Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | MAY 2014 | 13 of tanker movement that, although local residents recognise is neces- sary, can also create a lot of local inconvenience." "In the past we tended to put settlement tanks in where flow was pumped, with some form of straining to remove visible solids. However, that did not have a real effect in reducing ammonia levels or help improve BOD [biological oxygen demand] and oxygenation readings. We wanted to find a way to treat the flows to a higher standard on site, so we developed these new mobile treatment tanks from scratch." It is a three-fold treatment pro- cess, he says. "By using these mobile tanks we can remove some BOD and ammonia and end up giving 100 per cent saturated dissolved oxygen. "Basically it's the same technol- ogy as used at our wastetreatment plants, with a support media, such as furnace slag, clinker or plastic blocks that we use in wastewater treatment inside our submerged aerated filters, as a support mecha- nism to culture bacteria. With the mobile units we put a sewage feed onto the top of a plastic filter, water percolates down this, with the bacte- rial 'slime' digesting the BOD and ammonia." "The most difficult challenge was to try and get distribution right at the top of the unit. Working with Atac we designed a simple set of jets spraying water onto a series of plates that create a plume spray pattern. It's important to get an even distribution over the whole of the filter," Palmer says. It is a full treatment process. "The units tend to treat quite weak groundwater diluted sewage," Palmer says. The initial development plans had the units sized for 10 to 20 litres a second continuous flow, thus handling over 864 cubic metres a day. In terms of tankering, that is twenty five tanker loads. "We estimate that each tank can improve the quality of the water treated by as much as 40 per cent. Importantly, the tanks also allow more oxygen to be applied during the treatment process, helping to main- tain the natural balance of rivers and streams for local wildlife. We are tri- alling this innovative new approach, with a view to possibly using it on a wider scale in the future." The main driver for develop- ing the mobile treatment units was customer convenience, maintaining services by removing excess flows The most difficult challenge was to try to get distribution right at the top of the unit • Tank facts ● Each stainless steel tank is 3 metres high, 2.4 metres wide and 2.4 metres deep ● Each has six nozzles which distribute the water over a biological filter, with a maximum total throughput of 20 litres per second ● The filter is made of standard fixed film corrugated plastic and has a surface area equivalent to half a football pitch. Once 'seeded', it contains billions of waste-digesting bacteria ● It is protected from blocking by a layer of sponge to catch debris. This is changed on a regular basis ● Flow passing through the filter is collected on a benched false floor that discharges by gravity • Challenges 1 ● Design a system to operate independent of external power sources ● Treat wastewater at flow rate of 10 to 20 litres ps ● Reduce biological oxygen demand ● Reduce ammonia content ● Improve oxygen satu- ration ● Portability for use on roadsides or in fields • Perspectives Suppliers ● The project was in- spired by Southern Water, with initial design by Nigel Palmer and fabrica- tion by ATAC Solutions. ● There was nothing available on the mar- ket that met Southern Water's specific needs, especially the require- ment not to have an external power sup- ply. Palmer produced some initial designs and involved ATAC, already a framework supplier for Southern Water and with expertise in submerged aerated flooded filters. "They jumped at the chal- lenge and their location near us also meant we could see fabrication of the units at all stages," he says.

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