Preliminary Treatment

Grit Pumping Systems:
Adding Water to the Suction Line

It's not uncommon for grit pumping systems to use a centrifugal grit pump, to pump a grit slurry from a grit collector to a cyclonic gritseparator e.g. Pista Systems. Every time we were hit with high grit loads...the pumping system would fail. It took a long time (and seeing a competitor's design) to realize that the centrifugal pump won't work when the grit to run a water line to discharge near the end of the grit pump's suction pipe. No matter how much grit loading occurs... there's water to the suction pipe for the grit pump to work. We went from a system failure for every storm or 2nd pump starting to now...where it's only happened once since the system went on-line.

Lowering Level in Wet Well, to Reduce Grit Loading During Storms

By: Carl Michels and plant staff

Every time we had a storm, our screens and grit removal system were likely to fail. We looked at the well level at the suggestion of our consultant.

The designated operating levels made our entire interceptor a grit chamber. We had about 4 feet of sewage in a 5 foot interceptor...with flows ranging from 5,000 to 10,000 gpm on a normal day.

We liked the idea, but were concerned about the level of our pumps. We knew we had the potential of getting our pumps air bound. Our solution included:

1. We only lowered the set-point of the lead pump. The idea was to get the well lower, but we were going to need height to bleed other pumps. This also helped provide us flow equalization for free!!
2. We installed a solenoid valve on the pump bleed lines and modified the pump control circuit. Just before the pump is needed to start, the solenoid valve opens...allowing the pump to bleed at every cycle.
3. Eventually, we opted to pipe the bleeding system high enough to prevent flow when the solenoid valve opened (it just fills-up the pipe...reducing flow and blockages).

NOTE: EXTREME CAUTION IS NEEDED IF YOU MAKE A CHANGE THAT COULD RESULT IN GETTING PUMPS AIR BOUND. IF THERE'S A SHADOW OF DOUBT...GET HELP TO DO IT RIGHT!!!!

Note: It's entirely possible that even a minor well level decrease for the lead pump will help reduce problems. If nothing else...look at the situation and see if you have a grit trap too.

Note: With PLC pump control systems, it would be easy enough to have the solenoids operated directly by the PLC, prior to the time the raw pump is called for.

The idea may seem like a headache....but getting rid of the slug loads every storm made it well worthwhile.

Spray Bar on Fine Screen

By: Ed Artymuik

At Waukesha, our Parkson Aqua Guard fine screens do a good job of removing rags and debris from the wastewater. However, there is a lot of organic material (you know what I mean) that it picks up. To wash the screenings, this spray bar was built from a piece of one inch stainless steel pipe that was tapped for spray nozzles. The supply water is controlled by a 480 volt solenoid connected to two leads of the fine screen electric motor. Thus, when the motor starts, the solenoid opens allowing the water to wash the debris.

Fine Screen Spray BarThe picture to the left shows the wash system in operation. Though it may be hard to interpret from the picture, the screen conveyor rotates within the waste stream, trapping debris on the screen. As the screen conveyor travels by the wash bar it is cleaned of any free matter that is sticking to the rags.

The volume of rags has decreased by about a fourth and made the rag press (which dewaters the rags) a much cleaner operation without all the mess that used to be associated with the organic material that was squeezed out.