Sewage treatment is nothing but the degradation of organic molecules present in the wastewater. Present day practice is to degrade the biological mass either by using aerobic, anaerobic or by using facultative methods. i.e., the microbial organisms, microbes that are being generated in the aerobic or anaerobic or facultative states, degrade the biological mass. There are many technologies in today’s practice to treat the sewage aerobically or anaerobically or in combination. In this chapter, few treatment technologies which have proven track records of treating sewage to desirable standards of disposal or for its reuse. A brief description of those options with merits & demerits has been provided. They are:-
- Up flow Anaerobic Sludge Blanket (UASB) Technology
- Activated Sludge Process (ASP) Technology
- Extended Aeration (EA) Technology
- Sequencing Batch Reactors
- Waste Stabilization Ponds
- Moving Bed Bio Reactor
- Membrane Bio Reactor
The conventional sewage treatment technologies such as Activated Sludge Process (ASP), Waste Stabilization pond (WSP), Upflow Anaerobic Sludge Blanket (UASB) Reactor etc., are commonly adopted in sewerage system to treat wastewater up to secondary level as per the effluent standards. There are a number of newer treatment technologies that have come into practice in recent times and they do merit attention in their own way as under, but the difficulty is the design basis which is necessary to be standardized for adoption in projects funded by Governments. Recently, the following technologies such as Sequencing Batch Reactor(SBR) and Moving Bed Biofilm Reactor (MBBR)/ Fluidized Aerobic Bioreactor have been approved under JNNURM projects due to their advantages such as less requirement of land, high effluent quality etc. Small scale plants have been set up using MBR technology at various locations. These technologies are briefly described as under
Sequencing Batch Reactor (SBR)
Sequencing batch reactors (SBR) or sequential batch reactors are processing tanks for the treatment of wastewater. SBR reactors treat waste water such as sewage or output from anaerobic digesters or mechanical biological treatment facilities in batches. Oxygen is bubbled through the waste water to reduce biochemical oxygen demand (BOD) and chemical oxygen demand (COD) to make suitable for discharge into sewers or for use on land. While there are several configurations of SBRs the basic process is similar. The installation consists of at least two identically equipped tanks with a common inlet, which can be switched between them. The tanks have a flow through system, with raw wastewater (influent) coming in at one end and treated water (effluent) flowing out the other. While one tank is in settle/decant mode the other is aerating and filling. In one option of SBR there is at the inlet a section of the tank known as the bio-selector. This consists of a series of walls or baffles which direct the flow either from side to side of the tank or under and over consecutive baffles. This helps to mix the incoming Influent and the returned activated sludge, beginning the biological digestion process before the liquor enters the main part of the tank. Another option uses the same tank for mixing, mixing being done by a dedicated mixer. There are four stages of treatment in sequencing the treatment of sewage. In the first stage sewage will be filled in the container for aeration purpose.
This stage can be further divided ad mixed fill and react fill. In the second stage, the aeration of sewage will be carried to decompose biological mass and in third stage suspended biological masses will be made to settle to the bottom of the tank. Final stage involves the decanting operation that disposes supernatant liquid from the tank.
Advantages of SBR
- a) Complete treatment in a single basin: Separate chambers are not required as in the case of an ASP based STP.
- b) Control of reactor environment: The different phases of the system operation are fully controlled by a PLC depending the on the requirement of the waste water parameters.
- c) Shock loads: The Tanks can tolerate hydraulic and organic shock loads.
- d) Flexible aeration: The aeration of the basin can and is controlled throughout the application of air. This property enhances energy reduction.
- e) Increasing Settling area: Since the entire settling happens in the same basin, more complete settling occurs. Since there is no clarifier to accumulate sludge, there is no odour developed.
Moving Bed Bio Reactor (MBBR)/ Fluidized Aerobic Bioreactor FAB
This technology is essentially the same as activated sludge except that the media suspended in the reactor offers additional surfaces for the microbes to grow and this in turn maximizes the growth of microbes in a given volume of aeration tank compared to the conventional aeration without the media and to that extent, it does appear preferable. Diffused aeration is of course needed. FAB technology is akin to MBBR except that instead of the media in suspension, the media is kept stationary and fluidized in the aeration tank. Schematic flow diagram of Moving Bed Bio Reactor process is presented below
Advantages
- There are no limitations of height as long as compressors can be suitably used Circular structures can be used to economize on construction costs & time
- The structures can be easily covered for indoor air quality when needed. Requires lower footprints compared to conventional activated sludge
- Easy to operate and maintain
Membrane Bio Reactor (MBR)
The Membrane Bioreactor is combination of the activated sludge treatment process and the membrane filtration process. The wastewater enters the wastewater treatment facility and passes through the usual Preliminary Treatment and Primary Treatment processes. A very fine screen prefer 2 – 3 mm clear opening is need to place prior to the MBR reactors to remove small suspended particles. This step is designed to reduce the potential fouling of the membranes with these fine particles.
The dissolved BOD (sugars, starches, carbohydrates, etc) that is in the wastewater is then consumed by the microbes in the aeration basin, and subsequently converted into additional microorganisms, or becomes attached to the biological floc.
The Mixed Liquor Suspended Solids (MLSS) is usually fairly high in MBR units, around 10,000 mg/L 15000 mg/l. This high MLSS concentration allows for lower hydraulic retention times (HRT) which equates to smaller aeration basins. This also equates to an activated sludge that may be fully nitrifying, as the Mean Cell Residence Time (MCRT) is usually well above 10 days. Pumps are attached to the membrane modules, and pull a slight vacuum that pulls water from the tank through the perforations in the membranes leaving the microorganisms behind in the tank. Most all of the MBR facilities utilize fine bubble aeration in the aeration tanks, except for those areas that will have the MBR modules. These membrane module areas will usually have coarse bubble diffusers installed beneath them. Some facilities may use the single tank MBR process, or the double tank MBR process. In the single tank the filtration modules are placed near the opposite end from where the primary effluent enters the tank. In a double tank configuration, designers may have an aeration tank without a filtration module in it, followed by an aeration basin with the membrane filtration unit in it. The treatment process goal in both designs is to allow for suitable time for the conversion of BOD/COD into microbial cells or at least be absorbed/flocculated with the cellular masses prior to being placed near the membrane filtration units. . Schematic flow diagram of Submerged Membrane Bioreactor process is presented below
Schematic flow diagram of Submerged Membrane Bioreactor process
Advantages of Membrane Bio Reactor
- a) The effluent is of very high quality, very low in BOD (less than 5 mg/l), very low in turbidity and suspended solids.
- b) The “simple filtering action” of the membranes creates a physical disinfection barrier, which significantly reduces the disinfection requirements.
- c) The treatment process also allows for a smaller “footprint” as there are neither secondary clarifiers nor tertiary filters which would be required to achieve similar water quality results. It also eliminates the need for a tertiary backwash surge tank, a backwash water storage tank, and for the treatment of the backwash water.
Disadvantages of Membrane Bio Reactor
- a) The membrane modules will need to be replaced somewhere between five (5) and ten (10) years
- b) Energy consumption is very high.
- c) Fouling is troublesome, and its prevention is costly.
Criteria for Selection of Treatment Technology
Following are the criteria chosen for selection of treatment technology:
Treatment Standards & Possible Uses:
In the present projects, established Micro-STPs should achieve following treatment Standards:
Possible Uses of Treated Water: Prime use of treated water will be horticulture and gardening. However, user may use treated water in following reuses:
1- Construction Water
2- Industrial uses
3- Washing and floor cleaning
4- Boiler Water after further treatment if required