D.A. DANILOVICH*

*Danilovich Dmitrii Aleksandrovich, PhD in Engineering, Deputy Executive Director for Engineering and Environmental Work, Russian Association of Water Supply and Disposal, 119334, Russia, Moscow, Leninsky Ave, 38, bld. 2, tel.: (495) 939-19-36(495) 939-19-36, e-mail: da_danilovich@mail.ru

The experience of improving and evaluating the efficiency of aeration systems of the new line of the Lyubertsy wastewater treatment plant of the Moscow wastewater system is described. A biological nitrogen and phosphorus removal unit with a design capacity of 500,000 m³/day is used. After five years of operation Austrian AQUASTRIP plate-type membrane aerators were replaced with Russian АR-420Т disk membrane systems. This allowed significantly increasing wastewater flow to the new unit. In the course of studies two methods of evaluation of the aeration system efficiency were compared: the classical approach of direct “gas cap” measurement and the calculation method. In order to implement the calculation method a technique of evaluating the actual aeration efficiency of aeration tanks was developed based on the fundamental principles of the mass balance of the treatment processes. This technique allows calculating the actual efficiency of oxygen utilization supplied to the aeration system from the actual operation data for any time interval and with regard to any aeration tank number. In the course of the experiment about 9% divergence of the results of the two methods was obtained which corresponded to the measurement errors accepted as a basis of the calculations. The measurements and calculations showed that the specific efficiency of oxygen dissolution was around 6% per 1 meter of the aerator immersion depth, which complied with the information submitted by the manufacturer («Ecopolymer-M» CJSC), and by 30% higher than shown by AQUASTRIP aerators. The developed method can be used both in evaluating the aeration system efficiency for the certain wastewater treatment plant and in making the intra-sectoral analysis (benchmarking). The calculated values of the actual atmospheric oxygen dissolution efficiency and power consumption per 1 kilogram of the actually dissolved oxygen are recommended for being used as the target indicators of the wastewater treatment plant' improvement instead of the generally accepted air consumption (power consumption) per 1 m³ of wastewater.

The most modern and efficient structure for wastewater treatment in the Moscow sewage system is the nutrient removal block with a design capacity of 500,000 m³/day at the Lyubertsy wastewater treatment plant [1; 2]. The structure of the nutrient removal block includes four aeration tanks (Fig. 1), seven secondary sedimentation tanks, and a sludge pump station. Since the nutrient removal block does not have its own mechanical treatment facilities, clarified wastewater is supplied to it from other units. This is due to the fact that the block was built starting from the second half of the 1990s for unloading the biological treatment facilities of the remaining plant blocks in order to improve the overall treatment efficiency.

The UCT-process (the University of Cape Town technology) has served as the basis for the wastewater treatment technique used in the new nutrients removal block. Its distinctive feature is the preliminary denitrification of the sludge mixture fed to the anaerobic zone of the bioreactor.

Fig. 1. General view of the aeration tanks of the nutrients removal block

Clarified water in the nutrients removal block is supplied to the inlet of the first corridor (anaerobic zone) (Fig. 2). Here, the cells of phosphate-accumulating microorganisms release phosphates and absorb readily available organic substances. From the first corridor, the sludge mixture enters the second and third corridors (“carousel”) with anoxic and aerobic zones. The return biological sludge is fed to the inlet of the second corridor. Nitri-denitrification processes and partial oxidation of organic substances accumulated by phosphate-accumulating microorganisms occur in these corridors. The circulating sludge mixture (the so-called "Cape Town recycle") is supplied by submersible pumps from the outlet of the second corridor after denitrification process. In the fourth corridor (aerobic zone), the processes of nitrification and oxidation of organic compounds, including those accumulated by phosphate-accumulating organisms with simultaneous consumption of phosphates from the sludge mixture, are completed.

Internal recycling of denitrification is based on the "carousel" principle. With the help of mixers oriented along the flow axis, continuous movement of the sludge mixture through the aerobic and anoxic zones takes place in the second and third corridors of the bioreactor at a rate of 0.25-0.3 m/s.

In 2006, AETASTRIP T4,0EU-18 plate-type membrane aerators by Aqua Consult (Austria) were installed in the aeration tanks of the nutrients removal block. Air ducts and the base of the aerator are made of stainless steel; the membrane is made of polyurethane polymer.

Each aeration tank was equipped with 348 aerators. The length of each aerator is 4 m, width - 18 cm, the surface area of   the membrane is about 0.7 m² (Fig. 3), the total area of   the aerating surface is 244 m². The number of aerators was recommended subject to high efficiency, declared by the manufacturer. However, as experience has shown, this value was too overestimated.

The volume of air supplied to the aeration tanks of the nutrients removal block is regulated by a two-circuit automated system. In the primary circuit, individual for each aerated corridor, the air supply is regulated by the controller with the help of electrified valves in order to maintain the concentration of dissolved oxygen (determined on-line by the oxygen meter) within the set range. In the second circuit, common for the whole unit, the second controller, controlling the flow rate of the regulated HAFI turboblowers by the air pressure sensor signal (within 40-100% of the rated value), maintains the compressed air pressure in the common duct within the specified range. This system is designed to maintain an individual target concentration of dissolved oxygen in various aeration tanks and their zones, providing energy savings for aeration. The second circuit was working routinely from 2006 to the first half of 2013.

Fig. 2. Layout of the aeration tanks of the nutrients removal block

In the course of operation, shortly after the launch of the nutrients removal block, a progressive decrease in the capacity of aerators was recorded, which affected the concentration of dissolved oxygen in the aeration tanks. During the examinations, intensive biological fouling of the aerator membranes, as well as the deposition of hardness salts on the membrane surface was detected.

Fig. 3. AQUASTRIP aerator arrangement.  aeration tank is emptied for cleaning the aerators

By 2009, an examination of the aeration system showed that all the membranes of the aerators as a result of stretching acquired a residual curved deformation. A high degree of biofouling (colmatation) of the membrane surface was noted. The membranes in several aerators had tears of different degrees (Fig. 4). Due to the above reasons, the effective area of   the membranes of aerators was reduced by 25-40%.

Fig. 4. AQUASTRIP aerator’s membrane defects

To eliminate these defects, we shook membranes as recommended by the manufacturer, with a brief complete shutdown of the air supply - a simple, but, as it turned out, ineffective method. Manual mechanical cleaning of the aerators (Fig. 5) made it possible to remove part of the fouling. After purification, the intensity of aeration increased at once, but the effect did not last long.

Fig. 5. Manual cleaning of the AQUASTRIP aerator’s membranes

We conducted local experiments of chemical cleaning of the membranes of aerators using acetic acid. This treatment produced a pronounced positive effect. However, its implementation required a complex system of dosing and supplying the acid into the ducts. At the same time, it was necessary to replace the elements of the air-supply system using only acid-resistant materials. This method was recognized as unpromising, as it required both significant capital and operational costs.

The problems of operating the aeration system showed up mainly in summer. Thus, in summer 2010, in hot weather, the concentration of dissolved oxygen in the third corridors (the main aeration zone) of the aeration tanks of the nutrients removal block decreased to 0.1-0.3 mg/l. As a result, the capacity of the block in 2010 was significantly lower than the design one: the operation services tried not to supply more than 400-420 thousand m³/day of sewage to the block, i.e., not more than 80-85% of the design load.

Due to the high wear and reduced efficiency of the AQUASTRIP aerators in 2011, the Mosvodokanal has completely replaced the aerators in all the aeration tanks of the nutrients removal block (the design of this system did not in principle provide for the replacement of the dispersant only). The aeration system with AQUA-TOR (AR-420) disk aerators by Ecopolymer-M CJSC (Fig. 6) was installed. A distinctive feature of the AQUA-TOR aerators is the original toroidal structure, which creates circulation of the sludge mixture both outside and inside the aerator (through the central hole). This should prevent coalescence of air bubbles, which is the reason for the decrease in the efficiency of oxygen dissolution.

Fig. 6. AQUA-TOR aerators in the aeration tanks at the Lyubertsy wastewater treatment plant of Mosvodokanal JSC

Mosvodokanal JSC has accumulated a vast positive experience in operating AQUA-TOR aerators: as of November 2014, they are installed in aeration tanks with a total capacity of about 4,200 thousand m³/day of sewage. The total length of the air ducts is about 115 km, the total number of disks is over 140 thousand. The main characteristics of the AQUA-TOR aerators are given in Table 1.

Table 1

Parameter	Value
Aerator outer diameter, mm	420
Aerator inner diameter, mm	170
Aerating surface area, m2	ca. 0.115
Work airflow per aerator, m3/h	6–20
Optimum airflow per aerator, m3/h	10–12
Initial resistance to airflow (at 10 m3/h),  mH2O	0.24 ± 5%

 

Each aeration tank was equipped with 3325 disks (including 2338 in the third corridor, 987 in the fourth corridor), the total area of   the aerating surface was 382 m². When choosing the number of discs, the maintenance service took into account a significant increase in the load of oxidized contaminants on the block from the startup up to 50% and the necessary reserve to ensure operation under the conditions of the would-be implemented sediment acidification technique in the primary sedimentation (which would lead to an increase in the organic load). The introduction of a new aeration system allowed completely avoiding the limitation on oxidizing power; at certain periods, air was supplied to the block up to 25% above the designed load without deterioration in the treatment quality. The previously observed oxygen deficiency in aeration tanks, when the control valves were 100% opened by air, was eliminated. The generalized average annual operating data of the nutrients removal block of the Lubertsy wastewater treatment plant are given in Table 2 [2].

Table 2

Parameter, mg/l	2009	2010	2011	2012	2013	Average for 5 years	Design target	Efficiency, %
Suspended solids	86/5.2	108/6.2	98/6.2	113/6.5	139/6.4	109/6.1	–/8
BOD5	89/1.5–2.2	119/1.5-2.2	120/1.5–2.2	128/1.5-2.2	136/1.5–2.2	118/1.8	–/4
COD	278/31.9	325/26.6	318/27.9	323/32.8	361/34.4	321/30.7	–/30
Ntot (calc.)	30.5/8.2	–/8.9	40/10	41.8/11.8	43.3/9.9	38.3/9.7	30/*	75
N–NH4	24.4	28.7/1.37	32/1.13	33.4/0.89	34.6/0.9	30.6/0.96	25/1
N–NO2	–/0.03	–/0.04	–/0.05	–/0.07	–/0.21	–/0.08	–/0.02
N–NO3	–/7.1	–/7	–/8.3	–/10.3	–/8.3	–/8.2	–/9.1
Ptot	4.8/1	5.5/0.6	5.5/1.6	5.5/1.3	5.5/1.2	5.4/1.1	–/*	78
P–РО4	2.5/0.8	3/0.4	3.2/1.2	3.5/0.9	3/0.8	3/0.83	3.3/0.9
Note. The numerator - water supplied for treatment, the denominator - treated water. * Non-rated.

 

Information on the actual efficiency of aeration systems is important both for the analysis of operational parameters of aeration tanks, and for evaluation with further planning of works on the reconstruction of aeration systems.

Mosvodokanal JSC and other water service companies for this purpose use assessment of either process or power efficiency of aeration - air flow (or power consumption) per 1 m³ of wastewater being treated or 1 kg of incoming (or removed) BOD5. This approach has long been obsolete, since in the first case it does not take into account the concentration of pollutants, in the second - the amount of oxygen consumed to remove nitrogen, in both - the depth of nitrogen removal. In all cases, the sufficiency of air supply is not considered. For modern technologies, Mosvodokanal has offered to attribute power consumption to removed nitrogen, but this approach also obviously does not take into account a significant part of power consumption for BOD removal.

Indirect assessments are also used - a comparison with the supply of air to the same lines before and after the replacement of the aeration systems. This approach yields little information too, since it does not take due account of the load, the quality of treatment, and the presence (or absence) of air limitation.

Since the late 1990s, the engineering and technology center of Mosvodokanal has mastered and periodically used for scientific purposes and during commissioning works the method of exhaust gases ("gas cap") developed by the SRC "NII VODGEO" and described below. The method is based on a direct measurement of the oxygen concentration in the exhaust gases of the aeration tank. It provides accurate and objective information about the effectiveness of the aeration system, but has the following drawbacks (rather, limitations): it requires the use of several measuring instruments; the procedure for working with the cap is quit laborious; the collected information refers only to the sites studied and to a certain point in time.

The latter circumstance is highly important, since this limitation prevents from considering the unevenness of the contamination input and, accordingly, the consumption of oxygen during the day, as well as from assessing a considerable period of time. For this reason, the "gas cap" method more specifically characterizes the performance of the air dispersers and finds practical application, as a rule, during acceptation of new aeration systems and commissioning.

A more accessible and informative alternative may be an improved method of mass balance of oxygen and oxygen-oxidizable substances in the aeration-sedimentation tank system. It is based on a complete account of the actual dissolution of oxygen in the structure and the correlation of this amount with the amount of oxygen in the supplied air. That is, this approach is based on the calculation of the amount of useful oxygen consumed and refers to the entire aeration system - it generalizes the consumption throughout the plant as a whole and for the whole period of time taken. It also considers the overflow of air relative to the optimal demand, both as a result of imperfection of air distribution in   the aeration tank, and the lack (inadequacy) of flow regulation by the hour.

Thus, the mass balance approach that allows evaluating the aeration system in general is of the greatest interest for addressing the problems of current operation.

The efficiency of using air oxygen is calculated as the ratio of the mass of dissolved oxygen to the mass of supplied oxygen:

where M_O2 – useful oxygen consumed (including the saturation of the sludge mixture at the aeration tank outlet), kg/day; W_air - the actual volume of the supplied air (at atmospheric pressure), m³/day; S_O2  – mass concentration of oxygen in the air, kg/m³.

The obtained efficiency will correspond to actual conditions for the period of the measurements (temperature of the sludge mixture and air, atmospheric pressure, oxygen content). The reduction of the Э value to the standard conditions is described below.

The mass of dissolved oxygen corresponds to its consumption for wastewater treatment processes (the sum of the consumption for the processes of C-oxidation and N-oxidation) and the mass of dissolved oxygen in the sludge mixture after the aeration tanks. It is also necessary to take into account the flow of oxygen into the aeration tanks with sewage:

M_O2 = M_O2^COD + M_O2^N – M_O2^sew + M_O2^ex.

The oxygen consumption for the oxidation of organic contaminants is calculated on the basis of the following mass balance equations for COD:

M_COD^en = M_O2^COD + M_COD^eas + M_COD^ex,

where M_COD^en – the mass of COD in incoming sewage (hereinafter in mass balance description - in kg); M_O2^COD is the mass of COD subjected to oxidation during biological treatment (i.e., corresponding to the actual consumption of oxygen for oxidation of organic and other substances defined as COD); M_COD^eas – the mass of COD in excess biological sludge discharged from the system; M_COD^ex – the mass of COD in treated water.

M_O2^COD = M_COD^en – M_COD^eas – M_COD^ex.

For nitrogen, the balance of oxygen consumption is determined on the basis of the following equations (similar to ATV-131 [3]):

M_O2^N = M_O2^nitri – M_O2^deni;

M_N^nitri = 4,3(M_N–NO3^D + M_N–NO3^ex);

M_N^deni = 2,9M_N–NO3^D;

M_N–NO3^D = M_Ntot^en – M_Norg^ex – M_N–NH4^ex – M_N–NO3^ex – M_Norg^eas,

where M_O2^N – the mass of oxygen consumed for the oxidation of nitrogen; M_O2^nitri – the mass of oxygen consumed for nitrification; M_O2^deni – is the mass of oxygen of nitrates used to oxidize organic substances during denitrification (therefore, subtracted from oxygen consumption for nitrogen oxidation); M_N–NO3^ex – the mass of nitrogen of nitrates in treated water; M_N–NO3^D – the mass of nitrogen of nitrates subjected to denitrification; M_Ntot^en – mass of total nitrogen in the incoming wastewater; M_Norg^ex –  the mass of organic nitrogen in treated water; M_N–NH4^ex –  the mass of ammonium nitrogen in treated water; M_Norg^eas – the mass of organic nitrogen in excess biological sludge.

Then:

M_O2^N = 4.3(M_Ntot^en – M_Norg^ex – M_N–NH4^ex – M_N–NO3^ex – M_Norg^eas + M_N–NO3^ex) – 2.9(M_Ntot^en – M_Norg^ex – M_N–NH4^ex – M_Norg^eas);

M_O2^N = 1.4(M_Ntot^en – M_Norg^ex – M_N–NH4^ex – M_Norg^eas) – 2.9 M_N–NO3^ex.

The intake of nitrogen in the form of nitrates and nitrites in wastewater can be taken as insignificant.

For the balance components, such as COD and nitrogen in biological sludge, in the event of significant changes in the sludge mass in the aeration tanks (as a result of changes in the sludge dose and/or the number of operating aeration tanks), the accumulated (reduced) amount of sludge should be added (subtracted) to the amount of excess biological sludge deduced for the total time period for which the balance is calculated. If not all days of the time interval are used in the calculation, but only those when the detailed process control was carried out, the change in sludge mass should be made in proportion to the ratio of the number of such days to the total duration of the time interval.

The implementation of the methodology at the Lyubertsy wastewater treatment plant in Moscow did not require any additions to the established list of process control, but to measure the dissolved oxygen content in the incoming water (substantially saturated in the spillway of the distribution chamber) and organic nitrogen in treated water. The specific content of COD and nitrogen in excess biological sludge was taken from literature data, the meteorological parameters (to bring the parameters of the supplied air to standard conditions) were according to the Hydrometeorological Service.

The measurements were conducted in September 2013 for 23 days, where 17 days were for sampling and analyzing. Thus, data for these 17 days are used in the calculation.

The whole nutrients removal block (four aeration tanks and seven secondary sedimentation tanks) was the test object. The wastewater flow to the block was maintained at about 470,000 m³/day (at a design flow rate of 500,000 m³/day) and corresponded to the established operational practice, based on provision of the concentration of ammonium nitrogen in treated water in accordance with the process regulations of the facility. The performed analyses and the time of measurements are presented in Table 3.

Table 3

Parameter	Sample and time of measurement
aeration tank incoming sewage
Flow rate	Hourly average
Temperature	One-time, daily
COD	Hourly sampling for the cumulative sample*. Analyzed as a daily average
Total nitrogen
Dissolved oxygen concentration
Snap sampling, daily
Supplied air
Flow rate	Hourly average
Outdoor air temperature	Hourly average
Sludge mixture at the aeration tank outlet
Dissolved oxygen concentration	Two times in the measurement period every 24 hours, as well as according to the sensor readings (obtaining temperature information from the sensors is recommended)
Sludge dry matter	Daily
Excess biological sludge
Flow rate	Daily total
Dry matter	Three times per day
Treated wastewater
COD	Hourly sampling for the cumulative sample*. Analyzed as a daily average (shaken sample)
Ammonia nitrogen
Nitrate nitrogen
Organic nitrogen
* Due to the nature of water supply to the nutrients removal block, its hourly variation coefficient is negligibly low and was not taken into account when sampling. In other cases, snap samples should be taken (or taken into account in the cumulative sample) in a volume proportional to the inflow in the given hour.

Hourly sampling for the cumulative sample*. Analyzed as a daily average

Hourly sampling for the cumulative sample*. Analyzed as a daily average (shaken sample)

Hourly sampling for the cumulative sample*. Analyzed as a daily average (shaken sample)

Hourly sampling for the cumulative sample*. Analyzed as a daily average (shaken sample)

Despite the obviousness of the approaches used in the development of this technique, it was applied in Russia for the first time. Abroad, calculations based on mass balance are widely used at US and UK facilities. It is also known about the application of this technique for the purposes of benchmarking of wastewater treatment plant [4]. However, there is no any detailed description of the methodology, which allows its reproduction.

The main quantitative results of approbation of the technique on the nutrients removal block of the Lyubertsy wastewater treatment plant are given below. A detailed presentation of the specific results of the calculation is made for the convenience of the application of the procedure by specialists from other wastewater treatment plant.

The calculations were made for each day of measurements; the results of the mass balance were summed up. The average and (or) total results of instrumental management of flow rates are given in Table 4, the results of analytical control of incoming and treated water - in Table 5.

Table 4

Parameter	Daily average	During the experiment*
Supplied wastewater volume, thousand m3	488	8304
Excess biological sludge flow rate, thousand m3	8.4	142
Sludge mixture temperature, °С	23.5	–
Supplied air flow rate, thousand m3	1574	26,760
* Only the observation days accompanied with the tests were taken into account (i.e., working days).

 

Table 5

Parameter	Daily average	During the experiment*
Incoming sewage
COD, mg/l MCODen, t	373 –	– 3085
Total nitrogen, mg/l MNtoten, t	41 –	– 341
Dissolved oxygen, mg/l MO2sew, t	3 –	– 25
Treated water
COD, mg/l MCODex, t	36.6 –	– 302
Ammonia nitrogen, mg/l MN–NH4ex, t	0.49 –	– 4.1
Nitrate nitrogen, mg/l MN–NO3ex, t	7.8 –	– 64
Organic nitrogen, mg/l MNоргex, t	1.9 –	– 16.2
Dissolved oxygen (sludge mixture), mg/l MO2ex, t	6.1 –	– 68*
Mass of excess biological sludge discharged, t		1305
Mass of biological sludge accumulated in the aeration-sedimentation tank system for the days of sampling, t		76
Mass of biological sludge discharged with treated water, t		49
The total mass of the sludge formed during the measurement period, t		1430
Ash content in biological sludge, %		30.4
Specific COD of ashless substance [5], g/g		1.42
COD mass in the formed sludge, t		1456
Specific nitrogen in the formed sludge [5], g/g		0.075
Nitrogen mass in the formed sludge, t		74
Calculation results of oxygen consumption for treatment processes
COD mass, subjected to oxidation during biological treatment, MO2COD, t		1447
Nitrogen mass, subjected to oxidation during biological treatment, MO2N, t		535
Dissolved oxygen mass, MO2, t		1866
* Subject to the consumption of recycled sludge.

The flow rate of the supply air in the nutrients removal block at the inlet of each of the two independently controlled aeration zones located respectively in the third and fourth corridors of each of the four aeration tanks (Fig. 2). Data on the average flow rates over the days of measurement, as well as on overpressure are given in Table 6.

Table 6

Aeration tank	Aeration zone inlet overpressure, kPa		Aeration zone inlet air flow rate, thousand m3/day
	corridor 3	corridor 4	corridor 3	corridor 4
No.1	58	60.3	217	190
№ 2	59.7	59.8	211	105
No. 3	59.4	58.8	274	147
No. 4	59.8	58.9	234	177
Average	59.2	59.4	234	155

Aeration tank

The initial data were converted to normal (standard) conditions with respect to the assessment criteria for the aeration systems (T_n.c. = 20 °C = 293.15 K, P= 760 mm Hg = 1.013 bar) and also for air humidity 0% using the known conversion formulas.

The excess pressure in the ducts was transferred to absolute pressure, subject to the atmospheric pressure, taken from the data of meteorological observations (in kPa too).

the product of the pressure of saturated air vapors of the relative humidity of air was previously subtracted from the value of atmospheric pressure:

P_dry = P_meas – 0.01R_hP_sat,

where P_izm is the measured atmospheric pressure, kPa; R_h - relative air humidity, fraction; P_sat - the pressure of saturated water vapor at a given temperature, kPa.

The volume of supplied air was reduced to normal conditions by the formula following from the integral gas law:

The values   of the temperature of the air supplied to the aeration tanks are calculated according to the temperature of the atmospheric air, corrected for its heating due to compression based on the results of actual measurements carried out prior to the experiment by means of a temperature probe screwed into a special nozzle for each duct branch. The correction values   were 15 °C for the aeration tank closest to the main engine building of the plant, and 12 °C for the furthest one.

Thus, the average load per aerator (reduced to normal conditions) during the measurement period was: in the third corridor - 6.4 m³/h, in the fourth corridor - 10.3 m³/h (according to the manufacturer's recommendation, 20 m³/h, optimal - 10-12 m³/h).

The oxygen content in the air, equal to 0.287 kg/m³, was taken from the data of meteorological observations as average for September 2013 (subject to the reduction to normal conditions).

The actual oxygen transfer efficiency (OTE) was:

To verify the proposed calculation technique, simultaneously with the collection and processing of data for the implementation of the mass balance technique, direct measurements of the air oxygen transfer efficiency were carried out using the "gas cap" method. It must be noted that these measurements and the subsequent reduction of their results to the standard values   of oxygen transfer efficiency are not included in the methodology proposed for the implementation in the framework of the production analysis of the wastewater treatment plant operation. They were carried out solely for the approbation purpose.

Fig. 7. Gas collector

The gas collector is a metal rectangular cap equipped with floats with internal dimensions of 1200x1200x460 mm with a central outlet branch pipe and a hose (Fig. 7). The gas collector was held at a given point of the aeration tank surface and moved by means of cables. The hose of the gas collector was attached to a stand mounted with the parameter recording instruments (Fig. 8).

Fig. 8. A stand with measuring instruments.

From left to right – Testo speedometer, Alfa-Bassens AKPM oxygen meter (air oxygen), WTW dissolved-oxygen meter (dissolved oxygen).

Oxygen transfer efficiency was determined on the basis of a quantitative measurement of the oxygen and carbon dioxide content in the incoming and exhaust gases of the aeration tanks. To record the amount of oxygen in gases, an oxygen meter with an AKPM-01 sensor (Alfa-Bassens, Russia) was used.

The concentration of carbon dioxide in the air and in the gas discharged from the aeration tank was measured in experiments conducted in parallel with the oxygen transfer efficiency. The concentration of CO2 was determined by gravimetric method using absorption tubes; as an absorber, anhydrone (water vapor absorption) and ascarite (CO₂ absorption) were used.

Oxygen transfer efficiency (%) was calculated by formula:

where C – the calculated concentration of oxygen in the exhaust gas, corrected for the change in volume (see below); C₀ – the concentration of oxygen in the air.

The calculation of oxygen transfer efficiency considered the change in the volume of the gas discharged from the aeration tank with respect to the volume of air supplied for aeration, due to the absorption of oxygen by the biological sludge and saturation of the gas mixture with carbon dioxide and water vapor. The calculated concentration of oxygen in the exhaust gas is:

С = С_xK;

K = V/V₀,

where С_х – the measured oxygen concentration in the exhaust gas (according to the oxygen meter readings); K – a correction for volume changes; V₀ – the volume of air supplied to the aeration tank; V – the volume of air discharged from the aeration tank.

The ratio of the volumes of air supplied to and the gas discharged from the aeration tank was calculated by formula:

i.e., the volume of air discharged from the aeration tank is equal to the volume of air supplied to the aeration tank, minus the volume decrease due to O₂ uptake, plus the increase due to CO₂ saturation, plus the increase due to steam saturation, where C_x – the measured oxygen concentration in the exhaust gas (according to the oxygen meter readings), %; С_СО2^х – the concentration of СО₂ in the exhaust gas, %; С_СО2^в – concentration of СО₂ in air, %; С_Н2О – the concentration of water vapor in the exhaust gas, %; С_Н2О^в – the concentration of water vapor in air, %.

Since the concentration of carbon dioxide in the air is 0.03%, the subsequent calculations can ignore this value, and the correction for the change in the gas volume due to CO₂ is:

The partial pressure of water vapor in the air discharged from the aeration tank at a humidity of 100% and a temperature of 25-30 °C is approximately 2.3%.

Therefore:

The gas cap measurements were also to obtain data on the effect of horizontal flow velocity in the "carousel" of the aeration tank on oxygen transfer efficiency. To this end, in September 2013, simultaneously with the measurements as per the above described method, a series of direct measurements of efficiency were performed at different values   of the horizontal flow velocity. Table 7 shows the results of measurements for the regulatory flow rate in the carousel (about 0.28 m/s by average value) and in the absence of horizontal velocity (achieved by temporary disabling of the mixers in the “carousel”).

Table 5

Measurement No.	Oxygen transfer efficiency, %
	with horizontal flow	without horizontal flow
1	23.7	23.5
2	22.8	22
3	22.6	20.8
4	21.7	21.1
5	23.2	21.8
6	22.9	21
Average	22.8	21.7

Measurement No.

To compare the results of oxygen transfer efficiency obtained by different methods and under different conditions, the data should be reduced to normal (standard) conditions. For the supply air, this transition has already been made. With respect to the dissolution conditions in the sludge mixture under the conditions considered, the actual temperature of the sludge mixture should be taken into account; actual oxygen concentration (normal conditions impose complete oxygen deficiency); speed mode (added to standard procedures based on research, some of which are given in this paper).

In the foreign literature this relationship is expressed by the equation:

AOTE = T_t, T_d, T_p, T_v SOTE,

where AOTE – actual oxygen transfer efficiency; SOTE – standard oxygen transfer efficiency; T_t, T_d, T_p, T_v – corrections for temperature, oxygen concentration, atmospheric pressure and flow rate of the sludge mixture, respectively.

The desired SOTE is determined as follows:

The temperature correction for increase in the dissolution rate of oxygen with increasing temperature is taken into account by the equation:

T_t = 1.024^{t – 20}.

For average temperature of the sludge mixture, 23.5 °C: T_t = 1.087.

The real oxygen deficiency is taken into account by the formula:

where C_s – saturation concentration in real conditions, depending on the salt content (not taken in the present calculation), temperature, atmospheric pressure, depth of the aerator; C_x – the oxygen content in the sludge mixture; 9.09 – saturation concentration of oxygen under normal conditions.

The experimental saturation oxygen concentration is 8.61 mg/l at an average temperature of 23.5 °C.

A downward correction for pressure T_p (the Lyubertsy wastewater treatment plant is located around 140 m above sea level) is defined as the ratio of the average pressure (745 mm) over the measurement period to a pressure of 760 mm (normal conditions) and is equal to 0.98. Then, the saturation concentration in conditions other than normal will be C_t = 8.44.

The influence of the aerator depth (5.6 m) is taken into account by the formula [6]:

where K_H – actual pressure coefficient; K_H0 – normal conditions coefficient.

For calculation by the developed technique, C_x was determined on-line based on the oxygen meter readings, and subject to the oxygen concentration at the aeration tank outlet as a weighted average of air flow rate into two aerated corridors of the aeration tanks. At an average concentration of 2.3 mg/l in the third corridor, 4.5 mg/l in the fourth corridor and an air flow distribution of 0.6:0.4, the weighted average C_x was 3.2 mg/l. For data obtained by direct measurements, the dissolved oxygen concentration is determined by direct measurement with a portable instrument, and is equal to 1 mg/l.

The coefficient of transition from pure to waste water T_p in the literature is usually expressed as the α·β product (α is the coefficient based on the content of organic impurities, β is the saturation coefficient determined by the concentration of dissolved salts). As is known, the value of the coefficient α varies in an extremely wide range - from 0.25 (in sludge mixture with initial wastewater) to 0.9 (at the outlet of the plant). The presence of recycles in the corridor aeration tank significantly reduces the coefficient gradient along the length. The values   of these coefficients were experimentally non-measurable. To measure them, the tests for measuring the aeration efficiency in the same plant must be conducted using the classical method both in clean and in wastewater. In this study, to reduce the results of the calculated and direct measurements of efficiency to the same conditions, the values   of the coefficients α and β were taken from the literature (subject to the contamination level of the liquid phase in the studied corridors). For the calculation method, α was taken as a weighted average for the third and fourth corridors.

The developed technique also allowed us to process the available operational data for the nutrients removal block at the Lyubertsy WWTP for 2008 for the period when AQUASTRIP aerators were used (Table 8).

Table 8

Parameter	AQUA-TOR (2013)		AQUASTRIP (2008), calculation method
	for the calculation method	for direct “gas cap” measurement
Actual oxygen transfer efficiency (AOTE)
16.5	22.8	11.8
Temperature correction	1.087	1.087	1.024
Oxygen deficiency coefficient	0.59	0.82	0.67
Velocity correction	1.03	1.054	1.03
Coefficient α:    in corridor 3    in corridor 4    weighted average	0.7 0.85 0.77	0.7 – 0.7	0.7 0.85 0.76
Coefficient β	0.95
Standard oxygen transfer efficiency SOTE, reduced to depth of 6 m, %	34.9	37.8	24.3
Specific efficiency of SOTE (SSOTE), %/m	5.8	6.3	4.05
The discrepancy between the values   of SSOTE in two techniques, %	8.5		-

Parameter

AQUASTRIP (2008), calculation method

 

Thus, the simultaneously determined oxygen transfer efficiency has shown a fairly good convergence of the methods. The actual efficiency of oxygen transfer for the AQUA-TOR aerators was 30% higher than for the Austrian AQUASTRIP system.

It is interesting to compare the results with the available bench data for the standard oxygen transfer efficiency SOTE. Fig. 9 shows the results of testing the AQUA-TOR aerators (AP-420) in the Spanish test laboratory ATC. According to these data, SSOTE with loads on the disk, which took place during measurements in September 2013, is: 6.8%/m at 6.4 m³/h and 6.2%/m at 10.3 m³/h (unfortunately, measurements were made with the aerators submersed to a depth of only 2.75 m).

Fig. 9. AQUA-TOR aerator test certificate

The specific flow rate weighted average efficiency SSOTE for the loads during measurements at the Lyubertsy wastewater treatment plant was 6.44%/m, which corresponds to efficiency SOTE at a depth of 6 m, equal to 38.6%. This value differs by 2% only from the estimate obtained from the direct measurement by the "gas cap" method (as already mentioned, since the values of the coefficients α and β were not measured, but assigned, it is correct to speak about the estimation of SOTE).

It is important to estimate the specific power consumption based on the results of the experiment in 2013. The calculation of this value using the data of the operation service of the Lyubertsy wastewater treatment plant for September 2013 is given in Table. 9. Calculation of these parameters (except for the latter - the aeration power efficiency of under standard conditions) should also be included in the aeration efficiency analyzing technique.

Table 9

Parameter	Value
Electricity consumption for air supply to aeration tanks, thousand kWh/day	48.6
Wastewater flow rate in measurement days, thousand m3/day	488
Air flow rate in the aeration tanks in measurement days, thousand m3/day*	1574
Effective oxygen flow rate, t/day	111
Specific electricity consumption:    for wastewater treatment, kW·h/m3    for air supply, kW·h/1000 m3    for oxygen dissolution, kW·h/kg	0.1 31 0.44
Actual aeration power efficiency during the measurement period (September 2013), kg/(kWh)	2.3
Actual aeration power efficiency under standard conditions (SAE)**, kg/(kWh)	4.8
* Measured value at actual pressure. ** Reduced to standard conditions according to Table 9.

 

Summary

1. A detailed technique has been developed for assessing the actual efficiency of aeration systems in aeration tanks of wastewater treatment plant, based on known regularities of mass balance processes. This technique allows calculating the actual transfer (dissolution) efficiency of air oxygen during any interval of time and with reference to any number of aeration tanks according to actual operating data. A special feature of this method is an entire consideration of all factors that affect the actual need for structures in oxygen. The technique is based only on data from the analysis of the quality of incoming and treated water and measurement of the main flow rates of water, air, and sludge and does not require significant changes in the process control system.

2. This technique was tested at wastewater biological treatment facilities with the removal of nitrogen and phosphorus with a capacity of 500 thousand m³/day in comparison with the classical method of direct measurement in exhaust gases (a “gas cap” method). The discrepancy in the results (8.5%) corresponds to the accuracy of measurements of the main components of the mass balance.

3. The developed technique can be used both in evaluating the aeration system efficiency for the certain wastewater treatment plant and in making the intra-sectoral analysis (benchmarking). The calculated values of the actual atmospheric oxygen transfer efficiency and power consumption per 1 kilogram of the actually transferred oxygen are recommended for being used as the target indicators of the wastewater treatment plant' improvement.

4. The measurements, carried out both by the mass balance method and by the direct method of exhaust gases, showed high efficiency of the operation of the aeration system using the AQUA-TOR aerators (AR-420T). The oxygen dissolution efficiency corresponds to both the best world analogs and information provided by the manufacturer (Ecopolymer-M CJSC). The results of direct measurements of oxygen transfer efficiency (reduced to standard conditions) practically coincided with the data obtained during the tests of the AQUA-TOR aerator in a test laboratory in Spain.

5. Replacement of the Austrian AQUASTRIP aerators, which has not proved themselves in operation, with the AQUA-TOR aerators, which earlier showed positive results in other blocks of the Moscow wastewater treatment plants, ensured increase in the efficiency of oxygen transfer by 30%, enhanced the reliability of the block cleaning about 500 thousand m³/day of wastewater.

The author expresses his gratitude to the employees of Mosvodokanal S.A. Streltsov, N.A. Belov, S.N. Novikov, G.E. Khamidov, M.V. Kevbrina, and A.V. Akmentina for organizing the field measurements, and also to the team of Ecopolymer-M CJSC for the information provided.

REFERENCES

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