A dip into the world of scum

Scum, by the definition of the word, is a surface phenomenon. For any kind of process tank relying on gravity to settle out suspended solids (SS), the presence of floating scum is therefore a potential problem, as it clearly defies the main operating principle.

At one of Yorkshire Water’s largest WwTWs, Knostrop, in Leeds, scum formation in the primary tanks has occurred intermittently since the first works were established in the Victorian era. However, in a short space of time water engineers have now tested and installed a system called SCUDA (self contained underwater dipping apparatus), a patented electrically/electronically operated system, which is helping the company deal with Knostrop’s scum problems.

Knostrop WwTW serves a densely populated area of west Yorkshire. Industrial trade effluent accounts for 94% of the flow into its two low-level sewers and up to 30% into another pair of sewers entering at high level.

Grease and fats come predominantly from food manufacturing and constitute most of the solids rising to the surface of the primary tanks. This scum formation occurs as the solids decay, producing ammonia and nitrogen, which brings them to the top.

The resulting effect is potentially not only extremely unsightly, with thick scum covering the water surface, but the fats can also gave rise to odours, blockages and high maintenance costs.

Conventionally, the specified piece of equipment for dealing with scum and floating sludge is either the box and scraper type, which relies on the scraper to pull the scum up a ramp and into a hopper, or a cylindrical device that partially rotates in order to draw off scum through a longitudinal slot.

Unfortunately when the level of grease contamination is high, the scum bridges across the slot or box and the quantity of water taken in with the solids is insufficient to carry it into the receiving wells. The lines also become caked with scum and for several years at Knostrop, the lines have periodically required clearing, using a high-pressure water jet to relieve blockages.

Other difficulties can arise when solids build up to the extent that they spill over the scum board, or are forced underneath it. Residues then flow over the weirs and into the biological filters, leading to downstream contamination and maintenance problems.

Determined to resolve these problems, Yorkshire Water began working with the principal contractor, MWH and the site’s engineering contractors, MJ Gleeson, to improve scum removal from the primary tanks. In February 2001 MJ Gleeson brought in a SCUDA system, designed and patented by Thornwood Roche Developments, for trials on one of the problem primary tanks. New Haden Pumps, a sales distributor for the SCUDA, installed the system on the most contaminated tank.

The trial unit had an immediate impact, as Yorkshire Water senior operator, Keith Lowde acknowledged: “It was obvious almost straightaway that Thornwood Roche had the solution to our problem. The system operates automatically and the settings can be adjusted and optimised for the given influent and tank conditions. But even on the initial set-up the effect, after 48 hours, was unbelievable. The water had a clear surface after starting off with a thick covering of scum.”

Prior to the trial, the engineering contractor had looked at the possibility of using other mechanical scum removers, without testing them in-situ. However, two weeks into a trial period of nine months, MJ Gleeson engineers were confident the SCUDA should be installed on the remaining primary tanks. Consequently, the last in a total of 17 units has just been installed at Knostrop.

David Sykes, MJ Gleeson’s contract manager for the project added: “The effect of this dipping process is not just cosmetic. In the previous situation we had grease going into the scum boxes and, without sufficient water to flush it into the receiving well, the grease was clogging the holes in the filter beds. The system does a lot of work on the primary tanks towards ensuring the client meets environmental law and EU directives but we also think the system could be used effectively in secondary treatment tanks.

“Working closely with David Lee of New Haden Pumps and Ken Wooton of Thornwood Roche we were able to engineer a solution to our client’s specification. Starting with the basic SCUDA design they added braking to the motor specification and used components from Yorkshire Water’s preferred suppliers list. We had great support at every stage of the trials”.
David Lee’s experience with the system began while working at a WwTW in Lancashire, where the influent contained traces of residual metallic nitrates from a local catalyst manufacturing plant. The nitrate-enriched washings denitrified in the primary tanks, bringing nitrogen-containing sludge to the surface as a heavy scum. These primary tanks became the proving ground for the early SCUDA design. The graphs below illustrate the device’s performance at the site.

“The idea was at a very early stage of development when we put the first one into use. Nevertheless the results were pretty remarkable. The visible aspect of the problem, that is the scum, was removed but monitoring the plant performance before and after installation showed that it had a measurable effect on the water quality. We saw a 19% fall in suspended solids in the primary tank effluent and a corresponding 24% fall in the final effluent discharged from the humus tanks.

“Our results proved that the system worked on a site with primary tank scum problems. I would even say it restored some pride in the operators, who had become used to seeing the tanks in a terrible state and were struggling to keep them clear. Perhaps more importantly for Thornwood Roche, it allowed them to optimise the design in a working WwTW environment, under severe conditions.”

At Knostrop there is a considerable amount of renovation work taking place downstream of the primary tanks, making monitoring and analysis of the full system difficult for the time being. However, as Yorkshire Water’s Keith Lowde points out, the main influence of the SCUDA system is already clear: “We do not expect to see all the benefits for six or seven months, when the residues should have cleared the whole system. We will then be undertaking extra monitoring on the humus tanks and outfalls to give us a more detailed picture of the performance improvement. In the meantime, the change in the appearance of the primary tanks has been amazing.”

The main components of the SCUDA are a stainless steel hopper, a motorised dipping mechanism and a unique control panel. The hopper has a dipping depth of 95mm, with the top resting position of the hopper rim 50mm above the normal water level. This allows for fluctuating water levels. When the process tank’s rotating bridge activates a proximity switch some 2m before the SCUDA, a signal to the control panel initiates the dipping sequence.

The hopper dips to a level just below the surface, pre-set by an adjustable probe. Water then flushes into the hopper and exits through a flexible hose to the normal scum discharge pipework. Any scum or floating sludge pushed to the hopper by the bridge scraper is carried off in the flow of water.

The dipping sequence can be programmed at the control panel, which also has facilities for telemetry monitoring. Sequence control allows the cycle to be optimised, according to the nature of the scum. For example, the hopper and downstream pipes can be primed, or wetted by flushing with water before the bridge scraper approaches. This assists the removal of sticky scum during the main dip, which typically lasts for ten seconds. At Knostrop the hopper dips for a third time, after the scraper passes, giving the system a further rinse with water to minimise any likelihood of clogging.