Significance of biofilm activity in facultative pond design and performance

~

Pergamon

0273-1223(95)00499-8

Wal. Sci. T,ch. Vol. 31. No. 12, pp. 119-128. 1995. Copyright @ 1995 1AWQ Printed in Great BrilaUl. All rights reserved. 0273-1223/95 $9'50 + om

SIGNIFICANCE OF BIOFILM ACTIVITY IN FACULTATIVE POND DESIGN AND PERFORMANCE C. Polprasert and B. K. Agarwalla Environmental Engineering Program. Asain Institute a/Technology.

G.p.a. Box 2754. Bangkok 10501. Thailand

ABSTRACf Facultative ponds have found wide application in wastewater treatment as an economical systemwhere land area is available at reasonable cost Different approaches are available in the literature for the design of facultative ponds. Most research have dealt with only suspended biomass and considered major form of biomass responsible for substrate removal. However, the side walls and it as tile bottom of the facultative pond can provide support for the growth of attached (biofilm) biomass which also aids in the degradation of organic matters (substrate). This study demonstrates the significance of biofilm biomass growing on the side walls and bottom of these ponds to substrate utilization. A model for substrate utilization in facultative ponds is proposed which encompasses first-order reactions of both suspended and biofilm biomass. The biofilm activity is described with a diffusion type model, while the dispersed flow model is used for the pond hydraulics to include a wide range of pond dimensions and operating conditions. The proposed model, validated with observed data of two fullscale facultative ponds located in Bangkok, Thailand, and in New Mexico, U.S.A, was able to predict effluent BOD, concentrations of these iwo ponds reasonably well.

KEYWORDS Waste stabilization ponds, model, BOD removal, suspended biomass, biofilm, design.

LIST OF ABBREVIATIONS

a. at

C. C,

C

C.

c., d

D Dr

D. h J

specific surface area, m2/m3

v'l+4ktd

effluent substrate concentration, glm3 substrate concentration in biofilm, glm 3 influent substrate concentration, glm3 substrate concentration at the liquid sublayer and biofilm interface, glm] substrate concentration in bulk water, glm] dispersion number dispersion coefficient, m2/day diffusion coefficient in biofilm, m2/day diffusion coefficient in liquid sublayer, m2/day liquid depth in the pond, m substrate flux into the biofilm, gI(m2.day) 119

120

Je k

kr..

kla L 1.,

r....

4

MAD r

RMSE r.

t

T u

WSP

x y

z a

.6 Jj

t

C. POLPRASERT and B. K. AGARWALLA

substrate flux into liquid sUblayer, g/(mz.day) overall reaction rate = kll + a,a8/(a+8) first-order rate constant of biofilm biomass, dayl first-order rate constant of suspended biomass, dayl pond length, m biofilm thickness, m ~re~1 B005 loading rate, kg/(ha.day) liqUId sublayer thickness, m mean absolute deviation losses due to reactions, g/(m3.day) root mean square error substrate utilization rate by suspended biomass, g/(m3.day) mean hydraulic retention time, day absolute pond water temperature, oK uniform flow velocity, m/day waste stabilization ponds distance from inlet along the pond length, m substrate travel distance in the biofilm, m fractional distance along pond length 0...14 tanh( t )kr..LJt liquid viscosity, N.s/m z characteristic biofilm parameter

INTRODUcnON Up to the presen1, most literature on wastewater treatment in facultative ponds has dealt with only suspended microorganisms .\Od has consider them the major group of biomass responsible for organic matter decomposition ( Oswald and Gotaas, 1955; Gloyna, 1971; G1oyoa et 01., 1976; Mara, 1976; Middlebrooks et al., 1982). However, an early study by Reynolds et al. (1975) recognized the potential of biofllm biomass growing on the surface of baffles submerged in the pond water in organic stabilization; but probablyowing to scum layer formation, the baffled ponds were found to have lower organic removal efficiencies than the non-baffled ponds. Some previous research on self-purification in shallow streams and rivers has shown biofilm biomass growing in open-channel beds to be effective in biodegrading organic carbon compounds (Srinanthakumar and Amirtharajah, 1983; Boyle and Scott, 1984; Lau, 1990, Gantzer et 01., 1988 and 1991). Recent studies on attached-growth waste stabilization ponds (WSP) found the biofilm biomass growing on attached-growth media installed in the pond water to be beneficial in the removal of organic matter and nitrogenous compounds from the pond water (Shin and Polprasert, 1987 and 1988; Baskaran et 01., 1992). From their laboratory experiments, !GIani and Ogunrombi (1984) reported that WSP installed with baffles yielded better treatment performance than those without baffles; although they attributed the improved efficiencies to a reduction in dispersion number in the baffled ponds, the biofilm biomass, presumably growing on the baffle surfaces, could, together with the suspended biomass, contribute partly to the organic matter degradation. Because WSP normally have low flow velocities, but are subjected to higher organic loading rates than streams. and rivers, biofilm biomass can be expected to grow at the bottom and side walls of the ponds. Accordm¥ly, it might be reasonable to hypothesize that, in the pond environment, the biofilm biomass could assIst the suspended biomass in biodegrading the incoming substrate. !his study was und~~~ken with the overall objective of proposing a model for facultative ponds that mcorporates the actIVItIes of both suspended and biofilm biomass on substrate biodegradation. Specific objectives of this study are : • development of a mathematical model for substrate biodegradation in facultative ponds that encompasses the suspended and biofilm biomass activities

Biofilm activity in facultative JXlnd design

121

- determinations of some kinetic coefficients of biofilm growing in a facultative pond and relative contribution of the biofilm activity on substrate removal. The proposed model was validated with data from two full-scakfacultative pondslocated in New Mexico, USA and in Bangkok, Thailand. Batch tests were performed to estimate some kinetic coefficients of the facultative pond biofilm biomass.

MODEL DEVELOPMENT A conceptual illustration of the facultative pond model is shown in Figure 1. The model involves substrate mass balances in the bulk liquid flow and in the biofilm, with substrate transport through the liquid sublayer acting as a link between the two. For substrate removal by the biofilm, a model describing a diffusion type process in which the material is transported because of the existence of a concentration gradient is used. Once the substrate reaches the biofilm surface, its concentration is determined by a diffusion term and a reaction term. In the bulk liquid flow, the concentration is determined by an advection term, a dispersion term and a reaction term. Substrate transport in bulk . liquid flow by diffusion is considered negligible when compared with dispersion and advection. The biofilm growing on side walls and at the bottomofthepondisassumed to be ideal biofilm of constant thickness and density. Within the biofilm, diffusion and consumption of the substrate are assumed to occur simultaneously. Advective transport in the biofilm is considered negligible. In addition, the diffusion and kinetic coefficients are assumed to be constant throughout the pond length. A substrate mass balance in a control pond volume (bulk liquid and biofilm) can be written as: can be written as: Inflow - Outflow = Accumulation

+ Losses due to reactions

(1) Concentration profile

Advection

h

Removal due to suspended biomass

Dispersion

Cw

y

Fig. 1. Conceptual illustration of the biofilm model in waste stabilization pond. In mathematical terms, Equation 1 can be expressed as follows: dCw

- u dx

d 2 Cw + D__

dx 2

-

r

•

0

(2)

in which u is the uniform flow velocity (m/day), Cw is substrate concentration in bulk water (glm3); D is dispersion coefficient (m2jda)). x is distance from inlet along the pond length (m); r represents losses due to suspended and biofilm biomass reactions (day-I). Because substrate utilization is hypothesized to be due to both suspended and biofilm biomass, r' is expressed as:

C. POLPRASERT and B. K. AGARWALLA

122

(3)

in which, r. is substrate utilization rate by suspended biomass (gI(mJ.day»; J is flux of substrate into the biofilm (gI(mz.day)) and a. is specific surface area of the biofilm (mz/mJ). Substituting the value of r from Equation 5 into Equation 4, and replacing x by fractional distance (z), the following equation is obtained: dZe v d_ _

dz

2

dev + tl:

dz

•

+ ta J

•

(4)

in which d (dispersion number) is equal to D/(uL), t (mean hydraulic retention time) is equal to Uu (day); z (fractional distance along the pond length) is equal to xfL, and L is the pond length (m). Equation 4 is similar to the model previously proposed by Gantzer et al. (1988) in describing substrate removal in a streambed biofilm reactor. Substrate Consumption Rate by Suspended Biomass First-order kinetic has found wide application in descnbing substrate removal rate in WSP by suspended biomass (Marais, 1970; Thirumurthi, 1974; Mara, 1976), and is given by: (5)

r. = kraC.

in which, kr. is the first-order rate constant for suspended biomass (day-I). Substrate Consumption Rate in Biofilm Neglecting the accumulation of substrate on the biofilm surface, substrate mass balance within the biofilm (Figure 1) based on first-order kinetics can be written as (Lau, 1990): (6)

in which, D (" y and kr a are, respectively, substrate diffusion coefficient (mz/day), substrate travel distance (m), and first-order rate constant (day-I), in the biofilm. The solution of Equation 6 is given by Lau (1990) as: k L e J I'yeO • tanh(4!> cI> ra r •

(7)

in which C. is substrate concentration at the liquid sublayer.biofilm interface (glmJ), I, a characteristic biofilm parameter, is defined as J(kr.O)/Dr and Lr is biofIlm thickness (m). Substituting r. from Equation 5 and J from Equation 7 into Equation 4: (8)

Biofilm activity in

flLculL~tive

pond design

123

Assuming linear variation of substrate concentration across the liquid sublayer. the substrate flux (J.) is given by Fick's Law: (9)

in which D. is molecular diffusivity in the liquid sublayer (m 2/day), L. is liquid sublayer thickness (m). At steady-state conditions, Equations 7 and 9 can be combined to give: (10)

in which (11) tanh
It _ to'

Substituting

C. from Equation

~

fa

L

(12)

f

10 into Equation 8:

2

d Cw - dew d- + t (k f + a

dz2

dz

a

aA) C

~

au+p

w

(13)

1.. and Lr are assumed constant. the terms a and 8 will not vary along the pond length for a particular set of climatic conditions.

If Dw>

Based on the assumption that BOD, concentrations near the pond outlet do not change considerably or its gradient approaches zero at the pond outlet. the boundary conditions for Equation 13 are:

c..

de..tdz in which

=

Ct.

= 0,

at z ... 0 at z .. 1

Ct is influent substrate concentration (glmJ ).

For the above boundary conditions, Equation 13 can be integrated to give: 2 a 1 e 1 / 2d

(14)

in which C. is effluent substrate concentration (glmJ )

a1

-

";1

+

4ktd

(15)

C. POLPRASERT and B. K. AGARWALLA

124

k -

kf.+a.~ 0:+"

(16)

Equation 14 is the model for substrate biodegradation in facultative ponds that incorporates the reactions of both suspended and biofilm biomass. To solve this equation, the values of a.. d, and other model parameters need to be known; in this study, some of these model parameters were obtained from the literature, while others were determined from laboratory experiments. Details of the procedures employed to determine the model parameters were given by Agarwalla (1992). All sample analyses were undertaken according to the methods described in "Standard Methods" (APHA, AWWA, WPCF, 1989). Table 1 lists some of the model parameters used in validating Equation 14. TABLE 1 MODEL PARAMETERS

PIr.meler

Banlkok pond

New Mexico Pond

18.5l,1()<"

umolla

Peny and Cbilton

Uni!

Rcfc~DOe

Df

m'/day

3O.4x1()<·

0.'

m'lday

68.4xI()<·

41.7xl()<"

(2:O'C)

day'

1Sl.2

151.2

... (2:O'C)

day'

0.037

0.071

4'

m

1538d04

153&<104

L,

m

2OOc1()<

2OOc1()<

to.

(1976) (1973)

Thil Sludy Thinlmunbi

(1974)

Thil IIl1dy ANumed

'CorTec:ted for a>enle pond tempera'ure USlnllhe relationship of o.~rr or DrJ.'rr _ constanl (Peny and Qulton, 1973), 1ft whlCbli and T are liquid viaaloity and absolule pond temperalure, ....pectiYely. . 'The .. valuea of 0.037 and 0.071 were delermined from Th,rumunbi (1974) by usinlthe actual L. val.... of the Baaltok poncIand me New MexlCX) pond. 'A..rase of \be ranse of 1462 . 1615 ~m -Value at a..rale temperalure of 30.4 'C of the BanCtok pond "Value al annual a..nse tempera'ure of 12.5 'C of the New Mexico pond

MODEL VALIDATION The proposed facultative pond model (Equation 14) was validated by compa~ing the computed effluent BOD, concentrations (Ce ) with the observed effluent BOD, concentratIons of the two full-scale facultative ponds located in Bangkok, Thailand, and in New Mexico, USA. Details of the Bangkok WSP system were given by Polprasert et al. (1983). Comprising of two parallel sets of ponds in series, these WSP units treat most of the campus waste~ater and the treated effluent ~ discharged into a nearby irrigation canal. Due to a recent incr~ase I~ waste loa~s, the first pO~ds 10 series have become anaerobic, and causing the second ponds 10 senes.t0 functIOn as facultatIve ponds. The dimension of each facultative pond is 427m x 120.0m x l.3m : WIdth x leng~h x depth. The range of influent BOD, concentrations to the facultative ponds was 30 • 60 mgtl, WIth the average org~nic loading rate (L..) of 40 kg BODJ(ha.day). Detailed analysis of.t~e f~cultative pond efficiencies durmg the period of October-December, 1988 was conducted by Bhanudlpan (1988) and Mishra(1988). The New Mexico, USA. pond, also a small facultative pond, is located at the Inhalation Toxicology Research Institute, Kinland Air Force Base. It has a dimension of 84.2m x 78.1m x O.9m : width x length x depth, and is located at an elevation of 1718 m above mean sea level. The pond received a combination of municipal and animal wastewaters of relatively high strength. The weekly average range

125

Biofilrn activity in facultative pond design

of influent BOD$ concentrations to this pond was 270 • 800 mg/l, with the average I.." value of 103 kg BOD5I'(ha.day). The environmental and water quality of this pond was monitored in details, from Augustl,1972 to July 31. 1973 by Larsen (1974). The computed and observed effluent BOD, concentrations of the Bangkok pond and the New Mexico pond are shown in Figs. 2 and 3, respectively. During the data collection, the effluent BOD, concentrations of both ponds were measured on the same day as those of the influent BOD, concentrations. The mean theoretical hydraulic retention times of the Bangkok and New Mexico ponds were 13 and 48 days. To account for the hydraulic retention times, dispersion and short-circuiting effects, the effluent BOD, concentrations (both observed and predicted values) were plotted 10 days and 28 days later than the influent BOD, data collected on the same day for the cases of the Bangkok pond and the New Mexico pond, respectively. In Figure 3 the vertical lines represent the weekly maximum and minimum levels of the BOD, concentrations recorded. Week I in Figure 3 started on August 1, 1972.

70 .....- - - - - - - - - - - - - - , - - - - - - - - - - - , ~

o

60

Influent Effluent, observed

- - Effluent, predicted

50

....co e

OICI

o

30

a1

Oct30

Nov 4

Nov 9

Nov 14

Novl9

Nov24

Nov29

Day

Fig. 2 Computed and observed effluent BOD$ concentrations of the Bangkok pond.

Oec4

Dec 9

C. POLPRASERT and B. K. AGARWALLA

126

1 0 0 0 . - - - - - - - - - - - - - . - - - - - - - - -..... 6 Influent -

Effluent, predicted C Effluent, observed weekly overage Effluent, observ.d mallmum- minimum

.....

l:It

E

10

o

o

CD

O~..;;...

o

_ _..J.-

10

......

20

....L

Week

__JL__ __ _ '

30

40

Fig. 3. Computed and observed effluent BODs concentrations ofthe New Mexico pond. For the Bangkok pond, the root mean-square error (RMSE) and mean absolute deviation (MAD) of the computed and observed data are 6.S mgl1 and 5.3 mgl1, respectively. These values are close to the standard deviation of 4.24 mgl1 of the observed effluent BODs concentrations, indicating satisfactory predictions of the effluent substrate concentrations by Equation 14. This good correlation was to be expected because, as indicated in the preceding section (Table 1), some of the model par~meters were determined from the experiments conducted under the same climatic conditions and usmg the same type of wastewater as for the Bangkok pond. The results shown in Figure 2 suggest the applicability of Equation 14 in describing BODs removal in a facultative pond by the combined reactions of the suspended and biofilm biomass. The RMSE and MAD values between the predicted and observed effluent BODs concentrations of the New Mexico pond were 59.0 and 48.1 mgll, respectively. It should be noted th~t the standard deviations of the observed influent and effluent BODs concentrations of the New MeXICO pond were 138.4 and 38.4 mgl1, respectively. The normalized values of RMSE (or RMSE/C.) of the Bangkok pond and New Mexico pond were 0,48 and 0.40, respectively, indicating that Equation 14 yielded approximately the same statistical results in predicting the performance of these 2 ponds. However, ~e relatively high RMSE and MAD values of the New Mexico pond could be due to the fact that, In the absence of experimental data, the model parameters kIa, Lr and l. were assumed to be the same for both ponds, whereas the climatic and influent wastewater characteristics of both ponds were considerably different (se~ footnote of Table 1 for temperatures and descriptions of the 2 ponds in the earlier paragraph) whIch could possibly result in both ponds having different values of the above model parameters. CONTRIBUTION OF BIOFILM BIOMASS TO SUBSTRATE REMOVAL The relative importance of biofilm biomass to that of suspended biomass in organic removal in a pond can be determined using an equation of Gantzer et ol (1988):

Biofihn activity in facuJlative pond design

127

% of total removal rate due to biofilm biomass 100

(17)

Equation 17 indicates that the higher the a. value, the greater the percent contribution of the biofilm biomass in organic removal in the pond water. From the data shown in Table 1, the present tContributionsof biofilm biomass in BOD, removal in both Bangkok and New Mexico ponds were found to be 49 and 46, respectively (Table 2). These data clearly show the significant contribution of the biofilm biomass in biodegrading the influent BOD, compounds, although it was slightly less than that of the suspended biomass. TABLE 2 SUBSTRATE REMOVAL EFFICIENCIES BY SUSPENDED AND BIOFILM BIOMASS Bangkok pond

New Mexico pond

Percent contribution of biofilm"

49

46

Percent removal by combined effect of suspended and biofilm biomassb

65

83

Observed overall BOD, removal efficiency

66

80

"Determined from Equation 17 bDetermined from Equation 14 The predicted and observed overall BOD, removal efficiencies (based on average data) ofthe Bangkok and New Mexico ponds agree reasonably well. The data presented in Table 2 suggest the important role of the biofilm biomass on substrate utilization and the applicability of Equation 16 in predicting BOD, removal efficiencies in facultative ponds. SUMMARY AND CONCLUSIONS

This study demonstrates the significance of biofilm biomass growing on the side walls and bottom of facultative waste stabilization ponds, in addition to that of suspended biomass, on substrate utilization. A facultative pond model is proposed which considers· besides dispersion number, hydraulic retention time and organic loading rate - first-order reaction kinetics of the suspended and biofilm biomass as important parameters for BOD, removal. The proposed model, when validated with observed data of two full-scale facultative ponds located in Bangkok, Thailand, and in New Mexico, USA. was able to predict effluent BOD, concentrations of these 2 ponds reasonably well. The biofilm biomass was found to contribute about 46 - 49% of BODs removal, indicating the significance of these biofilm bacteria in organic matter degradation in facultative ponds. ACKNOWLEDGEMENTS

This research was supported by the Canadian International Development Agency and the AEON

C. POLPRASERT and B. K. AGARWALLA

128

Group Environment Foundation, Japan. The authors wish to thank Professor E. Joe Middlebrooks for his help on attached-growth pond research and Professor Bruce Rittmann for suggestions on the biofilm models.

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