Cold-climate constructed wetlands

Cold-climate constructed wetlands

~ Wat . Sci . Tech . Vol. 32. No . 3. pp. 95-101, 1995. Copyright e 1995 IAWQ Printed in Great Britain . AU rights reserved. 0273 -1223/95 $9'50 .. 0...

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Wat . Sci . Tech . Vol. 32. No . 3. pp. 95-101, 1995. Copyright e 1995 IAWQ Printed in Great Britain . AU rights reserved. 0273 -1223/95 $9'50 .. 0'00

Pergamon 0273- 1223(95)0 0609-5

COLD-CLIMATE CONSTRUCTED WETLANDS T. Meehlum, P. D. Jenssen and W. S. Warner JORDFORSK, Centre for Soil and Environm ental Research. N-J 432 As, Norway

ABSTRACT This paper outlines design considerations for constructed wetlands with horizontal subsurface flow treating domestic wastewater in cold climates of northern latitudes. Particular attention is devoted to the use of a filter medium with high phosphorus adsorption capacity. Experience from two Norwegian multistage systems consisting of an aerobic pretreatment step followed by constructed wetland units indicates purification processes are nearly the same during winter and summer seasons, witb quite high removal of organic matter (COD. BOD), phosphorus and nitrogen.

KEYWORDS Cold climate: constructed wetland : multistage system ; nitrogen ; organic matter; phosphorus, subsurface flow. INTRODUCTION In northern regions, cold winter climate can significantly affect hydraulics, chemical and biochem ical processes. Dormant vegetation and the slow reaction rate for soil or aquatic microbes at low temperatures may reduce both physical and biological activity, and thus affect system performance on a seasonal basis. It is, therefore, important to consider winter conditions when designing a system for cold climate regions. Most investigations studying natural treatment of wastewater are confined to systems operating in temperatures warmer than 5 "c. Since eviden ce of purification at cooler temperatures is limited, the winter performance of CWs is questioned. Lakshman (1992) describes Canadian experiences with CWs and winter storage of effluents. However, work in Sweden (Wittgren, 1988; Gumbricht, 1991). Denmark (Schierup et al., 1990), Austria (Navara, 1992) and northern USA and Canada (Herskowitz, 1986; Reed et al., 1984; Reed, 1993) shows that some CWs function in sub-zero environments. CWs are typically installed for wastewater treatment at a site where a wetland did not previously exist. There are several advantages to this approach , not the least of which is manipulating control of flow to co mpensate for icing and/or insulating portions of the system to control temperature dependent treatment processes which may be physical , biological and/or chemical. Regardless of climatic conditions, the basic features in a CW with horizontal subsurface flow are a uniformly graded (flat to slightly sloping) vegetated soil surface, a method for uniform wastewater distribution at the head of the system, and collecti on works at the end (Brix, 1987). The design variables include: characteristics of applied wastewater, hydraulic load ing, filter medium, the depth of liquid in the system , detention time, control of flow path, type of vegetation, and vegetation management. In Norway , where mean winter temperatures often drop below -10 "C, the major 95

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winter concerns are low operating temperatures with ice formation on the surface of the treatment system. freezing of equipment. low reaction rates. and dormant vegetation. MATERIAL AND METHODS Two experimental multistage systems have been installed for treatment of domestic wastewater. An outline of the system design is given in Fig. J. System

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An important design feature in the CWs is the aerobic pre-treatment stage. Aerobic pre-treatment may be an important design feature in a cold climate for various reasons (Jenssen et al . 1991a, 1993). In the winter, when plants are dormant. the oxygen supply to the wetland is reduced . Aerobic pre-treatment enhances nitrification prior to the wetland stages. This is assumed to improve the N removal. In addition the BOD load on the wetland stages is reduced, thereby reducing possible clogging of the inlet side of the wetland. Although each CW has a different pre-treatment design. each is based upon an aerobic system. Designed for equal loading rates (2 m3d- l) and equal water types (municipal. 10 PE), the two wastewater systems are somewhat similar. Both have no moving parts, except for tipping buckets registering the outflow. Based on the design flow rate. the estimated detention time in both systems is 14 days. Each system receives wastewater from 8 m3 septic tanks. The two wastewater systems are 90 em deep. The major differences in design are the aeration units and number of stages (Fig. 1). System 1 was designed as a two-phase system: aerobic pre-treatment followed by a single-unit CWo The 1 m 3 aeration tank was not in operation during the investigation: thus. the system operated as a single-phase system. Specifically, subsurface flow was through a 9 x 12 m basin filled with a fabricated porous medium (LECA, Light Expanded Clay Aggregates). LECA comes in variety of sizes (0-30 mm diameter), has a high initial hydraulic conductivity. and a P-adsorption capacity exceeding 4 kg 1m3 - estimated by a batch experiment using phosphate solution (Jenssen et al. , 199Ia). This combination of a high hydraulic conductivity, porous structure and high P-adsorption capacity makes LECA interesting as a filter media. P-adsorbent and insulating material in on-site wastewater treatment systems. System 2 improves the removal efficiency of traditional single- or double-unit CW systems (e.g. System I) with aerobic pre-treatment. A 4 m 3 vertical (non saturated) flow sand-filter (Stage 2) is placed between the septic tank (Stage I) and the first CW unit (Stage 3) to aerate and remove organic matter and nitrification. Stage 3 is primarily a 6 x 10m subsurface flow CW designed to denitrificate. but contains iron rich sand to remove P. albeit limited. Stage 4, containing fine-grain LECA (0-4 mm diameter). is the principle unit to remove P. Since biological growth on particle surfaces (biofilm) can reduce the Psorption capacity. the main P-removal unit is located as the terminal step. The CW systems are located at approximately 60 N latitude and share similar characteristics. The sites are inland and have a mean January

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temperature of -7 C . They use similar native plant material (Phragmites australis and Typha latifolia). The other commonalty is that the systems were designed to operate by gravity with minimal, if any, moving parts. In both systems, effluent samples were collected monthly and analysed for the following: pH, electrical conductance, temperature, BOD, COD, TOC, SS, NH 4-N, N03-N, Tot-N, P04-P, Tot-P, Cl, and faecal coliform bacteria (E. coli) according to Norwegian standards for water analysis. RESULTS AND DISCUSSION The effluent concentrations of BOD, COD, N-tot, P-tot, chloride and E.coli for the pilot plants System 1 and 2 are shown in Figs 2 and 3. Figure 4 shows the effluent concentrations of COD, N-tot and P-tot during the investigation period (January 1992 to June 1994) at System 2. The removal rates of System 1 and System 2 are shown in Table 1.

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Figure 2. Concentrations of BOD, COD, N-lo!, P-lot and Chloride from 2 different stages at System 1. Sampling sites: Septic tank (S I), and CW LECA (S2). The figures show mean (column), standard deviation and median.

Table 1. Removal efficiency (%)* of two wastewater treatment systems Parameter System I System 2

* Removal is not adjusted for dilution due to precipitation. (The removal efficiency in the different stages and the chloride concentrations and are shown in Figs. 2 and 3). Although eight parameters were measured (Table 1), we shall focus upon the removal of organic matter, P and N. which were hiah. The main removal of BOD in Svstem 2 (Fiz. 4) occurred in the vertical flow

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pretreatment filter, The BOD reduction in System 1 (Fig. 2) is also very high despite no aerobic pretreatment. It is surprising to find this removal in a premature system where the plant cover is not fully developed. The BOD-removal has also been relatively constant throughout the seasons, suggesting that the vegetation has not been important to the BOD-removal. The high removal in System 1 may be attributed to the high specific surface area of this system, which has LECA (0-2 mm) and a long retention time (14 days). Both systems show promising P-removal (>95%). Comparing the performance efficiency of the two systems is somewhat difficult because the systems have been in operation for 2 and 3 years, respectively, and part of that time without vegetation. Anaerobic or anoxic conditions prevail in a CW except close to the roots. I

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Figure 3. Concentrations of BOD, COD, Nstot, P·lOt, Chloride and E. coli from 4 different stages at System I. Sampling sites: Septic tank (SI), Sand filter (S2), CW sand (S3) and CW LECA (S4). The figures show mean (column), standard deviation and median.

It is uncertain how root development will influence P-removal. This may be critical during winter, because vegetation does not provide oxygen; hence, the systems might become anaerobic. Nevertheless, constant Premoval was observed for these two systems, independent of season. BOD-removal and P-removal were most efficient in System 1 although the aeration tank was not in operation during the investigation period. The loading and surface area of the single CW filtration unit (110 m 2) was similar to the total surface area of System 2's dual-unit system (106 m2), the composition of filter material was different. System 1 used only fine-grain LECA (0-2 mm) whereas System 2 used coarse-grain LECA (0-4 mm) and iron-rich sand, suggesting that the finer LECA with its relatively high specific surface area is a very promising media for use in smaller CW units, possibly without aerobic pretreatment. The P adsorption capacity of LECA used in System 1 has not been estimated yet, however, with a larger specific surface area, a higher adsorption capacity than the LECA 0-4 mm can be expected. In theory the total service life for P-removal for both Systems is more than 2 decades but, for the reasons mentioned above, this may be much shorter. If the Premoval drops below acceptable levels, replacement of the filter media will remediate the situation. The removed LECA constitutes a suitable soil conditioner, especially for heavy clay soils. The value of the used LECA as a P-fertiliser depends on the bioavailability of the associated P. Whether or not this will be a cost efficient P-removal method depends on the longevity of the LECA as a P-adsorbent, and its subsequent fertiliser value. Another means for remediating decreasing P-removal capacity is to take the CW unit out of service for a while and allow it to dryout and "rest".

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Figure 4. Effl uent concentrations from sample sites septic tank (S I) and CW LECA (S4) of COD, P-IO!and N-lot duri ng the investig ation perio d (January 1992 10Ju ne 1994) at Sys tem 2.

The overall N-removal has ranged from 50- 60% for both systems. The main removal of N at System 2 occurs in the first CW unit. At Syste m 2, nitrification was expected in the vertical flow pretreatment unit, however, this was not achieved. The influence of N to the first wetland unit, where the main N-removal occurred , was therefore dominated by ammo nia. The results do not reveal whe ther the N-reduction was due to ammonia adso rptio n or nitrification and subsequent denitrification. The pretre atm ent unit was recently rebuilt with a better distrib ution system and a coarser LECA 2-4 mm instead of 0-4 mm. The resu lts should give better aera tion as well as longer detention time. We expect these modifications to yie ld a higher BOD reduction and nitrification in the pretreatment unit. This should increase the potential for N removal in the subseq uent wetland unit provided s ufficient orga nic matter for the denitrification is available. In the future we therefore antic ipate better BOD and N-removal in System 2. The N-remova l of System I is also expected to increase as the aerobic pretreatment unit is put into operatio n, Relative constant N-removal was observed for System 2, inde pendent of season (Fig 4). System 2 stands out as being an effic ient system for remova l of faecal coliforms, 99.9% (Fig. 4, Table I). The effluent co ncentrations ranged <70 faecal coliformsl lOO m!. ln System I, coliform co unts were not

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taken, but this system has a detention time similar to System I and equally high coliform removal is expected. The overall purification performance (Table 1 and Fig. 4) for the winter months (December through March) for COD, P and N is quite similar to the performance during the other seasons (Jenssen et al., 1993).

The most critical frost period is before snow cover in the autumn. In Norway, the coldest average monthly temperature at 60· north latitude is seldom below -10 ·C; and more than two weeks with average temperatures below -20 "C is improbable - especially in the autumn and before snowfall. If the plant material is not harvested it will provide insulation, albeit limited. Additional insulation may therefore be helpful, especially the first winters after planting. The insulation value of System 2's vegetation was supplemented with 10-15 em of straw. The snow cover was less than 30 cm. For Systems 1, no supplementary insulation was added to the vegetation litter and snow cover. The hydraulics of both systems functioned properly through the past two winters, when recorded temperatures dropped below -20 "C. CONCLUSIONS CWs with a theoretical capacity to remove pollutants for decades were built using porous media (LECA) with a high adsorption capacity. After three years, the BOD- and P-removal are still greater than 85%, but long term removal - and removal mechanisms in full scale systems - need further documentation. In particular, the effect of temperature, precipitation and hydrological variables need to be addressed. Practical experience at a sub-surface CW in Norway, 60· N latitude, revealed that natural vegetation supplemented with 10-15 ern of straw, and 20-25 ern of snow, insulated the system well enough to maintain hydraulic flow throughout the winter - even though temperatures dropped below -20 ·C for more than two weeks. Despite the promising results of the two CW systems, the impact of long-term cold climate upon reliable performance remains in question. Widespread application in Norway is not recommended until more knowledge on the purification processes in cold climate is obtained. ACKNOWLEDGEMENT The projects are financed by The National Agricultural Inspection Service (Statens Landbrukstilsyn), The Norwegian Department of Environment, The Norwegian State Pollution Control Authority (SFT), NS Norsk LECA and Centre for Soil and Environmental Research (JORDFORSK). The authors greatly appreciate co-operation of Johan Ellingsen and Fredrik Lynghaug whose help and inspiring attitude has contributed to the realisation of these projects. REFERENCES Brix, H. (1987). Treatment of wastewater in the rhizosphere of wetland plants - the root zone method. Wat. Sci. Tech., 19(1f2). 107-118. Herskowitz, J. (1986). Town of Listowel artificial Marsh project, Project Report 128 RR. Ontario Ministry of Environment, Toronto. Ontario. Gumbricht, T. (l99l). Nutrient reduction using macropbyte systems in temperate climate. Licentiate thesis. Royal Ins. of Tech., TRlTA-KUTI9I:I064. Sweden. Jenssen, P. D., Krogstad, T., Briseid, T. and Norgaard, E.(l991a). Testing of reactive filter media (LECA) for use in agricultural drainage systems. Proc. International Seminar of the Technical Section of C.I.G.R. on Environmental Challenges and Solutions in Agricultural Engineering, Agricultural Univ. of Norway, As, Norway, July 1-4, 160-166. Jenssen, P. D., Krogstad, T. and Mzehlum, T.(l991b). Wastewater treatment by constructed wetlands in the Norwegian climate: Pretreatment and optimal design. In: Ecological Engineering for Wastewater Treatment, C. Etnier and B. Guterstam (eds), pp. 227-238. Proc, International Conference, Stensund, Bokskogen, Sweden. Jenssen, P. D., Mashlum, T. and Krogstad, T. (1993). Potential use of constructed wetlands for wastewater treatment in northern environments. Wat. Sci. Tech., 28(lO), 149-157. Lakshrnan, G. (1992). Design and operational limitations of engineered wetlands in cold climates - Canadian experiences. Paper presented at INTECOL's IV International Wetlands Conference, September 1992, Columbus, Ohio (in press). Navara, G. A. (l992). Constructed wetlands for extensive sewage treatment in the Alps. Paper presented INTECOL's IV International Wetlands Conference: Global Wetlands - Old World and New, Columbus, Ohio, USA (in press).

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Reed, S. C., Bastain, R., Blackand, S. and Khettry, R. (1984). Wetlands for wastewater treatment in cold climates. Proc. AWE Water Reuse III, AWE, Denver Colorado, 962-972. Reed, S. C. (1993). Subsurface flow constructed wetlands for wastewater treatment, U.S. Environmental Protection Agency, Office of Wastewater Enforcement and Compliance. Washington D.C. Sawhney, B. L. and Hill, D. E. (1975). Phosphate sorption characteristics of soil treated with domestic wastewater. 1. Environ. Qual., 4, 342-346. Schierup, H. -H., Brix, H. and Lorenzen, B. (I990).Wastewater treatment in constructed wetlands in Denmark - state of art. In: Use of Constructed Wetlands in Water Pollution Control, P. F. Cooper and B. C. Findlater (eds). Adv. Wat. Pollut. Control, 11,495-504. Wittgren, H. -B. (1988). Removal of wastewater nitrogen in a wetland filter. Thesis. Linkoping Studies in Art and Science, 29. Linkoping, Sweden.