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Screening: screening is clears large solid objects from sewage, such as garbage, stones, can, fabric scraps, plastic bags, branches, etc., in order to avoid obstruct pumps, valves and piping and damage other subsequent processing equipment. Coarse sieve gutter spacing is 50mm-100mm. Moreover, screening spacing of fine screening is 10mm, 6mm or 2.5mm etc. The fine spacing can be more efficient, but it will also faster to be junk clogging.
Grit removal: To prevent grit damaged on the operation of machinery and equipment, reduce precipitation in the pipeline and prevent accumulation at sludge digestion tank. The entrance of grit trap usually have adjustable angle of diversion plate to control the inflow of the sewage. The grit can be precipitate to the bottom, and then the grit remove arm will poke the grit to bottom side of the trap. The grip is removed by grip classifier.
Primary sedimentation: Nearly 50-70% of the solids and particulate matters of size larger than 0.1 mm settle down under influence of gravity. A reduction between 30-40% in the number of coli forms is obtained. The sludge and is removed by mechanically operated devices without disturbing the operation in the tank. To achieve a particular degree of solids removal the time of detention of a parcel of sewage must be such that all particles below a certain size can fall to the bottom of the tank after entering at top water level.
Figure 6: Typical sewage composition
The trajectory of the particle is shown in the simplified diagram above.
Time of detention = dbl/Q=d/Vs
Which simplifies to: VS =Q/A bl = area, A
Therefore VS = Q/A
Simultaneously a small amount of biological action also takes place in which microbes present in the sludge attack complex organic solids and break them down to simpler soluble substances and ammonia. The grease rises to the surface to form a scum which is removed by scum collector. Purpose of sedimentation is to remove suspended solids and reduce the strength of the sewage. Sedimentation is the settling and removal of suspended impurities which occur when water stands still or flows slowly through a basin or tank. There is negligible turbulence and hence particles of high density tend to settle down and form a sludge layer at the bottom of the tank whereas clarified water will be collected through the outlet.
Activated sludge process is the most common option in secondary treatment and is also a method of suspended cell sewage treatment method. It starts with aeration that encourages the growth of microbes in the waste. The microbes feed by the organic material, which then allows solids to settle out. Activated sludge is contain full of bacteria and continually re-circulated back to the aeration basin to increase the rate of organic decomposition. In raw sewage, bacteria predominate during the holding time in the aeration tank. The number of suspended bacteria decrease but those associated with floccules greatly increase in number. During the holding period, a portion of dissolved organic substrate is mineralized. Another portion is converted to microbial biomass. In the advanced stage of aeration, most of the microbial biomass is associated with floccules that can be removed from suspension by settling. The General design of aeration reaction time of sewage is about 6 hours to 8 hours. Temperature affects the reaction rate of micro-organisms, each additional 10 â„ƒ temperature can cause the double reaction rate. Furthermore, the common food to microorganism ratio is 0.1è‡³0.5kg BODï¼Âd.kg MLSS
Final sedimentation: The oxidized sewage from the aeration chamber is led into the final sedimentation tank where it is detained for 4 hours. The solids-liquid separation effected by gravity to produce a clarified effluent for discharge after biological treatment. The sludge that collects in the final sedimentation tank is called activated sludge. It differs from the sludge in the primary sedimentation tank in that it is practically inoffensive and is rich in bacteria, nitrogen and phosphates. Part of activated sludge is pumped back into the aeration tanks i n the activated sludge process and the rest is pumped into the sludge digestion tanks for treatment and disposal. (Microbiologyprocedure.com) (Yu Shu man)
There also will have bacteria after final sedimentation, so it is need to disinfection to ensure most of bacteria a dead. The most common methods in Hong Kong is applied ultra violet disinfection system and add the sodium hypochlorite solution before being discharged of effluent.
From the above description, we can see that the typical secondary sewage treatment works is quite large. For example the size of primary sedimentation tank, aeration tank and final sedimentation tank are over 2000m2 per tank at ShaTin sewage treatment works. Even the total size of one set of primary sedimentation tank, aeration tank and final sedimentation tank at Stanley is also over than 1000 m2.So the typical activated sludge process is not suitable for small area/ premises. So it can be apply
The small treatment capacity sewage treatment plants (small capacity
STPs). The design flow rate is about 600 m3/d (population equivalent of approximate 1500) is suitable of premises in Hong Kong. The evaluation criteria of the sewage treatment technologies for small scale STPs include: land use, operation and maintenance friendly, treatment efficiency, treatment flexibility and potential for Hong Kong application.
Small scale STPs are common application in Hong Kong:
Oxidation Ditch (OD)
Rotating Biological Contactor (RBC)
Sequencing Batch Reactor (SBR)
Membrane Biological Reactor (MBR)
Oxidation Ditch (OD)
The oxidation ditch is a sort of equipment used for a long-term aeration and is characterized by a long sludge age in the system. It consists of a long channel of an elliptical or circular shape equipped with a horizontally mounted rotor for generating a water flow and stirring water in the channel to supply oxygen for the biological process. (RD 2035) Cyclotron oxidation tank consists of a ring composed of sewage tank, 1 m to 1.5 m depth. Sewage rotary speed is 0.3 to 0.6 m/s
Thought it requires a relatively large area, the oxidation ditch has a simple structure and can be easily operated as well as being able to remove nitrogen. Thus, it is used in relatively small sewage treating plants.
Advantages of using the oxidation ditch are:
Robust to load fluctuations and smaller sludge quantity.
Aeration efficiency can be easily controlled by changing the rotation speed of the rotor and the dipping depth.
Little energy is required as the rotor is of high efficiency.
Perform nitrification and denitrification in the process.
Disadvantages of using oxidation ditch are:
It requires a large area because the tank is large and the depth is shallow,
It also require for small sedimentation tank after oxidation ditch.
Rotating Biological Contactor (RBC)
The Rotating Biological Contactor (RBC) is a unit consisting of a series of closely parallel discs mounted on a rotating shaft which is supported just above the surface of sewage to be treated. The design of both Biomass thickness and speed of rotation also are fundamental. Aerobic degradation of the pollutants takes place on the disc surface which Micro-organisms grow on it. There are least 85% removal rate of the biochemical oxygen demand (BOD) from domestic sewage should be designed in RBC process. (LAM Ho Kai)
Advantages of using the RBC technology are:
longer contact time and higher level of treatment
Simpler operation than activated sludge.
Less susceptibility to upset from changes in hydraulic or organic loading than the activated sludge process.
Sewage from domestic and other biodegradable industrial discharges also can be treated.
Process can also be designed to remove ammonia nitrogen.
Disadvantage of using the RBC technology is:
It also require for small primary and final sedimentation tank after RBC technology.
Though the size is much smaller than activated sludge process plant and oxidation ditch, plant of RBC is large.
Sequencing Batch Reactor (SBR)
The sequencing batch reactor (SBR) is a fill-and-draw activated sludge treatment system. It includes a generic system of variable volume activated sludge in which aeration, sedimentation and decant are combined in a single reactor. Therefore, there is no needed for secondary clarifier or return sludge facilities. The operation of the SBR, it consists of five distinct periods (Fill, React, Settle, Draw and Idle), which comprise one complete reactor cycle.
Fill: The raw sewage fed to the reactor. In Fill period can be static, mixed or aerated.
By manipulating the actual specific growth rates of the microbes and regulating the oxygen tension in the reactor, the organism selection can be controlled.
React: Aeration and mixing is take place at reactor.
Settle: Microbes are separate by sedimentation during a period. This is quiescent settling and separation of MLSS from the treated sewage
Draw: This is process which withdrawal of treated sewage from the reactor
Idle: This is process which removes the sludge from the reactor bottom.
Advantages of using the SBR process are:
No secondary clarifier and return activated sludge pumping process are required.
Footprint is small.
High tolerance for peak flows and shock loadings.
Avert mixed liquor suspended solids washout during peak flow events.
Clarification under ideal quiescent conditions than typical activated sludge treatment system.
Disadvantages of using the SBR process are:
Requirement of an equalization tank.
Requirement of process expertise in programming the cycles.
Maintenance is more complicated cause by complex Automatic control system
It is easily drawing the sludge when the extraction liquor process.
Membrane Biological Reactor (MBR)
Membrane bioreactor (MBR) process is a modification of the activated sludge process. The liquid-solids separation is carried out by highly specialized membranes, as compared with the secondary settler in the continuous- flow activated sludge process. Mixed liquor suspended solids (MLSS) levels maintained in 8000 to12000 mg/L in bioreactors can be increase the treatment efficiency. The membrane can be act as a filter to remove the pollutants from the sewage in either suspended or dissolved. MBR is combining the membrane technology and activated sludge process for the treatment of sewage, so it can be replacement for the final clarifiers. The membrane materials include polymeric materials or ceramics. These materials are porous and achieve solids and liquid separation mechanically. The pore sizes of the membranes are commonly from 0.03 to 0.4 microns. Providing a most effective pore size of less than 0.01 microns can be enhances the effectiveness of this filtration performance. The pore size of membranes which less than 0.01 microns can be stops the virus, bacteria.
The MBR process can exist in two different configurations. The first configuration is a side-stream system, in which the membrane is independent of the bioreactor. The MLSS of the bioreactors is pumped around a recirculation loop containing a membrane unit where the permeate is discharged and the retentate is returned to the bioreactor. The second configuration is a submerged MBR, in which membrane modules are immersed within the bioreactor. This is currently the most common MBR configuration for municipal applications. The MBR technology is no primary or secondary settlement stages required and no additional tertiary treatment or UV stages to achieve very high disinfection quality.
Advantages of using the MBR process are:
It has a high quality effluent after treatment cause by removal rate of suspended solids is least above 0.1 micron.
It has low energy consumption cause by the system is working at sub-pressure lower than bar up to 0.5 bar
It has high sludge concentration. The process enables working in high bio mass concentration. The concentration of sludge is 4 times higher than other processes of biological of sewage treatment.
The process is without chemical consumption. At Chemically Enhanced Primary Treatment and Secondary Treatment is required chemical for Enhance Settle and sludge thickening
Its operation is simplified. The operating system is simple and reliable in terms of manpower and maintenance. The membrane has high resistant for oxidation.
It requires small footprint. The biological dissolving, solid separation, effluent polishing and sludge thickening are also taken place in same tank.
Disadvantages of using the MBR process are:
The initial cost of membranes is high.
The incoming sewage requires screening before entering the membrane bioreactor tank.
Chemical cleaning after operation for a period is required.