Water & Wastewater Treatment Magazine
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Precious metals in sewage sludge The current regulatory focus on the commercialisation of sewage sludge makes it the ideal time to investigate how to recover metals including gold, silver and platinum from wastewater www.wwtonline.co.uk | WWT | OCTOBER 2018 | 31 DR ALAA HAMOOD, LECTURER AND COURSE LEADER IN CIVIL ENGINEERING UNIVERSITY OF WOLVERHAMPTON The Knowledge Sludge treatment methods of raw sewage sludge include applying physical and chemical processes to reduce the water content in preparation for the anaerobic digestion process to gener- ate electricity (Figure 1). The by-product of the digestion process is a stabilised sewage sludge which is currently used as fertilizers for agricultural applications. The chemical composition requirements of stabilised (treated) sewage sludge for agricultural applica- tions focuses on the presence of nitrogen, phosphorus, and organic matter, with safe levels of Potentially Toxic Elements (PTE) including chromium, zinc, copper, nickel, cadmium, lead, mercury, molybdenum, selenium, arsenic and fluoride. However, sewage sludge incor- porates other valuable materi- als, and these are currently being wasted in the farming industry. It is time for the sector to explore innovative practices and to recognise the value of raw sewage sludge as a resource rather than a waste product. This article proposes an additional step in the treat- ment process of raw sewage sludge to extract existing valuable materials. These valuable materials include any substances that have high commercial value and W ater and wastewa- ter companies in England and Wales recently published data on the quantities of raw sewage sludge generated at their treat- ment plants; it was reported that between 2016-2017 the sector generated 1,368,350 tonnes of dry solids (TDS) of raw sludge, equivalent to 41,279,894 tonnes of wet raw industrial applications, such as heavy metals, noble metals, and other industrial materi- als. The valuable materials enter the wastewater stream from different household and industrial sources including cleaning products, cosmetics, medication, human waste, storm water, water treatment chemicals, and unauthorised industrial wastewaters. There is a potential for vast quanti- ties of valuable materials to be extracted from sewage sludge. A pilot study was carried out at the University of Wol- verhampton to identify and quantify precious materials in raw sewage sludge. Five sam- ples of wet raw sewage sludge were collected from a sewage treatment works in the UK be- tween October and December 2017. The sludge samples were in the form of thick liquid that contained between 2.8 to 3.3% solid content of total mass. Collected samples were filtered on 5-13μm cellulose filter paper to separate the liquid, which was later analysed for its chemical composition using Inductively Coupled Plasma mass spectrometry (ICP). Rep- resentative dry solid samples were also obtained by drying the wet raw sewage sludge for 24 hours using a water bath. The dry solid samples were dissolved in acidic solution sewage sludge. This informa- tion was made available to encourage investors, within and outside of the wastewater sector, to devise any commer- cial opportunities that can be created through the optimal use of this material. This will also help the sector to meet the objectives of Ofwat's PR19 and Water 2020 vision. The current treatment