Vogelsang and Thames Water celebrate the 18th birthday of pumps installed at Crawley
20 November 2006, News release from Vogelsang Ltd
This is an article about pumps - the heart of most processes that involve liquids or sludges. Like the human heart, pumps in industrial processes do not really do anything specifically interesting but without them no other process could work properly. If you run an internet search on effluent treatment or primary sludge you can find lots of sites which describe the processes but none of them describe the pumps which are used to move materials between each process stage.
by Toby Clarke, Vogelsang UK.
The Thames Water effluent treatment works at Crawley uses nine Vogelsang positive displacement rotary lobe pumps to transfer primary sludge from settlement tanks to the next processing stage. Each pump is rated to pump 20 m3/h of sludge with approximately 2 m suction lift. These pumps were installed in 1988 so have been in operation now for over 18 years and the only down time has been for planned routine maintenance. There has been no break in service due to breakdowns or any underperformance issues.
For people not used to working in municipal effluent treatment, information about the duty these pumps perform may not mean very much, so we'll now outline a 'typical' sewage treatment works scheme. Firstly the incoming sewage passes through a fine screen. This removes the larger items that people have flushed down toilets, or items which have found their way into the drains such as plastics and pieces of wood. These materials are then chopped up, put into skips and taken to landfill sites. (Thames Water has a campaign in place to encourage their customers not to put household rubbish down the drain.)
The sewage then flows into channels where heavy particles of grit - a mixture of sand, ash, stones and other small items - will sink. This material is then removed and taken to landfill sites.
The screened sewage flows into large tanks where the remaining heavy solids will sink to the bottom. When fresh sewage or wastewater is added to a settling tank, approximately 50% of the suspended solid matter will settle out in about an hour and a half. This collection of solids is known as raw sludge or primary sludge and is said to be "fresh" before anaerobic processes become active. Once anaerobic bacteria take over, the sludge will become putrescent in a short time and must be removed from the sedimentation tank before this happens.
At the Crawley site these settlement tanks have a sloping bottom and the Vogelsang pumps are connected to off takes at the low point of the sloping base. This is at a lower level than the pumps so the pumps have to self prime and draw the effluent up approximately 2 metres. The separated sludge is transferred to a sludge treatment plant and the remaining liquid (or settled sewage) is treated with bacteria. (There is an animated diagram on the Thames Water web site which illustrates this process for anyone not familiar with the general processes here).
One of these pump installations is shown in figures 1 and 2 below. The first picture (Figure 1) shows the external installation adjacent to a settlement tank with a protective cover over the pump and motor. The second picture (Figure 2) shows the inlet and outlet connections to the pump head and a mechanical pressure relief valve with recycle loop which allows material to be recirculated around the pump if there is any blockage downstream of the pump.
Typically untreated primary sludge contains between 2 and 8 % total dry solids (TS), this primary sludge contains approximately 3 - 4% dry solids. The pumps operate for approximately 10 minutes every hour on the Crawley site which has been defined as the optimum time to remove most of the settled sludge without also pumping out too much of the liquid.
The sludge solids consists of grease and fats (6 - 35% of TS), protein (20 - 30% of TS), Nitrogen, phosphorous and potash (2.3 - 7.8% of TS), cellulose (8 - 15% of TS), Iron (2 - 4% of TS) and silica (15 - 20% of TS). The pH will be 5.0 - 8.0 and the organic acids 200 - 2,000 mg/l as HAc. This primary sludge is usually grey and slimy and, in most cases, has an extremely offensive odour. Primary sludge can, however, be readily digested under suitable conditions of operation. The specific gravity of these sludge solids is around 1.4 while the specific gravity of the sludge is 1.02.
The quantity of solids entering the wastewater treatment plant daily may be expected to fluctuate over a wide range. The plant is designed by considering the average and maximum rates of sludge production and the potential storage capacity of the treatment units within the plant.
In this installation the main duty requirements for the pumps are that they are self priming and can operate with a suction head, as well as the duty requirements that they can pump semi-solid waste reliably and operate in a stop/start mode. It is also important that the pumps operate reliably. Different types of pumps can be considered for this application, for example, centrifugal, progressive cavity, reciprocating piston pumps and rotary lobe. The pumps used on this site are rotary lobe type pumps.
Rotary lobe pumps offer a number of benefits for this duty. They are self priming (to 8 metres) and, although not a requirement for this particular site application, they can also run dry for a period of time. (For example, Vogelsang guarantee a 30 minute dry running time for their rotary lobe pumps, compared with centrifugal and progressive cavity pumps which have a very short or no dry running capability). The rotary lobe pumps installed on this site are fitted with the original design of rotors rather than the modern 'HiFlo pulsation-free' rotors invented and developed by Vogelsang and now used as standard on all their rotary lobe pumps, such as on the pumps installed on the Thames Water Reading site discussed below. This pulsation free operation is achieved by putting a 'twist' into each rotor lobe which then achieves a +/- 0% pulse compared with the +/- 3% pulsation from a progressive cavity pump.
This innovative development in rotary lobe pump design was developed in the Research and Development facility that Vogelsang operate in their German factory. Over 70 staff work there on new pump and applications development department alone.
A further benefit of these rotary lobe pumps is that the routine maintenance activities can be carried out on the pump head without disconnecting any pipework or removing the pump from its installed position. Removing the front cover of the pump head allows access to the rotor lobes, mechanical seals and wear plates. So carrying out a full routine service on a pump head will typically take one person less than one hour and no heavy lifting is involved.
The typical, expected life for any pump in this type of application used when calculating whole life costs is anywhere up to 25 years. The Vogelsang pumps in this application have operated for over 18 years now with no breakdowns and show no sign of stopping. Dean Perry, Thames Water Area Engineering Supervisor responsible for these pumps said "there is not a lot to say about them, you want pumps to do their job and work reliably and these ones do just that".
On another Thames Water effluent site at Reading, Vogelsang HiFlo rotary lobe pumps are in use at a later stage in the process. Many people will have seen this site when driving along the M4, a picture of the 'landmark' anaerobic digester vessels is shown in Figure 3. After being belt thickened and blended, the primary and secondary activated sludges enter the Alpha Biotherm pasteurisation plant's thermophilic aerobic digester through a macerator, where they are held at 70°C for one hour. The pasteurised sludge is subsequently pumped to four bottom-fed, egg-shaped, anaerobic digesters, each some 20 m high. These are the first of their kind in the UK, constructed entirely from reinforced concrete with external silver cladding. The heat required for the pasteurisation and digestion processes is provided either by digester biogas through CHP engines or three dual fuel (biogas / diesel) boilers.
Originally, centrifugal pumps were used to pump sludge to the pasteuriser plant's digester. At this stage in the process the sludge contains 8% dry solids and the centrifugal pumps failed because they could not maintain the required flow and pressure.
These centrifugal pumps were replaced by the Vogelsang rotary lobe pumps and a picture of this installation is shown in Figure 4. The pumps in this application have a positive feed pressure and have to operate for approximately 40 minutes every hour.
As can be seen from Figure 4, the space available for the installation was quite limited so the pump motors were mounted above the pump heads. This effluent treatment works was opened in January 2005 and Vogelsang and Thames Water look forward to writing an article about the performance of these pumps in 2024. Sludge is a generic term for solids separated from suspension in a liquid by a variety of processes. As has been discussed above, most commonly sludge refers to solid waste extracted in the process of sewage treatment; the term sewage sludge is used commonly. However, industrial wastewater solids are also referred to as sludge, whether generated from biological or physical-chemical processes. Some examples of other 'sludges' which are typically processed using the positive displacement rotary lobe pumps are:
Using rotary lobe pumps in these, and other, applications can provide a number of benefits. For example, due to the compact size of these pumps, they can be installed in locations where some other types of pumps could not be used, and the construction costs of pump houses can be reduced because they occupy approximately 1/3 of the space of progressive cavity pumps. Vogelsang's pumps typically use less energy than centrifugal pumps offering a cost saving and, as mentioned above, maintenance is simplified with the pump being left in-line while maintenance is performed. One additional feature of rotary lobe pumps is that the direction of pumping may be reversed by simply reversing the direction of rotation of the motor, the piping to the pump does not need to be changed. In the applications discussed in this article that is not a specific benefit but in some transfer applications it can be very useful to pump in either direction as required.
Figure 1 : View of one pump installation showing protective housing
Figure 2 : View showing pump head with inlet and outlet connections
Figure 3 : External view of anaerobic digester vessels on Thames Water Reading site
Figure 4 : Rotary lobe pumps installed in the location
where centrifugal pumps were removed
For further information please email Vogelsang Ltd