HK1208212A1

HK1208212A1 - Improved process and system for biological water purification

Info

Publication number: HK1208212A1
Application number: HK15108834.6A
Authority: HK
Inventors: 格特．彼得森; 雅各布．克拉格．安徒生
Original assignee: 恩维丹国际公司
Priority date: 2012-10-15
Filing date: 2013-10-10
Publication date: 2016-02-26
Also published as: WO2014059990A1; 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 method and system for biological water purification

Technical Field

The invention relates to a method for treating wastewater by using return sludge. More particularly, the present invention relates to a method for sidestream removal of nitrogen and phosphorous (which is removed in a wastewater treatment plant) using return sludge, wherein a small portion of the return sludge is not sidestream treated with external bacteria or chemicals for enhancing the method.

Background

Due to the increasing demand from legislators worldwide, processes and technologies for sanitary, municipal, commercial and industrial wastewater treatment have been constantly improved and new equipment is constantly being added.

The basic idea behind all biological processes for wastewater treatment is to introduce contact with microorganisms that feed on inorganic and organic matter in the wastewater. The concentration of contaminants in the wastewater, including inorganic or organic nitrogen and inorganic or organic phosphorus, as well as Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), and Total Suspended Solids (TSS) is thereby reduced. The principle behind biological treatment is the degradation and growth of microorganisms on the contaminants present in the wastewater. The organic material is converted by the metabolism of the microorganisms into cellular material, which is no longer in solution or dispersed, which can be separated from the aqueous phase in a secondary clarifier by simple gravity settling. The treated effluent leaving the system is then much cleaner than when it enters the Waste Water Treatment Plant (WWTP).

In a typical biological treatment based wastewater treatment plant, influent wastewater is pretreated in a primary treatment wherein a portion of the organisms, including suspended solids, are removed by settling. After this primary treatment, the wastewater is subjected to a biological treatment in which the contaminants in the wastewater are degraded in a biological process in which microorganisms are used to remove the remaining organic matter, nitrogen and phosphorus from the wastewater.

The biological treatment may include several treatment stages, including an anaerobic treatment zone and an aerobic treatment zone. The growth and metabolic activity of the microorganisms is thus controlled and exploited by using controlled process conditions in the different zones.

After biological treatment, the treated wastewater is allowed to settle/settle in a secondary settling tank for a predetermined period of time, after which effluent is extracted from the upper portion of the settling tank and discharged. A portion of the activated precipitation sludge, which contains biomass produced by the growth of microorganisms in the aeration tank during wastewater treatment, is removed for dewatering and disposal, while another portion of the remaining sludge may be returned to biological treatment as Activated Return Sludge (ARS) to enhance biodegradation. The activated return sludge may thus be returned to the biological treatment reactor without any subsequent treatment. Alternatively, the activated return sludge may be subjected to an initial aerobic treatment to increase the aerobic sludge age and improve nitrogen removal in the secondary treatment zone. In this way, low load systems can be easily upgraded, for example when new regulations require stricter limits on nitrogen removal.

The amount of activated sludge is rate limited when used in a treatment process in WWTP and existing WWTP will not be effective in meeting nitrogen effluent standards if there is no excess of COD to nitrogen, e.g., the ratio of COD to N is below 7, 6, or even 5. Moreover, aeration devices commonly used for return sludge treatment are idle for part of the time, typically 50% of the time in some WWTP concepts. This means that the amount of aeration equipment may have to be doubled and that all hydrolysed cod (hcod), which is produced in the anaerobic section of the wastewater treatment plant and which is not utilized by biological phosphorus removal (bio-P) microorganisms, will be degraded in the aerobic phase and then not used in the treatment tank for denitrification or biological phosphorus removal.

WO 2010/148044 describes an apparatus for removing biological phosphorus and nitrogen from raw water to reduce excess sludge production. This is achieved by treatment in an anaerobic side flow reactor in which hydrolysis and fermentation occur-in a preferred embodiment, the treatment is facilitated by providing acidic conditions in the side flow reactor (CSTR). In the side-flow reactor, the process includes the use of chemicals such as bases or acids, added digestive enzymes, sonication, ozone treatment or other oxidative chemical treatments, and heat treatment.

The method uses an acidic environment and heat to achieve benefits. One drawback of this method is that nitrifying bacteria cannot survive under acidic conditions and thus the promotion of the method and apparatus of WO 2010/148044 comes at the expense of nitrifying bacteria and their action.

Thus, despite the continuing development of wastewater treatment technology, there remains an urgent need for further improvements in biological wastewater treatment plants to provide an expanded range of use and still meet the high requirements for efficiency and removal of unwanted organic and inorganic compounds (e.g., total nitrogen, total phosphorus, total suspended solids, biological oxygen demand, and chemical oxygen demand) regardless of the available COD.

It is therefore an object of the present invention to provide an improved process for biological treatment with better energy utilization of the achievable COD and thus more efficient purification of the wastewater.

Disclosure of Invention

On the background of the above, in a first aspect it is an object of the present invention to provide a method for biological treatment of wastewater by an activated sludge process using the same bacterial population in the overall process, said method comprising the steps of:

a) the wastewater feed is made to flow into a treatment tank,

b) subjecting the wastewater in the treatment basin to a biological treatment process to provide a mixture of treated wastewater and activated sludge,

c) precipitating the mixture in a precipitation tank to provide treated wastewater and precipitated sludge, wherein the precipitated sludge is subjected to the steps of:

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

ii) subjecting the first return portion of the settled sludge to a process comprising aerobic and/or anaerobic treatment in a sidestream reactor,

d) flowing a first return portion of the settled sludge from the sidestream reactor and a second return portion of the settled sludge from the settling tank into a treatment tank.

Surprisingly, it has been found that by introducing two parts of return sludge into a treatment basin, wherein only the first part is subjected to a side stream treatment, the overall removal of inorganic and organic matter is more efficient than in conventional wastewater treatment plants, wherein one part of the return sludge is treated and returned to the treatment basin. In this particular process, a first portion of the activated sludge is subjected to aerobic and/or anaerobic treatment suitable for nitrification and denitrification processes as well as dephosphorization in a sidestream reactor and a second portion is returned to the treatment tank without any sidestream treatment.

By concentrating the time consuming biological reactions in a smaller volume of the side flow reactor (which has a sludge concentration 3-5 times higher than the treatment plant), the hydrolysis will be more efficient. The rate of hydrolysis under, for example, anaerobic treatment in the side-flow reactor continues at a constant rate up to a residence time of 40 hours.

The method uses the same bacterial population throughout the process, i.e. no external bacteria are added at any point of the process to enhance a specific process, nor are any chemicals added to enhance aerobic or anaerobic bacterial action.

In the context of the present invention by "same bacteria" is meant that no specific bacteria are added or grown at any point of the process and that the bacterial population is purely the result of the reactions taking place in the process, which is controlled by the volume, extent and time of aeration or absence of air.

By using a side-flow reactor according to the invention, phosphorus is mainly removed in the treatment cell a, whereas the bacteria in the system are ready for removal of P by optimal uptake using HCOD. Thus, the method can also be seen as a means for providing HCOD to the process tank for optimal removal of phosphorus.

This will provide a more flexible wastewater treatment plant when there is excess wastewater, since the COD content can be concentrated and/or utilized in the side-stream reactor, while the second portion of the return sludge in the settling tank is returned directly to the treatment tank, so that Hydrolysis (HCOD) providing easily accessible COD can be run without being affected by fluctuations in the flow of influent wastewater.

Furthermore, the system of the present invention provides a flexible solution with respect to the achievable COD content. Thus, with a low COD to nitrogen ratio, typically below 7, preferably below 6, more preferably below 5, there is still efficient denitrification. This is particularly useful in regions of the world where the ratio of COD to nitrogen is very low-or in other words, where the nitrogen content is high. This is the case, for example, in china, due to low fat food waste and due to frequent pre-treatment of industrial waste.

The pH is substantially constant throughout the process, about neutral or slightly alkaline, and therefore the pH is preferably greater than 6.5, more preferably greater than 7 and most preferably in the range of 6.8 to 7.5. Thus, no acidic or basic components need to be added to provide a particular pH, as the operation according to the invention ensures that the pH is within the preferred range.

The process of the present invention is also beneficial in that it removes phosphorus present in untreated wastewater more efficiently than prior art processes that focus primarily on denitrification.

Also, because smaller tubing and smaller sidestream treatment cells can be used, the installation costs for running the process of the present invention are lower. In addition to cost, this also affects the flexibility of implementing the present invention in existing wastewater treatment plants because the space required to improve the wastewater treatment process is limited.

Thus, in summary, the present invention relates to a process wherein hydrolysis occurs in a side-flow reactor with a-preferably constant-activated sludge flow rate, and wherein the resulting hydrolyzed oxidizable contaminants (HCODs) are available for bound nitrogen such as NO formed in the aerobic section of the side-flow reactor₃The denitrification, P removal in the anaerobic section of the side flow reactor and/or the remaining HCOD from the side flow reactor, may enhance denitrification and P removal in the treatment tank-i.e. for improved biological phosphorus and nitrogen removal by the microorganisms in the treatment tank.

The decomposition of complex organic compounds is broken down step by step to simple compounds and takes place in activated sludge distributed throughout the plant by the process of hydrolyzing these compounds to hydrolyzed cod (hcod).

Anaerobic treatment allows polyphosphate microorganisms (bacteria) to take up Volatile Fatty Acids (VFAs) present in wastewater and release phosphorus in the form of phosphate to increase the future potential for phosphate absorption from wastewater.

Aerating the wastewater to allow ammonia and R-NH₃ ⁺And NH₄ ⁺Nitration to nitrite NO₂ ^-And finally nitrated to nitrate NO₃ ^-. The nitrifying microorganisms are specialized and grow slowly. Thus, sufficient time must be available for aerobic treatment of the activated sludge to enable the growth of the microorganisms at a faster rate than the removal rate of the settled sludge from the discharge settling tank. Aeration also promotes COD and BOD consumption by the heterotrophic organisms.

In degrading COD materials, many COD removing microorganisms are able to utilize NO if oxygen is not available₃ ^-As an oxidant, and adding NO₃ ^-Conversion to free nitrogen, N₂。

Phosphorus removal is a biological removal by using special microorganisms present in the sludgeTwo-stage process with phosphorus (bio-P) bacteria. If the biological phosphorus removal bacteria are in the absence of oxygen and NO₃ ^-The biological phosphorus-removing bacteria will absorb and convert VFA-COD into organic polymer, PHB, by removing phosphate to the aqueous phase, which is stored in the cells as an energy source in the polymerized phosphate compounds.

When the biological phosphorus removing bacteria are kept under aerobic conditions in the next step, the biological phosphorus removing bacteria will absorb more phosphorus than it is removed, which is often referred to as an "over-uptake" process. The dephosphorizing bacteria utilized 4 times more HCOD in this step than was taken up in the first release step.

Biological phosphorous removal bacteria are HCOD dependent in both anaerobic and aerobic steps and in the first step the bacteria are inhibited by nitrate concentrations above about 1 mg/l. Thus, a key aspect of dephosphorization is to enhance the growth of the bacteria in the anaerobic zone and to make them release phosphate in preparation for the bacteria's uptake of phosphorus in the treatment tank.

The absorbed phosphorus will be removed together with excess sludge. The biological phosphorus removing bacteria can only easily use degradable compounds or Hydrolyzed Compounds (HCOD), preferably in soluble form, and the concentration of these compounds in the influx of the WWTP is not ideal for biological phosphorus removal in cold climate conditions, since bacterial growth is limited.

Anoxic and anaerobic treatment promotes nitrate to nitrogen (N)₂Which is released to the atmosphere). In the context of the present invention, anoxic is defined as a condition in which no free oxygen is present but oxygen is present by incorporation into nitrate compounds. Denitrifying microorganisms also consume COD but there are many species with denitrification capability. Thus, the denitrifying microorganisms may also use partially hydrolyzed COD compounds. The denitrification rate will be increased if readily degradable compounds are available.

Hydrolysis occurs throughout the plant, but in all aeration reactors or stages, the COD removing microorganisms rapidly convert readily degradable compounds with oxygen and leave no excess hydrolyzed COD (also known as easily degradable compounds) for biological phosphorus or nitrogen removal.

In a particular embodiment, the first return portion of the settled sludge is in the range of 5-30% (v/v), preferably 10-25% (v/v), more preferably 10-15% (v/v) of the first and second return portions of the settled sludge. Thus, the first return portion of the settled sludge constitutes only 3-10% (v/v), more preferably 4-8% (v/v), even more preferably 5-7% (v/v) of the wastewater stream feed (Q1).

Even at such low reflux volumes in the side-stream reactor, COD/HCOD utilization was found not to decrease, as the reactivity was found to be independent of the volume of the reflux portion and to be related to the concentration of sludge in the reflux portion. At the same time, the sidestream treatment is not affected by water flow fluctuations, as the second sidestream can 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 in the side-flow reactor as highly concentrated sludge, which ensures that the content of COD species is concentrated and hydrolysed at a constant rate but in a much smaller volume. Thus, in short, the production capacity is optimized because the same utilization is obtained in a smaller volume.

Regardless of the total demand for oxygen associated with COD conversion, in a side flow reactor, an amount proportional to the COD in the side flow reactor may be satisfied. The time the sludge is aerated in the sidestream reactor may be counted as Aerobic Sludge Age (ASA), which is measured to provide a sufficient population of nitrifying bacteria in the WWTP.

It has been observed that the rate of hydrolysis of COD material under anaerobic conditions continues at a constant rate of hydrolysis for residence times up to 40 hours. The residence time is defined as the flow into a particular process tank divided by the volume of the process tank (treatment tank, side flow reactor, settling tank, etc.). Thus, the phosphorus removal and nitrification and/or denitrification process as a whole is improved.

The flow rate of the first return portion of the settled sludge is preferably constant irrespective of the sludge return flow rate, i.e. irrespective of the sum of the first and second return portions of the settled sludge (sludge return flow rate). By operating the process in this manner, it is ensured that a very low volume is maintained in the side-stream reactor, irrespective of the influent flow into the wastewater treatment plant.

Controlling the sludge return flow rate can be adapted to changes in wastewater inflow, e.g., due to heavy rainfall, drought, etc. This results in a more robust wastewater treatment process. Therefore, WWTP can be operated more efficiently by constant optimal utilization of COD.

Furthermore, the lower flow rate of the sludge return leads to an increase in the depth of the sludge in the clarifier and thus to an increase in the concentration of return activated sludge provided to the treatment tank.

The second return portion may be varied according to the level of the inlet water. The purpose of the second reflux portion is to maintain a high concentration of sludge in the treatment tank. The second reflux portion constituted about 40% (v/v) of the wastewater feed.

There are many challenges in optimizing wastewater treatment, and particularly in efficiently operating a sidestream reactor, because nitrate is formed in the aerobic section and oxygen and soluble Nitrate (NO) are formed during at least a portion of the anaerobic section or during a portion of the non-aeration time₃-N) must be very low in concentration in order to make P removal in the side stream effective. Thus, if there is an excess of hydrolyzed cod (hcod), an improved effluent quality will be obtained if the side-stream reactor treatment is started with an aerobic step.

Thus, in a particular embodiment of the invention, the first return portion of the settled sludge in step c) ii) is continuously treated in a first step of aerobic treatment, followed by a second step of anaerobic treatment. This will ensure that the content of COD species is decomposed under aerobic conditions, releasing nitrogen. Further variations of this embodiment may be implemented to minimize nitrate levels in anaerobic treatment. In one embodiment, this is achieved by increasing the denitrification capacity in the anaerobic reaction zone by reducing the aerobic capacity in the aerobic reaction zone, such as by dividing the aerobic reaction zone into several sections, at least one of which is operated under intermittent aeration, which will ensure that HCOD is converted and that sufficient denitrification has occurred.

It is possible to set a predetermined maximum nitrate level in the first return portion of the settled sludge before it enters the anaerobic zone from the aerobic zone. Proper aeration prior to entry will ensure optimal operation of the anaerobic zone.

After flowing the first and second return portions of the settled sludge into the treatment tank, the remaining HCODs may be consumed by denitrification and/or biological phosphorus removal bacteria. If the ratio of COD to nitrogen is below 6, excess COD may be required in the treatment tank, which will ensure that full denitrification capacity is obtained.

Thus, in particular, this embodiment may be more advantageous when the ratio of COD to nitrogen in the treatment tank is low, i.e. below 6, such as below 6, 5, 4 or 3.

This embodiment is also advantageous if the ratio is higher, because the remaining HCOD will increase the denitrification rate and thus speed up the treatment process in the treatment basin, whereby the capacity of the apparatus is further increased.

Another challenge is that wastewater treatment plants are designed for assumed future loads, however the actual load and wastewater composition may differ from the assumed conditions.

The removal of phosphorus in the anaerobic phase will only be efficient in cases where both the oxygen and nitrate concentrations are very low, preferably close to zero. Thus, in another embodiment of the invention, the first return portion of the settled sludge in step c) ii) is treated continuously in the first step of anaerobic treatment, followed by the second step of aerobic treatment. Nitrate is thus discharged to the treatment tank and is absent during anaerobic hydrolysis. When NH is in the feed₃The method is particularly useful when the concentration of (a) is high.

In another embodiment of the invention, the first return portion of the settled sludge in step c) ii) is subjected to alternating anaerobic and aerobic treatment, such that the outflow from the side flow reactor is closed during the aerobic section in the treatment tank and open during the anaerobic section in the treatment tank.

The influent in the first portion from the settling tank to the side flow reactor is stored in the side flow reactor system and the water level will rise accordingly. This would be advantageous if the wastewater treatment plant was subjected to large fluctuations in the amount of wastewater fed to the treatment basin.

The capacity of the sidestream pump will then have sufficient capacity to increase the flow rate of the sidestream reactor during a shorter run time.

In another embodiment of the present invention, the sidestream reactor is divided into separate sections, wherein each separate section may be operated under aeration, intermittent aeration or no aeration. Reduction of NO for anaerobic treatment by reducing aeration in the zone of a sidestream reactor₃ ^-And the non-aeration treatment period is prolonged. This results in an increase in denitrification capability. It has been observed that partial denitrification on the order of 60-70% is sufficient to operate and control wastewater treatment plants efficiently.

In another embodiment the first return portion of the settled sludge in step c) ii) is first subjected to aerobic treatment in a section having a volume of 5-15%, more preferably 8-12%, such as about 10% of the volume of said anaerobic section, followed by a second step, i.e. anaerobic treatment in the anaerobic section, and finally a third step, i.e. aerobic treatment.

It has surprisingly been found that by inserting aerobic treatment into a smaller volume relative to the anaerobic treatment volume, the hydrolysis rate in the subsequent anaerobic treatment is significantly increased and more particularly doubled.

In yet another embodiment, the sidestream reactor may be provided with one or more sections, by dividing the sidestream reactor into two sections, for example by at least one mechanical weir (mechanical weir), and each section of the tank is provided with a manual or automatic outflow valve in addition to the inflow. The portions of the cell are in fluid communication with each other. In this embodiment, the side-stream reactor may be operated in stages 1, 2 and 3, wherein:

1) the feed is introduced into the aerobic section. The outflow valve of the aerobic section is opened and the outflow valve of the anaerobic section is closed. Then a part of the formed NO₃N will flow directly into the treatment tank without being subjected to denitrification by the anaerobic section of the side-flow reactor.

2) The feed is introduced to the anaerobic section of the sidestream reactor, wherein the valve is opened, whereas the valve of the aerobic section is closed. The anaerobic section thus receives fresh sludge from the treatment tank and HCOD can flow to the treatment tank where it facilitates denitrification and P removal/stripping; and finally

3) The feed is introduced to an aerobic tank. And closing the outflow valve in the aerobic tank and opening the outflow valve of the anaerobic part. The anaerobic section will then receive nitrate from the aerobic section and the HCOD will be used for denitrification. When the limit of denitrification capacity has been reached and the nitrate concentration exceeds a certain preset value, e.g. 1-5mg NO₃at-N/L, operation may be switched to segment 1 above.

If the cycle is run for 50% of the aeration time and 50% of the anaerobic time, the sludge hydrolysis process will produce, for example, 2-4% HCOD levels of sludge COD present in the reactor. NH generated from COD hydrolysis₄The amount of-N will include about 10-12% of the HCOD released, and thus the hydrolyzed HCOD and the NH released are removed simultaneously using a combined nitrification/denitrification process₄-N is possible.

If the reactor is run with 50% aeration/50% denitrification, the nitrate-N level from the combined process will be low and enhanced biological phosphorus removal will be initiated.

The effluent from the combined aeration/denitrification will contain the remaining HCOD to be used in the primary bioreactor for higher rate denitrification than in conventional activated sludge denitrification processes.

In a typical operating cycle, the total time for segments 1, 2 and 3 will be 4-8 hours; the duration of each stage may vary depending on the amount of ammonia-N, nitrate-N and phosphate-P in the reactor.

This arrangement may be varied in further embodiments described below in which the first portion of the sidestream reactor is generally fed to a sidestream reactor comprising a central aerobic section and a plurality of anaerobic sections, preferably two on either side of the aerobic section in a parallel manner, for example as shown in figure 4A. Each section is provided with an outflow valve. The inlet may be as shown in fig. 4A or adapted to provide an inlet for feeding to the desired section as shown in fig. 5A. In this arrangement, the sludge is treated in a series of steps, wherein the inlet is usually led to the aerobic section and wherein the operation is run, wherein

1) The outflow valve of the aerobic section is opened and the anaerobic outflow valve is closed. Thus, part of the formed nitrate will flow into the treatment tank without denitrification in the anaerobic section.

2) The outflow valve of one of the two anaerobic sections is opened and the other two outflow valves are closed. An anaerobic zone will then receive nitrate from the aerobic zone and HCOD will be used for denitrification. The P-bacteria removed from the P then flow into the treatment tank with excess HCOD which will enhance biological P removal and denitrification in the treatment tank; and finally

3) The outflow valve of the other anaerobic section (section different from step 2) and the other two outflow valves are closed. This will have the same effect as described in 2).

A typical time span for completing stages 1, 2 and 3 is 4-8 hours.

In one variation of the side-stream reactor arrangement just described, the first portion is typically reintroduced to the central aerobic section of the side-stream reactor. Preferably, sections capable of operating under aerobic or anaerobic conditions are provided on both sides. Preferably, the segments are arranged in a parallel manner, such as shown in fig. 4A. Each section is provided with an outflow valve. The inlet may be as shown in fig. 4A or adapted to provide an inlet for feeding to the desired section. In this arrangement, the sludge is also treated in a series of steps, wherein the inlet is usually led to the aerobic section and wherein real-time operation is run, wherein

1) The outflow valve of the central aerobic section is opened and the anaerobic outflow valve is closed. Thus, part of the formed nitrate will flow into the treatment tank without denitrification in the anaerobic section. The other two stages run anaerobically and due to the closing of the valves the nitrate formed in the previous cycle will be denitrified, so that when the nitrate concentration is sufficiently low, i.e. below 1-1.5mg/l, phosphorus will be released.

2) The outflow valves of the two side sections are opened and the central outflow valve is closed. The P released in the anaerobic section from step 1 is now allowed to flow into the treatment tank, while the nitrate from the central aerobic section flows into the anaerobic section, due to the shifted position of the valve. The nitrate will be denitrified in the side tank. In stage 2, one of the side tanks will be operated under aerobic conditions for a period of time until all the released nitrogen has been oxidized to nitrate, after which the central valve is opened and the side valves are closed; and finally

3) The valve of the section that has been operated separately under anaerobic conditions is opened and the two other valves are closed. The P released from the P-bacteria then flows into the treatment tank and the nitrate from the central tank flows into the open section where it is denitrified.

Thus, in various embodiments described above, the sidestream treatment comprises a plurality of real-time cycles, wherein the sludge is directed to different stages during different periods of the cycle. This will ensure a flexible and optimal utilization of the feed and bacteria to enable P and N removal at different point-high levels of the treatment process without the use of specially added bacteria or chemicals.

In another embodiment, the Mixed Liquor Suspended Solids (MLSS) concentration (kg/m) in the sidestream reactor³) 2-6 times, preferably 3-5 times higher than the MLSS concentration in the treatment tank, which makes it possible to obtain an increased rate of COD hydrolysis, nitrification, denitrification and phosphorus removal.

Thus, the side-flow reactor represents about 20-33% of the required volume if all of these processes occur in the treatment cell. Thus, the same overall conversion is obtained in a smaller volume.

According to a further aspect of the invention, there is provided the use of the method according to the invention for biological phosphorus removal and/or biological nitrification and denitrification.

The use of the method for biological nitrification and denitrification and/or biological phosphorus removal is in a particular embodiment preferably when the ratio of COD to total nitrogen is low, such as below 7, preferably below 6, and more preferably below 5, 4 or 3.

According to another aspect of the present invention, there is provided a computer program in which the above processes are performed by an instruction from a computer in which the computer program has been installed.

In another aspect, the present invention provides a system suitable for treating wastewater by the activated sludge process, said system comprising at least one treatment basin (a) connected to at least one settling basin (B), said settling basin (B) being connected to at least one side flow reactor (C), said side flow reactor (C) being connected to said at least one treatment basin (a), and said treatment basin (a) further having one inlet and said settling basin further having one or more liquid and/or solid outlets, wherein a piping system in direct communication is provided between said settling basin (B) and treatment basin (a).

It should be understood that by direct communication, a piping system is meant one or more interconnected pipes. Mixing of the different streams may occur (as shown in fig. 1A-C). Such a configuration is still within the definition of a direct communication piping system provided.

Drawings

FIGS. 1A, B and C are schematic illustrations of a biological wastewater treatment system according to the invention illustrating a side stream treatment stream V_TReflux at different locations.

Fig. 2 is a schematic representation of an embodiment of the sidestream reactor C shown in fig. 1A, B and C, wherein the first reflux portion of the activated sludge is first subjected to anaerobic treatment followed by aerobic treatment.

Fig. 3 is a schematic representation of an embodiment of the sidestream reactor C shown in fig. 1A, B and C, wherein the first reflux portion of activated sludge is first subjected to aerobic treatment followed by anaerobic treatment.

Figures 4A-B illustrate different embodiments of the invention wherein the aerobic and anaerobic treatment of the first section are in parallel.

Fig. 5A and B illustrate an embodiment wherein the sidestream reactor is operated under one stage aerobic conditions, anaerobic conditions and aerobic followed by anaerobic conditions.

Detailed Description

The present invention is described in more detail below. All of the features and details should be equally applicable to the various embodiments and aspects of the methods and uses.

The term "treatment tank" refers to a system in which organic and inorganic substances of wastewater are degraded in a biological process using microorganisms to remove organic compounds, nitrogen, phosphorus, and the like from the wastewater. The treatment tank may comprise different zones, such as anaerobic, aerobic and/or anoxic zones, each of which may be in different orders-in series and in parallel.

The residence time of each treatment cell is determined by dividing the flow into a particular treatment cell by the volume of the particular treatment cell. The residence time of the treatment tank can vary widely but is typically 2 hours to 3 days. However, the residence time is specific to each individual wastewater treatment plant and is also highly dependent on the type of wastewater to be treated and the ambient temperature. Therefore, the actual residence of each wastewater treatment plant depends on the conditions. The person skilled in the art can determine in what order the residence times should typically be.

The term "sidestream reactor" refers to a system, optionally subdivided into a number of separate sections in series, wherein the first reflux portion of the settled sludge is subjected to aerobic and anaerobic conditions. Each section may be operated independently so that the first return portion of the settled sludge is subjected to aerobic or anaerobic treatment, followed by anaerobic or aerobic treatment, respectively.

The residence time in the side flow reactor is determined by the flow entering the side flow reactor divided by the volume of the side flow reactor. The time for the side flow reactor can vary widely but is typically from 6 hours to 3 days, preferably from 12 hours to 2 days, more preferably from 20 hours to 30 hours. Since the side-stream reactor can be operated in separate stages, the residence time of each stage can be different. The residence time in each section of the side flow reactor may vary widely, however, but is typically from 3 hours to 1 day, preferably from 6 hours to 12 hours, more preferably from 10 hours to 15 hours. If the sidestream reactor is operated under batch conditions, i.e. only one stream enters the sidestream reaction section without any liquid flowing out of the section of the sidestream reactor, the residence time is calculated as the flow into the section of the sidestream reactor divided by the volume of the section of the sidestream reactor. The residence time of the sections of the side flow reactor is typically from 2 hours to 1 day, preferably from 4 hours to 15 hours, more preferably from 7 hours to 10 hours.

The term COD refers to the chemical oxygen demand for degrading oxidizable pollutants using strong oxidants. The COD test is a measure of the relative oxygen-consuming effect of wastewater pollutants. COD is measured by the ISO 6060:1989 standard (water quality-chemical oxygen demand determination).

The term hydrolyzed COD or HCOD refers to a measure of oxidizable contaminants. HCOD levels and amounts are formed by microbial hydrolysis of COD in aerobic and anaerobic treatments. The HCOD was measured by combining the differences in soluble COD and soluble PO4-P before and after biological treatment using the following expression: the difference between the activated return sludge treatment tank and the soluble PO4-P before and after the treatment tank, expressed using:

HCOD ═ Δ soluble COD +2.5 ×. Δ PO4-P

The COD and PO4-P were measured on the filtered samples using a 4 μm filter or the like.

The term "MLSS" refers to mixed liquor suspended solids expressed in kg per m³Total suspended solids amount.

Unless otherwise indicated, all percentages in the specification and claims are v/v%.

The term "alternating anaerobic treatment and aerobic treatment" refers to a process wherein a side flow reactor is periodically aerated so as to obtain a section of aerobic treatment of the first return portion of the settled sludge, followed by a section of anaerobic treatment of the first return portion of the settled sludge. The period of aeration and non-aeration may be 0.5, 1, 2, 5 hours or more. In addition, the periods of aeration and non-aeration may be different. Thus, the sidestream reactor may be operated with 2 hours of aeration followed by 1 hour of non-aeration.

Referring now to fig. 1A, B and C, the process of the present invention is illustrated wherein untreated wastewater or primary clarified wastewater is treated in accordance with the present invention.

Wastewater feed Q₁Is sent to a treatment tank A where the wastewater feed Q₁Subjecting the microorganisms to different biological treatments, such as anaerobic treatment, aerobic treatment and anoxic treatment, to provide a treated stream Q_T。

The treatment basin a may comprise several separate basins, each of which may be combined in series or in parallel and which operate independently of each other, i.e. the number, order and type of biological treatments in said treatment basin may differ. Thus, the biological treatment process may be just an aerobic treatment, or alternatively, as is often the case, the biological treatment process may be an anaerobic treatment followed or preceded by an aerobic treatment, which may be in one tank, or in several tanks.

The mixture of treated wastewater and sludge is used as treated stream Q_TFlows from the treatment tank A to the sedimentation tank B. In the sedimentation tank B, sludge is sedimented to provide a two-phase system comprising treated wastewater and sedimented sludge.

Containing only a small amount of sludge (effluent)) Treated wastewater Q of (2)₂Is discharged, possibly for further processing if necessary.

The precipitated sludge Q₃Is withdrawn from the bottom part of the sedimentation basin. It is conceivable that more than one sludge flow may be taken from the settling tank, but preferably from one pipeline, as this is easier to maintain and install. The stream is split into two reflux portions. First return portion V of the settled sludge₁At which it acts as a side stream treatment stream V_TIs subjected to further biological treatment in a side-flow reactor C before being refluxed to the treatment tank a. The treatment in the side-flow reactor C comprises at least one aerobic treatment and/or anaerobic treatment.

Second reflux fraction V of the precipitated sludge₂Returned to treatment tank a without any further biological treatment.

By the flow Q after said precipitation₃The division is controlled by suitably arranged flow meters, valves and pumps before or after the division. Two pumps downstream of the split are preferred.

Flow V of side stream treatment_TAnd a second reflux portion V₂Can be connected to the feed stream upstream of the treatment cell as a single feed to the treatment cell (FIG. 1A), can be fed independently to the treatment cell (FIG. 1B), or alternatively, can be two streams V_TAnd V₂May be mixed together before being fed into the treatment tank a (fig. 1C).

Final optional fraction Q of the precipitated sludge₄Is discharged and may be used for further processing if necessary, but most commonly is used to recover viable bacteria for further inoculation.

The system also includes piping, liquid moving devices, such as pumps, and valves or other devices for opening and closing the flow between the zones, sections and cells. These are all well known in the art.

To control the biological treatment of the first return portion of the settled sludge and the flow rates of the first and second return portions of the settled sludge, sensors and flow meters may be included at various locations in the process to measure a number of factors.

Factors to be measured include, but are not limited to, the input flow Q of untreated wastewater into the treatment tank₁Any internal flow(s) between the different treatment tanks, flow(s) from the treatment tanks to the settling tank, and flow of the first and second return portions of settled sludge,

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

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

NO in different process zones in the side-flow reactor, process tank(s) and settling tank₃ ^-Oxygen, O₂COD, BOD, HCOD, total nitrogen and total phosphorus and PO₄-the concentration of P.

The measured output is used to run the process through software developed specifically for controlling wastewater treatment, such as EnviStyr, available from EnviDan a/S.

Referring now to FIG. 2, a further variation of the sidestream reactor of FIGS. 1A, B and C is illustrated, a first portion V of settled sludge in the sidestream reactor C₁Is first subjected to anaerobic treatment in an anaerobic treatment zone C-1 and subsequently to aerobic treatment in an aerobic treatment zone C-2 to provide a side stream treatment stream V_T. The anaerobic and aerobic treatment zones, C-1 and C-2, may be divided into separate sections (shown by dashed lines).

Referring now to FIG. 3, a further variation of the side flow reactor of FIGS. 1A, B and C is illustrated, wherein the biological treatment of a first portion of the settled sludge is carried out in side flow reactor C, first subjected to aerobic treatment in aerobic treatment zone C-2, followed by anaerobic treatment in anaerobic treatment zone C-2Anaerobic treatment is carried out in the treatment zone C-1 to provide a sidestream treatment stream V_T. The anaerobic and aerobic treatment zones C-1 and C-2 may be divided into several separate sections (indicated by dashed lines).

Referring now to fig. 4A-B, an embodiment of the present invention is illustrated wherein the settled sludge is sidestream processed in a parallel sidestream reactor through a separate processing zone, a bypass of a second processing zone, or a combination thereof.

In the embodiment shown in FIG. 4A, the first return portion V₁Is divided into two parts, providing a first return part A, V_{1_A}And a first return portion B V_{1_B}. Said portion V_{1_A}And V_{1_B}Respectively carrying out aerobic treatment and anaerobic treatment. Thus, in this embodiment, the first fraction is treated by side streams by aerobic and anaerobic treatment, but in subdivided fractions a and B in parallel. This embodiment is particularly useful when the nitrogen content is very high. In a variation of this embodiment, there is only one side-flow reactor C having both aerobic and anaerobic zones, but after aerobic and/or anaerobic treatment, the treated sludge bypasses and is fed directly to the treatment tank, as illustrated in fig. 4B. Further variations and combinations of the described embodiments are contemplated and are within the scope of the present application. The invention will now be described by way of the following non-limiting examples.

A variant is shown in fig. 5A, in which the treatment tank is divided into two sections, an aerobic section (black blocks) and an anaerobic section (white blocks). However, the invention should not be limited to two stages, as three or more stages are envisioned, such as additional anaerobic stages. The inlets are shown as Inlet Distributors (ID) which can direct feed from the treatment tank (a) to different sections of the side-stream reactor. It is also envisioned that there may be one inlet to each section. Each section has an outflow opening and the portions are in fluid communication with each other.

The operation of the reactor is illustrated in more detail in fig. 5B, again with the black box being aerobic and the white box being anaerobic. In this embodiment, the sidestream reactor may be operated in stages 1, 2 and 3, wherein:

1) the feed is introduced into the aerobic section. The outflow valve of the aerobic section is opened and the outflow valve of the anaerobic section is closed. Then partially formed NO₃the-N will flow directly into the treatment tank without the need for denitrification in the anaerobic part of the side-flow reactor. This mode of operation may typically be within a time span of 0-2 hours when operated for a total of about 6 hours.

2) In the second stage, the feed is introduced to the anaerobic section of the sidestream reactor, with the valves open, whereas the valves of the aerobic section are closed. The anaerobic section thus receives fresh sludge from the treatment tank and HCOD can be returned to the treatment tank where it promotes denitrification and P removal/stripping, this mode of operation typically operating for one hour, i.e. sometimes 2-3; and finally

3) In the third stage, the feed is introduced into the aerobic tank. The outflow valve of the aerobic tank is closed and the outflow valve of the anaerobic section is opened. The anaerobic stage will then receive nitrate from the aerobic stage and the HCOD will be used for denitrification. When the limit of denitrification capacity is reached and the nitrate concentration exceeds a certain preset value, such as 2-4 hours and in particular 3 hours as illustrated, i.e. T-3-6. As described above, the operation may thereafter be transitioned to segment 1 again.

In a typical run, the total time for stages 1, 2 and 3 will be 4-8 hours, more specifically 6 hours as illustrated. The duration of each stage may vary depending on the amount of N and O in the reactor, however typical operation of each stage may be 1-3 hours in stage 1 and 1/2-1 in stage 2¹/₂Hours and 2-4 hours in 3 stages.

In all of the embodiments described, it is foreseen that the anaerobic section/basin may be provided with aeration means to enable air to be provided if deemed necessary. This will provide a more flexible setup.

Examples

The embodiment according to claim 4 of the present invention was tested on a large scale. I.e. a process with a side-flow reactor having first an aerobic treatment zone followed by an anaerobic treatment zone.

In the feed, the ratio of COD to N was 6. The total sludge volume in the plant was 355.5 tDS and the sludge content of the side-flow reactor (c) was predetermined to a level of 50% of the total volume of the plant, i.e. 177.8 tDS. The wastewater feed consists of COD 420mg/L, BOD 180mg/L, N70 mg/L and P8 mg/L.

The process was run under cold conditions, i.e. at 10 ℃. And setting the first reflux portion to V₁/Q₁At 12.3% (v/v), the flow is regulated by specially configured software such as EnviStyr available from Envidan A/S.

Table 1: measuring the Mass balance of the components (kg/day)

Characterisation/flow	Q₁	Q₂	Q₄
				COD	42000	3000	19400
N	7000	1500	1292

Table 2: flow rate (m)³Day)

Q₁	100000
		Q₃	35000
V₁	12240

From this example it can be seen that very efficient nitrogen removal and COD degradation is obtained with a small portion of the return sludge which is treated to activate the sludge for wastewater purification in the treatment tank.

Claims

1. A method for biological treatment of wastewater by an activated sludge process using the same bacterial population throughout the process, the method comprising the steps of:

a) the wastewater feed is made to flow into a treatment tank,

c) precipitating the mixture in a settling tank to provide treated wastewater and precipitated sludge, wherein the precipitated sludge is subjected to the steps of:

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

2. The method of claim 1, wherein phosphorus is removed by bacteria in the treatment basin.

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

4. A process according to claim 1, 2 or 3, wherein the first return portion of the settled sludge is 5-30% (v/v), preferably 10-25% (v/v), more preferably 10-15(v/v) of the first and second return portions of the settled sludge.

5. A process according to any one of claims 2-4, wherein the first return portion of the settled sludge in step c) ii) is treated continuously in a first aerobic treatment step and subsequently in a second anaerobic treatment step.

6. The process according to claim 5, wherein the aerobic treatment is divided into a plurality of sections, and wherein at least one of the sections is operated under intermittent aeration.

7. A process according to claim 2, 3 or 4, wherein the first return portion of the settled sludge in step c) ii) is treated continuously in a first anaerobic treatment step and subsequently in a second aerobic treatment step.

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

9. A process according to any one of claims 1 to 8, wherein the sidestream reactor is divided into separate sections, wherein each separate section may be operated under aeration, intermittent aeration or no aeration.

10. A process according to claim 8 or 9, wherein the first return portion of the settled sludge in step c) ii) is first subjected to aerobic treatment in a section having a volume of 5-15%, more preferably 8-12%, such as about 10%, of the volume of the anaerobic section, subsequently subjected to a second anaerobic treatment in the anaerobic section, and finally subjected to a third aerobic treatment.

11. The process according to any of the preceding claims 1-10, wherein the side-flow reactor is divided into separate sections, at least one aerobic section and at least one anaerobic section, wherein each section is provided with an outflow valve and is in fluid communication with each other, and wherein the first return portion is run in a series of sections for a specific period of time (a specific temporal period), comprising:

a first aerobic section wherein the first reflux portion is fed to the aerobic section and the aerobic outflow valve is opened and the anaerobic outflow valve is closed;

a second anaerobic section wherein the first reflux portion is fed to the anaerobic section and wherein the anaerobic outflow valve is opened and the aerobic outflow valve is closed; and

a third aerobic and anaerobic section, wherein the first reflux portion is fed to the aerobic section and wherein the aerobic outflow valve is closed and the anaerobic outflow valve is opened.

12. The process of any of claims 1-11, wherein the Mixed Liquor Suspended Solids (MLSS) concentration (kg/m) in the side-stream reactor³) 2-6 times higher, preferably 3-5 times higher, than the MLSS concentration in the treatment tank.

13. A method according to any of claims 1 to 12, wherein said first return portion of precipitated sludge constitutes 3-10% (v/v), more preferably 4-8% (v/v), even more preferably 5-7% (v/v) of the wastewater stream feed (Q1).

14. The method of any one of claims 1-13, wherein the wastewater feed is sewage, municipal wastewater, domestic wastewater, commercial wastewater, industrial wastewater, wastewater from septic sludge tanks (septic sludge tanks), or the like.

15. Use of the method according to any one of claims 1-14 for biological dephosphorization and/or nitrification and denitrification.

16. System suitable for treating wastewater by the activated sludge process, said system comprising at least one treatment basin (a) connected to at least one settling basin (B), said settling basin (B) being connected to at least one side flow reactor (C), said side flow reactor (C) being connected to said at least one treatment basin (a), and said treatment basin (a) further having one inlet and said settling basin further having one or more liquid and/or solid outlets, wherein a piping system in direct communication is provided between said settling basin (B) and treatment basin (a).