Restoration of Pyritic Colliery Waste with Sewage Sludge in the Midlands Coalfield, England. United Kingdom

Abstract

A trial was set up in 1990 in the Midlands coalfield in the United Kingdom (UK) to evaluate the use of sewage sludge to revegetate colliery waste tips containing 1 -2% sulfur as iron pyrites.The rate of sewage sludge application is currently restricted by legislation and codes of practice to maximum concentrations of potentially toxic elements (copper, nickel, zinc, etc.) in the soil or waste after application. Following this guidance, an application rate of 250 mt/ha dry solids was applied at the trial site. At this rate, the colliery waste became extremely acidic pH <4.0. From experience elsewhere, much higher levels have been found to be necessary to control acidification in the absence of other measures or treatments.In view of the restriction on the amount of sewage sludge that can be applied, it is recommended that the current practice of covering fresh colliery wastes with soil or low sulfur spoil to a minimum depth of 0.45m is continued in the UK. Where this is not possible, the sludge must always be applied with sufficient neutralising agent to control the potential acidity. If the acidity cannot be maintained above pH 5.0, the guidelines do not permit the application of sewage sludge.Additional Key Words: pyritic wastes, acidity, sewage sludge, soil cover. Pyritic Colliery Waste in the UK CoalfieldsCurrent coarse washery discard colliery wastes in the UK average between 0.2% and 2.0% sulfur in the form of iron pyrites (Glover 1984), although locally much higher levels are frequently recorded in the more pyritic wastes. The most pyritic wastes occur in the Midland coalfields of Nottinghamshire and Yorkshire and in the western Scottish coalfield. Modem waste tips are formed using earth-moving plant which, through compaction, has largely eliminated the risk of spontaneous combustion and reduced the oxidation of pyrite within the tip. There remains the potential for acid generation in the final surface layers, which generally are required to be uncompacted and permeable for the purpose of revegetation.While the modem construction methods for waste tips have much reduced the risk of acid mine drainage, there is still concern about surface acidification and subsequent pollution of watercourses and bodies by acidic runoff containing soluble salts and metals. Also, surface acidification can be of significance for restoration by causing poor vegetation growth and, in extreme cases, loss of vegetation. Both conditions can be time consuming and costly to rectify. Therefore, the prevention and treatment of acid potential and acidity has been, and continues to be, a priority in modern colliery waste restoration in the UK. https://doi.org/10.21000/JASMR94030324Practices for Treatment of Acidity Use of Neutralizing AgentsTraditionally, the method to control acid generation and to treat existing acidity in the surface layer of colliery waste has been to apply neutralizing agents, such as limestone (CaC03) or lime (CaO), as used in the management of agricultural soils (ADAS 198 I).Where the potential for aci...