Phosphorus removal in macrophyte based treatmen

Maimuna Nalubega (7225259)

Robinah Nakawunde (7227566)

English

Loughborough University Institutional Repository

F   SANITATION AND WASTE: NALUBEGA and NAKAWUNDE23421st WEDC Conference Kampala, Uganda, 1995Phosphorus removal in macrophyte basedtreatmentMaimuna Nalubega and Robinah Nakawunde, UgandaSUSTAINABILITY OF WATER AND SANITATION SYSTEMSEUTROPHICATION OF SURFACE water bodies due to highnutrient loads is a global concern. In Uganda, prolifera-tion of the water hyacinth (Eichhornia crassipes), and blue-green algae along the shores of Lake Victoria may be anindicator. The inefficiency of existing sewage manage-ment systems would greatly affect the lake’s ecosystemsave for wetlands which are reported to have wastewater purification capacity (e.g, Taylor, 1991; Breen,1990;  Muthuri, et al, 1989; Chale, 1989;), through nutrientdepletion and pathogenic die-off. Capacities, perform-ances and responsible processes are however still indefi-nite and a subject of study.Results of a six months’ model-study on phosphorusuptake capacities by cyperus papyrus indicate that papy-rus vegetation can effectively remove as much as 10.6 mgP m-2 d-1. In floating papyrus, subjected to frequentflushing and not attached to sediments, there are indica-tions of a sustenance requirement of at least 2 mg P/L.Levels in excess of 30 mg P/L seem to ultimately affectproper plant growth.BackgroundOver 70% of Uganda’s industrial and urbanized townsare within catchment areas of its many surface waterbodies. These towns depend on the water for multiplesocio-economic and public health functions includingwater supply, fishing, navigation and waste water dis-posal. The last function however has increased stresseson water resources when increasing quantities of pollut-ants are discharged into them. Mugidde (1993), for ex-ample, reported changes in phytoplankton species andmacrophytes in L. Victoria, attributable to water quality,and in particular, nutrient changes.The inefficiency of most waste water managementsystems, and total absence in some cases, is likely togreatly affect this aquatic ecosystem save for the manyfringing wetlands existing between the communitiesand sewer outfalls. Examples in Uganda include:Nakivubo swamp, Kampala; Nabajjuzi swamp, Masaka;and Kirinya swamp in Jinja. These wetlands, with Cyperuspapyrus as the dominant vegetation, have the capacity toremove nutrients from waste water as it flows throughthem by either plant/microbial uptake,chemicalprecipitation, adsorption onto sediments, or loss intoatmosphere (denitrification). Moreover, compared toextended conventional systems that would effectivelyremove nutrients, these systems are cheaper, needminimum maintenance, can be operated by non-skilledpersonnel and require minimum energy.Whereas nitrogen can be lost to the atmosphere throughdenitrification, phosphorus is a more conservative nutri-ent whose removal can only be the other mechanismsoutlined above. The capacity of sediments to retain phos-phorus is however limited and wetlands can actuallyexport it (Gehrels and Mulamoottil, 1989). Microbialuptakes are also limited under anaerobic conditions inwetlands. It is therefore believed that phosphorus canonly be effectively removed by papyrus harvesting. How-ever, the rates and amounts of P that plants can effec-tively remove from waste-water, optimum concentra-tions and frequency for harvesting which will ensurewetland sustainability are not certain and are a subject ofrelated studies under IHE-Delft, in the Netherlands.The overall objective of this study was to assess thecapacity of papyrus wetlands to control phosphorusrelated eutrophication in order to better understand andpromote this technology.Specifically, the study aimed at:1. Experimentally determining the P removal capacityof Cyperus papyrus.Figure 1. Map of Nakivubo wetland and InnerMurchison Bay of Lake Victoria, showing fieldsampling sites (1 to 3)F   SANITATION AND WASTE: NALUBEGA and NAKAWUNDE2352. Assessing  the effect of increased nutrient loads onuptake and plant growth rates.3. Relating total P uptake by experimental plants toresults of Nakivubo swamp in Kampala and estimat-ing the swamp’s capacity to receive higher pollutionlevels.Methodologyi) Experimental set-upTwo clones (one rhizomatous and the other seedling) ofyoung floating Cyperus papyrus plants were planted ineach of 10 pots on one of the open terraces of the Facultyof Technology, Makerere University. The pots were oncea week fed with waste water, from the end of Nakivubochannel (see site 2, figure 1), in which varying nutrientlevels based on phosphorus concentrations and an N:Pratio of 10:1 were added. The P and N sources wereNaH2PO4.2H2O, KNO3 and (NH4)2SO4. P concentrationsused were 0.5, 2, 4, 8, 30 and 50 mg P/L. The remaining4 pots consisted of a blank (only raw waste water) andduplicates for the 0.5, 4 and 30 mg P/L nutrient levelsrespectively. All 10 pots were fed with a uniform surpluscarbon source (sucrose) so that this element, which isabundant in the natural swamp is not limiting. Theplants were left in the open to experience nearly similarenvironmental conditions as field plants about 6 Kmaway.Raw water was collected once a week, analyzed fortotal P and kept at   4oC for < 48 hours. A three weeks’stabilization period was allowed for the plants to re-establish themselves prior to measurements.ii) Sample collection and analysis:At the end of each week, water samples were taken fromeach pot for immediate total P determination. In addi-tion, electrical conductivity and pH were measured us-ing appropriate WTW probes. Feed water was thenreplenished to re-establish the different P levels.Water samples for similar measurements were alsotaken from the Bugolobi sewage treatment plant effluent,end of Nakivubo channel (the major feed into the wetland)at the inlet point into the study wetland and at the InnerMurchison bay - swamp interface (sites 1 to 3 on Figure1).Total phosphorus was determined by the ascorbic acidmethod on a Hach DREL2000 spectrophotometer, fol-lowing potassium persulphate digestion (APHA/AWWA, 1992).iii)Biomass measurements:Plant growth was monitored during the study period byfortnightly measurements of shoot dimensions (heightand girth at base) and total increase in weight of theplants over time.Results and discussioni) Initial conditionsInitial experimental conditions and some supplemen-tary results are summarized in Table 1.ii) Phosphorus absorption by papyrusIt was observed that P uptake rates were a function ofboth time and concentrations. In figure 2a, the averagedaily P uptake was higher during the first 5 weeks andsubsequently reduced.Both figures 2a and 2b also reveal that weekly totalphosphorus removal rates from the wastewater were,after the fourth week, almost similar for concentrationsTable 1. Initial and some 7th week resultsFigure 2b. Weekly total Phosphorus uptakeas a function of time.Figure 2a. Average daily total P uptake rate,as a function of initial load.F   SANITATION AND WASTE: NALUBEGA and NAKAWUNDE236iii)Biomass growth rateFigures 3a to 3d relate to the biomass growth rate as afunction of P concentration and time. 3a shows that thedaily biomass growth is much higher for the first 4 weeksand reduces to near constant levels thereafter. This com-pares quite well with uptake rate trends in figure 2a.Figures 3b, c and d show that there is a steady rise in plantbiomass as a function of P concentrations. Optimum andsustainable growth occurs at concentrations of 4 - 8 mgP/L. Table 2 further elucidates this. For 30 and 50 mg P/Lhowever, there appears to be a luxury uptake of P whichis not reflected in the biomass growth rate. In fact, someparameters like the girth and total height are seen to belower, possibly as a result of nutrient toxicity (Reddy andDeBusk, 1987).Following an extremely dry spell (week 8) in whichsome plants withered due to excessive evapotran-spiration,it was decided to set up an additional pot tomonitor P levels and residual volumes for 1 week. Theresults were used to optimise regular water additions tothe pots.Results after week 8 therefore represent a rejuvenationeffort by the plants. To this end, nitrogen levels wereboosted for all the pots while P levels remained the same.This is reflected in the results for all concentrations forweeks 9 to 11.iv) Field resultsResults for samples from sites 1,2 and 3 in the field aresummarized in Table 3. Turning to Figure 2a again, forFigure 3a. Average daily increase in plant weightas % of initial weight.Figure 3b. Increase in plant biomass with time.Figure 3c. Increase in total plant girthcircumference at base with timeFigure 3d. Increase in total plant height with timeTable 2. Potted papyrus phosphorus absorptionas %ge of initial loadbetween 2 and 8 mg P/L. At lower concentrations ( 1 mgP/L), phosphorus uptake actually ceased after the 5thweek, and instead, P was released into the water. Puptake for higher concentrations (30 and 50 mg P/L)were generally higher but also more erratic. The aboveresults for weeks 1 and 6 are elucidated in Table 2. Prelease for lower concentrations may be attributed toimminent death of plants (see Table 1, week 7) due tolimited nutrients compared with competing demandse.g, by algae and other microbes.The rapid die-off of plants for low P concentrationsdoes not completely tally with field conditions. In thefield, papyrus has been seen to grow even in areas whereP levels were as low as 0.2 mg/L (pers. obs.). This is dueto recycling of nutrients on death of biota (Chale, 1989),leading to localised and sustaining high concentrations.The phenomenon would probably have occured for thepots if roots were not virtually rinsed on a weekly basis,during nutrient replenishment.F   SANITATION AND WASTE: NALUBEGA and NAKAWUNDE237Table 3. Field results (see Fig.1. for sample sites)4. At P concentrations  30 mg/L, algae and other possi-ble competitors lead to reduced biomass growth. Inthe field, this could correspond to a much lowernutrient level.5. The direct relationship between P concentrations andbiomass growth rates necessitates regular harvestingof plants to ensure sustained P removal.AcknowledgementsThe authors are grateful for technical and financial sup-port from IHE, Delft - The Netherlands and NTH (NUFU)- Norway.ReferencesAPHA, (1985). Standard methods for examination of waterand waste water. American Public Health Association,Washington. D C.Breen, P.F. (1990). A mass balance method for assessingthe potential of artificial wetlands in waste water treat-ment.  Wat. Res.  24: 689-692.Chale, F.M.M. (1987). Plant biomass and nutrient levelsof a tropical  macrophyte (Cyperus papyrus  L.) receiv-ing domestic waste water. Hydrobiol. Bull. 21: 167 - 170.Gaudet, J.J. (1977). Uptake, accumulation and loss ofnutrients in tropical swamps. Ecology 50: 415 - 422.Gehrels, J. and Mulamoottil, G. (1989). The transforma-tion and export of phosphorus from wetlands.Hydrobiological processes  3: 365 - 370.Mugidde, R. (1993).  The increase in phytoplanktonprimary production and biomass in Lake Victoria,Uganda. Verh. Internat. Verein. Limnol. 25: 646 - 649.Muthuri. F. M., Jones, M.B. and Imbamba, S.K. (1989).Primary productivity of papyrus  (Cyperus papyrus) ina tropical swamp; lake Naivasha , Kenya. Biomass 18: 1-14.Taylor, A.R. D. (1991). Status of Nakivubo and Luziraswamps in Kampala. Report  prepared for the Ministryof Environment Protection & Natural Resources.Wetlands conservation and management programme,Kampala- Uganda.the ‘optimum’ nutrient levels of 2-8 mgP/L, it can beestimated that stable P uptake is at a rate of 0.5 mg/d. Forthe experimental buckets of area=0.047 m2, this corre-sponds to an absorption capacity of 10.6 mg P/m2.d.Considering the Nakivubo swamp which has an area ofabout 1.8 km2, the allowable daily load is 19.1 kgP/d. Foran average flow of about 80,000 m3/d, the optimumconcentration that should flow to the swamp for totalremoval should be under 0.24 mg P/L.From these results, if all other variables are ignored, Premoval between sites 2 and 3 is equivalent to 0.45 mg/L. If an optimum concentration for papyrus uptake of0.24 mg P/L is assumed, then  about 0.21 is adsorbed ontosediments and peat material. This corresponds to a highpercentage of total P removal and hence should not beignored. A further check on field sediment adsorption /release capacities is recommended.Conclusions1. Papyrus plants, even minus a sub-stratum, have thecapacity to remove at least 0.5 mg P/m2.d and henceprotect surface waters from eutrophication.2. A minimum P concentration in the water is requiredto sustain papyrus growth. In experimental condi-tions, this corresponded to 2.0 mg/L. Below this,papyrus tends to be out-competed.3. Low nutrient concentrations lead to papyrus die offand subsequent release of more nutrients. This phe-nomenon may be responsible for sustenance of plantsin areas with low water-phase P concentrations.

Phosphorus removal in macrophyte based treatment

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Phosphorus removal in macrophyte based treatment

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