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A couple of sewage digesters turned out to be twice as big a challenge as the small company that built them expected. HUGH DE LACY investigates.
G&T was formed in 1994 by principal Lex Thompson, and has grown steadily to a current staff of 13 and turnover of around $6 million a year from a range of infrastructure projects in the Canterbury region. In late 2005 the company won the contract to build the two new digesters at the Bromley treatment plant that break down the solids in Christchurch’s sewage over a 20-day period, so the treated wastewater can be discharged into Pegasus Bay through the three-kilometre long ocean outfall. Work began early in 2006 and the project was ready to be commissioned 24 months later when leaks were discovered that required a complete review of the design, and a job that was supposed to have taken two years suddenly stretched to four. The project had begun with the installation of sheet-piling and dewatering systems following excavation for the digesters’ under-tank drainage system to a depth of five metres below the water table in marine silts. The drainage pump station required excavation a further two metres deep to allow groundwater to be drawn down so the tanks wouldn’t start floating during their occasional future maintenance draining.
The concrete in-fills between the panels were installed in a single pour using specially built formwork and a specific concrete formula supplied by Christchurch Readymix, pumped in by Grant Brothers. The tanks were then post-tensioned and sealants were installed over all internal wall and floor joints. While this was going on the steel roofs were being fabricated by Ewing Construction on a nearby unused tank. The roofs were built from steel sections rolled to radius, welded together on site to form the frame, then plated on the underside. An inflatable tent was used to encapsulate the completed roofs to allow sandblasting and painting to be carried out regardless of the weather. The project also included a 320 cubic metre fibreglass buffer tank for sludge storage. It was manufactured in Auckland and barged to Christchurch, entering the Avon/Heathcote estuary on a favourable tide one morning and, after a couple of gentle groundings, being off-loaded at a local yacht club near the Christchurch wastewater treatment plant oxidation ponds.
Local firm Lyttelton Engineering installed the array of pumps, heat exchangers, valves and pipework over a nine-months period. Melray Electric installed the 11Kv power supply and automated control system. Everything was ready to commission in early 2008 when testing of the digesters at their operating temperature revealed leakages from the tank floors. After a fair bit of head-scratching the designers decided that the best way to deal with the problem was to line the floors of both digesters with 8mm steel plate. This process began with a concrete ring beam being formed round the inside perimeter of the tanks as an anchorage for the floor plates. Sections of plate cut to profile were then craned into the tanks through the central roof opening, and welded into position. This took more than three kilometres of welding using standard electrodes.
The two digesters built by G&T Construction are part of a plant-wide upgrade which has included automation of most of the treatment process, and the company’s contract was expanded in 2008 to include the automation of four existing digesters – that latter work being commissioned later the same year. By last month the two new digesters had been seeded with sludge from the existing digesters, with further amounts being added in stages. Over the next few months they will be operated firstly in mesophyllic conditions before being heated later in the year to the designed thermophyllic operating temperature. G&T managing director Lex Thompson told Contractor the project was the largest the company had undertaken, “and it tested our resources at times”. It had been a shock to discover the leaks caused by the design flaw just as the digesters were originally about to be commissioned, and it took the willing support of staff and subcontractors to knuckle down again and effect the modifications needed to make them work properly. The company and staff will be on hand on March 24 for the Christchurch City Council’s button-pushing ceremony which will commission the Pegasus Bay ocean outfall, the country’s longest. For all the frustration G&T encountered with the digester tanks, it paled into comparison with the problems that plagued the outfall project itself, which took the lives of two workers in October 2008. The new outfall, fed by G&T’s digesters, will be commissioned just seven days before the expiry of the Environment Canterbury consent which previously allowed treated wastewater to be discharged directly into the Avon/Heathcote estuary, the source of consternation for years in environment-conscious Christchurch.
Contractor Vol.34 No.3 April 2010 |
It was a key forerunner to the biggest construction project ever undertaken by the Christchurch City Council – the soon-to-be-commissioned $87.2 million Pegasus Bay ocean wastewater outlet – but small local company G&T Construction found the building of two 7000 cubic metre digesters was a project fraught with bigger problems than it could have imagined.
The digesters operate at a temperature of 57 degrees Celsius, so sub-floor insulation had to be provided by way of a 200mm layer of polyurethane foam sprayed between the layers of site concrete. The concrete floors were then poured in situ, and the 12 metre high walls, in the form of pre-cast and pre-stressed panels weighing more than 40 tonnes each, were placed by a 40-tonne crawler crane.
After water-testing of the digesters, the 70-tonne roofs were hoisted into place by the crawler crane, which required 200 tonnes of ballast to counter the final lift radius. A plant room was built between the digesters for the mechanism and pipework required to heat and mix the contents, and distribute them to other parts of the treatment plant.
After sandblasting and painting, the tanks were filled with water and again tested for leaks at both 37 and 55 degrees Celsius. This time they performed as they were supposed to, and they were ready to be incorporated into the wider project.