The waste sludge from the water treatment process was originally pre-treated by means of the thermal 'Zimpro' process, and dewatered by filter presses, achieving a very high dry matter content. Despite its good dewatering performance, the Zimpro process is known for its high energy consumption, and intensive operation and maintenance requirements. When a thorough renovation of the Zimpro system became imminent, the Water Authority decided to replace Zimpro with a modern sludge treatment line, that produces biogas rather than consuming valuable fuel for thermal treatment.
Witteveen+Bos was awarded the process selection, design and construction supervision of the new sludge line. A system was chosen consisting of thickening, sludge digestion and dewatering in belt filter presses. An evaluation was made of the available options to make best use of the produced biogas. This resulted in the selection of combined heat and power generation that provides electricity and heat both to the treatment process and energy consumers outside the WWTP.
Energy and economic benefits
The biogas from the sludge digestion process is used as fuel in two combined heat and power plants (CHPs), each consisting of a biogas engine driving an electricity generator. These CHPs provide electricity and heat to the WWTP, resulting in a 50 % saving on the cost of energy purchase. The excellent performance of the sludge digestion makes more biogas available than expected. The extra biogas is transported by a gas main to a third CHP in the 'Haagse Beemden' residential area, that provides electricity and heat to the local homes.
A side-stream treatment plant for the rejection water from the sludge dewatering was added to the project at year-end 2013. By separately treating this nitrogen-rich stream, it is possible to save energy in the aeration process and to stop the effluent recirculation.
The new sludge line and side-stream treatment considerably improve maintenance, energy efficiency and operational economics and also contribute substantially to fulfilling the sustainability objectives of the Brabantse Delta Water Authority. The new systems are prepared for future implementation of thermophilic digestion, possibly in combination with thermal pressure hydrolysis. This will result in further energy savings and reduction of the residue streams.
An efficient industrial construction philosophy was adopted in all design steps of the new sludge line. Explicit choices were made regarding the capacity, size, redundancy and materials selection of process elements. The plant lay-out was optimised, aiming at optimum functional relationships, with special attention to the configuration of machines, installations and piping. This has resulted in short transport lines and good maintainability.
The design process also resulted in low construction costs without compromising on the quality objectives of the water authority. The cost reductions were achieved mainly through the simple arrangement of the buildings and the elimination of excess capacity and redundancy. In process situations with downtime for maintenance and inspection, it is still possible to continue processing the sludge.
Sludge digestion and biogas safety
The energy consumption of the sludge digestion system was minimised by the use of mechanical mixing and sludge heating without recirculation. The biogas transport system was greatly simplified. In design and operation much attention was given to the explosion safety of the biogas plant. The monitoring and control effort was reduced by incorporating a high degree of automation. Stringent inspections by an independent body have confirmed the high degree of safety.
The Witteveen+Bos water department has used its standard 3D design package, linked to a Building Information Model (BIM), for the design of buildings and installations. An innovative feature is that the disciplines of civil engineering, electrical engineering and mechanical engineering each work with their own design software on the same project in a central database. The central database records both three-dimensional spatial information and technical features of the parts. This gives enormous synergy between all of the disciplines. BIM has numerous advantages compared with a traditional 2D or 3D drawing, because information can be added to all the components that make up the model. By means of 3D visualisation the draft designs of buildings and installations were presented to the various stakeholders, allowing them to make comments and suggest improvements. This has led to a broadly accepted and improved design in terms of operations, maintenance and safety.
The new sludge processing system was put into service mid-2012. The project will be completed in 2014 with the construction of a new information building; by then the old Zimpro sludge treatment will have completely disappeared.