With the handling of sludge – often the most problematic area in the wastewater treatment process – two wastewater treatment facilities in Scotland and Germany installed non-clogging pumps for greater efficiencies and savings in maintenance.

Efficient solutions for wastewater treatment plants that handle large amounts of sludge are an expensive and complex operation. As treatment plants become larger and more sophisticated, problems become even more complex and costly. That is why it is important to make the right configuration and equipment choices, especially when choosing pumps that will efficiently handle difficult sludge applications.

In two separate applications in Europe, wastewater treatment plant operators installed non-clogging pumps to solve tough process applications.

Handling Problematic Sludge

Untreated domestic and industrial sewage in Scotland has, for a long time, been piped out to sea along the attractive Moray Firth coast. This pollution has resulted in a steady decline in the fishing industry and thinning tourist arrivals.

However, thanks to a scheme put in place for Scottish Water (Dunfermline, Scotland) by a consortium called Catchment, effluent from the local communities of 55,000 inhabitants is being carefully treated before discharge. Along with this treatment process, a valuable agricultural fertilizer is being produced.

Scottish Water and the local communities decided to meet the environmental demands. Centered at Lossiemouth, three sequencing batch reactor (SBR) treatment plants – Oakenhead Wood, Buckie and Macduff – were built to take care of the wastewater from 22 existing pump stations along the coast.

The plants process raw effluent, using the SBR method. This is an activated sludge process that forces bacterial breakdown of the crude sewage. As the incoming sewage consists of up to 7 percent dry solids, it was important to provide a solution that could handle this problematic sludge. Therefore, Scottish Water decided to install various sizes of non-clogging pumps with self-cleaning impellers and high-sustained efficiency.

Various non-clogging pumps play key roles in the transfer and recirculation of raw and processed sewage at the Oakenhead Wood, Buckie and Macduff waste-water treatment plants in Scotland. 

<caption>Various non-clogging pumps play key roles in the transfer and recirculation of raw and processed sewage at the Oakenhead Wood, Buckie and Macduff wastewater treatment plants in Scotland.

The Oakenhead Wood plant treats 10,000-cubic meters of raw sewage per day. Much of this input comes from its own catchment area, but large quantities of raw sewage are also imported by tankers from other locations in Scottish Water’s network. Also, 17 percent dry solids (DS) sludge cakes from Buckie and Macduff are transported to Oakenhead for transformation into agricultural fertilizer pellets. The plant has been designed with great attention to detail in order to provide trouble-free sludge handling pumps.

For instance, the pipe system design is simple and imposes minimal losses for the imported sludge transfer and recirculation applications. The suction pipe from the screened sludge tanks has been sized to present the sludge to the pump in the best manner possible.

One of the pumps recirculates the imported raw sludge in order to avoid sedimentation. After screening, the sludge is pumped to a storage tank. There another pump mixes the sludge with the incoming waste activated sludge from the SBR process. The treated effluent is discharged to sea while the sludge cake goes to a sludge dryer where all bacteria are killed by the 220-deg C temperatures within the dryer. This procedure transforms the sludge cake into 92 percent dry solid valuable fertilizer pellets, ready for agricultural use.

Due to the chosen treatment process, the final effluent is of good quality and is being discharged to sea. At the same time, nearly five tons a day of valuable bio-waste granules for use in agriculture are being produced. The choice of reliable non-clogging pumps has resulted in trouble-free sludge handling. This, in turn, brought significant savings in both capital and maintenance costs.

In all, 13 non-clogging pumps are in operation in the Oakenhead Wood plant, as well as another eleven others divided between the Buckie and Macduff treatment plants.

A Public Utility Treatment Solution

The municipal wastewater treatment plant in Zweibrücken (southern Germany) is a modern SBR treatment plant, designed for a population of 70,000. Although modernized between 1997 and 2000, the community-owned plant faced continual problems with clogging of its three screw impeller pumps, installed in the digester to circulate sludge from the digester across heat exchangers.

Although all solid material more than 6-mm in diameter had been filtered out before the sludge reached the digester, the sludge contained 4.5 percent dry solids (DS), mostly fine rags and sand. All three pumps were alternating: one was always in operation, a second pump started when new sludge was added to the cycle, and a third pump was on standby.

Rags built up on the front of the screw impeller. Those rags would compress into larger blockages that got stuck in the screw design. The pumps could be blocked several times a day and much time was spent by the service staff on dismounting, cleaning, and reassembling each clogged pump. It was obvious that these pumps were not suitable for this kind of application.

Having heard of non-clogging pumps, the community installed a trial pump. One of the problem pumps was replaced by a non-clogging pump running at 7.5-kW with a capacity of 100-m3/hour (28-l/s). The pump fit to the existing piping and electrical system. 

Due to continual clogging problems, a trial non-clogging pump was installed to replace one of the three screw pumps.

The impeller and volute of the pump inhibit clogging around the single vane. As a result of the open design of the impeller and a relief groove in the volute, material which could gather on the vane is washed out before it collects up and blocks the pump.

For the wastewater treatment plant in Zweibrücken, that means not only fewer stoppages but also smoother running. The new installed pump has, in a very short period, led to a more reliable process and remarkably lowered maintenance costs. 

Pumps & Systems, November 2006