Fruitful approaches towards a mountain of food waste
By Angela TAM
Have you enjoyed a buffet recently? Hong Kong is probably the buffet capital of the world, given the common love of food and the popularity of a dining format that allows people to sample different cuisines at the same meal.
This love of food, however, comes at a price: food waste. Although some charities have made arrangements to collect unconsumed food from various restaurants, hotels and supermarkets for distribution among the city's needy, the bulk of it is still discarded, which is a waste of resources and a burden on Hong Kong's landfills, which are fast running out of capacity.
Given the shortage of landfill space and opposition against their expansion, innovative ideas to overcome Hong Kong's waste problem are urgently needed.
To prevent more waste from being disposed of in landfills in the first place, the Environmental Protection Department (EPD) began a study in 2006 that looked into the collection of food waste from the commercial and industrial sectors for biological treatment. A pilot composting plant was set up in Kowloon Bay to treat 4 tons of food waste per day. The process essentially involves accelerated composting: a bulking agent is added to the waste, which is placed in a rotating drum for mixing and aerobic composting.
The composter's temperature was set at 55 deg C to kill off pathogens. The waste is semi-composted within about ten days and maturisation takes about six weeks, after which the composted material is passed through a screen to remove the bulking agent. Set up in November 2008 for a 12-month trial, the plant was used to treat food waste from the Olympic venues. During the trial, 200 tons of source-separated waste was collected and 30 tons of compost conforming to the requirements set down by the Hong Kong Organic Resource Centre was made.
Commercial interest in buying the compost suggested the approach had potential and was crucial in the Government's plans to develop an organic waste treatment facility (OWTF) in Hong Kong capable of treating 400-500 tons of food waste a day. Of two sites identified - one in Siu Ho Wan and the other in the North District - the 2 ha site in Siu Ho Wan was preferred as it is already zoned for government use and occupied by other waste management facilities; that is, a sewage treatment works and a refuse transfer station; as well as a water treatment works.
The idea is to have an OWTF that would recover as much waste as possible, taking into account the unique profile of Hong Kong's food waste, which tends to be greasier and more watery than those collected in the West. After exploring various treatment options, the department finally decided to choose anaerobic digestion plus composting. To avoid contamination, collected waste would be pre-treated to recover metals and reduce the waste volume. The biogas generated by the process is to be cleaned and used to power the facility through a combined heat and power generation unit while the digested material would be taken through post-treatment for water removal and composting.
The first phase of the OWTF is expected to reduce the volume of waste going to landfills by 60,000 tons a year, generate enough electricity to supply 3,000 households and produce 7,000 tons of compost.
The EPD completed prequalification for a design-build-operate contract for the facility a while ago and expects to complete the tendering process by the end of 2012. Under this schedule, the first-phase facility will be completed in 2014. A coastal site in Sha Ling in the New Territories which was previously used for livestock waste composting has been identified for a second-phase plant with the capacity to treat 300 tons of food waste a day. Commissioning of this plant is expected in 2016/17.
Whoever undertakes the development and operation of the plant may consider a treatment process that has been developed locally by researchers at the City University of Hong Kong (CityU). Dr Patrick Lee, assistant professor at CityU's School of Energy & Environment (SEE), has discovered a mixture of bacteria that could optimally generate bioenergy from food waste.
Using advanced DNA sequencing, Dr Lee studied a few hundred types of bacteria and how they would combine to produce methane in an anaerobic (oxygen-free) environment. Five bacteria that performed well in a mixture were then used to treat food waste collected from CityU's canteen. Laboratory results showed that the process could reduce the food volume by 50% during the conversion of the waste into methane; while the remaining residue, which is rich in nutrients such as nitrogen and phosphorous, could be turned into fertilisers through composting to further decrease the volume by 75%.
Dr Lee said his teamís research showed the microbial process was effective in producing methane to generate heat and electricity, thus reducing our dependence on fossil fuels. According to their research data, the amount of electricity generated through this process could potentially cover 1-2% of local electricity consumption if all of the approximately 1.3 million tonnes of food waste generated were converted.
"According to an overseas study, the conversion of food waste into methane could reduce 400 kg CO2 emissions per tonne of food waste treated, so the process has an additional benefit for our carbon footprint," Dr Lee said.
The process is also much more efficient than methane extraction from landfills. "The methane yield would be much higher because it's concentrated extraction," he said.
Dr Lee received HK$1 million in funding from the Research Grants Council for his research, which has reached the proof-of-concept stage in the laboratory. Now he is looking to scale up but acknowledges that the low cost of electricity in Hong Kong may deter interest as the capital cost of building a plant for the process can be substantial.
From waste to useful products
Also looking to scale up an alternative concept is Dr Lee's colleague Dr Carol Lin, also an assistant professor at SEE, whose research involves converting food waste back into commercial products.
Dr Lin's idea is to set up a biorefinery which will use enzymatic hydrolysis to break down the macromolecules in food waste into simple sugars and free amino nitrogen which can be subsequently utilised by bacterium for succinic acid fermentation. Succinic acid is commonly used as a flavouring agent by the food and beverage industry and has a wide range of pharmaceutical and industrial applications, from clothing fibres to paints and auto bumpers.
"Basically, [the process] needs bioreactors for the fungal fermentation and the subsequent bacterial fermentation for succinic acid production. Then a vacuum evaporation and distillation method is used to recover succinic acid," Dr Lin explained. "This is not an energy-intensive process since the operation temperature for both fungal and bacterial fermentation are mild (55 deg C and 37 deg C respectively)."
Dr Lin's team tried different bacteria, such as Actinobacillus susccinogenes and E coli, for succinic acid fermentation. They discovered that A succinogenes could produce the highest amount of succinic acid with the input of CO2 while an E coli mutant could only produce succinic acid with a lower concentration with the input of air.
The concept was tested using bakery waste as well as mixed food waste from CityU's canteens and shown to work well.
"Based on the techno-economic study, this food waste biorefinery strategy could generate profit by selling succinic acid produced and the solid biomass produced during fermentation as animal feed," Dr Lin said. "It will take five years for this plant to generate profit with a rate of return of 19.6%."
She added that the optimal size for a commercially viable biorefinery could be determined by using the software SuperPro Designer.
Due to concerns over climate change and a growing shortage of fossil fuels, the concept of biorefining has been gaining traction in recent years, with the World Economic Forum issuing a report on the subject in 2010 that looked at issues such as feedstock, conversion techniques and optimum plant size. However, many of the pilots set up around the world are based on using agricultural waste or crops that can grow on marginal land as feedstock. Hong Kong is unusual in that downstream waste is being targeted - with good reason: according to Dr Lin, the Hong Kong Organic Waste Recycling Centre currently collects 1 ton of unconsumed bakery per day. In the city as a whole, over 1.3 million tonnes of food waste accounting for one-third of municipal solid waste is generated a year. There is so much food waste, one university estimates that the food waste collected from its campus in one day is enough to produce compost for fertilising all its greenery for one month.
There is clearly commercial potential in the excess.
"Organic waste materials should no longer be treated as waste, but as a valuable resource that can be recovered and transformed into useful products. Through the work of our faculty members and researchers, we hope to harness the potential of food waste and contribute to a more sustainable and green environment for Hong Kong and around the world," said SSE dean Prof Johnny C L Chan.
While both CityU approaches will, if adopted, significantly reduce the amount of food waste disposed of in landfills, Dr Lee is frank about his ambitions.
"I don't mind working myself out of a job," he said. "After all, we shouldn't be wasting so much food. The food that we throw away needs energy to grow, package and distribute, so when we waste food we're also wasting energy. It would be better if we don't do that in the first place."
The Kowloon Bay pilot plant demonstrated the commercial viability of composting. Image: EPD & EMSD
The treatment process used at CityU is similar to the one proposed for the OWTF. Image: CityU
Compost produced at the Kowloon Bay pilot plant. Image: EPD & EMSD
The fermentation process used to convert food waste into succinic acid. Image: CityU
The succinic acid yielded by the treatment process. Image: CityU