Enhanced denitrification and organics removal in hybrid wetland columns: Comparative experiments

Abstract

This study investigated three lab-scale hybrid wetland systems with traditional (gravel) and alternative substrates (wood mulch and zeolite) for removing organic, inorganic pollutants and coliforms from a synthetic wastewater, in order to investigate the efficiency of alternative substrates, and monitor the stability of system performance. The hybrid systems were operated under controlled variations of hydraulic load (q, 0.3–0.9 m³/m² d), influent ammoniacal nitrogen (NH₄–N, 22.0–80.0 mg/L), total nitrogen (TN, 24.0–84.0 mg/L) and biodegradable organics concentration (BOD₅, 14.5–102.0 mg/L). Overall, mulch and zeolite showed promising prospect as wetland substrates, as both media enhanced the removal of nitrogen and organics. Average NH₄–N, TN and BOD₅ removal percentages were over 99%, 72% and 97%, respectively, across all three systems, indicating stable removal performances regardless of variable operating conditions. Higher Escherichia coli removal efficiencies (99.9%) were observed across the three systems, probably due to dominancy of aerobic conditions in vertical wetland columns of the hybrid systems.

Introduction

With unique advantages of low energy consumption and operating cost, constructed wetlands have been increasingly used in wastewater treatment. Pollutants are removed in a wetland system via a complex variety of physical, chemical and biological processes that are still not fully understood. The performance of the wetlands is often limited in terms of nitrogen removal, due to a lack of methods to balance conflicting conditions required for organics removal, nitrification and denitrification.

In a wetland system nitrogen is typically removed from wastewater primarily via the classic biodegradation route (i.e. ammonification, autotrophic nitrification, followed by heterotrophic denitrification), while other processes such as plant uptake and sorption–desorption also contribute to the removal (Vymazal, 2007). Regarding the biodegradations and eventual removal of nitrogen, significant alternative routes have been discovered, in particular anaerobic ammonium oxidation (Dong and Sun, 2007, Paredes et al., 2007). However, to date researchers have not been able to consistently implement the alternative biodegradation routes in the wetlands, and the classic denitrification route is still relied upon as the major mechanism for nitrogen removal.

One of the major problems associated with the classic nitrogen route is the lack of organic carbon for denitrification, due to the dependability of synthesis and activity of denitrifying enzymes on organic carbon availability (Lavrova and Koumanova, 2010). In an efficient wetland, substantial removal of organics (by aerobic decomposition) tends to occur prior to nitrification. As such, organics in the raw wastewater are not available for denitrification, and the classic route of nitrification cannot be completed; this often causes the TN level of the wastewater to remain virtually unchanged, while the nitrification of ammonia increases the nitrate level and acidity of the wastewater.

To overcome this problem, two options may be taken: (1) adding external sources of carbon directly into the wastewater, or (2) providing controlled release of organics from wetland media inside wetland matrices. The first option has already been put into practice (Songliu et al., 2009) at a cost of increased operating expenses. Regarding the second option, little information is available from the literature. The traditional substrates used in treatment wetlands are gravel and soil; neither provides a sufficient organic source for denitrification, and few studies have investigated the use of more organic-rich materials in the wetlands. In addition to nitrogen removal, existing literatures provide little information about the role of organic-rich materials in wetland systems for the removal of other pollutants (i.e. phosphorus, coliforms), despite their potential as alternative substrates (Gray et al., 2000).

This study had been planned to investigate the combination of three wetland systems with traditional (gravel) and alternative substrates (wood mulch and zeolite) for removing organic, inorganic pollutants and coliforms, emphasizing on meeting the conflicting conditions required for organics and nitrogen removal without adding external organics. The objectives of this study are two-fold: (1) to monitor the efficiency of alternative substrates, and (2) to investigate the stability of system performance under different operating conditions in terms of pollutant loadings.