Managing sludge for centralised AAD
Sludge quality, storage and transport have become critical factors driving performance and efficiencies. Mark Neal, of Saxlund International, explains.
Anglian Water needed to meet Ofwat’s obligation for it to provide enhanced digestion for 80% of its raw sludge production and to increase total renewable electricity output from biogas to 86.8GWh/year by 2015.
In considering its choice of technology – and its technology partner – Anglian Water sought tried and tested, robust solutions for storing increasing volumes of sludge at the satellite sites that feed its Colchester, Cliff Quay and Pyewipe hubs.
As part of a multimillion-pound AD scheme, Saxlund completed a series of projects for Anglian Water in 2014. The projects included the construction of raised storage silos, specially designed to allow de-watered sludge to be directly loaded into trucks below for transportation off site.
This new technology enables a much faster reception with less odour and less risk of contamination by foreign objects which could cause damage to plant and equipment or blockages downstream.
Population pressure, more restrictive discharge consents and a trend to reduce spreading to land, means sewage sludge volumes are increasing. As a consequence, consultants, contractors and operators increasingly have to look for the most efficient methods of dewatering, storage and transportation of sludge for treatment.
At the processing hub, anaerobic digestion (AD) process operators are looking for consistency in terms of calorific value, Dry Solids (DS%) content, temperature, cake rheology (the cell structure of the cake) and homogeneity, to ensure process uniformity and efficiency and ultimately optimum gas yields.
To do this, efficient operators are endeavouring to manage the mix of consistencies from their various satellite sites to balance the infeed to their digestion process. And to add a level of urgency, this needs to be achieved within shortened timeframes to benefit from the optimum rheology and life of their sludge cake.
Where the sludge has traditionally been produced in settlement tanks at 6% DS and pumped into tanker for shipment off site, operators are weighing up the cost / benefits of investing in centrifuges and downstream cake handling equipment against the resultant volumetric and mass, ie transportation, storage and handling of Slurry @ 6% DS compared with Cake @ 22% DS.
The indirect benefits are wider than first thought, firstly a reduction in “live storage” volume, plus fewer container movements, volume for volume. Additionally, this leads to a transition from container rental to dedicated tippers and a reduction in carbon and transportation costs, which means a 4:1 ratio of fewer road miles for the equivalent movement. Further benefits include an overall reduction in haulage labour costs with storage facilities able to manage weekend volumes without shunting of containers, quicker turnaround time for sludge lorry loading perhaps 15-20 minutes complete and minimised odour emissions.
It also means that unnecessary demurrage and reception blockage costs are minimised, as truck-loading silos (TLS) offer a quicker turnaround (an estimated 15 minutes loading as opposed to 30 minutes plus shunting time) and reduced physical strain on drivers, who remain in their cabs during loading. They also reduce odour, as a TLS is odour-controlled whereas hook lift containers are only covered.
Although, we do not have specific figures, industry sources suggest that the cost saving of transporting sludge stock to hubs is largely determined by distance and load volumes from its satellite sites. Typically, the saving is around £10,000 to £17,000 per month for a 100m3 truck loading silo compared with an existing 2-3 RO-RO (roll-on roll-off) container system. For a 240m3 truck loading silo the saving could be in the region of £27,000 a month.
Truck-loading silos (TLS) using the latest sliding-frame technology and screw conveyors enable greater volumes of sludge to be handled in a shorter space of time than traditional hook lift container storage systems. Ideally sludge should be discharged on a ‘first in first out’ basis, important when handling materials with a shelf-life. A sliding frame mechanism can be used to undercut the full section of the contents of the silo to prime a screw trough. Since the whole section is undercut, the pile section discharge is termed “mass-flow” (like laminar flow in a pipe) from the silo.
Truck loading silos can fill larger rigid trucks or articulated tipper trailers, reducing the waiting time for a truck delivery, the number of return trips required.
They can ensure greater security from foreign object debris (FOD) contamination which can damage pumps and cause narrow restrictions in downstream plant.
It is worth mentioning that, whilst some storage silos may be cheaper, they may be of an inferior design and quality which can result in problems with bridging and discharging. Storage systems need to be large enough to accommodate up to four days’ fill, taking account of reduced manpower over four-day-long bank holiday weekends: cheaper silos have been known to collapse when they cannot properly contain the weight of the sludge.
Dewatering sludge into cake not only makes sludge easier to transport and store, it also helps to make it more friable, stable, and safer to manage, while minimising odour. For those operators already using pumped cake, the move to chain or screw handling and storage benefits the cake composition, making it more AD-efficient and easier to spread by farmers.
Friable sludge, free from foreign objects, handled without excessive force and with an accurate DS%, is what AD operators require for optimum performance. A constant, precisely metered volumetric flow of friable sludge is critical to anaerobic digestion and thermal hydrolysis systems and requires properly engineered solutions that take account of the materials’ non-free flowing properties to avoid the risk of blockages.
A measured flow ensures a homogenous discharge to the next process. The thermal hydrolysis process involves heating sludge to destroy any pathogens and degrading cell structures to create better flowing, good quality sludge making it easier for bacteria to digest and to produce methane.
Selecting the right technical, environmental and economic solution, not only for water companies’ current requirements but also for their anticipated future needs, makes sound sense.
Extensive experience and a full understanding of the processes and risks are essential. It is also important for an industry partner to provide a complete, end-to-end quality solution for sludge storage and handling which includes design and construction, commissioning and maintenance.
Technological innovations such as these in use by Southern Water and Anglian Water are completely changing the way the wastewater industry deals with converting sewage sludge to energy. In order to maintain economy of scale, efficiencies in advanced anaerobic digestion (AAD) at each hub, indigenous waste needs to be topped up, which means transferring sludge from satellite sites to central hubs.
Robust bulk material handling solutions and advanced digestion systems are now commonplace in reducing fuel costs and carbon emissions whilst improving the efficient processing of wastewater and sewage sludge.
Mark Neal is water specialist at Saxlund International.
This article first appeared in the December 2016 issue of WET News.
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