WO2001038869A2

WO2001038869A2 - System for determination of wastewater biodegradability

Info

Publication number: WO2001038869A2
Application number: PCT/DK2000/000646
Authority: WO
Inventors: Jes Vollertsen
Original assignee: Jes Vollertsen
Priority date: 1999-11-22
Filing date: 2000-11-22
Publication date: 2001-05-31
Also published as: AU1383401A; WO2001038869A3

Abstract

A method of and a system for determination of wastewater biodegradability, whereby a sample of wastewater is introduced into a reaction chamber of a measuring apparatus. The sample of wastewater is subjected to oxygen-containing air, and measurements are performed of a dissolved oxygen (DO) content of the sample of wastewater. A rate of oxygen consumption is determined, and organic matter fractions of the sample of wastewater are determined on the basis of the determined rate of oxygen consumption of the sample and on the basis of microbial properties of said organic matter fractions. A model of the sample of wastewater, e.g. the organic matter fractions and their microbial properties, may be utilized for simulating the oxygen consumption rate of the sample.

Description

SYSTEM FOR DETERMINATION OF WASTEWATER BIODEGRADABILITY

Field of the invention

The invention relates to a method of determination of wastewater biodegradability according to claim 1, a system for determination of wastewater biodegradability according to claim 8, and a measuring apparatus for determination of the oxygen uptake rate of a liquid according to claim 15.

Background of the invention

When designing sewer systems and/or wastewater treatment plants, it is important to know the characteristics of the wastewater which will flow in a system and/or be treated by a treatment plant. Traditionally, gross characteristics of wastewater like COD (chemical oxygen demand) or BOD (biological oxygen demand) have been used for these purposes. However, for the purpose of modeling in-sewer transformations of organic matter and assessment of the impact which sewer system design and operation have on wastewater treatment plants, receiving waters and the sewer system itself, the traditional gross characteristics of wastewater like COD or BOD are inadequate.

Thus, an object of the invention is to provide a method and a system which will offer a more detailed characterization of the wastewater than that of prior art.

Another object of the invention is to provide a method and a system which will provide a characterization while taking into account the biomass activity as well as the substrate quality ofthe wastewater.

A further object of the invention is to provide a measuring apparatus for determina- tion of the oxygen uptake rate of a liquid, e.g. wastewater, which will allow measurements to be made with a high degree of accuracy. These and other objects are achieved by the invention as explained below.

Summary of the invention

The invention relates to a method of determination of wastewater biodegradability according to claim 1 , whereby

a) a sample of wastewater is introduced into a reaction chamber,

b) the sample of wastewater is subjected to oxygen-containing air,

c) measurements are performed of a dissolved oxygen (DO) content of the sample of wastewater, and a rate of oxygen consumption is determined, and

d) organic matter fractions of the sample of wastewater are determined on the basis of the determined rate of oxygen consumption of the sample and on the basis of microbial properties of said organic matter fractions.

Hereby, a characterization is made on the basis of the respiration activity of the wastewater biomass, e.g. the oxygen uptake rate (OUR) of the biomass. When combined with knowledge of the microbial properties of organic matter fractions of the sample of wastewater, the method allows the organic matter fractions to be determined when comparing measured values with values achieved on the basis of said knowledge.

When, as stated in claim 2, steps b and c are repeated one or more times before step d is performed, a more accurate determination of the fractions may be performed, e.g. as the time period during which the rate of oxygen consumption can be determined is prolonged, and thereby provide a more precise determination.

Preferably, as stated in claim 3, the measurement of the dissolved oxygen content of the sample is performed in a reaction chamber arranged to minimize and/or prevent biofilm growth, whereby accurate determinations of the fractions may be performed. Suspended growth of the microorganisms is essential for reliable interpretation of the obtained oxygen uptake rates (OUR), as the knowledge of the microbial properties of the organic matter fractions ofthe wastewater will not be influenced by the growth of biofilm.

In a preferred embodiment, as stated in claim 4, the organic matter fractions, determined on the basis of the determined rate or rates of oxygen consumption, are heterotrophic biomass, readily biodegradable organic matter, rapidly hydrolysable organic matter, and slowly hydrolysable organic matter.

Hereby, a relatively precise and detailed fractionation of the sample of wastewater is provided which will be practical in use, e.g. when designing sewer systems, waste- water treatment plants, etc. and also when assessing the operational impact on sewer systems, wastewater treatment plants, receiving waters etc.

Preferably, as stated in clam 5, the microbial properties of said organic matter frac- tions are facilitated by a model taking into account the interrelated effects of the fractions, whereby an advantageous embodiment is achieved.

In a preferred embodiment, as stated in claim 6,

- the determination of said organic matter fractions is performed on the basis of a simulation by a model of the sample of wastewater,

- values obtained by using said model are compared with determined values of oxygen consumption, and

- parameters of said model are changed in dependency of said comparison.

Hereby, an iterative determination may be performed, whereby the determination of the fractions of the sample will be determined with high precision, as the iterative process may be performed until an optimal, i.e. minimal, difference between the model value and the determined value is achieved or until a preset difference is achieved.

As stated in claim 7, the model of the sample of wastewater may be specified by the following equations:

X.

C\\ d(^~*$O) - _π Y ₊ 1 _JL „ ^S Y

^{(3) ~}aT ^{" 9Λ +} Y_H ^μ" κ^~ s_s ^~x*

with the following definitions:

μπ Maximum growth rate [d -^"h ] k_hi Hydrolysis rate for Xsi [d^"1] k_h2 Hydrolysis rate for X_S2 [d^"1]

Ks Saturation constant [gCOD m^~3]

K_X1 Saturation constant for X_Sι [gCOD gCOD^"1] Kχ₂ Saturation constant for X_Sι [gCOD gCOD^"1] q_m Maintenance energy requirement rate constant [d^" -h¹]

So Dissolved oxygen [gO₂ m^"3]

Ss Readily biodegradable substrate [gCOD m^"3]

XB Heterotrophic biomass [gCOD m^"3] Xsi Fast hydrolysable substrate [gCOD m^"3]

X_s2 Slowly hydrolysable substrate [gCOD m^"3]

Y_H Yield constant for X_B [gCOD gCOD^"1]

By using this model, the microbial properties of the fractions of the wastewater and the interrelated effects will be defined in a precise and true manner, whereby the fractions may be determined with a sufficiently high degree of accuracy by comparing measured values with corresponding values achieved by the described model.

The invention also relates to a system for determination of wastewater biodegradability according to claim 8, wherein the system comprises

- means for taking a sample of wastewater and introducing it into a reaction chamber,

- means for subjecting the sample of wastewater to oxygen-containing air,

- means for measuring a dissolved oxygen (DO) content of the sample of wastewater and for determining a rate of oxygen consumption, and

- means for determining organic matter fractions of the sample of wastewater on the basis of the rate of oxygen consumption of the sample and on the microbial properties of said organic matter fractions.

Hereby, a system is provided which involves means for performing a characteriza- tion based on the respiration activity of the wastewater biomass, e.g. the oxygen uptake rate (OUR) of the biomass. Further, as means are provided for combining this with the knowledge of the microbial properties of organic matter fractions of the sample of wastewater, the system allows the organic matter fractions to be determined by means of comparing measured values with values achieved on the basis of said knowledge. When, as stated in claim 9, the means for measuring the dissolved oxygen content of the sample comprises a reaction chamber arranged to minimize and/or prevent biofilm growth, accurate determinations of the fractions may be performed. Suspended growth of the microorganism is essential for reliable interpretation of the obtained oxygen uptake rates (OUR), as the knowledge of the microbial properties of the organic matter fractions of the wastewater will not be influenced by the growth of biofilm.

Preferably, as stated in claim 10, the means for determining the fractions of the sam- pie of wastewater comprises means for calculating the oxygen consumption by each said number of fractions, means for summing these fractions and means for comparing the resulting value with a corresponding value achieved by the measurement of the oxygen consumption of the sample of wastewater, whereby an advantageous embodiment is achieved.

In a further preferred embodiment, as stated in claim 11, the system comprises means for facilitating a model of a sample of wastewater, said model representing organic matter fractions with different microbial properties taking into account the interrelated effects of the fractions, whereby the system will take into account the relatively complex characterization of the sample, e.g. biomass activity as well as substrate quality in an advantageous manner.

Preferably, as stated in claim 12, the organic matter fractions of the contents, determined on the basis ofthe determined rate or rates of oxygen consumption, are heterotrophic biomass, readily biodegradable organic matter, rapidly hydrolysable organic matter, and slowly hydrolysable organic matter.

With such a system, a relatively precise and detailed fractionation of the sample of wastewater will be provided and offer results which will be practical in use, e.g. when designing sewer systems, wastewater treatment plants, etc. and also when as- sessing the operational impact on sewer systems, wastewater treatment plants, receiving waters etc.

In a further preferred embodiment, as stated in claim 13, the means for determina- tion of said organic matter fractions comprises means for performing a simulation on the basis of a model of the sample of wastewater, means for comparing values obtained by using said model with determined values of oxygen consumption, and means for changing the parameters of said model in dependency of said comparison.

Hereby, a system is achieved wherein an iterative determination may be performed, whereby the determination of the fractions of the sample will be determined with high precision, as the iterative process may be performed until an optimal, i.e. minimal, difference between the model value and the determined value is achieved or until a preset difference is achieved.

Preferably, as stated in claim 14, the model of the sample of wastewater is specified by the following equations:

₀₎ 5hμ_,_{Λ +} '-'i S V dt γ„

with the following definitions:

μ_H Maximum growth rate [d^" ] k_hi Hydrolysis rate for X_Sι [d^" ] k_h2 Hydrolysis rate for X_S2 [d^" ]

Ks Saturation constant [gCOD m^" 3η ]

Kχι Saturation constant for X_Sι [gCOD gCOD -^"h¹]

Kχ₂ Saturation constant for X_S1 [gCOD gCOD^"1] q_m Maintenance energy requirement rate constant [d^"1] So Dissolved oxygen [gO₂ m^" ]

Ss Readily biodegradable substrate [gCOD m^"3]

X_B Heterotrophic biomass [gCOD m^"3]

Xsi Fast hydrolysable substrate [gCOD m^"3]

Xs₂ Slowly hydrolysable substrate [gCOD m^"3] Y_H Yield constant for X_B [gCOD gCOD^"1]

By using this model in the system, the microbial properties of the fractions of the wastewater and the interrelated effects will be defined in a precise and true manner, whereby the fractions may be determined by the system with a sufficiently high de- gree of accuracy by comparing measured values with corresponding values achieved by the described model.

Further, the method and system according to the invention may be used in connection with the process of making decisions on how a sewer system is to be operated, maintained and/or renovated. For example, if a planned connection of a food processing industry will give rise to in-sewer problems, it may be necessary to modify, renovate or redesign the sewer system. Further, the invention may be utilized to design a sewer system in order to fulfill certain requirements. These requirements may for example be criteria concerning the level of odeur from the sewer system, the risk of corrosion in a sewer system, removal of easily transformable matter, production of easily transformable matter etc. The criteria concerning removal of easily transformable matter, e.g. removal of as much matter as possible, may be of particular impor- tance in connection with systems having mechanical treatment and no biological treatment, whereas the criteria concerning production of easily transformable matter may be of particular importance in connection with systems comprising nitrogen removal and/or biological phosphorous removal.

Finally, the invention relates to a measuring apparatus for determination of an oxygen uptake rate of a liquid, and in particular wastewater, according to claim 15, particularly in connection with a method of and/or a system for determination of waste- water biodegradability, wherein said measuring apparatus comprises a reaction chamber comprising a surface consisting of stainless steel at least internally.

Hereby, formation and growth of biofilm on the internal surfaces of the reaction chamber are prevented or minimized, whereby true measurements may be performed without unwanted influence from biofilm growth. This will be particularly advanta- geous in connection with determination of wastewater biodegradability, as accurate determinations of the fractions of the wastewater may be performed. Suspended growth of the microorganisms is essential for reliable interpretation of the obtained oxygen uptake rates (OUR), as the knowledge of the microbial properties of the organic matter fractions of the wastewater will not be influenced by the growth of bio- film. Also, when measurements have to be performed for a relatively long period of time, e.g. 1 - 2 days or more, the reduction of the biofilm growth is significant.

When, as stated in claim 16, the surface of stainless steel of the reaction chamber is treated in order to prevent and or reduce the growth of biofilm on the surface at least internally, an added effect is achieved.

Preferably, as stated in claim 17, the surface of stainless steel is treated by bead blasting, preferably by glass-bead blasting, which has shown an advantageous reduction ofthe biofilm growth.

Advantageously, as stated in claim 18, the reaction chamber is encompassed by a shield comprising means for controlling and/or regulating the temperature of the re- action chamber, whereby the conditions for the measurements may be controlled selectively and according to user specifications. For example, the temperature may be held constant within a variation of +/- 0,5°C, or may be varied according to a preferred sequence.

When, as stated in claim 19, the apparatus comprises means for adding oxygen- containing air to the reaction chamber, means for creating turbulence and/or flow in the liquid and/or means for circulating the liquid in the reaction chamber, an effective and/or speedy aeration of the sample in the reaction chamber may be performed, which is essential to the achievement of fast and reliable measurements and/or determinations of values based on said measurements.

Preferably, as stated in claim 20, the apparatus comprises means for measuring the oxygen utilization of a sample of liquid contained in the reaction chamber, preferably in the form of an oxygen probe.

In a further preferred embodiment, as stated in claim 21, the apparatus comprises means for measuring and/or recording the amount of dissolved oxygen (DO) in a sample contained in the reaction chamber and or means for measuring and or re- cording the temperature of said sample, which may be advantageous in many applications but will be especially advantageous in connection with determination of wastewater biodegradability according to the invention.

The figures

The invention will be described below with reference to the drawings of which

fig. 1 shows a first embodiment of a measuring apparatus according to the invention, fig. 2 shows an alternative embodiment of a measuring apparatus according to the invention, figs. 3a - 3c show a system for measuring biodegradability of a liquid, such as wastewater, having two reaction chambers, fig. 4 shows a block diagram of a model of the processes taking place in the wastewater, fig. 5 shows the measured content of dissolved oxygen versus time of a sample, fig. 6 shows the determined oxygen uptake rate versus time of a sample and a curve representing the corresponding simulated values, fig. 7 shows the result of a determination of the organic matter fractions of a sample, and fig. 8 shows a block diagram illustrating an application ofthe invention.

Detailed description

Fig. 1 shows a measuring apparatus 10 for measuring the dissolved oxygen content of a sample of liquid. The measuring apparatus comprises a reaction chamber 11 for containing a sample of a liquid and an oxygen probe 12, part of which will be submerged into the liquid when a sample of liquid is contained in the reaction chamber 11.

The reaction chamber 11 is surrounded by a cooling cap 13 which may be ring- shaped and which is adapted to contain fluid, e.g. a liquid, for cooling or possibly heating of the sample contained in the reaction chamber 11. For this purpose, the cap 13 is provided with connectors 14 and 15, e.g. couplings for tubes, pipes or the like, which serve as inlet and outlet for the cooling or heating fluid.

The reaction chamber 11 may be filled and emptied by means of a inlet/outlet 16, and the reaction chamber is upwards joined with a expansion chamber 18. A closing piston 19 can close the connection between the reaction chamber 11 and the expan- sion chamber 18. Further, the reaction chamber has inlet and outlet means 20 and 21 for air, e.g. atmospheric air, which may be provided by an air pump and/or a pressurized air tank. At the bottom of the measuring apparatus 10, e.g. just below the reaction chamber 1 1 or possibly in a bottom part of the reaction chamber, heating means 22 is arranged, for example in the form of electric heating means, for heating the liquid in the cham- ber, if necessary. Finally, a stirrer 23 is provided for optional stirring of the liquid in the reaction chamber.

The reaction chamber 11 , or at least the part of the reaction chamber in contact with the liquid of a sample, is made of a material which inhibits the formation of biofilm, such as stainless steel and in particular stainless steel designed to minimize and/or prevent formation of biofilm. According to the invention, the surface of the stainless steel reaction chamber may be bead-blasted, preferably by glass beads, in order to obtain a smooth surface which is sufficiently prohibitive of the formation of biofilm.

An alternative embodiment 110 of a measuring apparatus according to the invention is illustrated in fig. 2. This embodiment comprises a bottom or reaction chamber part 130 and a lid part 140. The bottom part 130 contains a reaction chamber 111 with double-walled sidewalls. The outer wall 113 constitutes a cooling (or heating) cap, which defines a substantially ring-shaped chamber for containing cooling (or heat- ing) fluid, e.g. a liquid as explained above in connection with fig. 1. Such a liquid can be introduced to the cooling cap and escape through inlet and outlet means 114 and 115, which may be formed as pipe stubs, couplings for tube or pipes etc. The inlet and outlet means 114 and 115 may be placed diametrically opposite each other instead of being placed as shown in fig. 1 , which may be preferable in some cases in order to obtain better circulation ofthe cooling (or heating fluid) in the cap.

The bottom part 130 is provided with inlet/outlet means 116 for filling and/or emptying the reaction chamber 111, and at the top of the bottom part, means are provided for fastening the lid part 140 to the bottom part 130. In the embodiment shown in fig. 2, these fastening means comprises an outer thread 131 which may cooperate with an inner thread part 141 on the lid part 140. Further, the bottom part 130 is provided with sealing means 132 which may comprise an O-sealing ring as illustrated in fιg.2. Sealing means may be provided on the lid part 140 instead of the sealing means 132 on the bottom part 130 or as supplementary sealing provisions.

As explained above, the lid part 140 is fastened to the bottom part by means of the thread part 141, but other fastening means may also be used. The lid part has two openings 146 and 148, which may be used for measuring means and/or for introducing fluids, e.g. air, into the reaction chamber 111 as will be described later. Finally, the lid part has a vertical pipe-shaped extension which has overflow outlet means 144 on the side, for example in the form of a pipe stub or the like. The inner volume of the pipe-shaped extension may serve as an expansion chamber 118.

The measuring apparatus 110 may be provided with a heater and/or stirring means corresponding to the heater 22 and the stirrer 23 described in connection with fig. 1.

Corresponding to the measuring apparatus 10 described in connection with fig. 1, the reaction chamber 111 or at least the part of the reaction chamber, e.g. the inner wall in contact with the liquid of a sample, is made of a material which inhibits the formation of biofilm such as stainless steel and in particular stainless steel which has been designed to minimize and/or prevent the formation of biofilm. According to the invention, the surface of the stainless steel reaction chamber may be bead-blasted, preferably by glass beads, in order to obtain a smooth surface which is sufficiently prohibitive ofthe formation of biofilm.

Preferably, the measuring devices are made of a stainless steel alloy of a type that is certified for food production and similar applications.

A suitable size of the reaction chambers 11, 111 of both embodiments of the measuring devices 10, 1 10 will be approx. 1 - 5 liters. In a preferred embodiment, the size is 2 - 3 liters and optionally 2,2 liters. The function of a measuring apparatus as shown in fig. 1 or 2 will now be described with reference to figs. 3a - 3c which illustrate a system according to an embodiment ofthe invention.

Figs. 3a, 3b and 3c illustrate a system for measuring biodegradability of a liquid such as wastewater, wherein two measuring devices 110 are used. However, the measuring devices described in connection with fig. 1 may be used as well.

Fig 3a illustrates these measuring devices, the heating and the cooling systems and the signal lines involved herein.

Fig. 3b illustrates these measuring devices with systems for introducing the liquid, e.g. the wastewater into the reaction chambers, emptying the chambers, cleaning the reaction chambers and the signal lines etc. for controlling these systems.

Fig. 3c illustrates these measuring devices and the system for introducing air into the reaction chambers, measuring the oxygen uptake rate (OUR) of the samples and the signal lines involved herein.

It will be understood that all parts and components shown in figs. 3a - 3c will be present in a system according to the illustrated embodiment of the invention. However, for reasons of clarity, the system has been illustrated in three sub-systems.

As shown in fig. 3a, each measuring apparatus 110 is placed on a heater 306, and a temperature sensor or measuring means 302 is placed in one of the openings 146, 148 shown in fig. 2. The temperature measuring means 302 is connected by means of signal lines 304 to control means 310 which also serves to collect measured data and transmit them to a computer 320, e.g. a PC, via connection lines 322.

The control means 310 may comprise a programmable logic computer, PLC, and will serve to control the operations of a system according to the invention. Measured data collected by the control means 310 will be transmitted to the computer 320, where data are stored, processed and/or interpreted. The computer means 320 also communicates data, for example set points etc. to the control means 310, and the computer means 320 contains program means for operating a system according to the invention.

As shown in fig. 3a, the heaters 306 are connected to the control means 310 by signal lines 308 in order to control the heating of each sample of liquid, e.g. wastewater, in a measuring device 110, in dependence on temperatures measured by the sensors 302.

The cooling of the samples may be controlled by means of cooling circuits comprising a cooling compressor 330. From this cooling compressor, cooling fluid may be circulated through tubes, pipes etc. 332 to and/or from the inlet and outlet means 114 and 115 of both measuring devices 1 10. Circulation control means 334 is provided in one of the pipelines 332 from the cooling compressor. The circulation control means serves as T-connections between the main cooling lines and the lines to the cooling cap of each measuring device. Furthermore, the circulation control means 334 comprises control or regulating means for controlling or regulating the flow to each measuring device 1 10 independently as illustrated by the control signal lines 336 from the control means 310 to each circulation control means 334. The control or regulation may be in the form of an on/off-control or more sophisticated control forms, e.g. Pi-control, PID-control etc.

As shown in fig. 3a, the measuring devices are also provided with means 312 for measuring the content of dissolved oxygen in the sample of liquid in each measuring device 110. The means 312, which will also be named oxygen meters or oxygen probes, is placed in one of the openings 146, 148 (fig. 2) in the lid part 140. These oxygen meters 312 may be commonly used measuring means for measuring the content of dissolved oxygen (DO) in a liquid. For example, Ingold oxygen sensors with 12 mm Teflon membranes may be used. Means for introducing the liquid, e.g. the wastewater, into the reaction chambers, emptying the chambers, cleaning the reaction chambers and the signal lines etc. for controlling these means will now be described with reference to fig. 3b.

A number of pumps 340 are provided in a system according to the illustrated embodiment. As shown, three pumps 340 are connected via pipes, tubes or the like 342 to the inlet/outlet means of each measuring device 110, and these pumps are controlled via control signal lines 346 from the control means 310. The pumps 340 are also connected to supply or outlet pipes or tubes 344 as indicated in fig. 3b.

A first pump in each group of three pumps 340 serves to pump liquid, e.g. wastewater, into the reaction chamber of each measuring device 110. A second pump serves to pump a cleaning fluid, e.g. clean water, into the reaction chamber of each measuring device 110, and a third pump serves to pump liquid, e.g. wastewater, cleaning fluid etc., out ofthe reaction chamber of each measuring device 110.

The pumps 340 may be of any suitable type of pump, preferably electrically driven pumps for easy control of pumping liquid. In a preferred embodiment, however, peristaltic pumps will be used in a system according to the invention. With these pumps, it will be possible to supply a precise amount of liquid into the reaction chamber as these pumps provide a precise and specific amount of liquid for each cycle, e.g. revolution.

The means for introducing air into the reaction chambers, for measuring the oxygen uptake rate (OUR) of the samples and the signal lines involved herein will now be described with reference to fig. 3c

As mentioned above, each measuring apparatus is provided with an oxygen meter 312 for measuring the content of dissolved oxygen in the sample of liquid. The measurement data are transmitted to the control means 310 via signal lines 350, and from here to the computer 320 for storing, processing and interpretation. The system comprises a source of air, e.g. atmospheric air or pressurized air, which may be supplied to each sample through air injector means 360 placed in one of the openings 146, 148 (fig. 2) in the lid part of each measuring apparatus 110. As shown, the air may be supplied by an air compressor 364 connected to the air injector 360 by means of air pipe or tube 362. The air compressors 364 are controlled by the control means 310 as indicated by the control signal lines 366.

A system according to the invention is operated as follows: A sample of wastewater is introduced into the reaction chamber of each measuring apparatus 110 by means of the pumps 344. The temperature of the sample can be controlled by means of heaters 306 and cooling circuits 330, 334, 332, controlled by the control means 310 and the computer 320 in order to obtain a desired time-dependent temperature or preferably a constant temperature, e.g. with a preferred temperature interval of +/- 0,5 °C.

The oxygen content of the sample will be measured by means of the oxygen meters 312, either continuously or periodically. The result of this is transmitted to the computer and stored for processing. When the measured oxygen content of a sample reaches a predetermined low level, or alternatively after a predetermined period of time, air containing oxygen, e.g. atmospheric air, is fed to the sample by means of the air supply means 364, 362, 360, for example for a given period of time, whereby the oxygen will dissolve in the liquid, e.g. the wastewater, of the sample. When air is fed to the sample, the reaction chamber is connected to the expansion chamber, for example by opening the closing piston 19 (fig. 1) or similar closing means in connection with the measuring apparatus shown in fig. 2. After ended aeration, the con- nection to the expansion chamber is closed, e.g. with the closing piston 19, after a preset time delay in order to ensure that no air bubbles are left in the suspension in the reaction chamber.

This cycle will be repeated until sufficient results, e.g. measurements of the dissolved oxygen content, have been obtained to determine the composition of the sample of wastewater. This will normally take several hours and as much as 24 to 48 hours, which is the reason for having two measuring devices 110 in the illustrated system. By having two measuring devices, it is possible to perform two measuring sequences simultaneously, possibly staggered and/or with different cycle periods, different temperatures etc., whereby more precise results can be obtained in a faster manner. It is evident that a measuring system using only one reaction chamber may be used in accordance with the invention, and it will also be possible to use more than two reaction chambers in a measuring system according to the invention.

Once the measuring sequence is finished, the reaction chamber is emptied for waste- water by means of a pump 340, and the reaction chamber will be cleaned by e.g. clean water pumped into the reaction chamber. The water will be pumped out again by a pump 340, and another portion of clean water may be pumped into the reaction chamber until the reaction chamber is sufficiently clean. Hereafter, the measuring apparatus will be ready for a new measuring sequence.

The measured data stored in and/or processed by the computer 320 will, partly during the measuring sequence and partly immediately after the measuring sequence is completed, be used to determine the composition of the sample of wastewater introduced into the reaction chamber.

This determination is performed by the use of a model of the processes taking place in the wastewater, i.e. the processes which will lead to use of the dissolved oxygen (DO) in the wastewater sample.

The processes are illustrated in fig. 4, wherein the content of dissolved oxygen (DO) is illustrated by the block 400. This oxygen is used partly for growth of biomass as illustrated by the arrow 410 and partly for maintenance of biomass as illustrated by the arrow 420. According to the model, the wastewater contains four fractions of biodegradable material, i.e. heterotrophic biomass 430, readily biodegradable substrate 440, slowly hydrolysable substrate 450 and fast hydrolysable substrate 460.

The heterotrophic biomass 430 will give rise to oxygen consumption and readily biodegradable substrate consumption for growth of biomass 410. Furthermore, the oxygen and readily biodegradable substrate is consumed for the maintenance of the biomass 420. The slowly hydrolysable substrate 450 and the fast hydrolysable substrate 460 will be processed into a readily biodegradable substrate 440 which will give rise to oxygen consumption for maintenance and growth of biomass 410, 420.

This model can be described with the following equations (1) to (5):

^{' qmXB ~}T ^H κ^~ -_s ^x»

where

μ_H Maximum growth rate [d^"1] k_hi Hydrolysis rate for X_S1 [d^"1] k_h Hydrolysis rate for X_S2 [d^"1]

Ks Saturation constant [gCOD m^"3] K_X] Saturation constant for X_S1 [gCOD gCOD^"1]

Kχ₂ Saturation constant for X_S1 [gCOD gCOD^"1] q_m Maintenance energy requirement rate constant [d^" -¹]

So Dissolved oxygen [gO₂ m^"3]

Ss Readily biodegradable substrate [gCOD m^"3] X_B Heterotrophic biomass [gCOD m^" ]

X_S1 Fast hydrolysable substrate [gCOD m^"3]

X_S2 Slowly hydrolysable substrate [gCOD m^" ]

Y_H Yield constant for X_B [gCOD gCOD^"1 ]

One of the objects of the invention is to determine the fraction Ss (readily biodegradable substrate), X_B (heterotrophic biomass), Xsi (fast hydrolysable substrate) and X≤₂ (slowly hydrolysable substrate) of the wastewater.

The model of a sample of wastewater facilitated by the equations (1) - (5) is stored in the computer 230, where it will be used to simulate values of the oxygen uptake rate (OUR) of a sample of wastewater.

An example of measured values of the content of dissolved oxygen (DO) versus time is illustrated in fig. 5. The measurements start at the time 501 , prior to which the sample has been aerated. Measurements are performed until the time 502, at which the sample is aerated again, bringing the dissolved oxygen content up to the value indicated by 503. The cycles are repeated until a sufficient number of measurements have been performed in order to obtain a good result.

The measurements are used to determine rates of oxygen uptake, e.g. the biological consumption of oxygen in the sample. An example of such determined values versus time is illustrated in fig. 6 as the curve 601.

In fig, 6 a curve 602 obtained by performing simulations using the model of the system is illustrated. A number of simulations are performed during the measuring sequences and/or afterwards, and a comparison is performed with the measured curve for each simulation. The comparison is performed by using commonly applied techniques, e.g. the least mean square-method (LMS). The parameters, e.g. the content of the organic matter fractions, of the model are altered by using commonly applied methods, and this iterative process is continued until a comparison results in a satisfactory low result and/or a minimum. The model will then represent the determined organic matter fractions of the sample. An example of such determined results is illustrated in fig. 7.

Fig. 8 shows a block diagram illustrating an application of the invention in relation to wastewater analysis. The block 801 illustrates the sampling of the wastewater in a catchment to be analyzed. The block 802 corresponds to the apparatus shown in figs. 3a - 3c, e.g. the hardware such as the reactors, the electronics, the pumps etc., and the software such as the software for control of hardware, interpretation of measurements etc. The block 802 thus also incorporates the iterative process for determina- tion of the parameters involved, e.g. the wastewater characteristics, i.e. the quantities of the model components X_B, Ss, Xsi, s2, and the model parameters. These are led to the block 803 which illustrates the software for analysis and prediction of in-sewer processes occurring in the catchment. Finally, the results from block 803 are led to block 804 which illustrates the process of making decisions on how the sewer system is to be operated, maintained and/or renovated. For example, if the planned connection of a food processing industry will give rise to in-sewer problems, it may be necessary to modify, renovate or redesign the sewer system. Further, the invention may be utilized to design a sewer system in order to fulfill certain requirements. These requirements may for example be criteria concerning the level of odeur from the sewer system, the risk of corrosion in a sewer system, removal of easily transformable matter, production of easily transformable matter etc. The criteria concerning removal of easily transformable matter, e.g. removal of as much matter as possible, may be of particular importance in connection with systems having mechanical treatment and no biological treatment, whereas the criteria concerning production of easily transformable matter may be of particular importance in connection with systems comprising nitrogen removal and/or biological phosphorous removal.

Claims

Patent Claims

1. A method of determination of wastewater biodegradability, whereby

a) a sample of wastewater is introduced into a reaction chamber,

b) the sample of wastewater is subjected to oxygen-containing air,

2. Method according to claim 1, characterized in that steps b and c are repeated one or more times before step d is performed.

3. Method according to claim 1 or 2, characterized in that the meas- urement of the dissolved oxygen content of the sample is performed in a reaction chamber arranged to minimize and/or prevent biofilm growth.

4. Method according one or more of claims 1 -3, characterized in that said organic matter fractions determined on the basis of the determined rate or rates of oxygen consumption, are heterotrophic biomass, readily biodegradable organic matter, rapidly hydrolysable organic matter, and slowly hydrolysable organic matter.

5. Method according one or more of claims 1 -4, characterized in that said microbial properties of said organic matter fractions are facilitated by a model taking into account the interrelated effects ofthe fractions.

6. Method according to one or more of claims 1 - 5, characterized in that

- the determination of said organic matter fractions is performed on the basis of a simulation by a model ofthe sample of wastewater,

- values obtained by using said model are compared with determined values of oxy- gen consumption, and

- parameters of said model are changed in dependency of said comparison.

7. Method according to claim 5 or 6, characterized in that said model of the sample of wastewater is specified by the following equations:

where

μH Maximum growth rate [d^"1] k_hi Hydrolysis rate for Xsi [d^"1] k_h2 Hydrolysis rate for X_S2 [d^"1]

Ks Saturation constant [gCOD m^*3]

K_X1 Saturation constant for X_Sι [gCOD gCOD^"1] K_x2 Saturation constant for X_Sι [gCOD gCOD^"1] q_m Maintenance energy requirement rate constant [d^" ]

So Dissolved oxygen [gO m^"3]

Ss Readily biodegradable substrate [gCOD m^"3]

X_B Heterotrophic biomass [gCOD m^"3] Xsi Fast hydrolysable substrate [gCOD m^"3]

X_S2 Slowly hydrolysable substrate [gCOD m^" ]

Y_H Yield constant for X_B [gCOD gCOD^"1]

8. A system for determination of wastewater biodegradability, wherein the system comprises

- means for subjecting the sample of wastewater to oxygen-containing air,

9. System according to claim 8, c h a r a c t e r i z e d i n t h a t the means for measuring the dissolved oxygen content of the sample comprises a reaction chamber arranged to minimize and/or prevent biofilm growth.

10. System according to claim 8 or 9, characterized in that the means for determining the fractions of the sample of wastewater comprises means for calculating the oxygen consumption by each said number of fractions, means for summing these fractions and means for comparing the resulting value with a corre- sponding value achieved by the measurement of the oxygen consumption of the sample of wastewater.

11. System according to claim 8, 9 or 10, characterized in that the system comprises means for facilitating a model of a sample of wastewater, said model representing organic matter fractions with different microbial properties while taking into account the interrelated effects ofthe fractions.

12. System according to one or more of claims 8 - 11, characterized in that said organic matter fractions of the contents, determined on the basis of the determined rate or rates of oxygen consumption, are heterotrophic biomass, readily biodegradable organic matter, rapidly hydrolysable organic matter, and slowly hydrolysable organic matter.

13. System according to one or more of claims 8- 12, characterized in that the means for determination of said organic matter fractions comprises means for performing a simulation on the basis of a model of the sample of wastewater, means for comparing values obtained by using said model with determined values of oxygen consumption, and means for changing the parameters of said model in dependency of said comparison.

14. System according to one or more of claims 11 - 13, characterized in that said model of the sample of wastewater is specified by the following equa- tions:

. dX_B S_s

(2) — dt - = μ_H ^H — κ_s + - —s_s ^XB^B

where

μπ Maximum growth rate [d^"1] k_hi Hydrolysis rate for Xsi [d^"1] k_h2 Hydrolysis rate for X_S2 [d^_1]

Ks Saturation constant [gCOD m^"3]

Kχι Saturation constant for X_Sι [gCOD gCOD^" -¹]

Kχ₂ Saturation constant for X_S1 [gCOD gCOD^"1] q_m Maintenance energy requirement rate constant [d^-1]

So Dissolved oxygen [gO₂ m^"3]

Ss Readily biodegradable substrate [gCOD m^"3]

X_B Heterotrophic biomass [gCOD m^"3]

Xsi Fast hydrolysable substrate [gCOD m^"3] X_S2 Slowly hydrolysable substrate [gCOD m^"3]

YH Yield constant for X_B [gCOD gCOD^"1]

15. A measuring apparatus for determination of an oxygen uptake rate of a liquid, and in particular wastewater, particularly in connection with a method of and/or a system for determination of wastewater biodegradability, wherein said measuring apparatus comprises a reaction chamber comprising a surface consisting of stainless steel at least internally.

16. Measuring apparatus according to claim 15, characterized in that the surface of stainless steel of the reaction chamber is treated in order to prevent and/or reduce the growth of biofilm on the surface at least internally.

17. Measuring apparatus according to claim 16, characterized in that the surface of stainless steel is treated by bead blasting, preferably by glass-bead blasting.

18. Measuring apparatus according to claim 15, 16 or 17, characterized in that the reaction chamber is encompassed by a shield comprising means for con- trolling and/or regulating the temperature ofthe reaction chamber.

19. Measuring apparatus according to one or more of claims 15 - 18, characterized in that the apparatus comprises means for adding oxygen- containing air to the reaction chamber, means for creating turbulence and/or flow in the liquid and/or means for circulating the liquid in the reaction chamber.

20. Measuring apparatus according to one or more of claims 15 - 19, characterized in that the apparatus comprises means for measuring the oxygen utilization of a sample of liquid contained in the reaction chamber, preferably in the form of an oxygen probe.

21. Measuring apparatus according to one or more of claims 15 - 20, characterized in that the apparatus comprises means for measuring and/or recording the amount of dissolved oxygen (DO) in a sample contained in the reaction chamber and or means for measuring and or recording the temperature of said sample.