IMPORTANT TERMS
- Unit processes
- Unit process refers to chemical conversion of reactants into desirable products. Ex: Oxidation, Nitration, Hydrogenation, desulphurization of sulphur, etc
- In unit process there is no physical change but chemical changes occur
- Unit process is secondary
- Unit process involve both physical and chemical changes
- Unit process is a chemical conversion of reactants into desired products.
- Unit operations
- Unit operations refer to separation of products by physical means. Ex: Absorption, Adsorption, Distillation, Membrane separation, mixing, agitation, drying, quenching, evaporation, crushing, grinding, crystallization, separation, etc
- Each step is one unit operation and the entire process is unit process
- In unit operations, no chemical changes but physical changes occur
- Unit operations is a physical change
- Unit operations need to be performed in unit process
- Unit operations involve physical changes only
- Unit operation is separation of products by physical means
- Biological unit process
- Biological unit process make use of microbes for removal of dissolved organic matter from wastewater. The two types of biological unit processes are:
- Aerobic biological unit process and
- Anaerobic biological unit process
- Biological unit processes carry out treatment of wastewater by microorganisms in the presence (aerobic) or absence of oxygen (anaerobic)
- Examples of aerobic processes are:
- Activated Sludge Process
- Trickling filters
- Aerobic stabilization ponds and
- Aerated lagoons
- Rotating Biological Contactors
- Oxidation ponds
- Examples of anaerobic processes are:
- Anaerobic sludge digestion
- Anaerobic contact process
- Anaerobic filters
- Anaerobic lagoons or ponds
- Treatment system
- The type of combination used from the available unit operations and processes for treatment of a particular waste is known as treatment system.
- Primary treatment system
- As per the ongoing technology, the wastewater treatment system that includes all units of the preliminary treatment system such as 'sump and pump house' , 'approach channel', 'bar screen', 'grit chamber', 'parshall flume or velocity control device', 'skimming tank' and the 'primary sedimentation tank' together are known as the primary treatment system
- Secondary treatment system
- The removal of colloidal and soluble organic content is carried out in the secondary treatment system consisting of an aeration basin with return sludge facility or a trickling filter and a secondary sedimentation tank is called a secondary treatment facility
- Waste stabilization ponds (Oxidation ponds)
- Waste stabilization ponds are large, man-made water bodies in which black water or grey water or fecal sludge are treated by naturally occurring processes under the influence of sunlight, wind, microorganisms and algae. They may be aerobic, anaerobic or facultative each having different treatment and design characteristics. These ponds are low on operation and maintenance and have high removal rate for pathogens and BOD. They require large surface areas and special expertise in design of such systems. The effluent contains high amount of Nitrogen and Phosphorus and hence can be used for agricultural purposes but not for recharge of groundwater.
- Oxidation lagoons (Aerated lagoons)
- Oxidation lagoons or aerated lagoons operate on the principle of the activated sludge process
- It consists of an earthen basin in which wastewater is fed only after screening
- The wastewater is treated with or without recycling of sludge
- Oxygen required is provided by surface aerators or submerged diffuse aeration system
- The system assumes complete mixing and the flow of wastewater is continuous
- Oxidation ditches (Extended Aeration Systems)
- An oxidation ditch is a modified activated sludge biological treatment process that utilises long solids retention time to remove biodegradable organics. Oxidation ditches are typically complete mix systems.
- Oxidation ditches are suitable for areas where land availability is high.
- They are easy to maintain and adapt easily to shock loads.
- Rotating Biological Contactor (RBC)
- Rotating Biological Contactors also known as Rotating Biological Filters are fixed bed reactors consisting of stacks of rotating discs mounted on a horizontal shaft. They are partially submerged and rotated as wastewater flows through. The microorganisms are alternately exposed to atmosphere and wastewater, allowing both aeration and assimilation of dissolved organic pollutants and nutrients thereby degrading the pollutants
- Up-flow Anaerobic Filter (UAF)
- An Up-flow Anaerobic Filter (UAF) is characterised by long detention time and production of anaerobic effluent. It uses physical removal mechanisms like flocculation, sedimentation and absorption. Anaerobic digestion also occurs in the bed. The bed is generally filled with gravel.
- Up-flow Anaerobic Sludge Blanket
- Up-flow Anaerobic Filter is also known as Up-flow Anaerobic Sludge Blanket Reactor. In this treatment process, a blanket of biologically formed sludge granules act as a filter through which the wastewater is passed where solid-liquid separation takes place. The separated settling solids return to the active blanket zone while the liquid passes over the weir as effluent. The waste solids get attached to the biomass and are stabilized. The gas produced as a result of stabilization is collected.
- This process is an important option for sewage treatment in warm countries
- Tertiary or Advanced Treatment System
- Tertiary treatment is the final cleaning process that improves wastewater quality before it is reused, recycled or discharged to the environment. The treatment removes remaining inorganic compounds and substances such as Nitrogen and Phosphorus
- Bacteria, viruses and parasites which are harmful to public health are removed in this stage
- Alum is used to remove additional phosphorus and group the remaining solids together for easy removal in the filters
- Due to addition of alum, tiny particles cluster together in masses called floc.
- Filters are back-washed every 24 hours to remove accumulated floc.
- Chlorine contact tank removes microorganisms in tertiary treated wastewater
- Remaining chlorine is removed by adding sodium bisulphite
- Granular media filtration
- Any process that removes suspended particles through a porous medium is defined as filtration
- Granular Media Filters can be:
- Slow sand filters
- Rapid sand filters
- Granular media types:
- Mono media - Silica sand
- Dual media - Anthracite coal or Granular Activated Carbon
- Multimedia - Garnet
- Ultrafiltration
- Ultrafiltration is a type of membrane filtration in which forces like pressure or concentration gradients lead to separation through a semipermeable membrane.
- Suspended solids and solutes of higher molecular weight are retained on the retentate while water and low molecular weight solutes pass through the membrane in the permeate.
- Ultrafiltration can be used to remove particulates and macro-molecules from raw water to produce potable water.
- It can replace secondary or tertiary treatment in conventional water treatment systems or can be used as a standalone system
- It is sometimes integrated in water treatment systems in the pre-treatment system
- After ultrafiltration, the recycled water can be reused for a number of industrial purposes like boiler or cooling tower feed, water supplementation, pH adjustment, washing equipment and vehicles, fire protection, process rinse water for production lines, toilet flushing, dust control, construction activities and concrete mixing
- Industries that consume large volumes of water or discharge toxic effluents employ ultrafiltration for reusing water
- Ultrafiltration is frequently used to pretreat surface water, sea water and biologically treated municipal water ahead of reverse osmosis unit
- Micro-strainers
- Biological nitrification/denitrification
- Ion exchange
- Air stripping
- Chemical processes
- Reverse osmosis
- Electrodialysis
- Chemical precipitation
- Adsorption
- Treatment process vs Treatment system
- Population projection
- Flow rates and their fluctuations
- Mass loading
- Design criteria
- Hydraulic flow diagram
- Total solids, suspended solids, Total dissolved solids, Volatile solids, Fixed solids, Miineral solids
- BOD
- COD
- Surfactants
- Oil and grease
- Alkalinity
- Chlorides
- Total Nitrogen as N
- Sulphur
- Total Phosphorus as P
- Alkalinity as CaCO3
- Heavy metals
- Strength of wastewater
- Design period
- Flow variations
- Daily flow rate
- Average daily flow rate
- Maximum daily flow rate
- Minimum daily flow rate
- Dry Weather Flow (DWF)
- Concept of mass load
- Detention period or Hydraulic Retention Time
- Flow through velocity
- Settling velocity
- Hydraulic Loading or Surface Loading Rate or Over Flow Rate
- Weir Loading Rate
- Volumetric Loading or Organic Loading (BOD or COD or VSS loading)
- Food to Microorganism ratio (F/M)
-
The ratio of incoming BOD to MLSS, the F/M ratio is also known as the
loading on the system and is calculated as kg of BOD per day per kg of
MLSS in the aeration tank.
- If the F/M ratio is low and the aeration period or detention time (in
the aeration tank) is long, the microorganisms make maximum use of
available food, resulting in a high degree of treatment. Such systems
are known as extended aeration systems and are mostly used for isolated sources (small developments).
- Mean Cell Residence Time or Solids Retention time
- Basin Geometry (L:B:D for rectangular tanks) or (Diameter & side water depth for circular tanks)
- Reactor
- Sizing of units
- Operation units are normally designed at peak flows
- Determination of surface area
- Determination of cross-sectional area
- Stoichiometry
- Rate of reaction
- Homogeneous reaction
- Heterogeneous reaction
- Rate of reaction
- Effect of temperature on reactions
- Treatment units (Reactors)
- Types of reactors
- CFSTR
- PFR
- BR
- AFR
- FBR
- PBR
- SBR
- Reactors in series
- Primary settling tank; Types of settling
- Concept of biological treatment
- Aerobic process
- Anaerobic process
- Facultative process
- Anoxic process
- Suspended growth process
- Attached growth or fixed film process
- Bio-kinetic coefficients (Growth constants)
- Microbial growth rate
- Substrate utilization rate
- Limiting substrate
- Endogenous decay
- Significant bio-kinetic coefficients
- Specific growth rate
- Yield coefficient
- Maximum Substrate Utilization Rate Constant
- Half velocity constant
- Endogenous decay coefficient
- Reactor biomass and biomass yield
- BOD rate constant
- CODtotal
- CODb = 1.6 * BOD
- BODu/BOD5 = 1.5 (for domestic wastewater)
- Organic loading
- Quantity of air required
- Diffuse aeration system
- Surface aeration system
- Power required for oxygenation
- Volatile Suspended Solids (VSS)
- Mixed Liquor Suspended Solids (MLSS)
- The combination of the liquid and microorganisms undergoing aeration is known as mixed liquor, and the suspended solids are called Mixed Liquor Suspended Solids (MLSS).
- Mixed Liquor Volatile Suspended Solids (MLVSS) - MLVSS/MLSS = 0.8
- Sludge Volume Index (SVI)
- Sludge wasting
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