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NATO ASI Series, Vol. G 30
Edited by 1. Bogardi and R. D. Kuzelka
©Springer-Verlag Berlin Heidelberg 1991
CONTROL OF NITROGEN SOURCES AND PRINCIPLES OF TREATMENT
Department of Environmental Engineering Istanbul Technical University, 80626 Maslak Istanbul, Turkey
Nitrogen-removal efficiencies were calculated in relation to sludge ages. The effluent of a physical-treatment plant was passed through a plant-scale biological unit where nitrification and denitrification occurred. A biological-treatment unit consisting of five cascades was used as a pilot-scale plant, and the respirometric activity and nitrogen concentration in different forms were measured. The variation of ammonium concentrations within the reactor under various recycle ratios and sludge load conditions is presented.
Especially in industrialized countries, it is no longer possible to use polluted surface waters for drinking-water supplies. Nitrogen pollution in surface waters originates mainly from domestic and industrial wastewater, and drainage and surface runoff from agricultural areas. The amount of nitrogen in domestic wastewater discharges is in the range of 8 to 15 gr/day/ person (Helmer and Sekoulov, 1979).
High nitrogen loads in receiving waters create several problems (Diesterweg, 1985):
- The greater part of ammonia nitrogen (NH4-N) in river waters readily converts to nitrate. The oxygen required for this process is drawn from the water; when river oxygen levels fall below 4 mg/L fish stocks are threatened. This process occurs more rapidly and has more serious consequences as water temperatures rise.
- At pH values over 7, the ratio of ammonium (NH4+) to free ammonia (NH3) shifts in favor of the free ammonia, which is toxic to fish and other aquatic organisms. The toxicity limit of ammonia is in the range of 0.2 to 2 mg/L for fish, and 0.2 to 9 mg/L for other aquatic animals.
- Increased levels of NH4+ and NO3– in stagnant waters promote plant growth and can lead to eutrophication.
- Drinking-water quality is threatened by high nitrate and nitrite concentrations, which can lead to infantile cyanosis.
The various forms of nitrogen that are present in nature and the pathway between them are shown schematically in Fig. 1. Table 1 shows concentrations of nitrogen compounds in domestic and some industrial wastewaters.
Table 1. Concentrations of nitrogen compounds in domestic and some industrial wastewaters
2. Nitrogen Removal in Wastewater
2.1. Treatment Stages
Generally, domestic wastewater is treated in a two-stage (mechanical and biological) process in which suspended solids and dissolved organic matter are removed. Advanced treatment methods are then used to remove the untreated or slightly treated pollutants. Fig. 2 shows different treatment alternatives (joint and separate) for industrial and domestic wastewaters. Biological, physical and chemical processes can be used to remove nitrogen, ammonia nitrogen and nitrate nitrogen at various points in the treatment process.
Fig. 1. Nitrogen cycle (after Young et al., 1975)
Fig. 2. Treatment alternatives for industrial and domestic wastewater (after U.S. EPA, 1973)
2.2. Nitrogen Removal in Biological Treatment
Up to 10% of the total nitrogen in wastewater can be removed in the mechanical-treatment stage. In the biological-treatment stage, part of the ammonia is used by the cell for microbial growth, and leaves the system as sludge. Approximately 0.05 kg nitrogen/kg COD (chemical oxygen demand) is removed in biological treatment, although the amount varies according to the sludge load. If the sludge load is nitrogen, for example, the amount can increase up to a maximum of 50%.
2.3. Nitrification – Denitrification
Using biological-treatment systems for nitrification, it is possible to convert a significant amount of organic nitrogen and ammonia nitrogen to nitrate and nitrite. Nitrate in turn converts to nitrogen in the denitrification stage and leaves the system as gas.
2.4. Evaluation of Nitrogen-removal Technologies 2.4.1 Suspended-growth Systems
Fig. 3 shows alternative processes used in nitrification and denitrification of domestic wastewater (Harremoes and Goneng, 1983). The different approaches can be classified according to the biomass configuration in the reactor for the treatment of domestic and industrial wastewater containing nitrogenous compounds. In suspended-growth systems (activated sludge), the bacteria are kept within the system by gravity separation and recirculation of the sludge. Fig. 4 shows a recycling system with pre-denitrification and post-nitrification.
2.4.2. Attached-growth Systems
In attached-growth systems (biofilm reactors), the bacteria are retained on solid media which are in contact with the wastewater. Activated-sludge systems were until recently the main systems used for nitrification and/or denitrification of nitrogenous wastewater. However, biofilm systems are used increasingly because of the ease of operation and low cost. Three types of reactors are commonly used as biofilm reactors (U.S. EPA, 1975): (1) trickling filters, (2) rotating biological contactors, and (3) submerged filters. The basic principles of each reactor type are presented schematically in Fig. 5.
Fig. 3. Four alternative nitrification-denitrification processes
Fig. 4. A recycling system with pre-denitrification post-nitrification
3. Nitrogen Removal in Biological Systems by Nitrification-Denitrification Process
In a study performed by the author at Stuttgart University (Samsunlu, 1986) effluent from a mechanical-treatment plant was sent to a model biological unit where nitrification and denitrification were performed, and nitrogen-removal efficiencies in relation to sludge ages were calculated.
Fig. 5. Biofihn reactors employed in wastewater treatment
3.1. Description of the Model
Raw sewage from the mechanical-treatment process was introduced to a biological-treatment unit (fig. 6) consisting of five cascades. The first part was stirred, and the subsequent four cascades were aerated with diffused air. The aeration pipes are placed at the two sides of the tank. Apart from the biological unit there was also a primary sedimentation tank and equipment and pumps for recycling. The first cascade was designed for the purpose of &nitrification, while the others were designed with the aim of nitrification. Iw order to achieve a higher nitrogen removal rate, the recycle ratio, QR, and the influent water flow rate, Qg were changed, and NH4-N, NO3-N, NO2-N measurements were performed in each cascade in the biological unit for every case. The oxygen level in the water was maintained at a constant 1.5 mg 021L, but some variation in the dissolved oxygen concentration occurred, and the DOC level occasionally fa below I mg/L.
In the study, the respirometric activity and nitrogen concentrations in different forms were measured, including NH4-N, total nitrogen, NO3-N, and NO2-N. Concentrations of the different forms of nitrogen were determined by standard methods (APHA, 1985), and pH, alkalinity, COD and BODE were measured. The measurements were performed on the ifluent (precipated raw sewage, every cascade of the biological unit (five cascades), and the effluent water.
Fig. 6. Biologkal nitrogen-removal unit model
3.2. Experimental Results
In the experiment, samples were taken from inlets and outlets of numbered cascades (from one to five). Alkalinity, suspended solids, COD, BOD5, total nitrogen, ammonia nitrogen,
nitrate nitrogen, nitrite nitrogen, total phosphorus, temperature, volatile suspended solids, dissolved oxygen, sludge volume index and respirometric activity tests were performed on the samples. The inlet flow rate is expressed as Qg and the recycled rate as QR. Influent flow rates, recycled flow rates, and variations of NH4-N in influent and recycle flow rates as maximum and average values are provided in Table 2.
In Fig. 7, the N}14-N values versus different recycle percentages are plotted. NH4-N concentrations in the effluent must be under 3 mg/L. Since this limitation is only met by recycle percentage of 600, it is possible to get a value under the limit in the second cascade. The number of the cascades can thus be reduced.
Fig. 7. Variation of ammonium concentrations within the reactor under various recycle-ratio and sludge-load conditions
Table 2. Performance data for various sludge loadings
The sludge load in the treatment plant can be 0.3 kg/kg-day and higher if carbon removal only is considered. However, if nitrification is also required, the sludge load must be approximately 0.1 kg/kg-day or lower (Samsunlu, 1986).
Nitrogen compounds play an important role in the ecological balance. Nitrogen is discharged every day in increasing amounts from domestic- and industrial-wastewater sources. If nitrogen is not controlled at the wastewater source, eutrophication can occur in surface waters.
Nitrogen removal rates in conventional-treatment systems are very low and nitrogen removal from wastewaters is very expensive. To remove nitrogen in biological-treatment systems by nitrification and denitrification processes, it is necessary to design and operate biological-treatment plants. Nitrification and denitrification can be successfully achieved by activated-sludge or submerged-filter systems. However, in order to achieve more effective nitrogen removal, it is necessary to add a tertiary physical-chemical treatment process following biological treatment.
APHA, AWWA, WPCF, (1985) Standard Methods for the Examination of Water and Wastewater, 16th Edition
Diesterweg G (1985) Tower-Biology and Its Applications for the Nitrification/Denitrification of Ammonia-Rich Wastewater. Proceedings of the 40th Industrial Waste Conference, Purdue University, Purdue, Illinois
Harremoes P, Ganeng ijE (1983) The Applicability of Biofilm Kinetics to Rotating Biological Contactors, Preprint for EWPCA/IAWPR International Seminar on Rotating Biological Discs. Stuttgart
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