V. B. SMIRNOV, chief engineer (Voskresensk Biological Treatment Station);
G. I. GETSINA, candidate of technological sciences (Research Institute of public water supply and water treatment)

AERATION TANK FUNCTIONING INTENSIFICATION AT BIOLOGICAL WASTEWATER TREATMENT STATION

Sewage from the cities of Voskresensk, Yegoryevsk as well as from neighboring enterprises and centers of population proceed to the Joint-Stock Company “Minudobreniya”. As to its contents the sewage consists of 60 % of municipal and 40 % industrial sewage. Industrial sewage is distinguished by its considerable diversity due the fact that it is produced at chemical plants, metal-working enterprises, light and food industry enterprises, various motor-transport enterprises. The sewage arriving to the sewage biological treatment station contains contaminating agents of municipal origin, considerable amounts of heavy metal salts, compound organic substances, oil products as well as nitrogen and phosphor compounds with the concentration of 60-100 mg/l.

The biological treatment station was projected under the traditional design of municipal aeration station design and consists of mechanized grids, horizontal sand catchers, primary and secondary circular settlers, two-corridor aerotanks and contact reservoirs. In addition to these equipment the gravel-sand filters with ascending water current developed by the Research Institute of public water supply and water treatment were put to service 15 years ago.

Due to the raised by the Moscow regional nature committee requirements as to the treated sewage quality (BODfull 3 mg/l, ammonia nitrogen 2 mg/l, phosphates 5 mg/l), the station personnel along with the Research Institute of public water supply and water treatment in 1993-1994 conducted investigations to determine the ways of the intensification of the process of biochemical oxidation of organic contamination and biogenic substances (nitrogen and phosphor compounds) in aerotanks. The ways of the most effective and economical oxidation in aerotanks by way of increasing the biomass, existent there, were sought after, which in its turn, stipulated the sludge load decrease, specific oxidation velocity increase, sludge age increase. Possible ways of biomass amount increase are: the immobilization of the biomass on various carriers, sludge recycling degree increase from the secondary settlers to aerotanks.

In the course of the research it had been established that if for the full biological treatment the sludge age does not exceed 2-3 days and nights, then the deep nitrogen removal requires the sludge age to be no less than 10 days and nights, sludge dosage more than 3.5 g/l, the duration of sludge residence in aerotank no less than 8-10 hours. At that an additional amount of oxygen is required for ammonia nitrogen nitrification out of calculation of 4,6 mg of oxygen per 1 mg of ammonia nitrogen plus oxygen consumption for carbon oxidation, aquatic life provision, organic substance stabilization, active sludge agitation in aerotanks.

Pilot tests have confirmed the possibility of ammonia nitrogen content decrease to 2 mg/l by way of maintaining large dosage of active sludge in aerotanks to 5-6 g/l at 100% active sludge recycling with the availability of dissolved oxygen in concentration of no less than 4 mg/l, that can be provided by the air delivery with specific consumption of 13.5 m³/m³.

For the provision of the required air amount for aerotanks the polyethylene tubular aerators of the research and production firm “Ekopolimer” production were chosen as the easiest to mount, exploit and providing the best economical indexes of air oxygen dissolution in aerotanks due to milli-bubbling aeration.

Aerotanks with increased sludge dosage, increased dissolved oxygen content, aeration duration of 9-10 hours have been exploited for more than a year.

The comparative data on the oxygen impregnation efficiency and ammonia nitrogen oxidation in aerotanks are given in the table 1.

It is necessary to note that after the introduction of the “Ekopolimer” aerators the ammonia nitrogen oxidation has increased up to 80-90 % (used to be 30-45 %) with the dissolved oxygen content increase up to 4,5 mg/l in comparison with the previous 1,5-2 mg/l with the air consumption decrease by two times.

Table 2 provides the data characterizing the active sludge status in the aerotanks mode of high-productivity aerotanks-displacers with increased dissolved oxygen content, sludge dosage and prolonged aeration (section № 1 with the "Ekopolimer" aerators).

The given microbiological analysis of active sludge existent in the aerotanks has testified to a great variety of micro-organisms as to their species: Arcella discoides, Euglycha laevis, Paramecium aurelia, Euplotes putella, Aspidisca costata, Vorticella aequilata, Rotaria rotatoria, Notommata ansata, Epistulis plicatilis, Thuricola similis. The presence of the given species in the biocenose in agile, active state testifies to the processes of deep biochemical purification, nitrification and mineralization of the active sludge existent in the aerotank.

The research on the aerotank oxidation ability is being done at the present time. This can be achieved by biomass increase at the expense of the attached forms of aquatic life and further dissolved oxygen content increase up to 7-8 mg/l with the help of the installed aerators of the research and production firm “Ekopolimer”.

Conclusions:

1. It is most appropriate to use the given aerotank type for municipal and concentrated difficult-to-oxidize industrial sewage containing organic substances, nitrogen, phosphor in increased concentrations.
2. Processes of nitrification-denitrification with sludge deep mineralization, decomposing its major part to water, nitrogen and carbon dioxide take place in such aerotanks with the increased sludge dosage (more than 4-5 mg/l) and 100% non-recycling of the active sludge with the dissolved oxygen content of more than 4 mg/l, which lets considerably decrease the volume of the disposed excessive sludge up to 20 %), that is to say, no less than by five times.
3. Usage of the tubular aerators of the research and production firm “Ekopolimer” has made it possible to effectively solve the task of equipping the aerotank, designed for deep treatment of municipal and industrial sewage as to its economical aspect. The expences for acquisition and mounting of the aerators is paid back during one year at the expence of energy saving saying nothing of sewage treatment quality improvement.