18 March 2008

Ice pigs offer novel way to check outfall

Take up to 10 large blocks of ice, a few ping pong balls, tennis balls and a crayfish buoy or two. Add half a dozen experienced divers and two days of low tides and good weather, and the scene is set for a novel attempt to clear the city’s 1.8km marine outfall pipe of any possible obstructions.

The cleaning, while scheduled, is an important part of getting the design right for the city’s new wastewater system and treatment plant. The aim is to move as much debris as possible today [subs: 18 March] weather permitting.

Divers from Wairoa’s Peter Blunden Diving Services will remove the pipe’s cap end, then remove and clean the diffuser ports before fastening them closed. The city’s wastewater will be held back for a time and, with all three pumps working to capacity, will be released in a large volume to purge the pipe. One pump will run throughout the night to move more. On Wednesday, a series of different sized “ice pigs” will be launched, starting with the smallest within which ping pong balls will be frozen, ranging to the biggest with crayfish buoys inside. This “pigging” operation, similar to that used in the petrochemical industry with “steel pigs”, will help identify any potential obstructions.

For the past fortnight, the Wastewater Project’s design consultants CH2M Beca have monitored the flow of wastewater through the outfall operating all three existing pumps continuously rather than allowing gravity discharge part of the time. They assessed how much energy was needed to pump each cubic metre of wastewater before the pipe was clean, and will repeat the exercise for another fortnight after the cleaning. Beca engineer Chris French expects a noticeable reduction in the energy required once the pipe is clean.

“The information we gain will help us more accurately gauge the size of the new pumps required, ensure their optimum efficiency and reduce ongoing energy requirements.”

The city’s wastewater currently flows down the pipe by gravity at low tide but is assisted by pumps as the tide rises and pressure builds up. Gravity and pumps will also be used in the new system.

Fulton Hogan engineering department manager Trevor Ward, who manages the city’s milliscreening and outfall operations, says the trial will help determine whether there are any physical blockages in the last 250 metres of the pipe. The ice and balls could take an hour to float down. The longer they take, the more likely there are blockages.

“We don’t want to put anything into the outfall that might jam it up. If the ice pig hits an obstruction, the ice will melt – aided by the warmth of the wastewater – and flow out.”

The city’s wastewater will be held up for about 30-40 minutes at a time before being let go in a rush. Divers will return to blast compacted sand, sticks, dirt and fat from the walls of the pipe if the pigging hasn't achieved the desirable level of cleaning expected.

Two of the entry points for debris have been closed. Before 1990 material could enter the outfall pipe through the open “stilling basin” within the milliscreening operation. Debris from the sea –animal bones, driftwood, crayfish floats, heavy duty rope and the like – was able to be sucked back into the last 250 metres of the outfall pipe through the 22 diffuser ports. In early 2007, these ports were fitted with stainless steel and rubber duckbill valves that allow wastewater to flow out but deny entry to debris from the sea.

The only debris able to enter the outfall now is through the milliscreens or with stormwater. Sand reduces the diffusers’ efficiency and is a particular problem. Much of it washes off roads, enters the stormwater system and infiltrates its way into the wastewater before discharge through the outfall. Gisborne District Council is working hard to reduce or eliminate the amount of stormwater getting into the wastewater system.

While alternative long-term cleaning methods are being assessed, the proof of the ice pig trial will be in how fast the balls pop out the end.

ENDS