Factors Affecting the Longevity of Subsurface Horizontal flow Systems Operating as Tertiary Treatment for Sewage Effluent

Compared with other forms of wastewater treatment, horizontal flow reed beds require very little by way of operational and maintenance input. Unfortunately, in practice this frequently results in them receiving little or no attention at all. Tertiary treatment reed beds are not a “fit and forget” solution, but they are often treated this way because they are very forgiving and abuse-tolerant. Severn Trent Water Ltd. installed their first tertiary reed beds in the late 1980s and the company now has almost 350 units, mainly at village works, to ensure that the effluent from existing secondary treatment units is polished to a produce a high-quality effluent well within the standard. ARM Ltd. has been designing and constructing reed beds for 20 years and has units on more than 250 sites, a large number of these are on Severn Trent Water sites. After a number of years a few of these tertiary reed beds have deteriorated to the extent that they are close to failing to comply with the regulator’s requirements. Severn Trent Water Ltd. recognised this situation and has committed a budget for a programme of reed bed maintenance. Initially a budget was allocated to refurbish the reed beds in the poorest condition. Resources are now being directed to regular maintenance of all reed bed sites. Between November 2002 and October 2005, ARM personnel carried out more than 300 primary surveys of reed bed sites in order to determine the condition of the reed beds and to establish what if any remediation was necessary. Since September 2004, when the first part of this chapter was presented at the IWA Avignon conference, more than 200 secondary surveys have been carried out when staff visited sites to carry out remedial work. The work is being further extended by use of ground penetrating radar (GPR) to try to assess the degree of clogging in different beds and in different parts of the beds. The preliminary results from this study are included. A series of tertiary surveys will be made in the future when the sites are revisited as part of a rolling maintenance contract.