Modelling sunlight disinfection in a high rate pond

Ecological Engineering 22 (2004) 113–122

Modelling sunlight disinfection in a high rate pond Rupert J. Craggs a,∗ , Alec Zwart b , John W. Nagels a , Robert J. Davies-Colley a a

National Institute of Water and Atmospheric Research (NIWA), P.O. Box 11-115, Hamilton, New Zealand b Department of Statistics, The University of Waikato, Private Bag 3105, Hamilton, New Zealand Received 5 September 2003; received in revised form 5 March 2004; accepted 10 March 2004

Abstract Studies of disinfection in conventional waste stabilisation ponds (WSPs) suggest that sunlight is an important factor, sometimes interacting with elevated dissolved oxygen and pH. Shallow depth and mixing in ecologically engineered high rate ponds (HRPs) enable greater exposure of wastewater to sunlight than in conventional WSPs, and we hypothesised that the reported efficient disinfection in HRPs may reflect this. We conducted two experiments on disinfection in a pilot-scale HRP treating dairy farm wastewater. The HRP was operated in batch mode so that removal of Escherichia coli could be followed by sampling over time (2 days). In both experiments, E. coli removal was rapid during daylight hours and slow overnight. The data were well-fitted by a simple model with a dark die-off term evaluated using night data, and a sunlight exposure term evaluated using day-time data. Dissolved oxygen and pH appeared to have little influence on inactivation rate over the measured range of pH (8.0–9.2) and DO (0–22 g m−3 ). Overall, about 75% of the total E. coli inactivation in the HRP was attributable to sunlight action. © 2004 Elsevier B.V. All rights reserved. Keywords: E. coli; Disinfection; High rate pond model; Solar radiation; Wastewater

1. Introduction Previous studies on disinfection in wastewater stabilisation ponds (WSPs) have shown that sunlight is an important factor in the inactivation of faecal indicators (Mayo, 1989; Curtis et al., 1992; Davies-Colley et al., 1997, 2000), and that sunlight interacts with other factors, notably dissolved oxygen and pH. However, sunlight disinfection in conventional WSPs may be limited by restricted light penetration. The euphotic depth of New Zealand WSPs is typically only 0.35 m (Davies-Colley et al., 1995). Furthermore, these ponds ∗ Corresponding author. Tel.: +64-7859-1807; fax: +64-7856-0151. E-mail address: [email protected] (R.J. Craggs).

are typically thermally stratified during daylight hours, which confines disinfection to the pond surface waters, so restricting overall disinfection. Several studies of WSP disinfection have been conducted with the aim of relating removal of indicator bacteria (usually total or faecal coliforms) to environmental factors. Mayo (1995) reviewed these studies and asserted that models of WSP disinfection that incorporated sunlight were much more satisfactory than those that did not. Unfortunately, several of the other factors that have been implicated in disinfection are difficult to study, because of co-variation on diurnal and seasonal time-scales. In particular, temperature, pH and dissolved oxygen, all vary diurnally almost in phase with each other, owing to solar radiation cycles and algal metabolism in pond water (see Fig. 1).

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Temperature ( o C)

Experiment 1

Experiment 2

30 25 20 15 9.5 9.3

pH

9.0 8.8 8.5 8.3 8.0 25

DO (g m-3)

20 15 10 5

Solar Irradiance (W m -2 )

0 1400 1200 1000 800 600 400 200 0 0:00

12:00

0:00

12:00

Time (NZST)

0:00

12:00

0:00

12:00

0:00

Time (NZST)

Fig. 1. Physico-chemical data for the HRP and solar irradiance (temporary declines due to cloudiness) collected during the 2 days of Experiment 1 (20–21 January 2000) and Experiment 2 (22–23 February 2000).

For these reasons, experimental investigation of the effects of different factors on disinfection is likely to be more robust than statistical studies using monitoring data.

Application of the findings of Curtis et al. (1992) and Davies-Colley et al. (1999) to modelling and prediction of disinfection in conventional WSPs is difficult because of the complex mixing processes in these

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systems. Specifically, it is not easy to ascribe a particular sunlight exposure (insolation) to a particular parcel of pond water in the presence of diurnal stratification. Therefore, the relative importance of sunlight disinfection versus other (dark) mechanisms of inactivation (such as protozoan predation and sedimentation) remains uncertain. High rate ponds (HRPs) have been shown to provide better disinfection than conventional WSPs (Oswald, 1988; Fallowfield et al., 1996; Bahlaoui et al., 1997). HRPs are ecologically engineered ponds that promote the symbiotic association between aerobic bacteria and algae. They are much shallower than conventional ponds (0.2–0.6 m deep compared to 1.0–1.5 m deep) and incorporate baffles to form a channelled—raceway around which wastewater is gently circulated using a paddlewheel (Oswald, 1988). Algae assimilate nutrients and through photosynthesis release oxygen which drives aerobic treatment of the wastewater, breaking down organic matter to carbon dioxide and nutrients, which are in turn assimilated by the algae (Oswald, 1988). The gentle mixing alters the ecology of the pond to favour the growth of large, colonial microalgae such as Scenedesmus sp. and Micractinium sp. that are easily settled in subsequent algae settling ponds. The shallow depth, large surface area, and paddlewheel-mixing ensure high exposure of the whole HRP volume to solar radiation, which increases the efficiency of disinfection over that achieved by conventional WSPs. Moreover, the intense daytime photosynthetic activity of the HRP algae (Oswald, 1988) can result in supersaturation of dissolved oxygen (DO) and pH > 9.0, which is expected to enhance disinfection. Therefore, high rate ponds provide a simpler experimental system than conventional WSPs for investigating and modelling disinfection, because the continuous mixing prevents stratification of the pond water and ensures uniform physico-chemical conditions and uniform average sunlight exposure. The aim of this study was to develop an improved understanding of disinfection (measured as inactivation of the faecal indicator Escherichia coli) processes that occur in HRPs. Here, we report the results of two experiments conducted in a pilot-scale HRP treating dairy farm wastewater. The HRP was run in batch mode so that E. coli decline due to sunlight exposure and other factors could be followed with time.

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The specific aims of the experiments were to: 1. test the hypothesis that sunlight exposure is the main cause of disinfection in HRPs; 2. determine the influence of DO and pH on E. coli inactivation by sunlight in HRPs; 3. develop a simple model of disinfection in HRPs. We expect that the model presented here will be useful to ecological engineers designing HRPs for particular applications where local solar insolation data are available or can be measured.

2. Model development A simple general first order model for the disinfection process is dC = −k(t)C dt

(1)

where C(t) is the E. coli concentration at time t, and k(t) is the decay rate, which may depend upon various factors such as irradiance, DO, and pH. Initially we assumed a simple model involving (independent) dark and light-dependent disinfection rates only, so that k(t) = kd + ks G(t)

(2)

where kd is the background or “dark” disinfection rate (s−1 ), ks is the “light only” disinfection rate coefficient (m2 J−1 ), and G(t) is the total solar irradiance (J m−2 s−1 ) incident upon the pond surface. Substituting Eq. (2) into Eq. (1) and solving yields the model of Sarikaya and Saatci (1987), namely: ln(C) = ln(C0 ) − kd t − ks S(t) where S(t) =




θ=t

θ=0

G(θ) dθ

(3)

(4)

is the total insolation incident upon the pond surface and, C0 = C(t = 0) is the initial E. coli concentration and θ is the exposure time. The above formulation relates pond disinfection to the total solar irradiance (G) incident on the pond surface because, pragmatically, we hoped to be able to predict disinfection in HRPs from climate station records of G. However, only a minor portion of the

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total solar irradiance that penetrates the water is responsible for disinfection. Davies-Colley et al. (1997, 1999) found that solar UV wavelengths in the range from 290 to 400 nm and centred around 340 nm were mainly responsible for solar disinfection. Furthermore, a proportion (of order 10%) of this “biologically active UV radiation” is reflected at the pond surface, and the remainder is attenuated down the water column. We could improve the generality of the model by reducing the total solar insolation by a factor that accounts for the reflection and attenuation of the biologically-active UV component of sunlight in the HRP water. However, transmission of the biologically active part of the solar UV spectrum is difficult to measure, and is only weakly related to (and much smaller than) the penetration of visible light into HRP water, so we have chosen to neglect this refinement for now. As discussed below, we measured the spectral light attenuation and other optical characteristics of the wastewater in our experiments so that in the future it will be possible to make predictions for HRPs in different seasons, with different pond depth, and for wastewaters with different optical properties.

3. Methods 3.1. HRP experimental system Experiments on disinfection rate were conducted using a pilot-scale HRP, which was part of a trial Advanced Pond System on a dairy farm at Anchor Products Hautapu, New Zealand (37◦ 52 S, 175◦ 29 E). The HRP received dairy farm effluent following pre-treatment in an anaerobic pond. The experimental HRP was a single-loop raceway (2.2 m wide, 18.0 m long, 37.5 m2 area) with semi-circular endwalls. The HRP was lined with high-density polyethylene (HDPE) plastic (Superseal Dam Liner, AGPAC Plastics) and a dividing wall (of the same material) separated the two (1.1 m wide) channels of the raceway. The HRP depth was maintained at 0.2 m by the height of the outflow stand-pipe (40 mm PVC pipe) to give a 7.5 m3 pond volume. A free standing, 1 m wide, galvanised steel paddlewheel circulated the wastewater around the HRP raceway. The paddlewheel had eight blades (0.65 m deep) and

was driven through a drive belt from a three-phase 0.125 kW gear (300:1) motor (SD13LIS, Parvalux, UK) controlled (frequency controller, FRU120S, Mitsubishi, Japan) to give a pond water mean surface velocity of 0.15 m s−1 . 3.2. Disinfection experiments Two experiments were conducted to evaluate the influence of solar radiation, and pond DO and pH, on the inactivation of E. coli during the southern summer. The experiments were conducted with the HRP operated in batch-mode, with no inflow or outflow. On the evening before each experiment a third of the HRP volume was replaced with anaerobic pond water, to provide an initial high concentration of the indicator bacteria. From 07:00 NZST on the first day of the experiment, field measurements and water samples were taken from the surface water of the HRP over 2 days. Samples were taken frequently (as often as 30 min) around mid-day, and less often (every 90 min) in the morning and evening of each day when inactivation was expected to be lower. At each sampling time field measurements were made of the pond water temperature and pH (WP81, TPS Ltd., Australia) and DO (WP82Y, TPS Ltd., Australia). Water samples were collected manually in sterile 100 ml vials for E. coli analysis. An autosampler (Manning, USA) was used to collect samples at 4-h intervals during the night. E. coli was analysed (Colilert method, which compares well with MPN and other standard methods (Eckner, 1998)) immediately after the sample was collected or on the following morning for samples collected overnight by autosampler. 3.3. Solar radiation and optical measurements Solar radiation was monitored on-site with a LiCor LI-200SA pyranometer (Li-Cor Inc., Lincoln, Nebraska, USA) connected to a datalogger (LiCor LI-1000) recording 10 min averages. The ambient weather conditions at the time of the experiment were also noted. Both experiments were conducted on days that began with fog (which cleared by late morning) and had scattered cumulus clouds through most of the afternoons. On day 1 of Experiment 1 there was a period of dense cloud around noon (Fig. 1).

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where G is the depth averaged total solar irradiance (MJ m−2 s−1 ) and z is the depth of the pond (0.2 m). In order to estimate the spectral absorption and scattering through the solar UV range (290–400 nm), spectral scans were performed on several water samples taken over the duration of the experiments with a Pye-Unicam PU8800 spectrophotometer (Phillips, Cambridge, England) fitted with an end-window photomultiplier (Davies-Colley et al., 1993). The spectral scanning procedure was too onerous to be done on more than a few samples, so we used the absorption coefficient at 340 nm (middle of the 290–400 nm solar UV range) to indicate change in UV penetrability of the pond water throughout the experiments. Measurements of absorption at 340 nm (g340 ) were made by spectrophotometer on membrane filtrates (Sartorius 0.2 ␮m pore size filters) of the pond water samples taken for microbiological assay. Measurements were also made (on the same filtered samples) at 440 nm (g440 ), to detect any change in light absorption in the visible part of the spectrum. Both the 340 and 440 nm absorption indices were corrected for light scattering by filter-passing colloids (Davies-Colley and Vant, 1987) using absorbance measurements at 780 nm—at which wavelength absorption is negligible.

ture, DO and pH in the HRP water all correlated broadly with incident solar radiation. The lower DO and pH of the HRP during the second experiment reflects the lower concentration of algae (Chl a: 2.2 g m−3 ) present than in the first experiment (Chl a: 4.7 g m−3 ). The temperature of the pond water during the experiments (reaching a maximum of about 33 ◦ C) is not expected to have contributed to inactivation of E. coli for which the threshold for thermal effects (including thermal interaction with optical inactivation) is about 45 ◦ C (e.g., McGuigan et al., 1998). 4.2. Optical conditions Solar UV radiation (290–400 nm) was attenuated much more strongly by the pond water than photosynthetically active radiation (PAR: 400–700 nm, symbol K(␭) in Fig. 2). Attenuation in the UV range is mainly due to absorption by dissolved yellow substance (humic material, symbol g(␭) in Fig. 2), which accounts for much of the colour in dairy wastewater (Sukias et al., 2001). In the visible PAR range in contrast, absorption is dominated by the algae—as suggested by the distinct peak of chlorophyll a in the red region (676 nm). A gradual downward trend of attenuation coefficients occurred with time throughout both experi-

Optical coefficients (m-1)

To determine the attenuation of solar UV radiation in the HRP water, spectral absorption and scattering coefficients in the UV range were calculated for pond water samples taken at the beginning, middle, and end of each experiment. These coefficients were in turn used to calculate spectral irradiance attenuation in the UV range, K(␭) (units m−1 , ␭ denotes wavelength), with an equation based on Monte Carlo modelling of photon behaviour given by Kirk (1994). Finally, the average UV irradiance over the 20 cm depth of the pond water was calculated with the (Morowitz, 1950) equation: G G= (1 − e−K(λ)z ) (5) K(λ)z

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350 Index wavelength of 340 nm

300 250

Absorption spectrum of all pond water constituents (ac)

200

K(

150 100

)

Chlorophyll-a peak

50

g(

0 300

)

400

500

600

700

800

Wavelength (nm)

4. Results 4.1. Physico-chemical conditions The physico-chemical data collected during Experiments 1 and 2 are shown in Fig. 1. Tempera-

Fig. 2. Spectra of optical coefficients measured at the start of Experiment 1 (08:00 h on 21 January 2000). The absorption spectra are shown for all water constituents (- - -) with symbol ac, as well as for the filtrate or yellow substance (—) with symbol g(␭). The irradiance attenuation spectrum ( ) with symbol K(␭), calculated from both the absorption spectrum and scattering spectrum (not shown) is also given.

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ments. This suggests that photo-oxidative bleaching of light-absorbing pond constituents was occurring, amounting to a total 25% reduction in K(PAR), 24% reduction in g440 , and 10% reduction in g340 in Experiment 1. However, penetration of UV radiation as indexed by K(340) reduced only slightly through the experiments (4% in Experiment 1) being dependent on light scattering as well as absorption. The average K(340) values were very similar at 199 and 208 m−1 for Experiments 1 and 2, respectively. These values are appreciably higher than the attenuation coefficient for visible light (=14 m−1 at the start of Experiment 1). The corresponding irradiance averaged over the 20 cm depth of the HRP, as calculated using the Morowitz equation together with the value of K(340) from Fig. 2, was about 2.5% of the incident (surface) value in the UV range, compared with 34% in the visible (PAR) range.

The dark inactivation rates kd for each experiment were estimated from the E. coli concentrations obtained during the night, when light levels were negligible (G(t) ≈ 0). Although only three such data points were taken for each experiment and there was appreciable scatter, there was a drop in E. coli concentration overnight. We assumed that there was no recovery of E. coli numbers through the night due to self-repair, and that the dark rates were independent of other environmental factors such as temperature. The dark rates were then obtained as the slope in the linear regression fit of In(C) versus time, which is consistent with Eq. (3) when irradiance is negligible (and hence insolation is constant over time). The estimated dark rates are shown in Table 1.

4.3. E. coli inactivation

4.5. Light inactivation rate

Very high rates of E. coli inactivation (∼2 log reduction in 2 days) occurred in both experiments (Fig. 3), with inactivation mainly occurring during the daylight hours, and much slower rates of reduction at night. The initial E. coli concentration was appreciably higher in Experiment 1 than Experiment 2, although both concentrations are well within the range reported for dairy anaerobic ponds in New Zealand (authors’ unpublished data).

Using estimates for the dark rate coefficients kd , we estimated the light rate coefficients ks by linear regression fits of In(C) + kd t data versus insolation, S (i.e., the light rate coefficient was estimated from the plot of measured E. coli counts, corrected for dark die-off). For each experiment, fits were made separately to each day’s data. Although there are apparent small shoulders at the beginning of each day in Experiment 1, no attempt was made to account explicitly for this

4.4. Dark inactivation rate

E. coli MPN (100 mL) -1

1000000 100000

Experiment 1

10000 1000 100 Experiment 2 10 0:00

12:00

0:00

12:00

0:00

Time (NZST) Fig. 3. E. coli concentration data (points) MPN (100 mL)−1 ± 95% confidence intervals (as given by MPN tables) and theoretical fits (lines) plotted against time during the 2 days of Experiment 1 (20–21 January 2000) and Experiment 2 (22–23 February 2000). Shading indicates period of darkness.

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Table 1 Estimates of ks and kd for each day of Experiments 1 and 2 with 95% confidence intervals (CIs) for the ks

Experiment 1 Experiment 2 Overall average

Day 1 ks (m2 MJ−1 )

Night kd (h−1 )

Day 2 ks (m2 MJ−1 )

0.064 (0.047–0.080) 0.114 (0.086–0.142) ks = 0.083

0.020 0.023 kd = 0.02

0.099 (0.076–0.121) 0.056 (0.033–0.079)

CIs (in parentheses) were calculated from the standard error of ks obtained from the regression fit, neglecting the uncertainties in the estimated kd values.

Fig. 4. E. coli concentration data (points) and theoretical fits (lines) plotted against insolation (MJ m-2 ) for Experiment 1 (20–21 January 2000) and Experiment 2 (22–23 February 2000).

Fig. 5. Normalised E. coli concentration data (C/C0 ) (points) and model fits (lines) plotted against insolation (MJ m−2 ) adjusted for assumed surface reflection (10%) and attenuation at 340 nm wavelength for Experiments 1 and 2.

phenomenon (i.e., “shoulder” data was not excluded). The resulting estimates of ks for each day of each experiment are given in Table 1. The data and model fits are compared in Fig. 3 (plotted against time) and Fig. 4 (plotted against insolation from the beginning of the experiments). It is noteworthy that the overall (2-day) data fit in Experiment 2 (0.085 m2 MJ−1 ) is very similar to that for Experiment 1 (0.082 m2 MJ−1 ) despite an order of magnitude difference in initial E.

coli count. Fig. 5 shows the same data plotted against insolation corrected for attenuation of 340 nm UV over the HRP water column.

5. Discussion Our experiments have shown that sunlight exposure accounts for most of the disinfection in HRP systems

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as measured by E. coli removal. We can achieve a fairly good curve fit to the experimental survival curve data with a simple model based on incident total solar irradiation alone (assuming a constant dark rate), ignoring the effects of variation in the proportion of UV in solar radiation with change in solar altitude through the day (e.g., McKenzie et al., 1997) and diurnal variation in pond pH and DO. The light inactivation rate coefficient ks averaged 0.083 m2 MJ−1 , which is similar to the value of 0.068 m2 MJ−1 reported by Davies-Colley et al. (2003) for a HRP treating sewage, despite greater depth (0.3 m cf. 0.2 m) and contrasting optical character (with lower UV attenuation, K(340)∼70 m−1 cf. 200 m−1 ). 5.1. Dark inactivation The processes of dark inactivation are not well understood but plausibly include settling and protozoan grazing. The dark contribution to inactivation of E. coli in both of our experiments appears to be between 1/5 and 1/3 of the total. There appears to be a dearth of literature reports of dark inactivation rates for E. coli, however kd values for faecal coliforms (comprised mainly of E. coli in sewage treatment systems) are given by Auer and Niehaus (1993) who accounted for settling (0.030 h−1 ), and by Mayo (1995) who conducted experiments in bottles—which presumably did not include settling (0.006 and 0.005 h−1 ). Literature values of kd for total coliforms range from 0.005 to 0.019 h−1 (Sarikaya and Saatci, 1987; Qin et al., 1991). The kd values we obtained for E. coli (0.020 and 0.023 h−1 ) are broadly consistent with these literature values for faecal and total coliforms. 5.2. Effects of DO and pH The light inactivation rate coefficient ks was higher on day 2 than on day 1 of Experiment 1 (Fig. 4, Table 1). This difference might be attributable to the higher DO and pH (Fig. 1) on the second day. However, if disinfection rate increased with increase in DO or pH, we would have expected to see the highest disinfection rate during the early afternoon, somewhat lagging the peak of solar irradiance at solar noon, but preceding the peak of DO/pH in the afternoon. Such patterns are not apparent in either experiment. This is not surprising for Experiment 2 when both the DO and

pH of the pond remained low, but is unexpected for Experiment 1 when relatively high DO and pH values occurred—in the ranges reported by (Davies-Colley et al., 1999) to interact with sunlight. We conclude that pH and DO are “second order” factors, neither of which need be explicitly accounted for in simple modelling of disinfection in HRPs unless (as yet undefined) extreme values are encountered. 5.3. Sunlight spectral effects Previous work (Davies-Colley et al., 1997, 2000) suggests that the short wavelength components of sunlight in the solar UV (300–400 nm) especially those in the UVB range (300–320 nm) are the main cause of E. coli reduction in WSP water. Because UVB is highest as a proportion of solar irradiance when the sun is at its highest altitude near solar noon, we would also have expected to see an increase in the inactivation rate at this time. However, our data (Fig. 3) show no sign of enhanced inactivation rate near solar noon. A possible explanation is that the most intrinsically damaging UVB was so rapidly attenuated within the highly pigmented dairy wastewater (K∼350 m−1 ), that the overall inactivation was weighted towards the less damaging, but more abundant and more penetrating radiation at longer UV and visible wavelengths. Because the UV penetration into the HRP was similar in the two separate experiments the survival curves are immediately comparable on the same graph as a function of incident insolation (Fig. 4). The similarity of the optical character of the HRP water in the two experiments meant that adjusting the insolation data for attenuation within the water column had very little influence on the shape of the survival curves (Fig. 5 versus Fig. 4). However, Fig. 5 may be useful for extending the modelling to HRPs of different optical depth (zK, where z is HRP depth and K is the attenuation coefficient for the bactericidal component of sunlight—here assumed to be 340 nm UV). 5.4. Solids association The inactivation rate on day 2 of Experiment 2 was appreciably slower than on other days (Table 1). We note that the initial concentration of surviving E. coli was also low on this day. A possible explanation is that these E. coli are a residual population strongly

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associated with algal/bacterial floc on the pond bottom. Residual populations of particle-associated bacteria have been observed in other disinfection systems. For example, Qualls et al. (1983) attributed a long tail of persistent total coliforms in secondary wastewater subject to inactivation by a UV-lamp to association of the bacteria with particles, an interpretation confirmed by later work such as that of Parker and Darby (1995). The algal/bacterial solids on the bottom of the HRP are one component of the HRP system that is not well mixed. E. coli associated with this settled floc would be largely screened from damaging UV radiation by the overlying pond water, so accounting for their persistence. 5.5. Practical example and further work HRPs are (with the exception of the bottom solids) well mixed, unlike (diurnally stratifying) conventional WSP systems. Thus, HRPs provide an excellent system for experimentation and modelling of WSP disinfection. Moreover, the results of experiments on HRPs are expected to have immediate practical relevance to design of improved disinfection in pond systems. Since the HRP (neglecting the bottom solids) may be treated as a complete-mix reactor (CMR), we can use the steady-state equation for a decaying constituent in such a reactor: C 1 = C0 1 + k(V/Q)

(6)

where (C/C0 ) is the ratio of pond to inflow E. coli, k is the total E. coli removal rate (=kd + ks S) (Eq. (3)), V is the HRP volume and Q is the flow through the pond, and (V/Q) is the hydraulic residence time. Using the average values of ks (0.083 m2 MJ−1 ) and kd (0.02 h−1 = 0.48 d−1 ) derived from our experiments (Table 1), and average total daily insolations for a particular location (e.g., for the Ruakura, Hamilton, Meteorological Station, some 15 km away from our experimental HRP, Ssummer = 22.8 MJ m−2 , Swinter = 6.1 MJ m−2 ) we can calculate the average summer and winter values of k: Summer : Winter :

k = 0.48 + (0.083 × 22.8) = 1.89 d k = 0.48 + (0.083 × 6.1) = 0.99 d−1

−1

121

Substituting these values in the CMR equation for a HRP hydraulic residence time of 8 days, predicts:   C 1 Summer = = 0.06 (94% removal) C0 1 + 1.89(8)   C 1 Winter = 0.11 (89% removal). = C0 1 + 0.99(8) Our simple model is expected to be adequate for indicative design purposes. However further work is desirable to refine this model with regard to: (1) the influence of DO and pH; (2) the spectral region of sunlight causing inactivation and the attenuation of these bactericidal wavelengths in HRP water; (3) the temperature coefficient of dark die-off (we would expect kd to be lower in winter); and (4) the inactivation rates of pathogens relative to indicator micro-organisms. The model could also be improved by using Monte Carlo modelling for the measured distribution of insolation rather than seasonal average values.

6. Conclusions Experiments on indicator bacteria removal, in a pilot-scale HRP treating dairy farm wastewater, show sunlight exposure to be the single most important inactivating factor, although dark processes contribute between 1/5 and 1/3 of overall bacterial inactivation during summer. A simple model, with a dark inactivation term (fit to overnight data), and a solar radiation term (fit to the daytime data, corrected for dark die-off) accounted for most of the features of the survival curves. The model should be useful for estimating inactivation in HRPs in different climatic regions, particularly, if it can be extended to account explicitly for exposure of the HRP water column to biologically active components of sunlight.

Acknowledgements We thank James Sukias for field assistance, Chris Tanner and Graham McBride for reviewing this manuscript, and Anchor Products Hautapu for their support of the experimental facility. This research was

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supported by the Foundation for Research Science and Technology through Contract No. C01X0010.

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