SE454508B - PROCEDURE FOR BIOLOGICAL CLEANING OF WASTE WATER - Google Patents

Description

454 508 l5 2 Despite these serious problems and the fact that large amounts of urban wastewater are discharged to sensitive recipients, such as lakes and bays with a relatively low water turnover and in watercourses that flow into such lakes and bays, only a small part of the urban wastewater is subjected to a purification to remove phosphorus.

In most cases when phosphorus is removed from wastewater, this is done by a chemical precipitation, namely by adding aluminum sulphate, ferric chloride, ferrous sulphate, calcium hydroxide or calcium oxide to the wastewater to remove dissolved phosphorus, after which precipitation products are removed. separated by sedimentation, flotation or filtration. This entails increased operating costs.

However, the extra sludge production that a chemical precipitate entails also creates increased operating costs for sludge treatment and possible capacity problems for the existing sludge stabilization and / or sludge drainage. It would therefore be desirable if the biological treatment of wastewater for nitrogen removal could be combined with a biological removal of phosphorus.

It would also be desirable for the phosphorus-containing product obtained during phosphorus removal to be in such a form that it could be used for fertilization purposes in order to thereby save currency for the introduction of phosphate minerals for the production of phosphorus-containing fertilizers. It has thus been estimated that phosphorus extracted from wastewater can cover 10-30% of the amount of phosphorus used in agriculture.

In a known process (Phoredox method) for simultaneous biological removal of nitrogen and phosphorus from wastewater by the activated sludge method, the wastewater is treated in series-connected reactors, by treating the wastewater in a first reactor anaerobically, in a second anoxic and in a third aerobically, before clarification, and the sludge precipitated during clarification is recycled to the anaerobic treatment zone.

At the same time, some of the liquid is recycled from the aerobic treatment zone to the anoxic treatment zone.

The sludge recycled by the Phoredox method has a not insignificant content of nitrate ions, and it has been found that these ions have an adverse effect on phosphorus removal.

In an attempt to solve this problem, a modified Phoredox method has been developed, the so-called UCT method, which comprises the same treatment sequence for the wastewater as the Phoredox method, but where the sludge from the clarification zone is recycled to the anoxic treatment zone and liquid is recycled therefrom. , which has a low nitrate content to the anaerobic treatment zone. This avoids that nitrate ions in the sludge from the clarification zone will destroy the anaerobic treatment of the wastewater and thereby the phosphorus removal.

When purifying wastewater in an activated sludge-type plant, only the amount of phosphorus corresponding to the content of the normal amount of sludge is normally removed. In practice, this amount constitutes about 30% of the phosphorus content in the wastewater, and the sludge generated usually contains 1.5-2.5% phosphorus on a dry matter basis.

However, it has been found that under certain hitherto unclear conditions, a significantly larger amount of phosphorus is removed at the same time as the removal of nitrogen than is expected to be included in the sludge. This condition has been further investigated, and on this basis it has been found that at the same time as a good removal of nitrogen from waste water, a more efficient and constant phosphorus removal can be achieved when the waste water, before undergoing the anoxic 454 508 4 and aerobic treatments, is treated during anaerobic conditions, and the sludge generated during clarification is recycled to the anaerobic treatment zone.

The process according to the invention is thus distinguished in that the waste water is brought into contact with microorganisms under anaerobic conditions, before it is subjected to the anoxic and aerobic treatments, that the residence time of the waste water in each treatment zone, in which it is treated under different anoxic and aerobic conditions, is long in relation to the length of these treatments, and that the sludge is recycled to the wastewater which is treated under anaerobic conditions.

Nitrogen and phosphorus removal in the process of the invention are believed to be dependent on the presence of the following three types of bacteria: (a) phosphorus accumulating heterotrophic bacteria, (b) non-phosphorus accumulating, denitrifying heterotrophic bacteria and (C) nitrifying bacteria.

During the anaerobic treatment of the wastewater, the phosphorus-accumulating bacteria, which contain accumulated polyphosphate, occupy low-molecular-weight organic matter, which is stored in the form of, for example, polyhydroxybutyrate (PHB), because the relatively energy-rich polyphosphate is used as an energy source. This forms orthophosphate ions, which are released into the liquid medium in the anaerobic treatment zone. During the aerobic treatment of the wastewater there is an oxidation of accumulated and hydrolysed, suspended organic matter, and the energy released thereby is used partly for the growth of the microorganisms and partly to accumulate orthophosphate ions from the liquid medium in the phosphorus accumulating bacteria in form of polyphosphate. There is thus a removal of dissolved phosphorus from the wastewater.

If the sludge recycled from the aerobic treatment zone has a high concentration of nitrate, it occurs in the anaerobic treatment zone that the non-phosphorus accumulating nitrifying bacteria are given the opportunity to metabolize loose, easily degradable organic matter, the amount of organic substance that the phosphorus-accumulating bacteria can use is reduced. This means that the growth and thus the effect of the latter is reduced. In the process in question, using two treatment zones, which are used alternately for anoxic and aerobic treatment of the waste water, several changes between anoxic and anaerobic treatments take place while the water is in a treatment zone. In other words, the residence time of the wastewater in a treatment zone is long in relation to the length of the individual phases.

During the anoxic phase, there is initially a significant amount of nitrate ions derived from the previous aerobic treatment. The concentration of these nitrate ions decreases during the anoxic phase at the same time as the NH3 concentration rises, and the conditions thereby become more and more anaerobic (which, as mentioned above, results in the release of phosphorus in the form of orthophosphate ions). In the anaerobic phase, NH3 is oxidized from the previous phase and at the same time the NO3 concentration rises.

By exposing the wastewater to several anoxic and aerobic treatments while it is in a treatment zone, a particularly low average nitrate concentration in the product leaving the treatment zone is achieved and thus also a low average nitrate concentration in the sludge recycled to the anaerobic treatment zone, which makes this particularly effective in view of the phosphorus removal in question. 454 508 6 The wastewater from the anaerobic zone is preferably led to an anoxic treatment zone, but it can also be led to a zone where aerobic treatment takes place.

It should be borne in mind that due to the natural content of calcium, magnesium and iron in the waste water and its pH value, there may be a chemical precipitation of phosphorus in the form of metal phosphates during the anaerobic treatment of the waste water. In areas with soft water (a low calcium content), however, such a purely chemical precipitation is considered to be of secondary importance. Since the phosphorus concentration in the waste water I in a treatment zone is high at the transition from an anoxic to an aerobic treatment of the waste water, a phase advance removal of the waste water from the treatment zones to the clarification zone is preferably carried out. In other words, the removal of the waste water to the clarification zone from a treatment zone is carried out only some time after there has been a transition from an anoxic to an aerobic treatment and vice versa. With a suitable selection of the collection times, it can be ensured that the waste water not only has a low phosphorus content but also a low nitrate and ammonia content.

A particularly high safety against the supply to the anaerobic treatment zone of an inappropriately large amount of nitrate together with the recycled sludge can be achieved by ensuring that the sludge in the clarification zone has an average residence time therein of at least 2 hours. This can be achieved by using a clarification tank with a particularly large sludge pocket.

During the extended residence time of the sludge in the clarification tank, a hydrolysis takes place, whereby an endogenous (ie without external supply of organic matter) denitrification occurs. Instead of increasing the residence time of the sludge in the clarification zone, the hydrolysis of the sludge to be recycled can take place in a separate hydrolysis zone, in which a part of the recirculated sludge is retained for at least 1 hour. 454 508 7 It is also advantageous to divide the recirculated the sludge in two fractions and lead, for example,% of the sludge to a pre-treatment zone, to which waste water is supplied, and from which the waste water is led to the anaerobic treatment zone. In this pretreatment zone, in which a precipitate of undissolved constituents takes place, a biological degradation occurs and thereby an oxygen generation which results in the material being in a form which is more easily degradable for the microorganisms. The precipitate from the pretreatment zone can be added to the anaerobic treatment zone. If an embodiment with both a pre-treatment zone and a separate container for sludge hydrolysis has been chosen, the part of the sludge that would otherwise be transported to the pre-treatment zone can be transported to the separate hydrolysis container. The precipitate from the pretreatment zone is transported to the separate hydrolysis tank, and the treated precipitate and sludge from this tank are transported to the anaerobic treatment zone. This embodiment can in some cases lead to a reduction in the plant's reactor volume. The supply of sludge to the anaerobic treatment zone can take place in the lower part of the treatment zone and in such a way that upwardly flowing sludge cover is formed here. In this case, a concentration of the sludge in the container is achieved and at the same time hydrolysis of suspending substances present in the waste water.

This hydrolysis means that biodegradable organic matter is converted into an easily degradable organic substance which is available for the phosphorus accumulating bacteria. With the latter embodiment, without the use of a separate hydrolysis vessel, substantially the same effect as when using a separate hydrolysis zone can be achieved.

The anaerobic treatment zone may instead be divided into at least two separate series-connected sub-zones. 454 508 8 In the first of these zones, nitrate supplied by recycled sludge is removed by reaction with the supplied waste water, thereby preventing anoxic conditions from arising in the subsequent zone.

The invention will be described in the following with reference to the drawing, which shows a flow diagram for a plant for carrying out the method according to the invention.

The wastewater is led through a line 1 into a tank 2, where it is mixed with recycled sludge, which is introduced into the tank 2 through a line 3 and kept under anaerobic conditions for a period of 1-4 hours.

The subsequent treatment takes place in two treatment tanks 5 and 7, where the wastewater can be treated both anoxically and aerobically. In the illustrated plant, the treatment is carried out in two phases, phase 1 and phase 2. In phase 1, the anaerobically treated wastewater is transported through a line 4 to the treatment tank, in which for a certain time, preferably the whole period, anoxic conditions are maintained, and from there it is transported through a connecting line 6, to the second treatment tank 7, in which aerobic conditions are maintained for a certain time, preferably all the time. From the tank 7, the waste water is transported through a line 8 to a clarification tank 9, from the bottom of which sludge is taken which is recirculated through the line 3 to the treatment tank 2. From the clarification tank 9, purified waste water is taken out through a line.

In phase 2, wastewater from the tank 2 is transported through a line 11 to the treatment tank 7, in which anoxic conditions are now maintained, from where they are transported through the connecting line 6 to the treatment tank 5, where aerobic conditions are maintained, and further through a line 12 to the clarification tank 9 u 454 508 EXAMPLES Such a treatment plant as shown in the drawing was used, however, the tank 2 was divided into three series-connected sub-zones where the raw waste water I together with recycled sludge I was transported to the first in a row of these sub-zones.

The tank 2 had a sludge volume of 200 liters and was equipped with a stirrer, which rotated at a sufficiently high speed to prevent precipitation of sludge. Treatment tanks 5 and 7 each had a volume of 830 liters and were both equipped with a stirrer to ensure sufficient agitation to prevent sludge precipitation. Both treatment tanks were equipped with nozzles for blowing air under pressure, so that in the aerobic phase an oxygen concentration of 2-3 mg / liter could be achieved in the waste water. The clarification tank had a volume of 1120 liters and a surface area of 0.18 m2 and was equipped with a centrally vertically mounted inlet pipe and overflow at the top edge of the tank wall. A guide screen was arranged over the mouth of the inlet pipe, which forces the vertically incoming wastewater down towards the bottom of the tank, before the stream is again led in another direction and the water flows up towards the overflow.

Waste water was fed to the described plant in an amount of 1-2 1.2 liters / min and with a C / N ratio of 12-13, and sludge was recirculated from the clarification tank in an amount of 0.5-0. 6 liters / min.

During an operating period of 3 months (February-May 1983), 34 samples of raw waste water and purified waste water were taken during operation with phase lengths of 140 minutes.

Analysis of the samples showed that on average a reduction of the phosphorus content of the wastewater was achieved from 7 mg P / liter to 0.54 mg PO 4 -P / liter with a standard deviation of 0.33 mg / liter. At the same time, the nitrogen content was reduced from 25 mg N / liter to 454 508 0.77 mg NH 3 -N / liter with a standard deviation of j; 0.99 mg / liter and 3.2 mg NO3-N / liter with a standard deviation of 2.5 mg / liter.

Claims (8)

10 15 20 25 30 454 508 ll PATENT REQUIREMENTS 1. A process for the biological treatment of waste water by means of the activated sludge method, in which the waste water is successively subjected to an anaerobic, anoxic and aerobic treatment and then passed to a clarification zone for sludge separation, which is recycled and mixed with the waste water to be subjected to an anaerobic treatment, characterized in that the anaerobic, anoxic and aerobic treatment is carried out in three separate tanks, and that the waste water anaerobically treated in the first tank is led alternately to the second and third tanks, whereby for an initial period it is treated anoxically in the second tank and aerobic in the third and for a second period anoxic in the third tank and aerobic in the second. 2. A method according to claim 1, characterized in that the waste water is withdrawn from the treatment zones during both the anoxic and the aerobic treatment. 3. A method according to claim 1, characterized in that the sludge is on average poured into the clarification zone for at least 2 hours. 4. A method according to claim 1, characterized in that an anaerobic treatment zone divided into at least two separate series-connected sub-zones is used. 5. A method according to claim 1, characterized in that a part of the sludge is recycled to a pre-treatment zone, to which waste water is supplied, and from which the waste water is transported to the anaerobic treatment zone. 6. A method according to claim 5, characterized in that sludge is removed from the pre-treatment zone and that the removed sludge is introduced into the anaerobic treatment zone. 7. A method according to claim 1, characterized in that a part of the sludge is recycled to a separate hydrolysis zone, to which sediment is also supplied from a pretreatment zone, and that the treated sludge and the sediment are from there transported to the anaerobic treatment zone. A method according to claim 1, characterized in that the recycled sludge is transported to the lower part of the anaerobic treatment zone and in such a way that a floating sludge blanket is formed therein.