Advanced Wastewater Treatment
Primary and secondary treatment remove the majority of BOD and Suspended Solids found in wastewaters. However, in an increasing number of cases this level of treatment has proved to be insufficient to protect the receiving waters or to provide reusable water for industrial and/or domestic recycle. Thus, additional treatment steps have been added to wastewater treatment plants to provide for further organic and solids removals or to provide for removal of nutrients and/or toxic materials.
For the purposes of this lesson, Advanced Wastewater Treatment will be defined as: any process designed to produce an effluent of higher quality than normally achieved by secondary treatment processes or containing unit operations not normally found in Secondary Treatment. The above definition is intentionally very broad and encompasses almost all unit operations not commonly found in wastewater treatment today.
Types of Advanced Wastewater Treatment
Advanced Wastewater Treatment may be broken into three major categories by the type of process flow scheme utilized:
Tertiary Treatment
Physical-Chemical Treatment
Combined Biological-Physical Treatment
Tertiary treatment may be defined as any treatment process in which unit operations are added to the flow scheme following conventional secondary treatment. Additions to conventional secondary treatment could be as simple as the addition of a filter for suspended solids removal or as complex as the addition of many unit processes for organic, suspended solids, nitrogen and phosphorous removal. Physical-chemical treatment is defined as a treatment process in which biological and physical-chemical processes are intermixed to achieve the desired effluent. Combined biological-physical-chemical treatment is differentiated from tertiary treatment in that in tertiary treatment any unit processes are added after conventional biological treatment, while in combined treatment, biological and physical-chemical treatment are mixed.
Another way to classify advanced wastewater treatment is to differentiate on the basis of desired treatment goals. Advanced wastewater treatment is used for:
Additional organic and suspended solids removal
Removal of nitrogenous oxygen demand (NOD)
Nutrient removal
Removal of toxic materials
In many, if not most instances today, conventional secondary treatment gives adequate BOD and suspended solids removals. Why, then, is additional organic and suspended solids removal by advanced treatment necessary? There are a number of good answers to this question.
Advanced wastewater treatment plant effluents may be recycled directly or indirectly to increase the available domestic water supply.
Advanced wastewater treatment effluents may be used for industrial process or cooling water supplies.
Some receiving waters are not capable of withstanding the pollutional loads from the discharge of secondary effluents.
Secondary treatment does not remove as much of the organic pollution in wastewater as may be assumed.
The first three reasons for additional organic removal through advanced wastewater treatment are simple. The fourth requires some explanation. The performance of secondary treatment plants is almost always measured in terms of BOD and SS removals. A well designed and operated secondary plant will remove from 85 to 95% of the influent BOD and SS. However, the BOD test does not measure all of the organic material present in the wastewater. An average secondary effluent may have a BOD of 20 mg/L and a COD of 60 to 100 mg/L. The average secondary plant removes approximately 65% of the influent COD. Thus, when high quality effluents are required, additional organic removals must be accomplished. In addition to the organic materials remaining in most secondary effluents, there is an additional oxygen demand resulting from the nitrogen present in the wastewater.
In wastewaters, much of the nitrogen is found in the form of ammonia. When secondary treatment is used, a great deal of this ammonia is discharged in the effluent. Bacteria can utilize this ammonia as an energy source and convert ammonia to nitrite and nitrate.
NH3 + O2 + Bacteria ® NO2 + O2 + Bacteria ®NO3
Another reason for advanced wastewater treatment may be to remove nutrients contained in discharges from secondary treatment plants. The effluents from secondary treatment plants contain both nitrogen (N) and phosphorous (P). N and P are ingredients in all fertilizers. When excess amounts of N and P are discharged, plant growth in the receiving waters may be accelerated. Algae growth may be stimulated causing blooms which are toxic to fish life as well as aesthetically unpleasing. Fixed plant growth may also be accelerated causing the eventual process of a lake becoming a swamp to be speeded up. Therefore, it has become necessary to remove nitrogen and phosphorous prior to discharge in some cases
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Toxic materials, both organic and inorganic are discharged into many sewage collections systems. When these materials are present in sufficient quantities to be toxic to bacteria, it will be necessary to remove them prior to biological treatment. In other cases, it is necessary to remove even small amounts of these materials prior to discharge to protect receiving waters or drinking water supplies. Thus, advanced wastewater treatment processes have been used in cases where conventional secondary treatment was not possible due to materials toxic to bacteria entering the plant as well as in cases where even trace amounts of toxic materials were unacceptable in plant effluents.
Nitrification
Biological nitrification may be used to prevent oxygen depletion from nitrogenous demand (NOD) in the receiving waters. Nitrification is simply the conversion of ammonia to nitrate in the treatment plant rather than in the receiving water. Nitrification may be carried out in the same tank as BOD removal or in a separate stage. Nitrification may be carried out either in activated sludge flocs or in fixed films.
Regardless of the particular scheme chosen, the same basic requirements for nitrification must be maintained:
Oxygen level
Loading rates
Solids retention time
Alkalinity
pH
Freedom from toxic materials
Temperature
Sufficient oxygen must be available for nitrification to occur. Approximately 4.5 pounds of dissolved oxygen are required for the conversion of 1 pound of ammonia to nitrate. Dissolved oxygen sufficient to satisfy the remaining BOD is also required. In activated sludge plants the mixing requirement of the basins must also be considered. Generally, dissolved oxygen levels of approximately 2 - 3 mg/L are recommended for nitrification. The bacteria responsible for nitrification reproduce at a much slower rate than those responsible for BOD removal. Thus, the danger always exists for the "wash out" of the nitrifying organisms. That is, unless the nitrifying bacteria reproduce at the same or greater rate than they are removed from the system (by waste sludge) then the population of bacteria will be insufficient to carry out nitrification. For this reason, nitrification systems are operated at higher return sludge rates than conventional secondary treatment. The amount of sludge to be wasted is significantly less than from a conventional activated sludge system.
Nitrification systems are sensitive to pH variation. Optimum pH has been found to be approximately 7.8 to 9.0. Reductions in nitrification have been found outside this range. Alkalinity is also destroyed during nitrification. Theoretically, 7.2 pounds of alkalinity are destroyed in converting 1 pound of ammonia to nitrate. In low alkalinity wastewaters, Quick lime (CaO) or Ca(OH)2 is often used to provide alkalinity and pH control.
Generally, the influent BOD to nitrification systems has not been found to effect performance. However, sufficient oxygen must be provided for the organic demand and organic shock loads must be avoided.
Biological Denitrification
Biological nitrification satisfies the nitrogenous oxygen demand by converting NH3 to NO3. In some applications, such as discharge into enclosed bodies of water or recycle to water supplies, nitrification may not be sufficient. When nitrogen removal is required, one of the available methods is to follow biological nitrification with biological denitrification.
Denitrification is accomplished under anaerobic or near anaerobic conditions by bacteria commonly found in wastewater. Nitrates are removed by two mechanisms: (1) Conversion of NO3 to N2 gas by bacterial metabolism and (2) conversion of NO3 to nitrogen contained in cell mass which may be removed by settling.
In order for denitrification to occur, a carbon source must be available. Most commonly, methanol is used. The methanol must be added in sufficient quantity to provide for cell growth and to consume any dissolved oxygen which may be carried into the denitrification reactor.
Usually 3 to 4 pounds of methanol per pound of nitrate are required. Careful control of methanol feed is necessary to prevent waste of chemicals. In addition, if excess methanol is fed to the system, unused methanol will be carried out in the effluent causing excessive BOD.
Denitrification may be carried out in either a mixed slurry reactor or in fixed bed reactors. Denitrification filters carry out both denitrification and filtration in the same unit. Mixed slurry systems consist of a denitrification reactor, reaeration basin and clarifiers. Reaeration prior to clarification is required to free the sludge from trapped bubbles of nitrogen gas.
Denitrifying bacteria grow very slowly and are extremely sensitive to temperature.
Denitrification rates have been shown to increase five-fold when the temperature is increased from 10°C to 20°C. Thus, operating parameters such as sludge age and retention time must be varied with temperature.
The pH in denitrification systems must be carefully controlled. The optimum pH is from 6.0 to 8.0.
Denitrification is a very sensitive and difficult process to operate. Little full scale operational experience is available. Constant monitoring of pH, methanol feed and temperature is essential to successful operation.
Filtration
Granular media filtration to remove those suspended and colloidal solids which are carried over from previous unit processes is a common unit process in advanced wastewater treatment. Effluents of less than 10 mg/L BOD and 5 mg/L suspended solids are not uncommon for effluents from biological treatment processes after filtration.
Gravity filters similar to rapid sand filters are sometimes used. Often a combination of filter medias, such as anthracite coal and sand are used to provide coarse to fine filtration as the water passes through the fi
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