Water & Wastewater Treatment

In the know Getting to grips with... sludge cake optimisation covered. What factors should be borne in mind when designing the reception facilities on site? A quick turnaround is ideal in relieving trucks / WwTW operators from unnecessary demurrage and reception blockage costs. 'Tip and go' has definite benefits against shovel loaders, albeit that consideration needs to be given to the effect of large loads on the discharge method. Inclined receptions need a high torque to move the equivalent rigid/ articulated load. Ideally, sludge should be discharged on a 'first in first out' basis. Silo discharge and conveying solutions (such as the latest Sliding Frame technology with screw conveyors) that can deliver a constant, precisely metered volumetric flow is critical to AD and thermal hydrolysis systems. This requires properly engineered solutions that take account of the materials' non-free flowing properties. How can the costs of the process be reduced? Switching to a truck-loading silo from RORO container system can result in typical savings of around £10,000 to £17,000 per month, for a 100m 3 truck loading silo compared with an existing 2-3 RORO container system. For a 240m 3 truck loading silo, the saving could be around £27,000 per month. The exact cost saving is largely determined by distance and load volumes from satellite sites. roll-on, roll-off (RORO) containers handling pumped or screw fed cake, and articulated tankers for liquid sludge. More recently, the alternative rigid and articulated trucks are being used for cake transport with up to double the capacity for transporting cake. Existing RORO containers are not completely enclosed, which presents a risk of contamination from foreign objects debris (FOD) which can damage pumps and cause narrow restrictions in downstream plant. They also suffer from an inherent problem of odour control. Why is sludge dewatering important? By dewatering sludge, for example through centrifuge or belt press, slurry of circa 6% dry solids can be dewatered into cake comprising 22% dry solids, representing a 4:1 reduction in volume. This means one truck could potentially be used instead of four for the equivalent mass (without water). This represents a massive potential saving in haulage costs (and carbon footprint contribution) of up to 75%. Dewatering, considerate sludge handling and storage in a silo makes the material more friable, stable, safer and easier to transport and store. What are the considerations in the choice of storage system? Friable sludge (free from FOD), handled without excessive force and with an accurate DS% is what the 28 | MARCH 2016 | WWT | www.wwtonline.co.uk A truck reception at Esholt WWTW, West Yorkshire AAD operator needs to manage the optimum performance of the digester. Security is therefore the first consideration, to prevent the risk of FOD contamination. Secondly, the infeed system (pumped or chain) has an effect on sludge consistency, in terms of storage and handling. Thirdly, the storage system must be large enough to accommodate up to four days' fill, taking account of reduced manpower over four-day-long bank holiday weekends. Why change from a hook li container storage system? The more recent silo storage systems that replace hook li™ containers are more efficient in construction and operation. They can fill larger rigid trucks or articulated tipper trailers. Larger truck-loading silos and truck reception silos enable greater volumes to be handled, reducing the waiting time for a truck delivery, the number of return trips required and importantly ensuring greater security from FOD. There is also a reduced cost of container rental and shunting, as a larger standard lorry can service more locations during normal working times. Truck Loading Silos (TLS) offer a quicker turnaround (an estimated 15 mins loading as opposed to a 30+ mins shunting time) and reduce physical strain on drivers, who remain in their cabs during loading. TLSs also reduce odour, as silos are odour controlled whereas hook li™ containers are only A truck loading silo at Harwich, Essex

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