Detection of Pollution by Toxics in Wastewater Treatment Plants

Copyright © lFAC Computer Applications in Biotechnology, Osaka, Japan, 1998

DETECTION OF POLLUTION BY TOXICS IN WASTEWATER TREATMENT PLANTS B. Weiss 1.Z, M. Ferry!, M.N. Pons! N. Rocbe!, J.L. Cecile1,z,

c. Prost!

Laboratoire des Sciences du Genie Chimique, CNRS-ENSIC-INPL I, rue Grandville BP 451 F-54001 Nancy cedex, France GEMCEA , 149 rue Gabriel ?eri, F-54500 Vandoeuvre-Ies-Nancy, France I

2

Abstract: A test based on the analysis of the exogeneous oxygen uptake rate (OURex) has been developed for the rapid detection of incoming inhibitory substances able to be detrimental to the normal operation of wastewater treatment plant by activated slugde. A Principal Component Analysis procedure is used for the OURex data treatment. The test has been validated by laboratory and field experiments. Copyright © J998 IFA C Keywords: Water pollution, Data management, Respirometry, Detection, Toxics

biomass (Kong, et al., 1996; Siepman and Teuscher, 1997 . Our purpose is to investigate the possibility of using the sludge taken from the plant to be protected, for the detection of incoming toxic substances with a detection based on the measurement of the exogeneous uptake rate of oxygen (OURex) and the shape of its evolution after the injection of waste water.

I. INTRODUCTION In the near future new regulations in terms of wastewater discharge into receiving bodies (river, lake, sea) will require the treatment in adequate facilities of all incoming wastewaters. Crisis situations may occur in the case of rainstorms or accidental release of toxic components in the sewers. In the later case, if some critical influents can be forecast (fire water) and taken care of by closing the sewer accesses and by temporary storage in on-site tanks, some others may not. It is however highly desirable to be able to detect such situations in order to protect the wastewater treatment plants, and especially their biological part.

After simulation and lab tests to develop the data treatment procedure, field tests have been conducted on the wastewater treatment plant of a large urban community (350 000 eq. inh) in France, for on-site assessment. 2. MATERIALS AND METHODS

It appears unrealistic to organize a specialized detection of toxics at the plant inlet: the spectrum of possible components is large: heavy metals (Hg, Pb, Cr, Zn, ... ), fuels , solvents, pesticides, herbicides, detergents .... A global and more robust detection based on respirometry is proposed. Respirometers have been tested in laboratory and on-site for estimation of biological parameters or pollution concentration using the sludge itself or some fixed

Figure I presents the experimental set-up for lab and field tests. The reactor has a volume of 1.5 l. For lab tests, a sludge sample is taken from the wastewater treatment plant at 8 a. m. transported to the lab and placed in the respirometer. There is approximatively a delay of one hour between the time of sampling and the injection of the synthetic

503

dc

substrate in the lab. After obtaining the response of the sludge without any toxic, HgS04 is added and after some minutes of stabilization a second dose of synthetic substrate is added. Synthetic substrate is composed of sodium acetate and ammonium chloride. For field tests, a sludge sample is taken from the wastewater treatment plant and transported to the on-site respirometer. There is approximatively a delay of 10 min between the time of sampling and the beginning of the experiment. 100 ml of waste water sampled after the grit removal unit is injected in the respirometer, with or without addition of a toxic substance (HgS04 or K2Cr207)

-dt = 0 = k Ja(c. - ce ) .,

OURend

(2)

where C e is the dissolved oxygen at steady state. Combining Eqs I and 2 gives: dc dt

= kJa(c e -c)-OURex

(3)

The oxygen mass transfer coefficient can be measured by the dynamic method involving a shutdown of the air flowrate for a short period of time (Bandyopadhyay, et aI., \9(7) The respirometer is filled with aerated sludge without any substrate. When air starts to flow again, Eq. 3 becomes: dc = k a(c - c) dt

J

(4)

e

The exogeneous oxygen uptake rate can then be obtained by:

The wastewater plant has an aeration basin of 3300 m3 , made of a lOOm long channel equipped with gas diffusers (pons, et aI., 1996).The sludge is taken at one third from the inlet.

OUReX=kJa(ce-c)-(~~)

(5)

There are different ways to model the effect of inhibitory substances on biomass growth. In the competitive inhibition case, the inhibitor is fixed on active sites in such a way that it limits normal substrate fixation. If the normal substrate uptake rate is represented as:

The behavior of the respirometer has been simulated using Matlab™ and data have been analyzed using Statlab™ (Sip Infoware, Nanterre, France).

rmax S r - --'., - K,.+S

1

(6)

I

::

where S is the substrate concentration, r max the maximal substrate uptake rate and K,. the saturation constant, it becomes under competitive inhibition:

I

I

i ~~~+rr

r, = K

3. BACKGROUND Basically oxygen consumption in the aeration tank of the plant as well as in the respirometer is related to the degradation of carbon and nitrogen-based substrates by heterotrophs and autotrophs. The oxygen uptake rate, OUR, is divided into two parts: - an endogeneous part (OURend), corresponding to the respiration of a live cell using its intracellular content as a substrate for the synthesis of various components, - an exogeneous part (OURex), induced by substrate biodegradation The mass balance on oxygen can be written as: dc - = kJa(c s -c) -OURex-OURend (I)

rmax ( I +

t )-1 S

(I+~) +s'

r, = K s

(8)

Kl

It is generally assumed that autotrophs are far more

sensitive than heterotrophs to toxic inhibition. This can be simulated by using different inhibition constants and even different inhibition types for both kinds of microorganisms. 4. SIMULATIONS

Due to its small volume the respirometer is supposed to be well-mixed and therefore its behavior is easily simulated. The Sludge Model 1 (Henze, et aI., 1987) is used to represent the bioreactions taking place. Figure 2 presents the behavior of the respirometer for an injection of synthetic substrate containing

dt

where c is the dissolved oxygen concentration and kJa the oxygen mass transfer coefficient. any

(7)

where K, is the inhibition constant and J the inhibitor concentration. In case of incompetitive inhibition the inhibitor affects also the maximal substrate uptake rate:

Fig. 1: Experimental set-up

At steady state, without substrate, Eq. 1 becomes:

.(1 + _ L)+ ,,,' ., Kl

biodegreadable

504

only soluble biodegradable carbon subtrate (Ss) and soluble nitrogen substrate as ammonium (SI.). Within a few minutes the available substrates have been consumed. The dissolved oxygen probe has its own response characteristics, which should be taken into account. It has been considered that the probe could be modeled as a first order system, with a characteristic time 1: = 30 sec:

- the cumulated ~URex, OURexr The initial slope of the dissolved oxygen curve was preferred to the inital slope of the OURex curve, as the OUR computation uses a time-derivative of the oxygen concentration, which creates a noisier signal. A Principal Component Analysis was then run on these four descriptors for various situations, with and without inhibition.

(9)

(a)

\

(a) X 10· ' 7~----~----~------------~------

.--------

(b)

Inrua! slope ~

OI;R"" l' ./

.'

'"<>"' M

M...Uf~ by th. oll)gefl proM

Fig. 3: Definition of the descriptors used for characterization of dissolved oxygen (a) and OURex (b) curves versus time

RNI 1 0

0 .05

0 .15

01

0 .2

0 .25

0 .2

02'

Figure 4 depicts the displacement of the representative points, in the space of the two first principal components, for cases with and without particulate biodegradable substrate (Si), as well as the effect of competitive and incompetitive inhibitions. Inhibition is supposed to affect only the growth of heterotrophs on soluble carbon substrate and the growth of autotrophs on soluble nitrogen substrate. The effect of the response characteristics of the oxygen probe are not taken into account here. It is clear that the dissolved oxygen curve and OURex will be affected by the available substrates as well as by the amount and type of inhibitor. The availability of substrates is an important point as the plant load is changing along the. The effect of the daily variations of soluble and insoluble pollution is shown in Figure 5b. The increase of the concentration in insoluble biodegradable substrate causes mainly the increase of OURexr. OURexmax and WOURex are mainly affected by Ss and Sn. In case of inhibition the cycle retains its general shape but is displaced. Due to possible superposition, it will be important to know the sampling time of the waste water or to have some external information on the pollution load.

Tlmo(h)

(b) 0 .3

025

jt' Re.,

I

0 .2

"

\

~

:-0.15

a:

::> 0

, ,,

0.1

, ,

, ~~ ""uted

,

\

,

0 .05

0

,

,, - .... _ _ . - - - - - - i

0

0 .05

01

Time (h)

0'"

Fig. 2: Theoretical bchavior of the respirometer: (a) Oxygen concentration (b) OURex after a pulse of C+N substrate To summarize the information contained in the OUR peaks, a Principal Component Analysis technique is used (Einax, et aI., 1997) It could be applied to the complete peak (Candwani, et aI. , 1995) or after preprocessing. The latter solution was adopted: to characterize the shape of the dissolved oxygen and OURex curve, the following descriptors have been selected (Figure 3): - the initial slope of the dissolved oxygen curve - the maximal value of OURex, OURexmax - the width of the OURex curve at OURexrnax/2 , WOURex

5. EXPEIIDAENTAL RESULTS 5.1 Without inhibition Figures 6 and 7 present the response of the respirometer during field tests. The sludge is sampled in the aeration tank at one third from the

505

waste water and recycled sludge inlet. It have been shown that, at that location, most of the readily soluble biodegradable substrate was already consumed and that the ammonium concentration was significantly reduced [3] .

o

2

1 p .m.

4 p~ ~

..

.J

.2

M

.1

""-

$!l. C)("j $." ·1

t!

.jEXre3~ e

-2

.. ~

Fig : 5: Effect on the respirometry test: (+) no inhibition, (e) competitive inhibition (Kr = 0.08 g/I fpr heterotrophs and Kr = 0.015 g/I for autotrophs, with I = 0.05 g/I), (~) incompetitive inhibition (Kr = O.OX g/I for heterotrophs and K r = (UlI 5 gll for autotrophs, with I = 0.005 g/I)

..........................................................:

Fig: 4: Central point: no inhibition, Ss = 0.023 g/l, Sn = 0.0024 g/l, Si = 0., (.) no inhibition, Ss and Sn decrease, Si = 0; (0) no inhibition, Ss = 0.023 g/l, Sn = 0.0024 g/l, Si increases; (.) competitive inhibition (Kr = 0.08 g/l for heterotrophs and Kr = 0.015 g/l for autotrophs), Ss = 0.023 g/l, Sn = 0.0024 g/l, Si = 0., (0 ) and (_) incompetitive inhibition (Kr = 0.08 g/l for heterotrophs and K r = 0.015 gll (0) and 0.03 g/l (_) for autotrophs), Ss = 0.023 g/l, Sn 0.0024 g/l, Si = O.

4,5

1

gu ~ 4

3

c

tu

o ~

~

~

is

0,9

2

.

~ .:

1.5 1 0,5

0,6 ~~ 0,6 E

~f ,4 ~

,

:

o~ ~ 0.2 ~6 0

!!i

..... _

', .... ;... ,:-- .', -_.:

..... _"':'.'

0+-----.-----+-----1----.:..---4 .0.1 SOO

1000

1500

2000

Time (s ec)

Fig: 6: Injection of waste water at 2 p.m.

After a stabilization period of about 10 min, the waste water sample is injected into the respirometer. It can be noticed that the dissolved oxygen, during the stabilization period, as well as at the end of the test remains around 4 mg/l: this means that the sludge metabolism is very active. The insoluble slowly biodegradable substrate, included in the waste water and in the sludge, is metabolized as well as residual ammonium during this period. It means also that the OURex measured is not the total one but reflects mainly the consumption of readily available substrates.

4.5

~

.

......... -..................................................... ........... .. . 0.9 0.8 0,7 0,6 0.5

4

E 3.5 ~

~

J

o

2,5 2

~

1.5

~

:

",

0,4

g

.,

1

is 0.5

c

E ~

_ OJ

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· :. . ... . ./

•• -

..

r"· ...-.J

o +----.-----+-----.------+ soo

1000

1500

0,3 M 0,2 a:: ~ 0.1 0 0 ·0 .1

2000

Time (sec)

Fig;. 7 '. Injection of waste water at 4 {l.m. 5.2 With inhibition

Figure 8 presents the experimental results of OURex monitoring on the plant. The sludge is sampled at the same location than previously. The Principal Components Analysis was run specifically for the field data. A 24hr-cycle exists, although not fully identical to the theoretical one depicted in Figure 5b. Branches are seen for the periods of the day at low load (around 4 a.m.) and high load (mid-day) . The cycle is function of the general human activity (a shift is observed for the mid-July data, when many people are on vacation) as well as of the weather conditions (shift after a rain event). Both types of events affect the load and the composition of the pollution.

Figure 8 presents the results of a lab-experiment with injection of HgS04 as a toxic pollutant. It can be noticed that the initial oxygen level is higher than in the field test, due to the delay for sludge transportation. Most of the available substrate has been consumed. The Principal Component Analysis is run specifically on the lab data. Figure 9 summarizes in the results obtained for various concentrations of HgS0 4. The sludge accepts relatively high concentrations of that toxic, as seen in Figure 9a. In Figure 9b, the sludge in the plant had been subject to a toxic shock ten days before the experimental data were obtained and had not yet

506

fully recovered from it: it is more sensitive to the toxic pollutant.

(a

~----------------------------~ 5

.0

o

4

2.-----------~~_.._--------,

3 1,5

-

2

N

0,5

0

~

·1 .{l,5

·2

-1

b

um..

-1,5

o

-2

..

4

2

OC

.,.,

f1

-2 -4

-2

0

2

4

"

4

Fig: 8: Experimental results without inhibition: (+) with time indications in italics: 24-hr monitoring of a normal dry weather day (July I't); (Q) with time indications in italics bold: monitoring between 8 and 12 a.m., dry weather, reduced human activity (July 17th); (0) with time indications underlined: monitoring between 1 and 5 p.m., after a rainstorm (August 26 th).

·······································0 ·············· ............ .

."

.0 .:

3

-

-

Cl

2

,-

rr-t;t

N

#

0

-. ....... 0

·1 -2

-1,5

-1

-0,5

0

f1

Figure 10 presents the behavior of the sludge, during a field test with injection of 6 mgll HgS0 4. The normal behavior should be the one shown in Figures 6 and 7. Significant changes of OURex curves are observed, with a decrease of OURexmax from 0.8 to 0.5 mg/l/min. Complete inhibition, with no respiratory activity, was observed with the injection of 30mg/1 HgS04 or 6 mg/I K2 Cr0 4. The effect of inhibition in the plane of the two first principal components is shown in Figure 11 : in each test the data point corresponding to the addition of the toxic is off the normal path at that time ofthe day.

Fig: 9: Effect of HgS04 on sludge activity (a) 6 to 23 mg/I HgS04 (b) 9 to 40 mg/I HgS04. (+) before toxic injection, (Q) after toxic injection. Details in text. 4,5

0.9

o.e

~

E 3,5 ~ 34

::; :::

0.7 "2 0.6 E 0.5 ~ 0.4 .§. 0.3 ~ er: 0.2 ::::> 0,1 0 0

)

! \,.

1 is 0,5

6. CONCLUSIONS

....;•..•.. / ...... _ .f'...,;.·;.....;.•/·,.. : i\...~ "" '.;"............... ... / .....::' o -I-------t---~~------t-~.:....:..;;'---4 -0,1

500

A method based on the analysis by Principal Component Analysis of the oxygen uptake rate by sludge has been tested for the detection of inhibition by toxic substances in a wastewater plant. Principal Component Analysis enables to take into account a more complete description of the phenomena taking place in the respirometer that the simple monitoring of one variable such as dissolved oxygen.

1000

1500

2000

Time (sec)

Fig: 10: Injection of waste water + toxic at 3 p.m. The detection is possible under the following conditions: - the sample time of the incoming raw water should be known or some additionnal information on the biological oxygen demand of the sample: it has been shown that the simple measurement of OURex was not sufficient to detect inhibition. The data should be compared to those that should be obtained under normal operation of the plant

507

- the limit of detection will depend on the toxic substance:for example mercury is better accepted by the microorganisms than chromium. Further tests are necessary for complete validation of the method and its full automation. The research project is on progress to build a larger database in the case of no inhibition, especially for different weather conditions and to combine the respirometric information with other pollution measurement. The response to different toxic substances is also currently under investigation.

REFERENCES Kong Z., Vanrolleghem P., Willems P. and Verstraete W. (1996) Simultaneous determination of inhibition kinetics of carbon oxidation and nitrification with a respirometer, Water Research 30, 825-836 Siepmann and Teuscher GmbH, STIPTOX Information Leaflet, Gross-Umstadt, Germany Pons M.N., Pereira L., Roche N., Weiss B., Prost C. and Corriou J.P. (1996) Methodology for the development of a wastewater treatment plant simulator, Computers chem. Engng, 20, S1395S1400. Bandyopadhyay B., Humphrey A.E. and Taguchi H (1967) Dynamic measurement of the volumetric oxygen transfer coefficient in fermentation systems, Biotechnol. Bioeng., 9,533-544. Henze M., Grady C.P.L. Jr, Gujer W., Marais G.v.R. and Matsuo T. (1987) Activated Sludge Model nO 1, lA WPRC, Scientific and Technol. Report n° 1, lA WPRC, London Einax J.W., Zwanziger H.W. and Geiss S. (1997) Chemometrics in Environmental Analysis, VCH, Weinheim Candwani R. M. , Thomhill N., Horstmann B. , Karstnas P. and Titchener-Hooker N.1. (1995) Exploratory diagnosis of large-scale chromatographic processes by PCA. In: Comp. App. in Biotechnol, (A. Munack and K. SchUgerl (Eds», pp 35-40, Pergamon, London.

(a) 3 2 ~

."

0 -1 -2

. ·..·f

-4 -3 -2 -1 0 1 2 3 4

1'1

(b) 3

.............................................. .

2

~ 0

~: .

-1

-2

_..-? ~

+-+-~r--t-+-+-+--i

-4 -3

-2 -1 0 1 2 3 4

1'1 (c)

3.....-------, 2

C!

1

-~

•.....~

-2 +-+-t-II-t---+-+-t--i -4 -3 ~ ~ 0 1 234

1'1

Fig: 11 : Detection of inhibition (a) 6 mg/l HgS04 (b) 30 mg/l HgS04 (c) 6 mg/l K2Cr207; (0) with continuous line = normal trajectory; (+) with dashed line = inhibition detection test

ACKNOWLEDGEMENTS The authors wish to thank the Grand Nancy Urban Community and the Regional Council of Lorraine for their support. A COST Short Term Mission grant is also acknowledged.

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