Complex business of equipment choice
Cake product hanleability and new legislation are the latest parameters affecting dewatering decision-making.
For the last two decades, sewage treatment works operators have used belt presses for general-purpose sludge dewatering. Historically, volumetric throughput has been the main selection driver. However, in the new economic climate, where whole life costs are playing a large part in decision making, water companies need to consider new and additional parameters, in particular cake product handleability and stackability. And while water companies are improving facilities at medium and large sewage treatment works (STW), legislation is driving demand for dewatering equipment at small STWs too.
No two sludges the same
Sludge is a complex material reflecting both the source of the wastewater and the process system that produced it. It is a combination of organic sludge, of imprecise and variable composition, and inorganic sludge that can be classified in terms of chemical composition and particle size. Water within the sludge can be described in three ways – free, intercellular and intracellular. The first two types are readily removed using mechanical thickening or dewatering technology, but intracellular water contained within the cell walls can only be released using heating, freezing or electro-induced forces.
In recent years, more biological sludge has been produced. At the same time, and partly as a result of this, wastewater has become more dilute as water companies adopt frequent desludging regimes, and reduce consolidation times in settlement systems. Hence, sludge dewatering demand has increased. This has stretched existing capacity to the full. Regulatory pressures and market drivers have also diminished disposal and recycling routes for sewage sludge. This has caused the water industry to take a broader view and the improvements made have resulted in a higher quality end product, sludge cake, with greater stackability and dry solids content.
Odour and maintenance are two more areas where the belt press scores highly as it produces significantly less odour nuisance when cake is stored in a cake pile, compared to other dewatering methods. Minimal downtime is a bonus for STW operators and Simon-Hartley has various packages designed to keep plant operational and maintain and increase process performance.
The belt filter press provides a means of continuously dewatering sewage sludge into a handleable cake. Using three distinct stages, the machine firstly conditions and flocculates the wastewater, agglomerating the solids using a polymer. Next, gravity allows the free water to drain away over a continuous, horizontal, porous, filter belt. The remaining sludge solid fraction is sandwiched between two tensioned filter belts and travels round a series of compression and shear rollers.
Choosing the most appropriate belt press for each situation depends on understanding the factors affecting the performance of the machine and how each can be improved:
- Effective conditioning and flocculation depends on ensuring the polymer is well matched to each site-specific sludge. Increased polymer use will improve dewatering efficiencies and filtrate liquor quality, but will increase OPEX costs. So a balance must be sought. Variable energy non-clogging mixers should ideally be used to optimise polymer usage.
- Gravity drainage is induced free drainage in the low-pressure zone. Inverting the sludge during this process maximises free drainage and filtration.
- Compression is the parameter that can most affect sludge cake quality. Pressure should progressively increase through the rollers and belt tension varied to increase dewatering forces on the filter cake.
Better use of space
Designs which increase the available filtration area have been sought in recent years. Modern belt presses make better use of the available space within the structural framework, and this has greatly increased the overall filtration area. Simon-Hartley’s Klampress provides cost effective dewatering, resulting in a consistent dry sludge cake. Some relatively new treatment processes generate 100% biological type sludge, using a combination of a gravity belt thickener (GBT) and belt press technology in a single unit. The 3 belt Klampress is designed specifically to meet this demand. The Aquabelt offers highly efficient filtrate drainage via the higher belt speeds while the slower speeds and high performance of the Klampress give high cake dry solid concentrations.
Manufacturers are now producing presses that produce a much higher quality sludge cake. The machines have a greater number of rollers than would have been used in the past, operating at considerably higher tension pressures. To cater for these increased filtration pressures, design engineers have had to make mechanical enhancements. The structural frames of the belt presses are now designed to withstand increasingly higher forces. In addition, improved high torque drive assemblies have been fitted to accommodate the increased hydraulic pressure.
Belt press technology is now fully automated, needing minimum operator input, and significant improvements have been made in PLC control instrumentation and information communication technology, such as linking to SCADA systems.
High performance belt presses are available that combine the above parameters cost-effectively. Modular construction of the Simon-Hartley belt press designs means that upgrades can easily be carried out requiring low capital investment. Thus presses can be optimised to changing incoming wastewater situations. Responding to legislative pressures and market demands Simon-Hartley has developed a compact belt press – ‘mini’ Klampress – to provide a simple dewatering unit.
When operators make the final decision on which equipment to buy, they must ensure that it will not be working at the top end of its performance limits, so that greater process efficiencies can be gained. Longer residence within the belt filtration section will maximise dry cake solids content and reduced sludge flow rates onto each machine will optimise polymer use.
‘Following these guidelines when using a belt filter press will cut down-time as fewer process shutdowns will result, and running costs will fall with minimal use of polymer,’ argues Peter McLoughlin. ‘When applied correctly, the new generation belt filter systems will typically have between 30-50% lower polymer consumption rates when compared to alternative dewatering technologies such as old generation belt presses or centrifuges,’ he said. ‘All of which adds to a much lower whole life cost.’