Two strategies for advanced nitrogen elimination in vertical flow constructed wetlands

e

Pergamon

Wal. Sci. T6Ch. VOl. 3'. No.'. pp. 71-77.1997.

C 1997 IA wQ. Published by E10evier Science LId

PH: S0273-1223(97)OOO54-1

Printed In Great Britain. 0273-1223197 $17'00+ ()ooo

TWO STRATEGIES FOR ADVANCED NITROGEN ELIMINATION IN VERTICAL FLOW CONSTRUCTED WETLANDS Johannes Laber, Reinhard Perfler and Raimund Haberl Institute/or Water Provision, Muthgasse 18.1190 Vienna, Austria

ABSTRACf Since 1991 the institute for water provision has run two vertical flow constructed wetlands for domestic wastewater treatment at two farm houses (8 p.e.) in Upper Austria. The systems are designed for elimination of organic compounds and for nitrification which was no problem even under winter conditions. In 1995 two methods were tested to achieve denitrification also in both systems. System A is a one-stage system. system B has two stages operated in series. The approach to the one-stage system consisted in pumping a part of the nitrified effluent from the soil filter back to the settling tank of the influent. where the raw wastewater mixes with the nitrified effluent-water. The approach to the two-stage system B consisted in adding an external carbon source (methanol) mto the second. water saturated stage. The research was undertaken during several experimental stages to test the influence of the recirculation ratio (system A) respectively of the feeding (system B; intermittent, continuous. batch). The highest elimination rates could be achieved with system B (dosage of methanol) during experimental stage 1 (intermittent loading four times a day). The mean elimination rates were 82% for N inorg and 78% for IN. The TN elimination performance of system A was only a little lower (72%). The elimination of COD. BODs and TOC was no problem throughout the whole experimental period (effluent concentrations of both systems were well below the Austrian standards). @ 1997 IAWQ. Published by Elsevier Science Ltd

KEYWORDS Constructed wetlands; denitrification; methanol dosage; nitrification; nitrogen; recirculation; single households; subsurface flow; vertical flow. INTRODUcnON

In order to improve water pollution control. removal of nutrients is of great importance. This fact has obviously influenced the water legislation in this field in many European countries (Haberl et al., 1995). Horizontal flow systems are an appropriate technology for high stability elimination of COD and BODS but nutrients removal only amounts to average values of 30 to 50 (60)% (e.g. Schierup et al., 1990; BOmer, 1992). To achieve higher elimination rates, vertical flow systems have been developed. Originally developed by Seidel (1978) such systems have been in operation for several years in Europe (Boutin, 1987; Burka and Lawrence. 1990; LMnard et al., 1990; Brix. 1992). But again removal of nutrients often turned out to be insufficient in all these designs of constructed wetlands. Therefore vertical flow systems (one or two stages) with intermittent loading have been developed. 71

J. LABER eral.

72

In 1991 the Institute for Water Provision - Universitat fUr Bodenkultur, Vienna/Austria, built up two constructed wetlands (system A, system B) for domestic wastewater treatment at two farm houses (8 p.e.) in Upper Austria. The systems are designed for elemination of organic compounds and for nitrification. The fulfillment of the Austrian effluent standards (for treatment plants < 500 p.e.) for COD (90 mgll), TOC (30 mgll), BODs (25 mgll) and NH4-N (10 mgll) was no problem for four years (winter and summer periods). In 1995 two methods were tested to improve the denitrification rate in order to achieve high Total Nitrogen (TN) elimination. METHODS System A System A is a one-stage subsurface flow constructed wetland (8 p.e., 5m2 surface/p.e., vertical flow, main layer: depth 8Ocm, mixture of sand and gravel with grain size 0-8 mm), planted with Phragmites australis and with a settling tank (3m 3) for mechanical pretreatment. The soil filter is operated without water saturation within the soil filter (low water level) and is fed four times a day. The approach to this one-stage system consisted in pumping a part of the nitrified effluent from the soil fllter back to the settling tank at the influent. In the settling tank the raw wastewater mixes with the nitrified effluent-water. The raw wastewater contains the necessary carbon compounds for the denitrifying bacteria and the necessary anoxic conditions exist in the settling tank. The denitrification rate can be controlled by varying the recirculation rate. The increase in hydraulic load caused by recirculation may cause a decrease of nitrification perfonnance at high recirculation rates. The experimental period was split into two stages. During the first stage the recirculation rate was kept at 50 to 60% of the effluent quantity. During the second stage the water level within the fllter was raised from 0 to 20 cm. By creating this buffering zone within the intermittently loaded filter the quantity of recirculated effluent could be raised to 80%. In addition anoxic conditions in this saturated zone should promote denitrification. Automatic samplers were used to collect 24h composite samples. SystemB System B is a two-stage subsurface flow constructed wetland (8 p.e., 5 m 2 surface/p.e., both stages vertical flow, main layer: 60 cm depth, mixture of sand and gravel with grain size 1-4 mm) planted with common reed and with a settling tank (3 m 3) for mechanical pretreatment. The first stage is operated without water saturation within the filter (low water level) and fed four times a day. The second stage, which is connected in series, is operated with a completely saturated soil body (high water level). Through this water saturation of the soil filter anoxic conditions for denitrification are created. This two-stage constructed wetland showed high elimination rates for COD, BODs, TOC and NH 4-N even in winter (well beyond the Austrian effluent limits - see introduction). However, a further elimination of total nitrogen could not be achieved. The organic compounds were almost completely eliminated in the first stage. Therefore there was not enough carbon available for the denitrifying bacteria. The litter layer on the surface of the filter cannot be assumed as sufficient "internal" carbon source and the release of carbon by the root zone system amounts to 0.7-1.3 glm2/d (Stengel, 1985). The approach to this system consisted in adding an external carbon source (methanol) into the second stage. Experience gathered by this research could lead to the dimensioning of a bypass of raw wastwater directly into the second stage. The disadvantage of a bypass with mechanical treated wastewater is the additional loading of ammonia which cannot be eliminated in the water saturated second stage. In this case the hydraulic dimensioning (flow rate) of the bypass is decisive for the treatment efficiency. Methanol was added to the influent of the second stage by a membrane dosing pump four times a day corresponding to the intermittent loading of the first stage. The feeding rate for methanol of 60 ml/d was calculated with respect to the optimal CIN-ratio for denitrification. Again 24h composite samples were collected by automatic devices.

Advanced nitrogen elimination

73

RESULTS AND DISCUSSION System A - Recirculation Experimental stage 1. 50% of the nitrified effluent were recycled to the settling tank. Figure 1 shows the development of the effluent concentration of N0 3-N and NH4-N throughout the whole experimental period (June-August).

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The drop of nitrate concentration is clearly visible whereas the ammonia concentration remains low. The higher figures from June, 19th and 20th are due to a defect of the intermittent feeding system. The intermittent loading is particularly important for nitrification: on the one hand it is necessary for even distribution on the filter surface and on the other hand oxygen is sucked into the ruter by the flush loading.

74

J. LABER et al.

After the defect had been repaired, the figures decreased again (Fig. I). In the first week of July the recirculation pump was turned off, to see how quickly the sytem reacts to different operational conditions. During this first experimental stage (till 25.7.1995) an average N0 3-N effluent concentration of 40.6 mgll could be achieved. Compared to the average concentration before starting the recirculation (January to July 1995: 59 mgll) the concentration during the recirculation was reduced by 32%. The average NH 4-N effluent concentration during the first experimental stage (0.63 mgll) was almost as low as before (0.40 mgll). The mean NOrN concentrations amounted to 0.20 mg/l. The elimination rate of system A for inorganic nitrogen (Ninorg) could be raised from 17% to 42%. The effluent concentration of N inorg is given in Fig. 2.

Experimental stage 2. In this stage the filter has been operated with a saturated zone of 20 cm of the main layer (80 cm). As a result the mean effluent concentration of N03-N could be lowered to 20.6 mgll. Compared to stage 1 of the experiment, the concentration was reduced by 50% (compared to the period before the start of the recirculation: 65%). After the water level had been raised by 20 cm, the ammonia concentration increased for a short adaptation period (up to 7 mgll NH 4-N). The average effluent concentration during the second experimental stage was 1.7 mgll NH 4-N. Because of reduction of the unsaturated zone and the higher hydraulic loading rate (increasing recirculation ratio) nitrification was reduced. The concentration was still well beyond the Austrian effluent limit of 10 mg/l NH 4-N (for treatment plants < 500 p.e.). Problems with meeting the effluent limit could occur with further increase of recirculation rate andlor low temperature conditions. The average effluent concentration of N inorg was 22.5 mg/l, which corresponds to an elimination rate of 69% (four times higher than without recirculation). If you add the organic nitrogen (influent mean: 25.1 mgll; effluent mean: 4.7 mgll), the TN elimination rate is 72%. Figure 3 shows that the mean effluent concentrations (throughout the whole experimental period) of COD (41 mgll) and TOC (12.4 mg/l) clearly fell below the Austrian limits (COD: 90 mgll; TOC: 30 mgll). Before the experiment however, the effluent concentrations of COD and TOC were lower (30 mgll COD and 9.5 mgll TOC). The main reason for these lower elimination efficiencies lies in the increased hydraulic loading rate (due to the recirculation) of the constructed wetland. This becomes clearly visible if you compare the average concentrations of both experimental stages (stage 1: 33.6 mg/l COD and 10.8 mgll TOC; stage 2: 45 mg/l COD and 14.7 mg/l TOC). I--COD -Austrian effluent standard TOC 100

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Advanced nitrogen elimination

75

System B - POSaie ofmethanol Experimental stage 1 - Intermittent feeding. On the 31st of May 1995 the dosage of methanol into the second, water saturated bed started. The methanol was pumped into the inlet of the bed four times a day. Figure 4 shows a high denitrification efficiency after a two-week-phase of adaptation of the microorganisms. The mean N03-N effluent concentration after adaptation was 4.0 mg/!. The influent concentration (influent of the second stage = effluent of stage one) was 36 mg/!, which means that the elimination rate conceming nitrate was 89%. Before the research started the mean N0 3-N effluent concentration was 44.4 mg/!, which proves that there was no denitrification due to the lack of a carbon source.

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Compared with the time before the start of the dosage, the NH 4-N effluent concentration was clearly higher (5.4 mg/! in contrast to 0.65 mg/!). This is due to the saturation of the second bed of the constructed wetland. because less aerobic soil body was available. Nitrification could take place only within the first stage, which led to a lower nitrification rate than before the raising of the water level of the second bed. However, the first experimental stage was still clearly influenced by the adaptation of the microorganisms. At the end of the fust experimental phase and during the first month of the second phase the ammonia concentration was already down to about one mg/!. The Ninorg concentration of both effluents (bed 1 and 2) is presented in Fig. 5. The elimination performance of the second bed was 79% and that of the whole constructed wetland was 82%. If you add the mean concentration ofNorg (20.8 mg/! in the influent and 6.8 mg/! in the effluent), the elimination of total nitrogen is 78%, as opposed to 28% before the start of the dosage (Table 1). This more or less corresponds to a tripling of the TN elimination performance. Experimental stage 2 - Continuous feeding. From the 7th of July 1995 the constructed wetland was not fed intermittently any more but continuously over an overflow. Therefore the N0 3-N concentration increased rapidly up to a mean effluent concentration of 15.7 mg/!. The denitrification performance fell to 58% (compared to 89% during experimental stage 1; Table 1). The way of feeding undoubtedly influences the denitrification performance. This is due to the uneven distribution of methanol during the continuous feeding. The quantity of water which flows through the distribution system is too small for the whole surface of the constructed wetland to be fed. On the other hand, the methanol was very well distributed through the flush of the intermittent feeding. Because of the small quantity of methanol (60 mIld) which was needed, the methanol was in both cases added four times a day into the second stage of the constructed wetland.

J.LABERetal.

76

Therefore the denitrifying bacteria did not get enough carbon during continuous feeding. The Ninorg elimination rate of the second experimental stage was 59% (Figure 5). The TN elimination was 61%. The high NH4-N concentrations in the second half of August were due to much higher hydraulic loading rates during a few days (caused by a defect), which led to a lower nitrification rate. After the defect had been repaired, the ammonia concentration dropped quickly again (Figure 4).

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Experimental stage 3 - Batch feeding. At the end of September the feeding system was restored to batch feeding once a day. This is also a kind of intermittent feeding but with much longer intervals than in the rust experimental stage. Samples were taken throughout two periods (at the beginning of stage 3 and during December - winter conditions). The elimination performance for N03-N was 72%, which means that it was in between the results of the two former experimental stages (Table 1). The average N0 3-N effluent concentration of the first soil fllter (=influent of the second) was 45.5 mg/l, that of the second soil fllter was 12.74 mg/l. The nitrification performance was clearly lower than before (7 mg/l mean NH 4-N effluent concentration), which was due to the system of feeding and the low water temperature during December « 4°C). The elimination rate of Ninors of the constructed wetland was 62%, the TN elimination rate was 63%.

Advanced nitrogen elimination

n

CONCLUSIONS The highest elimination rates could be achieved with system B (dosage of methanol) during experimental stage 1 (intennittent loading four times a day). The mean elimination rates were 82% for Ninorg and 78% for TN (Table 1). These elimination rates are similar to the results of Platzer (1996), who found a maximum elimination rate of 70% for TN in a two-stage constructed wetland with a bypass of raw wastewater into the second stage. The TN elimination of system A (recirculation) was 72%. Both approaches (recirculation and dosage of methanol) showed that the treatment system "Constructed wetland" can achieve high performances of nitrification as well as denitrification. Recirculation of nitrified effluent water of a one-stage vertical flow constructed wetland into the settling tank is especially a practical and simple solution which can be operated with low maintenance. This is of great importance for onsite, low tech treatment plants like constructed wetlands for single households or small communities. The results show that such systems could be of significance for: countries with strict effluent limitations for nitrogen (as for example Austria); constructed wetlands which are built up beside small recipients; infiltration of the effluent water of the constructed wetland. The elimination of COD, BODs and TOC was no problem throughout the whole experimental period. The effluent concentrations of both systems were well below the Austrian standards. Only after the start of the methanol dosage there was a period of adaptation which is necessary for the microorganisms and often referred to in literature (Hillenbrand and Bohm, 1996).

REFERENCES Boutin, C. (1987). Domestic wastewater treatment in tanks planted with rooted macrophytes: case study; description of the system; design criteria; and efficiency. Wat. Sci. Tech., 19(12),29-40. Bllmer. T. (1992). Einflu6faktoren fUr die Leistungsfahigkeit von PflanzenkHiranlagen. Schriftenreihe Wasserversorgung, Abwasserbeseitigung und Raumplanung, TH Darmstadt, 58.

Brix. H. (1992). Constructed wetlands for municipal wastewater treatment in Europe. Fresented at Wetland Systems in Water Pollution Control. Sydney, 23-25 November. Burica, U. and Lawrence, P. C. (1990). A new community approach to wastewater treatment with higher water plants. Constructed Wetlands in Water Pollution Control (Adv. Wat. Pollut. Control no. Il). P. F. Cooper and B. C. Findlater (eds). Pergamon Press, Oxford, UK, 359-371. Haberl. R.. Perfler. R. and Mayer. H. (1995). Constructed wetlands in Europe. Wat. Sci. Tech., 32(3). 305-316. Hillenbrand, T. and Bllhm. E. (1996). Ma6nahmen zur Verbesserung der Denitrifikation. Korrespondenz Abwasser. 3196, 393404.

A., Boutin. C. and Esser, D. (1990). Domestic wastewater treatment with emergent hydrophyte beds in France. in Water Pollution Control (Adv. Wat. Pollut. Control no. ll). P. F. Cooper and B. C. Findlater (eds), Pergamon Press. Oxford. UK, 183-192. Perfler, R. and Haberl. R. (1993). Actual experiences with the use of reed bed systems for wastewater treatment in single households. Wat. Sci. Tech .• 28(10), 141-148. Platzer, Chr. (1996). Enhanced nitrification and denitrification by a combination of vertical and horizontal flow reed beds. Natural and Constructed Wetlands for Wastewater Treatment and Reuse - Expuiences. Goals and Limits. R. Ramadori. L. Cingolani and L. Cameroni (eds). Perugia, Italy, 201-211. Schierup. H-H., Brix. H. and Lorenzen. B. (1990). Wastewater treatment in constructed reed beds in Denmark - state of the art. Constructed Wetlands in Water Pollution Control (Adv. Wat. Pollut. Control no. ll), P. F. Cooper and B. C. Findlater (eds). Pergamon Press, Oxford, UK, 495-504. Seidel. K. (1978). Gewllsserreinigung durch hahere Pflanzen. Zeitschrijt Garten und Landschaft. HI. 9-17. Stengel. B. (1985). Perspektiven der Nitratelimination in kllnstlichen Feuchtgebieten. Grundlagtn und Praxis natumaher KllJrve/fahren. Liebenburg, Germany. Li~nard.

Constructed Wetlands