Energy efficiency upgrade: HUBER Drum Screen LIQUID instead of primary clarifier sets new standards at the Vohburg and Kahla wastewater treatment plants

The topic of replacing a conventional primary clarifier with a mechanical screen has already been the subject of numerous publications. HUBER SE was the first company in Germany to take up this new challenge several years ago. The performance of the plants already installed, their reliable operation and the resulting energy savings in the operation of the wastewater treatment plants are already a success story, as the Vohburg and Kahla projects impressively demonstrate.

Vohburg wastewater treatment plant: complete modernisation for energy performance optimisation

Vohburg on the Danube is a town in the Upper Bavarian district of Pfaffenhofen an der Ilm. The town lies on the northern border of the district, around 15 kilometres down the Danube from Ingolstadt. The old town centre is framed by the rivers Danube, Little Danube and Paar. The Ilm also flows along the southern edge of the town. Vohburg could therefore be called a ‘four-river town’

The municipality of Vohburg and its neighbouring areas are growing steadily, but unfortunately the capacity of the wastewater treatment plant is not. As a result, it has become necessary to expand the capacity of the Vohburg wastewater treatment plant from 9,000 PE to 14,000 PE. The scope of the refurbishment includes a fundamental conversion of the process management from aerobic to anaerobic sludge stabilisation. One advantage of anaerobic process management is that up to 30% aeration energy can be saved. The costs for the aeration energy of an aerobic sludge stabilised wastewater treatment plant are one of the largest items in a municipality's budget. For wastewater treatment plants, this means that approx. 60% of the total costs are incurred by the aeration energy of the biological treatment stage. An additional advantage of converting to anaerobic digestion is that 50% of the electricity will be produced by the sewage treatment plant itself in future. 

Many factors are important in order to be able to make this process conversion project a reality. Firstly, the space required for a conventional primary clarifier must be available and secondly, there must be sufficient space to accommodate a digester and a CHP unit. If the wastewater treatment plant does not yet have a sludge dewatering system, a mechanical dewatering system must also be planned.

In the Vohburg project, the technical effort required to build a primary clarifier was not economically feasible. The planning engineering company BBI Ingenieure GmbH and the city of Vohburg therefore analysed the possibility of a mechanical alternative to the primary clarifier. However, the use of the HUBER Drum Screen LIQUID to remove suspended solids from the wastewater flow upstream of the biological stage should only be approved if prior tests with a HUBER demonstration plant show that the HUBER Drum Screen LIQUID achieves the required reduction performance.

HUBER SE therefore provided a test plant for 3-4 weeks. This trial was supervised by the Nuremberg University of Applied Sciences and a HUBER team. The Drum Screen LIQUID had to compare itself with a competitor's belt screen system on this wastewater treatment plant. As the results showed, the HUBER Drum Screen LIQUID was able to demonstrate clear advantages due to its significantly better reduction performance in terms of filterable solids and COD as well as the operational reliability of the system. Despite the very low concentration of filterable solids, the HUBER solution showed a reduction performance of 70%. And in spite of the sometimes high external water input and a ratio of 50% particulate / 50% dissolved COD, a separation performance of 29% total COD was still achieved. An excellent result under these conditions.   

The maximum inflow to the Vohburg wastewater treatment plant is 360 m³/h. The HUBER Drum Screen LIQUID installed there is also designed for 360 m³/h and contributes to relieving the biological stage. In concrete terms, this means a reduction in aeration costs of up to 30%. HUBER Disc Thickeners S-DISC are installed on the wastewater treatment plant to process the sludge from the Drum Screen LIQUID. A digester was installed to utilise the thickened sludge for energy recovery. The digested sludge is then dewatered by a HUBER Screw Press Q-PRESS®. Furthermore, it was necessary to integrate a CHP unit which converts the gas yield from the digester into electricity. As already mentioned, the self-generated electricity can be used for own consumption or fed into the public power grid. Commissioning took place at the end of 2019. 

Reduction of operating costs by 35,000 euros per year

At the time of planning, a reduction in operating costs of 35,000 euros/year was expected for this wastewater treatment plant with an expansion stage of 14,000 PE. As the most important component of the overall concept, the HUBER Drum Screen LIQUID has made a significant contribution to the successful energy-efficient refurbishment of the Vohburg wastewater treatment plant. This project impressively demonstrates how the process conversion from aerobic to anaerobic sludge stabilisation can conserve valuable resources, generate financial added value for the wastewater treatment plant operator and at the same time protect the environment. 

Measures:

• HUBER Drum Screen LIQUID<
• HUBER Disc Thickener S-DISC
• Digester incl. CHP
• HUBER Screw Press Q-PRESS®

Benefits:

• 24 % savings in aeration energy after reaching the expansion stage
• 50% self-supply of electricity for the wastewater treatment plant (approx. 140,000 kWh/a)
• 40 % reduction in disposal costs
• Utilisation of waste heat from CHP for building heating 

Savings due to reduced cost for purchased electricity and for sludge disposal amount to approx. 80,000 €/year

New paths – new opportunities: the energy upgrade of the Kahla wastewater treatment plant

Kahla is a small town in the middle part of the Saale river valley, south of Jena. Kahla is the seat of the Südliches Saaletal administrative community for the southern Saale valley, but is not a member of the community itself. Kahla is famous for its porcelain, which has been produced there for over 150 years.

The Kahla wastewater treatment plant benefited from a European funding programme. In order to improve the energy efficiency of a wastewater treatment plant, process optimisation measures are often carried out for wastewater treatment plants of 8,000-50,000 PE. For small to medium-sized wastewater treatment plants in particular, this means converting the concept of the aerobic sludge-stabilised wastewater treatment plant originally planned for plant sizes to the new concept of the anaerobic sludge-stabilised wastewater treatment plant. From a size of 50,000 PE, wastewater treatment plants are generally only operated with anaerobic sludge stabilisation anyway.

In the Kahla project with a capacity of 15,000 PE, the HUBER Drum Screen LIQUID was integrated into the hydraulic connection line between the grit chamber and aeration via a branch line. For this purpose, an empty basin was used to accommodate the machine. Therefore, no new structure had to be built. Another decisive factor was the higher degree of separation compared to a primary clarifier After all, the wastewater in Kahla is not the conventional type. There is a predominant proportion of soluble COD, which comes from food industry operations. This shifts the ratio between soluble COD and particulate COD in favour of soluble COD. As the design of a primary clarifier is largely determined by the hydraulic load, an unnecessarily large structure must be built to ensure the desired degree of separation of particulate COD (only this can be separated by mechanical-physical processes). This ratio shift plays a subordinate role in the design of an ultra-fine screening system.

The local food industry had significantly increased its production capacity, so that an aeration tank enlargement would have been necessary. However, due to the advantages of this technology described above, the necessary aeration tank enlargement could be avoided by installing an ultra-fine screen. 

This drum screen was planned in such a way that no additional adjustments to the structure were necessary. Only one pipeline into and one pipeline out of the HUBER Drum Screen LIQUID tank was required. An emergency bypass was already in place.

HUBER Disc Thickeners S-DISC are installed to process the sludge from the HUBER Drum Screen LIQUID. A 2-phase digester was installed to process the thickened sludge for energy recovery. Furthermore, it was necessary to integrate a gas turbine CHP unit, which converts the gas yield from the digester into electricity. 

Commissioning took place at the end of 2019. Thanks to the excellent cooperation with the engineering company Arequa, it was possible to develop this extraordinary, innovative and sustainable concept. The citizens will be grateful if these measures for the energy efficiency optimisation of the wastewater treatment plant enable the costs for wastewater charges to be kept stable in the medium term despite rising electricity and sludge disposal prices. 

Measures:

• HUBER Drum Screen LIQUID
• HUBER Disc Thickener S-DISC
• Digester incl. CHP

Benefits:

• Consistent aeration energy despite increased loads (COD / filterable solids)
• 50% self-supply of electricity for the wastewater treatment plant (approx. 146,000 kWh/a)
• 20 – 30% less sludge for dewatering (reduction in disposal costs)
• Utilisation of waste heat from CHP to heat the digester.