Alexandros Stefanakis

Surplus activated sludge dewatering in pilot-scale sludge drying reed beds

A pilot-scale experiment on dewatering of surplus activated sludge (SAS) is presented, where two pilot-scale vertical flow, sludge drying reed beds (SDRBs), planted with Phragmites australis are used. The bottom of the beds is filled with cobbles, connected to the atmosphere through perforated PVC ventilation tubes, in order to achieve oxygen diffusion through the overlying porous medium that is colonized by roots and an abundant nitrifying biomass. Two layers of gravel, of decreasing size from bottom to top, make the drainage layer where the reeds are planted. The two beds were fed according to the following cycle: one week feeding with SAS at rates one 30 kg/m(2)/year and the other 75 kg/m(2)/year, and resting for three weeks. The results show that planted SDRBs can effectively dewater SAS from domestic sewage, the produced residual sludge presents a high dry weight content, the degree of volume reduction depends upon the initial SAS concentration and can be of the order of 90%, and decomposition of organic matter and high levels of mineralization can be achieved. Furthermore, the percolating water is not septic. The fertilizer value of the treated SAS, which contains no added chemicals, is comparable to that of SAS treated by other methods.

Heavy metal fate in pilot-scale sludge drying reed beds under various design and operation conditions

Thirteen pilot-scale sludge drying reed bed (SDRB) units have been constructed and operated under various settings. The beds included a cobbles lower layer, where perforated PVC aeration tubes were placed, and two gravel layers on top. The setup included planted beds with common reeds and control units. Three sludge loading rates (SLR) were examined: 30, 60 and 75 kg dm/m(2)/yr. Heavy metal (HM) accumulation in the residual sludge layer was negligible or low, and was found to increase with sludge layer depth. Plant uptake was low; the belowground biomass accumulated significantly more HMs compared to the aboveground biomass. Less than 16% of the influent HM left the bed through drainage. HM accumulation in the gravel layer was the major metal sink in the mass balance. On the whole, the HM content of the residual sludge was below the legal limits proposed by the EU for land application.

Stability and maturity of thickened wastewater sludge treated in pilot-scale sludge treatment wetlands

Thickened wastewater activated sludge was treated in 13 pilot-scale sludge treatment wetlands of various configurations that operated continuously for three years in North Greece. Sludge was loaded for approximately 2.5 years, and the beds were left to rest for the remaining period. Three different sludge loading rates were used that represented three different population equivalents. Residual sludge stability and maturity were monitored for the last year. Sludge was regularly sampled and microbial respiration activity indices were measured via a static respiration assay. The phytotoxicity of sludge was quantified via a seed germination bioassay. Measurements of total solids, organic matter, total coliforms, pH and electrical conductivity were also made. According to microbial respiration activity measurements, the sludge end-product was classified as stable. The germination index of the final product exceeded 100% in most wetland units, while final pH values were approximately 6.5. The presence of plants positively affected the stability and maturity of the residual sludge end-product. Passive aeration did not significantly affect the quality of the residual sludge, while the addition of chromium at high concentrations hindered the sludge decomposition process. Conclusively, sludge treatment wetlands can be successfully used, not only to dewater, but also to stabilize and mature wastewater sludge after approximately a four-month resting phase.

Modeling of Vertical Flow Constructed Wetlands

This chapter summarizes modeling efforts in vertical flow constructed wetlands (VFCWs). VFCW models include empirical (i.e., first-order k - C model, regression equation models, etc.) and mechanistic (i.e., hydrodynamic pollutant removal) models. The two most comprehensive models used in VFCW modeling, FITOVERT and Constructed Wetland 2-Dimensions model, are presented, along with a series of other models for pollutant removal prediction.

Processes and Mechanisms in Sludge Treatment Wetlands

This chapter describes the main processes and mechanisms responsible for sludge dewatering and mineralization in Sludge Treatment Wetlands. Dewatering in Sludge Treatment Wetlands is mainly achieved through water draining and evapotranspiration. Sludge mineralization is achieved through the decay of organic matter, nitrogen, phosphorus, and other organic micropollutants content and minimization of the microorganism activity (respiration activity). The various transformation/removal processes for the different pollutants are more or less the same as in Vertical Flow Constructed Wetlands with wastewater treatment, as described in part A.

Performance of Sludge Treatment Wetlands

This chapter presents the global efficiency of Sludge Treatment Wetlands from various countries, in terms of: • dewatering efficiency,

General Aspects of Sludge Management

This chapter presents general information of sludge produced during wastewater treatment. In detail, this chapter includes: (a) wastewater sludge main characteristics, (b) common treatment methods for sludge treatment, and (c) European and US legislation for sludge management.

Techno-Economic Aspects of Vertical Flow Constructed Wetlands

This chapter presents an economical and an environmental evaluation of VFCWs as a wastewater treatment technology. A discussion on the global cost of this technology takes place as also on their cost-effectiveness, environmental benefits and life cycle compared to conventional treatment technologies. Overall, VFCWs possess low investment and minimum operational costs compared to other conventional wastewater treatment methods and represent an environmental friendly technology for wastewater and sludge treatment, which places them among the sustainable treatment technologies.

Treatment of Special Wastewaters in VFCWs

This chapter presents VFCW experiments/applications on treating special wastewaters. These include a variety of industrial (tannery and textile), agro-industrial (dairy and animal farm, olive mill, and winery) wastewater and landfill leachate, as also remediation of contaminated groundwater. These specific types of waters/wastewaters were selected due to the high pollutant loads and toxic substances (i.e., azo-dyes, phenols) they contain. In this chapter, the effects of different design and operational parameters (i.e., pretreatment stages, plant species, and porous media) in pollutant removal efficiency are discussed.

Domestic/Municipal Wastewater Treatment with VFCWs

This chapter covers all the aspects of design, facility layout, operation, and performance of Vertical Flow Constructed Wetlands (VFCWs) for domestic and municipal wastewater treatment. Specifically, the design, construction, and operation part presents: – the basic parameters used in the design of the systems in various countries,