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Light penetration in waste stabilization ponds
War. Res. Vol. 28, No. 5, pp. 1031 1038, 1994 Copyright ~~ 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0043-1354/94 $6.00 + 0.00
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L I G H T P E N E T R A T I O N IN WASTE STABILIZATION PONDS T. P. CURTISI*~, D. D. MARA I ~ , N. G. H. Dtxo 2 and S. A. SlLVA 3 ~ ~Department of Civil Engineering, University of Leeds, Leeds LS29JT, England, 'Conj. Jardim Amazonas, Rua F, Condominio Juaperi, BIoco B, Apto 203, Bairro Parque 10, 59055, Manaus-AM, Brazil and 3EXTRABES, Caixa Postal 306, 58100 Campina Grande-PB, Brazil (First received November 1991; accepted in revised form September 1993) Abstract--The penetration of light into waste stabilization ponds was studied because of its importance in pathogen removal and algal productivity. The attenuation of light in ponds was dominated by light absorbtion by gilvin (also called dissolved yellow matter or humic substances) and algae, light scattering processes (turbidity) being of no importance. Gilvin exerts a strong influence over the spectral variation and longer wavelengths penetrate much better than short wavelengths. Estimates of attenuation coefficients in the u.v. suggest that these wavelengths are less penetrating than previously reported. Differences in algal concentrations cause the differences in light attenuation seen between ponds, though they also cause some spectral variation because short wavelengths are more affected by changes in algal biomass than long ones. In the absence of algae there appears to be a lower limit to the clarity of ponds dictated by gilvin and other substances. Secehi disks were found to be reliable instruments for measuring light penetration. Key words--lagooning, solar radiation, light penetration, photosynthetic activity, algae, humic matters, Secchi disk
NOMENCLATURE
PAR = photosynthetically active radiation U.V. = ultraviolet E = irradiance (Wm -2 or/zeinsteins m -2 s -I) Ed= downward irradiance (Wm -2 or/~einsteins m-2 s-i ) Eo= upward irradiance (Wm -' or/teinsteins m -2 s -I ) 2 : depth (m) Kd= attenuation coefficient for downward irradiance (m -t ) R = irradiance reflectance a = absorbtion coefficient (m- i ) b = scattering coefficient (m i) bb = back scattering coefficient (m- l ) G440 the absorbance of gilvin at 440 nm (m- i) P440= the absorbance of tripton at 440 nm (m- i ) re= the constant relating light attenuation to chlorophyll a concentration (#g 1-1) A = the constant that represents the attenuation coefficient in the absence of algae (m -~) INTRODUCTION
Light penetration is of fundamental importance to the functioning of facultative and maturation ponds, affecting both pathogen survival (Curtis et al., 1992a, b; Troussellier et al., 1986) and the concentration and productivity of the algal population (Kirk, 1983). A study of the penetration of light in *All correspondence and reprint requests should be addressed to: Dr Tom Curtis, Department of Civil Engineering, Cassie Building, University of Newcastle, Newcastle upon Tyne NEI 7RU, England.
waste stabilization ponds (WSP) is an essential first step in the modelling of these processes. A proper understanding of the impact of light on pathogenic organisms and productivity requires a knowledge of how light penetration varies from wavelength to wavelength as well as from WSP to WSP. Previous workers who have measured the attenuation of light in WSP (Moeller and Calkins, 1980; Calkins eta/., 1976; Sarikaya et al., 1987) have not considered either spectral or inter-pond variation, neither have they sought to explain the mechanisms governing light penetration. This paper describes a study undertaken to elucidate the fundamental factors governing the penetration of all wavelengths of sunlight into WSP. Fortunately, due to the importance of light penetration in aquatic photosynthesis the subject has been widely studied. The optics of fresh and marine waters has been comprehensively reviewed by Kirk 0983)° The bulk of the work on, and instrumentation in, this field is devoted to PAR, which falls between 400 and 700 nm. Bactericidal sunlight can have a wavelength of anything from 290 to over 700 nm (Webb and Brown, 1979; Whitlam and Codd, 1986; Curtis et aL, 1992a) some wavelengths may be more bactericidal than others, and some may not be bactericidal at all. Ideally therefore, a knowledge of the aquatic optics of bactericidal light should encompass the penetration of a range of wavelengths between 290 and 700 nm. This requires a submersible u.v.-visible
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T.P. CURTISet al.
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spectroradiometer and only one of these has ever been built (Smith et al., 1979). The instrument has been used to develop predictive empirical models for seawater (Baker and Smith, 1982) but at algal concentrations (maximum chlorophyll a = 10/~g l - l ) far below those seen in WSP. Fortunately the same principles apply to the penetration of sunlight, whatever the wavelength. Consequently the ability of u.v. light to penetrate in a given body of water can generally be inferred from what is known about the penetration of visible light. Light is usually measured as irradiance (E) expressed as the number of photons, or the amount of energy, per unit area per unit of time. The former is measured as #einsteins m -2 s -j (1 einstein ~ 6 x 1023 photons) and the latter as W m -2. Downward (Ed) and upward (Eu) irradiance are measures of the light shining on horizontal surfaces facing up and down respectively. Ed is the type of irradiance most usually measured. The ratio of upward to downward irradiance (Eu/Ea) is known as the irradiance reflectance (R). E d diminishes with depth as follows:
Edf~)=
Ed~0)e-Kdz
(1)
where Edcz) and Ed~0) are the values of the downward irradiance at a depth z (m) and just under the surface, respectively; Kd ( m - ' ) is the attenuation coefficient for downward irradiance and it measures how easily light penetrates a body of water, which in turn depends on what is suspended and dissolved within it. Eu and R also diminish with depth and may be described by equations analogous to that above (Kirk, 1983). Only one of two things can happen to a photon of light in a WSP: it can be scattered or it can be absorbed. The probabilities of these two events happening are measured by the absorption and scattering coefficients, a (m -~) and b (m-~). These are inherent properties of a given body of water but for most purposes it is not necessary to know the value of b and a, only the ratio between the two. This ratio can be easily calculated from R (Kirk, 1981b), and the value of R at the surface (R0) with the sun at an angle of 90 ° is related to the amount of light scattered backwards measured by bb, the back scattering coefficient, and a by the following expression (Kirk, 1983): R0 = 0.33 bb/a (2) For turbid waters, such as WSP, bb/0.019 = b (Kirk, 1983) and equation (2) becomes: R 0 = 17.4 b/a (3) The reflectance, and hence the ratio of b to a, of algal rich waters is usually very low. The chances of a photon being absorbed or scattered before reaching a certain depth will also depend on the angle at which it is penetrating the water. The more acute the angle the less chance it has of reaching a significant depth. The angles over which the light field are distributed are summarized as the average
cosine, which may be thought of as the average value of the cosine of the angle (relative to the vertical) of all the photons in an infinitesimally small volume. At a given depth the average cosine depends on, and increases with, the b/a ratio. In water of low reflectivity with the sun at 90 °, the average cosine will not usually change much with depth. Taking the idea that a photon may be either scattered or absorbed, Kirk (1981a) has completed a Monte Carlo study of photons entering waters with different b/a ratios. From this study he derived the following expression relating the attenuation coefficient at the point where light has 10% of its surface value to the coefficients a and b: Kd = (a 2 + 0.256 ab )J/2
(4)
Thus in waters with b/a ratios of less than 1 the attenuation coefficient is principally dependent only on a and therefore (Kirk, 1983): Kd ~ a
(5)
The coefficients of absorbtion and scattering are themselves governed by the light absorbing and light scattering components of the water. The light absorbtion properties of natural waters are attributable to four components: the water, gilvin (dissolved yellow matter), algae and tripton (inanimate particulate matter). The total absorbtion at a given wavelength can be obtained by adding together the absorbtion of these four separate components. Although apparently colourless, water absorbs light moderately well at wavelengths <550nm. Gilvin absorbs strongly in the u.v. and is known to be present in sewage and WSP (Curtis, 1990; Haag et al., 1986; Draper and Crosby, 1983). It is quantified by the absorbance at 440 nm (G440) of a 0.2/~m filtrate of the water in question. Tripton has an absorbtion spectrum similar to that of gilvin, though it typically absorbs very much less light. Tripton is important in water with many particles derived from soil erosion and the resuspension of sediment. It is quantified by the absorbance of particulate matter at 440nm (P440). Unfortunately P~0 is difficult to measure because of the particulate nature of tripton which means that special equipment is required to distinguish between the substances scattering and absorbing properties. Algae, being photosynthetic, have large quantities of pigments, which also impede light penetration. In productive waters, such as WSP, light absorbtion by algae may be very important, often limiting the growth of the algae themselves. The absorbtion spectra of algal cells largely reflects the absorbtion of the photosynthetic pigments, with peaks at around 440 and 680 nm. Changes in the attenuation coefficient at a given wavelength associated with a change in algal biomass will reflect the absorbance of the algae at that wavelength. The relationship between the two is approximately linear, with the absorbance due to algae being related to the amount of chlorophyll a (an indicator of
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Light penetration in stabilization ponds algal biomass) by a c o n s t a n t Kc. Because o f the t u r b i d n a t u r e o f a n algal p o p u l a t i o n its a b s o r b a n c e can only be m e a s u r e d with a specially a d a p t e d spectrophotometer. Scattering is caused b o t h by the water itself a n d particles, such as bacteria, within the water. The latter p r e d o m i n a t e s in all n a t u r a l waters. Scattering increases the a t t e n u a t i o n coefficient of a body o f water by increasing the p a t h l e n g t h o f the p h o t o n s , a n d light is not " u s e d u p " in any way. Light is scattered in roughly the same way in all turbid waters: m o s t o f it is scattered forward, a n d the 2 % o f light t h a t is scattered in the direction from which it came is k n o w n as back-scattered light. The objective of this investigation was to characterize the f u n d a m e n t a l aspects o f W S P optics a n d to discover the n a t u r e a n d cause of spectral a n d interp o n d variations in light penetration. This report describes studies using submersible radiometers a n d spectroradiometers t h a t have discovered that absorbtion r a t h e r t h a n scattering d o m i n a t e s light attenuation a n d t h a t a b s o r b t i o n is principally a t t r i b u t a b l e to gilvin a n d algae. The former is responsible for m u c h o f the spectral variation in light a t t e n u a t i o n whilst the latter causes m o s t of the differences in light p e n e t r a t i o n seen between WSP. MATERIALS AND METHODS
Location and design of ponds The fieldwork was carried out at the Federal University of Paraiba's experimental wastewater treatment station (EXTRABES) Campina Grande, northeast Brazil (7 ° 13' 11" S, 35 ° 52' 31" W and 550 m above mean sea level). Three types of pond were used: deep maturation and facultative pilot-scale ponds and shallow model scale ponds. The maturation ponds (prefix M) were part of a series of 5 identical ponds comprising an anaerobic pond (A8) fed with raw sewage, followed by a secondary facultative pond (F 16) and three maturation ponds (MI2-M14); the maturation ponds were 10.0 m long, 3.35 m wide, 2.20 m deep and so had areas and volumes of 33.5 m 2 and 73.7 m 3 respectively. The first pond in the deep series was fed with raw domestic sewage (BOD ~ 20 mg 1-1 ) that was pumped from a header tank by a variable speed peristaltic pump (model HRSV, Watson Marlow, Falmouth) at a rate which gave a volumetric load in A8 of ~,40.0 g BODm -3 day t and each pond a mean hydraulic retention time of 5 days. The facultative ponds (prefix F) had a length, width and depth of 25.4, 7.1 and 2.3 m respectively and consequently had an area of 181.0 m 2 and a volume of 417.7 m 3. Each of the facultative ponds was fed by a peristaltic pump and had a retention time of approx. 15 days and an areal load of about 300 kg ha- l day - ~. The model-scale ponds (prefix N) comprised a series of four, 1 m ~ asbestos cement tanks, which were fed with effluent from an anaerobic digester by a peristaltic pump, had a depth of 0.7 m, a retention time of 5 days and an areal load on the first pond (N l) of approx. 170kgha-I day i.
calibrated Crump 550 voltmeter. All underwater light measurements were taken when the sun was at an angle of 85-90 ° and not hidden by cloud. The quantum sensor was lowered into the water by hand in a cradle also manufactured by Crump, the upward irradiance was measured by turning the cradle over. The spectroradiometer was placed in a specially made waterproof box that allowed the wavelength to be changed whilst the meter was submerged. The box was weighted and hung from a wire at each corner, the wires passing through two aluminium posts which were themselves placed over strong rails positioned over the pond. This elaborate arrangement was required to hold the instrument firmly, the pond water being so absorbent that even small movements could affect the reading obtained. The depth of the box was measured with a ruler and the window of the water-proof box was wiped with a tissue to remove any algae, readings were then taken quickly before the window became obscured again. Readings below 420 nm were not taken as the predominately red underwater light tended to distort the result at these wavelengths. The Secchi disk used was a white circle of metal ~ 300 mm in diameter; the disk was lowered into the water on marked cord and the depth that it became invisible noted.
Other measurements Chlorophyll a was estimated by the methanol extraction method of Pearson (1986), no correction was made for phaeophytin. The filtrates were prepared by passing pond water through a sterile 0.2/am filter on a sterile membrane filtration apparatus; the water was then decanted into a sterile 25 ml bottle. The absorbances of the filtrates were measured in a 40 mm quartz cuvette on a scanning spectrophotometer (DMS 80, Varian, Walton on Thames, U.K.) in Leeds, U.K. within 5 weeks of collection. The statistical methods used are described in Sokal and Rohlf (1981). RESULTS
General features o f light penetration Nine different p o n d s were studied, P A R and m o n o c h r o m a t i c light decreased exponentially a n d the rate o f a t t e n u a t i o n was sometimes lower near the surface. Different wavelengths were also a t t e n u a t e d at different rates, the longer wavelengths penetrating more t h a n the shorter ones. This caused a shift in the spectrum in the underwater light field, the light becoming redder as it penetrated the water. The variation o f light a t t e n u a t i o n with wavelength in three p o n d s is summarized in Fig. 1. A t any given wavelength there was considerable difference in K d between ponds: the higher values were found in those p o n d s with more algae. Nevertheless the Kd spectra o f p o n d s M I 3 and N3 were similar in shape, with lower a t t e n u a t i o n occurring at long wavelengths a n d increasing in p r o p o r t i o n to the wavelength. Pond F I9, which h a d the highest Kd values exhibited a slightly different pattern, the a t t e n u a t i o n coefficient peaking at 470 nm rather than showing a steady increase with decreasing wavelength.
Measurement of the underwater light field
The causes ~[" light attenuation and its t~ariation
Two types of meter were used to measure the underwater light: a Crump 552 underwater quantum sensor (Crump Scientific Instruments, Billericay, U.K.) that detected downward PAR (400-700 nm), and a Crump 556 spectroradiometer that could detect downward monochromatic light between 390 and 720 nm. Both instruments were read via a
The values for reflectance (R0), the ratio o f upwelling to downwelling P A R at the surface of the WSP, are shown in Table I. The ratio of scattering to a b s o r b t i o n (b/a) was calculated using e q u a t i o n (3). A l t h o u g h the values of b/a vary somewhat, they are
1034
T.P. CURTlSet al. 120
O)
100
16
•
-
14
"o c
~I
80
12
6o
10
411
8
20
- 6
~
::::I
Z /
E
ff
0
I 420
I 440
I 460
I 480
I 500
I 520
t 540
I° 580
I 620
I 660
I 700
4
Wavelength (nm) Fig. 1. The spectral variation in the vertical attenuation coefficient (Kd) in ponds MI3 ( ), FI9 (...) and N3 ( - - - ) . The chlorophyll a concentrations were 112, 1628 and 689 #g 1-~, respectively.
all very low and the ponds absorbed light far more than they scattered it. Therefore the attenuation of light is almost entirely dictated by absorbtion and equation (5) applies. All the pond water filtrates examined absorbed light in a m a n n e r similar to that shown in Fig. 2. The absorbances of the pond filtrates at 4 4 0 n m are compared in Table 2. There was relatively little difference between the absorbances of filtrates of water from different sources, which contrasts with the large variations in Kd values observed between ponds. Nevertheless, all the filtrates absorbed far more light at longer wavelengths than shorter ones (Fig. 2), just as the pond water column did (Fig. 1). Indeed the shape of the typical filtrate absorbance spectra was very similar to the attenuation coefficient spectra found in N3 and M12 (Fig. I), especially at wave-
Table 1. The reflectance (R 0) and the scattering absorbtion (h/a) ratio at the surface of a number of pilot scale WSP at EXTRABES
Pond
Date
Ro
h /a
NI N2 N3 N4 N4 FI8 FI9 FI9 F19 MI2 MI2 M 13 MI4 M 14
27/10/89 27/10/89 27/10/89 26/10/89 25/10/89 3/I 1/89 24/10/89 30/10/89 3/I 1/89 23/10/89 24/10/89 24/10/89 20/10/89 24/10/89
0.0089 0.0090 0.0014 0.0000 0.0095 0.0095 0.0135 0.0250 0.0028 0.0130 0.0225 0.0065 0.0200 0.0200
0.0270 0.0273 0.0042 -0.0288 0.0288 0.0409 0.0758 0.0085 0.0394 0.0681 0.0207 0.0606 0.0606
lengths < 520 nm. The absorbances in the filtrate of M I3 at these shorter wavelengths were found to be strongly correlated with Kd in N3 and M13 in the same range (r = 0.991 and 0.992 respectively); and a highly significant (P <0.001) regression was also found between the two. Using these regression equations the attenuation coefficients in the u.v. were estimated from the absorbances of M I 3 filtrates (Table 3). A significant regression was not found (P = 0.576) for the more eutrophic pond FI9. The algal concentration, unlike filtrate absorbance, varied considerably from pond to pond and a significant (P = 0.0058; r = 0.69) model II regression was found between chlorophyll a and Kd(eAalin the ponds 25 -
20 i-
"T E
15
C
=o 10 ,< .¢3
5
~
0 300
-,I 350
400
450
500
550
I
I
I
600
650
700
Wavelength (rim) Fig. 2. The absorbance of a 0.2pm filtrate of pond water from MI3 (. . . . ) and MI4 ( - - ) .
Light penetration in stabilization ponds Table 2. The absorbance at 440 n m o f 0 . 2 ~ m filtrates o f E X T R A B E S W S P waters and raw sewage and typical K d at 440 n m Source
Abs~0 (m I)
Ka~ 0 (m i )
1.80 2.45 1.90 2.53 1.85 2.55 2.55 2.53 1.80
NA ND ND 55.45 97.01 ND 18.38 13.42 6.02
R a w sewage FI6 F 17 FI8 FI9 A8 M12 M 13 M 14
N D = not done. N A = not applicable.
studied. Similar regressions for attenuation coefficients between 440 and 700 nm were all significant (P between 6.42 x 10-3 and 1.513 x 10-6) with high correlation coefficients (r =0.81--0.99). Only Kd(42o) was found not to regress significantly with chlorophyll a (P = 0.6046), though elimination of the facultative ponds from the calculation made the regression very significant (P = 3.71 x 10 -4, r = 0.99). The regression coefficient (Kc) relating algal concentration to the attenuation coefficient also varied with wavelength (Fig. 3). Light in the 440-500 nm band was much more sensitive to the presence of algae than light at longer wavelengths. The absorption spectrum of cultured algal cells (Chlorella pyrenoidosa) is similar to that of Kc (Fig. 3). The shape of the spectra of the constant (A) (Fig. 3) was similar to that of the pond filtrates (Fig. 2) though the former was larger than the latter and fluctuated more at the longer wavelengths. The values of Kc calculated for 580 and 620 nm (0.0173 and 0.0156) in the WSP are very similar to that calculated by Ganf (1974) (0.016) for 589nm in a shallow, eutrophic, equatorial lake. Unfortunately Ganf (1974) only published the regression coefficient for the most penetrating wavelength, so it is not possible to make comparisons at other wavelengths. The use o f the Secchi disc This cheap and simple device gave reproducible results, a good linear relationship was found between the reciprocal of KdPAR (m - i ) and the Secchi depth (m) (l/PAR =0.0222 +0.261 x Secchi depth, P > 0.001, r = 0.96, n = 13). A relationship was also
Table 3. Estimates of the attenuation coefficients in ponds N3 and M 13 based on linear regression between attenuation at a given wavelength (Ka) and typical absorbance o f a pond ( M I 3 ) filtrate at the same wavelength (Abs). K d ( M I 3 ) = 3.35 + 4,20 Abs; K~ (N3) = - I 1.2 + 4 1 . 2 Abs Wavelength (nm)
Abs (m ' )
K d (N3) (m ~)
Kd ( M I 3 ) (m ~)
400 380 360 340 320
3.69 4.97 6.97 9.88 14.56
140.73 192.52 275.92 395.65 588.75
18.83 24.22 31.25 44.83 64.51
Chlorophyll a concentrations o f N3 and M I 3 were 689 and 1121zgl i respectively.
1035
found between the algal concentration (chlorophyll a, # g l -~) and the reciprocal of the Secchi depth (m) [Chl a = (252 × l/Secchi) - 620, P > 0.001, r = 0.88, n = 13]. DISCUSSION
The penetration of light in WSP is essentially similar to that seen in any other water body with high levels of giivin and algae, for example Lake George in Uganda (Ganf, 1974). Since algae are found in all facultative and maturation ponds (Mara and Pearson, 1986) and gilvin is present at roughly similar levels in raw sewage and wastewater effluent in all parts of the world (Curtis, 1990; Haag et al., 1986; Draper and Crosby, 1983) we believe that our findings will be applicable to WSP worldwide. The observed exponential decrease in the light is common to all waters. The variations in light attenuation seen between depths, ponds and wavelengths are a consequence of the contents of the pond. The very low reflectivity observed over a very wide range of turbidities and algal densities suggests that low reflectivity is a feature common to all ponds. We therefore conclude that light penetration in WSP is dependent on the absorbative properties of the water and changes in turbidity will not influence the optical properties of pond water (Kirk, 1983). u.v. Light was not detectable by the quantum sensor used, but as light absorbtion increases below 400 nm, reflectivity is probably also very low and our conclusions about the unimportance of turbidity also apply in this waveband. The spectral variation in attenuation coefficients seems to be governed by the absorbance of the dissolved substances in the water. The relationship between the absorbance of gilvin at a given wavelength and the attenuation coefficient was striking. As gilvin is found in all domestic sewage the same pattern of spectral variation will be found in all WSP. In some industrial effluent other colours may predominate. The shift to the red seen in the ponds is also found in other water bodies with high gilvin concentrations, such as Lake George in Uganda (abs~0 = 3.4m ~) (Ganf, 1974). It appears that relatively little u.v. light can penetrate the water and the reports by Calkins and his co-workers (Moeller and Calkins, 1980; Calkins et aL, 1976) that u.v.-B (280-320 nm) has an attenuation coefficient of 16 m are probably incorrect. A filtrate of pond water has an attenuation coefficient of more than 16 m ~in the u.v.-B, so it seems impossible for the water column to have such a low attenuation coefficient at these wavelengths. The instrument used was probably detecting red light and in the manner discussed below. These findings cast doubt on thc validity of Calkins work on the mechanisrns by which WSP remove faecal coliforms. As well as influencing FC removal the strong attenuation of u.v. light may well protect the algae from photoinhibition (Paerl et al., 1985) and
1036
T.P. Cog.s et al. 0.07
.,~ C ~
16
.Q 14 ~
Chlorella
oldosa
12
0.05
}
lO 0.114
a
co 0.03 ._~ (o "o
.
l
-
~
6
0.02
~E" ~ 0.01 ~E o
""N. ,
,
2 ""~ ,
........
.....
,'-G'"'r
Wavelength
.'""" . . . . . . . . . .
e,
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o
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Fig. 3. The spectral variation of the constant A ( - - A - - ) that represents the minimum possible attenuation coefficient, and the regression coefficient K¢ ( - - O - - ) relating the algal biomass to light attenuation. The absorbance of whole algal cells (Chlorella pyrenoidosa) is included (...I-:1...) for comparison.
so increase algal productivity and therefore the pH and DO of the WSP. The spectral variation in the facultative ponds was slightly different as it peaked and declined between 480 and 420 nm. This decline is rather strange since light appears to penetrate better in facultative WSP than in less eutrophic ponds. This could reflect a difference between the algal populations of facultative and maturation ponds; if so, it is odd that this discrepancy was only apparent at short wavelengths. A more plausible explanation is that the spectroradiometer readings may have been distorted by the rapid attenuation of the blue light in facultative ponds. As the photodiode used was much more sensitive to red than blue light, a small amount of stray red light leaking past the monochromator in the presence of little or no blue could distort the results. The consequent exaggeration of the amount of blue light would increase with depth and lead to the calculation of erroneous attenuation coefficients. The difference in attenuation coefficients observed between ponds cannot have been caused by gilvin because its absorbance was relatively constant. The good correlations and significant regression equations found between chlorophyll a concentrations and attenuation coefficients suggest that pond to pond variation in light penetration can be almost entirely attributed to differences in algal biomass. The poor relationship between Kd(420)and chlorophyll a was probably due to the distortion introduced by red light in facultative ponds we have
discussed above. This same mechanism may have led to an underestimation of the value o f / ~ at 440 and 460 nm. It is clear from the calculated Kc values that the shorter wavelengths are more affected by algae than the longer wavelengths. The latter are available for photosynthesis, which increases pH, but are not very good at removing faecal coliforms (Curtis, 1990). Therefore changes in algal biomass might affect photooxidation (light) mediated killing more than pH mediated killing, though an increase in oxygen concentration (also required for photooxidation) might offset a decrease in light penetration. The relationship between light penetration and algal concentration at various wavelengths would play an important part in any model of sunlight and pathogen removal. It would therefore be interesting to repeat this work with a spectroradiometer that functioned in the u.v. In the absence of such an instrument the values of Kc between 320 and 400 nm could be crudely estimated from the data in Table 3. The regression coefficient Kc depends on the algal species, chlorophyll a concentration and water colour (Kirk, 1983). Nevertheless, the good agreement found between this work and that of Ganf (1974) suggests that the values of K¢ presented in this paper may be applicable in all WSPs with chlorophyll a concentrations <2000#g1-1. Maximum algal concentrations in maturation ponds never exceed this limit, but facultative ponds occasionally do (Silva, 1982; Troussellier et al., 1986). Light attenuation reported by Mills (1987) in facultative ponds were not as great
Light penetration in stabilization ponds as the high (3-5000 # g l - l ) chlorophyll a concentrations would lead one to expect. The regression equations may not therefore be extrapolated and more data are required. The constant in the linear equation (A) represents the absorbtion that can be attributed to non-algal material. The good fit obtained at most wavelengths implies that the non-algal absorbance is quite stable and represents the minimum Kd possible in a WSP. The constant is a reflection of the absorbance of the gilvin, this conclusion is born out by its spectral variation. The differences between the minimum K d values derived from the regression equations and the laboratory measurements of gilvin absorbance were probably caused by the turbidity and the absorbance of tripton and water. As the constant A is dependent on the absorbance of gilvin and tripton it will increase in the u.v. The notion that WSP have a stable and irreducible minimum attenuation coefficients with high absorbance in the u.v. questions the usefulness of James (1987) suggestion that the removal of FC from ponds may be accelerated by removing the algae to increase killing by the sun. Especially when one considers that the algae provide oxygen which is vital to the killing of faecal coliforms by light (Curtis et al., 1992a). The observation that light penetration, above a certain minimum, is controlled by algae may be employed to predict the algal concentration in a pond by using Kirk's (1983) adaption of Talling's (1957) model relating depth to the attenuation coefficient and the ratio of respiration rate and the light saturated photosynthetic rate. This will be an important first step in future pond design equations since the algal population affects pathogen removal (Troussellier et al., 1986), and the maximum possible organic load as well as the pH and dissolved oxygen, which in turn influence nutrient and pathogen removal (Mara and Pearson, 1986). Finally although this study has employed sophisticated and expensive equipment the good relationship between the attenuation coefficient for P A R and Secchi depth suggest that light attenuation may be studied by very modestly equipped laboratories. The relationship between the algal concentration and Secchi depth could be used by unskilled maintenance workers to monitor the stability of the algal population of a WSP system. Acknowledgements--We should like to thank Dr Adrianus Van Haandel and Sr Lenimar Andrade de Oliveira for help during the field work.
REFERENCES
Baker K. S. and Smith R. C. (1982) Biooptical classification and model of natural waters--lI. Limnol. Oeeanogr. 27, 500-509. Calkins J., Buckles J. D. and Moeller J. R. (1976) The role of solar ultraviolet radiation in natural water purification. Photochem. Photobiol. 24, 49 57. WR 28/5- D
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Curtis T. P. (1990) The mechanisms of removal of faecal coliforms from waste stabilization ponds. Ph.D. thesis, University of Leeds. Curtis T. P., Mara D. D. and Silva S. A. (1992a) Influence of pH, oxygen and humic substances on ability of sunlight to damage fecal coliforms in waste stabilization pond water. AppL envir. Microbiol. 58, 1335-1343. Curtis T. P., Mara D. D. and Silva S. A. (1992b) The effect of sunlight on faecal coliforms in ponds. Wat. Sci. Technol. 26, 1729-1738. Draper W. M. and Crosby D. G. (1983) The photochemical generation of hydrogen peroxide in natural waters. Arch. Envir. Contain. Toxicol. 12, 121-126. Ganf G. G. (1974) Incident solar irradiance and light penetration as factors controlling the chlorophyll a content of a shallow lake (Lake George, Uganda). J. Ecol. 62, 593-609. Haag W. R., Hoigne J., Gassman E. and Brunn A. (1986) Singlet oxygen in surface waters, part III: Photochemical formation and steady-state concentrations in various types of waters. Envir. Sci. Technol. 20, 341-348. James A. (1987) An alternative approach to the design of waste stabilization ponds. Wat. Sci. Technol. 19, 213-218. Kirk J. T. O. (1981a) A Monte Carlo study of the return of the underwater light field in, and the relationship between the optical properties of, turbid yellow waters. Aust. J. Mar. Freshwat. Res. 32, 517-532. Kirk J. T. O. (1981 b) Estimation of the scattering coefficient of natural water using underwater irradiance measurements. Aust. J. Mar. Freshwat. Res. 32, 533-539. Kirk J. T. O. (1983) Light and Photosynthesis in Aquatic Ecosystems. Cambridge Univ. Press, Cambridge, U.K. Mara D. D. and Pearson H. W. (1986) Artificial freshwater environments: waste stabilisation ponds, Chap. 4. In Biotechnology, (Edited by Schoenborn W.), Vol. 8 pp. 177-206. VCH Verlagsgellscahft, Weinheim. Mills S. W. (1987) Sewage treatment in waste stabilization ponds: physiological studies on the microalgal and faecal coliform populations. Ph.D. thesis, University of Liverpool. Moeller J. R. and Calkins J. (1980) Bactericidal agents in wastewater lagoons and lagoon design. J. Wat. Pollut. Control Fed. 52, 2442-2451. Paerl H. W., Bland P. T., Dean Bowles N. and Haibach M. E. (1985) Adaptation to high intensity, low wavelength light among surface blooms of the cyanobacterium Microcystis aeriginosa. Appl. envir. Microbiol. 49, 1046-1052. Pearson H. W. (1986) Estimation of chlorophyll a as a measure of algal biomass in waste stabilization ponds. Regional Seminar on Waste Stabilization Pond Research, CEPIS, Lima. Sarikaya H. Z., Saatchi A. M. and Abdulfuttah A. F. (1987) Effect of pond depth on bacterial die-off. J. em, ir. Engng 113, 1350-1362. Silva S. A. (1982) On the treatment of domestic sewage in waste stabilization ponds in N.E. Brazil. Ph.D. thesis, University of Dundee. Smith R. C. and Baker K. S. (1981) Optical properties of the clearest natural waters. Appl. Opt. 20, 177-184. Smith R. G., Ensminger R. L., Austin R. W., Bailey J. D. and Edwards G. D. (1979) Ultraviolet submersible spectroradiometer. Soc. Photoopt. lnstrum. Engng 208, 27 140. Sokal R. R. and Rohlf F. J. (1981) Biometry, 2nd edition. Freeman, New York. Tailing J. F. (1957) The phytoplankton population as a compound photosynthetic system. New Phytol. 56, 133-149. Troussellier M., Legendre P. and Baleux B. (1986) Modelling the evolution of bacterial densities in a eutrophic ecosystem (sewage lagoons). Microbial Ecol. 12, 355 379.
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T. P. CURTIS et al.
Webb R. B. and Brown M. S. (1979) Action spectra for oxygen dependent and independent inactivation of Escherichia coli WP2 from 254 to 460 nm. Photochem. Photobiol. 29, 407-409. Whitlam G. C. and Codd G. A. 0986) Damage
to
microorganisms
by light. In Microbes in (Edited by Herbert R. A. and Codd G. A.). Special Publications of the Society for General Microbiology 17, Academic Press, London. Extreme
Environments
Porg,.o.
L I G H T P E N E T R A T I O N IN WASTE STABILIZATION PONDS T. P. CURTISI*~, D. D. MARA I ~ , N. G. H. Dtxo 2 and S. A. SlLVA 3 ~ ~Department of Civil Engineering, University of Leeds, Leeds LS29JT, England, 'Conj. Jardim Amazonas, Rua F, Condominio Juaperi, BIoco B, Apto 203, Bairro Parque 10, 59055, Manaus-AM, Brazil and 3EXTRABES, Caixa Postal 306, 58100 Campina Grande-PB, Brazil (First received November 1991; accepted in revised form September 1993) Abstract--The penetration of light into waste stabilization ponds was studied because of its importance in pathogen removal and algal productivity. The attenuation of light in ponds was dominated by light absorbtion by gilvin (also called dissolved yellow matter or humic substances) and algae, light scattering processes (turbidity) being of no importance. Gilvin exerts a strong influence over the spectral variation and longer wavelengths penetrate much better than short wavelengths. Estimates of attenuation coefficients in the u.v. suggest that these wavelengths are less penetrating than previously reported. Differences in algal concentrations cause the differences in light attenuation seen between ponds, though they also cause some spectral variation because short wavelengths are more affected by changes in algal biomass than long ones. In the absence of algae there appears to be a lower limit to the clarity of ponds dictated by gilvin and other substances. Secehi disks were found to be reliable instruments for measuring light penetration. Key words--lagooning, solar radiation, light penetration, photosynthetic activity, algae, humic matters, Secchi disk
NOMENCLATURE
PAR = photosynthetically active radiation U.V. = ultraviolet E = irradiance (Wm -2 or/zeinsteins m -2 s -I) Ed= downward irradiance (Wm -2 or/~einsteins m-2 s-i ) Eo= upward irradiance (Wm -' or/teinsteins m -2 s -I ) 2 : depth (m) Kd= attenuation coefficient for downward irradiance (m -t ) R = irradiance reflectance a = absorbtion coefficient (m- i ) b = scattering coefficient (m i) bb = back scattering coefficient (m- l ) G440 the absorbance of gilvin at 440 nm (m- i) P440= the absorbance of tripton at 440 nm (m- i ) re= the constant relating light attenuation to chlorophyll a concentration (#g 1-1) A = the constant that represents the attenuation coefficient in the absence of algae (m -~) INTRODUCTION
Light penetration is of fundamental importance to the functioning of facultative and maturation ponds, affecting both pathogen survival (Curtis et al., 1992a, b; Troussellier et al., 1986) and the concentration and productivity of the algal population (Kirk, 1983). A study of the penetration of light in *All correspondence and reprint requests should be addressed to: Dr Tom Curtis, Department of Civil Engineering, Cassie Building, University of Newcastle, Newcastle upon Tyne NEI 7RU, England.
waste stabilization ponds (WSP) is an essential first step in the modelling of these processes. A proper understanding of the impact of light on pathogenic organisms and productivity requires a knowledge of how light penetration varies from wavelength to wavelength as well as from WSP to WSP. Previous workers who have measured the attenuation of light in WSP (Moeller and Calkins, 1980; Calkins eta/., 1976; Sarikaya et al., 1987) have not considered either spectral or inter-pond variation, neither have they sought to explain the mechanisms governing light penetration. This paper describes a study undertaken to elucidate the fundamental factors governing the penetration of all wavelengths of sunlight into WSP. Fortunately, due to the importance of light penetration in aquatic photosynthesis the subject has been widely studied. The optics of fresh and marine waters has been comprehensively reviewed by Kirk 0983)° The bulk of the work on, and instrumentation in, this field is devoted to PAR, which falls between 400 and 700 nm. Bactericidal sunlight can have a wavelength of anything from 290 to over 700 nm (Webb and Brown, 1979; Whitlam and Codd, 1986; Curtis et aL, 1992a) some wavelengths may be more bactericidal than others, and some may not be bactericidal at all. Ideally therefore, a knowledge of the aquatic optics of bactericidal light should encompass the penetration of a range of wavelengths between 290 and 700 nm. This requires a submersible u.v.-visible
1031
T.P. CURTISet al.
1032
spectroradiometer and only one of these has ever been built (Smith et al., 1979). The instrument has been used to develop predictive empirical models for seawater (Baker and Smith, 1982) but at algal concentrations (maximum chlorophyll a = 10/~g l - l ) far below those seen in WSP. Fortunately the same principles apply to the penetration of sunlight, whatever the wavelength. Consequently the ability of u.v. light to penetrate in a given body of water can generally be inferred from what is known about the penetration of visible light. Light is usually measured as irradiance (E) expressed as the number of photons, or the amount of energy, per unit area per unit of time. The former is measured as #einsteins m -2 s -j (1 einstein ~ 6 x 1023 photons) and the latter as W m -2. Downward (Ed) and upward (Eu) irradiance are measures of the light shining on horizontal surfaces facing up and down respectively. Ed is the type of irradiance most usually measured. The ratio of upward to downward irradiance (Eu/Ea) is known as the irradiance reflectance (R). E d diminishes with depth as follows:
Edf~)=
Ed~0)e-Kdz
(1)
where Edcz) and Ed~0) are the values of the downward irradiance at a depth z (m) and just under the surface, respectively; Kd ( m - ' ) is the attenuation coefficient for downward irradiance and it measures how easily light penetrates a body of water, which in turn depends on what is suspended and dissolved within it. Eu and R also diminish with depth and may be described by equations analogous to that above (Kirk, 1983). Only one of two things can happen to a photon of light in a WSP: it can be scattered or it can be absorbed. The probabilities of these two events happening are measured by the absorption and scattering coefficients, a (m -~) and b (m-~). These are inherent properties of a given body of water but for most purposes it is not necessary to know the value of b and a, only the ratio between the two. This ratio can be easily calculated from R (Kirk, 1981b), and the value of R at the surface (R0) with the sun at an angle of 90 ° is related to the amount of light scattered backwards measured by bb, the back scattering coefficient, and a by the following expression (Kirk, 1983): R0 = 0.33 bb/a (2) For turbid waters, such as WSP, bb/0.019 = b (Kirk, 1983) and equation (2) becomes: R 0 = 17.4 b/a (3) The reflectance, and hence the ratio of b to a, of algal rich waters is usually very low. The chances of a photon being absorbed or scattered before reaching a certain depth will also depend on the angle at which it is penetrating the water. The more acute the angle the less chance it has of reaching a significant depth. The angles over which the light field are distributed are summarized as the average
cosine, which may be thought of as the average value of the cosine of the angle (relative to the vertical) of all the photons in an infinitesimally small volume. At a given depth the average cosine depends on, and increases with, the b/a ratio. In water of low reflectivity with the sun at 90 °, the average cosine will not usually change much with depth. Taking the idea that a photon may be either scattered or absorbed, Kirk (1981a) has completed a Monte Carlo study of photons entering waters with different b/a ratios. From this study he derived the following expression relating the attenuation coefficient at the point where light has 10% of its surface value to the coefficients a and b: Kd = (a 2 + 0.256 ab )J/2
(4)
Thus in waters with b/a ratios of less than 1 the attenuation coefficient is principally dependent only on a and therefore (Kirk, 1983): Kd ~ a
(5)
The coefficients of absorbtion and scattering are themselves governed by the light absorbing and light scattering components of the water. The light absorbtion properties of natural waters are attributable to four components: the water, gilvin (dissolved yellow matter), algae and tripton (inanimate particulate matter). The total absorbtion at a given wavelength can be obtained by adding together the absorbtion of these four separate components. Although apparently colourless, water absorbs light moderately well at wavelengths <550nm. Gilvin absorbs strongly in the u.v. and is known to be present in sewage and WSP (Curtis, 1990; Haag et al., 1986; Draper and Crosby, 1983). It is quantified by the absorbance at 440 nm (G440) of a 0.2/~m filtrate of the water in question. Tripton has an absorbtion spectrum similar to that of gilvin, though it typically absorbs very much less light. Tripton is important in water with many particles derived from soil erosion and the resuspension of sediment. It is quantified by the absorbance of particulate matter at 440nm (P440). Unfortunately P~0 is difficult to measure because of the particulate nature of tripton which means that special equipment is required to distinguish between the substances scattering and absorbing properties. Algae, being photosynthetic, have large quantities of pigments, which also impede light penetration. In productive waters, such as WSP, light absorbtion by algae may be very important, often limiting the growth of the algae themselves. The absorbtion spectra of algal cells largely reflects the absorbtion of the photosynthetic pigments, with peaks at around 440 and 680 nm. Changes in the attenuation coefficient at a given wavelength associated with a change in algal biomass will reflect the absorbance of the algae at that wavelength. The relationship between the two is approximately linear, with the absorbance due to algae being related to the amount of chlorophyll a (an indicator of
1033
Light penetration in stabilization ponds algal biomass) by a c o n s t a n t Kc. Because o f the t u r b i d n a t u r e o f a n algal p o p u l a t i o n its a b s o r b a n c e can only be m e a s u r e d with a specially a d a p t e d spectrophotometer. Scattering is caused b o t h by the water itself a n d particles, such as bacteria, within the water. The latter p r e d o m i n a t e s in all n a t u r a l waters. Scattering increases the a t t e n u a t i o n coefficient of a body o f water by increasing the p a t h l e n g t h o f the p h o t o n s , a n d light is not " u s e d u p " in any way. Light is scattered in roughly the same way in all turbid waters: m o s t o f it is scattered forward, a n d the 2 % o f light t h a t is scattered in the direction from which it came is k n o w n as back-scattered light. The objective of this investigation was to characterize the f u n d a m e n t a l aspects o f W S P optics a n d to discover the n a t u r e a n d cause of spectral a n d interp o n d variations in light penetration. This report describes studies using submersible radiometers a n d spectroradiometers t h a t have discovered that absorbtion r a t h e r t h a n scattering d o m i n a t e s light attenuation a n d t h a t a b s o r b t i o n is principally a t t r i b u t a b l e to gilvin a n d algae. The former is responsible for m u c h o f the spectral variation in light a t t e n u a t i o n whilst the latter causes m o s t of the differences in light p e n e t r a t i o n seen between WSP. MATERIALS AND METHODS
Location and design of ponds The fieldwork was carried out at the Federal University of Paraiba's experimental wastewater treatment station (EXTRABES) Campina Grande, northeast Brazil (7 ° 13' 11" S, 35 ° 52' 31" W and 550 m above mean sea level). Three types of pond were used: deep maturation and facultative pilot-scale ponds and shallow model scale ponds. The maturation ponds (prefix M) were part of a series of 5 identical ponds comprising an anaerobic pond (A8) fed with raw sewage, followed by a secondary facultative pond (F 16) and three maturation ponds (MI2-M14); the maturation ponds were 10.0 m long, 3.35 m wide, 2.20 m deep and so had areas and volumes of 33.5 m 2 and 73.7 m 3 respectively. The first pond in the deep series was fed with raw domestic sewage (BOD ~ 20 mg 1-1 ) that was pumped from a header tank by a variable speed peristaltic pump (model HRSV, Watson Marlow, Falmouth) at a rate which gave a volumetric load in A8 of ~,40.0 g BODm -3 day t and each pond a mean hydraulic retention time of 5 days. The facultative ponds (prefix F) had a length, width and depth of 25.4, 7.1 and 2.3 m respectively and consequently had an area of 181.0 m 2 and a volume of 417.7 m 3. Each of the facultative ponds was fed by a peristaltic pump and had a retention time of approx. 15 days and an areal load of about 300 kg ha- l day - ~. The model-scale ponds (prefix N) comprised a series of four, 1 m ~ asbestos cement tanks, which were fed with effluent from an anaerobic digester by a peristaltic pump, had a depth of 0.7 m, a retention time of 5 days and an areal load on the first pond (N l) of approx. 170kgha-I day i.
calibrated Crump 550 voltmeter. All underwater light measurements were taken when the sun was at an angle of 85-90 ° and not hidden by cloud. The quantum sensor was lowered into the water by hand in a cradle also manufactured by Crump, the upward irradiance was measured by turning the cradle over. The spectroradiometer was placed in a specially made waterproof box that allowed the wavelength to be changed whilst the meter was submerged. The box was weighted and hung from a wire at each corner, the wires passing through two aluminium posts which were themselves placed over strong rails positioned over the pond. This elaborate arrangement was required to hold the instrument firmly, the pond water being so absorbent that even small movements could affect the reading obtained. The depth of the box was measured with a ruler and the window of the water-proof box was wiped with a tissue to remove any algae, readings were then taken quickly before the window became obscured again. Readings below 420 nm were not taken as the predominately red underwater light tended to distort the result at these wavelengths. The Secchi disk used was a white circle of metal ~ 300 mm in diameter; the disk was lowered into the water on marked cord and the depth that it became invisible noted.
Other measurements Chlorophyll a was estimated by the methanol extraction method of Pearson (1986), no correction was made for phaeophytin. The filtrates were prepared by passing pond water through a sterile 0.2/am filter on a sterile membrane filtration apparatus; the water was then decanted into a sterile 25 ml bottle. The absorbances of the filtrates were measured in a 40 mm quartz cuvette on a scanning spectrophotometer (DMS 80, Varian, Walton on Thames, U.K.) in Leeds, U.K. within 5 weeks of collection. The statistical methods used are described in Sokal and Rohlf (1981). RESULTS
General features o f light penetration Nine different p o n d s were studied, P A R and m o n o c h r o m a t i c light decreased exponentially a n d the rate o f a t t e n u a t i o n was sometimes lower near the surface. Different wavelengths were also a t t e n u a t e d at different rates, the longer wavelengths penetrating more t h a n the shorter ones. This caused a shift in the spectrum in the underwater light field, the light becoming redder as it penetrated the water. The variation o f light a t t e n u a t i o n with wavelength in three p o n d s is summarized in Fig. 1. A t any given wavelength there was considerable difference in K d between ponds: the higher values were found in those p o n d s with more algae. Nevertheless the Kd spectra o f p o n d s M I 3 and N3 were similar in shape, with lower a t t e n u a t i o n occurring at long wavelengths a n d increasing in p r o p o r t i o n to the wavelength. Pond F I9, which h a d the highest Kd values exhibited a slightly different pattern, the a t t e n u a t i o n coefficient peaking at 470 nm rather than showing a steady increase with decreasing wavelength.
Measurement of the underwater light field
The causes ~[" light attenuation and its t~ariation
Two types of meter were used to measure the underwater light: a Crump 552 underwater quantum sensor (Crump Scientific Instruments, Billericay, U.K.) that detected downward PAR (400-700 nm), and a Crump 556 spectroradiometer that could detect downward monochromatic light between 390 and 720 nm. Both instruments were read via a
The values for reflectance (R0), the ratio o f upwelling to downwelling P A R at the surface of the WSP, are shown in Table I. The ratio of scattering to a b s o r b t i o n (b/a) was calculated using e q u a t i o n (3). A l t h o u g h the values of b/a vary somewhat, they are
1034
T.P. CURTlSet al. 120
O)
100
16
•
-
14
"o c
~I
80
12
6o
10
411
8
20
- 6
~
::::I
Z /
E
ff
0
I 420
I 440
I 460
I 480
I 500
I 520
t 540
I° 580
I 620
I 660
I 700
4
Wavelength (nm) Fig. 1. The spectral variation in the vertical attenuation coefficient (Kd) in ponds MI3 ( ), FI9 (...) and N3 ( - - - ) . The chlorophyll a concentrations were 112, 1628 and 689 #g 1-~, respectively.
all very low and the ponds absorbed light far more than they scattered it. Therefore the attenuation of light is almost entirely dictated by absorbtion and equation (5) applies. All the pond water filtrates examined absorbed light in a m a n n e r similar to that shown in Fig. 2. The absorbances of the pond filtrates at 4 4 0 n m are compared in Table 2. There was relatively little difference between the absorbances of filtrates of water from different sources, which contrasts with the large variations in Kd values observed between ponds. Nevertheless, all the filtrates absorbed far more light at longer wavelengths than shorter ones (Fig. 2), just as the pond water column did (Fig. 1). Indeed the shape of the typical filtrate absorbance spectra was very similar to the attenuation coefficient spectra found in N3 and M12 (Fig. I), especially at wave-
Table 1. The reflectance (R 0) and the scattering absorbtion (h/a) ratio at the surface of a number of pilot scale WSP at EXTRABES
Pond
Date
Ro
h /a
NI N2 N3 N4 N4 FI8 FI9 FI9 F19 MI2 MI2 M 13 MI4 M 14
27/10/89 27/10/89 27/10/89 26/10/89 25/10/89 3/I 1/89 24/10/89 30/10/89 3/I 1/89 23/10/89 24/10/89 24/10/89 20/10/89 24/10/89
0.0089 0.0090 0.0014 0.0000 0.0095 0.0095 0.0135 0.0250 0.0028 0.0130 0.0225 0.0065 0.0200 0.0200
0.0270 0.0273 0.0042 -0.0288 0.0288 0.0409 0.0758 0.0085 0.0394 0.0681 0.0207 0.0606 0.0606
lengths < 520 nm. The absorbances in the filtrate of M I3 at these shorter wavelengths were found to be strongly correlated with Kd in N3 and M13 in the same range (r = 0.991 and 0.992 respectively); and a highly significant (P <0.001) regression was also found between the two. Using these regression equations the attenuation coefficients in the u.v. were estimated from the absorbances of M I 3 filtrates (Table 3). A significant regression was not found (P = 0.576) for the more eutrophic pond FI9. The algal concentration, unlike filtrate absorbance, varied considerably from pond to pond and a significant (P = 0.0058; r = 0.69) model II regression was found between chlorophyll a and Kd(eAalin the ponds 25 -
20 i-
"T E
15
C
=o 10 ,< .¢3
5
~
0 300
-,I 350
400
450
500
550
I
I
I
600
650
700
Wavelength (rim) Fig. 2. The absorbance of a 0.2pm filtrate of pond water from MI3 (. . . . ) and MI4 ( - - ) .
Light penetration in stabilization ponds Table 2. The absorbance at 440 n m o f 0 . 2 ~ m filtrates o f E X T R A B E S W S P waters and raw sewage and typical K d at 440 n m Source
Abs~0 (m I)
Ka~ 0 (m i )
1.80 2.45 1.90 2.53 1.85 2.55 2.55 2.53 1.80
NA ND ND 55.45 97.01 ND 18.38 13.42 6.02
R a w sewage FI6 F 17 FI8 FI9 A8 M12 M 13 M 14
N D = not done. N A = not applicable.
studied. Similar regressions for attenuation coefficients between 440 and 700 nm were all significant (P between 6.42 x 10-3 and 1.513 x 10-6) with high correlation coefficients (r =0.81--0.99). Only Kd(42o) was found not to regress significantly with chlorophyll a (P = 0.6046), though elimination of the facultative ponds from the calculation made the regression very significant (P = 3.71 x 10 -4, r = 0.99). The regression coefficient (Kc) relating algal concentration to the attenuation coefficient also varied with wavelength (Fig. 3). Light in the 440-500 nm band was much more sensitive to the presence of algae than light at longer wavelengths. The absorption spectrum of cultured algal cells (Chlorella pyrenoidosa) is similar to that of Kc (Fig. 3). The shape of the spectra of the constant (A) (Fig. 3) was similar to that of the pond filtrates (Fig. 2) though the former was larger than the latter and fluctuated more at the longer wavelengths. The values of Kc calculated for 580 and 620 nm (0.0173 and 0.0156) in the WSP are very similar to that calculated by Ganf (1974) (0.016) for 589nm in a shallow, eutrophic, equatorial lake. Unfortunately Ganf (1974) only published the regression coefficient for the most penetrating wavelength, so it is not possible to make comparisons at other wavelengths. The use o f the Secchi disc This cheap and simple device gave reproducible results, a good linear relationship was found between the reciprocal of KdPAR (m - i ) and the Secchi depth (m) (l/PAR =0.0222 +0.261 x Secchi depth, P > 0.001, r = 0.96, n = 13). A relationship was also
Table 3. Estimates of the attenuation coefficients in ponds N3 and M 13 based on linear regression between attenuation at a given wavelength (Ka) and typical absorbance o f a pond ( M I 3 ) filtrate at the same wavelength (Abs). K d ( M I 3 ) = 3.35 + 4,20 Abs; K~ (N3) = - I 1.2 + 4 1 . 2 Abs Wavelength (nm)
Abs (m ' )
K d (N3) (m ~)
Kd ( M I 3 ) (m ~)
400 380 360 340 320
3.69 4.97 6.97 9.88 14.56
140.73 192.52 275.92 395.65 588.75
18.83 24.22 31.25 44.83 64.51
Chlorophyll a concentrations o f N3 and M I 3 were 689 and 1121zgl i respectively.
1035
found between the algal concentration (chlorophyll a, # g l -~) and the reciprocal of the Secchi depth (m) [Chl a = (252 × l/Secchi) - 620, P > 0.001, r = 0.88, n = 13]. DISCUSSION
The penetration of light in WSP is essentially similar to that seen in any other water body with high levels of giivin and algae, for example Lake George in Uganda (Ganf, 1974). Since algae are found in all facultative and maturation ponds (Mara and Pearson, 1986) and gilvin is present at roughly similar levels in raw sewage and wastewater effluent in all parts of the world (Curtis, 1990; Haag et al., 1986; Draper and Crosby, 1983) we believe that our findings will be applicable to WSP worldwide. The observed exponential decrease in the light is common to all waters. The variations in light attenuation seen between depths, ponds and wavelengths are a consequence of the contents of the pond. The very low reflectivity observed over a very wide range of turbidities and algal densities suggests that low reflectivity is a feature common to all ponds. We therefore conclude that light penetration in WSP is dependent on the absorbative properties of the water and changes in turbidity will not influence the optical properties of pond water (Kirk, 1983). u.v. Light was not detectable by the quantum sensor used, but as light absorbtion increases below 400 nm, reflectivity is probably also very low and our conclusions about the unimportance of turbidity also apply in this waveband. The spectral variation in attenuation coefficients seems to be governed by the absorbance of the dissolved substances in the water. The relationship between the absorbance of gilvin at a given wavelength and the attenuation coefficient was striking. As gilvin is found in all domestic sewage the same pattern of spectral variation will be found in all WSP. In some industrial effluent other colours may predominate. The shift to the red seen in the ponds is also found in other water bodies with high gilvin concentrations, such as Lake George in Uganda (abs~0 = 3.4m ~) (Ganf, 1974). It appears that relatively little u.v. light can penetrate the water and the reports by Calkins and his co-workers (Moeller and Calkins, 1980; Calkins et aL, 1976) that u.v.-B (280-320 nm) has an attenuation coefficient of 16 m are probably incorrect. A filtrate of pond water has an attenuation coefficient of more than 16 m ~in the u.v.-B, so it seems impossible for the water column to have such a low attenuation coefficient at these wavelengths. The instrument used was probably detecting red light and in the manner discussed below. These findings cast doubt on thc validity of Calkins work on the mechanisrns by which WSP remove faecal coliforms. As well as influencing FC removal the strong attenuation of u.v. light may well protect the algae from photoinhibition (Paerl et al., 1985) and
1036
T.P. Cog.s et al. 0.07
.,~ C ~
16
.Q 14 ~
Chlorella
oldosa
12
0.05
}
lO 0.114
a
co 0.03 ._~ (o "o
.
l
-
~
6
0.02
~E" ~ 0.01 ~E o
""N. ,
,
2 ""~ ,
........
.....
,'-G'"'r
Wavelength
.'""" . . . . . . . . . .
e,
[] ,
o
(nm)
Fig. 3. The spectral variation of the constant A ( - - A - - ) that represents the minimum possible attenuation coefficient, and the regression coefficient K¢ ( - - O - - ) relating the algal biomass to light attenuation. The absorbance of whole algal cells (Chlorella pyrenoidosa) is included (...I-:1...) for comparison.
so increase algal productivity and therefore the pH and DO of the WSP. The spectral variation in the facultative ponds was slightly different as it peaked and declined between 480 and 420 nm. This decline is rather strange since light appears to penetrate better in facultative WSP than in less eutrophic ponds. This could reflect a difference between the algal populations of facultative and maturation ponds; if so, it is odd that this discrepancy was only apparent at short wavelengths. A more plausible explanation is that the spectroradiometer readings may have been distorted by the rapid attenuation of the blue light in facultative ponds. As the photodiode used was much more sensitive to red than blue light, a small amount of stray red light leaking past the monochromator in the presence of little or no blue could distort the results. The consequent exaggeration of the amount of blue light would increase with depth and lead to the calculation of erroneous attenuation coefficients. The difference in attenuation coefficients observed between ponds cannot have been caused by gilvin because its absorbance was relatively constant. The good correlations and significant regression equations found between chlorophyll a concentrations and attenuation coefficients suggest that pond to pond variation in light penetration can be almost entirely attributed to differences in algal biomass. The poor relationship between Kd(420)and chlorophyll a was probably due to the distortion introduced by red light in facultative ponds we have
discussed above. This same mechanism may have led to an underestimation of the value o f / ~ at 440 and 460 nm. It is clear from the calculated Kc values that the shorter wavelengths are more affected by algae than the longer wavelengths. The latter are available for photosynthesis, which increases pH, but are not very good at removing faecal coliforms (Curtis, 1990). Therefore changes in algal biomass might affect photooxidation (light) mediated killing more than pH mediated killing, though an increase in oxygen concentration (also required for photooxidation) might offset a decrease in light penetration. The relationship between light penetration and algal concentration at various wavelengths would play an important part in any model of sunlight and pathogen removal. It would therefore be interesting to repeat this work with a spectroradiometer that functioned in the u.v. In the absence of such an instrument the values of Kc between 320 and 400 nm could be crudely estimated from the data in Table 3. The regression coefficient Kc depends on the algal species, chlorophyll a concentration and water colour (Kirk, 1983). Nevertheless, the good agreement found between this work and that of Ganf (1974) suggests that the values of K¢ presented in this paper may be applicable in all WSPs with chlorophyll a concentrations <2000#g1-1. Maximum algal concentrations in maturation ponds never exceed this limit, but facultative ponds occasionally do (Silva, 1982; Troussellier et al., 1986). Light attenuation reported by Mills (1987) in facultative ponds were not as great
Light penetration in stabilization ponds as the high (3-5000 # g l - l ) chlorophyll a concentrations would lead one to expect. The regression equations may not therefore be extrapolated and more data are required. The constant in the linear equation (A) represents the absorbtion that can be attributed to non-algal material. The good fit obtained at most wavelengths implies that the non-algal absorbance is quite stable and represents the minimum Kd possible in a WSP. The constant is a reflection of the absorbance of the gilvin, this conclusion is born out by its spectral variation. The differences between the minimum K d values derived from the regression equations and the laboratory measurements of gilvin absorbance were probably caused by the turbidity and the absorbance of tripton and water. As the constant A is dependent on the absorbance of gilvin and tripton it will increase in the u.v. The notion that WSP have a stable and irreducible minimum attenuation coefficients with high absorbance in the u.v. questions the usefulness of James (1987) suggestion that the removal of FC from ponds may be accelerated by removing the algae to increase killing by the sun. Especially when one considers that the algae provide oxygen which is vital to the killing of faecal coliforms by light (Curtis et al., 1992a). The observation that light penetration, above a certain minimum, is controlled by algae may be employed to predict the algal concentration in a pond by using Kirk's (1983) adaption of Talling's (1957) model relating depth to the attenuation coefficient and the ratio of respiration rate and the light saturated photosynthetic rate. This will be an important first step in future pond design equations since the algal population affects pathogen removal (Troussellier et al., 1986), and the maximum possible organic load as well as the pH and dissolved oxygen, which in turn influence nutrient and pathogen removal (Mara and Pearson, 1986). Finally although this study has employed sophisticated and expensive equipment the good relationship between the attenuation coefficient for P A R and Secchi depth suggest that light attenuation may be studied by very modestly equipped laboratories. The relationship between the algal concentration and Secchi depth could be used by unskilled maintenance workers to monitor the stability of the algal population of a WSP system. Acknowledgements--We should like to thank Dr Adrianus Van Haandel and Sr Lenimar Andrade de Oliveira for help during the field work.
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