In paper mills, either only aerobic processes or a combination of aerobic and anaerobic technology is used for biological wastewater treatment. There is much potential for lowering costs, especially in the integration of an anaerobic high-load stage. In this case, a high-capacity anaerobic reactor such as the R2S or the E2E from Voith is located upstream of the aerobic treatment stage. A reactor of this type breaks down 60%-80% of the load, as organic materials in the absence of oxygen are transformed into biogas within a few hours. This gas is available as an additional energy source and can be used for steam or power generation. In paper mills with a daily production of around 1,400 metric tons, up to 780 Nm³ methane per hour can be generated from the anaerobically treated wastewater. At their best, these wastewater treatment systems actually operate with a positive primary energy balance. The energy costs of the paper mill can thus be lowered by up to 5%.

Voith offers two types of anaerobic reactors: the R2S reactor, which is especially suitable for wastewater containing a lot of calcium with a high COD content, and the E2E reactor which is suitable for diluted low-calcium wastewater. The first anaerobic reactor which Voith installed in the Italian town of Lucca in 2007 is an R2S reactor. From the beginning the reactor has always worked very well as the operator confirms to Voith technicians who regularly service it on site.

Meanwhile, a further 25 models of this reactor type have been sold all over the world and 14 units of the E2E reactor are already being used worldwide.

At its newly constructed mill in Partington, Great Britain, the paper manufacturer Saica has chosen a wastewater treatment system with an R2S reactor from Voith. That one reliably breaks down approx. 80% of the load. Each day up to 25,000 Nm3 of biogas is produced. The secret behind this success is the rugged design and very exact adaptation to the specific conditions of the paper mill, which produces 1,400 metric tons of corrugated board base paper daily from 100% recovered paper and generates up to 6,000 m3 of wastewater.

The start-up at the Partington site was difficult, because the coordination of processes was not as simple as initially assumed. Nonetheless, the goals that were set were achieved. The customer is very satisfied today as Federico Asensio, the Deputy General Manager and R&D Group Director at Saica, is happy to confirm: “For a customer, it is important to rely on the technical know-how and commitment of the supplier, especially if there is an unexpected situation. Thanks to the strong collaboration between Voith and Saica, the initial problems have been solved and the wastewater treatment system finally achieved our original expectations. Its operation is smooth and stable.”

This constant stable behavior is one of the advantages of the integrated approach. Axel Gommel, a Voith engineer, confirms from a manufacturer’s perspective that integrated process designs such as those at Saica generally lead to noticeably more stable operating behavior than is seen with conventional systems. There are other advantages: there are fewer dissolved substances in the circulating water during production, the legal limit values for the wastewater are reliably adhered to with low personnel requirements and the excess sludge accumulated is reduced by 70%. Depending on the paper grade, it is possible to reduce the fresh water consumption of the paper mill by reusing purified water.