Undeterred by the dirty jobs

It's a piece of critical infrastructure that all urban dwellers rely on, but one which they’d probably prefer was unseen, unheard and most importantly un-smelt – the local wastewater treatment plant. These facilities are hugely expensive and complex and require frequent monitoring to ensure that they continue to run efficiently and comply with relevant regulations and consents.

The Challenge

The Waimakiriri District Council needed to measure the depth of sludge in the Rangiora and Waikuku treatment ponds, and so they asked Fox & Associates to step in (as it were). Previously the Council had commissioned manual measurements of the physical sludge depth, however this didn’t provide an accurate picture of how full the ponds were, the pattern of sludge buildup or how much capacity remained. The Council also didn’t have any accurate records of the completed dimensions of the ponds at the time they were built.

Before diving in, it was important to think about how the survey could be done safely. The ponds contain all manner of unpleasant (not to mention hazardous) substances, and the ponds, and their surrounds can be dangerously slippery. Some ponds are aerated, which dramatically reducing the density of the water potentially meaning that boats (and people) are less buoyant and are at danger of sinking to the bottom. The ponds are also in continuous operation, therefore not able to be shut down while the survey was being conducted.

The Solution

It was decided to undertake the survey using our Bathycat USV (Unmanned Surface Vessel – a radio-controlled boat) which is equipped with survey-grade GPS to record accurate positions and a sonar sensor to measure the depth of water beneath the hull. This system can produce a rich dataset while also keeping our staff out of harm’s way. A LiDAR survey using one of our UAVs (Unmanned Aerial Vehicles - drones) would enable modelling of the surrounding ground pond above the waterline.

The work at Rangiora and Waikuku was undertaken over the course of 3 days with staff working in pairs to deploy, retrieve (and importantly clean) the boat, carry out the UAV survey and undertake check measurements at safely accessible locations around the ponds to verify the sonar and LiDAR readings. The field team don’t rate this type of work as one of their favourite jobs ever, but the work was completed safely and efficiently. While the Bathycat is equipped with an autopilot that can follow a preset pattern this was not used for these surveys due to the large number of obstructions within the ponds. Interestingly, having now piloted the boat manually around the pond, knowing where the obstructions lie, we think it would be possible to automate future surveys by having the Bathycat follow the track of the first survey.

Turning Data Into Insight

Once the fieldwork was complete, the captured and processed data and combined into one CAD software output. One of the challenges with the sonar data was to ‘clean’ the data to remove data points representing floating objects suspended in the water beneath the boat – it doesn’t bear thinking about! We also had to construct a digital 3d model of the empty ponds, based on a combination of the design plans and measurements to clear areas within the ponds. Once we had the model of the empty ponds we could illustrate and quantify the amount, location and pattern of sludge build up.

All of this information was presented to the client in the form of CAD models and plans which clearly illustrate the pattern of material build up and clear channels in the ponds formed by the action of the various pumps, aerators and other devices. The data will enable the client to validate the theory of how the systems are supposed to work against the reality on the ground (or under the water, in this case). The client can then better manage the aeration of the pond and the overall management of waste within the system.

It's not the kind of work we undertake every day, but far from merely ‘going through the motions’ this leaned heavily into the Fox value of looking for a better way to help a client manage a facility that is of benefit to the whole community.