EP1015390A1

EP1015390A1 - Bioreactor control

Info

Publication number: EP1015390A1
Application number: EP19980903300
Authority: EP
Inventors: Rune Bakke
Original assignee: Hifo TECH AS
Current assignee: Hifo TECH AS
Priority date: 1997-02-06
Filing date: 1998-02-06
Publication date: 2000-07-05
Also published as: AU6007098A; NO970550D0; WO1998034878A1

Abstract

A method for biological conversion of waste in bioreactors, which require more than one conversion step to produce desired end products, where the concentrations of relevant parameters in the reactor are caused to fluctuate with time through systematic changes in physical or chemical input, is described. The method can be used for biological nitrogen removal from wastewater in a one compartment continuous flow bioreactor, wherein the oxygen concentration in the reactor is caused to fluctuate with time. The method can be used in a bioreactor for nutrient removal from wastewater in a one compartment continuous flow manner, wherein the reactor is provided with aeration means, means for measuring the oxygen concentration in the reactor and means to regulate the aeration as a response to the measurements of oxygen concentration. Oscillations of relevant parameters can be advantage in a variety of bioreactors to cause intermittent favourable conditions for the various process steps required and to obtain enhanced signals from the process through the monitored parameters.

Description

BIOREACTOR CONTROL

Field of the invention

The present relates to a method to control biological conversion in bioreactors through oscillations in parameters influencing the organisms involved, where the conversion to desired end products requires two or more process steps, such as enhancing biological nitrogen and phosphorous removal from waste water.

Background

Biological conversion of components in wastes, such as nitrogen and/or phosphorous removal from waste water and methane production from organic waste, is receiving increasing attention. Continuous flow process for biological nitrogen removal consists, for example, of more than one reactor/chamber/zone for the various microbial cultures involved and the compartments are normally connected by flow through and recycle lines. Cost of construction and operation of nitrogen removal plants are, therefore, significantly higher than for conventional biological waste water treatment without nitrogen removal requirements.

Biological nitrogen removal normally consists of three main groups of processes and microbial cultures: 1) Nitrogen containing organic matter is degraded by a variety of organisms and ammonium (NH₄ ) is released, 2) Nitrification: ammonia is converted to nitrate by certain autotrophic bacteria, 3) Denitrification: nitrate is then, through several steps, converted to nitrogen gas (N₂), which is released to the atmosphere, by another mixed culture that uses nitrate as electron acceptor (as opposed to free oxygen (O₂) used in processes 1 and 2) in the metabolism of organic carbon (See Figure 1).

The most comprehensive descriptions of all known method for biological nutrient removal is probably found in the textbooks "Biological nutrient removal"¹ and "Nutrient removal from waste waters"". The methods described fall into three main categories:

1) Aerobic carbon removal and nitrification followed by denitrification in a separate anoxic compartment with external carbon source added.

2). "Pre-denitrification" where a high degree of waste water recycles (up to 4 times the total flow through the plant) allows for the anoxic denitrification compartment to precede the aerobic nitrification (to eliminate the need for an external carbon source). 3) Sequential batch reactors (SBR) in which the waste water is introduced into the reactors in batches and then undergoes the complete treatment with nitrification and denitrification and sedimentation at different times. The SBR concepts still require more than one reactor, even though the whole process is carried in a single compartment, since the flow of waste water to the plant is more or less continuous (typically more than four reactors are applied). A good description of an SBR operated for biological nitrogen and phosphorous removal is given by Goncalves et al. (1994)¹".

Kerrn-Jespersen et al. (1994)^ιv applied alternating redox conditions in a continuous flow system (with biofilms) for waste water treatment, but not for the purpose of nitrification and denitrification, just to study denitrification and phosphorous release mechanisms.

Timberlake et al. (1988)^v constructed a continuous flow biofilm reactor in which nitrification and denitrification take place in a single reactor, but in different zones through the biofilm. They achieve this by reversing the oxygen/redox gradient compared to normal bioreactors, but this requires a rather intricate (expensive) physical adaptation of reactors. The method has, therefore, not obtained widespread use, if any, outside the laboratory.

Hao and Huang (1996a)^vl and (1996b)^vn designed a system based on redox oscillations with "real-time" control using oxidation-reduction probes (ORP) to measure the redox level in the bioreactor and turn aeration on and off based on these and other measurements (dissolved oxygen and pH). Hao and Huang turn the aeration on when the ORP measurements indicate that the nitrate is removed (denitrification complete), and back off when nitrification is complete. This strategy implies slow oscillations, since the rate of both nitrification and denitrification slows down towards completion, making the strategy relatively inefficient. A sequence, therefore, typically takes hours and the waste water detention time required to reach a high degree of nitrogen removal is, therefore, also quite high, implying large reactor volumes and high construction costs.

Problems with prior methods

The main problem in complex biological processes arises when the different process steps in a sequence of processes have differing optimal conditions, such as in nitrogen removal systems where nitrifiers require oxygen, while oxygen impedes denitrification. The different cultures are, therefore, either grown in separate zones or under some average sub-optimal conditions. In nitrogen removal systems this, generally speaking, has the consequence that unwanted aerobic heterotrophic bacteria have ideal conditions and are, therefore, too efficient at consuming oxygen and organic carbon that, respectively, the nitrifiers and the denitrifiers need. This is solved by either a) recirculating the nitrate containing waste water to the inlet zone where organic carbon is available, or b) by adding a carbon source for denitrification at the end of the process line, or c) as Hao and Huang (1996a)^vιn and (1996b)^lx did, through slow oscillations between acceptable conditions for the two cultures.

The prior solutions imply the use of more than one reactor/compartment and high energy expenditures either to a)pump large volumes of liquid in recycle or b)supply excess oxygen and an extra carbon source, or c) large reactor volumes. This in turn implies significantly increased investments and operating cost compared to conventional secondary biological treatment plants.

Summary of the invention

It is an object of the present invention to provide a method to control the environment in bioreactors through rapid variations in conditions which influence the activity of the organisms, such as for nitrogen removal from waste water, wherein the entire process thereby can occur in fewer, and preferably a single stage/reactor, in which mechanical complexity is kept at a minimum, energy expenditures are minimised and cost of construction and operation is significantly reduces.

It is another object of the invention to provide a cost efficient single stage reactor with a minimum of mechanical complexity, minimum energy expenditure and low cost of construction and operation.

According to a first aspect the invention is directed to a method for control of biological conversion of waste in bioreactors, wherein the conversion of the waste to the desired end product(s) requires two or more process steps, wherein the value of relevant reaction parameter(s) in the reactor is caused to fluctuate in cycles between values desirable for the individual process steps at frequencies, where the fluctuation periods are shorter than the time period required for the individual process steps to be completed. This implies that the fluctuation period will depend on the type of process and the process loading, but it will typically be less than 1 hour in most high efficiency reactors. In a preferred embodyment the bioreactor(s) contains a consortium of microorganisms where the different microorganisms requires different conditions to perform their part of the convertion and that the relevant reaction parameter(s) in the bioreactor is caused to fluctuate at a high frequency in such a manner that the organisms intermittently are exposed to conditions during which they can perform the desired conversion. More preferrably the relevant reaction parameter(s) is continuously monitored to determine the status and requirements of the process and that the parameter(s) is caused to fluctuate as a response to the status based on pre-set (programmed) rules and predetermined process requirements.

The reaction parameters can be caused to fluctuate by intermittent addition of a minimum factor in one or more of the conversion reactions. In a method for biological removal of nitrogen from waste water is the minimum factor oxygen that is added by means of intermittent aeration.

According to a preferred embodiment the bioreactor is a one compartment continous flow bioreactor.

In a further aspect the invention is directed to a bioreactor for nutrient removal from wastewater in a one compartment continuos flow manner, wherein the reactor is provided with aeration means, means for measuring the oxygen concentration in the reactor and means to regulate the aeration as a response to the measurements of oxygen concentration, wherein the aeration is started when the oxygen concentration falls below a pre-set value that that is set between 0 and 2 mg/1, preferably between 0,5 and 1,5 mg/1, and is stopped when the oxygen concentration is above pre-set value that is set between 2 and 7 mg/1, preferably between 1,5 and 6,5 mg/1.

The method according to the present invention is based on variations in time rather than space, so that the desirable micro-organisms have short time periods during which the conditions are optimal, rather than having separate compartments in which conditions are ideal. The combination of a single reactor and high frequent fluctuation in oxygen concentration permits effective nitrogen removal in a one compartment continuos flow reactor.

The present method can be applied as a new way of controlling conventional waste water treatment processes to remove nitrogen. It can, therefore, be applied both in many existing plants and in the design of new plants. A mechanical prerequisite to implement the present method in nitrogen and phosphorous removal is the ability to turn the aeration on and off and sufficient aeration capacity to reach the oxygen concentration level required for nitrification.

The process control must be automated such that aeration is turned on and off at proper intervals. The level of control required depends on local conditions such as type of bioreactor, loading rates, aeration capacities and variations in operating conditions. A timer turning the air off and on set according to process simulations and manual control, may be sufficient in a system with low and stable loads and not very stringent requirements. A system with high and variable loads, short liquid detention time and stringent effluent requirements will, on the other hand, require a more advanced control system, based, for example, on a computer and PLS and real-time control based on continuous monitoring of process parameters such as oxygen level, flow rates, temperature, pH, ammonia, nitrates, oxidation-reduction potential and/or conductivity.

The invention is generally directed to a general bioreactor control concept where high frequency oscillations in any relevant process parameter will yield enhanced treatment and signals from the process. The monitored oxygen or ammonia concentrations, pH or other parameters will contain more available information regarding the conditions in the process when the conditions are oscillating compared to conventional stable operation. Stronger oscillations yield stronger signals, reflecting the conditions in the process, on which basis the imposed oscillations are continually adjusted. Other process regulations, such as addition of nutrients and required pH adjustments, are also adjusted and optimised on the basis of continuous interpretations of such signals.

Brief description of the figures

The invention will now be more fully described with nitrogen removal as example, with reference to the attached figures wherein; Fig. 1 is an illustration the three main groups of processes that is normally involved in biological nitrogen removal; and Fig. 2 is an illustration of the total process of nitrogen removal.

Detailed description of the invention This is an invention to optimise biological systems for treatment of waste, which require more than one conversion step to produce desired end products, where optimal conditions are different for the individual steps. Such treatment processes can, according to the invention, be most efficiently carried out in bioreactor systems with oscillating conditions. Application of the invention, therefore, imply that some physical condition or parameter, such as aeration, liquid phase mixing, addition of chemicals and/or input of waste, must be changed in a systematic way; oscillated, so that concentrations of relevant parameters in the conversion environment (bioreactor(s)) oscillate between desirable levels. The primary example of this invention is a single stage, continuous flow process for nitrogen removal. Biological phosphorous removal results are also - briefly mentioned, while other applications, such as enhanced conversion of organic waste to methane gas is not yet tested.

Micro-organisms that normally do not coexist due to differences in environmental requirements (e.g. nitrifiers need free oxygen while denitrifiers will not denitrify efficiently with free oxygen available) can coexist because of frequent fluctuation between acceptable conditions for the required organisms such that they can be intermixed within the same micro niche. Mathematical simulations of bioreactors have been used to find conditions required for the micro-organisms involved i nitrogen removal (nitrifiers and denitrifiers) to coexist as desired.

According to the simulation models most of the oxygen supplied is utilised for nitrification, most of the organic carbon in the waste water is utilised for denitrification, and the aerobic heterotrophs are, therefore, out competed and get a much less dominating role compared to conventional processes for nitrogen removal.

Figure 1 illustrates biological nitrogen removal and the three main groups of processes normally involved in the total process:

1) Nitrogen containing organic matter ("COHNS") is degraded by a variety of organisms dominated by aerobic heterotrophs (Aer.Het.) and ammonium ( NH₄ ⁺ ) is released,

2) Nitrification: Ammonium (from the raw waste water and from step 1) is converted to nitrate by certain autotrophic bacteria called nitrifiers (Nitri),

3) Denitrification: Nitrate is then converted to nitrogen gas (N₂), which is released to the atmosphere, by another bacterial culture (Denitri.) which uses nitrate as electron acceptor (as opposed to free oxygen (O ) used by the nitrifiers) in the metabolism of organic carbon. Note that step 1 and 2 require oxygen, while free oxygen will slow down or stop step 3.

Figure 2 illustrates the consumption of oxygen in the HFO concept. Most of the oxygen supplied is utilised by nitrifiers (Nitri) for nitrification, most of the organic carbon in the waste water is utilised by the denitrifiers (denitri), and the aerobic heterotrophs are to a significant extent out competed.

According to the mathematical simulations the optimal oscillations will vary from reactor to reactor and from time to time, but in general, the models suggest and tests have confirmed that short sequences / high frequency oscillation (HFO) is ideal, e.g. frequency > 1 per hour, more preferably > 2 per hour, and most preferably >4 per hour to keep the process robust and efficient.

The observations that the optimal oscillations vary with load variations etc., imply that the aeration sequences must be regulated and adapted to local conditions based on measurements of process parameters and mathematical simulations of the process.

The HFO concept can be applied as a new way of controlling conventional waste water treatment processes to remove nitrogen. It can, therefore, be applied both in many existing plants and in the design of new plants. A mechanical prerequisite to implement the HFO is the ability to turn the aeration on and off and sufficient aeration capacity to reach the oxygen concentration level required for nitrification.

The process control must be automated such that aeration is turned on and off at proper intervals. The level of control required depends on local conditions such as type of bioreactor, loading rates, aeration capacities and variations in operating conditions. A timer turning the air off and on set according to process simulations and manual control, may be sufficient in a system with low and stable loads and not very stringent requirements.

A system with high and variable loads, short mean liquid detention time and stringent effluent requirements will, on the other hand, require a more advanced control system, based, for example, on a computer and PLS and real-time control based on continuous monitoring of process parameters such as oxygen level, flow rates, temperature, pH, ammonia, nitrates, oxidation-reduction potential and/or conductivity. The aeration sequences must, in all cases, be regulated and adapted to local conditions based on measurements of process parameters and mathematical simulations of the process.

It is theorised that the system must have at least 4 cycles per mean liquid detention time in the bioreactor to get an efficient nitrogen removal, i.e. a system having two hours mean liquid retention time must operate at a frequency of >2 cycles per hour.

The main parameter for regulation of the process is the O2 concentration. Experiment has indicated that the additional parameters, i.e. pH, conductivity, oxidation reduction potential and temperature, are correction factors and indicators to confirm that the process is in balance.

Example

The HFO concept can, according to mathematical simulations, be applied in both biofilm and activated sludge systems, but has so far only been tested in full scale biofilm processes (rotating biofilm reactor) at the øksnevad biological waste water treatment plant (0BR). 0BR is a plant receiving waste water from private homes, schools and businesses amounting to about 200 persons equivalents. Pre-treatment is limited to communition. Plant design and waste water composition are well documented.

Aeration of the reactor was started and stopped as a response to O2 measurements. The aeration was turned on when the O2 concentration fell below 1,5 mg/1 and was turned off at a concentration of 3,5 mg/1. This gave a frequency of about 4 cycles per hour. The pH in the sludge was relatively stable during the period and varied from about 6,5 to 7,0.

The results from the 0BR, obtained during Spring and Summer (~6 months) of 1995, are summarised in Table 1 below. Note that the results demonstrate that the HFO concept, although designed to remove nitrogen, also lead to increased phosphorous removal.

Table 1

Results from 0BR treatment plant reported as percentage removal of total nitrogen (Tot. N) and total phosphorous (Tot. P) from inlet to outlet of a single reactor with sedimentation at a) normal continuous aeration operation and HFO operation at low temperatures (6<T<10C) and higher temperatures (T>10°C).

It is probable that the same requirements in terms of "sludge age" (θ_c >~10days) apply to the HFO concept as it does to other bioreactors for nitrification. It is usually not a problem to reach such a high sludge age in biofilm reactors, while this is not always the case in activated sludge systems due to limitations in the sedimentation process. Successful utilisation of the HFO concept in some treatment plants may, therefore, require some process modification or expansion. It is, however, observed in test runs that the culture "looked healthier" (less foam and filaments) and the sedimentation process appeared to perform better, implying that the HFO concept could make it easier to obtain the required sludge age than conventional methods.

The bioreactor in waste water treatment applications normally represents the main steps in a series of processes and it is therefore influenced by other processes. The performance of a bioreactor in an activated sludge system is, for example, as discussed above, very dependent on the physical sedimentation process down stream to separate and return the culture/sludge. The pre-treatment up stream, such as screening or comminution, will also influence the performance of bioreactors in general, but probably not in any way unique to the HFO concept. The same is also the case for the fact that large diurnal variations in mass- and volumetric loading on waste water treatment plants are normal, which require adjustable and robust processes. It is, however, important that such factors are taken into consideration in the design of each HFO implementation, in which case it is expected that the HFO concept will make most bioreactors more adjustable and robust to unpredictable changes.

The observation that HFO operation can lead to biological phosphorous removal also implies that high frequency oscillations have a more general applicability beyond the simulated nitrogen removal application. The mechanism involved in phosphorous removal is that the phosphorous accumulating bacteria are exposed to alternating aerobic and anaerobic conditions, similar to the conditions in conventional plants where they are pumped through aerobic and anaerobic zones. High frequency oscillations (HFO) of relevant parameters can be of advantage in a variety of bioreactors, where a series of processes is required, to cause intermittent favourable conditions for the various processes and/or organisms and to obtain enhanced signals from the process through the monitored parameters.

References

" C.W. Randall, J.L. Barnard and H.D. Stensel. 1992. "Design and retrofit of wastewater treatment plants for biological nutrient removal". Water Quality Management Library Vol. 5. Technomic Publishing AG.

Basel.

" N.J. Horan, P. Lowe and E.I. Stentiford. 1995. "Nutrient removal from wastewaters". Technomic

Publishing AG, Basel.

"' R.F. Goncalves, L. Le Grand and F. Rogalla. 1994. "Biological phosphorous uptake in submerged biofilters with nitrogen removal". Water Science and Technology, Vol. 29, No. 10- 1 1 , pp. 135-143.

^,v J.P. Kerrn-Jespersen, M. Henze and R. Strube. 1994. "Biological phosphorous release and uptake under alternating anaerobic and anoxic conditions in a fixed-film reactor". Water Research, Vol. 28, No. 5, pp.

1253- 1255. ^v D.L. Timberlake, S. Strand and K.J. Williamson. 1988. "Combined aerobic heterotrophic oxidation, nitrification and denitrification in a permeable-support biofilm". Water Research, Vol. 22, No. 12, pp.

1513- 1517.

^V1 O.L. Hao and J. Huang. 1996. "Alternating aerobic-anoxic process for nitrogen removal: Process evaluation". Water Environment Research, Vol. 68, No. 1 , pp.83-93. ^v" O.L. Hao and J. Huang. 1996. "Alternating aerobic-anoxic process for nitrogen removal: Dynamic modeling". Water Environment Research, Vol. 68, No. 1 , pp. 94-104. ^v'" O.L. Hao and J. Huang. 1996. "Alternating aerobic-anoxic process for nitrogen removal: Process evaluation". Water Environment Research, Vol. 68, No. 1, pp.83-93.

'^* O.L. Hao and J. Huang. 1996. "Alternating aerobic-anoxic process for nitrogen removal: Dynamic modeling". Water Environment Research, Vol. 68, No. 1 , pp. 94-104.

Claims

P a t e n t C l a i m s

1.

A method for control of biological conversion of waste in bioreactors, wherein the conversion of the waste to the desired end product(s) requires two or more process steps, characterised in that the value of relevant reaction parameter(s) in the reactor is caused to fluctuate in cycles between values desirable for the individual process steps at frequencies, where the fluctuation periods are shorter than the time period required for the individual process steps to be completed.

2.

The method according to claim 1, characterised in that the bioreactor(s) contains a consortium of microorganisms where the different microorganisms requires different conditions to perform their part of the convertion and that the relevant reaction parameter(s) in the bioreactor is caused to fluctuate at a high frequency in such a manner that the organisms intermittently are exposed to conditions during which they can perform the desired conversion.

3. A method according to claim 1 or 2, characterised in that the relevant reaction parameter(s) is continuously monitored to determine the status and requirements of the process and that the parameter(s) is caused to fluctuate as a response to the status based on pre-set (programmed) rules and predetermined process requirements.

4.

A methd according to claim 1,2 or 3, characterised in that the parameters are caused to fluctuate by intermittent addition of a minimum factor in one or more of the conversion reactions.

5.

A method according to claim 4, characterised in that the method is a method for biological removal of nitrogen from waste water and that the minimum factor is oxygen and that oxygen is added by means of intermittent aeration.

6.

The method according to one or more of the claims 1 to 5, characterised in that the bioreactor is a one compartment continuos flow bioreactor.

7.

The method according to claim 6, characterised in that the system undergoes at least four cycles per mean liquid detention time for the waste water . -

8. The method according to claim 5, characterised in that the reactor is aerated when the oxygen concentration falls below a pre-set value that is set between 0 and 2 mg/1, preferably between 0,5 and 1,5 mg/1, and is stopped when the oxygen concentration is above pre-set value that is set between 2 and 7 mg/1, preferably between 1,5 and 6,5 mg/1.

9.

The method according to claim 5, characterised in that the fluctuations in oxygen concentration are caused by intermittent aeration wherein the aeration is controlled by a timing device.

10.

A bioreactor for nutrient removal from wastewater in a one compartment continuos flow manner, wherein the reactor is provided with aeration means, means for measuring the oxygen concentration in the reactor and means to regulate the aeration as a response to the measurements of oxygen concentration, characterised in that the aeration is started when the oxygen concentration falls below a pre-set value that that is set between 0 and 2 mg/1, preferably between 0,5 and 1,5 mg/1, and is stopped when the oxygen concentration is above pre-set value that is set between 2 and 7 mg/1, preferably between 1,5 and 6,5 mg/1.