О. B. KURNILOVICH, О. А. КОLESNICHENKО, engineers (Industrial department of water supply and sewerage economy, city of Krivoy Rog)

THE "AERATION TANK-SECONDARY SETTLER" SYSTEM CONTROL

The instability and diversity of the input stream as to its qualitative and quantitative parameters considerably affect the processes existent in the aerotank biosystem. That is why it is impossible to construct the accurate model of the process.

The method of settling the practical tasks of the “Aerotank-secondary settler” system control has been developed at the Krivoy Rog aeration station with the application of а mathematical model, describing the control process.

The mechanism of the joint functioning of the aerotanks and secondary settlers can be described by two functional dependences:

For aerotanks

Q_a= K_ta a_i, (1)

For secondary settlers

Q₂= K(_t2 (1/a_i), (2)

where Q_a, Q₂ is the maximum productive capacity of aerotanks and secondary settlers, m³/per hour;
K_ta – technological coefficient determining the aerotank functioning conditions;
К_t2 - technological coefficient determining the secondary settler functioning conditions;
а_i – sludge dosage in the sludge mixture incoming from the aerotanks to the secondary settlers, g/l.

The value of the technological coefficient of the aerotanks depends on the biocontamination in the incoming water, aerotank volume, regeneration percentage, recycling degree, sludge mixture temperature in the aerotank and parameters determining the sludge biomass activity degree (ash percentage, inhibition coefficient).

The technological coefficient of the secondary settling depends on the volume of the secondary settlers, design peculiarities determining the volume usage coefficient, sludge index, general coefficient of irregularity of sewage income.

If the dependence of the aerotank productive capacity Q_a and secondary settler productive capacity Q₂ on the active sludge dosage in the sludge mixture stream incoming from the aerotanks to the secondary settlers is drawn graphically, supposing that the conditions determining the coefficients K_is and K_it, remain virtually invariable for a certain period of time, then the graphics will represent the tolerance region (TR) of the technological parameters for a specific “Aerotank-secondary settler” controlled system (see the picture).

The tolerance range of the technological parameters of the “aerotank – secondary settler” system

In the picture the curve 1 denotes the boundary of the values of aerotank productive capacity for the given coefficient K_ta, and the curve 2 denotes the boundary of the value of the secondary settlers productive capacity with the given coefficient К_t2. The curves 1 and 2 limit from both sides the productive capacity tolerance region S_A and corresponding to these productive capacities sludge dosage in the mixture incoming from the aerotanks to the secondary settlers.

Let us consider settling of certain practical tasks by the example shown in the picture. If the sewage income Q, equaling 4500 м ³/per day and night, and the active sludge dosage in the mixture from aerotanks is 1,75 g/l then the point, corresponding to these parameters (point A) belongs to TR and consequently the specified effectiveness of the system is ensured.

If the active sludge dosage is 1 g/l (point А') the aerotanks will not ensure the specified oxidation process, if the dosage is 4 g/l (point А ") the settlers would be the weak point of the system, though the required standard of the biological process would be ensured.

The tolerance range of the sludge dosage is within the limits from 1,25 (minimum dosage) to 3,2 g/l (maximum dosage). If the income of the sewage makes up more than 7000 m³/ per day and night the system will be overloaded.

If it is revealed that the point, corresponding to the factual income and the sludge dosage previously belonging to the TR has gone beyond the boundary of the region the study of the roots that caused the TR boundaries transition and reduction of the region is conducted. Those may be: the biochemical oxygen demand5 increase in the incoming water, shutting down one of the aerotanks or secondary settlers, sludge index increase, coefficient of inflow irregularity increase.

The model of the aerotank control is based on the following dependence at the Krivoy Rog aeration station:

Where r is the reactivation tank volume;
R_i – recycling degree;
s - ash percentage of the sludge;
W¹_a volume of one section of aerotank, m³;
n_a -number of aerotank sections;
Т_w – temperature of sludge mixture in aerotanks, ⁰С;
q_i biochemical oxygen demand ₅, peak load mg/(g.per day and night);
L _sinlet, L _soutlet - biochemical oxygen demandз of organic substances in the incoming biologically purified water, mg/l;
j - decomposition product of active sludge inhibition coefficient, l/g.

For the sewage settling process description in the secondary settlers the formula (67) of the building code 2.04.03-85, point. 6.161 is used after certain correction of its coefficients.

For the calculation of the required air consumption the formula (49) of the building code 2.04.03-85, point 6.157, is used, supplemented by the parameters considering the extra oxigen consumption for ammonium joint oxidation.

Wherein the coefficient К₁ of the formula (49) of the building code needs to be paid specific attention, it needs to be corrected in case of filtering aerators application due to their depreciation in the course of time.

The polyethylene aerators of the “Ekopolimer” firm, introduced at the Krivoy Rog aeration station, are amongst the most practically feasible, resistant to hydropneumatic effects, easy to mount and dismount new types of aerators. The experience has shown that the coefficient value К1 can be assumed constant within a long period of time. The applied at the Krivoy Rog aeration station control model was created with consideration of local peculiarities but the stated in the present article principles and definitions may be employed at other aeration stations as well.