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AGROIWATECH

Cost-effective technologies for wastewater treatment

and waste biodegradation in agro-industries withreclamation of resources

EU-INCO: International Scientific Cooperation Projects

Contract number: ICA2-2001-10004

Deliverable D1 Review Report

Part 2:

On-line measuring techniques for agro-industrywastewater

Lettinga Associates FoundationPO Box 500

NL-6700 AM WageningenThe Netherlands

Tel: +31 317 482023Fax: +31 317 482108

http://www.leaf-water.org

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Contents

1.1 Scope.............................................................................................................. 31.2 Definitions ....................................................................................................... 31.3 Delineation...................................................................................................... 43.1 Introduction ..................................................................................................... 53.2 Basic measuring principles ............................................................................. 6

3.2.1 Chromatography ..................................................................................... 63.2.2 Electrochemistry...................................................................................... 73.2.3 Spectrometry........................................................................................... 83.2.4 Titrimetry ................................................................................................. 93.2.5

Observers (software sensors)................................................................. 9

3.2.6 Other principles..................................................................................... 10

3.3 Measuring principles for wastewater characteristics..................................... 103.3.1 Alkalinity................................................................................................ 103.3.2 BOD ...................................................................................................... 103.3.3 COD ...................................................................................................... 123.3.4 Dissolved oxygen.................................................................................. 123.3.5 Nitrate.................................................................................................... 133.3.6 Odor ...................................................................................................... 133.3.7 pH.......................................................................................................... 143.3.8 Redox potential ..................................................................................... 143.3.9 Temperature.......................................................................................... 143.3.10 Toxicity .................................................................................................. 153.3.11 VFA ....................................................................................................... 15

3.4 Conclusions .................................................................................................. 164.2 Data trends ................................................................................................... 164.3 Data patterns ................................................................................................ 164.4 Smoothing..................................................................................................... 174.5 Multivariate analysis...................................................................................... 17

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1 Introduction

1.1 Scope

In the agro-industry a large number of unit operations generate wastewater, from single

processes or after mixing streams from several different processes. In addition, these

wastewaters vary strongly in composition and concentration of pollutants, not only

between different regions and agro-industries, but also within individual industries. For

the realization of a cost-effective and sustainable management of these wastewaters

reliable treatment techniques and safe re-use strategies need to be implemented,

despite all the variations occurring in the wastewaters. To achieve this goal it is

imperative that adequate measurement techniques be established, in order to

characterize the wastewater composition and concentration. On the basis of this

characterization the best available treatment technique can be determined. For this

purpose on-line (semi-) continuous measurements are preferred because they allow

efficient monitoring and optimal (automatic) control of the treatment process.

Deliverable D1 consists of two separate parts: 1) a review report on characteristics

affecting anaerobic treatment and 2) a review report on on-line (semi)-continuous

measuring techniques for agro-industry wastewaters. This report concerns the second

part, whereas the first part is delivered in another report.

In this report the state-of-the-art on-line measuring techniques are evaluated for their

ability to characterize agro-industry wastewaters with respect to their treatability in an

anaerobic wastewater treatment system. Also, on-line measurements are the basis for

(automatic) process control.

1.2 Definitions

Instruments are on-line when they can send data to a computer or other data

acquisition system. When an instrument is off-line it may still be able to measure,

display and store data, but data are not readily available on a computer or other data

acquisition system. In-line instruments are instruments that are placed directly into a

stream of the process, and that naturally provide on-line data. These instruments range

from simple (solid state) probes to complex analyzers including automatic sampling

unit. Jeppsson et al. (2002) provide an overview of on-line sensors (or instruments) and

thei usage in wastewater treatment in Europe, and they conclude that sensors no


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longer represent the main bottleneck for the implementation of instrumentation control

and automation, rather the lack of plant flexibility is more troublesome. Continuous

means that a measurement is done without interruption. In practice, however, data are

always gathered and stored with a certain measurement interval. Therefore, a

measurement is considered continuous if the measurement interval is within the time

scale of the dynamics of the measured variable. Monitoring, in general terms, means

measuring for the purpose of supervision, regulation or control, including the collection

of measurement data, their storage as well as their presentation in a suitable form. This

definition includes manual sampling and analysis in the laboratory. More specifically,

monitoring denotes on-line, in-line and continuous measurement using instruments.

In summary, on- and off-line denote the way data is transferred between instrument

and data acquisition system and in-line indicates that the measurement takes place

directly in the process. Continuous refers to measurement frequency and monitoring

implies the use of on-line, in-line and continuous measurement instruments.

It is not possible to make a clear distinction between sensors and instruments. Usually,

the term sensor (or probe) is reserved for a device that responds directly to a

parameter to be measured and converts the response into a more usable form

(Bateson, 1991), whereas a measuring instrument includes sampling device, one or

more sensors, and data processing. Typically, the difference between sensor and

measuring instrument is based on a visual perception: if the technology is miniaturized

and hidden we tend to call the instrument a sensor. A special type of sensor is the

biosensor that consists of a chemical or physical sensor in combination with biological

material. Yet another type of sensor is the software sensor: an algorithm for the on-line

estimation of variables that are not measurable in real time, on the basis of related

measurements that are more easily measurable.

1.3 Delineation

This report focuses on measuring techniques for wastewater only. Measurement

techniques for treatment processes and effluents are not considered, although many

techniques apply to all these media. Exceptions include for example activity

measurements that only are relevant for biomass in a biological treatment system and

off-gas measurements that are related to the performance of the treatment system.

Further, only measuring techniques are considered that can be applied on-line, or have

the potential to be used on-line. The next section summarizes the main results from the

first part of Deliverable D1: a review report on agro-industry wastewater characteristics


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affecting anaerobic treatment. In section three basic measurement principles are

explained first, and next specific measuring principles for the characterization of

wastewater are reviewed. An overview is provided of the most frequently measured

variables in the agro-industry. Section four presents the most relevant data processing

methods. Correct data handling is very important when on-line measurements are

used, because data are often processed automatically, that is: without human

interference.

2 Wastewater characteristics

The report Agro-industry wastewater characteristics affecting anaerobic treatment

reviews all possible characteristics of a selection of agro-industrial wastewaters that

may influence the performance of biological (anaerobic) wastewater treatment

systems. The selected industries include: fruit and vegetable processing, sugar factory,

brewery and potato processing industry. Characteristics are extracted from literature

and provided by project partners. The report also reviews the above industries and

their major wastewater sources and components. Further, some basics of anaerobic

treatment technology are presented, as this technology is considered the key tool in the

management of agro-industrial wastewaters. The review shows that the agro-industry

is a large consumer of water, and that many different production processes produce

likewise many different wastewater streams that are characterized by high variability in

concentration and composition. Generally, these wastewaters represent high

concentrations of COD with high fractions of biodegradable material, making anaerobic

treatment technology the pertinent approach to reduce the organic load and recover

valuable resources. The most frequently reported (and presumably measured)

parameters in the literature are COD, followed by pH, N tot, SS and BOD5. Alkalinity,

which is a very important parameter with respect to the effect on a biological

wastewater treatment process, is reported rarely.

3 Measurement principles

3.1 Introduction

Measurement principles for wastewater characteristics rely on a limited number of

physical, chemical or biological techniques, or combinations of these. Some

measurement principles can be used to assess various characteristics, and some

characteristics can be assessed using various principles. For example the very

commonly measured COD may be measured using titrimetry or spectrometry.


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Technically, all measurement principles can be applied to construct instruments that

can be used to measure on-line and in-line certain characteristics in a wastewater

stream, although these instruments may not be available commercially. For example

an analyzer that measures alkalinity by automatic sampling, titration and data

processing may be typically used in the laboratory, but may also be employed at the

site to measure directly in the wastewater stream. Because of the harsh environmental

conditions common to wastewater handling practice special requirements, however,

will be needed with respect to the degree of automation, reliability, robustness,

resistance to water and chemicals, etc. In this chapter first the main measuring

principles will be described (method oriented), and then the measurement of the most

important parameters will be reviewed (parameter oriented). As stated earlier only

wastewater parameters are considered, and the review will be limited to measuring

techniques that can be applied on-line, or have the potential to be used on-line.

3.2 Basic measuring principles

3.2.1 Chromatography

Chromatography is based on the separation of substances by their different affinity

between a mobile phase and stationary phase. The mobile phase is usually a liquid or

a gas, whereas the stationary phase is usually a solid but may be an immobilized

liquid. Differences in affinity may be based on relative solubility, adsorption, size or

charge. Differences in solubility are expressed by partitioning between the mobile and

stationary phases. Adsorption differences cause the separation of molecules in a non

aqueous environment. Permeation (gel permeation) chromatography is based on

smaller molecules being retained by inclusion within smaller pores of the gel.

Separation by ion-exchange chromatography is based on the exchange of ions in the

mobile phase with ions on the stationary phase. Commonly used chromatography

methods are high performance/pressure liquid chromatography (HPLC), gas liquid

chromatography (GLC) and thin layer chromatography (TLC).

Chromatographic measurement involves the dissolution of a sample in the mobile

phase (which may be a gas or a liquid). The mobile phase is then forced through the

immobile stationary phase. The phases are chosen such that components of the

sample have differing affinities for each phase. A component that is quite soluble in the

stationary phase will take longer to travel through it than a component that is not very

soluble in the stationary phase but very soluble in the mobile phase. As a result ofthese differences in mobilities, sample components will become separated from each


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other as they travel through the stationary phase, and they can be separately detected

for qualitative and quantitative analysis.

Many organic components can be measured using chromatography, including those

present in very low concentrations such as micro-pollutants. Typical wastewater

components that can be assessed using chromatography are (volatile) fatty acids.

3.2.2 Electrochemistry

Electrochemistry is based on the measurement of electrical potential, current or

resistance using electrodes in a liquid containing the components being measured. The

ion-selective electrode (ISE) is based on the exchange of the ion being measured at a

specific sensitive membrane. When equilibrium is reached an electrical potential has

been built up across the membrane that is proportional to the concentration of ions in

the solution. The most well known example of an ISE is the pH-electrode that is

specific for H+ ions. ISEs are currently available for a number of commonly occurring

ionic species including ammonium, potassium, sodium, calcium, various heavy metals,

carbonate, sulphide and nitrate. Principal concerns with ISEs are the calibration and

the interference by other ions. ISEs are invaluable for on-line monitoring, robust,

unaffected by color and turbidity and can be operated over a wide temperature range.

If instead of a selective membrane an inert electrode such as platinum is used then it is

possible to measure the potential of redox couples in the solution, which can be

characterized as a sum parameter.

Voltammetric measurements are based on the conversion of components at an inert

electrode (working electrode) and enable qualitative and quantitative assessment by

evaluating current-potential curves. Voltammetry allows the assessment of heavy

metals simultaneously, and is especially suited for water samples with high salt

concentrations and complex matrices, such as wastewater. Under certain conditions

speciation can be done, because the voltammetric signal provides information on the

oxidation state and binding form (complexes) of the metal species. In principle all

components that can be reduced at the working electrode can be assessed. A well

known example of a voltammetric technique is the measurement of dissolved oxygen

concentration.

While most electro-analytical methods are based on the measurement of processes atthe electrode, the electric conductivity method is based on the resistance of a solution


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between two inert electrodes. As all ions contribute to the conductivity, the method

provides an indication of the total ion content of the liquid.

Whatever the electro-analytical technique, a reference electrode is always required in

order to obtain a useful signal. In most on-line electro-analytical instruments working

electrode and reference electrode are combined into one probe. A new development is

the multiple probe where various (ion-selective) electrodes are combined in one body

to measure several parameters simultaneously. Another new development is the Pd

metal oxide semiconductor (Pd-MOS) sensor, a solid state sensor for the measurement

of dissolved gases such as hydrogen.

3.2.3 Spectrometry

Spectrometric methods measure the absorbance, transmission, diffusion, or

fluorescence of radiation in the ultraviolet, visible and infrared range. Molecular

spectrometry is based on the measurement of components directly in the liquid,

whereas atomic spectrometry measures the components after volatilization in the gas

phase. UV absorbance is correlated with the concentration of aromatic and

polyaromatic compounds. This is used to measure COD and BOD5 on-line in

wastewaters (Muzio et al., 2001). Many dissolved inorganic compounds absorb light at

wavelengths

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demonstrated to suitable for the in-line measurement of VFA, alkalinity, COD and TOC

(Steyeret al., 2002a,b).

Mass spectrometry is a powerful tool for the qualitative identification of compounds.

The technique relies on the ionization of a compound under vacuum conditions and the

subsequent characterization of the patterns that are produced. Applications as an on-

line technique in wastewater treatment are scarce. Membrane inlet mass spectroscopy

was described as a method to determine dissolved hydrogen and was also

demonstrated to be used to measure VFA, after correction for pH changes were made

(Heinzle, 1992). The technique was demonstrated on a pilot anaerobic digester.

Generally, spectrometric instruments consist of a radiation source, a wavelength

selector, sample cell, reagent dosing unit (for VIS spectrometry), detector, and data

treatment and readout unit, and they are suitable for automated in-line measurements.

Robust solid state spectrometers have been developed especially for the use as

probes in wastewater treatment processes, such as nitrate sensors based on UV-

absorbance.

3.2.4 Titrimetry

Titrimetric methods are based on the measurement of the amount of reagent (the

titrant), mostly measured in units of volume of reagent solution that reacts with the

component to be assessed (the analyte). The concentration of component can be

derived from the amount of reagent, provided that the stoichiometrics of the reaction is

known and appropriate end point detection is employed. The end point detection may

be based on the measurement of abrupt changes in the solution by using for example

electrochemical or spectrometric techniques. Successful applications as an in-line

technique include the measurement of VFA, alkalinity, bicarbonate concentration and

ammonium concentration (Bouvieret al., 2002; Van Vooren et al., 1995).

3.2.5 Observers (software sensors)

An observer is an algoritm for the on-line estimation of variables and parameters that

are not measurable. The observer uses measurements that are easily assessable on-

line and model knowledge. Observers have been described that generate for example

measurements of alkalinity and COD on the basis of actually measured reactor influent

flow rate, CO2 gaseous flow rate, volatile fatty acids and total inorganic carbon

(Alcaraz-Gonzalez et al., 1999).


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3.2.6 Other principles

A number of measuring principles exist that use measurements of the interaction

between biomass and wastewater to obtain information about certain parameters in the

latter. For this purpose the reactor receiving the influent may be used, or a mini-reactor

running in parallel with the full-scale process. Interactions between biomass and

wastewater may be measured using any convenient technique such as pH, biogas

production rate, oxygen respiration rate, or heat production (calorimetry). A special

version of such a mini-reactor is the biosensor. This is a device that incoporates a

biological material (biomass from the process or from another source) or a biomimic

(e.g. cell receptors, enzymes, antibodies, nucleic acids etc.), intimately associated with

or integrated within a physicochemical transducer or transducing microsystem, which

may be optical, electrochemical, thermometric, piezoelectric or magnetic. Biosensors

combine the selectivity of biological substances with the processing power of

microelectronics and opto-electronics, and are suitable for in-line applications.

Typical wastewater characteristics that can be measured using biosensors include

biodegradability and toxicity, but also nitrate, that is commonly measured using

electrochemical or spectrometric techniques, can be measured with a biosensor

(Larsen et al., 2000).

3.3 Measuring principles for wastewater characteristics

The following parameters are commonly measured in wastewaters. Some of these,

such as redox potential and dissolved oxygen concentration, are also measured in the

plant treating the wastewater or its effluent.

3.3.1 Alkalinity

Partial alkalinity (PA) and total alkalinity (TA) are commonly measured (APHA), and

various in-line implementations have been developed based on titrimetric methods(Dochain et al., 2000) and spectrometric methods (Bjrnsson et al., 2000; Bouvieret

al., 2002; Jantsch and Mattiasson, 2003). Hawkes et al. (1994) developed a method

based on continuous measurement of flow rate of CO2 evolved from a sample after

saturation with gaseous CO2 and subsequent acidification with excess acid.

3.3.2 BOD

The biochemical oxygen demand (BOD) is the amount of oxygen per volume unit of

water consumed by the available micro-organisms in a period of five days (henceBOD5) at a temperature of 20C. BOD analysis was developed in England around 1900


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because of the need for a measure of pollution in river-water. The five-day testing

period is based upon the thought that the average retention time of British rivers is 5

days. The significance of BOD5 is limited due to the following reasons:

The period of 5 days is arbitrary. Quite often, not all biodegradable matter has been

oxidized after 5 days; that is why there is also a BOD20, to give a more reliable

indication of the actual amount of degradable matter.

The amount of degradable matter in wastewater is much higher than in river-water

or effluent. For this reason, wastewater needs to be diluted, resulting in less easily

interpreted analyses results.

Usually, the operation temperature of a water purification plant is less than 20C.

Therefore, the BOD5 of wastewater is not representative for the oxygen demand

during the purification process.

The concentration of micro-organisms in the purification process is much higher

than that during the BOD5-analyses. Therefore, the conditions of the analyses are

not representative of those in the purification plant.

Nitrification can form a major part of the BOD5, introducing uncertainty because this

process is relatively sensitive to the conditions during the analysis. In particular, the

presence of toxic substances can have a dramatic effect on the nitrification.

Sometimes, nitrification is suppressed by adding nitrification-inhibitors (e.g. Allyl-

thio-urea, ATU), but the effect on the oxidation of organic material is unclear. BOD5 has limited significance for anaerobic treatment because biodegradability is

different under anaerobic and aerobic conditions. If a wastewater is to be treated

anaerobically an anaerobic BOD measurement is preferred. Such a measurement

can be carried out by measuring the decrease of COD and/or CH4 production after

mixing a sample of wastewater with anaerobic sludge.

Despite the limitations of BOD5, this measurement is still frequently applied. The

reasons for this are:

BOD5 has, despite better alternatives, become a standard and a regular part of the

measuring programmes of purification plants all over the world.

The analyses can be carried out with relatively simple means.

BOD5 is a biological parameter; thus, it is relevant for the application of biological

purification processes.

BOD5 of effluent has significance, because it gives a prediction of the oxygen

demand after discharge of the effluent in the receiving water.


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Because automation of the traditional procedure for BOD5 measurement for in-line

purposes is impractical, many attempts have been made to correlate this parameter to

a more easily measurable parameter, the most regular being the short-term

biochemical oxygen demand (BODST). Techniques for the measurement of BODST

include mini activated sludge reactors (Spanjers et al., 1993, 1994) and biosensors (Liu

et al., 2000, 2001).

3.3.3 COD

The chemical oxygen demand or COD is the most widely used parameter for

wastewater characterization. It measures the oxygen equivalent of the organic matter

content that is susceptible to oxidation by a strong chemical oxidant. Analytical

methods are described in the Standard Methods Handbook by the APHA. Standard

methods lists three methods; the open reflux method, the titrimetric closed reflux

method and the colourimetric closed reflux method. All three methods use dichromate

as an oxidant, which makes the description standard dichromate method, used by

quite a lot of the authors, but not very specific. Other methods for COD analysis include

the use of test kits or reagent sets. Automated systems for in-line COD measurement

according to the abovementioned methods are available on the market, as can be seen

in for instance magazine advertisements. They are not mentioned in literature so far.

Alternatively, an IR-spectrometric method (Steyeret al., 2002a) and software sensors

(Aubrun et al., 2001; Alcaraz-Gonzalez et al., 2002) are reported to yield reliable in-line

measurements of the COD.

3.3.4 Dissolved oxygen

Measurement of the dissolved oxygen concentration (DO-concentration) is very

important in the practice and research of aerobic treatment. Initially, this measurement

was carried out with chemical analyses (e.g. Winkler-method), but now this is done

using an amperometric (polarographic, voltammetric) DO-sensor. The DO-sensor

exists of two, sometimes three, electrodes in an internal electrolyte solution separated

from the solution to be measured by means of a semi-permeable membrane. Dissolved

oxygen molecules diffuse from the bulk liquid through the membrane into the internal

solution. These molecules are reduced at the cathode, creating an electrical current.

This current is proportional to the diffusion rate of the oxygen molecules through the

membrane, which in turn is proportional to the DO-concentration in the bulk. Usually,

the relationship between the electrical current and the DO-concentration is linear. Since

several physical factors, such as temperature and permeability of the membrane, affect

this relationship, the DO-sensor needs calibration. It can be demonstrated that, for a


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DO-sensor, water-saturated air is equivalent to oxygen-saturated water. This

characteristic is used when calibrating the sensor on 100% DO in water-saturated air,

which is much easier to obtain than oxygen-saturated water. Most DO-meters (the

complete instrument, including the DO-sensor) have a display on which it is also

possible to read in mg DO per litre (or ppm). The conversion from %-saturation to DO-

concentration takes place within the meter by means of tables. The relationship

between these two variables depends on the temperature and the atmospheric

pressure.

3.3.5 Nitrate

The nitrate ion-selective electrode is a relatively common technique for in-line nitrate

measurements. Several authors reported from long-term experience with this type of

sensor that the calibration value drifted in a rather random and unpredictable way

(Petersen et al., 2002; Malisse, 2002).

Nitrite, and nitrate after reduction to nitrite, can be measured using spectrometric

methods in the visible range after reaction with reagents to form coloured complexes.

The UV-based nitrate measuring principle is suitable for robust solid state

implementation (Langergraberet al., 2003). As a limitation of the UV-based technique

the interference of organic compounds in the complex matrix of wastewater to the

nitrate absorbance at 205 nm is reported (Lynggaard-Jensen, 1999, 2003).

The nitrate biosensor (Larsen et al, 2000) consists of a N2O transducer immersed in a

small bio-chamber that is separated from the medium by a nitrate ion selective

membrane. The measurement principle is based on the diffusion of nitrate from the

medium through the ion selective membrane into the bio-chamber, where it is reduced

to nitrogen dioxide N2O by specialized bacteria present in the chamber, and the

produced N2O is then measured by the N2O transducer. The signal of this electrode is

proportional to the nitrate concentration.

3.3.6 Odor

Dewettinck et al. (2001) describe an electronic nose consisting of 12 metal oxide

sensors to monitor volatile organic compounds in the effluent of a domestic wastewater

treatment plant. To process measurement signals they propose two new concepts:

relative sensorial odor perception and relative fingerprint.


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3.3.7 pH

pH is a measure of the proton-concentration. It can be measured by means of an

electrochemical sensor, consisting of a glass-electrode and a reference-electrode.

Calibration of a pH-meter is based on the determination of the relation between the

sensor-potential and the pH of a solution. For that purpose, two buffer-solutions are

used: one with pH=7 to determine the offset and one with pH>7 or pH7. The slope is temperature dependent, which means that a

(calibrated) pH-meter gives larger variations due to temperature changes at pH7, than is the case at pH=7.

3.3.8 Redox potential

The redox potential (also Oxidation Reduction Potential, ORP) of a solution is the

potential of an inert electrode (e.g. platinum) when present in a solution. It can be

considered as an indication of the oxidative status of the wastewater or mixed liquor.

The potential, which is always measured with respect to the reference-electrode, is the

result of reactions of different redox-couples in solution at the electrodes. The history of

a platinum-electrode (oxidising or reducing conditions), as well as the pollution can

strongly influence the behaviour of the electrode. As a result, it is almost impossible to

attach an absolute value to the redox potential. However, it can be used to trace

changes in the redox-couples. The redox potential also is being used in addition to the

DO-measurement to indicate anoxic conditions. For example, when the DO-

concentration is very low (

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dependent resistance. This sensor is not very accurate, but it can detect very small

changes in temperature.

3.3.10 Toxicity

Toxicity is not an absolute variable, but is always related to a certain biological process.

Measurements are, therefore, always based on the effect of a wastewater or a

compound to be investigated on the process, that is: the interaction between

wastewater and biomass. It is obvious that with wastewater purification one should

determine the effect on one of the relevant biological processes, for instance

methanogenesis , by measuring the gas production rate. It is also possible is to

measure the effect on the respiration rate of the activated sludge. Another possibility is

to measure the effect of a toxicant on a specific micro-organism, or group of micro-

organisms, for example in commercial toxicity meters. An example is the Microtox, in

which the emission of light by the bio-luminescent marine bacterium Photobacterium

Phosphoreum is measured. The test involves the measurement of changes in light

production when the bacteria are exposed to a wastewater, effluent or toxicant.

3.3.11 VFA

The most common technique to assess volatile fatty acids (VFA) is gas

chromatography, although titrimetry (Bouvieret al., 2002; Feitkenhauer, 2002; Lahav et

al. 2002) seems to be more suitable for in-line implementation. Nevertheless Pind et al.

(2002, 2003) describe a technique based on gas chromatography to monitor VFA on-

line in difficult media. An in-situ pre-filtration technique by a rotating filter, was followed

by an ultra-filtration step, and finally acidification prior to GC analysis. Alternatively,

VFA can be measured in-line using IR-spectrometry (Steyer et al., 2002) or, as

reported in one case, using mass spectrometry (Heinzle, 1992).

Lahav et al. (2002) report on a new titration method suitable for on-site measurement

of VFA and carbonate alkalinity. In contrast to other titration methods, this method can

be applied generally (irrespective of VFA/carbonate species ratio). The method takes

into account the effects of phosphate, ammonium, sulfate and sulfide weak acid

subsystems on the titration results. The method involves eight pH observations and

model calculations, and the procedure takes typically 15 minutes. The method was

verified using three industrial effluents. Very high accuracy (>97%) and good

reproducibility was obtained for VFA concentrations above 100 mg/l as acetic acid.


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Cruwys et al. (2001) have reviewed the possibilities to develop a VFA monitor based

on metal oxide semiconductor (MOS) gas sensing technology to measure headspace

concentration of VFA.

3.4 Conclusions

Measurement principles for wastewater characteristics rely on a limited number of

physical, chemical and biological techniques, or combinations of these. Some

measurement principles can be used to assess various parameters, and some

parameters can be assessed using various principles. For example the very commonly

measured COD may be measured using an automated version (including automatic

sampling) of the conventional method, titrimetry or various spectrometric techniques. In

the literature experiences and results with many prototypes and some commercial

implementations are described (see also Appendix).

4 Data processing

Correct data handling is very important when on-line measurements are used, because

data are often processed automatically, that is: without human interference. Typically,

time series are most commonly used for presenting measurement data. This provides a

good overview but does not reveal the full information content or, contrary, it shows too

much information for a good interpretation. In the following the most important data

processing methods will be reviewed. For detailed information on these and other

methods, see for example Olsson et al. (2004).

4.2 Data trends

Data trends display overall development during a certain measuring period. They show

whether a parameter is increasing and decreasing and are used to predict a general

development in data. When evaluating trends it is important to assure that the period is

sufficiently long, so that cyclic phenomena do not bias the trend. Spreadsheet

programs usually provide built-in trend functions.

4.3 Data patterns

Many data series of wastewater parameters show a repeated pattern, such as a diurnal

variation, or a variation related to a batch process operation. Knowledge of the normal

variation pattern is valuable in detecting abnormal events and taking the right control

action to prevent treatment process upsets. Data patterns can be identified andevaluated by using percentiles. For example a 60% percentile is the value below which


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60% of the measured values are to be found. Limits in percentiles can be used to

analyze large data sets to assess the portion of time that a parameter is outside its

normal boundaries. Alternatively, variations in the data can be highlighted by

calculating the standard deviation.

4.4 Smoothing

Measured data is always subject to noise, which can be a problem in the correct

interpretation of the measurements. One way to deal with noise is to smooth the data

by using a numerical method called filtering. The most common numerical filter is the

moving average. This filter substitutes each data point with the average of a given

number of points before and after that point. The number of points used to calculate the

average defined the window of the filter. The larger the window, the higher the degree

of smoothing. As a too high a degree of smoothing may hide significant effects, a

balance must always be achieved between inaccurate smoothing and excessive noise.

An improved smoothing technique is the exponential filter that corrects the filtered

value as soon as a new measurement is made. It calculates the new filtered value by

combining a weighted version of the previous filtered value and the latest measurement

value. This filter requires setting a weighting factor between 0 and 1.

4.5 Multivariate analysisWhen several sensors are used to monitor continuously the amount of data may be

such that it leads to confusion and inability to identify the important information, and

thus a loss of information. In order to help extract the essential information from large

date series from several different sensors a large number of statistical methods is

available. One of these, the multivariate analysis is increasingly being used in

wastewater treatment (Rosn and Lennox, 2001; Le Bont, 2003). The most basic

multivariate analysis method is the principal component analysis

5 Conclusions

Measuring principles for wastewater characteristics rely on a limited number of

physical, chemical or biological techniques, or combinations of these. The most

important include: chromatography, electrochemistry, spectrometry and titrimetry.

Some wastewater parameters can be assessed by several of these measuring

principles. Several data processing methods exist to cope with the vast amount of data

that is generated when several in-line instruments are continuously in operation.


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Appendix: Overview of manufacturers of on-line

Instruments

Overview of manufacturers of on-line instruments (Lynggaard-Jensen et al., 2003)

Producer Web-site NH4 NOx PO4 TN TP Org. SS SlSp

ABB www.abb.com x x x

Applikon www.applikon.com/newadi/Default.htm x x x x

Bran&Luebbe www.bran-luebbe.de/eng/index.html x x x x x x

Danfoss Analytical www.danfoss.com/analytical/index.asp x x x

Dr.Lange-Contronicwww.drlange.com/drlange-en/langeen.html x x x x x x x

Endress&Hauser

Staiger Mohilowww.endress.com x x x x x

Gl International www.gliint.com, www.hansbuch.dk x x

Greenspan www.greenspan.com.au x x

Hach www.hach.com/Prod/piproces.htm x x x x x

ISCO/STIP www.isco.com/html/prdprocess.html x x x

Maihak www.maihak.de x

Marklandwww.sludgecontrols.com

www.insatech.comx x

MJK Automation www.mjk.dk x x

Royce www.royceinst.com, www.danova.dk x x

Siemenswww.siemens.dk/energimiljo/

miljo/spildevand.htmlx x x

WTW www.wtw.de/gb/index.html x x x x

ZellWeger www.zelana.com x x x x x

Zllig www.zuellig.ch/e/messtechnik.htm x

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