Water & Wastewater Treatment

In the know Technically speaking: wastewater treatment Grit under the microscope. Contrary to standard assumptions, particles are frequently smaller than 200 microns and irregular in shape 44 | SEPTEMBER 2015 | WWT | www.wwtonline.co.uk 2008) make assumptions about the nature and form of grit. Similar to the current USA standards (Water Environment Federation Method of Practice 8 and Metcalf and Eddy) they make some key assumptions for 'convenience'. These define grit as a clean silicate sand particle of above 200 microns in size, which has a uniform, smooth spherical shape, and a specific gravity (SG) of 2.65. Based on these assumptions, the standards have established an all-purpose particle settling-out rate depending on flow rate, which defines conventional treatment and equipment design parameters. Conventional grit removal equipment is designed to remove up to 95% of this size and shape of grit. That is - if working at peak design efficiency. Grit size matters The trouble is, the sampling we are conducting at a number of plants in the UK is consistently showing that this cosy picture of grit is wrong. Wastewater grit sampling underway at one of Hydro's first UK pilot sites Overall, we are finding that around 60% of grit is smaller than 200 microns, and in some places an even greater proportion than that. The indication is that most grit removal systems may actually be removing as little as 20% to 30% of the incoming total grit load. From our experience working with more than 200 installations in North America we know that, in fact, the majority of particles can be in the 106 micron to 75 micron range, or even smaller. Grit shapes are seldom spherical, and shape significantly affects the particle settling rate. Actual particles are frequently rough in texture, which allows even smaller particles and organics to bind to them. The particles frequently carry organics, such as soaps, fats, oils and grease, which expand the surface area or clump together, and thus significantly affect the settling rate. Conventional equipment is designed around notional rapid settling rates seldom actually seen in raw wastewater, so does not trap slower settling particles, i.e. the smaller and larger, non-round, low density particles that form the majority of the grit intake. False assumptions about grit are exacerbated by standard industry inlet channel sampling techniques that sample from the bottom of the channel, rather than from across the whole channel depth. Advanced Grit Management In the USA there is growing acceptance and support for grit removal techniques that are able to remove grit particles 75 microns and larger. They focus on methods to remove the 106 micron to 75 grit particles, depending on the needs of the plant, while at the same time also removing larger particles above 106 micron. Behind the successful deployment of these techniques is an approach which Hydro calls Advanced Grit Management (AGM). The basis of AGM is sampling inlet waters to gain an accurate profile of the incoming grit, and then recommending and tailoring a combination of technologies to meet those conditions for an individual plant. The premium collection technology uses a stacked tray design called HeadCell, which uses a large surface area and short settling distances to capture fine particles. HeadCell achieves up to 95% removal of all grit 75 microns or larger. To guarantee these levels of performance, HeadCell is accompanied by effective washing and dewatering technology, so that all organics are effectively removed while preventing smaller particles being resuspended. GritCup washer and classifier and SpiraSnail dewatering units have been developed for optimum integration with HeadCell. The effects of high-performance grit collection on the typical wastewater treatment plant operation vary from plant to plant, but are usually considerable and can be calculated for every plant. Hydro International is currently undertaking sampling at WWTPs across the UK and has a pilot unit available to install on site free of charge; it is also discussing opportunities with UK water companies to partner a full equipment trial for a HeadCell system. With innovation high on the agenda, it looks like grit could turn out to be an unlikely game changer for the UK water industry.

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