STUDYING THE PROPERTIES OF CONSTRUCTED WETLAND SEDIMENTS FOR THEIR POSSIBLE APPLICATION AS FERTILIZERS

The processes of sediment formation and their biogeochemical properties were studied in 2005-2007 at the experimental constructed wetland site ("Bioplato") designed for treatment of domestic effluents in Ukraine, nearby the city of Kharkiv. The constructed wetland consists of three units: one with vertical filtration applying fine gravel (0.5 m in depth) and coarse sand (0.3 111) as filtering media, one with horizontal filtration applying middle sand (0,8 m) as a filtering medium, and one surface flow unit with natural wetland soil applied, The site also includes a septic tank and a small sludge-drying field. The constructed wetland with a capacity of 50 111 3 of treated domestic wastewater per day was established in 1998 and since then it has been operated continuously until now. Macrophytes were planted in I 998-200 I, and recently the reed (Phragmites australis), cattail (Typha latifolia), and a number of sedge species (Carex Jpp.) dominate over the rest of ca. 30 plant species with average cover of 85-90%. Treatment efficiency of 90-95% of BOD 5 and suspended solids removal, as well as 98-99% of removal of pathogenic microorganisms allows discharging the treated wastewater into adjacent artificial lake, Sludge was sampled at each unit followed with standard laboratory analyses of its main characteristics: total organic carbon (TOC), total nutrients (N, P), contents of trace elements, abundance of pathogenic microorganisms, and general toxicity. The results of analyses confirmed that the top layer of sludge from each unit of the assessed constructed wetland site could be used as a source of fertilizers for grain and leguminous crops.

treatment of domestic and certain kinds of industrial wastewater, agricultural and urban runoff, as well as for purification and protection against pollution of natural waters [5,12,16,18,19,26,27,29,30,33]. Among different types of constructed wetlands the vertical and horizontal filtration (subsurface) systems and free surface wetlands are applied the most often for the above mentioned purposes [I, 2, 6, 12, 19, 26, 28, 33], All such systems consist of mineral substrate where aquatic helophyte and hydrophyte vegetation are established to provide the media for associated roots and rhizomes microorganisms and to serve as a complex natural "device" utilizing nutrients, accumulating metals and decomposing other pollutants. Processes of sedimentation, filtration, decomposition, accumulation and nutrient cycling take place simultaneously in the constructed wetlands, thus proving their general efficiency and feasibility.
To date, great amount of data is collected on constructed wetlands biotic components' ability to reduce significantly contents of pollutants (COD, BOD, suspended solids, nutrients, metals, synthetic compounds, oil products, etc.) and pathogens in the treated wastewater [I, 3, 4, 5, 6, 7, 14, 15, 17, 18, 19, 29, 32, 34], Less known are processes of sediment formation and properties in the constructed wetland systems featuring mineral and organic-mineral substrates as filtering media, Such knowledge is necessary when restoration or re establishment of artificial wetland systems is planned after their maturation/ageing accompanied with clogging and reduction of overall efficiency [8,28,30,31 ]. In those cases when removal of sediments is foreseen it is important to understand their chemical, toxicological and sanitary (bacteriological and parasitological) characteristics in order to identify the future possible ways of sediments utilisation and/or minimising their negative impacts on the environment. In the most of relevant literature a critical age of artificial wetland systems, after which restoration or regeneration would be desirable, is suggested as 7 to IO years, This article summarises preliminary investigations of the properties of sediments at an experimental constructed wetland system that has been established and used as the only mean for purification of domestic wastewater from a small village in Ukraine for nine years.
Research questions were defined as the following: How sedimentation processes in the constructed wetlands with vertical and horizontal filtration in temperate climates effect mechanical composition of the filtering medium at the wetland's maturation? What is mechanical composition of sediment layer of the free-surface constructed wetland after continuous exploitation and maintenance for several years without regeneration? How much organic matter and nutrients are contained in constructed wetland sediments ? What are patterns of metal accumulation in sediments ? What are bacteriological and parasitological characteristics of sediments? Do chemical, toxicological and sanitary characteristics of investigated sediments allow their potential application in agriculture if the sediments would be removed during reconstruction/regeneration of the constructed wetland site? Field data were collected and results of laboratory analyses of sampled sediments were made during 2005-2007 and used for answering the above questions, unit has separate outlet to the gully. The outlet from the third unit is used as general outlet while wastewater runs consequently through the first to the third treatment units. Each unit is insulated with polymer film in order to prevent infiltration to groundwater. To supply development of macrophytes, rhizomes of the six most frequent local species -Reed Phragmites communis, Cattail Typha latifolia, Wood Bulrush Scirpus �ylvaticus, Sedges Carex acuta, C. nigra, C. hirta -were taken from natural wetland below the lake and placed on the third unit's surface together with wet soil in late May -early June 1998. The reeds and cattails were planted on unit 2 in July 1998 and July 1999. On unit I, macrophytes were planted in 2000-200 I. Recently, these species dominate over the rest of ca. 30 plant species with average total cover of 85-90%.

Experimental constructed wetland site
For the whole period of wetland's operation since 1998, the units featured high efficiency in pollutant removal as certified with the results of water quality control measurements done by the Kharkiv Regional Sanitary and Epidemiological Authority. Thus, treatment efficiency was 90-95% for BOO 5 and 90-97% for suspended solids removal, as well as 98-99% of removal of pathogenic microorganisms (Table /). Wastewater after treatment satisfies the requirements of national regulations and therefore is discharged into adjacent artificial lake.   .__j Average figures of pollutants removal suggest that there was no significant reduction in efficiency of the Yelyki Prokhody constructed wetland over 9 years of continuous operation.

Sampling design
A stratified random sampling design was applied to the study. Sediments were sampled in May 2006 at each unit of the experimental constructed wetland site, from sedimentation tank and from the lake at the dam. At each sampling point but the unit I, sediments were collected from two strata -0-5 cm and 20-30 cm below the bottom's surface. At the unit I water depth did not allow to do so at the moment, and samples were taken from the upper stratum only. Total number of samples was 21.
Constructed wetland and lake sediments were composed of mineral filtering substrate, solid particles settled from the wastewater during sedimentation, products of wastewater organic matter decomposition, plant residuals and products of their decomposition, as well. Before laboratory analyses of sediments all vegetation residuals were removed from the samples.
Collection, preservation and handling of samples were done according to international standards [8,9,10].

Laboratory analyses of samples
Mechanical composition and particles' density of sampled sediments were measured by the gravimetric technique at the Environmental Analytical Laboratory of the National Academy of Municipal Economy Kharkiv.
Measurements of total organic carbon, total nitrogen, phosphorus and pH of water extract of sediments were done at the Chemical Analytical Laboratory of the Ukrainian Research and Design Institute of Water Supply, Sewerage Systems and Environmental Engineering "UkrYodGeo", Kharkiv, with application of standard techniques [21,22,23,24].
Sanitary bacteriological and parasitological properties of sediments were studied with application of standard detection techniques at the Analytical Laboratory of Kharkiv Regional Sanitary and Epidemiological Authority [25].
The contents of 9 metal elements in sediment samples were measured by emission spectroscopy techniques (DS259/SM3 I 20ICP for Fe, Cd, Cr, Cu, Ni, Sr, Zn; SM3 I 13HGAAS for Ag and Sn) at the Eurofins Sverige AB, Sweden.
Results of the laboratory analyses are presented in Figure 3 and Tables 2-4.

Mechanical composition and pH of substrate at different constructed wetland units
Sediments in the sedimentation tank and constructed wetland units differed by their mechanical composition ( grain-size distribution) and degree of heterogeneity (Figure 3). In the sedimentary tank coarse and middle sand particles were accumulated with heterogeneity coefficient of 6.7. In the constructed wetland units with vertical and horizontal filtration fine sand fraction was the best represented, and coefficient of sediment heterogeneity was 1.8 to 3.8. In the third unit sediments featured fine and middle sand fractions with coefficient of heterogeneity of 6.4 that makes them similar to the sand filter. This can be explained by the [ I 3 ].
Kalmar ECO-TECH '07 KALMAR, SWEDEN, November [26][27][28]2007 presence of middle and fine sand substrate at the unit provided for drainage upon which the soil and rhizome layer was established. Here sediment particles' density was 2.51-2.52 g/cm 3 , and the sand's density ( from drainage substrate) -2. 66 g/cm 3 , Mechanical composition of sediments from adjacent lake is quite typical for water bodies of such kind,  Measured pH of water extract from sediments at each unit (6.8-7.2) showed their neutral properties.

Total organic carbon and nutrient contents of sediments
Sediments from investigated constructed wetland contained organic matter and nutrients in concentrations less or almost equal to those of artificial lake ( Table 2). The highest total organic carbon's ( TOC) contents were found in the upper strata of sediments from the sedimentation tank and unit I. Significantly less amounts of TOC were found in the sediments of units 2 and 3 that may suggest more extensive processes of organic matter decomposition comparing to previous stages of wastewater treatment Although TOC contents of sediments at all sampling points were much less than those required for organic fertilizers, this measured index appeared very similar to average TOC contents in typical chemozem soils of the region  [26][27][28]2007 lower in constructed wetland sediments than in the lake excepting nitrogen in sediments from the septic tank where its concentration was the highest. Total phosphorus content in tank's sediments was several times higher than in wetland's sediments but still slightly less than in the lake. There is clear trend of increasing phosphorus availability in sediments from the first unit with vertical filtration wetland through the second unit with horizontal filtration wetland to the third unit with free surface wetland. For total nitrogen, although its content was the highest in sediments of the third unit comparing to the first two, differences among units were rather insignificant. By nutrients contents, constructed wetland sediments are much richer than typical chernozem soils of the region, and just slightly differ to the less from requirements stated in the standards for organic fertilizers. However, sludge from the sedimentation tank has greater potential for the use as a fertilizer since nutrients contents in it is significantly higher than minimum requirements. Sediments sampled from deeper sand-containing strata at constructed wetland units 2 and 3 featured much lower contents of organic matter and nutrients: 0.059%, 0.255% and 0.035% for TOC, total nitrogen and total phosphorus, respectively.

Metal contents of sediments
Contents of all studied metal elements and arsenic in all sampled sediments were less than limits permitted by standards for organic fertilizers; however these were generally several times higher than background contents in typical soils of the region ( Table 3).
Although detailed discussion on patterns of metal and arsenic accumulation in sediments of constructed wetlands of different types compared to patterns of the same elements accumulation and speciation in plants of the associated vegetation will be the subject of another paper, some preliminary observations can be made.
The iron was greater accumulated by sediments of the sedimentation tank and the top stratum at the third unit than in other cases. Cadmium's concentrations in sediments were the highest in deeper strata at the second and third units. For copper, nickel and chromium the pattern is that highest concentrations were found in the sediments of unit 3 comparing to other sites.

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Kalmar ECO-TECH '07 KALMAR, SWEDEN, November 26-28, 2007 Two later elements also showed significantly higher concentrations in deeper comparing to upper sediments' strata. For strontium, the highest and similar contents were found in sediments of the sedimentation tank and unit 3, and in sediments of other wetland units it had much less contents. The highest in the series concentration of arsenic was observed for sediments of the first unit, with further strong trend of reduction to almost background concentrations at the subsequent wetland units. The highest content of zinc was observed in sediments of the sedimentation tank with further reduction up to 11 times towards the third unit. Lead was the only element with the contents less than the background concentration at each stage of wastewater treatment, however clear pattern of its distribution in the sampled sediments was not revealed.  n/a n/a limits [20] 00

Sanitary bacteriological and parasitological properties of wetland sediments
Data on sanitary and epidemiological characteristics of sampled sediments suggest that only sediments from the unit 3 and the lake confonn to the standard requirements set up for organic fertilizers ( Table 4). By the contents of faecal coli-fom1s, sediments from the sedimentation tank and units I and 2 do not suit the standards [25], and disinfection (through e.g. thermal treatment) should precede their possible application as fertilizers.
Taking into account the fact the experimental constructed wetland site was designed as a chain of different units to fonn the whole and comparing outflow to inflow concentrations of faecal coli-forms we can conclude that investigated constructed wetland system is highly effective and efficient mean of wastewater disinfection. Continuous exploitation of constructed wetland treatment systems with stationary or quasi stationary technological parameters for long time is not obviously associated with reduction in treatment effectiveness and efficiency after 7-1 0 years of operation, Although there are general recommendations to restore/re-generate constructed wetlands' ability to remove pollutants with this periodicity, we can say that period of continuous operation may be much longer, and the needs for reconstruction/re-generation should be identified based on actual performance of constructed wetlands.