WO2014059990A1

WO2014059990A1 - Improved process and system for biological water purification

Info

Publication number: WO2014059990A1
Application number: PCT/DK2013/050318
Authority: WO
Inventors: Gert Petersen; Jacob Kragh ANDERSEN
Original assignee: Envidan International A/S
Priority date: 2012-10-15
Filing date: 2013-10-10
Publication date: 2014-04-24
Also published as: HK1208212A1; CN104703928A

Abstract

The present invention relates to a process for side stream removal of nitrogen and phosphorous in a waste water treatment plant using the activated sludge method wherein a small fraction of the return sludge is side stream treated and without the use of external bacteria or chemicals for enhancing the process.

Description

Improved process and system for biological water purification

The present invention relates to a process for the treatment of waste water using return sludge. More specifically, the present invention relates to a process for side stream removal of nitrogen and phosphorous stripping in a waste water treatment plant using return sludge wherein a small fraction of the return sludge is side stream treated without the use of external bacteria or chemicals for enhancing the process. Background

Due to increasing demands from legislators throughout the world, processes and technology for the treatment of sanitary, municipal, commercial and industrial waste water has continuously been improved and new equipment added.

The basic idea behind all biological methods of waste water treatment is to introduce contact with microorganisms, which are fed on the inorganic and organic materials in the waste water. Thereby, the concentrations of pollutants in the waste water are reduced, including inorganic or organic nitrogen and inorganic or organic phosphorous as well as the biochemical oxygen demand (BOD), the chemical oxygen demand (COD) and total suspended solids (TSS). The principle behind biological treatment is that microorganisms degrade and grow on pollutants present in the waste water. The organic material is transformed through the microorganism's metabolism into cellular mass, which - no longer in solution or dispersed - can be separated from the water phase in a secondary clarifier by simple gravity sedimentation. The treated effluent leaving the system is then much clearer than when it entered into the waste water treatment plant (WWTP).

In a typical waste water treatment plant based on biological treatment, the incoming waste water is pre-treated in a primary treatment where- in a portion of the organics, including suspended solids is removed by sedimentation. Following this primary treatment, the waste water is subjected to a biological treatment wherein the pollutants in the waste water are degraded in a biological process wherein microorganisms are utilized to remove remaining organics, nitrogen and phosphorous from the waste water. The biological treatment can included several treatment stages comprising an anaerobic treatment zone and an aerobic treatment zone. Microorganism growth and metabolic activity is thus controlled and exploited through the use of controlled process conditions in the various zones

After the biological treatment the treated waste water is allowed to clarify/settle in a second sedimentation tank for a predetermined time period after which effluent is drawn from the upper portion of the sedimentation tank and discharged. A part of the activated settled sludge, which comprises biomass produced during treatment of the waste water by the growth of mi- croorganisms in aeration tanks is removed for dewatering and disposed, whereas another part of the remaining sludge, may be returned as activated return sludge (ARS) to the biological treatment in order to enhance the biological degradation. The activated return sludge may be returned to the biological treatment reactors as such without any sequential treatment. Alternative- ly, the activated return sludge may be subjected to an initial aerobic treatment in order to increase the aerobic sludge age and to improve the removal of nitrogen in the secondary treatment zone. In this way, a low-loaded system can be easily upgraded, e.g. when new regulations require stricter restrictions on nitrogen removal.

The amount of activated sludge is rate limiting for the treatment process in WWTPs and if there is not an excess amount of COD to nitrogen, such as lower than a ratio of COD: N of 7, 6, or even 5, existing WWTPs are not as effective in meeting nitrogen effluent standards. Further, the aeration equipment commonly used in the treatment of returned sludge is idle part of the time, in some WWTP concepts, typically 50 % of the time. This means that the amount of aeration equipment potentially must be doubled, and that all hydrolysed COD (HCOD), which is produced during the anaerobic phases in the waste water treatment plants and which is not utilized by bio-P microorganisms will be degraded during the aerobic phases and are then not uti- lized for de-nitrification or biological phosphor removal in the treatment tank(s).

WO 2010/148044 describes an apparatus for removal of biological phosphorous and nitrogen from raw water to reduce the production of surplus sludge. This is achieved by treatment in an anaerobic side stream reactor where hydrolysis and fermentation takes place - in preferred embodiments the treatment is accelerated by providing acidic conditions in the side stream reactor (CSTR). In the side stream reactor the method includes the use of chemicals, such as bases or acid, added digesting enzymes, sonication treatment, ozone treatment, or other oxidizing chemical treatments as well as heat treatment.

The method uses the acidic environment and heat to achieve the benefits. One drawback of this method is that the nitrifying bacteria are not capable of surviving acidic conditions and hence the acceleration of the method and apparatus of WO 2010/148044 is at the expense of the nitrifying bacteria and their action.

Thus, despite the continuous development in waste water treatment technologies, there remains an urgent need for further improvement of biological waste water treatment plants that can provide a broadened range of uses and still meet the high demands in relation to efficiency, as well as re- moval of undesired organic and inorganic compounds such as total nitrogen, total phosphorus, total suspended solids, Biological Oxygen Demand and Chemical Oxygen Demand, regardless of the COD available.

Therefore, it is an object of the present invention to provide an improved method for biological treatment in which there is a better capacity utilization of the COD available and hence a more efficient purification of the waste water.

Summary of the invention

With this background, it is an object of the present invention in a first aspect to provide a process for biological treatment of waste water by the activated sludge method with the use of the same bacteria population throughout the process, said process comprising the steps of

a) feeding a waste water feed to a treatment tank,

b) subjecting the waste water in the treatment tank to a biological treatment process to provide a mixture of treated waste water and activated sludge,

c) allowing the mixture to settle in a sedimentation tank to provide treated waste water and settled sludge, wherein the settled sludge is subjected to the steps of

i) separating a first and a second return fraction of the settled sludge,

ii) subjecting the first return fraction of the settled sludge to a process comprising aerobic treatment and/or an anaerobic treatment in a side stream reactor,

d) feeding the first return fraction of the settled sludge from the side stream reactor and the second return fraction of the settled sludge from the sedimentation tank to the treatment tank.

It has surprisingly been found that by introducing two fractions of return sludge to the treatment tank, wherein only a first fraction is side stream treated, the overall removal of inorganic and organic materials is more efficient than in traditional waste water treatment plants where one fraction of the return sludge is treated and returned to the treatment tank. In this specific method the one first fraction of the activated settled sludge, is subjected to aerobic and/or anaerobic treatment suitable for nitrification and denitrifica- tion processes and phosphorus stripping in a side stream reactor and the second fraction is returned to the treatment tank without any side stream treatment.

By concentrating the time consuming biological reactions in a smaller volume in the side stream reactor having 3 to 5 times higher sludge concen- tration than the treatment plant the hydrolysis will be more efficient. The hydrolysis rate under e.g. anaerobic treatment in the side stream reactor continues with a constant rate up to 40 hours retention time.

The process uses the same bacteria population throughout the process, that is no external bacteria are added at any point of the process to enhance specific processes, neither are any chemicals added to enhance either aerobic or anaerobic bacterial actions.

By "same bacteria" in the context of the present invention is meant that no specialized bacteria are added or grown at any point of the process, and the bacteria population is solely the result of the reactions occurring in the process controlled by volume and the degree and timing of aeration or absence of air.

By utilizing the side stream reactor in accordance with the invention phosphorous is primarily removed in the treatment tank A, as the bacteria in the system have been prepared for optimal uptake by utilizing HCOD for P removal. Hence, the process may also be seen as a means for providing HCOD to the treatment tank for optimal removal of phosphorous.

This will provide a more flexible waste water treatment plant in times with excess waste water as the COD content can be concentrated and/or utilized in the side stream reactor while a second fraction of the return sludge in the sedimentation tank is directly returned to the treatment tank, such that the hydrolysis providing the easily accessible COD (HCOD) can operate unaffected by the fluctuation in the flow of incoming waste water.

Also, the process of the invention provides a solution which is flexible in relation to the available content of COD. Thus, in situations where the COD to nitrogen ratio is low, which is typically lower than 7, preferably lower than 6, more preferably lower than 5, there will still be an efficient denitrification. This is of particular benefit in areas of the world where the COD to nitrogen ratio is very low - or in other words that the nitrogen content is high. This is for example the case in for example China due to low fat food waste and due to frequent pretreatment of the industrial waste

Throughout the process pH is substantially constant around neutral or slightly basic, thus pH is preferably above 6.5, more preferred above 7 and most preferred in the range of 6.8 - 7.5. Thus, there is no need for adding acidic or basic components for providing a specific pH, since the operation according to the invention ensures a pH in the preferred range.

The process of the invention also has the benefit that the phosphorus present in the untreated waste water is more efficiently removed than in prior art methods primarily focusing on denitrification.

Also, the installation costs for operating the process of the present invention are smaller since smaller pipes and tanks for the side stream treatment can be used. In addition to costs, this also affects the flexibility in implementing the present invention into existing waste water treatment plants, since the space needed to improve the waste water treatment process is limited accordingly.

Thus, in summary, the present invention relates to a process wherein hydrolysis takes place in a side stream reactor provided with a - preferable constant - flow of activated sludge and wherein the produced hy- drolysed oxdizable pollutants (HCODs) can be utilized for de-nitrification of nitrogen bound as N0₃ formed in an aerobic section of the sidestream reac- tor, P stripping in an anaerobic section of the sidestream reactor and/or sur- plus HCOD from the sidestream reactor, can enhance de-nitrification and P removal in the treatment tank - that is for improving the biological phosphor removal and de-nitrification by microorganisms in the treatment tank.

Decomposition of complex organic compounds are broken down step by step to simple compounds and takes place in the activated sludge all over in the plant by the process of hydrolysing these compounds to hydrolysed COD (HCOD).

Anaerobic treatment allows poly-phosphate microorganisms (bacteria) to uptake volatile fatty acids (VFA) present in the wastewater and to re- lease phosphorus as phosphates in order to increase future potential for absorbing phosphates from the waste water.

Aeration of the waste water permits nitrification of ammonia, R- NH₃ ⁺, and NH₄ ⁺ to nitrites, N0₂ ^~, and ultimately to nitrates, N0₃ ^". The nitrifying microorganisms are specialized and grow slowly. Thus, there must be suf- ficient time for the aerobic treatment of the activated sludge, in order to enable these microorganisms to grow faster than the removal rate by discharged settled sludge in the sedimentation tank. Aeration also promotes COD and BOD consumption by heterotrophic organisms.

Many of the COD removing microorganisms are able to utilise N0₃ ^" as oxidant when degrading COD material, if no oxygen is available, and to convert N0₃ ^" to free nitrogen, N₂.

Removal of phosphorus is a two step process which takes place with specialized microorganisms present in the sludge, the bio-P bacteria. If bio P- bacteria are kept without oxygen and without N0₃ ^", the bio-P bacteria will absorb and transform VFA-COD into an organic polymer, PHB, by stripping phosphate, stored as an energy source in polymerized phosphate compounds in the cells, to the water phase.

When the bio P-bacteria as a next step are kept at aerobic conditions, the bio P-bacteria will absorb more phosphorus than what was stripped, a process which is commonly known as the "luxury uptake". In this step the P-bacteria utilize 4 times more HCOD than during the uptake of HCOD in the first stripping step.

Bio P-bacteria are dependent on HCOD in both the anaerobic and the aerobic steps and in the first step the bacteria will be inhibited by a concen- tration of nitrates above app. 1 mg/l. Thus, a crucial aspect in phosphor re- moval is to enhance the bacteria growth in the anaerobic phase and strip them for phosphate in order to prepare them for phosphor uptake in the treatment tank.

The absorbed phosphor will be removed with the surplus sludge. The bio P-bacteria can only use easily degradable compounds or hydrolysed compounds (HCOD), preferably in soluble forms, and the concentration of these compounds in the influent to a WWTP is not ideal for biological phosphorus removal under cold climate conditions, since the bacteria growth is limited.

Anoxic and anaerobic treatment promotes denitrification of the ni- trates to nitrogen gas, N₂, which is released to the atmosphere. In the context of the present invention anoxic is defined as conditions where no free oxygen is present but oxygen is present bound as nitrate compounds. The de-nitrifying microorganisms also consume COD but there are many species which have the ability to de-nitrify. Therefore, the de-nitrifying microorgan- isms can also use partly hydrolysed COD compounds. If easy degradable compounds are available, the de-nitrification rate will increase.

Hydrolysis takes place all over the plant, but in all aerated reactors or aerated phases, the COD removing microorganisms will rapidly convert easy degradable compounds with oxygen and leave no surplus of hydrolyzed COD, also known as easily degradable compounds, to the biological phosphorus removal or de-nitrification.

In a particular embodiment the first return fraction of the settled sludge is within the range 5-30% (vol/vol) of the first and second return fractions of the settled sludge, preferably 10-25% (vol/vol), more preferably 10- 15 % (vol/vol). Thereby, the first return fraction of the settled sludge only constitutes from 3 to 10 % (vol/vol) of the waste water flow feed (Ql), more preferred 4 - 8 % (vol/vol), even more preferred 5 - 7 % (vol/vol).

Even with such low return flow volumes in the side stream reactor, it was found that the COD/HCOD utilization rate was not compromised, since it was found that the reaction rate was independent of the volume of the return fraction but rather was dependent on the concentration of the sludge in the return fraction. At the same time the side stream treatment is unaffected by fluctuations in the water flow since the second side stream may be adjusted to meet these fluctuations.

Thus, it was found that up to 60% of the total sludge in the entire WWTP could be present as highly concentrated sludge in the side stream reactor which will ensure that the content of COD material is concentrated and hydrolysed at a constant rate but in a much smaller volume. Thus, in short, the capacity is optimized since the same utilization is obtained in a smaller volume.

Out of the total demand for oxygen related to COD conversion, an amount proportional to the amount of COD in the side stream reactor can be covered in the side stream reactor. The time in which the sludge in the side stream reactor is aerated can be counted to the aerobic sludge age (ASA), which is a measure to provide a sufficient population of nitrifying bacteria in the WWTP.

It has been observed that the hydrolysis rate of COD material under anaerobic condition continues with a constant hydrolysis rate with up to 40 hours of retention time. The retention time being defined as the flow into the specific process tank divided by the volume of the process tank (treatment tank, side stream reactor, sedimentation tank, etc). Thus, phosphorus removal and nitrification and/or denitrification processes are overall improved.

The flow of the first return fraction of the settled sludge is preferably constant independently of the sludge recirculation flow, i.e. the sum of the first and the second return fraction of the settled sludge (sludge recirculation flow). By operating the process at this mode, it is ensured that a very low volume is kept in the side stream reactor regardless of the inflow to the waste water treatment plant.

The control of the sludge recirculation flow can be adapted to varia- tions in the inflow of waste water, e.g. due to heavy rainfall, draught etc. This results in a more robust waste water treatment process. Thus, the WWTP can be operated more efficiently with a constant optimal utilization of the COD.

Further, a lower flow of sludge recirculation results in an increased sludge depth in the clarifier tank and hence an increase in the concentration of the return activated sludge provided to the treatment tank.

The second return fraction may vary in response to the incoming water levels. The purpose of circulating the second return fraction is to maintain a high concentration of sludge in the treatment tank. The second return fraction constitutes about 40 % (vol/vol) of the waste water feed.

There are a number of challenges in optimizing waste water treat- ment and in particular in effectively running the side stream reactor, as nitrate is formed in the aerobic section and at in at least a part of the anaerobic section or part of the non-aerated time the concentration of both oxygen and soluble nitrate, (NO₃-N) must be very low in order for the P stripping in the side stream to be effective. Thus, if there is an excess of hydrolysed COD (HCOD) an improved effluent quality will be achieved if the side stream reactor treatment is initiated with an aerobic step.

Thus, in a particular embodiment of the invention the first return fraction of the settled sludge in step c) ii) is treated in series in a first step by aerobic treatment, followed in a second step by anaerobic treatment. This will ensure that the content of COD material is degraded under aerobic conditions whereby nitrogen is released. A further variation of this embodiment can be made in order to minimize the level of nitrates in the anaerobic treatment. This can in one embodiment be obtained by increasing the de-nitrification capacity of the anaerobic reaction zone by reducing the aerobic capacity in the aerobic reaction zone, such as by dividing the aerobic reaction zone in several sections, wherein at least one section is operated with intermittent aeration, this will ensure that both the HCOD is converted and that sufficient denitrification has taken place.

It is possible to set a predetermined maximal nitrate level before the first return fraction of the settled sludge enters into the anaerobic zone from the aerobic zone. Proper aeration before entry will ensure optimal operation of the anaerobic zone.

After feeding the first and second return fractions of the settled sludge to the treatment tank, the remaining HCOD may be consumed by the denitrifying and/or the bio-P bacteria. Remaining HCOD may be needed in the treatment tank if the COD to nitrogen ratio is lower than 6, this will ensure that a full denitrification capacity is obtained.

Thus, this embodiment may in particular be even more advanta- geous when the COD to nitrogen ratio in the treatment tank is low, that is lower than 6, such as lower than 6, 5, 4, or 3.

If the ratio is higher, this embodiment is also advantageous since the remaining HCOD will increase the denitrification rate and thus speed up the treatment process in the treatment tank whereby the capacity of the plant is further improved. Another challenge is that a waste water treatment plant is designed for an assumed future load whereas the actual load and the composition of the wastewater may be different from the assumed conditions.

Stripping of phosphor in the anaerobic stage will only be efficient if both the oxygen and the nitrate concentration are very low, preferably close to zero. Therefore, in another embodiment of the invention the first return fraction of the settled sludge in step c) ii) is treated in series in a first step by anaerobic treatment, followed in a second step by an aerobic treatment step. Hereby nitrates are discharged to the treatment tank and not present during the anaerobic hydrolysis. This solution is particularly useful when the concentration of NH₃ in the feed is high.

In another particular embodiment of the invention the first return fraction of the settled sludge in step c) ii) is subjected to alternating anaerobic treatment and aerobic treatments such that the effluent from the side stream reactors be closed during aerobic phases in the treatment tank and can be open during anaerobic phases in the treatment tank.

The influent from the sedimentation tank to the side stream reactor fed in the first fraction is stored in the side stream reactor system and hence the water level will increase. This may be advantageous if the waste water treatment plant is subjected to large variations in the amount of waste water being feed to the treatment tank.

The capacity of the side-stream pumps should then have a capacity which is sufficient to increase the side stream reactor flow during the shorter operation time.

In another particular embodiment of the invention, the side stream reactor is divided in individual sections wherein each individual section can be operated with aeration, intermittent aeration, or no aeration. By reducing the aeration in the zones of the side stream reactor, the transport of N0₃ ^" for anaerobic treatment is reduced and non-aeration treatment period is pro- longed. This results in an increased de-nitrification capacity. It has been observed that a partial de-nitrification in the magnitude of 60-70% is sufficient to operate and control waste water treatment plants efficiently.

In yet another embodiment the first return fraction of the settled sludge in step c) ii) is first subjected to an aerobic treatment in a section hav- ing a volume constituting 5 to 15 %, more preferred 8 to 12, such as app. 10% of the volume of an anaerobic section, followed in a second step by an anaerobic treatment in the anaerobic section, and, finalized in a third step by an aerobic treatment.

It was surprisingly found that by inserting an aerobic treatment in a small volume relative to the anaerobic treatment volume, the hydrolysis rate in the following anaerobic treatment was increased dramatically, and more specifically the hydrolysis rate was doubled.

In still another embodiment the side stream reactor may be provided with one or more parts by dividing the side stream reactor for example by at least one mechanical weir into two parts and provide each of the parts of the tank with manual or automatic effluent valves in addition to influent openings. The sections of the tank are in fluid communication with each. In this embodiment the side stream reactor may be operated in phases 1, 2 and 3 where:

1) Influent is led to the aerobic part. The effluent valve in the aerobic section is open and the effluent valve in the anaerobic section is closed. Part of the N03-N formed will then flow directly to the treatment tank without being subjected to denitrification in the anaerobic section of the side stream reactor.

2) Influent is led to the anaerobic section of the side stream reactor in which the valve is open, whereas the valve in the aerobic section is closed. The anaerobic section thus receives new sludge from the treatment tank and the HCOD can flows to the treatment tank in which it facilitates de- nitrification and P removal/stripping; and finally

3) Influent is led to the aerobic tank. The effluent valve in the aerobic tank is closed and open in the anaerobic part. The anaerobic section will then receive nitrate from the aerobic section and HCOD will be utilized for de- nitrification. When the limit of the de-nitrification capacity has been reached and the nitrate concentration exceeds a specific preset value, has been reached and the nitrate concentration exceeds a specific preset value, such as 1-5 mg N0₃-N/l, and the operation may be shifted to phase 1 described above.

If the cycle is run with 50% of aerated time and 50% of anaerobic time, the sludge hydrolysis process will produce a HCOD level of e.g. 2 to 4% of the sludge COD present in the reactors. The amount of produced NH4-N from the hydrolysis of the COD will comprise around 10 to 12% of the released HCOD, and hence it is possible, at the same time, to remove the hy- drolysed HCOD and the released NH4-N using the combined nitrifica- tion/denitrification process.

If the reactors are operated with 50% aeration /50% denitrification, the level of nitrate-N out of the combined process will be low and enhanced biological phosphorus removal will be induced.

Effluent from the combined aeration/denitrification will contain surplus HCOD to be used in the main biological reactors for higher rate denitrifi- cation compared to a conventional activated sludge denitrification process.

In a typical operation cycle, the total time of phase 1, 2 and 3 will be 4 to 8 hours; the duration of each phase may be varied depending on the amount of ammonia-N, nitrate-N and phosphate-P, in the reactor.

This setup may be varied in further embodiments described below in which the side stream reactor the first fraction is always fed to the side stream reactor which is comprised of a central aerobic section and a number of anaerobic sectors, preferably two positioned at each side of the aerobic sector in a parallel manner for example as shown in figure 4A. Each section is equipped with an effluent valve. The inlet may be as shown in figure 4A or a directing inlet adapted to provide inflow to the section desired, as illustrated in figure 5A. In this setup the sludge is treated in a series of steps where the inlet is always directed to the aerobic section and where an operation is run in which

1) the effluent valve of the aerobic section is open and the anaerobic effluent valves are closed. Hence, part of the nitrate formed will flow to the treatment tank without denitrification in the anaerobic sections.

2) the effluent valve of one of the two anaerobic sections is open and the other two effluent valves are closed. One anaerobic section will then received nitrate from the aerobic section and HCOD will be utilized for de- nitrification. Subsequently P-bacteria stripped from P are flow to the treatment tank with excess HCOD which will enhance the biological P removal and denitrification in the treatment tank; and finally

3) the effluent valve of the other anaerobic section (a section different from that of step 2), and the other two effluent valves are closed. This will have the same effect as described under 2). A typical time span to complete phases 1, 2 and 3 is 4-8 hours.

In still a variation of the setup described immediately above of the side stream reactor, the first fraction is again always fed to a central aerobic section of the side stream reactor. Preferably on each side, sectors are pro- vided capable of operating at either aerobic or anaerobic conditions. Preferably the sections are positioned in a parallel manner for example as shown in figure 4A. Each section is equipped with an effluent valve. The inlet may be as shown in figure 4A or a directing inlet adapted to provide inflow to the section desired. In this setup the sludge is also treated in a series of steps where the inlet is always to the aerobic section and where a real time operation is run in which

1) the effluent valve of the central aerobic section is open and the anaerobic effluent valves are closed. Hence, part of the nitrate formed will flow to the treatment tank without denitrification in the anaerobic sections. The other two sections are run anaerobically and due to the closed valves nitrate formed in the previous cycles will be de-nitrified whereby P will be released when the nitrate concentration is sufficiently low, i.e. below 1 - 1.5 mg/l.

2) the effluent valve of both side sections are open and the central effluent valve is closed. The released P from step 1 in the anaerobic sections is now allowed to flow to the treatment tank while nitrate from the central aerobic section flows to the anaerobic sections due to the shifted position of the valves. This nitrate will be de-nitrified in the side tanks. In phase 2, one of the side tanks will be operated at aerobic conditions for a period until all released nitrogen has been oxidized into nitrate whereafter the central vaaeve is opened and the side valves are closed; and finally

3) the valves of the section which has solely operated under anaerobic conditions is opened and the two other valves are closed. P released from the P bacteria then flows to the treatment tank, and nitrate from the central tank flows to the open section where it is de-nitrified.

Hence in the various embodiments described above the side stream treatment comprises a number of real time cycles where the sludge is directed to various sections at various time slots of the cycle, This will ensure a flexible and optimal utilization of the feed and the bacteria in order to be able to removed P and N at high levels without using specially added bacteria or chemicals at various points of the treatment process.

In another particular embodiment, a mixed liquid suspended solid (MLSS) concentration (kg/m³) in the side stream reactor is 2 to 6 times higher than the MLSS concentration in the treatment tank, preferably 3 to 5 times higher which makes it possible to obtain an increased rate of COD hydrolysis, nitrification, de-nitrification and phosphorus removal.

Thus, the side stream reactors constitute about 20-33% of the required volume if all these processes were to take place in the treatment tank. Consequently, the same overall conversion is achieved in a smaller volume.

According to another aspect of the invention, there is provided the use of the process according to the invention for biological phosphor removal and/or biological nitrification and denitrification.

The use of the process for biological nitrification and denitrification and/or biological phosphor removal is in a particular embodiment preferable when the ratio of COD to total nitrogen is low, such as lower than 7, preferably lower than 6, and even more preferably lower than 5, 4, or 3.

According to another aspect of the invention there is provided a computer program wherein the above processes are carried out by instructions from a computer into which the computer program has been installed.

In yet another aspect the invention provides a system suitable for treating waste water by the activated sludge method said system comprising at least one treatment tank (A) connected to at least one sedimentation tank (B) said sedimentation tank (B) being connected to at least one side stream reactor (C), said side stream reactor (C) being connected to the at least one treatment tank (A), and said treatment tank (A) further having an inlet and said sedimentation tank further having one or more liquid and/or solid outlets, wherein a direct communication pipe system is provided between the sedimentation (B) tank and the treatment tank (A).

It should be understood that by direct communication, pipe system is meant one or more pipes interconnected. Mixing of various streams may occur (such as illustrated in figures 1 A - C supra). Such constructions are still within the definition of a direct communication pipe system being provided.

Figures

Figures 1A, B and C is a schematic overview of a biological waste water treatment system according to the present invention illustrating return of the side stream treated stream V_T in various positions.

Figure 2 is a schematic overview of an embodiment of the side stream reactor, C, illustrated in figures 1A, B and C wherein the first return fraction of the activated sludge is first subjected to anaerobic treatment followed by an aerobic treatment.

Figure 3 is a schematic overview of an embodiment of the side stream reactor, C, illustrated in figures 1A, B and C wherein the first return fraction of the activated sludge is first subjected to aerobic treatment fol- lowed by an anaerobic treatment.

Figures 4 A to B illustrate various embodiments of the invention where the aerobic and anaerobic treatments of the first fraction are in parallel.

Figures 5 A and B illustrate an embodiment where the side stream reactor is operated with a period of aerobic conditions, anaerobic conditions and aerobic followed by anaerobic conditions.

Detailed description of the invention

In the following the present invention is described in more detail. All features and details should be equally applied to each embodiment and aspect of the process and use.

By the term "treatment tank" is meant a system wherein organic and inorganic matter of the waste water are degraded in a biological process where microorganisms are utilized to remove organic compounds, nitrogen and phosphorous, etc., from waste water. The treatment tank system may comprise different zones, such as anaerobic aerobic and/or an anoxic zone which each can be placed in different orders - both serial and in parallel.

The retention time of each treatment tank is determined by the flow into the specific treatment tank divided by the volume of the specific treat- ment tank. The retention time of the treatment tank can vary much but is typically 2 hours to 3 days. However, the retention time is specific for each individual waste water treatment plant and depends also highly on the type of waste water to be treated and the ambient temperatures. Thus, the actual residence time of each waste water treatment plant depends on the condi- tions. It is within the skilled of the art to determine in which order the resi- dence time typically should be.

By the term "side stream reactor" is meant a system optionally subdivided in a number of individual sections in series wherein the first return fraction of the settled sludge is subjected to aerobic and anaerobic conditions. Each section can be operated individually such that the first return fraction of the settled sludge is either subjected to an aerobic or anaerobic treatment followed by an anaerobic or aerobic treatment, respectively.

The retention time in the side stream reactor is determined by the flow into the side stream reactor divided by the volume of the side stream reactor. The time of the side stream reactor can vary much but is typically 6 hours to 3 days, preferably 12 hours to 2 days, more preferably 20 hours to 30 hours. As the side stream reactor can be operated in individual sections, the residence time of each section may be different. However, the residence time of each section of the side stream can vary much but is typically 3 hours to 1 day, preferably 6 hours to 12 hours, more preferably 10 hours to 15 hours. If the side stream reactor is operated in batch conditions, i.e. there is only a flow into a section of the side stream reaction without any liquid flow out from the section of the side stream reactor, the residence time calculated as the flow into the section of the side stream reactor divided by the volume of the section of the side stream reactor. The residence time of the section of side stream reactor is typically 2 hours to 1 day, preferably 4 hours to 15 hours, more preferably 7 hours to 10 hours.

By the term COD is meant the measure of the chemical oxygen demand of oxidizable pollutants liable to be degraded using strong oxidizing agents. The COD tests are a measure of the relative oxygen-depletion effect of waste water contaminants. The COD is measured by the ISO 6060: 1989 standard (Water quality -- Determination of the chemical oxygen demand).

By the term hydrolysed COD or HCOD is meant the measure of the oxidizable pollutants. The content and amount of HCOD is formed by hydroly- sis of COD by the microorganisms during the aerobic and anaerobic treatment. The HCOD is measured by a combination of the difference in soluble COD and soluble P04-P before and after the biological treatment, using the following expression : activated return sludge process tank and the difference in soluble P04-P before and after the process tank using the following ex- pression : HCOD = Asoluble COD + 2.5*Δ P04-P

The COD and P04-P is measured on filtered samples using 4μηι filters or similar.

By the term "MLSS" is meant mixed liquor suspended solids, which is expressed as amount of total suspended solids in kg per m³.

Unless otherwise stated all percentages throughout the description and claims are % vol/vol.

By the term "alternating anaerobic treatment and aerobic treatment" is meant a process wherein the side stream reactor is periodically aerated such that a period of aerobic treatment of the first return fraction of the settled sludge is obtained, followed by a period of anaerobic treatment of first return fraction of the settled sludge. The period of aeration and non-aeration may be 0.5, 1, 2, 5 hours or even more. Further, the period of aeration and non-aeration may be different. Thus, the side stream reactor could be operated with 2 hours of aeration followed by 1 hour with no aeration.

Referring now to Figures 1A, B and C the process of the invention is illustrated in which untreated waste water or, alternatively, primary clarified waste water, is treated according to the invention.

The waste water feed Qi is fed to a treatment tank, A, where the waste water feed, Qi, is subjected to different biological treatments by microorganisms , such as anaerobic treatment, aerobic treatment and anoxic treatment to provide the treated stream Q_T.

The treatment tank, A, may be comprised of several individual tanks which each may be combined in series or in parallel and operated independently of each other. I.e. the number, order and type of the biological treatments in the treatment tank may differ. Hence, the biological treatment process could be an aerobic treatment only, or, alternatively, as may be the normal case, the biologic treatment process could be an anaerobic treatment followed by or preceded by an aerobic treatment either in one tank or in several tanks.

A mixture of treated waste water and sludge flows from the treatment tank, A, to a sedimentation tank, B, as the treated stream Q_T. In the sedimentation tank B, the sludge settles to provide a two phase system com- prising treated waste water and settled sludge.

The treated waste water, Q₂, comprising only small amounts of sludge (effluent), is discharged, possibly for further processing if necessary.

A fraction of the settled sludge Q₃ is taken from the bottom section of the sedimentation tank. It is contemplated that more than one stream of the sludge may be taken from the sedimentation tank, but one line is preferred, as this is easier in maintenance and installation. The stream is split in two return fractions. A first return fraction, Vi, of the settled sludge is subjected to a further biological treatment in a side stream reactor, C, before it is returned to the treatment tank, A, as the side stream treated stream V_T. The treatment in the side stream reactor, C, comprises at least an aerobic treatment and/or an anaerobic treatment.

A second return fraction, V₂, of the settled sludge is returned to the treatment tank A without any further biological treatment.

The splitting of the settled stream, Q₃ is controlled by flow meters, valves and pumps positioned as appropriate either upstream from the splitting of the streams or after the splitting. Two pumps downstream from the splitting are preferred.

The side stream treated stream V_T and the second return fraction V₂ may be connected to the feed stream upstream from the treatment tank for one single feeding to the treatment tank (figure 1A), be fed to the treatment tank individually (figure IB) or the two streams V_T and V₂ may be combined before feeding to the treatment tank A (figure 1C).

A last optional fraction, Q₄, of the settled sludge is discharged, pos- sibly for further processing if necessary, but most often for recovering active bacteria for further inoculating.

The system further comprises pipes, liquid moving means, such as pumps, as well as valves or other means for opening and closing communication between zones, sections and tanks. These are within the skill of the art.

In order to control the biological treatment of the first return fraction of the settled sludge and the flow of the first and second return fractions of the settled sludge, sensors and flow meters can be included a number of positions in the process for measuring a number of factors.

Factors to measure include but are not limited to the inlet flow, Qi, of untreated waste water to the treatment tank, any internal flow(s) between different treatment tanks, the flow(s) from the treatment tanks to the sedimentation tank, and the flow of first and second return fraction of the settled sludge,

the level of liquid in the treatment tank(s), the different treatment zones in the side stream reactor, and the sedimentation tank,

the MLSS content in the treatment tank(s), the different treatment zones in the side stream reactor and the sedimentation tank,

the concentrations of N0₃ ^~, oxygen, 0₂, COD, BOD, HCOD, total nitrogen and total phosphorous and P0₄-P in the different treatment zones in the side stream reactor, the treatment tank(s) and the sedimentation tank.

The output of the measurements is used to operate the process by software specially developed for controlling waste water treatment, such as EnviStyr available from EnviDan A/S.

Referring now to Figure 2 the side stream reactor of figures 1A, B and C is illustrated with the further modification that the biological treatment of the first fraction, Vi of the settled sludge in the side stream reactor, C, is first subjected to an anaerobic treatment in an anaerobic treatment zone, C- 1, followed by an aerobic treatment in an aerobic treatment zone, C-2, to provide the side stream treated stream V_T. The anaerobic and aerobic treat- ment zones, C- l and C-2, can be divided in several individual sections (indicated by dashed lines).

Referring now to Figure 3 the side stream reactor of figures 1A, B and C is illustrated with the further modification that the biological treatment of the first fraction of the settled sludge is in the side stream reactor C first subjected to an aerobic treatment in an aerobic treatment zone, C-2, followed by an anaerobic treatment in an anaerobic treatment zone, C-l, to provide the side stream treated stream V_T. The anaerobic and aerobic treatment zones, C- l and C-2, can be divided in several individual sections (indicated by dashed lines).

Referring now to figures 4 A to B embodiment of the invention are illustrated where the settled sludge is side stream treated in parallel side stream reactors either by single treatment zones, bypassing of second treatment zones or a combination.

In the embodiment illustrated in figure 4A the first return fraction, Vi, is split in two, providing first return fraction A, Vi _A and first return frac- tion B Vi B- The fractions Vi__A and Vi__B are treated by aerobic and anaerobic treatments respectively. Thus, in this embodiment the first fraction is side stream treated by both aerobic and anaerobic treatment, but in subdivided fractions A and B treated in parallel. This embodiment is particularly useful when the N content is very high. In variations of this embodiment there is only one side stream reactor C having both an aerobic and an anaerobic zone but after aerobic and/or anaerobic treatment the treated sludge is bypassed and fed directly to the treatment tank, this is illustrated in figure 4B. Further variations and combinations of the embodiments illustrated are contemplated and are within the scope of the application. The invention will now be illustrated by the following non limiting example.

In figure 5 A a variation is shown in which the treatment tank is divided in two sections, an aerobic (bold) and an anaerobic section (white). The invention should however not be limited to two sections, as three or more sections are contemplated, such as an additional anaerobic section. The inlet is illustrated as an inlet distributor (ID), which can feed the inlet from the treatment tank (A) to the various sections of the side stream reactor. It is likewise contemplated that there may be an inlet to each part. Each section- has an effluent outlet and the parts are in fluid communication with each oth- er.

In figure 5 B the operation of the reactor is given in more details, again the bold boxes are aerobic treatment whereas the anaerobic boxes are white. In this embodiment the side stream reactor may be operated in phases 1, 2 and 3 where:

1) Influent is led to the aerobic part. The effluent valve in the aerobic section is open and the effluent valve in the anaerobic is closed. Part of the N03-N formed will then flow directly to the treatment tank without requiring denitrification in the anaerobic part of the side stream reactor. When operating for around 6 hours in total this mode operation may is typically within the time span of time 0 - 2 hours.

2) In the second phase influent is led to the anaerobic section of the side stream reactor in which the valve is open, whereas the valve in the aerobic section is closed. The anaerobic section thus receives new sludge from the treatment tank and the HCOD can be fed back to the treatment tank in which it facilitates de-nitrification and P removal/stripping this mode of operation is typically operated for an hour, i.e. at time 2-3; and finally

3) In the third phase influent is led to the aerobic tank. The effluent valve in the aerobic tank is closed and open in the anaerobic part. The anaerobic section will then received nitrate from the aerobic section and HCOD will be utilized for de-nitrification. When the limit of the de-nitrification capacity has been reached and the nitrate concentration exceeds a specific preset value, such as 2-4 hours and specifically as illustrated 3 hours, i.e. T= 3- 6. Hereafter, the operation may be shifted to phase 1 again as described above.

In a typical operation, the total time of phase 1, 2 and 3 will be 4 to 8 hours, more specifically 6 hours as illustrated. The duration of each phase may be varied depending on the amount of N and O in the reactor; however a typical operation of each phase may be 1-3 hours in phase 1, V2 to I V2 hours in phase 2 and 2 to 4 hours in phase 3.

In all embodiments described it is contemplated that the anaerobic sections/tanks may be equipped with aeration means in order to be able to provide air if deemed necessary. This will provide a more flexible setup.

Example

An embodiment of the invention according to claim 4 was tested in large scale. That is, the process with a side stream reactor having first an aerobic treatment zone followed by an anaerobic treatment zone.

In the feed the COD to N ratio was 6. The total sludge amount in the plant was 355.5 t DS and the sludge content of the side stream reactor (c) was predetermined to a level of 50% of the total volume of the plant, i.e. 177.8 t DS. The composition of the waste water feed was COD: 420 mg/l, BOD 180 mg/L, N 70 mg/l and P 8 mg/L.

The process was operated at cold conditions i.e. at 10°C. And the first return fraction was set so that the relationship Vl/Ql was 12.3% (vol/vol), the flow was adjusted by special adapted software such as EnviStyr available from EnviDan A/S.

Table 1 : mass balance of measured constituents (kg/day)

Measure/Stream Qi Q2 Q₄

COD 42,000 3,000 19,400 N 7,000 1,500 1,292

Table 2: flow rates (m³/day)

From this exam ple it was seen that a very effective N removal and COD deg radation was obtained with a small fraction of the return sludge being treated to activate the sludge for purification of the waste water in the treatment ta nk.

Claims

P A T E N T C L A I M S

1. A process for biological treatment of waste water by the activated sludge method with the use of the same bacteria population throughout the process, said process comprising the steps of

a) feeding a waste water feed to a treatment tank,

c) allowing the mixture to settled in a sedimentation tank to provide treated waste water and settled sludge, wherein the settled sludge is subjected to the steps of

i) separating a first and a second return fraction of the settled sludge,

2. A process according to claim 1, wherein, phosphor is removed by the bacteria in the treatment tank.

3. A process according to claim 1 or 2, wherein step ii) comprises both aerobic and anaerobic treatment.

4. A process according to claims 1, 2 or 3, wherein the first return fraction of the settled sludge is within the range 5-30% (vol/vol) of the first and second return fractions of the settled sludge, preferably 10-25% (vol/vol), more preferably 10-15% (vol/vol).

5. A process according to any of the claims 2 to 4, wherein the first return fraction of the settled sludge in step c) ii) is treated in series in a first step by aerobic treatment, followed in a second step by anaerobic treatment.

6. A process according to claim 5, wherein the aerobic treatment is divided in several sections and wherein at least one of these sections is operated with intermittent aeration.

7. A process according to claims 2, 3 or 4, wherein the first return fraction of the settled sludge in step c) ii) is treated in series in a first step by anaerobic treatment, followed in a second step by an aerobic treatment.

8. A process according to claims 2, 3 or 4, wherein the first return fraction of the settled sludge in step c) ii) is subjected to alternating anaerobic treatment and aerobic treatment.

9. A process according to any of the claims 1 to 8, wherein the side stream reactor is divided in individual sections, wherein each individual section can be operated with aeration, intermittent aeration, or no aeration.

10. A process according to claims 8 or 9, wherein the first return fraction of the settled sludge in step c) ii) is first subjected to an aerobic treatment in a section having a volume constituting 5 to 15 %, more preferred 8 to 12%, such as app. 10% of the volume of an anaerobic section, followed in a second step by an anaerobic treatment in the anaerobic section, and, finalized in a third step by an aerobic treatment.

11. A process according to any of the preceding claims 1 to 10, wherein the sidestream reactor is divided in individual sections, said individual sections being at least one aerobic section and at least one anaerobic section, where each section is provided with an effluent and are in fluid communication with each other, and wherein the first return fraction over a specific rime period time is operated in a series of phases comprising

a first aerobic phase where the first return fraction is fed to the aerobic section and the aerobic effluent is open and the anaerobic effluent is closed;

a second anaerobic phase where the first return fraction is fed to the anaerobic section and where the anaerobic effluent is open and the aerobic effluent is closed; and

a third aerobic and anaerobic phase where the first return fraction is fed to the aerobic section and where the aerobic effluent is closed and the anaerobic effluent is open.

12. A process according to any of the claims 1 to 11, wherein a mixed liquid suspended solid (MLSS) concentration (kg/m³) in the side stream reactor is 2 to 6 times higher than the MLSS concentration in the treatment tank, preferably 3 to 5 times higher.

13. A process according to any of the claims 1 to 12, wherein the first return fraction of the settled sludge constitutes from 3 to 10 % (vol/vol) of the waste water feed (Ql), more preferred 4 - 8 % (vol/vol), even more preferred 5 - 7 % (vol/vol).

14. A process according to any of the claims 1 to 13, wherein the waste water feed is sewage, municipal waste water, residential waste water, commercial waste water, industrial waste water, waste water from septic sludge tanks, and the like.

15. Use of a process according to any of the claims 1 to 14 for biological phosphorous removal and/or nitrification and denitrification.

16. A system suitable for treating waste water by the activated sludge method said system comprising at least one treatment tank (A) con- nected to at least one sedimentation tank (B) said sedimentation tank (B) being connected to at least one side stream reactor (C), said side stream reactor (C) being connected to the at least one treatment tank (A), and said treatment tank (A) further having an inlet and said sedimentation tank further having one or more liquid and/or solid outlets, wherein a direct commu- nication pipe system is provided between the sedimentation (B) tank and the treatment tank (A).