Assessment of treatment efficiency of constructed wetlands in East Ukraine
Introduction
Constructed wetlands (CWs) are engineered ecosystems which act as biofilters to eliminate nutrients, pathogenic microorganisms, persistent organic pollutants, xenobiotics and trace elements (TE) from industrial and domestic wastewater streams within a semi-controlled environment (Kadlec and Knight, 1996, Brix, 1997, Maine et al., 2007 Marchand et al., 2010, Zhi and Ji, 2012). A common classification divides CWs according to their hydrology onto: (i) surface flow wetlands, (ii) horizontal subsurface flow wetlands, (iii) vertical subsurface flow wetlands, and (iv) hybrid systems (Vymazal, 2001, Arias and Brown, 2009).
A broad range of mechanisms affects organics (Vymazal, 2007, Vymazal and Kropfelova, 2009, Marchand et al., 2010) and TE (Marchand et al., 2010) removal in CWs. Organic matter (OM) is decomposed in CWs with horizontal sub-surface flow by both aerobic and anaerobic microbial processes as well as by sedimentation, filtration of particulate OM and uptake by the consortium plant/microorganisms. Highest removal efficiencies for BOD5 (Biological Oxygen Demand, which represent the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material over a 5-day period) and COD (Chemical Oxygen Demand, which indicates the mass of oxygen consumed to degrade OM) were reported in systems treating municipal wastewater while the lowest efficiency was recorded for landfill leachate, partly due to the fact that municipal wastewaters contain predominantly labile organics (Vymazal and Kropfelova, 2009).
Processes that affect removal and retention of nitrogen during wastewater treatment in CWs are manifold and include ammonia volatilization, nitrification, denitrification, nitrogen fixation, plant and microbial uptake, mineralization (ammonification), nitrate reduction to ammonium (nitrate-ammonification), anaerobic ammonia oxidation (ANAMMOX), fragmentation, sorption, desorption, burial, and leaching. However, only few processes ultimately remove total nitrogen from the wastewater while most processes just convert nitrogen to its various forms. Therefore, different types of CWs may be combined with each other in order to exploit the specific advantages of the individual systems, such as a succession of aerobic and anaerobic conditions (Taylor et al., 2005, Vymazal, 2007).
Phosphorus transformations during wastewater treatment in CWs include adsorption, desorption, precipitation, dissolution, plant and microbial uptake, fragmentation, leaching, mineralization, sedimentation (peat accretion) and burial. The major phosphorus removal processes are sorption, precipitation, plant uptake (with subsequent harvest) and peat/soil accretion. However, the first three processes are saturable and soil accretion occurs only in free water surface CWs with an emergent plant. Removal of phosphorus in all types of constructed wetlands is low unless special substrates with high sorption capacity are used (Vymazal, 2007). It is now recommended that nutrients, particularly nitrogen and phosphorus, must be recovered in marketable form (Cai et al., 2013, Miksch et al., 2015).
Surfactant are defined as “any organic substance and/or mixture used in detergents, which has surface-active properties and which consists of one or more hydrophilic and one or more hydrophobic groups of such a nature and size that it is capable of reducing the surface tension of water, and of forming spreading or adsorption monolayers at the water–air interface, and of forming emulsions and/or microemulsions and/or micelles, and of adsorption at water–solid interfaces” (AISE, 2013). CWs provide good surfactant abatement (Sima and Holcova, 2011, Tamiazzo et al., 2015).
The CWs efficiency depends on inlet contaminants concentrations, hydraulic loading, pH, redox conditions, temperature, and the presence/absence of the consortium plants/bacteria (Kadlec and Wallace, 2008). The elimination of contaminants in influents also depends on the inter- and intra-specific variability of the consortium macrophytes/microorganisms (Brisson and Chazarence, 2009, Marchand et al., 2010, Marchand et al., 2014a, Marchand et al., 2014b, Marchand et al., 2014a, Marchand et al., 2014b), climate conditions (Kushck et al., 2003, Maine et al., 2007) and anthropogenic factors (construction type, wastewater quality, operating conditions, etc.) (Saeed and Sun, 2012, Zhi and Ji, 2012). CWs generally show high efficiency in removal of suspended solids (SS), BOD5 and COD whereas removal efficiency regarding nutrients (N and P) is lower and more variable (Song et al., 2006, Garcia et al., 2010). Overall efficiency of CWs regarding removal of a wide range of pollutants has been proved for both warm and cold climates by many studies (Mander and Jenssen, 2002a, Mander and Jenssen, 2002b, Mander and Jenssen, 2002a, Mander and Jenssen, 2002b). Moreover, the net life-cycle energy output of CWs can be used for biofuel production and enhanced through optimising the nitrogen supply, hydrologic flow patterns and plant species selection (Liu et al., 2013).
Since the 1960s, CWs have been developing in different countries simultaneously and various synonymous names are used to indicate the same class of objects: “man-made wetlands” (Breckenridge et al., 1983), “artificial wetlands” (Gersberg et al., 1984), “reed beds” (Biddlestone et al., 1993), “engineered wetlands” (Zhang et al., 2010) and “bioplato” (Stolberg, 2002). The last name of CWs is mainly used in Ukraine and Russia.
In Ukraine CWs have been first developed and applied for purification of polluted surface waters in lakes, rivers and canals yet in 1980s. The first CW (“bioplato”) facilities for domestic wastewater treatment have been designed and constructed in 1998 at the Velyki Prokhody village near the city of Kharkiv (Stolberg, 2002). In 1997–2003, Kharkiv region has been a pilot in implementing the CWs ecological technology to wastewater treatment in small to medium-sized rural communities. Here the Regional Programme on Constructed Wetlands (‘Bioplato’) Implementation for 2011–2015 has been accepted by the Regional Council that made possible to design and build about 15 treatment facilities of such kind in 2011–2013. Besides this region, CWs were built in several other regions both in the West, Central and East Ukraine during 2003–2014. The further wide implementation of this low-cost, efficient eco-technology in the country could be facilitated with adoption by the Ukrainian Government of the Guidelines on Design and Operation of CWs developed by O.M. Beketov National University of Urban Economy in Kharkiv.
During the last decade the number of CWs in Ukraine has increased to somewhat 50 operation sites as these are considered an efficient alternative to conventional wastewater treatment systems, cost-effective and environmentally friendly bio-processes for purification of contaminated water (Magmedov et al., 1995, Rousseau et al., 2004, Paulo et al., 2013, Vera et al., 2013). Most of CWs in Ukraine are operated in small rural settlements which are located far away from the central sewage grid, have limited financial resources and energy supply shortages together with a lack of qualified staff for the maintaining and operating more sophisticated treatment systems (Vera et al., 2013).
This work aimed at assessing the performance of CWs in Kharkiv region, Ukraine by (i) monitoring common chemical parameters (pH, BOD5, COD and SS), surfactants, orthophosphates and total nitrogen concentrations) of wastewaters effluents and influents on 9 operated CWs in rural areas; (ii) evaluating the operating conditions of the CWs and their efficiency to treat domestic wastewaters and (iii) comparing their treatment efficiency to the conventional wastewater treatment facilities.
Section snippets
Environmental conditions
All studied CWs are located in the Kharkiv region, East Ukraine. The region (ca. 3000,000 inhabitants, 2014) is one of the most industrialised and urbanised areas of Ukraine. In spite of the high population and industries density, the region suffers from the shortage of available water resources (Vystavna et al., 2012). The climate of the study area is a typical for the Forest-Steppe natural zone i.e. moderate with distinct 4 seasons. The duration of the cold period (winter) varies from 125 to
Sampling and analytical method
Wastewater samples were taken at each site from inlet and outlet points during the period of the extreme oxygen depletion from late July to mid-October 2012 (Vystavna et al., 2012). Each studied site was sampled twice using the grab sampling technique (ISO/TS 13,530:2009). Actual wastewater discharge was measured on-site at inlet and outlet applying the measure jar and stopwatch at time when samples were collected. Where available, the measured values were corrected with the data collected over
Chemical parameters of influents and effluents
At almost all studied CWs, the pH was steady for both inlets and outlets and in the range 7.2–8.7 (Fig. 3). Only at B5, pH decreased from 8.1 at the inlet to 7.0 at the outlet. At the inlets of studied CWs, the mean value of DO in wastewaters was 0.12 ± 0.05 mgO2 L−1 and generally did not exceed 0.5 ± 0.1 mgO2 L−1. At the outlets, this parameter reached up to 1.0 ± 0.2 mgO2 L−1 and for three out of the nine studied outlets (B2, B8 and B9), it increased up to 6.0 ± 1.1 mgO2 L−1 (Fig. 3).
The inflow SS contents
Removal efficiency of constructed wetlands
Taking into account that all studied CWs had the same construction type, were located in quite similar environmental conditions (temperature, solar radiation and humidity) and were planted with the same macrophyte species (Hijosa-Valsero et al., 2012), it was assumed that differences in RE may be attributed to differences in inflow concentrations (Vymazal, 2011, Comino et al., 2013), construction work quality and operational conditions (Saeed and Sun, 2012, Vera et al., 2013).
All studied CWs
Conclusions
Our results revealed that CWs in Kharkiv region were efficient to remove organics (by BOD5 and COD indicators) and SS from domestic wastewaters of small rural communities. The RE of contaminants at CWs was mainly dependent on inflow concentration, design, maintenance parameters and operating conditions. At the same time the RE of nutrients were less than 50%. The comparisons on the RE of conventional WWTF and CWs showed that CW has the same level of the RE of organics (measured as BOD5 and COD)
Acknowledgements
The research has been financed by the Kharkiv Regional State Administration in the framework of the project “The technical audit and control of treatment efficiency of the phytotechnology facilities ‘bioplato’ designed and operated in the Kharkiv region” (Contract N16 of 12.06.2012). One of the author was granted by SAIA, National Scholarship of Slovak Republic.
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2020, Environmental ResearchCitation Excerpt :A removal rate of 3.86 log10 was also observed in the system assessed (Gikas and Tsihrintzis, 2010). In Ukraine, Vergeles et al. (2015) evaluated five hybrid systems which consisted of a sedimentation tank + VF-CW + HSSF-CW. Although the monitored systems varied in terms of size and concentrations of pollutants in the inlet and outlet, results showed overall removal efficiencies of 72.1% for TSS, 77.3% for COD, 82.6% for BOD, 51.5 for N–NH3, 50,1% for Ntotal and 49.5% for P-PO43-.