(part 3 of "Aerobic stabilisation of pharmaceutical wastewaters using large scale extended aeration activated sludge process" by Saugath Lahiri..JNTUniv, Hyd, INDIA jan 2002)
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3. MANUFACTURING PROCESS IN PHARMACEUTICAL INDUSTRY
The pharmaceutical manufacturing industry encompasses the manufacture, extraction, processing, purification and packaging of chemical materials to be used as medication.
The wastewater in a pharmaceutical industry generally comes from the synthesis and formulations of drugs. Most of the active ingredients marketed and sold as drugs are manufactured by chemical synthesis. Chemical synthesis is the process of manufacturing pharmaceuticals using organic and inorganic chemical reactions.
Chemical Synthesis TechniquesEmployed in the manufacturing of pharmaceuticals are:
Fermentation: It is a biochemical reaction within a reactor in the presence of selected active microbes or enzymes. Reactions are carried out under mild chemical and physical conditions. Various drugs like antibiotics, enzymes, hormones, vitamin biz, vaccines etc., are manufactured by the process of fermentation.
Organic Synthesis:--A large number of pharmaceuticals are manufactured by organic synthesis. This involves several steps like oxidation, reduction, nitration, sulphonation, halogeneration, amination, aminolysis, friedel-crafts acetylation alkylation, esterification, crystallization, hydrogenation, precipitation etc.
Products produced by organic synthesis are like chloramphenicol, sulfadrugs, quinolines, dexamenthosuner antidiabetics, antihelminthic, antifilamical, antileptrotic, antimalaria, anti T.B., antipyretic, analgesis and vitamins. Wide ranges of chemicals are used in different processes for production and purification of various drugs.
A variety of priority pollutants released during chemical synthesis are the reaction and purification solvents. These include benzene, chlorobenzene, chloroform, chloromethane, O-dichlorobenzene, 1,2-dichloroethane, methylene chloride, phenol, toluene and cyanide. The top five non-conventional pollutants associated with chemical synthesis are methanol, ethanol, isopropanol, acetone and ethyl acetate. Six member ring compounds such as xylene, pyridine and toluene are also widely used as organic solvents because they are stable compounds that do not easily take part in chemical reactions. These compounds are used either in the manufacture of synthesized pharmaceuticals or are produced as a result of unwanted side reactions.
Solvents are used in the chemical synthesis process to dissolve gaseous, solid or viscous reactants in order to bring all the reactants into close molecular proximity. Solvents also serve to transmit heat to or from the reacting molecules. By physically separating molecules from the each other, solvents slow down some reactions that would otherwise take place too rapidly, resulting in unwanted reactions and excessive increases in temperature.
Some solvents are also used to control the reaction temperature. Many plants operate solvent recovery units that purify contaminated solvents for reuse. These units usually contain distillation columns and may also include solvent-solvent extraction operations in which a second solvent is used to separate impurities. These operations result in aqueous wastes that contain residues fully or partially saturated with residual solvent. Wastewater is generally produced with each chemical modification that requires filling and emptying the batch reactors. This wastewater can contain un reacted raw materials as well as some lolvents along with a large number of compounds that differ due to the varied chemical reactions performed (ex: Nitration, Amination, Halogination, Sulphonation, Alkylation etc.) the pollutants in chemical synthesis wastewater/effluent vary with respect to toxicity and biodegradability. The production steps may generate acids, metals and other pollutants, while the waste process solutions and vessel wash water may contain residual organic solvents.
The primary sources of wastewater from chemical synthesis operations are
Wastewaters from chemical synthesis plants can be characterized as having high BOD, COD and TSS concentrations and extremely variable pH values ranging from 1.0 to 11.0.
Pharmaceutically active ingredients are generally produced by batch processes in bulk form and must be converted to dosage form for consumer use. The common dosage forms are tablets, capsules, liquids and ointments.
Tablets are prepared by blending the ingredient, filler and binder. The filler (starch, sugar) is required to dilute the active medicinal ingredient to the proper concentration and binder (ex: corn syrup or starch) is necessary to bind the tablet particles together. A lubricant (magnesium stearate) may be added for proper tablet machine operation. Various other physical methods employed for the preparation of formulation products are mixing, grinding, sieving, filtration, washing, drying, milling, encapsulation and packing.
Sources of Waste Water in a Pharmaceutical Industry
The wastewater in any industry comes from the different processes done in the industry and this can be called as ‘Process Wastewater’. Process wastewater can be defined as any water which, during manufacturing or processing, comes into direct contact with or results from the production or use of any raw material, intermediate product, finished product, by-product or waste product.
Water is used and wastewater is generated in pharmaceutical manufacturing processes as follows:
Other sources of process wastewater associated with pharmaceutical manufacturing operations include:
In addition to process wastewater other types of wastewater may be generated during pharmaceutical manufacturing. This wastewater may include non-contact cooling water (used in heat exchangers), non-contact ancillary water (boiler blowdown, bottle washing), sanitary wastewater and wastewater from other sources (stormwater runoff).
Wastewater Treatment Technologies in the Pharmaceutical Manufacturing Industry
The major wastewater treatment technologies used in the pharmaceutical manufacturing industry include:
1.Advanced biological treatment
2. Multimedia filtration
3. Polishing pond treatment
4. Cyanide destruction
5. Steam Stripping and Steam Stripping with Rectification
6. Granular activated carbon adsorption
7. pH adjustment/neutralization
8. Equalization
9. Air stripping
10. Incineration.
Advanced Biological Treatment
Advanced biological treatment is used in the pharmaceutical manufacturing industry to treat BOD , COD, TSS, and to degrade various organic constituents. The term "advanced" is used to refer to treatment systems that consistently surpass, on a long-term basis, 90% BOD reduction and 74% COD reduction in pharmaceutical manufacturing wastewater. To provide reduction of ammonia in the wastewater using advanced biological treatment, nitrification is necessary. Biological systems can be divided into two basic types: aerobic (treatment takes place in the presence of oxygen) and anaerobic (treatment takes place in the absence of oxygen). The four most common aerobic treatment technologies in the industry are activated sludge systems, aerated lagoons, trickling filters, and rotating biological contactors (RBC).
In aerobic biological treatment processes, oxygen-requiring microorganisms decompose organic and nonmetallic inorganic constituents into carbon dioxide, water, nitrates, sulfates, organic byproducts, and cellular biomass. The microorganisms are maintained by adding oxygen and nutrients (usually nitrogen and phosphorous) to the system. Activated sludge and aerated lagoon processes are suspended-growth processes in which the microorganisms are maintained in suspension within the liquid being treated. The trickling filter and RBC processes are attached- growth processes in which microorganisms grow on an inert medium (e.g., rock, wood, plastic).
Equalization of flow, pH, temperature, and pollutant loads is necessary to perform consistent, adequate treatment.
Some key design parameters for activated sludge systems include nutrient-to-microorganism ratio, mixed liquor suspended solids (MLSS), sludge retention time, oxygen requirements, nutrient requirements, sludge production, substrate removal rate constant (K), and percent BOD of effluent TSS. Pharmaceutical manufacturing industry averages for some of these parameters are presented in the following table.
| Parameter | Average | Average |
| Food to Microorganism Ratio (lb/lb/day) | 0.561 | 0.054 |
| MLSS (mg/L) | 5,521 | 3,443 |
| Sludge Retention Time (hours) | 33.0 | 22.9 |
| K | 11.14 | 2.06 |
| %BOD of TSS | 23 | 24 5 |
Ammonia treatment by nitrification is achieved in biological treatment units by incorporating two additional sets of autotrophic microorganisms. The first set of microorganisms (Nitrosomonas bacteria) converts ammonia to nitrites and the second set (Nitrobacter bacteria) converts nitrites to nitrates. These microorganisms are maintained in the treatment tank in a similar fashion as the microorganisms described above (addition of oxygen, nutrients, etc). Nitrification can be accomplished in either a single or two-stage activated sludge system. Indicators of nitrification capability are 1) biological monitoring for ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) to determine if nitrification is occurring, and 2) analysis of the nitrogen balance to determine if nitrifying bacteria reduce the amount of ammonia and increase the amount of nitrite and nitrate.
Multimedia Filtration
Multimedia filtration is used in the pharmaceutical manufacturing industry to reduce TSS in wastewater. This technology may also serve to treat BOD in wastewater by removing BOD 5 associated with particulate matter. A multimedia filtration system operates by introducing a wastewater to a fixed bed of inert granular media. Suspended solids are removed from the wastewater by one or more of the following processes: straining, interception, impaction, sedimentation, and adsorption. This operation is continued until there is either solids "breakthrough" (solids concentration increases to an unacceptable level in the discharge from the bed), or the head loss across the bed becomes too great (due to trapped solids) to operate the bed efficiently. If either of these conditions occurs, the bed must be cleaned by backwashing before it can be operated effectively again. Backwashing usually is accomplished by reversing the flow to the bed and introducing a "clean" stream of wash water. Wash water is introduced until the bed becomes fluidized (expanded). At this point, the solids are washed from the bed and carried away from the unit. It is common to return the backwashed solids stream to the biological treatment system (if applicable).
In multimedia filtration, a series of layers, each with a progressively smaller grain size medium (traveling from inflow to outflow of the bed) are used in the filtration bed. This design allows solids to penetrate deeper into the bed before becoming fixed, thus increasing the capacity of the bed and decreasing the buildup of head loss in the unit. Typical filtration media include garnet, crushed anthracite coal, resin beads, and sand. Though down flow (gravity flow) systems are the most common, up flow and biflow (influent is introduced above and below the filter medium, and the effluent discharges from the center of the filter medium) filtration units can also be used.
Some key design parameters associated with multimedia filtration units include wastewater flow rate, hydraulic loading rate, and filter medium depth. The following table shows ranges of values for each of these parameters for treatment units currently operated in the pharmaceutical manufacturing industry.
Parameter Range Units
Flow Rate 0.03 - 2.18 MGD
Hydraulic Loading Rate 2.0 - 5.0 gpm/ft
Depth of Medium 6 - 72 inches
Polishing Pond
Polishing ponds are used in the pharmaceutical manufacturing industry to remove TSS from wastewater using gravity settling. Some BOD removal associated with the settling of suspended solids may also occur. The wastewater is introduced at one end of the pond and ultimately flows out the other end. The pond is designed such that the water retention time is long enough and the water velocity is slow enough to allow solids to fall out of suspension. If the flow is too fast, or other mixing is added to the system, solids may be maintained in suspension and discharged from the pond. To avoid anaerobic conditions in the bottom portion of the pond, these units must be designed to be shallow, which may require a large land area if flow to the unit is high. Depths of polishing ponds currently used in the industry range from 2.5 to 14 feet. Retention times range from 0.2 to 14.6 days. In the past, polishing ponds have been designed with an earthen liner only; however, current regulations require installation of a minimum of two liners and a leak detection system for most new applications to this industry. Polishing ponds will accumulate solids over time and will therefore require periodic cleanout.
Steam Stripping and Steam Stripping with Rectification
Steam stripping and steam stripping with rectification are used both in industrial chemical production (for chemical recovery and/or recycle) and in industrial waste treatment to remove gases and/or organic chemicals from wastewater streams by providing steam to a tray or packed column. Under both technologies, differences in relative volatility between the organic chemicals and water are used to achieve a separation. The more volatile components of the feed mixture concentrate in the vapor, while the less volatile components concentrate in the liquid residue (bottoms). Steam stripping and steam stripping with rectification are effective treatment for a wide range of aqueous streams containing organics and ammonia. Appropriately designed and operated columns can treat a variety of waste streams ranging from wastewaters containing a single volatile constituent to complex organic/inorganic mixtures. Steam stripping and steam stripping with rectification can be used both as in-plant processes to recover concentrated organics from aqueous streams and as end-of-pipe treatment to remove organics from wastewaters prior to discharge or recycle. For most effective wastewater treatment, columns should be placed after the process generating the wastewater and before the wastewater is combined with other wastewater that does not contain the pollutants being treated. Wastewater with high concentration and low flow is easier and less expensive to treat than wastewater with high flow and/or low concentration. In addition, the amount of volatiles emitted to the air can be minimized if columns are placed prior to exposure of the wastewater stream to the atmosphere.
General Description
Steam stripping and steam stripping with rectification can be conducted as either a batch orcontinuous operation in a packed tower or fractionating column (sieve tray or bubble cap) withmore than one stage of vapor-liquid contact. In a steam stripping column, the wastewater feedenters near the top of the column and then flows downward by gravity, countercurrent to thesteam which is introduced at the bottom of the column. In a steam stripping with rectificationcolumn, the wastewater feed enters lower down the column to allow for a rectification above thefeed. In the rectification section, a portion of the condensed vapors are refluxed to the column tocountercurrently contact the rising vapors. This process concentrates the volatile components inthe overhead stream.Steam may either be directly injected or reboiled, although direct injection is more common. Thesteam strips volatile pollutants from the wastewater, which are then included in the upward vaporflow. As a result, the wastewater contains progressively lower concentrations of volatilecompounds as it moves toward the bottom of the column. The extent of separation is governedby physical properties of the volatile pollutants being stripped, the temperature and pressure atwhich the column is operated, and the arrangement and type of equipment used.The difference between steam stripping columns and steam stripping with rectification columns isthe location of the feed stream. Stripping columns have a feed stream located near the top of thecolumn while steam stripping with rectification columns have a feed stream located further downthe column. Pollutants that phase separate from water can usually be stripped from thewastewater in a steam stripper (a column without rectifying stages). Pollutants that are notphase-separable, such as methanol, need a column with rectifying stages to achieve a highconcentration of the pollutants in the overhead stream.The ancillary equipment used in conjunction with steam stripping and steam stripping withrectification columns includes a condenser and subcooler, pumps for the feed, overhead, bottoms,and reflux streams, a feed preheater and bottoms cooler, a decanter, a storage tank, a distillatetank, and an air pollution control device to contain any vapors from the condenser. The7-21condenser and subcooler condense and cool the overhead stream to a temperature amenable forstorage and disposal. The pumps supply the force to move the waste stream: either into thecolumn at the feed position or at a point above the feed in the case of a reflux stream. Thebottoms pump moves the bottoms from the stripping column to the bottoms cooler, and theoverheads distillate pump moves the distillate from the decanter to the distillate receiver tank.The feed preheater/bottoms cooler is a heat exchanger that heats the feed before it enters thecolumn at the same time it cools the bottoms stream so that it can be sent to a storage area ortreatment system. The decanter separates the aqueous layer from the organic layer after thestream comes from the condenser and subcooler. The aqueous layer can be refluxed back to thecolumn while the organic layer is usually disposed of or reused. The storage tank provides asteady feed for the steam stripper column, equalizing flow and waste variability. An air pollutioncontrol device may be needed to contain any pollutants that do not condense in the condenser andwould otherwise escape to the air. Wet scrubbers, carbon adsorption devices, or venting to acombustion device may be used to control air emissions. Figure 7-3 shows a flow diagram of atypical steam stripping treatment system and Figure 7-4 shows a flow diagram of a typical steamstripping with rectification treatment system.The typical construction material for steam stripping and steam stripping with rectificationcolumns in the pharmaceutical manufacturing industry is stainless steel. If a wastewater stream ishighly corrosive, a more corrosion-resistant material, such as Hastelloy or Teflon®-lined carbonsteel, may be required for construction of the column. The majority of pharmaceuticalmanufacturing facilities which currently use steam stripping and/or steam stripping withrectification columns to treat their wastewater use stainless steel.Salts and other pollutants may contribute to scaling and corrosion inside the column. Timelymaintenance should be provided to deter scaling problems. Costs of these measures are discussedin 10.The key design parameters for stripping columns are the steam-to-feed ratio and the number oftrays or equilibrium stages in packed columns. These parameters are calculated using the7-22equilibrium ratio of the least strippable contaminant in the wastewater stream and the removalefficiency required to treat the contaminant to the desired concentration. Typical ranges forsteam-to-feed ratios vary from 1:3 to 1:35, and the typical number of trays or equilibrium stagesvary from 2 to 20. Generally, columns with smaller diameters are packed while columns withlarger diameters have trays. Typical column packings are Pall rings, Rashing rings, Berl saddles,and Intalox saddles.
Granular Activated Carbon Adsorption
Granular activated carbon (GAC) adsorption is used in the pharmaceutical manufacturing industry to treat BOD , COD, or organic constituents in wastewater. Adsorption is a process in which soluble or suspended materials in water are bonded onto the surface of a solid medium. Activated carbon is an excellent medium for this process because of its high internal surface area, high
attraction to most adsorbates (the constituents to be treated), and the fact that it is hydrophobic (water will not occupy bonding sites and interfere with the adsorption process). Constituents in the wastewater bond onto the GAC grains until all surface bonding sites are occupied. At this point, the carbon is considered to be "spent", and requires regeneration, cleaning, or disposal. Activated carbon is normally produced in two standard grain sizes: powdered activated carbon (PAC) with diameters less than a 200 mesh, and GAC with diameters greater than 0.1 mm. PAC is generally added to the wastewater, whereas GAC is normally used in flow-through fixed bed units.
For treatment units, GAC is packed into one or more beds or columns. Multiple beds are more common, and are normally operated in series because this design allows for monitoring between beds, and therefore minimizes the risk of discharging wastewater from the system with concentrations above acceptable levels. Wastewater flows through a bed and is allowed to come in contact with all portions of the GAC. The GAC in the upper layers of the bed is spent first as bonding sites are occupied, and the GAC in progressively lower regions is spent over time as the adsorption zone moves down through the unit. When contaminant concentrations begin to increase at the bottom of the bed above acceptable levels, the bed is considered to be spent and must be removed. The above description assumes that beds are operated in downflow mode; however, it is also possible to use an upflow design for GAC systems.
Once a bed is spent, the carbon can be treated in three ways: regeneration, backwash, or disposal. Normally, it is possible to use high heat (1,500 to 1,700E F), steam, or chemical treatment to regenerate the spent carbon. These processes remove contaminants from the carbon without significantly affecting the carbon itself; however, some carbon is lost each time this procedure is performed, and carbon performance decreases slightly with each regeneration. If the carbon cannot be regenerated or backwashed, it must be disposed of as a solid waste.
The performance of GAC treatment units can be affected by several factors. Three important design criteria are saturation loading, wastewater TSS concentration, and hydraulic loading. Saturation loading is a treatment performance coefficient relating mass of contaminant adsorbed versus mass of carbon used. If this coefficient is very low (as is the case for highly soluble constituents), a GAC system will not perform efficiently. Parameters that effect solubility (i.e., pH and temperature) must also be considered when calculating a design saturation loading for a system. High TSS concentrations in wastewater will foul the GAC system. Solids will occupy bonding sites on the carbon and will get plugged in the pore spaces between GAC grains. If this happens, head loss may occur and a portion of the carbon bed will not be used for treatment. Flushing to remove solids can upset the mass flux zone in the GAC system. In some cases, it may be necessary to install some type of filtration prior to GAC treatment to keep TSS concentrations within acceptable limits. The effectiveness of GAC can only improve with lower TSS, and ideally, TSS levels in the influent should be as close to zero as possible. The amount of time the wastewater spends in contact with the GAC is directly related to hydraulic loading rate. If this
time is not long enough, effluent contaminant concentrations will be higher than expected.
pH Adjustment/Neutralization
Because many treatment technologies used in the pharmaceutical manufacturing industry are sensitive to pH fluctuations, pH adjustment, or neutralization, may be required as part of an effective treatment system. A pH adjustment system normally consists of a small tank (10 to 30 minutes retention time) with mixing and a chemical addition system. To adjust pH to a desired value, either acids or caustics can be added in the mixing tank. Some treatment technologies require a high or low pH to effectively perform treatment (e.g., air stripping of ammonia requires a pH of 10 to 11). pH is generally adjusted to between 6 and 9 prior to final discharge.
Equalization
Because many of the treatment technologies listed in this are performed continuously and some are sensitive to spikes of high flow or high contaminant concentrations, it is necessary to include equalization as a part of most treatment systems. Equalization is normally performed in large tanks or basins designed to hold a certain percentage of a facility's daily wastewater flow. Equalization will equalize high- and low-flow portions of a typical production day by allowing wastewater to be discharged to downstream treatment operations at a constant flow rate. Equalization can also provide a continuous wastewater feed to operations such as biological treatment that perform more effectively under continuous load conditions. The mixing that occurs in an equalization basin minimizes spikes of various contaminants in the discharged wastewater. This equalization will prevent loss of treatment effectiveness or treatment system failures associated with these spikes.
BIOLOGICAL TREATMENT PROCESS
Objectives:
The objectives of the biological treatment of wastewater are to:
Role of Micro-Organisms:
The role of micro-organisms to accomplish the above objectives involves conversion of the colloidal and dissolved carbonaceous organic matter into various gases and into cell tissue. Because cell tissue has a specific gravity slightly greater than that of water, the resulting cells can be removal from the treated liquid by gravity settling.
Microorganisms in Biological Treatment
The micro-organisms that are involved in a biological treatment process are:
Algae are also important in biological treatment process because of their involvement in nutrient (Phosphorus or nitrogen or both) removal.
Application of Biological Treatment Process
Biological Treatment Process
The major biological process used for wastewater treatment are categorized into five groups:
Biological treatment process can further be classified as suspended growth systems
and attached growth systems. The principal suspended growth biological treatment processes used for the removal of carbonaceous organic matter are
ACTIVATED SLUDGE PROCESS
Activated Sludge Process:
Activated sludge process is described as continuous flow biological treatment system characterized by a suspension of aerobic micro-organisms (MLSS) maintained in a relatively homogenous state by the mixing and turbulence induced in conjunction with the aeration process. Basically, the activated sludge process used micro-organisms in suspension to oxidize soluble and colloidal organics in the presence of molecular oxygen. During oxidation process a portion of the organic material is synthesized into new cells. A part of the synthesized cells then undergo auto oxidation reaction. These processes may be represented stoichometrically as follows.
Oxidation and Synthesis:
BACTERIAL
COHNS + O2 + nutrients -- (through bacteria)----> CO2 (organic matter) + NH3 +C5H7NO2 (+new bacterial cells)+ other end products.
Endogenous Respiration:
BACTERIAL
C5H7NO2+ 5O2 --> 5CO2 + 2H2O + NH3 + energy
To operate the process in a continuous basis the solids generated must be separated in a clarifier. The major portion is recycled to the aeration took to maintain the desired MLSS and the excess sludge is withdrawn from the clarifier for additional handling and disposal.
2.3.4. Process Microbiology:
To design and operate an ASP efficiently, it is necessary to understand the importance of the microorganism in the systems. In the ASP, bacteria are the most important microorganisms because they are responsible for the decomposition of the organic material in the effluent. A portion of the orginal waste is actually oxidized to low energy compounds such as NO3-, SO42- and CO2 and the remainder is synthesized into cellular material. All types of bacteria present in ASP are chemohetrotrophic bacteria.
The metabolic activities of other microorganisms are also important in the activated sludge process. For eg. Protozoa and rotifers, which act as effluent polishers. Protozoa consume dispersed bacteria that have not flocculated and are essential in the operation of biological treatment processes because they maintain natural balance/equilibrium among different groups of microbes. Rotifers consume small biological floc particles that have not settled.
The biomass is transformed from small particles (bacteria) to larger particles (protozoa and rotifers). These being bulkier are easier to sediment at the settling stage. The protozoa can also engulf particulate dead organic residues and by a process called Pinocytosis, take up soluble nutrient.
2.3.5. Basic Design Factors of ASP:
Basic factors in design of an ASP system are presented below in two categories
Dependent Variables
Independent Variables
To design an effective activated sludge treatment plant though several parameters needs to be considered predictably the main consideration will be given to the following parameters..
2.3.5.1. Organic Loading Rate:
The organic loading rate is defined as the amount of BOD/COD rate applied per unit volume of the reactor. If this is not adequately calculated the effluent quality will be poor as a result of inappropriate sludge settling rates from optimum.
Organic Loading Rate (Kg/m3/d) = Incoming flow (m3 d-1) * BOD of incoming liquid Kg/m3
where Reactor Volume is (m3)
2.3.5.2. Hydraulic Retention Time:
Aeration time of the effluent in the activated sludge tank is an important parameter that is monitored regularly. The HRT is calculated by dividing the reactor volume by the total incoming flow.
Hydraulic Retention time= Reactor Volume (m3) / Total Daily Flow (m3/d)
If this parameter is too low then the liquor will be poorly oxygenated and little BOD < ultimately removed.
2.3.5.3. Sludge Loading Rate:
The most useful loading parameter is the sludge-loading rate. This a measure of the ratio of the microbiological input (derived from return sludge flocs) to the nutrients, which is represented by the flow of the influent of a particular BOD/COD into the reactor.
It is expressed as,
Sludge loading rate =
BOD of incoming liquid (Kg/m3) * Influent flow (m3/day) / Reactor Volume (m3) * Reactor Solid (Kg/m3)
This parameter is particularly valuable as it can be readily controlled by altering the quantity if returned bioflocs. It is usually manipulated to effect the largest achievable reduction in BOD within the reactor, but alternatively an effective degree of nitrification is sought, the desired result may also be achieved by controlling the sludge-loading rate. Environmental factors are of utmost importance in the reactor VIZ., temperature, pH and alkalinity. Temperature is significant because changes in the wastewater temperature can effect the biological oxidation rate. Alkalinity and pH are important, particularly in the operation of nitrification processes. Low pH may inhibit the growth of nitrifying organisms and encourage the growth of filamentous organisms. So pH adjustment may be essential.28
2.3.5.4. Oxygen Uptake Rate:
Microorganisms in the activated sludge process use oxygen as they consume food. The rate at which they use oxygen, the oxygen uptake rate is the measure of biological activity. High OUR indicate high biological activity. The value of OUR is obtained by taking a sample of mixed liquor, saturated with DO, and with a DO probe measuring the decrease in DO with time.
Oxygen uptake is most valuable for plant operations when combined with VSS data. The combination of the OUR with the concentration of MLVSS yields a value termed Specific Oxygen Uptake Rate. SOUR indicates the amount of oxygen used by microorganisms and is reported as mg O2/g MLVSS.h.
2.3.5.5. Dissolved - Oxygen Control:
The amount of oxygen transferred in the aeration tank theoretically equals the amount of oxygen required by the microorganisms in the activated sludge system to oxidize the organic material and to maintain residual dissolved oxygen operating levels. When oxygen limits the growth of microorganisms, filamentous organisms may predominate, and the settleability and quality of the activated sludge may be poor. In practice the dissolved-oxygen concentration in the aeration tank should be maintained at about 1.5 to 4mg/L in all the areas of the aeration tank, 2mg/L are a commonly used value.
2.3.5.6. Foaming:
Viscous brown foam that covers the aeration basins and secondary clarifiers causes major problems in an ASP. The foam is associated with a slow growing filamentous organism of the actinomycetes group, usually of the Nocardia genus. Some of the possible causes of foaming are
2.3.6. Process Design Consideration
2.3.6.1. Loading Criteria: Controlling either mean cell residence time 0c or F/M ratio can control the effluent quality or the treatment efficiency. The mean cell residence time is given by the equation
qc= VX / QwX+(Q-Qw)Xe
Where V = Volume of the aeration tank
Q= influent wastewater flow
Qw = rate of excess sludge waste
X = concentration of MLVSS
Xe = concentration of VSS in effluent
The high rate activated sludge process designed for a q c of 3-5 days has a low substrate removal efficiency and may exceed the effluent standards of 30mg/l of total BOD and 30mg/l of suspended solids. q c in the range of 5-15 days provides a process efficiency resulting in effluent of satisfactory quality. The food to microorganism (F/M) ratio is given by the equation.
F/M = QSo /VX = So/q X
Where X= Concentration of microorganisms in reactor
So= initial BOD concentration
V= Volume of the reactor
Q= rate of inflow
q= retention time
The specific substrate utilization rate is the substrate utilization rate divided by
the concentration of biomass in aeration tank. It is given by the equation.
U= QSo/VX * ( So - Se ) / So
Where Se = BOD of effluent.
The relational between F/M and U can be expressed as
U =(F/M) E/ 100
The relation between qc and F/M is given by the equation
1 / q c = ( Y (FM)E --Kd ) / 100
whereY = Growth yield
Kd = microbial decay coefficient
E = Soluble BOD removal
For the activated sludge process to continue aeration tank volume can be calculated as:
V= q cY Q (So-Se) / X (1+Kd q c)
The rate of production of excess volatile suspended solids is given by
dx/dt = Y dF/dT -- Kd Y
or directly by dxX/dt =X /q c
where dF/dT = rate of substrate utilization.
dt
The total volatile suspended solid production per unit time
= dX/dT = XV/ q c
The above equation gives the mass of excess volatile suspended solid production, from which the total suspended solid production, and hence the sludge production can be estimated. The rate of return sludge flow Q2 depends on VSS concentration in secondary settling tank underflow (Xr) and is given by the equation.
The volume of actual air requirement=Theoretical requirement/ Oxygen transfer efficiency
2.3.6.2. Selection of the reactor type:
The selection of reactor type to be used in the treatment process is important in any of the biological processes. Operational factors to be considered for the reaction are
2.3.6.3. Sludge Production and Process Control
It is important to know the quantity of sludge to be produced per day because it will effect the design of sludge handling and disposal facilities.
2.3.6.4. Oxygen Requirements:
The oxygen requirements can be obtained by knowing 5 day BOD of waste and the amount of organisms wasted from system per day. If all the BOD5 were converted to end products the total oxygen demand can be known by converting BOD5 to BODL using appropriate conversion factors. It is known that a portion of waste is converted to new cells that are wasted form the system. Therefore if BODL of wasted cells is subtracted from the total, the remaining amount represents the amount of oxygen that must be supplied to the system.
The air supply must be adequate to
2.3.6.5. Nutrient Requirement:
If the biological system is to function properly nutrients must be available in adequate amounts. The basic nutrients are Nitrogen and Phosphorus. Other nutrient required by most of the biological systems are Na+, K+, Ca+, Mg+, PO3-, Cl-, SO2-4, HCO3- in substantial amounts and Fe+, Cu2+, Mn2+, Zn2+ in trace quantities.9,30
2.3.6.6. Environmental Requirements:
The important environment factors are temperature and pH, which are prescribed for the reactor.
2.3.6.7. Solid Separation:
The design of the reactor should be such that the biological solids can be separated from the treated wastewater. If the solids cannot be separated and returned to the aeration tank the activated sludge process will function properly i.e., the solids may interfere in the functioning of the process.
2.3.6.8. Effluent Characteristics:
Organic content is the major parameter of effluent quality. The organic content of effluent from biological treatment process is usually composed of the following constituents:
Soluble Biodegradable Organics
Non-Biodegradable Organics
2.4. ACTIVATED SLUDGE PROCESS – MODIFICATIONS
The activated sludge process is very flexible and can be adapted to almost any type of biological waste treatment problem. Several of the conventional activated sludge processes and some of the modification that have become standardized are given below.
2.4.1. Conventional Plug Flow:
In this process settled wastewater and recycled activated sludge enters the head end of the aeration tank and are mixed by diffused-air or mechanical aeration. Air application is generally uniform throughout tank length. During the aeration period, adsorption, flocculation and oxidation of organic matter occur. Activated sludge solids are separated in a secondary-settling tank.
2.4.2. Complete Mix System:
This process is an application of the flow regime of a continuos-flow stirred tank reactor. Settled wastewater and recycled activated sludge are introduced typically at several points in the aeration tank. The organic load on the aeration tank and the oxygen demand are uniform throughout the tank length.
2.4.3. Tapered Aeration:
Tapered aeration is a modification of the conventional plug-flow process. Varying aeration rates are applied over the tank length depending on the oxygen demand. Greater amounts of air are supplied to the head end of the aeration tank, and the amounts diminish as the mixed liquor approaches the effluent end. Tapered aeration is usually achieved by using different spacing of the air diffusers over the tank length.
2.4.4. Step-Feed Aeration
Step feed aeration is a modification of the conventional plug-flow process in which the settled wastewater is introduced at several points in the aeration tank to equalize the F/M ratio, thus lowering peak oxygen demand. Flexibility of operation is one of the important features of this process.
2.4.5. Modified Aeration:
Modified aeration is similar to the conventional plug-flow process except that shorter aeration times and higher F/M ratios are used. BOD removal efficiency is lower than other activated sludge processes.
2.4.6. Contact Stablization:
In this process two separate tanks or compartments are used for the treatment of the wastewater and stabilization of the activated sludge. The stabilized activated sludge is mixed with the influent wastewater in a contact tank. The mixed liquor is settled in a secondary settling tank and the return sludge is aerarted seperately in a reaeration basin to stabilize the organic matter. Aeration volume requirements are 50 percent less than the conventional plug-flow.
2.4.7. Extended Aeration:
Extended aeration process is similar to the conventional plug-flow process except that it operates in the endogenous respiration phase of the growth curve, which requires a low organic loading and long aeration time.
2.4.8. High Rate Aeration:
High rate aeration is a process modification in which high MLSS concentrations are combined with high volumetric loading. The combination aloows high F/M ratios and low mean cell residence times with relatively short hydraulic retention times. Adequate mixing is very important.
2.4.9. High Purity Oxygen:
High purity oxygen is used instead of air in the activated sludge process. Oxygen is diffused into covered aeration tanks and is recirculated. A portion of the gas is wasted to reduce the concentration of carbondioxide. pH adjustment may also be required. The amount of oxygen added is about four times greater than the amount that can be added by conventional aeration systems.
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