CS251844B1 - Method of wastewater treatment by intensification of biological flotation process - Google Patents

Abstract

The method of wastewater treatment by intensification of the biological flotation process is carried out by adding an alkaline salt, preferably calcium hydroxide, to the continuously stirred sludge suspension in the flocculation unit of the bioreactor and adjusting the pH of the suspension to a value of 8.5 to 10.0. The waste heat of the digestion units is used to heat the sludge, and to supplement the organic carbon, sludge water from the methanation stage can be added to the sludge suspension in an amount of 1 to 25% by volume. The essence of the process is the enzymatic activity of denitrifying bacteria, which are present in the mixed culture of activated sludge.

Description

The invention relates to a method of wastewater treatment by intensification of the biological flotation process, namely by increased activation of the denitrification activity of the mixed cell population.

The main factors in the treatment process are the high effect of wastewater treatment and the disposal or use of excess activated sludge with minimal costs and energy requirements.

One of the intensive methods of coalification of sewage sludge is the biological flotation process (author's certificate No. 228 403). This method of thickening biological, especially activated sludge by changing their physical structure consists in adding oxygenated nitrogen compounds, preferably nitrates or nitrites, to the sludge in an amount of 50 to 2,000 gm ^- ^ at a temperature of 5 to 45 °C and at a pH of 5 to 8.5 under anaerobic conditions. The change in the physical structure also allows for easier drainage of the sludge.

The essence of the process is the enzymatic activity of denitrifying bacteria, which are present in the mixed culture of activated sludge. These facultative anaerobic bacteria use nitrates as electron acceptors in the process of nitrate respiration in the absence of molecular oxygen while simultaneously utilizing organic substrate. The gases produced, especially gaseous nitrogen, are the cause of the flotation process.

The conditions for high efficiency of the biological flotation process are, in particular, suitable temperature and oxygen conditions, an excess of NO2 ions, which are dosed into the system, and ensuring high mass transfer between the individual phases.

The produced gas forms microscopic bubbles with a diameter of 10 to 220 µm, which adhere to the surface of the flakes or are part of their internal structure.

The amount of gas produced by the bacteria in each floc should be sufficient to float it. Gas bubbles should not be released from the particles during the process, as they cause disturbances in the flotation process due to high upward velocity, combined surface tension, etc. For these reasons, the presence of trace amounts of heteropolar substances (residual oils, surfactants, etc.) is appropriate, which reduce the wettability of the activated sludge flocs.

The process of bubble dispersion formation during the denitrification process can be influenced by changing the conditions in the bioreactor. These factors, which can significantly affect the denitrification enzyme efficiency and the amount and composition of denitrification gases, include, in particular, the pH of the environment, temperature and suspension mixing.

The optimum pH range for the denitrification process is reported to be 6.5 to 7.5, with pH 6 to 8 achieving 70% efficiency (Moore, S.F. and Schroeder, E.D., Water Has., 5, 452-5, 1971).

Another factor that significantly affects the rate of the denitrification reaction is temperature. The denitrification process occurs at temperatures from 0 to 50 °C. The increase in the rate of enzyme reactions with increasing temperature can be expressed by the temperature quotient O, _o , i.e. the ratio of the reaction rate at a certain temperature to their rate at a temperature 10 C lower. For denitrifying microorganisms, this quotient ranges from 1.5 to 2.0 (Dawson, RN and Murphy, KL: Adv. in Water Pollut. Res., 6, 671-83, 1972).

However, the reaction rate of enzyme reactions increases according to O,q only up to a certain temperature. After exceeding the temperature, when the rate of the catalyzed reaction reaches a maximum, a reversal occurs and the enzyme activity decreases very rapidly due to thermal inactivation of the protein component of the enzymes.

The reaction rate of the enzymatic denitrification reaction is further influenced by the amount of mass transfer between the liquid medium and the nitrate and biomass flakes present in the suspended phase. The mass flow intensity depends on the physical properties of the system, the concentration distribution, the hydrodynamic conditions of the process and especially the size of the mesiphase surface. In an effort to intensify the biological flotation process, obtain a microbial suspension with a high specific denitrification efficiency and achieve greater effluent pollution, optimal process conditions were found.

At the same time, it is not known that the thickening process is used simultaneously to remove dissolved substances from wastewater.

The subject of the invention is a method of wastewater treatment by intensification of the biological flotation process, the essence of which is that an alkaline compound, preferably calcium hydroxide, is added to a continuously stirred sludge suspension in a flocculation unit of a bioreactor and the pH of the suspension is adjusted to a value of 8.5 to 10.0.

Waste heat from the digesting units can be used to heat the sludge suspension, and to replenish organic carbon, sludge water from the methanation stage can be added to the sludge suspension in an amount of 1 to 25% by volume.

It was found that changing the pH significantly affects the rate of the denitrification reaction.

The rate of conversion of nitrate to nitrogen gas depends on the concentration of nitrite in the reaction mixture. The above nitrite concentrations in the environment have a feedback inhibitory effect on the activity of the first enzyme in the denitrification process - nitrate reductase, and thus significantly inhibit the conversion of nitrate.

The accumulation of nitrites in the environment is caused by the lower activity of the second enzyme in the nitrite reductase chain in the more acidic pH range (pH<7.2). As the pH decreases, the percentage of NgO in the denitrification off-gas increases significantly, as it is extremely soluble in water and therefore does not form a bubble dispersion suitable for the flotation process, thus reducing the separation effect.

It was found that by increasing the pH to pH 10, the denitrification process can be accelerated and thus the residence time of the suspension in the bioreactor can be shortened by more than 50%, while simultaneously utilising the organic substrate more intensively.

Increasing the pH also leads to the accumulation of phosphorus in the suspended phase, thereby achieving a significant reduction in the content of inorganic phosphates in the effluent. The principle of eliminating organic phosphates from the biological suspension of sewage sludge is based on their precipitation into insoluble forms with calcium ions, which are captured in biological flocs. In this reaction, calcium ions supplied by the flotation agent, i.e. calcium nitrate, can be used, or calcium ions present in the wastewater can be used.

The removal of inorganic phosphate from wastewater is also influenced by the ratio of the organic carbon and nitrogen source to phosphorus. When nitrogen is limited, phosphate accumulation also occurs. The lack of a macronutrient source can be addressed by adding sludge water. The accumulated phosphate can also serve as a source of phosphorus for cell growth in the absence of external phosphate. It was found that by increasing the temperature from 25 °C to 35 °C using a mixed culture of activated sludge, it is possible to achieve = 2.1 and with a simultaneous increase in pH to 8.5 O _1Q * 2.4.

Controlled mixing of the suspension in the bioreactor significantly increases the interfacial surface and thus the mass transfer necessary for the rapid course of the enzymatic reaction, without releasing gas bubbles from the flocs formed during the denitrification process. Controlled mixing was carried out on a model device using a slow-speed mixer (10 to 20 min ¹ ) and on a pilot-scale model using a static mixer. An increase in the specific denitrification rate of 200% was achieved compared to an unmixed system.

Biflotation treatment is accompanied by a reduction of organic pollution and thus secondary treatment of the effluent. It was found that the organic pollution of the effluent, indicated by the value of biochemical oxygen demand, is reduced on average by 60%, according to the value of chemical oxygen demand by 40%.

251S44

Organic compounds in wastewater serve as electron donors for the denitrification process and at the same time they are biosorbed in the spatial structure of the sludge.

By intensification of the flotation process, or denitrification process, under the above conditions, there is also a more significant reduction in organic pollution.

Hall:

VL

POJCHSK

BOD ₅ ^In the NOj analytical evaluation of the biological flotation process the following quantities were used= all substances, dimension (gl ^-1 ) = nitrate concentration, dimension (mg.I ^- ') = inorganic phosphate, dimension (mg.l ¹ ), ³ chemical oxygen demand, dimension (mg.l ¹ ) ³ biochemical oxygen demand, dimension (mg.l ¹ ) = specific denitrification rate ^{3 3} initial conc. NO? - residual conc. NO] -i ^J __ ^J (mg.g~ .h ) all substances . Sas

All substances (VL) were determined gravimetrically by drying the sample at 105 °C to constant weight. Spectrophotometric methods were used to determine the concentration of nitrates and phosphates; in the first case by reaction with sodium salicylate, in the second case with molybdenum after reduction with ascorbic acid; chemical oxygen demand (COD), indicating organic water pollution, was carried out by the dichromate method; biochemical oxygen demand (BOD^) by the dilution method. The measurements were carried out according to the recommended methods of Horáková et.al (Methods of chemical analysis of water, ecriptum VŠCHT Prague 1981).

The invention is further illustrated by examples, without being limited thereto. The invention is also connected with the construction of a bioreactor, elements of measurement and regulation of the process, which is being developed for the purpose of intensification of biological denitrification flotation.

Example 1

500 ml of initial return activated sludge from a mechanical-biological wastewater treatment plant with a total solids content of 8.38 g l ¹ , pH 6.95, temperature 18.5 °C and a specific denitrification rate of 17.25 mg.g” ¹ .h” ¹ was used. The sludge suspension was divided into five parts. In the first part, the original pH was maintained, in the second part, the pH was adjusted to 6.0 with concentrated sulfuric acid and in the other parts with 30% sodium hydroxide to pH 7.9; 8.5 and 9.0. In all parts of the sludge suspension, the nitrate content was increased by adding 10% wt. solution of technical calcium nitrate to a concentration of 173.0 mg.] ¹ MO] .

In all proportions, the course of nitrate degradation was monitored for 60 minutes. Table 1 shows the resulting specific denitrification rates for the sludge suspension under the original conditions and at the adjusted pH value. The increase or decrease in the specific denitrification rate is also expressed in wt. % of the specific denitrification rate of the untreated activated sludge.

Table I

6.95 6.00 7.90 8.50 9.0 mg.g ¹ .h ^-1 17.25 15.49 21.71 24.30 25.70 % wt. 100.00 89.80 125.80 140.90 149.00

By increasing the pH, for example, to 9.0, the specific denitrification rate increased by 49.0 wt%.

Example 2

1 sludge suspension from a mechanical-biological wastewater treatment plant with a total substance content of 4.79 gl” ¹ , pH 7.0 and temperature of 25 °C were divided into three parts and the denitrification conditions were further adjusted. In the first part, the original pH and temperature were maintained, in the second part, the pH was maintained at 7.0 and it was placed in a thermostat at a temperature of 35 °C, in the third part, the pH was adjusted to 8.5 with sodium hydroxide and the sludge suspension was also placed in a thermostat at a temperature of 35 °C. The nitrate content was increased in all three cases by the addition of a 10% solution of technical calcium nitrate to 173.24 mg.l” ¹ .

The course of nitrate elimination and organic pollution reduction was monitored at selected time intervals. The results are summarized in Table 11, where the specific denitrification is also shown.

fication rate and decrease in chemical Table 11 oxygen consumption in % by weight, after a reaction time of 105 minutes. pH 7.0 T = 35 °C pH 8.5 T = 35 °C Time Bin pH 7.0 T= 25 °C no; J mg.l COD mg.l ^-1 no; J -1 mg.l COD mg.l" ¹ no; J - -1 mg.l ' COD mg.l 0 173.24 541.0 173.24 54, ,0 173.24 541.0 15 120.00 57.94 57.94 30 91.00 33.24 25.29 45 81.00 362.0 14.71 326.0 8.68 289.0 60 52.94 2.57 1.47 75 51.00 1.47 1 .03 105 31 .03 238.0 1.47 208.0 1.03 169.9 ⁱⁿ N07 3 16.97 35.63 40.62 mg.g“'!» ¹ Reduction COD % wt. 56.01 61.55 68.76

The temperature quotient Q,q can be expressed as the ratio of specific denitrification rates at temperatures differing by .0 °C. By increasing the temperature from 25 °C to 35 °C, the temperature quotient Q _]0 = = 2.10 was obtained.

By increasing the temperature from 25 °C to 35 °C while simultaneously increasing the pH from 7.0 to 8.5, a temperature quotient Q _1O = = 2.39 was obtained.

Example 3

In 200 l of sludge suspension from a mechanical-biological wastewater treatment plant with a total solids content of 5.59 gl” ¹ , inorganic phosphate of 38.8 mg.l” ¹ , pH of 7.43, at a temperature of .7 °C and a specific trification rate of 15.04 mg.g” ¹ .!»“ ^{1 ,} the pH was adjusted to 8.5 with calcium hydroxide and then maintained at a constant value for 120 minutes.

The sludge suspension was dosed with potassium nitrate in a stirred vessel to a nitrate concentration of 100 mg.l. The course of nitrate and inorganic phosphate degradation was monitored at selected time intervals for 120 minutes. The specific denitrification rate and the percentage of removed phosphate were calculated from the obtained results. The specific denitrification rate of 21.57 mg.g” ¹ . h” ¹ was achieved, i.e. its increase compared to untreated sludge by 43% by weight. When the pH was increased to 8.5, 28.9% by weight of the present inorganic phosphate was removed, while in untreated sludge it was 12.4% by weight.

Example 4

Returned activated sludge from the mechanical-biological wastewater treatment plant with an average concentration of all substances of 4.5 g/l ^- ', organic matter content according to the chemical oxygen demand value of 410 mg.l ^- ', according to the biochemical oxygen demand of 198 mg.l ^- ', pH 6.95 and temperature 16.5 °C was thickened in a biological flotation unit with a usable flotation volume of 10 np in an amount of 5.5 m^.h ^- ' and a retention time of 1.0 h.

A 10% solution of calcium nitrate was used as the flotation agent. The sludge suspension was dosed to a nitrate concentration of 120 mg/l. The nitrate concentration in the effluent from the flotation bioreactor and the reduction of organic pollution were monitored for one month.

Over the next month, the pH of the sludge suspension and the inflow to the flotation unit was adjusted to a value of 8.5 with foamed hydroxide and the retention time was reduced from 1.0 hour to 0.5 hour.

The average nitrate concentration in the flotation unit effluent was 25 mg.l**'. The calculated specific denitrification rate was 11.7 mg.g**'.h**', which increased to 17.0 mg.g ^-, .h ^- ' by adjusting the pH to 8.5.

The organic pollution of the sludge water flowing out of the flotation unit at pH 6.95 was on average 229 mg.l ^- ' according to COD, resp. 82.mg.l~' according to BOD^. These values correspond to 44.1 wt. % (according to COD) and 58.6 wt. % (according to BOD^) reduction of organic pollution.

When the pH was adjusted to 8.5, the organic pollution of the sludge water flowing out of the flotation unit was on average 206.1 mg.l ^- ^ according to COD, resp. 69.7 mg.]*' according to BOD^. These values correspond to a 49.7% by weight (according to COD) and 64.8% by weight (according to BOD^) reduction of organic pollution.

Example 5

Biological flotation tests of a mixture of return activated sludge and sludge water from the methanation stage β with high organic contamination were carried out on a biological flotation unit with a usable flotation volume of 10 m^. The tests were carried out at a high hydraulic load of the system of 19 to 22 m^.m^.d ^- '. The amount of sludge water supplied to the test flotation ranged from 290 to 1,360 lh ^- ', with 1,100 lh**' corresponding to the total production of sludge water from the methanation stage.

The residence time of the mixture of activated sludge and sludge water in the flotation unit ranged from 0.81 to 0.96 hours, the pH of the mixture was adjusted in the flocculation unit with 30% sodium hydroxide to a value of 8.5, the temperature of the mixture was 20 °C.

The measured and calculated values are given in Table III. During the flotation process, there was a significant reduction of organic matter in the mixture of activated sludge and sludge effluent from the methanation stage. The reduction was 43% by weight (based on COD) and 73% by weight (based on BOD).

Table 111

Flow rate Q lh ¹ Dwell time h T °c PH NOZ dose mg.l ¹ ATTACK BOD _R NO mg.l“ VL β·Γ' OUTFLOW COD mg.l“ BOD _{K mo} 1-1 mg.l no; mg.l" ^{1 VL} -1 g.1 ¹ COD mg.l" ¹ Returnable activated sludge 7 200 290 0.96 20 8.5 76 6,708 858 434 13.4 2,716 441 171 30 Sludge water 2,508 940 535 41 .0

Returnable asset- _7 144 20 8.5 64 8,900 2,508 1,037 '1,940 473 535 21.5 41.0 ⁹ ·®53 678 41 36 sludge, Sludge water 360 0.92 Returnable asset- 8 571 7,100 180 7.7 sludge, Sludge water 095 0.81 20 8.5 55 11,306 820 — 5.998 12.3 176 14 Returnable ύ ^activated sludge Sludge water 638 915 0.90 20 8.5 65 7,570 5,440 691 1,567 354 535 14.2 6.189 31.4 431 106 27

The process according to the invention reduces the pollution of the effluent from the flotation bloreactor, indicated by the value of biochemical oxygen demand, from 50 to 80% by weight, according to chemical oxygen demand from 30 to 70% by weight, while simultaneously thickening the activated sludge. The content of inorganic phosphate is also reduced, up to 30% by weight. The process according to the invention can shorten the residence time of the sludge suspension in the flotation bloreactor by an average of 50%.

Claims (3)

SUBJECT OF THE INVENTION Process for the purification of waste water by intensifying the biological flotation process, characterized in that an alkaline compound, preferably calcium hydroxide, is added to the continuously stirred sludge suspension in the flocculation unit of the bloreactor and the pH of the suspension is adjusted to a value of 8.5 to 10.0. 2. A process for the purification of waste water by intensifying the biological flotation process according to claim 1, characterized in that the waste heat of the expanding units is used to heat the sludge suspension. 3. A process for the purification of waste water by intensifying the biological flotation process according to claim 1 or 2, characterized in that, to replenish the organic carbon, the sludge slurry from methanization stage is added in an amount of from 1 to 25% by volume.