process developed around 1912-1914 [33], while the first plants appeared in
the early 1960s and were mainly extended aeration plants or oxidation ditches. The most
important advantages of using activated sludge were freedom from the fly and odor
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problems associated with bacteria beds and a reduced requirement for land area. [34]
Currently over 90% of the municipal wastewater treatment plants within the United States
use it as the core part of treatment process ([31]). Overall, the activated sludge process is an
efficient and economic way to treat organic wastewater not only SS and BOD but also
nitrogen and phosphorus. It can provide high quality effluent.
3.6 .2 Trickling filter
3.6.2.1 Definition of Trickling filter
Trickling filter consists of support media, usually rocks, gravel or plastic.
Wastewater flows from the top to the bottom of trickling filter. Microorganisms attach to
the surface of bed media and form aerobic layer and anaerobic layer. Aerobic condition is
maintained by forced air flowing through the bed or natural convection of air [35].
Trickling filters remove organic pollutants by microbial films which attach to the
surface of media in trickling filter. The biofilter which use microorganism to reduce
organic develops on the top of media surface where oxygen and food is very sufficient.
When wastewater from the setting tank trickles over the bed of media, the pollutant in the
water will be removed by the biofilm. Finally, the biofilm became thick and heavy, it will
fall from the media then the new process start over again [36].
3.6.2.2 Purposes of Trickling Filter
The primary purpose of a trickling is to remove organic pollutant. In addition,
trickling filters can be used to remove nitrogen. Nitrification which oxidize ammonia to
nitrite and then to nitrate by microorganism begins after organic removal since nitrifers
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need more time to grow and heterotrophic bacteria dominant the biofilm at first. Bacteria
attach to the surface of support media and utilize the organic material as food.
Bacteria generate an extrapolymetric matrix which allows them to firmly adhere to the
surface [37].
There are many factors that determine a successful performance of a trickling filter.
A major component known as carrier material determines the performance of biofilter. The
materials of media include natural, intert, synthetic and mixtures of both. The media is not
only used to support the biofilm, but also trap contaminants which can be degraded further
by bacteria [38].
3.6.2.3 Selection of Biofilm Media
According to Odd-Ivar Lekang, there are several factors affect the selection of
biofilter media. These factors include void ratio, specific surface area, weight,
homogeneous water flow, and economics. Void ratio is the ratio of void volume and
volume of media. Specific area is defined as the total surface area of filter media per unit of
mass of media. The media with light weight can be easily handled. Homogeneous water
flow refers to the water flow that has the same or similar nature. The media has the
reasonable price and cost effective [39].
3.6.3 Membrane bioreactor
The membrane bioreactor (MBR) combines biological treatment with membrane
filtration process. MBR is more effective than conventional biological treatment process
to remove COD, BOD and SS. For example, suspended solid removal complete by
filtration rather than gravity, MBR has independent solid retention time and hydraulic
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retention time. According to the application, membrane bioreactor can be divided into
three categories— filtration membrane, gas diffusion membrane and extractive membranes
[40].
An MBR consists of two parts— biological suspended growth reactor and another
part is membrane. Two common types, microfiltration and ultrafiltration are frequently
used to treat wastewater. Membrane acts as separate unit to remove certain particles. In
regards to treatment of wastewater, MBR and activated sludge operate in the similar way;
however MBR does not need a clarifier. The advantages of MBR are that smaller plant size,
completely suspended solids removal and high treatment efficient. But MBR is much more
expensive to install, operate and maintain [41].
3.6.4 Wetland
Wetlands are lands saturated with water. It is a very important for a large spectrum
of habitats, such as temporary shallow waterbodies, lake margins, large river floodplains,
coastal beaches, coral reefs and beds of marine algae or seagrasses [42].
Wetlands use a high nutrient tolerance root system of reeds such as elephant to treat
or renovate wastewater [43]. An alternative type of wetland beyond natural wetlands is
constructed [44].
A wetland is one of the most effective ecosystems in the world. Plants play an
important role in nutrients cycling. One of the most common plants, floating macrophytes
are widely used for wastewater treatment. Nutrient-use efficiency (NUE) is considered to
be an important plant factor which combines a variety of nutrient uptake and release
process. NUE changes with types of plants and nutrient availability [46].
Compared with stabilization ponds and lagoons, wetlands can offer high degree of
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process control and develop applicable design and cost criteria for a given and desired level
of wastewater. Recently some studies document the possibility of using wetland to remove
waterborne pollutants [47].
Wetlands are used to remove aquatic pollutants. Aquatic pollutants are removed by
wetlands through physical, chemical and biological process, such as sedimentation, soil
adsorption, and biological transformation. There are more than 500 wetlands used in the
world to treat municipal and industrial wastewater [48].
Wetlands have different hydraulic retention times which range from 2 to 20 days.
High hydraulic retention is applied when wetlands are used for BOD removal and
nitrification by diffusion aeration. On the contrast, low hydraulic retention time is usually
employed to get higher quality treated wastewater and other design objectives, such as
denitrification and habitat enhancement [49].
Wetlands have been used to enhance wastewater discharge from municipal
wastewater treatment. Some food industries also use wetland for the tertiary treatment for
processing wastewater [50]. An alternative type of wetland beyond natural wetlands is
constructed.
Constructed wetlands are artificial wetlands that follow the natural processes to treat
wastewater. There are two types of constructed wetlands- surface flow and subsurface
flow. Wastewater flows from the top of the soil is called surface flow; wastewater flows
through porous medium is called subsurface flow [45]. As an engineered and managed
'natural system', constructed wetlands use less energy, more reliable and less cost than
reactor tanks and basins. A wetland system can also be served as wild life habitat or nature
centre. Author Hammer indicates that there are four factors in pollutant removal— wetland
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microbial populations, wetland macrophytes, wetland substrates, and water column [51].