WASTE WATER TREATMENT OVERVIEW
Wastewater is generated from two major sources. A) Industrial B) Sewage
Generated from the industrial chemical reactions, washings, blow downs, Fitrations & separations etc.
Generated from human activities like toilets, Baths & Kitchens etc.
If the wastewaters remain untreated and discharged directly to the environment the receiving waters would become polluted and water borne diseases would be widely distributed.
Waste Water Treatment can be broadly divided in three categories.
- Removal of suspended particulates.
- Removal of dissolved biodegradable compounds.
- Removal of dissolved non – biodegradable compounds (salts and in – organics).
We are briefly discussing all above schemes one by one in the below following notes.
1. Removal of suspended Particulates:
The wastewater contains suspended and coagulated particulates. These particles are removed by one or the below operations in series.
The coarse solids like plastics, cotton etc. are removed by screens of suitable mesh size. The screens can be manual or automatic.
Coagulation & Flocculation
The solids which are in suspension in wastewater are neutralized by the addition of coagulants like alum so as they adhere together to form flocs. Addition of flocculant like Polyelectrolyte is for the formation of bigger and heavier flocs to settle down.
The solids which are not removed by the above two methods are removed by Filtration. Filtration can be by passing through the pressure Sand Filter or Micron Filter etc. depending upon the size of the particle to be removed.
2. Removal of dissolved compounds
Generally for removal of dissolved organics / compounds biological treatment process are used.
Majority of the waste waters can be treated biologically, where the dissolved compounds are actually eaten up by the numerous naturally occurring bacteria to produce CO2, H2O and new cells bacteria (growth).
Proper environment (pH, Temperature etc.) has to be provided for the efficient functioning of the biological systems.
The biological treatment processes can be broadly divided into three types in terms of functioning:
2.1 Aerobic – Biological treatment process that occur in the presence of oxygen. Basic types of Aerobic process are:
2.1 a) Single Stage Activated Sludge Process
2.1 b) Double Stage Activated Sludge Process
2.1 c) Moving Bed Biological process (MBBR)
2.1 d) Membrane Bioreactor (MBR)
2.2 Anoxic – The process by which nitrate nitrogen is converted biologically to nitrogen gas in the absence of oxygen. This process is also known as denitrification
2.3 Anaerobic – Biological treatment process that occur in the absence of oxygen
In terms of treatment the biological process are divided in three broad types.
a. Suspended growth – Biological treatment processes in which the microorganisms which convert the organic matter or other constituents in the wastewater to gases and new cells are maintained in suspension within the liquid. Activated Sludge process, Upflow Aneaorobic Sludge Blanket Process (UASB) are examples of Suspended growth process.
b. Attached growth – Biological treatment process in which the microorganism which convert the organic matter or other constituents in the wastewater to gases and new cells are attached to some inert medium such as specially designed ceramic or plastic materials. Attached – growth treatment process are also known as fixed-film process
c. Combined growth – Combined process are those which combine both suspended and attached growth process.
2.1 Aerobic Biological Treatment Process
This process is also referred to as activated sludge process. They consist of predominantly heterotrophic bacteria mass in suspension in aeration tanks. The Bacteria are naturally occurring. Some of the BOD / bCOD is used for respiration and the rest in used for cell growth. During Nitrification a good fraction of autotrophic Bacteria are present which convert the ammonia nitrogen to nitrate. The general stoichiometric reaction occurring in Aerobic Biological Treatment. The Nitrifying bacteria (converting Ammonia nitrogen to nitrate) have low growth rates as compared to heterotrophic bacteria.
Note: The other nutrients are also needed but ignored in the above reaction.
Oxygen required for the above process is transferred by mechanical means. Diffused Aeration system are most commonly employed for supplying oxygen. Oxygen is used for respiration by the bacteria to produce energy needed for carrying out various cellular activities.
The common terms used in Biological Treatment Processes.
- Chemical Oxygen Demand (COD) – Chemical Oxygen Demand is the measure of the organic and inorganic compounds in water. It is generally expressed in mg/lt or ppm which corresponds to the mass of oxygen consumed per litre of solution. It includes both soluble and suspended compounds. It gives the measure of compounds that can and cannot be oxidized biologically.
- Biological Oxygen Demand (BOD) – Biological oxygen demand also referred to as biochemical oxygen demand is the measure of the compounds that are can be oxidized only biologically. It is also
Expressed in mg/lt or ppm which also indicates the mass of oxygen consumed per litre for biological degradation.
- Ultimate Biological Oxygen Demand (UBOD) – The 5 day BOD measures the amount of oxygen consumed by Biochemical Oxidation in the waste water in 5 days. The total amount of oxygen consumed when the biochemical reaction is allowed to proceed to completion is called the ultimate BOD. It is also expressed in mg / lt or ppm.
- Food to Microorganism Ratio (F/M): The F/M ratio is the food to Micro-organism ration and is defined as the rate of BOD or COD applied per unit volume of mixed liquor.
Q = Volume of the influent waste water m3/d
So = Influent BOD concentration mg/lt
S = Effluent BOD concentration mg/lt
V = Aeration Tank Volume m3
X = Mixed Volatile liquor concentration in aeration tank.
- Solids Retention Time (SRT) – SRT represents the average period of time during which the sludge has remained in the system. SRT is the most critical factor for design of activated sludge process as it determines the aeration tank volume, sludge wasted, Air required. SRT is expressed in days.
- Sludge wasted (Px) – Kgs / day – Px is the amount of the sludge that needs to be purged or wasted from the system to maintain the desired concentration of MLVSS in the aeration tank.
- Mixed Liquor Suspended Solids (MLSS) & Mixed Liquor Volatile Suspended Solids (MLVSS) – The biomass solids in a bioreactor are commonly measured as Total Suspended Solids (TSS) and Volatile Suspended Solids (VSS). The mixture of solids resulting from combining recycled biosludge from clarifier with incoming waste water in the bio reactor is termed as Mixed Liquor Suspended solids (MLSS) and Mixed Liquor Volatile Suspended Solids (MLVSS). The solids consists of biomass, Non Bio – degradable Volatile Suspended Solids (nbvss) and Inert inorganic Total Suspended Solids (ITSS)
Advantage of Activated Sludge Process
- High quality of outlet effluent is achieved
- Easy to operate and maintain
- Can easily accommodate shock loadings of effluent
- Can acclimatize to very toxic waste waters and produce good quality of outlet waste water
- If activated sludge operated plant is disturbed it can be set right within a short period of time.
The activated sludge process can be single stage or double stage depending on the inlet effluent quality and the desired outlet effluent quality. For low concentrations of inlet effluent like sewage, the activated sludge is operated in single stage.
2.1 a) Single Stage Activated Sludge Process
In the single stage activated sludge process only one Aeration tank in place. The soluble biological COD is removed by the bacteria. The biological COD is then converted to biomass, CO2 & H2O. The overflow of the aeration tank enters the Settling Tank, where the bio sludge settles down and is recycled back to the aeration tank to maintain the sludge concentration.
Excess sludge in wasted as Px.
2.1 b) Two Stage Activated Sludge Process
In two stage process two Aeration tank are used. Both the aeration tanks have separate clarifiers. The settled bio sludge in both the Settling tanks in recycled back to the respective aeration tanks. The overflow of 1st stage clarifier enters the 2nd stage aeration tank. The 1st stage aeration tank in designed for lower SRT than 2nd stage aeration tank. Sludge is wasted from both the aeration tanks separately. The soluble biological COD is degraded by the bacteria in both the aeration tanks. The biological COD is then converted to Biomass, CO2 & H2O.
2.1 c) Biological Treatment with Membrane Separation (MBR)
The concept of MBR (Membrane Bioreactor) consists of utilizing a bioreactor and microfiltration as one unit process for waste water treatment thereby replacing the need for Secondary Clarifier required for solids separation. The MBR are operated at much higher MLSS than conventional Activated sludge processes.
The MBR module is generally situated outside the Bioreactor. The mother liquor is pumped through the module. The clarified treated effluent is separated by the module and Mother liquor containing the Bio-solids are returned batch to the Bioreactor.
MBR provides following advantages.
- Operating at higher MLSS means lower retention time for Bioreactor and hence less footprint area than conventional activated sludge process.
- Longer SRT gives less sludge wasting.
- Eliminates the need for secondary clarification.
- The quality of the effluent produced is better in terms of turbidity, bacteria, solids content.
- Much higher costs than the conventional activated process.
- Higher operating cost.
- Much care has to be taken for membrane fouling.
- Membrane replacement is needed.
2.1 d) MBBR – Moving Bed Biological Reactor
In the MBBR the synthetic packing material are suspended in activated sludge aeration tank. These media packing offer greater surface area for Biomass to stick and hence offer greater MLSS concentrations in the aeration tank. Accordingly the volume of the aeration tank as compared to the activated sludge process is less. Majority of the times the MBBR aeration tank is followed by conventional activated sludge process aeration tank for polishing. Settled Bio Sludge in the settling tank is not recycled to the MBBR but to conventional aeration tank.
2.2 Pre Anoxic Process for Ammonical Nitrogen Removal
The Anoxic Tank is placed before the Aeration Tank and is operated with minimal amount of oxygen. The fresh effluent enters anoxic tank. The nitrates that are formed in the Aeration Tank by the aerobic nitrifying bacteria. The nitrates are returned back to the Anoxic Tank by the internal recirculation pump. In the Anoxic Tank the Nitrates are converted to nitrogen gas by denitrifying bacteria. The soluble biological COD is used as carbon source by the denitrying bacteria. The soluble biological COD is degraded in the aeration tank by the heterotrophic bacteria. The over flow of the aeration tank enters the secondary settling tank where the biosludge settles down and is recycled back to the Anoxic tank.
2.3 Upflow Anaerobic Sludge Blanket (UASB) Process
The UASB is the most commonly followed Anaerobic process for waste water treatment.
The effluent to be treated is pumped from the Bottom of the Anaerobic reactor evenly across the surface at a desired velocity. The waste water rises through the sludge blanket where the bacteria degrade the biological COD. This biological COD is then converted to methane Gas & Biomass. The entire reactor operates in the absence of oxygen.
The advantages of Anaerobic treatment are
- Lower operating cost
- Generation of energy in the form of methane.
Disadvantage of Anaerobic Treatment
- Much Long time for startup as compared to Aerobic.
- Doesn’t give the efficiency in colder temperatures. Many a times it is required to increase the temperature of feed water.
- Is not good at handling shock loads.
- Does not produce a high quality of effluent. Anaerobic process always have to be followed by Aerobic process to remove the remaining Biological COD.
- Cannot remove Biological Nitrogen & Phosphorus.
- Cannot handle toxic substances unlike Aerobic system.
- Requires Alkalinity.
3. Removal of dissolved non – biodegradable compounds (Inorganic Salts etc.)
The inorganic salts are removed by either the Reverse Osmosis Process followed by Evaporation of the reject stream of the Reverse Osmosis Unit or in some cases the wastewater is directly fed to the Evaporator for Evaporation where the TDS (Total Dissolved Solids) are very high and the Reverse Osmosis is not suitable to Handle it.
3.a) Reverse Osmosis Unit:
Reverse Osmosis Unit is basically used as separation unit. The Treated effluent / Any feed water after Pretreatment is pumped to the Reverse Osmosis Unit at high Pressure. During Osmosis water flows from the dilute solution to the concentrated solution which are separated by a semipermeable Membrane. During Reverse Osmosis the water flows from the concentrated side to the dilute side by the application of required Pressure on the concentrated side.
The product water separated during Reverse Osmosis is sent for reuse and the concentrated reject water is sent for Evaporation to recover / remove the salts by evaporation of water.
The type of membrane selected depends solely on the characteristics of the inlet water and the product quality required. Membranes are susceptible to scaling and fouling so the recovery rates of the membranes are planned accordingly.
Evaporation is a process of concentrating solution by vaporizing a portion of the solvent. The solution comprises of solvent which is generally water and solute in the form of dissolved salts added due to different processes in the manufacturing unit. Evaporation is followed by crystallization or Filtration or drying based on type of salt in the solution. Evaporation can be Single Effect or Multiple Effect. For more details please see Evaporator overview.