Principles of Biological Nutrient Removal (1)

1/Biological Nitrogen removal

• Biological nitrogen removal requires a two-step process. In the first step ammonia is oxidized to nitrate (nitrification) and various process configurations are then employed to provide the nitrate as an electron acceptor for biological respiration so that it can be reduced to molecular nitrogen (denitrification). Fundamental considerations will be presented first for the nitrification step in nitrogen removal. Included in this review are the microbiology, basic biochemical reactions, biological kinetics, and the factors that affect the kinetics and performance of biological nitrification and denitrification reactions

2/BIOLOGICAL NITRIFICATION

2.1.Microbiology

• Early work by Schloesing and Muntz (1877) investigating the percolation of sewage through a sand column found that the conversion of ammonia to nitrate and nitrite was due to living organisms, since the reaction could be stopped by introducing chloroform vapor into the column. Classical experiments by Winogradsky (1890) led to the isolation of autotrophic nitrifying bacteria, Nitrosomonas and Nitrobacter, which oxidize ammonia sequentially to nitrite and then to nitrate, respectively. Painter (1970) has listed other autotrophic bacteria genera capable of obtaining energy from the oxidation of ammonia, Nitrosococcus, Nitrospira, Nitrosocyctis and Nitrosoglea, and nitrite, Nitrocystis, but the nitrification in soil or wastewater treatment processes is attributed primarily to Nitrosomonas and Nitrobacter.

• Painter (1970) summarizes other literature sources that show a large number or heterotrophic bacteria capable of forming nitrite or nitrate. In one study by Eylar and Schmidt (1959) of about 1000 heterotrophic organisms isolated from soil, only fifteen wrere found to be able to nitrify. A review by Focht and Chang (1975) indicated that heterotrophic nitrification is possible with diverse genera of bacteria, fungi, and actinomycetes. However, it is doubtful that significant quantities of nitrate are generated by heterotrophic organisms, since autorophic nitrification rates are about ten times greater. Heterotrophic nitrification may be more prominent in atypical environments with either very alkaline or acidic pH conditions.

• Bock et al. (1988) reported on the ability of Nitrobacter to grow and reduced nitrate while using acetate, formate, pyruvate, or glycerine as organic substrates. They further showed in an aerobic/anoxic conditions. This observation has not been studied in wastewater treatment.

2.2.Oxidation and Synthesis relationships

The energy-yielding two-step oxidation of ammonia to nitrate is generally accepted to be as follows:

• Nitrosomonas

2NH4+ + 3O2 → 2NO2- + 4H+ +2H2O

• Nitrobacter

2NO2- + O2 → 2NO3-

• Total reaction

NH4+ + 2O2 → NO3- + 2H+ + H2O

• The standard free energy release associated with ammonia oxidation has been estimated to be 66 to 84 kcal/mole of ammonia, and for nitrite oxiation it has been estimated to be 17.5 kcal/mole of nitrite (Painter, 1970). If the amount of cell prodcution is proportional to the energy released, a greater amount of biomass should be formed from the oxidation of ammonia to nitrite than from the oxidation of nitrite to nitrate.

• Based on the above, the oxygen required for complete oxidation of ammonia is 4.57 g/g N oxidized with 3.43 g/g used for nitrite production and 1.14 g/g used for nitrate production. The amount of oxygen required is less than 4.57 g/g N when synthesis is considered in addition to oxidation due to oxygen obtained from fixation of carbon dioxide and nitrogen into cell mass. Wezernak and Gannon (1967) found that the actual total oxygen consumption was 4.33 g/g N with 3.22 g/g N used for ammonia oxidation and 1.11 g/g N used for nitrite oxidation. The following equation, presented in the U.S.EPA Nitrogen Control Manual (1975), accounts for both synthesis and oxidation and shows an oxygen requirement of 4.2 g/g N. The equation was developed using cell yield coefficients of 0.15 g/g NH4-N oxidized and 0.02 g/g NO2-N oxidized

NH4+ + 1.83O2 + 1.98HCO3- → 0.21C5H7O2N + 0.98NO3-+1.041H2O + 1.88H2CO3

• For activated sludge designs employing nitrification, a single yield coefficient that includes the growth of both Nitrosomonas and Nitrobacter is more convenient and some of the more recently used values.

Testing Wastewater Treatment Efficiency

1/Biochemical Oxygen Demand (BOD)

The biochemical oxygen demand test has been used widely by regulatory agencies to gauge overall treatment plant efficiencies. The traditional BOD measurement of the plant influent, grit removal influent, and the final effluent gives the most common measure of treatment plant efficiency. The BOD of wastewater is a common indicator of the fraction of organic matter that may be degraded by microbial action at a given time period at a temperature of 20 degrees Centigrade. The test is related to the oxygen that would be required to stabilize the waste after discharging to a receiving body of water. The drop in BOD from grit removal effluent to final effluent is usually used in calculating the solids growth rate in the aeration tank. The BOD test is too slow to provide timely information to the operator for control purposes. It can, however, provide the operator with the historic results of previous operating conditions. Tests for BOD are to be made on composite samples daily. BOD tests run for at least 20 days are also to be made on the effluent periodically to determine the oxygen requirements of the nitrogen compounds present in the effluent.

2/Chemical Oxygen Demand (COD)

Chemical oxygen demand is another means of measuring the pollutional strength of wastewater. By using this method, most oxidizable organic compounds present in the wastewater sample may be measured. COD measurements are preferred when a mixed domestic-industrial waste is entering a plant or where a more rapid determination of the load is desired. The chemical oxygen demand test has a major advantage over the biochemical oxygen demand analysis because of the short time required for performance - a few hours as opposed to five days for the standard BOD test. Since this test can be run in several hours, it gives the operator a more timely idea of what is entering the plant and how the plant is performing. This permits closer operational control of the treatment process.Generally, COD values are higher than BOD values. The reason is that biochemical oxygen demand measures only the quantity of organic material capable of being oxidized, while the chemical oxygen demand represents a more complete oxidation. Typical COD values for domestic waste range from 200 - 500 mg/L.

3/Total Organic Carbon (TOC)

Total organic carbon measurements have been used as a method for determining pollutional levels of wastewater for many years. The organic carbon determination is free of many of the variables involved in the COD and BOD analyses, with somewhat more reliable and reproducible data being the result. The need for rapid determination of wastewater strength has led to the development of organic carbon analyzers and their introduction into some treatment plant laboratories. All of the available instruments measure the organic carbon content of aqueous samples, although there are several methods by which this is accomplished. The TOC values will generally be less than COD values, because a number of organic compounds may not be oxidized in the total organic carbon analysis. Typical values of TOC for domestic waste range from 100 - 300 mg/L.

4/Total Oxygen Demand (TOD)

Another method of measuring organic matter in wastewater involves the oxidation of the sample to stable end products in a platinum-catalyzed combustion chamber. Total oxygen demand is determined by measuring the oxygen content of the inert carrier gas, nitrogen. TOD measurements are becoming more popular because of their quickness in determining what is entering the plant and how the plant is responding. Analysis time is approximately 5 minutes. The results obtained generally will be equivalent to those obtained in the COD test.

5/Solids Determinations

Laboratory determinations of suspended solids (SS) in the influent, primary effluent, and final effluent are standard measurements used to indicate treatment plant efficiency. The SS measurements are used in calculating the sludge volume index (SVI) and sludge density index (SDI) - both important control tools. There is a distinction between total suspended solids (TSS) and total volatile suspended solids (TVSS). TSS measures both the active bacterial mass and the inert materials in the waste or mixed liquor. TVSS is a more accurate estimate of the mass of active microorganisms in the mixed liquor and is the parameter to be used in calculating the food-to-microorganism (F:M) ratio.

6/Sludge Density Index / Sludge Volume Index

To determine what the return sludge pumping rate should be and to get some idea of sludge settling characteristics, sludge indices have been proposed. One of the most common is the Donaldson Index, SDI:

SDI = (MLSS (%) x 100) / % volume MLSS after 30 min settling

The other common index is the Mohlman Index, SVI:

SVI = % MLSS volume after 30 min / % MLSS (mg/L MLSS) = ml settled sludge x 1000

These indices relate the weight of sludge to the volume the sludge occupies. They show how well the liquids-solids separation part of the activated sludge system is performing its function on the biological floc that has been produced and is to be settled out and returned to the aeration tanks or wasted. The better the liquid-solids separation is, the smaller will be the volume occupied by the settled sludge and the lower the pumping rate required to keep the solids in circulation.

7/Sixty-Minute Settling Test

The 60 minute settling test is a reasonable approximation of what is happening in the final settling tank. So that solids do not accumulate in the final settling tank, they must be removed at an average rate equal to that at which they are applied.

8/pH

pH is a method of expressing the acid condition of wastewater. The pH scale ranges from approximately 1 - 14, with a pH of 1 - 7 considered the acid range and 7 - 14 considered the base range. pH 7 is defined as neutral. pH is a vital tool of the wastewater treatment plant operator when determining unit operations.

9/Alkalinity

This is a measure of a wastewater's capacity to neutralize. The bicarbonate, carbonate, and hydroxide ions are the primary contributors to alkalinity. The determination of alkalinity levels at various points in a plant will be an aid to the proper understanding and interpretation of the treatment process. For example, if chemical addition is used to coagulate wastewater for solids removal, hydrogen ions may be released and cause the pH to decrease. Alkalinity will tend to neutralize the acids formed and permit coagulation to proceed in the proper pH range. Some other processes dependent on pH are disinfection, digestion, and sludge preparation and conditioning.

Controlling Wastewater Treatment

1/Wastewater Plant Management

Success or failure in operating an activated sludge plant depends on proper control of the mass of active organisms in the plant. For example, variation of the solids retention time (SRT) will modify the entire process performance, including denitrification rates, sludge production and stability, mixed liquor concentration, oxygen uptake rates, and possibly the extent of nitrification. Increasing the SRT generally will increase oxygen requirements, the stability of the process, the extent of sludge stabilization, the extent of nitrification, and mixed liquor in concentration. Longer SRTs will decrease sludge production and denitrification rates in the post-aeration anoxic tanks. For pre-aeration anoxic tanks, longer SRTs increase the mixed liquor suspended solids (MLSS) and result in a greater denitrification rate.

2/Solids Control

The determinations of various forms of residue are useful in the control of a wastewater treatment plant. Total solids (TS), suspended solids (SS), dissolved solids (DS), and their volatile and fixed fractions, may be used to assess wastewater strength, process efficiency, and unit loadings. Measurements of the various residue concentrations are necessary to establish and assure satisfactory operation control.Total solids (TS), is a term applied to the weight of material per unit volume of a sample remaining after evaporation at a temperature of 103 - 105 degrees Centigrade. TS is equivalent to the sum of filterable and non-filterable residue. In wastewater, the term suspended solids (SS) corresponds to filterable residue. The term dissolved solids (DS) corresponds to the non-filterable residue. TS, DS, and SS may be further categorized as volatile or fixed (nonvolatile). Volatile solids are more or less equivalent to the fraction of residue that is organic, while nonvolatile solids approximate the inorganic fraction. Because solids determinations are empirical in nature, a consistent procedure should always be used and the procedure stated in the presentation of results.It has been estimated that, under normal operating conditions, about 1/3 of the incoming useable substrate is used for oxidation, while the remaining 2/3 is used for synthesis. Large portions of the incoming wastes are inert and not easily utilized. The result is that much of the substrate removed by the activated sludge floc remains in the floc and accumulates as either inert or living solids. Because of this collection and production of solids, the final settling tanks would eventually fill with solids, then flow over the effluent weir. Increasing the return sludge pumping rate without wasting some sludge would not solve the problem, because the sludge is just being moved around in the system and is not being removed.Ultimate control of the system, no matter what intermediate operating decisions are made, will always be based on solids wasting. There are three commonly used methods to decide how much sludge to waste: mixed liquor volatile suspended solids (MLVSS), food-to-microorganism (F:M) ratio, and sludge age.

3/Constant MLVSS

With this method, the operator is maintaining a constant mass of organisms to use the incoming food supply. Simply, if the operator finds that a mixed liquor volatile suspended solids (MLVSS) concentration of 2000 mg/L works effectively at the plant, that level will be maintained. If the solids in the aeration tanks increase above 2000 mg/L, the operator will waste more until the MLVSS level is again 2000 mg/L. If the MLVSS drops below 2000 mg/L, the operator will waste less and allow the solids concentration to increase.This system of solids control is simple to understand and manage, involves a minimum amount of lab work, and can produce good results. This method, however, has a rather severe limitation in that the important F:M ratio is ignored. Suppose, for example, that the biochemical oxygen demand (BOD) of the incoming waste increased by 50% over a substantial period of time. The increased solids production from the higher BOD load would be wasted to maintain the MLVSS level. The result, however, would be that the F:M ratio is 50% higher than the previously maintained ratio. The resulting high F:M or organic overload could easily lead to plant inefficiency or failure.

4/Constant F:M Ratio

Another method for control is wasting to maintain a constant food-to-microorganism (F:M) ratio. With this method, the operator will try to increase or decrease the MLVSS to match an increase or decrease in the BOD entering the plant. Most plants will operate best at a specific F:M ratio between 0.05 - 0.1. If the optimum F:M has been determined from experience and can be maintained, a good effluent may be produced with consistent plant operation.The F:M ratio is to be calculated at least weekly and related to the efficiency of treatment plant operation. An F:M ratio between 0.05 - 0.15 BOD/lb MLSS is usually considered acceptable for an extended aeration process. The food is calculated as the BOD in the influent that enters the aeration tank. If 175 mg/L BOD is the influent concentration at a flow rate of 3.0 MGD, the food available to the organism would be:

F = 175 mg/L x 3.0 MGD x 8.34 lb (mil gal)(mg/L) = 4375 lb/day

The mass of microorganisms is the average MLVSS concentration in the aeration tanks. For example, if the total aeration tank capacity were 2.06 mil gal, and the average MLVSS concentration in the aeration tanks 3000 mg/L, the estimated mass of organisms in the aeration tank would be:

M = 2.06 mil gal x 3000 mg/L x 8.34 lb (mil gal)(mg/L) = 51540 lb

The F:M ratio would then be:

4375 (lb BOD/day) / 51540 (lb MLVSS) = 0.8 lb BOD/day/lb MLVSS

This method of control would be most successful if the plant received a waste with very predictable variations; it requires a large amount of lab work because it is necessary to know both the amount of food added to, and the mass of, organisms in the system. Also, if the influent does not exhibit very predictable variations, the BOD or COD tests are too slow to allow MLVSS adjustments according to incoming waste strength.

5/Constant Sludge Age

The term sludge age is also commonly termed the sludge residence time (SRT), or mean cell residence time of the activated sludge system. Average sludge age will be simply the total amount of solids in the system divided by the amount leaving the system each day. Historically, sludge age has been calculated as a ratio of total solids (TS) in aeration to the weight of total solids (TS) in the aeration tank influent.

Sludge Age or Solids Retention Time or Mean Cell Residence Time, days


TS in aeration tank/solids wasted + solids lost in effluent per day

Therefore, if it takes 5 days to get rid of an amount of sludge equal to that contained in the system, then the sludge will be there 5 days, or have a sludge age of 5 days.

SRT = VaX/Qw Xu + Qe Xe

in which:

SRT = sludge age, days
Va = volume of aeration tanks, gal
X = average activated sludge concentration in the aeration tank, mg/L
Qw = flow rate of sludge being wasted, gpd
Xu = average concentration of activated sludge in final settling tank underflow,mg/L
Qe = flow rate of wastewater leaving plant, gpd
Xe = average solids concentration in the effluent, mg/L

The operator can set the sludge age (SRT) that has been found to work well for the particular plant. Therefore, to calculate the wasting rate (Qw), the operator needs to know only the volume of his aeration basin, the suspended solids (SS) concentration of the mixed liquor (X), and the return sludge (Xu). The only lab work involves determination of the mixed liquor and return sludge concentrations. The X:Xu ratio can be approximated very quickly by centrifuging samples of both the aeration mixed liquor and the return sludge and assessing the relative solids concentrations. If normal settling tests are done, the X:Xu ratio can also be approximated by taking the solids heights in a one L graduated cylinder after 30 minutes settling and dividing that height by the initial solids height in the cylinder. The overriding advantage of this method of solids control is that it is inherently simple and requires a minimum of lab work.

6/SRT(total) vs. SRT(oxic)

One operational parameter which deserves special mention is the oxic Solids Retention Time. Keeping a proper SRT(oxic) is important in the BioDenipho process so that efficient and reliable nitrification can be maintained. The SRT(oxic) is calculated by finding the process volume which is undergoing aeration (nitrification).For example, assuming the phase lengths are:

Phases A & D: 30 minutes (each)

Phases B & E: 60 minutes (each)

Phases C & F: 30 minutes (each)

*The oxic volume is 63% of the total. The aerated volume for the Water Reclamation Facility would equal:

(volume of the oxidation ditches) x (oxic volume fraction)

or

(1.32 MGD) x (0.63) = 0.83 MG

Therefore, in order to find the SRT(oxic), 0.83 MG would be the process volume. The SRT(oxic) would then be calculated as SRT(total) with only the applied volume being different:

SRT(oxic) / QwXu = X x V(oxic)

Common Wastewater Terms

1/Activated Sludge
The term activated sludge refers to the brownish flocculent culture of organisms developed in an aeration tank under controlled conditions. Also, sludge floc produced in raw or settled wastewater by the growth of zoological bacteria and other organisms in the presence of dissolved oxygen. A good quality of activated sludge is shown by brown color, good settling characteristics, and DO present.

2/Alkalinity
The capacity of water to neutralize acids, a property imparted by the water's content of carbonates, bicarbonates, hydroxides, and occasionally borates, silicates, and phosphates.

3/Anaerobic
A biological environment that is deficient in all forms of oxygen, especially molecular oxygen, nitrates and nitrites. The decomposition by microorganisms of waste organic matter in wastewater in the absence of dissolved oxygen is classed as anaerobic.

4/Anoxic
A biological environment that is deficient in molecular oxygen, but may contain chemically bound oxygen, such as nitrates and nitrites.

5/Bacteria
Bacteria are living organisms (animals) that cannot be seen by the naked eye. They are a group of universally distributed, rigid, essentially unicellular, microscopic organisms lacking chlorophyll. They are characterized as spheroids, rod-like, or curved entities, but occasionally appearing as sheets, chains, or branched filaments.

6/Biochemical Oxygen Demand (BOD)
The BOD test is used to measure the strength of wastewater. The BOD of wastewater determines the milligrams per liter of oxygen required during stabilization of decomposable organic matter by aerobic bacteria action. Also, the total milligrams of oxygen required over a five-day test period to biologically assimilate the organic contaminants in one liter of wastewater maintained at 20 degrees Centigrade.

7/Bulking Sludge
A phenomenon that occurs in activated sludge plants whereby the sludge occupies excessive volumes and will not concentrate readily. This condition refers to a decrease in the ability of the sludge to settle and consequent loss over the settling tank weir. Bulking in activated sludge aeration tanks is caused mainly by excess suspended solids (SS) content. Sludge bulking in the final settling tank of an activated sludge plant may be caused by improper balance of the BOD load, SS concentration in the mixed liquor, or the amount of air used in aeration.

8/Chemical Oxygen Demand (COD)
The milligrams of oxygen required to chemically oxidize the organic contaminants in one liter of wastewater.

9/Composite Sample
To have significant meaning, samples for laboratory tests on wastewater should be representative of the wastewater. The best method of sampling is proportional composite sampling over several hours during the day. Composite samples are collected because the flow and characteristics of the wastewater are continually changing. A composite sample will give a representative analysis of the wastewater conditions.

10/Denitrification
A biological process by which nitrate is converted to nitrogen gas.

11/Digestion
The biological decomposition of organic matter in sludge resulting in partial gasification, liquefaction, and mineralization of putrescible and offensive solids.

12/Disenfection
The killing of pathogenic organisms is called disinfection.

13/Dissolved Oxygen (DO)
The oxygen dissolved in water, wastewater, or other liquid. DO is measured in milligrams per liter. If the DO of a sample of water is 2 mg/L, it means that there are 2lbs of oxygen in 1 mil lb of water.

14/Dissolved Solids
Solids that cannot be removed by filtering are dissolved solids.

15/Extended Aeration
A modification of the activated sludge process which provides for aerobic sludge digestion within the aeration system.

16/Floc
Clumps of bacteria and particles that have come together to form clusters, or small gelatinous masses. The floc mass in an activated sludge aeration tank generally consists of microorganisms.

17/Grease
In wastewater, a group of substances, including fats, waxes, free fatty acids, calcium and magnesium soaps, mineral oils, and certain other non-fatty materials.

18/Milligrams per Liter (mg/L)
A unit of concentration of water or wastewater constituent. It is 0.001 g of the constituent in 1000 ml of water. The unit parts per million is identical to milligrams per liter.

19/Mixed Liquor (ML)
The mixture of activated sludge, wastewater, and oxygen, where in biological assimilation occurs.

20/Mixed Liquor Suspended Solids (MLSS)
The milligrams of suspended solids per liter of mixed liquor that are combustible at 550 degrees Centigrade. An estimate of the quantity of MLSS to be wasted from the aeration tank of an extended aeration plant may be determined by the rate of settling and centrifuge tests on the sludge solids.

21/Nitrification
The conversion of nitrogen matter into nitrates by bacteria.

22/Nitrogen
Nitrogen is present in wastewater in many forms: total Kjeldahl nitrogen, ammonia nitrogen, organic nitrogen.

23/Nitrogen Cycle
The cycle of life, death, and decay involving organic nitrogenous matter is known as the nitrogen cycle. In the nitrogen cycle ammonia is produced from proteins.

24/Orthophosphate
A simple compound of phosphorous and oxygen that is soluble in water.

25/Oxic
A biological environment which contains molecular oxygen; aerobic.

26/Polyphosphate
A large compound formed of several orthophosphate molecules connected by phosphate-storing microorganisms.

27/Raw Wastewater
Wastewater before it receives any treatment.

28/Reactor
A tank where a wastewater stream is mixed with bacterial sludge and biochemical reactions occur.

29/Return Sludge
Settled activated sludge returned to mix with incoming raw or primary settled wastewater. When the return sludge rate in the activated sludge process is too low, there will be insufficient organisms to meet the waste load entering the aerator.

30/Return Activated Sludge
Activated return sludge is normally returned continuously to the aeration tank. Recycling of activated sludge back to the aeration tank provides bacteria for incoming wastewater. Its should be brown in color with no obnoxious odor and is often also returned in small portions to the primary settling tanks to aid sedimentation. Settled activated sludge is generally thinner than raw sludge. Some activated sludge will be wasted to prevent excessive solids build up.

31/Sludge Age
In the activated sludge process, a measure of the length of time a particle of suspended solids has been undergoing aeration, expressed in day. It is usually computed by dividing the weight of the suspended solids in the aeration tank by the weight of excess activated sludge discharged from the system per day.

32/Sludge Digestion
The purpose of sludge digestion is to separate the liquid from the solids to facilitate drying. The proper pH range for digested sludge is 6.8 - 7.2.

33/Sludge Index
Properly called sludge volume index (SVI). It is the volume in millimeters occupied by 1 g of activated sludge after settling of the aerated liquid for 30 minutes.
Sludge Reaeration
The continuous aeration of sludge after initial aeration for the purpose of improving or maintaining its condition.

34/Splitter Box
A division box that splits the incoming flow into two or more streams. A device for splitting and directing discharge from the head box to two separate points of application.

35/Wastewater
Domestic wastewater is 99.9% water and 0.1% solids. Fresh wastewater is usually slightly alkaline. If the pH of the raw wastewater is 8.0, it indicates that the sample is alkaline. If wastewater has a pH value of 6.5, it means that it is acid. Wastwater is said to be septic when it is undergoing decomposition.

Aerobic and Anaerobic Digestion and Types of Decomposition

1/Introduction
Microorganisms , like all living things, require food for growth . Biological sewage treatment consists of a step-by-step, continuous, sequenced attack on the organic compounds found in wastewater and upon which the microbes feed.
In the following sections we will look at the processes of aerobic and anaerobic digestion and the decomposition of waste in each process.

2/Aerobic Digestion
Aerobic digestion of waste is the natural biological degradation and purification process in which bacteria that thrive in oxygen-rich environments break down and digest the waste.
During oxidation process, pollutants are broken down into carbon dioxide (CO 2 ), water (H 2 O), nitrates, sulphates and biomass (microorganisms). By operating the oxygen supply with aerators, the process can be significantly accelerated. Of all the biological treatment methods, aerobic digestion is the most widespread process that is used throughout the world.

3/Biological and chemical oxygen demand
Aerobic bacteria demand oxygen to decompose dissolved pollutants. Large amounts of pollutants require large quantities of bacteria; therefore the demand for oxygen will be high.
The Biological Oxygen Demand (BOD) is a measure of the quantity of dissolved organic pollutants that can be removed in biological oxidation by the bacteria. It is expressed in mg/l.
The Chemical Oxygen Demand (COD) measures the quantity of dissolved organic pollutants than can be removed in chemical oxidation, by adding strong acids. It is expressed in mg/l.
The BOD/COD gives an indication of the fraction of pollutants in the wastewater that is biodegradable.

4/Advantages of Aerobic Digestion
Aerobic bacteria are very efficient in breaking down waste products. The result of this is; aerobic treatment usually yields better effluent quality that that obtained in anaerobic processes. The aerobic pathway also releases a substantial amount of energy. A portion is used by the microorganisms for synthesis and growth of new microorganisms.

5/Aerobic Decomposition
A biological process, in which, organisms use available organic matter to support biological activity. The process uses organic matter, nutrients, and dissolved oxygen, and produces stable solids, carbon dioxide, and more organisms. The microorganisms which can only survive in aerobic conditions are known as aerobic organisms. In sewer lines the sewage becomes anoxic if left for a few hours and becomes anaerobic if left for more than 1 1/2 days. Anoxic organisms work well with aerobic and anaerobic organisms. Facultative and anoxic are basically the same concept.

6/Anoxic Decomposition
A biological process in which a certain group of microorganisms use chemically combined oxygen such as that found in nitrite and nitrate. These organisms consume organic matter to support life functions. They use organic matter, combined oxygen from nitrate, and nutrients to produce nitrogen gas, carbon dioxide, stable solids and more organisms.

7/Anaerobic Digestion
Anaerobic digestion is a complex biochemical reaction carried out in a number of steps by several types of microorganisms that require little or no oxygen to live. During this process, a gas that is mainly composed of methane and carbon dioxide, also referred to as biogas, is produced. The amount of gas produced varies with the amount of organic waste fed to the digester and temperature influences the rate of decomposition and gas production.

Anaerobic digestion occurs in four steps:
• Hydrolysis : Complex organic matter is decomposed into simple soluble organic molecules using water to split the chemical bonds between the substances.
• Fermentation or Acidogenesis: The chemical decomposition of carbohydrates by enzymes, bacteria, yeasts, or molds in the absence of oxygen.
• Acetogenesis: The fermentation products are converted into acetate, hydrogen and carbon dioxide by what are known as acetogenic bacteria.
• Methanogenesis: Is formed from acetate and hydrogen/carbon dioxide by methanogenic bacteria.

The acetogenic bacteria grow in close association with the methanogenic bacteria during the fourth stage of the process. The reason for this is that the conversion of the fermentation products by the acetogens is thermodynamically only if the hydrogen concentration is kept sufficiently low. This requires a close relationship between both classes of bacteria.
The anaerobic process only takes place under strict anaerobic conditions. It requires specific adapted bio-solids and particular process conditions, which are considerably different from those needed for aerobic treatment.

8/Advantages of Anaerobic Digestion
Wastewater pollutants are transformed into methane, carbon dioxide and smaller amount of bio-solids. The biomass growth is much lower compared to those in the aerobic processes. They are also much more compact than the aerobic bio-solids.


9/Anaerobic Decomposition
A biological process, in which, decomposition of organic matter occurs without oxygen. Two processes occur during anaerobic decomposition. First, facultative acid forming bacteria use organic matter as a food source and produce volatile (organic) acids, gases such as carbon dioxide and hydrogen sulfide, stable solids and more facultative organisms. Second, anaerobic methane formers use the volatile acids as a food source and produce methane gas, stable solids and more anaerobic methane formers. The methane gas produced by the process is usable as a fuel. The methane former works slower than the acid former, therefore the pH has to stay constant consistently, slightly basic, to optimize the creation of methane. You need to constantly feed it sodium bicarbonate to keep it basic.

Nitrification in the BOD test

Noncarbonaceous matter, such as ammonia, is produced during the hydrolysis of proteins. It is now known that a number of bacteria are capable of oxidizing ammonia to nitrite and subsequently to nitrate. The generalized reactions are as follows:

*Conversion of ammonia to nitrite (as typified by Nitrosomonas):
NH3 + 3/2O2 → HNO2 + H2O

*Conversion of nitrite to nitrate (as typified by Nitrobacter):
HNO2 + 1/2O2 → HNO3

*Overall conversion of ammonia to nitrate:
NH3 + 2O2 → HNO3 + H2O

The oxygen demand associated with the oxidation of ammonia to nitrate is called the nitrogenous biochemical oxygen demand (NBOD).

Because the reproductive rate of the nitrifying bacteria is slow, it normally takes from 6 to 10 days for them to reach significant numbers to exert a measurable oxygen demand. However, if a sufficient number of nitrifying bacteria is present initially, the interference caused by nitrification can be significant.

When nitrification occurs in the BOD test, erroneous interpretations of treatment operating date are possible. For example, assume the effluent BOD from a biological treatment process is 20 mg/L without nitrfication and 40 mg/L with nitrification. If the influent BOD to the treatment process is 200 mg/L, then the corresponding BOD removal efficiency would be reported as 90 adn 80 percent without and with nitrification, respectively. Thus, if nitrification is occurring but is not suspected, it might be concluded that the treatment process is not performing well, when in actuality it is performing quite well

Biochemical Oxygen Demand (BOD)

The most widely used parameter of organic pollution applied to both wastetwater and surface water is the 5-day BOD (BOD5). This determination involves the measurement of the dissolved oxygen used by microorganism in the biochemical oxidation of organic matter. Despite the widespread use of the BOD test, it has a number of limitations. It is hoped that, through the continued efforts of workers in the field , one of the measures of organic content, or perhaps a new measure, will ultimately be used in its place. Why, then, if the test suffers from serious limitations, is further space devoted to it in the text? The reason is that BOD test results are now used (1) to determine the approximate quantity of oxygen that will be required to biologically stabilize the organic matter present, (2) to determine the size of waste-treatment facilities, (3) to measure the effeciency of some treatment processes, and (4) to determine compliance with wastewater discharge permits. Because it is likely that th BOD test will continue to be used for some time, it is important to know the details of the test and its limitations.

1/Basic for BOD test.

-If sufficientoxygen available, the aerobic biological decomposition of an organic waste will continue until all of the waste is consumed. Three more or less distinct activities occur. First, a portion of the waste is oxidized to end products to obtain energy for cell maintenance and the synthesis of new cell tissue.

-Simultaneously, some of the waste is converted into new cell tissue using part of the energy released during oxidation. Finally, when the organic matter is used up, the new cells begin to consume their own cell tissue to obtain energy for cell maintenance. This third process is called endogenous respiration. Using the term COHNS (which represents the elements carbon, oxygen, hydrogen, nitrogen, and sulfur) to represent the organic waste and the term C5H7NO2 [first proposed by Hoover and Porges (1952)] to represent cell tissue, the three processes are defined by the following generalized chemical reactions:

+Oxidation:

COHNS + O2 + bacteria → CO2 + H2O + NH3 + other end products + energy

+Synthesis:

COHNS + O2 + bacteria + energy → C5H7O2N (New cell tissue)

+Endogeneous respiration:

C5H7O2N + 5O2 → 5CO2 + NH3 + 2H2O

If only the oxidation of organic carbon that is present in the waste is considered, the ultimate BOD is the oxygen required to complete the three reactions given above. This oxygen demand is known as the ultimate carbonaceous or first-stage BOD, and is usually denoted as BOD.

2/BOD test procedure

In the standard BOD test, a small sample of the wastewater to be tested is placed in a BOD bottle (volume = 300 ml). The bottle is then filled with dilution water saturated in oxygen and containing the nutrients required for biological growth. To ensure that meaningful results are obtained, the sample must be suitaby diluted with a specially prepared dilution water so that adequate nutrients and oxygen will be available during the incubation period. Normally, several dilutions are prepared to cover the complete range of possible values. Before the bottles is stoppered, the oxygen concentration in the bottle is measured.

-After the bottle is in cubated for 5 day at 20oC, the dissolved oxygen concentration is measured again. The BOD of the sample is the difference in the dissolved oxygen concentration values, expressed in milligrams per liter, divided by the decimal fraction of sample used. The computed BOD value is known as the 5-day, 20oC biochemical oxygen demand. When testing waters with low concentrations of microorganism, a seeded BOD test is conducted. The organisms contained in the effluent from primary sedimentation facilities are used commonly as the seed for the BOD test. Seed organisms can also be obtained commercially. When the sample contains a large population of microorganisms (e.g., untreated wastewater), seeding is not necessary.

-The standard incubation period usually 5 days at 20oC, but other lengths of time and temperatures can be used. Longer time periods (typically 7 days), which correspond to work schedules, are often used, especially in small plants where the laboratory staff is not available on the weekends. The tenperature, however, should be constant throughout the test. The 20oC tenperature used is an average value for slow-moving streams is temperate climates and is easily duplicated in a incubator. Different results would be obtianed at different temperature, because biochemical reaction rates are temperature-dependent. After incubation, the dissolved oxygen of the sample is measured and the BOD is calculated using Equations below:

*When the dilution water is not seeded:

BOD, mg/L = (D1-D2)/P

*When the dilution water is seeded:

BOD, mg/L = [(D1-D2)-(B1-B2)f]/P

where D1 = dissolved oxygen of diluted sample immediately after preparation, mg/L

D2 = dissolved oxygen of diluted sample after 5-day incubation at 20oC, mg/L

B1 = dissolved oxygen of seed control before incubation, mg/L

B2 = dissolved oxygen of seed control after incubation, mg/L

f = fraction of seeded dilution water volume in sample to volume of seeded dilution water in seed control

P = fraction of wastewater sample volume to total combined volume

**Note: Biochemical oxidation theoretically takes an infinite time to go to completion because the rate of oxidation is assumed to be proportional to the amount of organic matter remaining. Within a 20-day period, the oxidation of the caronaceous organic matter is about 95 to 99 percent complete, and in the 5-day period used for the BOD test, oxidation is from 60 to 70 percent complete.

Solid of Wastewater

Today, I want to share some physical characteristics of wastewater with you.
As you know, the most important physical characteristic of wastewater is its total solids content, which is composed of floating matter, settleable matter, colloidal matter, and matter in solution. Other important physical characteristic include particle size distribution; turbidity; color; transmittance; temperature; conductivity; and density, specific gravity, and specific weight. Odor, sometimes considered a physical factor, is considered in the following section.

Wastewater contains a variety of solid materials varying from rags to colloidal material. In the characterization of wastewater, coarse materials are usually removed before the sample is analyzed for solids. The various solid classifications are identified in below

-Total solids (TS): The residue remaining after a wastewater has been evaporated and dried at a specified temperature (103 to 105oC).

-Total volatile solids (TVS): Those solids that can be volatilized and burned off when the TS are ignited (500±50oC).

-Total fixed solids (TFS): The residue that remains after TS are ignited (500±50oC)

-Total suspended solids (TSS): Portion of the TS retained on a filter with a specified pore size, measured after being dried at a temperature (105oC). The filter used most commonly for the determination of TSS is the Whatman glass fiber filter, which has a nominal pore size of about 1.58µm

-Volatile suspended solids (VSS): Those solids that can be volatilized and burned off when the TSS is ignited (500±50oC).

-Fixed suspended solids (FSS): The residue that remains after TSS are ignited (500±50oC).

-Total dissolved solids (TDS = TS – TSS): Those solids that pass through the filter, and are then evaporated and dried at specified temperature. It should be noted that what is measured as TDS is comprised of colloidal and dissolved solids. Colloids are typically in the size range from 0.001 to 1µm

-Total volatile dissolved solids (VDS): Those solids that can be volatilized and burned off when the TDS are ignited (500±50oC).

-Fixed dissolved solids (FDS): The residue that remains after TDS are ignited (500±50oC).

-Settleable solids: Suspended solids, expressed as milliliters per liter that will settle out of suspension within a specified period of time.