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TECHNOLOGY OF AMMONIA NITROGEN REMOVAL FROM WASTEWATER IN AERATION TANKS EQUIPPED WITH ECOPOLYMER FINE-BUBBLE POLYMER DIFFUSERS

G. P. Verbitsky, O. B. Kurnilovich
Water Supply and Sewage Services Administration Kryvoi Rog, Ukraine

For removal of nitrogen containing compounds from wastewater methods of nitrification and denitrification that require alternation of aerobic and anaerobic zones at one and the same plant have found worldwide application. However, application of these methods at operating plants is problematic. This is explained by complexity of plants' reconstruction using special equipment. Researches conducted in recent years point at real opportunity to implement in conventional aeration tanks one-stage nitrification and denitrification process. It is proved that due to the impact of oxygen aerobic microorganisms screen anaerobic denitrificators in the particles of activated sludge. For this reason, both processes can run simultaneously [1].

The most critical factor for the efficiency of nitrification and denitrification processes is the increase of activated sludge age up to 20 to 30 days. In this case, 97-98% of ammonia nitrogen is removed, while denitrification exponent of power grows up to 50-65%.

According to regulations of fish farms required age of activated sludge for treatment of low concentrated wastewater can be lower, within the range of 10-15 days.

Conditions necessary to provide the specified above parameters exist at many WWTPs. This can be explained by a number of reasons:

1. A majority of currently operating aeration stations were designed and constructed before the year of 1990 according to the design standards issued in 1974 (СНиП 11-32-74) and earlier. The latter were valid till the year of 1986 when a new standard СНиП 2-04.03-85 was issued. It was known that such standards provided considerable reserve of volume of the aeration tanks, because they were designed at low working doses of activated sludge, particularly, 1.5 mg/l in the aeration tank, and 4.0 g/l in the regenerator.

2. Volume and concentration of wastes present in influent wastewater of a number of WWTPs, nowadays, are much lower than it was designed. Previous regulations, as well as existing ones, do not expect regenerators to be used for low concentrated influent wastewater with total BOD not higher than 150 mg/l.

In this case there is, as a rule, an opportunity to store biomass of activated sludge in the aeration tanks by means of growing its average dose up to 4 to 5 g/l and to increase sludge age up to 12 to 15 days and more. Thus, conditions for nitrification and denitrification process are created in the aeration tanks. Growth of sludge in the aeration tanks, as well as its excessive mass in them is also lower than it was planned.

It is very important to mention that compared to the processes of complete biological treatment these processes require larger airflow rate. However, methods for calculation of airflow considering oxidation of nitrogen containing compounds appeared only during the last years [2C3].

Oxygen plays the most important role in providing nitrification and denitrification processes, while its absence restricts their development. For efficient and cost-effective usage of oxygen modern types of aeration tanks should be used. Traditional, still widely used ceramic filtrate plates are capable to provide intensive diffusion. However, it is very difficult to use them under existing conditions. In course of operation intensive leakage of air occurs due to loose spaces between the plates, as well as due to their frequent damage. Aeration systems made of filtrate plates are characterized by uneven aeration along the aeration tank, higher resistance and clogging with dust, corrosion products and biological sedimentation.

Among modern types of diffusers the most perfect ones are polyethylene diffusers manufactured by scientific-industrial firm Ecopolymer that is located in Kharkov, Ukraine. Ecopolymer diffusers almost do not possess the above mentioned drawbacks. They have a very long service life, are convenient for installation and operation, as well as economically efficient.

Taking into consideration the fact that situation at Severnaya aeration station in the city of Kryvoi Rog was similar to the ones described above there was made a decision to use Ecopolymer diffusers during expansion and reconstruction of the station. Besides, after commissioning of the aeration tanks for the purpose of nitrogen containing compounds removal it was planned to make optimization of technological modes by means of ensuring conditions necessary for the processes of nitrification and denitrification.

Influent wastewater at Severnaya aeration station has the following features: BOD₅ 117 mg/l (BOD₅ 154 mg/l), while ammonia nitrogen concentration is equal to 16 mg/l. Volume of influent wastewater is kept at present within the designed capacity of the station, i.e. 44 thousand m³/day.

Before starting expansion and reconstruction of the station effluent BOD₅ was equal to 8.9 mg/l, ammonia nitrogen concentration in treated water was 4.6 mg/l, nitrate nitrogen - 2.8 mg/l.

Concentration of activated sludge in the aeration tanks was kept not higher than 1.0 to 1.5 g/l, while sludge age did not exceed 4 to 6 days. Filtrate plates were used as aerators.

Existing four-passage aeration tank is still equipped with filtrate plates. Three aeration tanks, each divided into three passages, were shut down for major overhaul.

Two new four-passage aeration tanks (48 m x 36 m x 3.2 m) were equipped with Ecopolymer polyethylene diffusers (two aeration lines in the first two passages, and one aeration line in the third and fourth passages).

Taking into account that concentration of organic waste in wastewater was low, plant operators managed to provide sludge age of 10 to 12 days and higher at average activated sludge dose that does not exceed 2.5 to 3.0 g/l. Volume of regenerator was decided to be equal to 25% (in this case a passage with higher concentration of activated sludge is called regenerator by tradition). BOD₅ per one unit of biomass of activated sludge was equal to 100 to 120 mg/l per day.

A new mode resulted in reduction of ammonia nitrogen concentration in biologically treated water up to 1.0 mg/l, while the level of nitrate nitrogen lowered up to 2.2 mg/l.

Wastewater treated in the aeration tanks equipped with polyethylene fine-bubble diffusers contained 0.3 to 0.5 mg/l of ammonia nitrogen. This figure meets the requirements of fish farm water bodies.

Thus, after the usage of polyethylene diffusers at Severnaya aeration station of the city of Kryvoi Rog and application of the technology of higher activated sludge age (10 to 12 days and more) efficiency of removal of nitrogen of ammonia salts from wastewater grew from 71.2% to 93.7%. Meanwhile, total concentration of non-organic nitrogen containing substances reduced from 53% to 80%. Considering the level of BOD₅ wastewater treatment efficiency improved, too. Particularly, earlier effluent BOD₅ was equal to 8.9 mg/l, while after that it lowered up to the level of 6.5 mg/l.

It is important that to obtain such results we did not have to increase airflow rate for the aeration tanks. Moreover, we were able to reduce it by 10-15%. Besides, active sludge growth lowered, too.

Results obtained at Severnaya aeration station of the city of Kryvoi Rog coincide with those received at other similar municipal wastewater treatment plants perfectly well. They prove a possibility to run simultaneously the processes of nitrification and denitrification in conventional aeration tanks when nitrogen compounds are removed up to the level required by fish farm water bodies [4].

LITERATURE:

1. A.S.Shelomkov., N.V. Zakhvatayeva. The technology of one-stage nitrification-denitrification process// Water supply and sanitary equipment. – 1996. - №8. – Pages 17-18.
2. S.V. Yakovlev, B.N. Shevtsov, I.V. Skirdov, A.A. Bondarev. Technological design of modern facilities of sewage biological purification// Water supply and sanitary equipment. – 1994. - №2. – Pages 2-5.
3. Y.M. Meshengisser, R.A. Galich, A.I. Shchetinin, Y.G. Marchenko. Peculiarities of calculation of the “Ekopolimer” aeration system// Ecology, technology, economy of water supply and sewerage system: Selected reports of International Congress (Yalta, April, 15-19, 1997). – Kharkiv: Printing factory “Globus”, 1997. – Pages 5-76.
4. V.B. Smirnov, G.I. Getsina.. Aerotank functioning intensification at sewage biological treatment station// Water supply and sanitary engineering.1995. № 12. – Pages 24-25.