Clean separation achieved

Sewage sludge is prepared for thermal processing by means of a centrifuge. A comprehensive set of sensors enables precise monitoring of the plant, both on site and remotely. Moreover, thanks to IO-Link the installation, commissioning and service processes are particularly efficient.

Almost 2 million tonnes of sewage sludge annually are produced in around 10,000 sewage treatment plants in Germany. While previously most sewage sludge has been used as fertiliser in agriculture, more and more sludge is now being recycled thermally for environmental reasons. Legal requirements in Germany require that all sewage sludge must be used for phosphorus recovery in mono-incineration plant from 2029 at the latest.

However, before sewage sludge can be used as fuel in the first place, its solid contents must be separated from the high water content - which makes up over 97 per cent of the mass.

This is where the company ISV-Umwelt from Langelsheim in Lower Saxony comes into play. Founder and Managing Director Sven Penkwitt explains: "We build mobile biomass dewatering plants and rent them to our customers either on a temporary or permanent basis.

One of these customers is the municipal sewage treatment plant in Diepholz where ISV-Umwelt sets up one of its plants for a one week period about four times a year. "In this period, around 72 tonnes of dry matter, which can then be used for thermal recycling, are separated from around 2400 cubic metres of digested sludge that accumulates in large basins over a period of three months."

The two main bearings of the centrifuge are subject to extreme forces when the heavy drum spins the sometimes inhomogeneous sewage sludge at enormous speed. Therefore two VVB vibration sensors have been screwed into each bearing. They permanently monitor the vibration behaviour.

A vibration pattern that deviates from the norm allows to quickly detect whether the mass cannot be processed, i.e. separated, correctly because e.g. the consistency or viscosity is not right or coarse lumps get into the centrifuge. Then the service staff can intervene and, for example, reduce the speed of the centrifuge to prevent damage.
Wear on the drum bearings is also detected at an early stage via an increasing vibration amplitude and reported as a maintenance signal. What is more, the vibration sensor has an integrated temperature sensor. Increasing temperature values indicate increased friction due to wear.   "The vibration sensor allows us to detect early on when a bearing needs to be replaced. We can assess the urgency and, at best, plan the maintenance work on the affected plant to take place between two assignments at the customer site. In any case, we are always in the know. We can reliably prevent or at least reduce to a minimum any unforeseen damage associated with downtime at the customer's premises”, says Sven Penkwitt.

The precisely metered inflow of sewage sludge into the centrifuge is crucial for an efficient separation process. For this purpose, a magnetic-inductive flow sensor is installed in the inflow. This sensor reliably detects the flow of a wide variety of liquid and viscous media, in this case the amount of biomass or sewage sludge that is added and that ranges from typically 15 to a maximum of 56 cubic metres per hour. This measured value is combined with both the feed pump and the centrifuge control in the plant controller. The sensor operates on the magnetic-inductive measuring principle. It offers the following advantage: the measuring section is free of any measuring element or other component that solid components of the sewage sludge could adhere to, blocking the pipe.

In addition, the sensor also measures the temperature of the conveyed sewage sludge. The viscosity of this sludge is higher in the cold seasons - a decisive factor that must be taken into account when feeding the centrifuge.

The storage tank with the polymer concentrate is monitored via the KQ1000 level sensor. This sensor is mounted on the outside of the tank and detects the fill level through the tank wall. Three switching signals with different priorities remind you to refill the tank. Sven Penkwitt has found a particularly pragmatic solution here: "Depending on the delivery time and the required quantity of the product, the sensor’s installation position on the supply tank may be higher or lower, which gives us flexibility in the lead time of the order."

The broad product portfolio makes it possible: with sensor technology from ifm and all the advantages of IO-Link, even a complex plant with many variables that have to be taken into account for maximum efficiency can be completely automated. However, it is not only the "hardware” that makes a difference, service, too, counts. This is how Sven Penkwitt sums it up: "The people at ifm understand what is at stake. They understand and support you, whether by phone and on site. The availability is also excellent: I call by 3pm and have my sensor the next day. This is of major importance for us in maintenance and service, because ultimately I am responsible towards my customers. I know of hardly any supplier in the electrotechnical sector that comes even close to this performance. That's why we rely on ifm wherever possible. Because we get everything we need from them: competence, reliability and speed."