Characterisation of microbial biocoenosis in vertical subsurface flow constructed wetlands

Abstract

In this study a quantitative description of the microbial biocoenosis in subsurface vertical flow constructed wetlands fed with municipal wastewater was carried out. Three different methods (substrate induced respiration, ATP measurement and fumigation–extraction) were applied to measure the microbial biomass at different depths of planted and unplanted systems. Additionally, bacterial biomass was determined by epifluorescence microscopy and productivity was measured via ¹⁴C leucine incorporation into bacterial biomass. All methods showed that > 50% of microbial biomass and bacterial activity could be found in the first cm and about 95% in the first 10 cm of the filter layer. Bacterial biomass in the first 10 cm of the filter body accounted only for 16–19% of the total microbial biomass. Whether fungi or methodical uncertainties are mainly responsible for the difference between microbial and bacterial biomass remains to be examined. A comparison between the purification performance of planted and unplanted pilot-scale subsurface vertical flow constructed wetlands (PSCWs) showed no significant difference with the exception of the reduction of enterococci. The microbial biomass in all depths of the filter body was also not different in planted and unplanted systems. Compared with data from soils the microbial biomass in the PSCWs was high, although the specific surface area of the used sandy filter material available for biofilm growth was lower, especially in the beginning of the set-up of the PSCWs, due to missing clay and silt fraction.

Introduction

Wastewater purification in constructed wetlands is a result of the interactions between plants, soil and microorganisms. Until now, constructed wetlands have often been seen as a “black box” and were investigated only with regard to their removal efficiencies for nutrients and hygienically relevant bacteria (e.g. Vacca et al., 2005). Only a few publications attempted to obtain a more detailed description of the microbial biocoenosis within the filter body of constructed wetlands. Some of these papers focused on microbial activity and productivity (e.g. Baptista et al., 2003); others investigated the microbial biomass and biofilm development (e.g. Larsen and Greenway, 2004), and only a very small number of papers have been found that investigated the microbial diversity within constructed wetlands by describing the bacterial community composition (e.g. Truu et al., 2005). Thus, there is a significant lack of information concerning the microbial community, their biomass, productivity and community diversity in constructed wetlands; but this information is highly needed for a better understanding of the degradation processes in these systems.

In natural wetlands it is established knowledge that the major part of the degradation and transformation processes regarding organic compounds is carried out by bacteria and fungi. Bacteria are dominating these processes in the wet sediment and the water body itself, both under aerobic and anaerobic conditions (Kirschner and Velimirov, 1999). In soil, and on plant litter in streams, and on dead shoots of marsh plants, on the other hand, the biomass of fungi often exceeds bacterial biomass (Bailey et al., 2002, Buesing and Gessner, 2006). However, the contribution of bacteria and fungi to degradation of organic matter in natural wetlands is still in debate (Buesing and Gessner, 2006) and there is no consistent knowledge on the situation in constructed wetlands.

In addition, it is a long time discussed topic (e.g. Brix, 1997) whether plants do significantly contribute to the organic matter and nutrient removal in constructed wetlands. It is well known that roots stimulate the microorganisms by the so called “rhizosphere-effect” (Stottmeister et al., 2003) but, to our knowledge, there are only few studies available investigating the microbial degradation processes and biocoenosis in planted compared to unplanted wetlands.

The aim of the present study was to get a detailed description of the microbial and bacterial biomass and bacterial productivity in planted and unplanted subsurface vertical flow constructed wetlands. Several methods based on different measuring principles were applied to quantify the microbial biomass within the filter body of constructed wetlands and a method for estimating bacterial productivity in sediments (Buesing and Gessner, 2003) was adopted. In addition, the results shall provide a basis for a future determination of stoichiometric and kinetic factors of CW2D, a model developed to simulate degradation and transformation processes in subsurface flow constructed wetlands (Langergraber and Šimůnek, 2005).