Secondary Wastewater Treatment

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Secondary wastewater treatment systems are biological systems with the purpose of processing primary wastewater system effluent. The main goal of secondary wastewater treatment is to remove soluble pollutants from wastewater. During secondary waste water treatment, consideration must be given to handling of sludge, where microbial organisms under proper environmental conditions, perform a wastewater decontamination function. Descriptions of the major types of secondary wastewater processes are presented, accompanied by equations for the chemistry involved and information on equipment design and performance parameters.


Type | Common name | Use | Aerobic processes:

Suspended growth Activated sludge Conventional (plug flow) Continuous flow stirred tank Step aeration Pure oxygen Modified aeration Contact stabilization Extended aeration Oxidation ditch Carbonaceous BOD removal (nitrification) Suspended growth nitrification; Nitrification Aerated lagoon Carbonaceous BOD removal (nitrification) Aerobic digestion Conventional air Pure oxygen Stabilization Carbonaceous BOD removal High-rate aerobic algal pond Carbonaceous BOD removal Attached growth Trickling filter Low rate High rate Carbonaceous BOD removal (nitrification) Roughing filter Carbonaceous BOD removal Rotating biological contactor Carbonaceous BOD removal (nitrification) Packed bed reactor Nitrification Combined processes Trickling filter, activated sludge Activated sludge, trickling filter Carbonaceous BOD removal (nitrification)

Anoxic processes:

Suspended growth Denitrification; Denitrification; Attached growth; Denitrification; Denitrification

  • Anaerobic processes
  • Suspended growth
  • Attached growth

Anaerobic digestion Standard rate single stage High rate single stage Two stage Anaerobic contact process Anaerobic filter Anaerobic lagoon (pond)

Stabilization, carbonaceous BOD removal

Carbonaceous BOD removal Carbonaceous BOD removal, stabilization (denitrification)

Carbonaceous BOD removal, stabilization

Aerobic/anoxic or anaerobic processes

Suspended growth Single stage Attached growth Nitrification/denitrification; Combined processes Facultative lagoon (pond)

Maturation or tertiary pond Anaerobic-facultative lagoon Anaerobic-facultative-aerobic lagoon

Carbonaceous BOD removal, nitrification, denitrification Nitrification/denitrification Carbonaceous BOD removal Carbonaceous BOD removal (nitrification) Carbonaceous BOD removal Carbonaceous BOD removal

TBL. 1 Major Biological Processes Used for Wastewater Treatment


General Principles of Secondary Treatment

Secondary treatment processes in the overall waste treatment system have three major purposes:

1. Biological oxidization of soluble organic matter that remains after primary treatment

2. Absorption of suspended solids carried over from primary treatment as well as settling of solids generated as a result of the biological process

3. Biological removal of certain nutrients like ammonia, nitrate, and phosphorus that are dissolved in wastewater Secondary biological treatment requires availability of many micro organisms (bacteria), good contact between these organisms and organic material, availability of oxygen, sufficient nutrients, favor able temperature conditions, favorable pH ranges, and adequate time for organisms to work. Microorganisms in secondary treatment systems need a source of food or energy [biochemical oxygen demand (BOD)], time (the biological process retention time), and a source of oxygen to function effectively and efficiently.

Secondary or biological waste treatment processes can be classified by the presence or absence of dissolved oxygen (DO). Terms usually applied to this classification are:

• Aerobic: Usually DO is maintained higher than 0.5 mg/L. Optimum is 1 to 2 mg/L or slightly higher in specific cases.

• Anoxic: DO should be less than 0.5 mg/L and most preferably non-detectable.. Combined oxygen such as nitrate and nitrite can be present.

• Anaerobic: DO should be non-detectable. and no nitrate or nitrite source exists.

Biological treatment processes can also be classified according to their means of providing medium for the biological organisms' growth:

• Attached growth: A mass of individual microorganisms attaching themselves to a fixed media (e.g., plastic rings) in a slime film (sometimes called fixed film)

• Suspended or slurry growth: A process where microorganisms are kept in a suspended state, as individual organisms, or as a mass of organisms (sludge flocs), that are mixed with the wastewater being treated in a solids suspension termed mixed liquor In modern waste treatment facility designs, there are usually two common components of a typical waste treatment system:

1. A biological reactor or vessel, in which wastewater comes in contact with the microbial population

2. A clarifier, in which biological solids are settled and collected in the sludge blanket and thereby separated from clarified effluent, which passes to the receiving waters Tbl. 1 shows the major biological treatment processes used for wastewater treatment.

Basic Methods of Secondary Treatment

Secondary wastewater treatment methods include activated sludge, aerated lagoons, trickling filters, and rotating biological contactors.

Activated Sludge

The activated sludge process is a suspended growth biological waste water treatment technique, in which a mixture of wastewater and biological mass (microorganisms) is agitated and aerated. The bio logical mass is subsequently separated from treated wastewater in a clarifier, and returned to the aeration process to maintain a balance of biological solids and wastewater being treated. The activated sludge process derives its name from the biological mass formed when air is continuously injected into nutrient-rich wastewater. Microorganisms are mixed thoroughly with organics under conditions that stimulate use of organics as a food source. As microorganisms grow, multiply, and mix by agitation with air, individual organisms clump together (flocculate), to form an active mass of microbes called activated sludge. Variations of the activated sludge process are discussed in a later section.

Aerated Lagoon

An aerated lagoon is a basin in which wastewater is treated on a flow-through basis. Oxygen is usually supplied by surface aerators or submerged aeration devices.

Trickling Filter

The trickling filter process is a fixed-film biological process that uses slag, rock, stones, plastic, or wood as medium on which microorganisms grow. Wastewater is typically applied as a spray from moving distributors. As wastewater trickles through the bed, microorganisms grow on the surface of the packing in a fixed film. Wastewater passes over and through the medium to provide needed contact between microorganisms and organics. Biological growth sloughs off the medium when the inner biological area can no longer receive oxygen. A clarifier is used after the trickling filter to remove this bio logical material before discharging treated water. Sometimes, effluent from the clarifier is recycled back to the trickling filter to increase BOD removal efficiency, and maintain optimum hydraulic loading on the filter.

Rotating Biological Contactor

This process is a fixed-film biological process that uses rotating discs mounted on shafts and placed in a tank, with about 40% of the disc area immersed in the tank (wastewater) and the remainder exposed to the atmosphere. A biological film or biomass grows on the surface of the discs. Rotation brings biomass in contact with wastewater for removal of organics and with the atmosphere for absorption of oxy gen. A secondary clarifier collects excess solids that are produced on the biodiscs.

Principles of Biological Waste Treatment

In the biological treatment of wastewaters, a mixed population of microorganisms uses colloidal and dissolved organics found in effluent from primary treatment as its main food supply. In consuming these organics, microorganisms use part of the organic substances to obtain energy needed for their life activities.

Biological respiration, in the presence of dissolved oxygen, produces products such as carbon dioxide, water, sulfates, nitrates, and phosphates. The remaining part of consumed organics is used as building blocks in a series of synthesis (reproduction) reactions, which result in an increased microorganism population (cell growth). Therefore, colloidal and dissolved organics originally present in wastewater, are transformed partly into a stable. form, such as carbon dioxide, and partly into a viable biological mass. This biochemical reaction is active in all biological treatment processes.

The biological mass is subsequently separated from the wastewater in secondary clarifiers, to ensure a proper degree of treatment within effluent and water quality standards.

Important Microorganisms

In the activated sludge process, microorganisms are dispersed through out the water phase; in trickling filters, rotating biological contactors, and other fixed-film processes, microorganisms are attached to a fixed surface, forming a biological film. In either process, microorganisms are doing the work, and therefore, all precautions must be taken to ensure a favorable environment for them.

Microorganisms considered important in biological treatment are: bacteria, fungi, algae, protozoa, rotifers, and worms.


Bacteria are single-cell microorganisms. They use soluble food and, in general, are found wherever moisture and a food source are avail able. Their usual mode of reproduction is by binary fission (i.e., by dividing, the original cell becomes two new organisms), although some species reproduce sexually or by budding. Even though there are thousands of different species of bacteria, their general form falls into one of three categories: spherical, cylindrical, and helical. Bacteria vary widely in size. Representative sizes are 0.5 to 1.0 µm in diameter for the spherical, 0.5 to 1.0 µm in width by 1.5 to 3.0 µm in length for the cylindrical (rods), and 0.5 to 5 µm in width by 6 to 15 µm in length for the helical (spiral).

In general, two types of bacteria can be distinguished: floc forming and filament forming. Floc-forming bacteria have the capability, under the right conditions, to clump together using excreted exo-cellular polymer to form a floc that is large and heavy enough to settle.

Filament-forming bacteria also remove organics from wastewater but are characterized by stringy or threadlike forms that are extremely light and easily washed out from the clarifier. It is clear that floc formers are preferred in a biological treatment plant. The character and type of wastewater as well as the environment (regime in which they live), dictate which forms will be the majority.

Temperature and pH play a vital role in the life and death of bacteria. The rate of reaction for microorganisms increases with increasing temperature, doubling with about every 18°F (10°C) of temperature rise, until some limiting temperature is reached. According to the temperature range in which they function best, bacteria may be classified as cryophilic (psychrophilic), mesophilic, or thermophilic. Typical temperature ranges for bacteria in each of these categories are presented in Tbl. 2.

Type | Temperature, deg. C Range Optimum Cryophilic* 10-30 12-18 Mesophilic 20-45 25-40; Thermophilic 45-75 55-65

The vast majority of secondary treatment plants is designed for mesophilic organisms and needs to be operated in the 25 to 40°C range for best treatment.

The pH of a solution is also a key factor in the growth of organisms.

Most organisms cannot tolerate pH levels above 9.5 or below 4.0. Generally, optimum pH for bacteria growth lies between 6.5 and 7.5.

Fungi In biological treatment systems, fungi are considered multicellular, non-photosynthetic, heterotrophic organisms.

Most fungi are strict aerobes. They have the ability to grow under low moisture conditions and can tolerate an environment with relatively low pH. Optimum pH for most species is 5.6; the range is 2 to 9.

Fungi have a low nitrogen requirement, needing only approximately one-half as much as bacteria.

The ability of fungi to survive under low pH and nitrogen conditions makes them very important in the biological treatment of some industrial wastes.


Algae are unicellular or multicellular, autotrophic, photosynthetic organisms. In oxidation ponds, algae are valuable in that they have the ability to produce oxygen through photosynthesis. At night, when light is no longer available for photosynthesis, algae consume oxygen in respiration, producing carbon dioxide (CO2 ). Oxygen is also consumed by biological decomposition of dead algae. (High die off of algae results in stagnant ponds with their associated odors.) Respiration also occurs in the presence of sunlight; however, the net reaction is production of oxygen. Equations (1) and (2) represent simplified biochemical reactions for photosynthesis and respiration:


CO H O light CH O O H O 22 2 22 2 ++?++

CH2 O represents new algae cells.


CH O O CO H O 22 22 +? +

In an aquatic environment, it can be seen that this type of metabolic system produces a diurnal variation in dissolved oxygen. The ability of algae to produce oxygen is vital to the ecology of the water environment. In some types of biological treatment (oxidation pond), algae are needed to supply oxygen to aerobic, heterotrophic bacteria.

Algae are also considered a nuisance in waste treatment. Algae cells may form large floating mats that reduce the oxygen transfer ability of a treatment tank or clog filters. The die off of large quantities of algae at night depletes dissolved oxygen, and the pond becomes odorous due to anaerobic decomposition of dead algae. They also cause an increase in effluent total suspended solids (TSS) levels from lagoon systems or activated sludge clarifiers, particularly during summer months.


Protozoa are motile, microscopic organisms that are usually single cells. The majority of protozoa are aerobic heterotrophs, although a few are anaerobic. Protozoa are generally an order of magnitude larger than bacteria and often consume bacteria as an energy source.

In effect, protozoa act as polishers of effluents from biological waste treatment processes by consuming free swimming bacteria and particulate organic matter.


Movement by means of cilia is characteristic of these protozoa. Cilia are hair-like extensions from the cell membrane. Besides being responsible for movement, they are also important in assisting protozoa to capture solid food. Sanitary engineers usually consider Ciliata to be divided into two types: free-swimming and stalked. The free-swimming type must swim after bacteria. They require a great deal of food, because they expend so much energy in swimming.

Paramecium is a free-swimming ciliate that is important in waste water treatment. Stalked ciliates may be attached to something solid and must catch food as it passes. Because their movement is limited or free-floating, they require less food for energy. Vorticella is a stalked ciliate that is important in biological treatment processes, especially in the activated sludge process.


The rotifer is an aerobic, heterotrophic, multicellular organism. Its name is derived from the fact that it has two sets of rotating cilia on its head, which are used for motility and capturing food. Rotifers are very effective in consuming dispersed and flocculated bacteria and small particles of organic matter. Their presence in mixed liquor indicates a highly efficient aerobic biological purification process.


Worms are characteristic higher life forms that appear in activated sludge systems with very high sludge age.

ILL. 1 Typical bacteria growth curve.

Bacterial Growth

Bacteria generally reproduce by binary fission. The time required for each fission, which is termed the generation time, can vary from days to less than 20 min. The general growth pattern of bacteria in a batch culture is shown in Fig. 1. Initially, a small number of organisms are inoculated into a culture medium, and the number of viable organisms is recorded as a function of time. The growth pattern based on the number of cells has four more or less distinct phases:

1. Lag Phase: Upon addition of an inoculum to a culture medium, the lag phase represents the time required for organisms to acclimate to their new environment.

2. Log Growth Phase: During this period, cells divide at a rate determined by their generation time and their ability to process food (constant percentage growth rate).

3. Stationary Phase: Here, the population remains stationary. Stationary growth occurs because cells have exhausted substrate or nutrients necessary for growth, and growth of new cells is offset by death of old cells.

4. Log Death Phase: During this phase, the bacterial death rate exceeds production of new cells. The death rate is usually a function of the viable population and environmental characteristics. In some cases, the log death phase is the inverse of the log growth phase.

The growth pattern is described in terms of the variation of the mass of microorganisms with time. This growth pattern consists of the following four phases:

1. Log Growth Phase: There is always an excess amount of food surrounding the microorganisms, and the rate of metabolism and growth is related to the ability of the microorganism to consume food.

2. Stationary Phase: The availability of food and the microorganisms are in balance.

3. Declining Growth Phase: The rate of growth and hence the mass of bacteria decrease because of limitations in the food supply.

4. Endogenous Phase: Microorganisms are forced to metabolize their own protoplasm without replacement because the concentration of available food is at a minimum. During this phase, a phenomenon known as lysis can occur, in which nutrients remaining in dead cells diffuse out to furnish remaining live cells with food.

It is important to note that the preceding discussions concerned a single population of microorganisms. Biological treatment units are composed of complex, interrelated, mixed biological populations, with each particular microorganism in the system having its own growth curve. The position and shape of a particular growth curve in the system, on a time scale, depends on food and nutrients available and environmental factors such as temperature, pH, and whether the system is aerobic or anaerobic.

Bacterial Oxidation

As previously discussed, secondary treatment or the conversion of organic matter to stable. end products is accomplished aerobically, anaerobically, or facultatively (alternating presence and absence of oxygen), using suspended growth or attached growth systems. A portion of organic material is oxidized to obtain energy necessary for synthesis of new cell mass. As organics are removed, microorganism cell matter is metabolized to stable. end products. In most biological treatment systems, these processes occur simultaneously.

The three processes may be represented as follows for an aerobic process:

1. Oxidation:

Organic matter COHNS O bacteria CO H O () ++ ? ++ 2 22 o other energy end products (3) 11

2. Synthesis:

Organic matter COHNS O bacteria energy C () ++ + ? 2 5 57 2 H NO new bacterial cells ()

3. Endogenous respiration:

CHNO O CO NH HO energy 57 2 2 2 3 2 55 2 +? + + + In Eqs., COHNS represents organic matter in wastewater. The formula C5 H7 NO2 , which represents bacteria cells, is a generalized value obtained from experimental studies.

Although the endogenous respiration reaction is shown as resulting in relatively simple end products and energy, stable. organic end products are also formed.

Biological Treatment Control Parameters

To ensure a favorable environment to promote the reactions involved in the biological treatment process, the following parameters must be controlled:

• pH and alkalinity

• Temperature

• Oxygen requirements

• Nutrient requirements

• Solids separation

• Biological solids recirculation

• Aeration capacity

• Mixing energy

• Hydraulic retention time

• Solids retention time

Biological waste treatment removes organic matter in wastewater in much the same manner as naturally occurring stream biota would in surface receiving waters. However, there is usually not as much time to break down solid organic matter in biological treatment. Generally, microorganisms in biological waste treatment work most efficiently on dissolved organic matter. These active microorganisms are a relatively small fraction of the total biological process biomass. Certain constituents adversely affect the biological treatment process.

Major constituents and their effects on the biological treatment process are shown in Tbl. 3.

TBL. 3 Biological Treatment System's Critical Constituents


Ammonia nitrogen Phosphate Calcium and magnesium Chloride Mercury Other heavy metals Sulfate Sulfide Petrochemicals Phenol compounds Surfactants

- -


Needed for growth; too low a level can inhibit growth; high levels of unionized ammonia can inhibit nitrification Needed for growth; too low a level can inhibit growth Very small amounts needed for growth Corrosive; toxic to microorganisms at very high levels Toxic to microorganisms at designated levels Toxic to microorganisms at designated levels Needed in small amounts Corrosive; depletes oxygen; toxic for nitrification bacteria Toxic to microorganisms at high levels Toxic to microorganisms at high levels until organisms acclimate; Cause foaming and can decrease oxygen transfer efficiency

TBL. 4 Relative Biodegradability of Organic Compounds

Easily biodegradable; Slower or moderately biodegradable; Less easily biodegradable

Sugars Alcohols Ketones Phenol compounds Organic acids Esters Ethers Cellulose Fats Lignins Polymeric compounds Hydrocarbons:



Alkyl, aryl Chlorinated aromatics

Certain substances present in municipal and industrial waste waters are more biodegradable than others. A relative comparison of biodegradability of various constituents commonly found in waste water is shown in Tbl. 4.

pH and Alkalinity

pH of wastewater is not always a problem. However, in biological treatment, pH can drop mainly because of nitrate and carbon dioxide generation from BOD and nitrogen. Operation of most biological processes is limited to a pH range of 5 to 9 (optimum is 6.5 to 7.5). If wastewater does not contain enough alkalinity (bicarbonate), biological production of carbon dioxide and nitrate can drop pH out of the optimum range. If pH drops below the optimum range, caustics, limes, or other alkalis can be added as needed.

Alkalinity is the measured capacity of a solution's ability to react with acid [usually sulfuric acid (H2 SO4 )] to a predetermined pH such as 4.2. The higher the alkalinity, the higher the demand is for the neutralizing agent (or acid) for a pH drop. Therefore, pH does not drop much even with carbon dioxide and nitrate formation when waste water alkalinity is high.

Systems designed for nitrification and denitrification require pH between 7.2 and 8.0. At pH above or below this level, nitrification may slow down. At pH < 6, nitrification completely stops.


Temperature affects all biological processes. Biological oxidation rates increase to a maximum at about 95°F (35°C) for most treatment systems. At temperatures greater than 95°F (35°C), treatment efficiency decreases by reducing bacterial floc formation. Temperatures in excess of 99°F (37°C) show a definite effect on biological systems. It is possible, however, in certain wastes, to operate efficiently at somewhat higher temperatures. Lower temperatures than 50°F (10°C) also affect performance of biological processes, especially nitrification efficiency.

The rate of biological activity is influenced by temperature because of the depth of penetration of oxygen into the floc or film. Oxygen penetration increases as temperature decreases, since oxygen is not used as quickly at floc surfaces and greater numbers of organisms per unit surface can react. Oxygen solubility also increases as temperature decreases.

ILL. 2 Carbon pathways in wastewater. Organic matter is removed by two pathways; Oxidation and Synthesis; Organic Matter consumed by microorganisms; Oxidation: serve as source of energy to drive metabolic processes Synthesis: essential for building and maintaining cells + nitrogen + phosphorus + trace elements new cells energy carbon waste 60%, 40% carbon waste

Sludge Production

Sludge production in a biological treatment system is expressed as the net effect of the following two processes:

1. Synthesis of new organisms resulting from assimilation of organic matter removed

2. Reduction of the mass of organisms under aeration by the process of die off and oxidation over an extended period (known as endogenous respiration) Ill. 2 shows the pathway of carbon contained in wastewater.

As a result, net sludge production is mainly functions of total BOD treated in the process and sludge retention time (SRT), which is also known as mean cell residence time (MCRT). For the activated sludge process where SRT is 5 to 10 days, sludge yield is typically 0.5 to 0.6 lb as dry mass when 1 lb of domestic BOD (0.5 to 0.6 kg/kg) is treated, and lower with longer SRT. Sludge yield is different for different types of industrial wastewaters.

Oxygen Requirements Theoretical oxygen requirements can be determined from the five day BOD (BOD5 ) of wastewater, less the amount of organisms wasted from the system per day. If all of the BOD5 were converted to end products (CO2 and H2 O), total oxygen demand would be computed by converting the BOD5 to BODL (all carbonaceous BOD converted to end products), using an appropriate conversion factor. A portion of the waste is converted to cell structure, and is removed from the system, however. Therefore, if BODL of the wasted cells is subtracted from the total, the remaining amount represents the amount of oxy gen that must be supplied to the system. It is known that one mole of cells is equal to 1.42 times the concentration of cells [see Eq. (6)]. The theoretical oxygen requirement for removal of carbonaceous organic matter in wastewater by an activated sludge system is OD BOD C =- - [( ) / ] . QS S f f P Cx ' 1 42 (6) where ODC = carbonaceous oxygen demand, lb O2 /d (kg O2 /d)

Q = influent wastewater flow rate, mgd (m3 /d)

S' = influent BOD, mg/L

S = effluent BOD, mg/L

f C = unit conversion factor, 8.34 lb/gal for U.S. units [(1000 L/m3 ) × (1 kg/10 6 mg) = 1/1000 for metric units]

f BOD = conversion factor for converting BOD5 to BODL ,dimensionless (0.68 for municipal wastewater)

Px = net sludge production in terms of volatile solids, lb/d (kg/d)

When nitrification occurs, the total oxygen requirement is that required for removal of carbonaceous organic matter [Eq. (6)], plus oxygen required for conversion of ammonia to nitrate as follows:

ODN =- 457 0 .( ) QN N f C (7) where ODN = nitrification oxygen demand, lb O2 /d (kg O2 /d)

Q = influent wastewater flow rate, mgd (m3 /d)

N0 = influent total Kjeldahl nitrogen (TKN), mg/L

N = effluent total Kjeldahl nitrogen (TKN), mg/L

4.57 = conversion factor for amount of oxygen needed for

complete oxidation of TKN f C = unit conversion factor, 8.34 lb/gal for U.S. units [(1000 L/m3 )(1 kg/10^6 mg) = 1/1000 for metric units] Based on the oxygen requirement, the aeration rate is calculated considering oxygen transfer efficiency, and the oxygen content in air using Eq. (8). Oxygen transfer efficiency depends on mixed liquor suspended solids (MLSS) in the aeration basin, diffuser type, temperature, basin depth, residual dissolved oxygen, and the like. In an aeration basin having a depth of 15 ft (4.6 m), overall oxygen transfer efficiency is typically 8 to 12% with coarse bubble air diffusers.

Fine bubble diffuser efficiency is in the range of 20 to 30%. Many plants have converted from coarse to fine bubbles to save energy.

AR =?/[ ( )( )] 4 e d (8) where AR = aeration rate, m3 air/min (1 m3 = 35.3 ft3) O = oxygen requirement, kg O2 /d

e = specific oxygen transfer efficiency, /m (commonly 0.015 to 0.03/m)

d = aeration basin depth, m A very rough rule of thumb for the total amount of oxygen that should be supplied to a biological wastewater treatment plant is approximately 2 to 4 lb O2 /lb BOD removed (2 to 4 kg/kg) for low loaded systems [0.05 lb BOD/lb MLSS (0.05 kg/kg)], and 1.5 to 2.5 lb O2 /lb BOD removed (1.5 to 2.5 kg/kg) for medium loaded systems [0.1 to 0.2 lb BOD/lb MLSS (0.1 to 0.2 kg/kg)].

TBL. 5 Typical Substrate and Nutrient Requirements for Wastewater with 200 mg/L BOD (300 mg/L COD) Element; Bacterial composition, mg/g biomass COD*; Minimum influent concentration, mg/L N 87 10 P 17 2 K 10 1.2 Ca 10 1.2 Mg 7 0.8 S 6 0.7 Na 3 0.4 Cl 3 0.4 Fe 2 0.2 Zn 0.2 0.02 Mn 0.1 0.01

Nutrient Requirements

In general, a ratio of BOD/nitrogen/phosphorus of 100/5/1 is recommended for maintaining the best biological conditions. For healthy growth, it is important to have sufficient nutrients by maintaining a small excess in the final effluent. Approximately 1or 2 mg/L nitrogen [as ammonia (NH3 )] and soluble orthophosphate (filtered sample) in the effluent is sufficient. Phosphoric acid and mono, di, and trisodium phosphate are used as sources of phosphorous. Poly phosphates and hexametaphosphate are not a readily available source of phosphate for microorganisms, and should not be used. Anhydrous or aqueous ammonia and urea are used as sources of nitrogen.

Ammonium phosphate can also be used, but it is difficult to meet the different demands for nitrogen and phosphorus additions.

In addition to nitrogen and phosphorous, many other mineral elements are essential for proper metabolic activity of microorganisms involved in waste treatment. Tbl. 5 summarizes minimum nutrient requirements for wastewater having a BOD of 200 mg/L. At higher BOD, minimum requirements of the nutrients increase proportionally.

Municipal wastewaters usually contain enough of all micro nutrients. However, many industrial wastewaters suffer from nutrient deficiency due to a narrow range of raw materials. For example, wastewaters in the chemical and pharmaceutical industries often contain low levels of mineral elements, while organic contents represented by BOD and chemical oxygen demand (COD) are very high. For proper biological treatment, micronutrients such as K, Ca, Mg, S, Fe, and the like, should be added as needed along with N and P.

Solids Separation

One of the most important aspects of biological waste treatment is in the design of facilities used to separate biological solids from treated wastewater. Secondary sedimentation must perform two functions:


1. Separate mixed liquor suspended solids from treated waste water

2. Thicken return sludge and waste activated sludge

Both functions are affected by surface area and depth of the sedimentation basin. Ample volume must be provided for storage of solids during periods in which sustained peak plant loadings are experienced. In addition, peak daily flow rate variations must be considered because they affect sludge removal requirements. The secondary clarifier/ thickener must be sized for either the hydraulic loading [gpd/ft2 or m3 /(h · m2 )], or solids flux [lb solids/(d · ft2 ) or kg/(h · m2 )]. In most cases, solids flux or loading is the critical parameter.

Return Sludge Requirements

The purpose of the return of settled solids is to maintain a sufficient concentration of activated sludge in the aeration tank, so that the required degree of treatment can be obtained in the time interval desired. Return of activated sludge from the clarifier to the inlet of the aeration tank is essential to the process.

Solids tend to form a sludge blanket in the bottom of the clarifier/ thicker, which varies in thickness from time to time. If solids thickening capacity of the secondary clarifier is inadequate, solids may fill the entire depth of the tank at peak flows. Return sludge pump capacities vary between 25 to over 200% of influent flow, depending upon operational strategy and condition.

ILL. 3 Conventional plugflow activated sludge process. Untreated Wastewater; Primary Clarifier, Sludge; Aeration Tank; Secondary Clarifier; Excess Sludge; Return Sludge; Effluent

Activated Sludge Process

The activated sludge process was developed in England in 1914, and was so named because it involves production of an active mass of microorganisms capable of stabilizing organic content of waste in the presence of dissolved oxygen. Activated sludge is probably the most versatile of biological treatment processes. The process has found wide application in both domestic and industrial wastewater treatment.

Activated sludge is a biological contact process where bacteria, protozoa, and small organisms such as rotifers are commonly found.

Bacteria are the most important group of microorganisms, because they are responsible for the structural and functional activity of the activated sludge floc. All types of bacteria (except pathogens) make up activated sludge. The predominant type is determined by the nature of organic substances in wastewater, mode of operation of the plant, and environmental conditions present for organisms in the process.

The conventional activated sludge process consists of an aeration tank, secondary clarifier, method of returning sludge to the aeration tank, and means of wasting excess sludge from the system (see Fig. 3). Sludge wasting is accomplished from the recycle or mixed liquor line. The flow model is plugflow with clarifier under flow recycle. Both influent settled wastewater, and recycled sludge enter the tank at the head end, and are aerated for a period of 4 to 12 h. Influent wastewater and recycled sludge are mixed by the action of diffused or mechanical aeration, which is constant in a well-operated system as mixed liquor moves down the tank.

During this period, absorption, flocculation, and oxidation of organic matter occur. Mixed liquor is settled in the activated sludge clarifier/thickener, and sludge is returned at a rate of approximately 25 to 100% of the influent flow rate.

Activated Sludge Process Equipment All activated sludge processes have certain process equipment similarities. These are discussed in the following paragraphs.

Aeration Tanks

Aeration tanks used in activated sludge processes are usually constructed of reinforced concrete and left open to the atmosphere. A cross section of a typical aeration tank is shown in Fig. 4.

The rectangular shape permits common-wall construction for multiple tanks. If total capacity exceeds 5000 ft3 (142 m3 ), the total aeration tank volume required should be divided among two or more units capable of independent operation. Total capacity required is determined from the biological process design.

If wastewater is to be aerated with diffused air, geometry of the tank may significantly affect aeration efficiency and amount of mixing obtained. Depth of wastewater in the tank should be between 10 and 16 ft (3 and 4.9 m), so that diffusers can work efficiently.

Freeboard should be 1 to 2 ft (0.3 to 0.61 m) above the waterline.

Width of the tank in relation to its depth is important, if spiral-flow mixing is used. The width-to-depth ratio for such tanks may vary from 1.0:1 to 2.2:1. This limits the width of a tank channel to 20 to 36 ft (6.1 to 11 m).

ILL. 4 Cross section of typical activated sludge aeration tank.

In large plants, channels become quite long, sometimes longer than 500 ft (152 m) per tank, and tanks may consist of one to four channels with round-the-end flow in multiple-channel tanks.

Large plants should contain not less than four tanks, and prefer ably six to eight or more. Some of the largest plants contain 30 to 40 tanks arranged in several groups or batteries.

Ill. 4 is based on the use of coarse bubble air diffusers for transfer of oxygen to the liquid. Today, the use of fine bubble diffusers installed across the width and length of the aeration basin are preferred, to reduce energy use. In fact, many plants originally built with coarse bubble diffusers have been modified to fine bubble dif fusers to reduce energy costs.

Secondary Clarifier

A secondary clarifier is constructed and operated very much like a primary clarifier, except that the secondary tank follows the bio logical treatment process (i.e., trickling filter or activated sludge). The function of secondary clarifiers varies with the method of bio logical treatment used. Clarifiers following trickling filters are used to separate biological solids that have broken away from the filter media. Clarifiers in an activated sludge system, however, serve two purposes. Besides providing a clarified effluent, they provide a concentrated source of return sludge to the aeration basin for process control.

Like primary clarifiers, secondary tanks may be round or rectangular. These tanks may be designed for natural settling or chemically aided settling, with tank size being related to one of the following:

• Surface loading rates in gallons per square foot (m3/m2 ) of floor area per day

• Solids loading rate in pounds (kilograms) of solids per square foot (m2 ) of floor area per day

• Flow-through velocity in feet per minute (m/min) (rectangular tanks)

• Weir placement and loading rates in gallons (m3 ) per day per linear foot (meter) weir length

• Retention time of settled sludge in hours Clarifiers in activated sludge systems must be designed not only for hydraulic overflow rates, but also for solids loading rates (solids flux). This is because both clarification and thickening are needed in activated sludge clarifiers. At higher MLSS values (i.e., more than about 1500 mg/L), ability of the clarifier to thicken solids becomes the controlling factor. Solids loading rate becomes critical in determining tank size and operation limitations. As a result, design of clarifiers following the activated sludge process is usually based on average and peak overflow rates and solids loadings.

Performance of secondary wastewater treatment systems is determined by comparing the quality of overflow from secondary clarifiers to that of incoming wastewater. The biological treatment unit converts some of the soluble and insoluble organics to suspended organic solids. However, the treatment process is successful only if these organic solids are removed in the secondary clarifiers/thickeners. Secondary clarifier operational variables have the most critical effect on overall plant performance. Tbl. 6 provides typical operational loading parameters of secondary clarifiers for treatment of various industrial wastewaters by bio logical treatment.

TBL. 6 Typical Biological Secondary Settling Tank Operational Loading Parameters

Solids flux Overflow rate Industry lb/(d · ft2) kg/(d · m2) gpd/ft2 m3/(d · m2) Pulp and paper 18-25 88-122 400-800 16-33 Petrochemical 10-15 49-73 300-600 12-24 Refinery biological contactor 8-12 39-59 300-600 12-24 Secondary 20-29 98-142 400-800 16-33 Nitrified effluent 17-24 83-117 400-600 16-24

TBL. 7 Description of Diffuse Air Aeration Devices

Bubble size:

  • Fine Medium Coarse

Transfer efficiency:

  • High
  • Medium
  • Low


Ceramic-bonded grains of fused crystalline aluminum oxide Vitreous silicate bonded grains of pure silica Resin bonded grains of pure silica

Plastic wrapped diffuser tubes Woven fabric sock or sleeve diffusers

Various orifice devices Sparger air escapes from periphery of flexible or rigid disc that is displaced when manifold pressure exceeds the head on the disc Slot orifice injectors

Aeration Equipment

The three basic methods of aerating wastewater are:

1. Introduce air or oxygen into wastewater with submerged porous diffusers.

2. Agitate wastewater mechanically to promote solution of oxygen from the atmosphere above the liquid being aerated (mechanical aerator).

3. Use aspirators (eductors) with or without blowers (jet aeration).

The amount of air used per pound (kilogram) of BOD removed varies greatly from one plant to another and for different industries.

Comparing air use at different plants is risky because of different loading rates, control criteria, biological treatment rates, and operating procedures.

Diffused Aeration

A diffused air system consists of diffusers that are submerged in wastewater, header pipes, air mains, and blowers and appurtenances through which air is supplied.

The diffusers most commonly used in aeration systems are designed to produce fine, medium, or coarse (relatively large) bubbles. Fine bubble diffusers constructed of neoprene and other materials are most commonly used today due to their high oxygen transfer efficiency (lower power consumption).

Mechanical Aerators

The most commonly used types of mechanical aerators are surface and submerged turbine aerators. With surface aerators, oxygen is entrained both from the atmosphere and from air or pure oxygen introduced in the tank bottom. In either case, the pumping action of the aerator, and that of the turbine, help to keep the contents of the aeration tank mixed. Both types are described here, along with aerator performance and energy required for mixing.

ILL. 5 Surface mechanical aerator.

ILL. 6 Simplex cone mechanical aerator. Turnbuckle Stay Rods Support Rods Shaft Housing Drive Unit Base Drive Unit Drive Ring Support Legs Water Level Uptake Tube

ILL. 7 Turbine mechanical aerator. Air Mixer Blades Water Level Drive Air Sparger Ring

Surface Aerators Mechanical surface aerators are the simplest type of aerators (). They are available in sizes from 1 to 100 hp (0.75 to 75 kW). They consist of submerged or partially submerged impellers that are attached to motors, which are mounted on floats or fixed structures. Impellers are fabricated from steel, cast iron, noncorrosive alloys, and fiberglass-reinforced plastic, which are used to agitate wastewater vigorously, entraining air in the wastewater and causing a rapid change in the air-water interface to facilitate solution of oxygen.

Surface aerators may be classified as low or high spec, according to the speed of rotation of the impeller. In low-speed aerators, the impeller is driven through a reduction gear by an electric motor. Motor and gear box are usually mounted on a platform that is supported either by piers extending to the bottom of the tank, or by beams that span the tank. They have also been mounted on floats. The propeller for high speed aerators is driven at the electrical motor speed. The high speed of the propeller and the smaller diameter (as compared to low-speed units) results in lower oxygen transfer efficiency.

Submerged Turbine Aerators

Most mechanical surface aerators are upflow types that rely on violent agitation of the surface, and air entrainment for oxygen transfer efficiency. However, with turbine may be used with either upflow or low-speed models in deep aeration tanks. The draft tube is a cylinder with flared ends mounted con centrically, with the impeller extending from just above the floor of the aeration tank to just beneath the impeller.

Mechanical Aerator Performance

Surface aerators are rated in terms of their oxygen transfer rate expressed as pounds (kilograms) of oxygen per horsepower-hour (kilowatt-hour) at standard conditions, which exist when the temperature is 68°F (20°C), initial dissolved oxygen is 0.0 mg/L, and test liquid is tap water. Commercial-size surface aerators now available range in efficiency from 2 to 3.5 lb O2/(hp · h) (1.2 to 2.1 kg/kWh) at standard conditions. The lower value is for motor speed aerators, while the higher is for low-speed units.

Energy Requirement for Mixing As with diffused air systems, size and shape of the aeration tank are very important, if good mixing is to be achieved. Aeration tanks may be square or rectangular and may contain one or more units. Water depth may vary from 4 to 12 ft (1.2 to 3.7 m) when using surface aerators. Depths up to 35 ft (10.7 m) have been used with draft tube mixers.

In diffused air systems, the air requirement to ensure good mixing varies from 20 to 30 ft3/1000 ft3 (20 to 30 m3/1000 m3) of tank volume.

Typical power requirements for maintaining a completely mixed flow regime with mechanical aerators vary from 0.6 to 1.15 hp/1000 ft3 (16 to 30 kW/1000 m3 ), depending on design of the aerator and geometry of the tank, lagoon, or basin. In the design of aerated lagoons for treatment of domestic wastes, it is extremely important that the mixing power requirement be checked, because in most instances it is the controlling factor. With both the diffused air systems and mechanical aerators, power required for oxygen transfer is usually less than that required for mixing to keep solids in suspension.

Jet Aeration Jet aeration consists of an eductor (aspirator) pump and blower. Pump suction is from the aeration basin and discharges to the inlet of the eductor. The blower supplies air to the venturi throat of the eductor. Water velocity and size of the venturi throat create small air bubbles for efficient oxygen transfer to liquid in the aeration basin.

It has been found in some cases that pumping biological flocs through the eductor breaks up the floc, resulting in solids carryover from secondary clarifiers.

Tbl. 8 shows typical performance characteristics for air dif fusers used in wastewater treatment.

Type of aerator | Water depth, ft (m) Oxygen transfer efficiency, % | Aerator rating: lb O2/(hp · h) kg O2/kWh Fine bubble Tubes-spiral roll Domes-full floor coverage Coarse bubble


Spargers Jet aerators Static aerators Sur face aerators

Low speed 1

High speed 8

[15 (4.6) 15-20 6-8 3.6-4.9 15 (4.6) 25-35 11-12.5 6.7-7.6 15 (4.6) 10-13 4-5 2.4-3 14.5 (4.4) 6-8 2-3.5 1.2-2.1 15 (4.6) 15-24 4-5 2.4-3 15 (4.6) 10-11 4-4.5 2.4-2.7 12 (3.7) - 3-3.5 1.8-2.1 8 (2.4) - 2-2.5 1.2-1.5]

Oxygen Equipment

There are two pure oxygen generator designs: the traditional cryogenic air separation process for large applications and a pressure swing adsorption (PSA) system for the somewhat smaller and more common plant sizes.

The cryogenic air separation process involves liquefaction of air, followed by fractional distillation to separate it into its components (mainly nitrogen and oxygen).

The PSA system uses a multi-bed adsorption process to provide a continuous flow of oxygen gas. Feed air is compressed and passed through one of the adsorbers. The adsorbent removes carbon dioxide, water, and nitrogen gas, and produces relatively high-purity oxygen. While one bed is adsorbing, the others are in various stages of regeneration.

The concept of the PSA generator is that oxygen is separated from feed air by adsorption at high pressure, and the adsorbent is regenerated by "blowdown" to low pressure. The process operates on a repeated cycle having two basic steps: adsorption and regeneration. continue

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