120019865_ESE38_06_R1_032403
JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH Part A—Toxic/Hazardous Substances & Environmental Engineering Vol. A38, No. 6, pp. 1073–1085, 2003
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Removal of Total Suspended Solids from Wastewater in Constructed Horizontal Flow Subsurface Wetlands
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T. Manios,1,* E. I. Stentiford,2 and P. Millner3
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School of Agricultural Technology, Technological and Education Institute of Crete, Heraklion, Crete, Greece 2 School of Civil Engineering and 3School of Biochemistry and Molecular Biology, Leeds University, Leeds, UK
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ABSTRACT
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Subsurface horizontal flow experimental wetlands (reed beds), were designed and built based on a combination of two design methodologies, that of the WRc and Severn Trent Water plc (1996) and that of the USA, EPA (1988). Four different growing media were used with a combination of top soil, gravel, river sand, and mature sewage sludge compost, to determine the best substrate for total suspended solids (TSS) removal. Eight units were constructed, two for each growing media. One bed for each pair was planted with Typha latifolia plants commonly known as cattails. Primary treated domestic wastewater, was continuously fed to the beds for more than six months. All eight beds performed very well. The best performance was achieved by the gravel reed beds with an almost constant removal rate above 95% and an average effluent concentration of less than 10 mg/L. Soil based beds containing top soil and sand, managed to reach values of removal around 90%. The wetlands containing compost in their substrate, produced an effluent with average concentration of less than 30 mg/L and a percentage removal between 80% and 90%. As expected, there was no significant difference in the performance of planted and unplanted wetlands.
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*Correspondence: T. Manios, School of Agricultural Technology, Technological and Education Institute of Crete, Heraklion, 71500, Crete, Greece; E-mail:
[email protected].
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1073 DOI: 10.1081/ESE-120019865 Copyright & 2003 by Marcel Dekker, Inc.
[24.3.2003–8:33am]
[1073–1086]
[Page No. 1073]
1093-4529 (Print); 1532-4117 (Online) www.dekker.com
I:/Mdi/Ese/38(6)/120019865_ESE_038_006_R1.3d
Journal of Environmental Health and Science (ESE)
120019865_ESE38_06_R1_032403
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Manios, Stentiford, and Millner Key Words: Total suspended solids; Wetlands; Reed beds; Sewage sludge; Compost; Gravel; Sand; Wastewater; Typha latifolia.
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INTRODUCTION
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Constructed wetlands or reed beds are claimed to be low cost, low technology systems able to treat a variety of wastewaters. In Europe, such systems have been successfully used for treating domestic sewage for small communities, of less than 2,000 people equivalent.[1,2] The main parameters affecting total suspended solids (TSS) removal in a subsurface flow reed bed (SF) are the hydraulic (structural) and microbiological characteristics of the substrate. Organic solids removal is achieved mainly through filtration, followed by either aerobic or anaerobic microbial degradation on the surface or within the media respectively. According to Davies and Cottingham[3] the majority of the solids, almost 75% of the original concentration, existing in wastewater are retained in the first third of a SF bed. Similarly, Zachritz and Fuller[4] suggested that 60% of the TSS were removed in the first one third of a SF reed bed. In SF systems the microorganisms responsible for the degradation of organic matter (whether soluble or suspended), are generally associated with slimes or films that develop on the surfaces of soil particles, vegetation and litter.[5,6] Originally the use of soil that had been extracted from the site during the bed construction was proposed as a substrate, to reduce the cost of construction. However, solids in the wastewater easily block soil’s pore spaces reducing the hydraulic conductivity.[7,8] Plant roots failed to increase or stabilize the hydraulic conductivity of the soil as expected.[9–11] As a result, the flow in a significant number of SF systems is over the surface which negates many of the perceived design advantages. In order to avoid clogging when using fine media, many engineers started to use larger rock sizes (10–15 cm) with larger void spaces which would also offer less resistance to flow over long distances. This approach did not solve the problem. The vegetation, which was used in these systems, normally grows in soil and the root network could not develop properly in the large void spaces. In addition, the larger rocks provided a smaller surface area for the support of microbial growth compared to the smaller rock sizes. Considering all these factors it was proposed that smaller size rocks (
