Water Journal : Current May 2016
53 www.awa.asn.au CASE STUDY: TRASH TO TREASURE This year, Yarra Valley Water (YVW) will notch up an Australian first, as the Victorian utility flicks the switch on a new plant generating biogas using 100% food waste. "Others in the industry have been doing [co-digestion], but we're going further into organic waste than anyone has before," Commercial Services Manager Andrew Edney said. The $27 million plant was designed to slash the utility's rising power bills and increase revenue, which is why sewage feedstock will not be used beyond the 2016 commissioning phase. "If we put sewage in [the anaerobic digester], the business case is a little more problematic," Major Projects Engineer Ian Donald said. Local food and beverage companies will pay YVW to take their waste, which will be converted into enough energy to power the equivalent of utility's four largest treatment plants. "The Aurora Sewage Treatment Plant next door will use about 30% of the electricity produced and the rest will then be sent out into the electricity grid," Donald said. Technology provider Aquatec- Maxcon will operate the 1MW capacity plant for the first two years. Solid organic waste will be placed in two solids hopper feeders, shredded and chopped up and mixed with liquid organic waste, before being pumped either direct to the digesters, or into short-term storage to be fed to the digesters automatically outside of business hours. The system is designed to pump 100 tonnes of organic waste into two stainless-steel mesophilic single- stage digesters each day. The biogas from the digesters is collected in the domed roofs of the digesters and sent via a carbon-scrubbing unit to two Combined Heat and Power (CHP) 500kW engines. The planning and approvals process for the ambitious facility was extensive, with the board, waste industry groups and regulators engaged as early as 2011. "Liaising with the Environmental Protection Agency, getting them on-board early and getting all the rough edges out meant when we got to the design and construction phase, we didn't have any barriers," Edney said. "One of the key messages is the opportunity for the waste and water industries to work more collaboratively together; leveraging the water industry's land buffer zones and ability with technical solutions." Donald said the project required a challenging shift in mindset: from service provider to business. "We had to ring-fence the project so that if it wasn't making money, we weren't going to cannibalise people's water bills to pay for it," he said. extract, while still removing pollutants. Advancements are not only being made in co-digestion, but also algal biomass and upfront carbon extraction. One way to increase energy production from wastewater is to encourage algal growth. Flinders University s Howard Fallowfield is at the forefront of Australian research in this area and said trials at Melbourne Water s Werribee site show that by making wastewater lagoons shallower (30--50cm rather 1.2--1.4m) and incorporating mixing, you can produce 70 tonnes of dry matter per hectare annually. "The algae are two to three times more productive than a sugar cane crop would be per hectare," Fallowfield said. "We re looking at between 40 and 160 megawatt hours per hectare (of algae), variable with season." The algae could be used to generate biogas or biodiesel and may be particularly appropriate for wastewater treatment plants in rural communities. Another idea being explored internationally is boosting biogas yields by extracting organic carbon directly from wastewater, rather than sewage treatment sludge. The problem with this concept has been that removing carbon first left none for essential de-nitrification purposes. However, University of Queensland Advance Water Management Centre Director Professor Zhiguo Yuan said his team has come up with a way to remove the nitrogen with little-to-no carbon. They do this using a biocidal compound UQ discovered five years ago called free nitrous acid, which can be produced from a waste stream in a sewage treatment plant. Pilot studies are still needed, but Yuan anticipates the breakthrough could help existing wastewater treatment plants triple biogas production with only minor retrofitting. "All we need to do is incorporate a new, very simple tank to be operated at normal temperature and normal atmospheric MARCO POLO OBSERVED THE CHINESE EXTRACTING BIOGAS FROM COVERED SEWAGE POTS, AND IN 1890S ENGLAND GAS FROM SEWAGE TREATMENT POWERED STREETLIGHTS.
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