GB2130570A - Method for the elimination or prevention of blockages in bottom aerators in water treatment and sewage treatment under operating conditions - Google Patents

Abstract

Blockages in bottom aerators in the treatment of water and sewage are eliminated or prevented under operating conditions by introducing formic acid into the gas fed into the aerators without taking the aerators out of operation. The acid converts the blocking material at least partly into a form which is soluble in the water or sewage to be treated.

Description

SPECIFICATION Method for the elimination or prevention of blockages in bottom aerators in water treatment and sewage treatment under operating conditions The present invention is concerned with a method for the elimination or prevention of blockages in bottom aerators in water treatment and sewage treatment under operating conditions.

Bottom aerators, i.e. aerators that are placed near the bottom of the aerator basin, have a lower energy requirement and a better air introduction capacity than aerators placed laterally in the aerator basin.

During the operation of aeration systems in-water or sewage treatment plants, depositions are formed in the pores of the aerators in the course of a shorter or longer time. As a rule, those deposits mainly consist of calcium carbonate and/or iron phosphate and/or organic substances. How soon these deposits result in blockage of the aerators, does not depend only on the material of which the aerators are made (e.g., various plastics, such as polyethylene, polypropylene or polystyrene, or ceramic sinter materials) and on the air transmission capacity, but in the first place, on the composition of the water to be treated. The deposits deteriorate the oxygen introduction capacity of the aerators and cause an increase in the energy consumption. Deposits occur both in so-called disc aerators (filter domes) and in candle aerators.

In a method of cleaning bottom aerators the aerator basin is emptied, the aerators are disassembled soaked and washed (in an acid bath mostly consisting of hydrochloric acid) and assembled again, and the basin is again taken into operation. Since the aerator basins could include up to 1,000 or even more disc aerators, this procedure is highly laborious and time-consuming. Moreover, thereby the functioning of the clarifier plants is deteriorated, or the sewage water must even be allowed to by-pass the clarifier plants. In certain plants, such a cleaning must take place several times per year.

There are also pivotable aerator systems, which can be pivoted out of the aerator basin for the purpose of cleaning, but such systems are expensive owing to the necessary articulated joints.

There are also methods, wherein the cleaning is effected with the aerators in place by using e.g. chlorine or hydrogen chlorine, see for instance R.B. Jackson, "Maintaining Open Diffuser Plates With Chlorine", Water Works & Sewerage, September 1942, pages 380-382, W.M. Franklin, "Purging Diffuser Plates With Chlorine", Water Works & Sewerage, June 1939, pages 232-233, "Manual of Practice No 5", Federation of Sewage and Industrial Wastes Associations, Champaign, Illinois, 1952, pages 60-61, U.S. Patent No. 2,686,138 and EP application No.49154. These methods are characterized by the use of strong acids and oxidizing agents, which in practice cause several problems, e.g.

- corrosion of pipes and diffusers, - the handling of the very dangerous gases stored in pressure vessels, is riskful and disturbance in the use of dosage devices may be hazardous, - the risk, that the biological purification of the water will be disturbed or that the bacterial culture will be destroyed, - due to corrosion and viewpoints of safety the arrangement of apparatuses is expensive.

In the method of the present invention the disadvantages mentioned above have been eliminated.

The present invention is concerned with a method for the elimination or prevention of blockages in bottom aerators in water treatment and sewage treatment under operating conditions. The method is characterized in that formic acid is introduced into the gas fed into the aerators without taking the aerators out of operation.

The gas fed into the aerators is, as a rule, air or oxygen, e.g. compressed air. Formic acid is supplied preferably in the form of gas or vapour, but it may also be introduced in the form of a fine spray.

Compared to the mineral acids mentioned above the organic acids are weaker. However, even small amounts of formic acid is strong enough to dissolve normal blockages caused for instance by Ca and Fe deposits and to prevent sliming under operating conditions. Formic acid is also volatile enough to be sprayed into the pipe distributing the gas to be fed into the aerators.

An essential requirement is that the cleaning chemical does not cause corrosion of the air distribution network or aerators. The air distribution network above the basin is usually made of Cr, Ni-alloyed steel. In certain cases galvanized pipes are used. In the following table the effects of formic acid and hydrochloric acid on steel containing 18 % Cr and 9 % Ni have been compared (data taken from "Korrosionstabellerför rostfria steel, Jernkontoret, Stockholm, D226, Stockholm 1979, Carlson press offsetstryckeri AB, pages 20, 45,46 and 58) Corrosion effect Cont. Temp.HCOOH HCI % oC 0.1 20-50 - 1, P 0.5 70 0 1, P 0.5 50 - 2 1 40-50 0 2 5 20 0 2 10 20 0 2 25 20 0 2 50 20 0 2 0 Corrosion rate < 0.1 mm/yearthe material is corrosion-resistant 1 Corrosion rate 0.1 - 1.0 mm/year the material is not corrosion-resistant, but it can be used in certain cases.

2 Corrosion rate > 1.0 mm/year P Spot corrosion is possible It is known that hydrochloric acid strongly dissolves zink. In order to determine the effect of formic acid the action on a galvanized sheet (160 g/m2) was studied. In the test the sheets were subjected to saturated formic acid vapour. Corrosion right through the zink was observed after 400 hours. According to the general heat galvanization standards the thickness of the zink layer of a 1-10 mm pipe is 420 g/m2. According to the test such a layer would have been corroded right through after about 1000 hours.

In periodic cleaning (see examples) the yearly cleaning time is less than 10 hours and the conditions are not as severe as the test conditions. Thus formic acid is also harmless to galvanized pipes.

Compared to the risks associated with the storing and the dosage of chemicals in the known methods and to the complicated apparatuses used therein the use of formic acid is easy and simple. The dosage device may be a conventional dosage pump for chemicals provided with a nozzle by which the formic acid is sprayed into the air. Formic acid can be sucked directly from a transport container, e.g. a plastic can, whereby the handling of the chemical is reduced to a minimum. When an adjustable dosage pump is used a flow indicator is unnecessary.

As a pump a high-pressure paint sprayer can be used. A pump with a feeder pipe may also be concerned; in this way, readily volatile or gaseous chemicals can be introduced. However, the chemical is preferably dosed by means of a pump and a carburator, e.g. an overflow, float or injection carburator, into the gas.

Thereby it is guaranteed that no non-gasified formic acid vapours are separated in the gas distribution network, but the formic acid vapours are distributed uniformly to the individual aerator apparatuses.

Combinations of such dosage devices may also be used.

Formic acid may be introduced into the gas supply pipe, preferably a pipe of compressed air, before or after the compressor. For example, the chemical may be fed into the principal gas distribution pipe or into the supply pipe system of the individual aerator sectors.

Formic acid is highly suitable, because it is biologically decomposable and non-toxic for the biocenosis in the organic sediments in the sewage treatment plants. Formic acid diluted in a great amount of water acts as a substrate for aerobic bacteria. The formic acid used is normally 50 to 85-% formic acid. Preferable is 85-% technical formic acid because it is a cheap commercial product and because of its low water content.

By means of the method in accordance with the present invention, extensive blockages of the aerators may already be dissolved after a treatment time of few hours, so that the pressure loss at the aerators is, as a rule, reduced almost to the value of new aerators. The chemicals may, however, also be dosed continually as smaller quantities, whereby blockages of the pores are prevented efficiently. Appropriate dosage quantities can be found out readily by means of experiments.

Since in weakly buffered water, e.g. soft water, the pH-value of the water to be treated might become excessively high on addition of larger quantities of acids, in such cases, practically a buffer agent, such as sodium bicarbonate, is added to the aerator basin. Under normal conditions this is, however, unnecessary.

The invention will be described more closely in the following with reference to the attached drawings.

Figure 1 shows an arrangement of trials for testing a continuously clogfree plant in a large-scale installation.

Figure 2 shows the pressure trend at the manometer according to Figure 1 using aeration with three different blower stages, measured in bar during one year.

Example 1 Experiments were carried out in a sewage treatment plant having an approx. 70,000 population equivalent.

The waste water accumulation comes mainly from a major slaughterhouse and a factory producing animal feed; in addition approx. 5000 inhabitants are linked to the municipal sewage network.

The aeration tanks studied were likewise equipped with Nokia disk aerators, type HKL-215with the following numerical distribution in both stages: 1568 aerators per aeration tank, Ist stage, 528 aerators per aeration tank, 2nd stage.

After a run for about two years all the aerators already had to be replaced twice. Quite precise analyses have revealed that the disks were caked up not only on the surface exposed to the water but also in their pores.

The grime was composed primarily of calcium carbonate. Clogging in the first stage was generally heavier than in the second stage.

The order of the tests is presented in Figure 1. The pressure air pipework from the blower to the aeration tank consists of zinc-plated steel, in the aeration tank itself of V4A steel. The air distributing grids on the bottom of the tank are made of PVC plastic, diameter 120 mm.

The formic acid is fed into the V4A pipeline with two dosing pumps Qmax = 18 I/h. In order to prevent backing up of the acid into the blower, the acid in the vertical pipe section leading down to the tank bottom is sprayed into the air flow through a jet.

Figure 2 shows the pressure trend, expressed in bars over a measured period of time, of blowers 1, 1 + 2 and 1 + 2 + 3. To facilitate interpretation three phases were distinguished: Phase I presents the conditions shortly before the last changing of the aerators. Blower 3 could not, however, any longer be switched on since its air would have been dissipated through the relief valve. On 2 December, the beginning of Phase II, the installation was again put into operation with the cleaned replacement disks. Within three months of operation without the addition of formic acid a clear pressure build-up was already to be noted again. From 2 March on, 85% formic acid was introduced into the pressure air with the successful result that during Phase lIthe pressure almost sank to the readings of new-ranking aerators.

When working out the acid dosage, a distinction must be made between the first and the second stage.

After a number of preliminary tests in which by and large an excessively large amount of acid was introduced into the pressure air, the following standard ratings can be recommended: q = 0.8 ml/(disk.d), q = 0.9 ml/(disk.d), where q corresponds to the daily dose of acid per disk. The acid dosing should preferably be carried out once to twice weekly since owing to the long-term effect one can expect the next days to be without the occurrence of caking or crusty formations. A quarter hour suffices to carry out the dosing. In the event that during the subsequent two to three months no pressure build-up was noted with dosage in this amount, the specific acid quantity q can be reduced by approximately 30 %.

In the sewage treatment plant studied, the annual requirement for technical formic acid (85%) was approx.

680 litres, which translates into chemical costs of about sFR 900. In comparison with the otherwise high costs of current, the large-scale cleaning works and the shutdown and return to operation of the aeration tank, this expense is so small as to be negligible. It has been established that the method has no influence on the biological process, does not attack the pipework or aerator and exerts a tolerable environmental load.

Example 2 The sewage treatment plant of the city of Turku, Finland, comprises five aerator basins in which the total number of disk aerators is 5250.

Ferrosulphate was added to the activated sludge process to remove the phosphor. After a run for 2.5 years the back pressure of the aerators had increased 0.06 bar due to the ferrosulphate and power failure. The aerators of one basin at the time were cleaned by feeding formic acid through a nozzle connected by a sleeve joint with the pipe directed downwards. A high-pressure paint sprayer was used as a pump. The dosage were 0.4 kg/aerator, i.e. about 2,100 1 85% technical formic acid during two hours. As a result of the cleaning the back pressure of the aerators decreased 0.06 bar, i.e. to the initial value. Because of the fall of the back pressure the energy consumption of the plant was reduced from 7450 kwh/d to 6550 kwh/d.The cleaning with formic acid did not affect the biological function in any way, which is proved by the results of the analysis of an average sample on a cleaning day: pH COD BOD7 Susp. solids Total -P Influent to plant 7.5 76 196 345 9.3 Influent to aeration 7.5 46 103 132 3.4 Outlet from secondary sedimentation 7.5 12 19 (ATV8) 9 0.3 These data correspond to the normal operation data of the sewage treatment plant.

Example 3 The effect of formic acid on sliming was tested at the Toronto Lakeview waste water treatment plant. Two Nokia Nopol HKP-600 diffusers were mounted on pipes of an individual length of 3/4" and installed in the aeration tank. The air supply to each diffuser was monitored and controlled at weekly intervals. The diffusers were pulled up above the mixed liquid level and inspected. One of the pipe diffusers acted as a control. 30 g HCOOH/week was fed into the other pipe diffuser. The essential results were as follows: - The control diffuser fouled quickly; a slime coating of a thickness of more than 1 cm grew on this diffuser within a period of 3-4 weeks.

- The test diffuser also fouled at the beginning of the test period. However, dosages of 30 g formic acid/week injected into the diffuser air stream over a period of 0.5 h reduced substantially the biofouling on this diffuser.

Claims (11)

1. Method for the elimination or prevention of blockages in bottom aerators in water treatment and sewage treatment under operating conditions, characterized in that formic acid is introduced into the gas fed into the aerators without taking the aerators out of operation.

2. Method as claimed in claim 1, characterized in that the gaslo be fed into the aerators is air or oxygen.

3. Method as claimed in claim 1 or 2, characterized in that the bottom aerators are formed as disc aerators or candle aerators.

4. Method as claimed in any of claims 1 to 3, characterized in that the formic acid is fed in the form of gas or spray.

5. Method as claimed in any of claims 1 to 4, characterized in that the formic acid is introduced into the gas to be fed into the aerators by means of a dosage device, such as a high-pressure pump and a nozzle with a small hole, of the type used, e.g., in high-pressure paint sprayers, a pump with a feeder pipe for the supply of readily volatile or gaseous chemicals, or preferably a pump and a carburator, or by means of a combination of such dosage devices.

6. Method as claimed in claim 6, characterized in that the formic acid is introduced into the gas supply pipe, which is preferably a pipe of compressed air, before or after the compressor.

7. Method as claimed in claim 5 or 6, characterized in that the formic acid is introduced into the principal gas distribution pipe or into the supply pipe system of the individual aerator sectors.

8. Method as claimed in any of claims 1 to 7, characterized in that the formic acid used is 50 to 85% formic acid, in particular 85-% technical formic acid.

9. Method as claimed in any of claims 1 to 8, characterized in that, in the case of waters and sewage waters that are only weakly buffered, e.g. waters or sewage waters with low carbonate hardness, a buffer agent, e.g. sodium bicarbonate, is introduced into the aerator basin.

10. A method according to claim 1 and substantially as hereinbefore described.

11. Any novel feature or combination of features described herein.