PDL motor controllers play important part in Feilding sewage treatment

APPLICATION BULLETIN - OCTOBER 1994 | BACK TO INDEX

PDL motor controllers play important part in Feilding sewage treatment

The tanks at Fileding Sewage Station are part of the effluent breakdown process.

The Manawatu District Council have recently finished giving their Feilding Sewage Treatment Plant a major upgrade and at the heart of the new treatment system is a series of PDL Electronics-supplied Microflo and Microdrive variable speed motor controllers.

"Management of the varying flows to the sewage treatment plant would have been very difficult without the PDL variable speed drive units and we would have been limited in the design approaches we could take,' says David Stewart, an associate of Dunedin-based Royds Consultants. "The only other way would have been to use the conventional stop/start on the pumps which would have made the whole flow control issue much more difficult. In this particular plant all flow has to be pumped and the need to match flows through different parts of the process means that the PDL speed control units are essential. Their performance has been excellent."

The upgrade was carried out in five main steps with the major construction work taking place over 1992 and 1993 and involved:

Construction of a second aerated lagoon.
Removal of existing pumps and piping from the influent (incoming) pump well and installation of larger pumps with PDL Microflo controllers along with piping to enable all influent sewage to be pumped to the aerated lagoons for treatment (with the exception of storm flows).
Reconfiguration of the sedimentation tank from a primary half and a secondary half to become a single parallel flow primary tank.
Construction of a solids contact tank and a clarifier to convert the plant to the Trickle Filter - Solids Contact (TF-SC) process, which extracts the by-products of the biological breakdown process.
Modification to the pump well at the end of the sedimentation tank and installation of new pumps with Microflo controllers to pump the effluent from the trickle filters to the solids contact tank and hence to the clarifier, before discharge to the nearby Oroua river.

Fielding plant manager Russell Dean is pleased with the way the conversion to the new technology has gone. "We'd shopped around and seen a lot of other people going to variable speed units so we knew they'd work well in sewage treatment. We've had the PDL controllers up and running for a year now and never had any problem with them at all. They're pretty easy to operate and as a result we're fitting most of our older pumps with them as well.

"In the sludge treatment area we can divert the sludge either back through the plant to help with clarification or, if we've got a surplus amount of sludge, transfer that back to sedimentation into the digester. So we've actually put a variable speed unit on that pump as well, which gives us a more accurate control of our return sludge. We've never had such a good range of sludge return before.

Aerated Lagoons where the influent sewage is treated.

"Apart from the plant pumps we've also installed a PDL Microdrive to control the scraper on the clarifier. It's the speed of the clarifier bridge rotation which controls the scraping of the sludge - the scraper is attached to the bridge and is a suction type designed to suck sludge from the base of the clarifier rather than disturbing it and causing re-suspension. Just at the moment I've got it ticking over at about one revolution every four hours, whereas if we get a buildup of sludge we can speed it up to cope with the load. The PDL drives certainly give us a tremendous amount of flexibility over how we control the plant. We're more than happy with them."

Sid Wildbor, an electrician with Feilding-based Dewe Electrical, responsible for the hardwiring of the plant, expands on the role of the PDL controllers at the input end of the Fielding plant.

"The normal flow rate fluctuates between about 150 - 450 cubic metres an hour over the day with an average flow rate per day of around 5,000 - 6,000 cubic metres. Pumps P1 and P2 at the intake end are identical Flygt 3201 submersible pumps. Both pumps are speed-controlled by PDL Microflo drives. The signals to the Microflos on P1 and P2 are generated by a Millitronics OCM11 level controller situated in the pump well (#1). It has a 4-20mA output which feeds into the two Microflos. They have a different trigger level. One is set to turn off it drops below 35 Hertz and the other one is set to drop off below 30 Hertz so it means that one is the duty pump. The sewage treatment plant operator can alter which one is duty pump on the keyboard, simply by altering the set points for the level at which the pumps trip out.

"The Millitronics sensor is what is called multi-ranger ultra-sound controller - that is, it bounces a signal off the surface of the effluent in the pump well - and on that we've got our bottom set point which is the limit for the output to start reading and we set the span over which we want to work. It doesn't necessarily mean though, that anything above zero is going to start pumping. All it means is that at zero or above, it will start registering a frequency on the Microflo.

"Trimate Industries didn't really want the pumps running below 30 Hertz. So we set one at 30 and one at 35. The level has to come up a reasonable amount in the pump well before the pumps actually start pumping. When they do start, they start at 30 Hertz so they're running at about 60% speed and because it's a soft start you don't get any water hammer or the wear problems you get with direct-on-line starting. Anything below 25 Hertz they won't pump because the head is too low. Through matching our sensor and PDL Microflos to one another, we can get very close control over flow rates. We're very pleased with the way the Microflos perform. You couldn't get the control we're getting without them."

In the final count, the upgraded plant is very much in touch with the latest thinking on sewage disposal.

"The plant upgrade was not only aimed at dealing with the increased industrial loading from Fielding,' explains David Stewart, "it was also aimed at improving the standard of treatment so that the final discharge to the Oroua River was able to meet existing discharge consent conditions under the Resource Management Act. The plant now has the capability of producing 30(BOD):30(SS)' quality for flows up to 10003/h, which includes moderate stormwater inflows - that's a significant quality improvement over the old plant.

"The next development at the plant will route the discharge into a wetlands, which will give it a final scrubbing before arriving at the river. That's one of the favoured environmentally sensitive solutions to modern waste water treatment and will result in a very high quality discharge to the Oroua River.
"We're very satisfied with the contribution PDL's technology has made to the overall success of this scheme."