Tracer experiments are of concern to wastewater treatment engineers and researchers because of the importance of determining hydraulic regimes and retention times in wastewater treatment units. In this work, a pilot-scale maturation waste stabilisation pond (WSP) was spiked with

Rhodamine WT, in order to determine how suspended organic matter would interfere with its performance as a tracer in a domestic wastewater treatment unit which had a high content of suspended algal biomass. A primary maturation pond was spiked in three separate runs with different levels of algae (high, medium and low), with a known amount of Rhodamine WT (20% v/v); the tracer was measured in the pond effluent in real time every 20 min for 3θ (the theoretical retention time, θ = 17 d). Algal biomass was monitored weekly from influent, column and effluent

water samples by chlorophyll-a determination. The results show that algal biomass has a strong influence on the behaviour of Rhodamine WT as a tracer and therefore the hydraulic characteristicsm calculated from tracer curves may be affected by tracer adsorption on suspended organic matter

Camargo Valero, M.A.

Mara, D.D.

English

White Rose Research Online

The influence of algal biomass on tracer experiments in maturation ponds

promoting access to White Rose research papers    White Rose Research Online   Universities of Leeds, Sheffield and York http://eprints.whiterose.ac.uk/     White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/8622/    Published paper Camargo Valero, M.A. and Mara, D.D. (2009) The influence of algal biomass on tracer experiments in maturation ponds. Desalination and Water Treatment, 4  eprints@whiterose.ac.uk  Desalination and Water Treatmentwww.deswater.com1944-3994 / 1944-3986 © 2009 Desalination Publications.  All rights reserved.4 (2009) 89–92* Corresponding author.Presented at the 2nd International Congress, SMALLWAT ’07, Wastewater Treatment in Small Communities,11–15 November 2007, Seville, SpainThe influence of algal biomass on tracer experiments in maturation pondsM. Alonso Camargo Valeroa*, Duncan MarabaSección de Ingeniería Ambiental, Universidad Nacional de Colombia, Bogotá, ColombiaTel. +57 (1) 3165000 ext 13334; Fax +57 (1) 3165555; email: macamargov@unal.edu.cobSchool of Civil Engineering, University of Leeds, Leeds LS2 9JT, UKTel. +44 (113) 3432276; Fax +44 (113) 3432265; email: d.d.mara@leeds.ac.ukReceived 15 April 2008; Accepted in revised form 21 June 2008A B S T R A C TTracer experiments are of concern to wastewater treatment engineers and researchers because ofthe importance of determining hydraulic regimes and retention times in wastewater treatmentunits. In this work, a pilot-scale maturation waste stabilisation pond (WSP) was spiked withRhodamine WT, in order to determine how suspended organic matter would interfere with itsperformance as a tracer in a domestic wastewater treatment unit which had a high content ofsuspended algal biomass. A primary maturation pond was spiked in three separate runs withdifferent levels of algae (high, medium and low), with a known amount of Rhodamine WT (20% v/v);the tracer was measured in the pond effluent in real time every 20 min for 3θ (the theoreticalretention time, θ = 17 d). Algal biomass was monitored weekly from influent, column and effluentwater samples by chlorophyll-a determination. The results show that algal biomass has a stronginfluence on the behaviour of Rhodamine WT as a tracer and therefore the hydraulic characteristicscalculated from tracer curves may be affected by tracer adsorption on suspended organic matter.Keywords: Algal biomass; Hydraulic characteristics; Maturation ponds; Tracer experiments1. IntroductionTracer studies have been used extensively by hydrolo-gists to determine the transport, mixing and diffusion ofharmful substances discharged to a water system or to awater body. These studies are carried out by tracking thefate of an appropriate tracer through time and space; trac-ers may include any current natural material or pollut-ant (e.g., chlorides), as well as materials or substancesintentionally injected, such as floats, salts, radioisotopesand fluorescent tracers. Fluorescent materials (naturaland synthetic) are able to emit radiation (light) immedi-ately upon irradiation from an external source, but emis-sion ceases as soon as the source of excitement is re-moved; fluorescent materials likely to be found in somestreams include algae, natural organic matter (e.g., hu-mic substances), certain minerals, paper and textile dyes,certain petroleum distillate products, and laundry-de-tergent brighteners [1]. The use of fluorescent manmadesubstances in hydrological tracing was reported byPritchard and Carpenter [2] and they are still extensivelyused due to their essential properties for water tracingsuch as being (a) water soluble, (b) highly detectable –strongly fluorescent, (c) fluorescent in a part of the spec-trum not common for materials generally found in wa-ter, thus reducing the problem of background fluores-cence, (d) harmless in low concentrations, (e) inexpen-sive, and (f) reasonably stable in a normal water envi-ronment.The most commonly used fluorescent tracers are Fluo-rescein (C20H10O5.2Na) and Rhodamine. The latter is morewidely used and it is available in a number of variants,including Rhodamine B (C28H31ClN2O3) and RhodamineWT (C29H29N2O5.Cl.2Na) characterized by the presenceof a xanthene nucleus (C13H10O). Rhodamine B is consid-erably cheaper than Rhodamine WT, but has the disad-90 M. A. Camargo Valero, D. Mara / Desalination and Water Treatment 4 (2009) 89–92vantage of a greater tendency to adsorb onto sedimentand other waterborne particles which may not behave inthe same hydraulic manner as the water under study.Rhodamine WT was specifically produced for water trac-ing, although it still has a slight tendency to be adsorbedand its fluorescence varies with temperature and con-ductivity. Its xanthene nucleus is strongly fluorescent inthe visible spectrum and wavelengths corresponding tomaximum excitation and emission intensity, and thismakes Rhodamine WT easily detectable with fluoromet-ric equipment.Tracer studies are also of concern to wastewater treat-ment engineers and researchers because of the impor-tance of flow analysis and hydraulic characterization inmunicipal and industrial wastewater facilities. The per-formance of a wastewater treatment unit depends mostlyon adherence to hydraulic design and a phenomenonsuch as short-circuiting deeply affects the facility’s over-all effectiveness and efficiency. Several tracer studies havebeen conducted in waste stabilisation pond (WSP) sys-tems for hydraulic characterization using indigo blue dye[3], lithium chloride [4], Rhodamine B [5], and Rhoda-mine WT [6–8]. However, there is little information onhow tracer experiments with Rhodamine WT performin wastewater treatment units which have a high con-tent of suspended biomass. In this work, a pilot-scaleprimary maturation WSP was spiked with RhodamineWT under three levels of suspended organic matter(mainly algae) content in order to identify the influenceof algal biomass on the results of tracer experiments.2. Methodology2.1. Pilot-scale WSP systemThis work was carried out at Esholt Wastewater Treat-ment Works in Bradford, West Yorkshire, UK, where theUniversity of Leeds has a pilot-scale WSP system.Screened sewage containing 50% domestic and 50% in-dustrial wastewater was fed in to a primary facultativepond (PFP) using a peristaltic pump (model 624S, WatsonMarlow Bredel Inc., Wilmington, USA); the PFP (9.9× 3.4×1.5 m) was loaded at 80 kg BOD/ha d (8 g BOD/m2d)with an average nominal retention time (θ) of 60 d. Theeffluent from the PFP was pumped out with two peri-staltic pumps in parallel (504S; Watson Marlow) at anaverage rate of 0.6 m3/d, in order to feed a primary matu-ration pond (M1) which discharged by gravity in to asecond maturation pond in series (M2). M1 (6.3×3.5×1.0 m) had an average θ of 17 d within the experimentaltimeframe reported herein.2.2. Tracer experimentsM1 was spiked in three separate runs with 50 ml of asolution containing a known amount of Rhodamine WT(20% w/v). The first run was undertaken on 19 July 2005with 4.0793 g of Rhodamine WT; the corresponding datesand mass of Rhodamine WT for the second and thirdruns were: 22 June 2006, 4.0873 g; and 20 December 2006,2.3001 g. Tracer concentrations in the M1 effluent weremeasured in-situ, every 20 minutes for 1θ before spikingand for 3θ afterwards, with a Rhodamine WT fluoromet-ric sensor (model YSI 6130, YSI Inc., Yellow Springs, USA)coupled to a multiparameter sonde (YSI 6820; YSI Inc.)with continuous data-logging system; dissolved oxygen(DO), temperature and pH were also recorded simulta-neously.2.3. Flow and water quality surveysThe M1 inlet flow was measured weekly following avolumetric method (readings from stopwatch and a mea-suring cylinder were taken); the effluent flow was calcu-lated from a water balance — net evaporation (rainfallminus evaporation) was estimated from weekly readingsusing a hook gauge evaporimeter (Casella CEL Ltd.,Bedford, England). Additionally, weekly samples werecollected from the M1 influent (Sample A), the pondwater column (Sample B) and the effluent (Sample C)and analyzed for chlorophyll a using the method ofPearson et al. [9].3. Results and discussionRhodamine WT concentrations in the M1 effluentwere normalised against the spike concentration (Co) byassuming complete mixing in the pond, to facilitate di-rect comparison of the tracer experiments undertaken.The Rhodamine WT results were also corrected for back-ground content based on results from readings recordedbefore tracer injection (negative values were taken as zeroas they included only Rhodamine WT) The normalisedtracer responses in the M1 effluent were plotted againstnormalised time (t/θ), as shown in Fig. 1. The curves fromthe three tracer experiments are not similar, even thoughthe inlet and outlet flow rates compared separately fromeach run were not significantly different (p <0.05). Re-sults from run 1 showed that the peak of the tracer tookabout 0.25θ to be reached, followed by an unsteady de-crease until a second broader peak appeared after 2.20θ.For the second run, the data exhibit a rapid rise to a firstpeak, followed by a rapid steady decrease with tracervalues very close to background values after only 2.20θ;in this particular case a second sharp peak occurred veryrapidly, reaching a C/Co value of almost 1.0. A third, butsmaller, peak appeared afterwards. In run 3 the C/Co”t/θcurve was very suggesting that the hydraulic regime inthe pond was close to complete mixing.Data from tracer experiments were also processedfollowing the method described by Levenspiel [10] fordispersion number (δ), actual retention time and Rhoda-mine recovery; the dead-space and short-circuiting indi-M. A. Camargo Valero, D. Mara / Desalination and Water Treatment 4 (2009) 89–92 91Fig. 1. Normalised tracer response curves for the RhodamineWT spikes in pond M1.0,00,20,40,60,81,00,0 0,5 1,0 1,5 2,0 2,5 3,0Normalised time, t/Normalised concentration, C/Co(a) Run 1(b) Run 2(c) Run 3ces were calculated by the method given by Kilani andOgunronbi [3]. The hydraulic characteristics of M1 fromeach run are summarized in Table 1.Hydraulic characteristics calculated from the tracerexperiments show that the retention time in run 1 washigher than the average nominal retention time, whichmay suggest that Rhodamine was temporary stored in-side the pond and released within the experimentaltimeframe as tracer recovery was 92%; this explanationis supported by the presence of a second broader peak atthe end of the concentration-time series. For run 2, thetracer recovery suggests that in this case approximately48% of the Rhodamine remained inside the M1 pond af-ter 3θ; experimental observations of sludge feedback atthe end of this run, in conjunction with the presence ofsecond and third peaks, would indicate that an impor-tant amount of tracer was stored in the pond sludge layer.Results from run 3 make a closer description of the hy-draulic regime in the pond and this run is selected as thebest of the three tracer experiments, mainly because thetracer behaviour in the pond effluent throughout the runwas very steady and tracer recovery was very high (95%);therefore, we can say that M1 has an intermediate flowpattern (δ = 0.648) with 14.3 d of hydraulic retention time.Algal biomass content may explain the differencebetween tracer experiment results. Fig. 2 shows chloro-phyll concentrations from the pond influent, water col-umn and effluent. The highest content of algal biomassas chlorophyll a occurred during run 1, followed by run 2;run 3 had the lowest content. Although Rhodamine WThas an only slight tendency to be adsorbed, this may beenough to affect the hydraulic characteristic results de-termined from the tracer experiments reported herein.The typical content of suspended organic matter in en-vironmental conditions during successful tracer experi-ments undertaken in water bodies cannot be comparedwith those expected in a WSP system when primary pro-ductivity has reached its maximum rate (e.g., summerconditions). Therefore, it is suggested that tracer experi-ments with Rhodamine WT in maturation ponds shouldbe carried out under conditions of low suspended algalbiomass in order to minimize tracer adsorption and thusavoid unrepresentative hydraulic characteristic results.Table 1Hydraulic characteristics of pond M1 from tracer experimentsRun Mean nominal retention time, d Retention time, d Dispersion number Rhodamine recovery, % Index of dead spaces Index of  short-circuiting Hydraulic regime Run 1 17 20.4 0.474 92 1.20 0.80 Intermediate Run 2 17 13.3 1108 52 0.78 0.93 Complete mixing Run 3 17 14.3 0.648 95 0.84 0.91 Intermediate 0,00,20,40,60,81,00,0 0,5 1,0 1,5 2,0 2,5 3,0Normalised time, t/Normalised concentration, C/Co0,00,20,40,60,81,00,0 0,5 1,0 1,5 2,0 2,5 3,0Normalised time, t/Normalised concentration, C/Co92 M. A. Camargo Valero, D. Mara / Desalination and Water Treatment 4 (2009) 89–924. ConclusionsThe results show that algal biomass has a strong in-fluence on the behaviour of Rhodamine WT as a tracerand therefore the hydraulic characteristics calculatedfrom tracer concentration-time series may be affected bythe adsorption of tracer onto suspended organic matter.Tracer experiments in maturation ponds should be un-dertaken when the pond algal biomass is low — i.e., attimes of the year when primary productivity rates arelow.AcknowledgmentsWe are extremely grateful to all our funders: the En-gineering and Physical Sciences Research Council (GR/S98382/01), the University of Leeds, the UniversidadNacional de Colombia, COLFUTURO, and especially toYorkshire Water who kindly provided the site and gaveus almost day-to-day operational support in one way oranother at Esholt.Fig. 2. Chlorophyll concentration box-plot for samples collected from M1 pond influent (Sample A), water column (Sample B)and effluent (Sample C) during trace experiments.Run 3Run 2Run 1Chlorophyll a, ug/L16001400120010008006004002000Sample ASample BSample CReferences[1] J.F. Wilson, Jr., E.D. Cobb and F.A. Kilpatrick, Fluorometric pro-cedures for dye tracing: US Geological Survey Techniques ofWater Resources Investigations, Book 3, Chapter A12. US Geo-logical Survey, Washington DC, 1986.[2] D.W. Pritchard and J.H. Carpenter, Measurement of turbulentdiffusion in estuarine and inshore waters. Intern. Assoc. Sci.Hydrology Bull., 20 (1960) 37–50.[3] J.S. Kilani and J.A. Ogunrombi, Effects of baffles on the perfor-mance of model waste stabilization ponds. Water Res., 18(8)(1984) 941–944.[4] O. Zimmo, Nitrogen transformations and removal mechanismsin algal and duckweed waste stabilisation ponds. PhD Thesis,International Institute for Infrastructural, Hydraulic Environ-mental Engineering, Wageningen University, Delft, The Neth-erlands, 2003.[5] J.J. Torres, A. Soler, J. Saenz, M.L. Leal and M.I. Aguilar, Studyof the internal hydrodynamics in three facultative ponds of twomunicipal WSPS in Spain. Water Res., 33(5) (1999) 1133–1140.[6] K.A. Mangelson and G.Z. Watters, Treatment efficiency of wastestabilization ponds. J. Sanit. Eng. Div., ASCE, 98(SA2) (1972)407–425.[7] A. Shilton, T. Wilks, J. Smyth and P. Bickers, Tracer studies on aNew Zealand waste stabilisation pond and analysis of treat-ment efficiency. Water Sci. Technol. 42(10–11) (2000) 323–348.[8] N. Bracho, B. Lloyd and G. Aldana, Optimisation of hydraulicperformance to maximise faecal coliform removal in matura-tion ponds. Water Res., 40 (2006) 1677–1685.[9] H.W. Pearson, D.D. Mara and C.R. Bartone, Guidelines for theminimum evaluation of the performance of full-scale waste sta-bilization pond systems. 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The influence of algal biomass on tracer experiments in maturation ponds

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