Organic matter stabilization in reed bed systems: Danish and Italian examples

Kinetics and mineralization fraction of organic matter from sewage sludge mixed with soil under controlled laboratory conditions

The objective of this work was to evaluate the kinetics and mineralization fraction of organic matter from sewage sludge mixed with the soil under controlled laboratory conditions. For this, organic material samples accumulated in layers 0-5 cm, 5-10 cm and 10-15 cm in relation to the surface of a vertical flow constructed wetland system (VF-CW) used for treatment of septic tank sludge, in addition to samples of dewatered sludge from a septic tank and a UASB reactor and non-dewatered sludge from an anaerobic digester were mixed with material collected from the surface layer of a Red Yellow Argisol at rates equivalent to the applications, via organic residues, of 300 and 600 kg ha^-1 year^-1 of total nitrogen. It was found that the two-phase kinetic equation fit better to the mineralization data of labile and recalcitrant organic carbon. For the lowest nitrogen application rate in the mixtures, the mineralization fractions of the total organic carbon were higher than 73%, while at the highest dose there was a reduction in the mineralization of organic matter by 17% and 63%, respectively in samples collected in layer 10-15 cm from the VF-CW and in the septic tank sludge dewatered in the drying bed.

Sustainable Dewatering of Industrial Sludges in Sludge Treatment Reed Beds: Experiences from Pilot and Full-Scale Studies under Different Climates

Sludge treatment reed beds (STRBs) are an established sludge treatment technology with multiple environmental and economic advantages in dewatering sludge generated during domestic wastewater treatment. However, little is reported regarding their appropriateness and efficiency for the treatment of sludge produced during industrial wastewater treatment and from water works. These sludge types may have significantly different quality characteristics than typical domestic sludge and may contain constituents that could affect their dewaterability. Therefore, the dewatering of these industrial sludge types is usually tested in small-scale pilot STRBs before the construction of full-scale systems. This paper presents and summarizes the state-of-the-art experience from existing pilot and full-scale STRB systems from various countries and climates treating sludge from various industrial sources, evaluates the suitability and the advantages of this sustainable treatment technology, and proposes the required dimensioning for efficient full-scale STRB operation and performance.

Comparative characterization of surface sludge deposits from fourteen French Vertical Flow Constructed Wetlands sewage treatment plants using biological, chemical and thermal indices

Due to their design and mode of operation, French Vertical Flow Constructed Wetlands (VFCWs) accumulate suspended solids from the inflow wastewater in the form of a sludge layer at the surface of the first filter. In order to maintain the treatment performance over the long term, the characteristics of the sludge deposits and their evolution have to be well described. In this objective, a panel of sludge deposit samples taken from 14 French VFCW sewage treatment plants was investigated. Elemental composition and organic matter content, nature and reactivity were analyzed. Results clearly revealed two categories of sludge deposits, namely the "young-age plants" type (1 year of operation and less) and the "mature plants" type (3 years of operation and more). Sludge deposits from the "mature plants" exhibited same biological, physical and chemical properties. Their organic matter was globally less abundant, more humified and less biodegradable than in the young-age plants type. Their overall contents in trace metals were also higher, although in a limited manner. The effect of additional treatments, particularly FeCl₃ injection for phosphorus precipitation, was observable in the "young-age plants" group. Finally, the sludge deposits sampled from one particular plant with specific operating conditions were found to exhibit very different characteristics from those of either groups identified. This observation underlined the influence of local conditions on the typology of the sludge deposits.

Sewage sludge and waterworks sludge stabilization in sludge treatment reed bed systems

In this study, results about sludge stabilization in sludge treatment reed bed (STRB) systems in two different systems, Hanningfield STRB 1 (England), treating waterworks sludge, and Stenlille STRB 2 (Denmark), treating surplus activated sludge, are presented. The study mainly focused on the effectiveness of the STRBs systems in stabilizing sludge organic matter; in fact, parameters correlated to biochemical and chemico-structural properties of organic sludge matter were determined. Dewatering and sludge stabilization were effective in both STRBs, as highlighted by total and volatile dry solids trend. β-glucosidase, phosphatase, arylsulphatase, leucine amino-peptidase and butyrate esterase activities, enzymes related to C, P, S, N and overall microbial activity, respectively, significantly declined along the profile in both STRBs. The determination of humic carbon highlighted the formation of a stable nucleus of humified organic matter in both STRBs in the deepest layers, thus meaning the successful stabilization of sludge organic matter for both kind of sludges. Similar conclusions can be drawn from pyrolysis gas chromatography analysis (Py-GC), which enables the characterization of soil organic matter quality from a chemical-structural point of view. The pyrolytic indices of mineralization and humification showed that in both STRBs the sludge organic matter is well stabilized.

Sludge quality after 10-20 years of treatment in reed bed systems

The effect on the environment of the operation of sludge treatment in reed beds (STRB) system is seen as quite limited compared to traditional sludge treatment systems such as mechanical dewatering, drying and incineration with their accompanying use of chemicals and energy consumption. There are several STRB systems in Denmark receiving sludge from urban wastewater treatment plants. Stabilization and mineralization of the sludge in the STRB systems occur during a period between 10 and 20 years, where after the basins are emptied and the sludge residue typically is spread on agricultural land. In the present study, the sludge residue quality after treatment periods of 10-20 years from four Danish STRBs is presented. After reduction, dewatering and mineralization of the feed sludge (dry solid content of 0.5-3 %) in the STRB systems, the sludge residue achieved up to 26 % dry solid, depending on the sludge quality and dimensioning of the STRB system. The concentration of heavy metals and hazardous organic compounds in the sludge residue that are listed in the Danish and EU legislation for farmland application of sludge was below the limit values. The nitrogen and phosphorus concentrations as an average in the sludge residue were 28 and 36 g/kg dry solid (DS), respectively. In addition, mineralization on average across the four STRB systems removed up to 27 % of the organic solids in the sludge. The investigation showed that the sludge residue qualities of the four STRBs after a full treatment period all complied with the Danish and European Union legal limits for agricultural land disposal.

Organic matter and pollutants monitoring in reed bed systems for sludge stabilization: a case study

In this study, results about sludge stabilization and pollutant monitoring in a reed bed system (RBSs) situated in Central Italy (Colle di Compito, 4,000 p.e.) were presented. In order to evaluate the process of sludge stabilization, parameters that highlighted the biochemical, chemical and chemico-structural properties of organic sludge matter have been followed during the entire period of operation (7 years). Moreover, the trend of heavy metals (bioavailable fractions and total content) and toxic organic compounds (LAS, NPE and DEHP) was monitored during all the period. The trend of all parameters related clearly demonstrated that sludge stabilization successfully proceeded in RBS. Moreover, through statistical analysis modelling, it is possible to determine how the stabilization process proceeded in terms of organic matter mineralization and humification, and how these processes influenced the content of pollutant compounds present in the stabilized sludges.

Economic assessment of sludge handling and environmental impact of sludge treatment in a reed bed system

The effect on the environment of the establishment and operation of a sludge treatment reed bed system (STRB) is quite limited compared to mechanical sludge dewatering, with its accompanying use of energy and chemicals. The assessment presented here of the investment, operation and maintenance costs of a typical STRB, and of the related environmental impact, is based on the experiences gained from the operation of a large number of STRB in Denmark. There are differences in the environmental perspectives and costs involved in mechanical sludge dewatering and disposal on agricultural land compared to STRB. The two treatment methods were considered for comparison based on a treatment capacity of 550 tons of dry solids per year and with land application of the biosolids in Denmark. The initial capital cost for STRB is higher than a conventional mechanical system; however, an STRB would provide significant power and operating-cost savings, with a significant saving in the overall cost of the plant over 20-30 years. The assessment focuses on the use of chemicals, energy and greenhouse gas emissions and includes emptying, sludge residue quality and recycling. STRB with direct land application is the most cost-effective scenario and has the lowest environmental impact. A sludge strategy consisting of an STRB will be approximately DKK 536,894-647,636 cheaper per year than the option consisting of a new screw press or decanter.

Greenhouse gas emissions from sludge treatment reed beds

Sludge treatment reed bed systems (STRBs) are considered as an alternative technology for surplus sludge treatment. Organic matter is decomposed by various microbial reactions, resulting in gases such as CO₂and CH₄emitting into the atmosphere. The aim of this study is to investigate gas emission from STRBs. The static transparent chamber was adopted to measure gas emission; it allows sunlight to enter and plants to photosynthesise. The comparison of total solids and volatile solids showed STRBs have a higher efficiency in dewatering and mineralization than a conventional unplanted sludge drying bed (USDB). The CO₂emission ranged from 28.68 to 100.42 g CO₂m⁻² d(-1) in USDB, from 16.48 to 65.18 g CO₂m⁻² d⁻¹ in STRBs; CH₄emission ranged from 0.26 to 0.99 g CH₄ m⁻² d⁻¹ in USDB, from 0.43 to 1.95 g CH₄m⁻² d⁻¹ in STRBs. Both gas fluxes decreased towards the end of vegetation and reached the highest rates during the hot and dry summer. After the system was loaded by sludge, the fluxes of CO₂and CH₄significantly decreased in the USDB, whereas they increased in STRBs. In terms of CO₂equivalent, the global warming potential of CH₄was 13.13 g CO₂eq m⁻² d⁻¹ and 15.02 g CO₂eq m⁻² d⁻¹ in USDB and STRBs, respectively.