Improving Sludge Handling and Quality for Centralized AD

    Cover Photo Courtesy of Saxlund International

    Today, forward thinking water companies are exploiting sludge as an important fuel, indeed many have well developed sludge strategies, building advanced anaerobic digestion (AAD) plant and thermal hydrolysis infrastructures to convert sludge to energy. In the UK, and other parts of the world, centralized AD processing hubs, fed by a network of outlying satellite stations, have become a popular model. In the process sludge quality, storage and transport have become critical factors driving performance and efficiencies. 

    “…dewatering sludge to produce cake not only makes it easier to transport and store, it also helps to make it more friable, stable, and safer to manage, while minimizing odor”

    As this trend continues and sewage sludge volumes grow, operators increasingly have to look for the most efficient methods of dewatering, storage and transportation of sludge for treatment. At the processing hub, 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, efficiency and ultimately optimum gas yields. To do this, efficient operators are endeavoring 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. Sludge has traditionally been produced in settlement tanks at 6% DS and pumped into tankers for shipment off site. But dewatered sludge at 22% DS significantly changes the transportation, storage and handling issues and is an increasingly attractive option. The indirect benefits are wider than first thought. Firstly there is significant 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. Typical savings through sludge cake optimization dependent on application can be estimated at around USD13,000 to USD22,000 per month for a 100m3 truck loading silo compared with an existing 2-3 RORO container system.  And around USD35,000 per month for a 240m3 truck loading silo. Further benefits include an overall reduction in haulage labor costs with storage facilities able to manage weekend volumes without shunting of containers, quicker turnaround time for sludge lorry loading, and minimized odor emissions. It also means that unnecessary demurrage and reception blockage costs are minimized, as truck-loading silos (TLS) such as those developed by Saxlund International offer a quicker turnaround (an estimated 15 minutes loading as opposed to a 30 minutes plus shunting time) and reduced physical strain on drivers, who remain in their cabs during loading. They also reduce odor, as a TLS is odor-controlled whereas hook lift containers are only covered. Truck-loading silos (TLS) using the latest Saxlund 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. Sludge is 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, and the number of return trips required.  Foreign object debris (FOD) contamination, which can damage pumps and cause narrow restrictions in downstream plant, can also be reduced. 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 holiday weekends: cheaper silos have been known to collapse when they cannot properly contain the weight of the sludge. 

    “A constant, precisely metered volumetric flow of friable sludge is critical to anaerobic digestion and thermal hydrolysis systems”

    So, dewatering sludge to produce cake not only makes it easier to transport and store, it also helps to make it more friable, stable, and safer to manage, while minimizing odor. 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. A constant, precisely metered volumetric flow of friable sludge is critical to anaerobic digestion and thermal hydrolysis systems, requiring 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. Selecting the right technical, environmental and economic solution, for a water companies’ current requirements and future needs, makes sound sense. But extensive experience and a full understanding of the processes and risks are essential. Two key players in the UK are Southern Water and Anglian Water. Both have deployed Saxlund sludge cake handling and silo solutions to optimize sludge transfer to centralized hubs in support of their advanced AD infrastructure. As the race to build more centralized AD facilities continues we see growing demand for robust bulk handling and storage solutions across the industry to support quality sludge dewatering and processing. Experience will of course always count, and choosing the right engineering partner will be essential. Economies of scale and the drive for ever more efficient and centralized AD processes is likely to see increased demand for robust bulk material handling solutions across Europe and beyond. Of course, experience counts so choosing the right partner to improve sludge handling and quality will be essential.

    Mark Neal
    Biosolids Materials Handling Sales Manager
    Saxlund International Ltd.